ALBERT

Description: [1]  We investigated the evolution of seismicity and deformation in the unstable eastern flank of Etna volcano over a thirty-year period (from 1980 to 2012). A significant temporal correlation has been revealed between periods of flank acceleration and intensified seismic activity by comparing seismicity along the northern border (Pernicana fault system) of the sliding flank and the deformation of the eastern flank. Two marked phases have been observed in 1984-1986 and in the years following 2002. These two phases are separated by an intermediate phase from 1987 to 2001, in which the flank sliding slowed down and the seismicity dropped drastically. This common temporal evolution in the deformation rate and seismic release supports the hypothesis that the seismicity in the northern border can be viewed as a marker of the response to accommodate the stress exerted by the traction of the eastern flank sliding. This interplay has also been corroborated by Finite Element Method (FEM) numerical computations that highlight a good correlation between the seismicity pattern and areas of positive stress changes induced by the sliding surface. The two intense phases of flank acceleration are representative of two main different sources: volcano flank instability stretching the eastern sector in the first 1984-1986 phase and magmatic intrusions pushing the eastern flank seaward since the 2002-03 eruption. Establishing the relationship between flank acceleration and seismic activation, therefore, contributes to understanding Etna's mechanical behavior, and provides insights into the processes regulating the unstable flank response.

Description: [1]  Numerous studies in the Central Pyrenees have provided evidence for a rapid phase of exhumation of this mountain belt during the Late Eocene (37–30 Ma). Simultaneously, the closure of the Ebro foreland basin allowed the accumulation of sediments at the southern Piedmont, which partially covered the fold-and-thrust belt from Late Eocene ( e . g . when it was still actively deforming) to Miocene times. We aim here at understanding the consequences of such syn-tectonic sedimentation on the Southern Pyrenean fold-and-thrust belt by using a 2-D numerical model that reproduces the development of a thin-skinned wedge subject to different modes of sedimentation and erosion. The results show contrasting fold-and-thrust belt behavior when applying aggrading or prograding sedimentation, which we link to the critical state of the wedge. When the sediments are sourced from the hinterland (progradation), the thrusting propagates toward the foreland; whereas when the sediments aggrade from the basin, the thrusting sequence migrates backward. This latter mode shows patterns of deformation that compare favorably to the Pyrenean thrusting sequence observed during Eocene-Miocene times.

Description: [1]  In southern California, fault slip rate estimates along the San Andreas fault (SAF) and Garlock fault from geodetically-constrained kinematic models are systematically at the low end or lower than geologic slip rate estimates. The sum of geodetic model slip rates across the Eastern California Shear Zone is higher than the geologic sum. However, the ranges of reported model and geologic slip rate estimates in the literature are sufficiently large that it remains unclear whether these apparent discrepancies are real, or attributable to epistemic uncertainties in the two types of estimates. We further examine uncertainties in geodetically-derived slip rate estimates on major faults in southern California by conducting a suite of inversions with four kinematic models. Long-term-rigid elastic block models constrained by the geologic slip rates cannot fit the present-day GPS-derived velocity field. Deforming (permanent off-fault strain) elastic block models and viscoelastic earthquake cycle block models constrained by geologic slip rates can fit the present-day GPS-derived velocity field with 28-33% of the total geodetic moment rate occurring as distributed deformation off of the major faults. Models incorporating viscoelastic mantle flow predict systematically higher slip rates than purely elastic models on many of the the major southern California faults with ranges of (elastic/viscoelastic) 29-34/30-37 mm/yr for the Carrizo SAF segment, 20-24/20-32 mm/yr for the Mojave SAF segment, 14-17/18-22 mm/yr for the Coachella SAF segment, 13-19/14-22 mm/yr for the San Jacinto fault, and 5-11/5-11 mm/yr for the western Garlock fault.

Description: This paper proposes a method to identify blocking onset and decay by means of two stability indicators: enstrophy advection and its integral. The key to this technique is the use of local Lyapunov exponents for the barotropic vorticity equation which can be approximated by the integral of enstrophy (IRE) over a fixed, finite, region. The IRE can then be viewed as a measure of stability. However, by differentiating the IRE with respect to time, two measures of stability can be derived to assess blocking onset and decay: (i) the integral of enstrophy advection (DIRE), for which a time series is used to assess stability; and (ii) enstrophy advection, for which contours are plotted in conjunction with 500 hPa heights to locate blocking. One year of Northern Hemisphere blocking events from July 2011-July 2012 are studied to demonstrate that the integral of enstrophy advection is a useful diagnostic. In particular, time series of IRE and DIRE for four of the blocking cases are presented, while contour plots of enstrophy advection for one case are presented. In all cases studied, the diagnostics were seen to detect the instability in an incipient blocking event and in its decay.

Description: The Weather Research and Forecasting (WRF) model was used to simulate the evolution of Tropical Storm Ivan (2004) in the southeast (SE) US using both the Yonsei University (YSU) and Mellor-Yamada-Janjić (MYJ) boundary layer parameterizations. In contrast with tropical cyclone (TC) simulations over the ocean, the effect of surface layer becomes secondary for a dissipating hurricane along its terrestrial track. Although these two schemes can reproduce Ivan reasonably well, our results suggest that the mixing properties for damped mechanical turbulent conditions (weakly stable) are strongly underestimated by both parameterizations. This underestimation impacts the thermodynamic properties of the storm, leading to significant differences in the storm areal extent and the simulated precipitation fields. Suggestions for further improvements are provided. An evaluation of the impact of using or not using a convective parameterization, specifically the Kain-Fritsch (KF) scheme, at 3 km grid spacing shows marginal impact on storm coverage, intensity and precipitation, except for the presence of widespread light rainfall in the Piedmont east of the mountains when the KF is employed. Analysis of the thermal structure of the simulated storm indicates that, in the inner-storm region, the KF is either not activated or primarily produces ( parameterized ) shallow convection. As a result, the net heating tendency associated with adiabatic and diabatic processes is almost unaltered inside the storm, together with a nearly equivalent surface momentum sink, leading to similar storm areal extent and intensity. Light rainfall to the east of the mountains can be due to the trigger mechanism of KF, which depends on boundary layer convergence, forcing parameterized deep convection near the coast, where surface roughness changes enhance convergence.

Description: Heavy precipitation events (HPEs) affect the south-eastern area of France frequently during the months of September to November. Very high amounts of rain can fall during these events, with the ensuing flash-floods causing widespread damage. The cases of the 6th of September 2010 and the 1st to the 4th of November 2011 represent the different large-scale conditions in which these episodes can occur. These HPEs are forecast with differing levels of skill by the Méso-NH model at a 2.5 km resolution. The case of the 6th of September is used to test different methods of addressing cloud physics parameterisation uncertainties. Three ensembles are constructed, where the warm process microphysical time tendencies are perturbed by different methods. Results are compared by examining the spatio-temporal distribution of the precipitation field as well as looking at ensemble statistics. The ensemble methodology which induces the most dispersion in the rainfall field is deemed the most suitable. This method is then used to examine the sensitivity of four cases from November 2011 to errors in the microphysical and turbulent parameterisations. It appears that according to the model skill for the HPE, the sensitivity to microphysical perturbations varies. Events where the model skill is high (low) show low (moderate) sensitivity. These cases show a stronger sensitivity to perturbations performed upon the turbulent tendencies, while perturbing the microphysical and turbulent tendencies together produces even further dispersion. The results show the importance and the usefulness of ensembles with perturbed physical parameterisations in the forecasting of HPEs.

Description: This paper analyses the annual mean vertical and latitudinal structure of the Brewer-Dobson circulation in the CMIP5 models. The strength of the tropical mass upwelling is found to increase at all altitudes throughout the stratosphere due to climate change. However, the width of the tropical upwelling region narrows below about 20hPa, and widens above 20hPa, suggesting different physical mechanisms may play a role in this change above and below 20hPa. In the lower stratosphere, an equatorward shift in the stationary wave critical line allows waves to propagate further into the tropics. However, in the upper stratosphere, where the behaviour is dominated by what happens during the winter, an increase in the extratropical zonal mean westerly jet leads to a reduced equatorward refraction of planetary waves. The seasonal cycle of the change in the Brewer-Dobson circulation is also considered, and differences are found in the latitudinal structure of the increased extratropical downwelling between the Northern and Southern Hemispheres in winter.

Description: Sub-seasonal forecasts have been routinely produced at ECMWF since 2002 with re-forecasts produced "on the fly" to calibrate the real-time sub-seasonal forecasts. In this study, the skill of the re-forecasts from April 2002 to March 2012 and covering a common set of years (1995 to 2001) has been evaluated. Results indicate that the skill of the ECMWF re-forecasts to predict the Madden Julian Oscillation has improved significantly since 2002, with an average gain of about 1 day of prediction skill per year. The amplitude of the MJO has also become more realistic, although the model still tends to produce MJOs which are weaker than in the ECMWF re-analysis. As a consequence, the ability of the ECMWF model to simulate realistic MJO teleconnections over the northern and southern Extratropics has improved dramatically over the 10-year period. Forecast skill scores have also improved in the Extratropics. For instance, weekly mean forecasts of the North Atlantic Oscillation Index are more skillful in recent years than ten years ago. A large part of this improvement seems to be linked to the improvements in the representation of the Madden Julian Oscillation. Skill to predict 2-metre temperature anomalies over the northern Extratropics has also improved almost continuously since 2002. Changes in the horizontal and vertical resolutions of the atmospheric model had only a small impact on the skill scores, suggesting that most of the improvements in the ECMWF sub-seasonal forecasts were due to changes in model physics which were primarily designed to improve the model climate and medium-range forecasts. The impact of changes in the data assimilation system and in the observing data has not been considered in this study, since all the re-forecasts used for this study were initialized from the same re-analysis over a common set of years.

Description: Dual-polarisation radar measurements provide valuable information about the shapes and orientations of atmospheric ice particles. For quantitative interpretation of these data in the Rayleigh regime, common practice is to approximate the true ice crystal shape with that of a spheroid. Calculations using the discrete dipole approximation for a wide range of crystal aspect ratios demonstrate that approximating hexagonal plates as spheroids leads to significant errors in the predicted differential reflectivity, by as much as 1.5dB. An empirical modification of the shape factors in Gans's spheroid theory was made using the numerical data. The resulting simple expressions, like Gans's theory, can be applied to crystals in any desired orientation, illuminated by an arbitrarily polarised wave, but are much more accurate for hexagonal particles. Calculations of the scattering from more complex branched and dendritic crystals indicate that these may be accurately modelled using the new expression, but with a reduced permittivity dependent on the volume of ice relative to an enclosing hexagonal prism.

Description: The parameterisation of diabatic processes in numerical models is critical for the accuracy of weather forecasts and for climate projections. A novel approach to the evaluation of these processes in models is introduced in this contribution. The approach combines a suite of on-line tracer diagnostics with off-line trajectory calculations. Each tracer tracks accumulative changes in potential temperature associated with a particular parameterised diabatic process in the model. A comparison of tracers therefore allows the identification of the most active diabatic processes and their downstream impacts. The tracers are combined with trajectories computed using model-resolved winds, allowing the various diabatic contributions to be tracked back to their time and location of occurrence. We have used this approach to investigate diabatic processes within a simulated extratropical cyclone. We focus on the warm conveyor belt, in which the dominant diabatic contributions come from large-scale latent heating and parameterised convection. By contrasting two simulations, one with standard convection parameterisation settings and another with reduced parameterised convection, the effects of parameterised convection on the structure of the cyclone have been determined. Under reduced parameterised convection conditions, the large-scale latent heating is forced to release convective instability that would otherwise have been released by the convection parameterisation. Although the spatial distribution of precipitation depends on the details of the split between parameterised convection and large-scale latent heating, the total precipitation amount associated with the cyclone remains largely unchanged. For reduced parameterised convection, a more rapid and stronger latent heating episode takes place as air ascends within the warm conveyor belt.

Description: Motivated by observations of the mean state of tropical precipitable water (PW), a moist, first baroclinic mode, shallow water system on an equatorial β -plane with a background saturation profile that depends on latitude and longitude is studied. In the presence of a latitudinal moisture gradient, linear analysis of the non-rotating problem reveals large-scale, symmetric, eastward and westward propagating unstable modes. The introduction of a zonal moisture gradient breaks the east–west symmetry of the unstable modes. The effects of rotation are then included by numerically solving the resulting eigenvalue problem on an equatorial β -plane. With a purely meridional moisture gradient, the system supports large-scale, low-frequency, eastward and westward moving neutral modes. Some of the similarities, and some of the discrepancies of these modes with intraseasonal tropical waves are pointed out. Finally, a zonal moisture gradient in the presence of rotation renders some of the aforementioned neutral modes unstable. In particular, as per observations of large-scale, low-frequency tropical variability, it is seen that regions where the background saturation profile increases (decreases) to the east favour eastward (westward) moving moist modes.

Description: In this study changes in the Northern Hemisphere winter storm tracks during the 20th century are investigated based on the individual 56 ensemble-members of the 20th Century Reanalysis dataset. It is found that the 20th century trends in storm track activities exhibit large discrepancies between the upper and lower troposphere. In the upper troposphere, a substantial intensification is identified at the poleward and downstream regions of the North Pacific and North Atlantic storm track activities, indicating a large northeastward expansion of storm tracks in the late 20th century. However, in the lower troposphere the synoptic eddy activities, especially in terms of the eddy kinetic energy (EKE) and meridional eddy heat flux, tend to be significantly weakened over the high-latitudes of central-western North Pacific and the upstream regions of the North Atlantic storm tracks. Further inspections find that such strengthening (weakening) of storm tracks in the upper (lower) troposphere are mainly attributed to the increase (decrease) of the baroclinic instability, which is predominantly determined by the meridional temperature gradient changes. Moreover, from a local energetic perspective, the baroclinic generation and barotropic damping of the synoptic eddies are found to be substantially enhanced at the upstream and downstream regions of the two storm tracks in the upper troposphere, respectively, while in the lower troposphere the baroclinic energy conversion to eddies are generally decreased.

Description: We derive a family of ideal (nondissipative) 3D sound-proof fluid models that includes both the Lipps-Hemler anelastic approximation (AA) and the Durran pseudo-incompressible approximation (PIA). This family of models arises in the Euler-Poincaré framework involving a constrained Hamilton's principle expressed in the Eulerian fluid description. The derivation in this framework establishes the following properties of each member of the entire family: the Kelvin-Noether circulation theorem, conservation of potential vorticity on fluid parcels, a Lie-Poisson Hamiltonian formulation possessing conserved Casimirs, a conserved domain integrated energy and an associated variational principle satisfied by the equilibrium solutions. Having set the stage with the derivations of 3D models using the constrained Hamilton's principle, we then derive the corresponding 2D vertical slice models for these sound-proof theories.

Description: [1]  Determining the scale-length, magnitude, and distribution of heterogeneity in the lowermost mantle is crucial to understanding whole mantle dynamics, and yet it remains a much debated and ongoing challenge in geophysics. Common shortcomings of current seismically-derived lowermost mantle models are incomplete raypath coverage, arbitrary model parameterization, inaccurate uncertainty estimates, and an ad hoc definition of the misfit function in the optimization framework. In response, we present a new approach to global tomography. Apart from improving the existing raypath coverage using only high quality cross-correlated waveforms, the problem is addressed within a Bayesian framework where explicit regularization of model parameters is notrequired. We obtain high resolution images, complete with uncertainty estimates, of the lowermost mantle P-wave velocity structure using a hand-picked dataset of PKPab-df, PKPbc-df, and PcP-P differential traveltimes. Most importantly, our results demonstrate that the root mean square of the P-wave velocity variations in the lowermost mantle is approximately 0.87%, which is three times larger than previous global-scale estimates.

Description: [1]  The Dzhungarian strike-slip fault of Central Asia is one of a series of long, NW-SE right-lateral strike-slip faults that are characteristic of the northern Tien Shan region, and extends over 300 km from the high mountains into the Kazakh Platform. Our field-based and satellite observations reveal that the Dzhungarian fault can be characterised by three 100 km long sections based on variation in strike direction. Through morphological analysis of offset streams and alluvial fans, and through OSLdating, we find that the Dzhungarian fault has a minimum average late Quaternary slip rate of 2.2 ± 0.8 mm/yr and accommodates N-S shortening related to the India-Eurasia collision. This shortening may also be partly accommodated by counter-clockwise rotation about a vertical axis. Evidence for a possible paleo-earthquake rupture indicates that earthquakes up to at least Mw 7 can be associated with just the partitioned component of reverse slip on segments of the central section of the fault up to 30 km long. An event rupturing longer sections of the Dzhungarian fault has the potential to generate greater magnitude earthquakes ( Mw 8), however long time periods (e.g. thousands of years) are expected in order to accumulate enough strain to generate such earthquakes.

Description: The surface wind response to SST and SST meridional gradient is investigated in the Gulf of Guinea by using daily observations and reanalyses in the 2000–2009 decade, with a focus on boreal spring and summer months (May to August), where quasi-biweekly fluctuations in the position of the northern front of the equatorial cold tongue induce quasi-biweekly equatorial sea surface temperature (SST) anomalies. Following a large-scale wind acceleration (deceleration), an equatorial SST cold (warm) anomaly is created within a few days. In order to explain the local atmospheric response to this SST anomaly, the two following mechanisms are invoked: first, a colder (warmer) ocean decreases (increases) the vertical stability in the marine atmospheric boundary layer, which favors a weaker (stronger) surface wind; and second, a negative (positive) anomaly of SST meridional gradient induces a positive (negative) anomaly of sea level pressure meridional gradient, which decelerate (accelerate) the surface wind. The first mechanism has an immediate effect in the equatorial belt between 1°S-1°N (and to a lesser extent between 3°S and 1°S), while the second takes one or two days to adjust and damps anomalous southeasterlies up to 800 hPa in the low troposphere between 7°S and 1°N, through reversed anomalies of meridional SST and pressure gradient. This negative feedback leads to weaker (stronger) winds in the southeastern Tropical Atlantic, which forces the opposite phase of the oscillation within about a week. Around the equator, where the amplitude of the oscillation is found maximal, both mechanisms combine to maximize the wind response to the front fluctuations. Between the equator and the coast, a low-level secondary atmospheric circulation takes control of the surface wind acceleration or deceleration around 3°N, which reduces the influence of the SST front fluctuations.

Description: The purpose of the operational-oriented system COALITION ( C ontext and Scale Oriented Thunderstorm Satellite Predictors Development) is to automatically detect severe thunderstorms early in their development and consequently help weather forecasters to increase lead time when issuing severe weather warnings. This new object-oriented system integrates data provided by different sources. Data from Meteosat Second Generation Rapid Scan Service, weather radar and numerical weather prediction, as well as climatology are utilized by the system. One of its primary purposes is to use all the best operationally available information about convective processes and to integrate it into a heuristic model. Furthermore the orographic forcing, which is often neglected in heuristic nowcasting models, is taken into account and included in the system as an additional convective triggering mechanism. This is particularly important for areas characterized by complex orography like the Alpine region. The COALITION algorithm merges evolving thunderstorm properties with selected predictors. The forecasted evolution of the storm is the result of the interaction between convective signatures and surrounding storm environment. Eight different "object-environment" interactions are analysed in eight modules, providing ensemble nowcasts of thunderstorm attributes (satellite- and radar-based) for the following 60 minutes. All ensemble nowcasts are then combined through a weighting and thresholding scheme and the results are summarized into a single graphical map in order to facilitate user interpretation. The COALITION nowcast system has an update frequency of 5 minutes. The output highlights the cells having a high probability of severe thunderstorm development within the next 30 minutes. Verification statistics confirm that COALITION is able to nowcast the intensity of developing convective cells with sufficient skill up to a lead time of about 20 minutes.

Description: In late February 2010 the extraordinary windstorm Xynthia crossed over Southwestern and Central Europe and caused severe damage, affecting particularly the Spanish and French Atlantic coasts. The storm was embedded in uncommon large-scale atmospheric and boundary conditions prior to and during its development, namely enhanced sea surface temperatures (SST) within the low-level entrainment zone of air masses, an unusual southerly position of the polar jet stream, and a remarkable split jet structure in the upper troposphere. To analyse the processes that led to the rapid intensification of this exceptional storm originating close to the subtropics (30°N), the sensitivity of the cyclone intensification to latent heat release is determined using the regional climate model COSMO-CLM forced with ERA-Interim data. A control simulation with observed SST shows that moist and warm air masses originating from the subtropical North Atlantic were involved in the cyclogenesis process and led to the formation of a vertical tower with high values of potential vorticity (PV). Sensitivity studies with reduced SST or increased laminar boundary roughness for heat led to reduced surface latent heat fluxes. This induced both a weaker and partly retarded development of the cyclone and a weakening of the PV-tower together with reduced diabatic heating rates, particularly at lower and mid levels. We infer that diabatic processes played a crucial role during the phase of rapid deepening of Xynthia and thus to its intensity over the Southeastern North Atlantic. We suggest that windstorms like Xynthia may occur more frequently under future climate conditions due to the warming SSTs and potentially enhanced latent heat release, thus increasing the windstorm risk for Southwestern Europe.

Description: We present a series of idealized numerical model experiments to investigate aspects of deep convection in tropical depressions, including the effects of a boundary layer wind structure on storm structure, especially on vertical vorticity production and updraught splitting, and the combined effects of horizontal and vertical shear on vertical vorticity production, with and without background rotation. In warm-cored disturbances such as tropical depressions, the vertical shear and horizontal vorticity change sign at some level near the top of the boundary layer so that, unlike in the typical middle-latitude ‘supercell’ storm, the tilting of horizontal vorticity by a convective updraught leads not only to dipole patterns of vertical vorticity, but also to a reversal in sign of the updraught rotation with height. This finding has implications for understanding the merger of convectively-induced vorticity anomalies during vortex evolution. Ambient cyclonic horizontal shear and/or cyclonic vertical vorticity favour amplification of the cyclonically-rotating gyre of the dipole. Consistent with an earlier study, storm splitting occurs in environments with pure horizontal shear as well as pure vertical shear, but the morphology of splitting is different. In both situations, splitting is found to require a relatively unstable sounding and relatively strong wind shear.

Description: [1]  We performed shock compression experiments on preheated forsterite liquid (Mg 2 SiO 4 ) at an initial temperature of 2273 K and have revised the equation of state (EOS) that was previously determined by shock melting of initially solid Mg 2 SiO 4 (300 K). The linear Hugoniot, U S  = 2.674 ± 0.188 + 1.64 ± 0.06 u p km/s, constrains the bulk sound speed within a temperature and composition space as yet unexplored by 1-bar ultrasonic experiments. We have also revised the EOS for enstatite liquid (MgSiO 3 ) to exclude experiments that may have been only partially melted upon shock compression and also the EOS for anorthite liquid, which now excludes potentially un-relaxed experiments at low pressure. The revised fits and the previously determined EOS of fayalite and diopside were used to produce isentropes in the multicomponent CaO-MgO-Al 2 O 3 -SiO 2 -FeO system at elevated temperatures and pressures. Our results are similar to those previously presented for peridotite and simplified “chondrite” liquids such that regardless of where crystallization first occurs, the liquidus solid sinks upon formation. This process is not conducive to the formation of a basal magma ocean. We also examined the chemical and physical plausibility of the partial melt hypothesis to explain the occurrence and characteristics of ultralow velocity zones. We determined that the ambient mantle cannot produce an equilibrium partial melt and residue that is sufficiently dense to be a ULVZ mush. The partial melt would need to be segregated from its equilibrium residue and combined with a denser solid component to achieve a sufficiently large aggregate density.

Description: [1]  High resolution sparker and crustal-scale airgun seismic reflection data, coupled with repeat bathymetric surveys, document a region of repeated coseismic uplift on the portion of the Alaska subduction zone that ruptured in 1964. This area defines the western limit of Prince William Sound. Differencing of vintage and modern bathymetric surveys shows that the region of greatest uplift related to the 1964 Great Alaska earthquake was focused along a series of sub-parallel faults beneath Prince William Sound and the adjacent Gulf of Alaska shelf. Bathymetric differencing indicates that 12 m of coseismic uplift occurred along two faults that reached the sea floor as submarine terraces on the Cape Cleare bank southwest of Montague Island. Sparker seismic reflection data provide cumulative Holocene slip estimates as high as 9 mm/yr along a series of splay thrust faults within both the inner wedge and transition zone of the accretionary prism. Crustal seismic data show that these megathrust splay faults root separately into the subduction zone décollement. Splay fault divergence from this megathrust correlates with changes in mid-crustal seismic velocity and magnetic susceptibility values, best explained by duplexing of the subducted Yakutat terrane rocks above Pacific plate rocks along the trailing edge of the Yakutat terrane. Although each splay fault is capable of independent motion, we conclude that the identified splay faults rupture in a similar pattern during successive megathrust earthquakes and that the region of greatest seismic coupling has remained consistent throughout the Holocene.

Description: [1]  We perform a time-lapse analysis of Rayleigh and Love wave anisotropy above an underground gas storage facility in the Paris Basin. The data were acquired with a three-component seismic array deployed during several days in April and November 2010. Phase velocity and back azimuth of Rayleigh and Love waves are measured in the frequency range 0.2-1.1 Hz using a three-component beamforming algorithm. In both snapshots, higher surface wave modes start dominating the signal above 0.4 Hz with a concurrent increase in back azimuth ranges. We fit anisotropy parameters to the array detections above 0.4 Hz using a bootstrap approach which also provides estimation uncertainty and enables significance testing. The isotropic phase velocity dispersion for Love and Rayleigh waves match for both snapshots. We also observe a stable fast direction of NNW-SSE for Love and Rayleigh waves which is aligned with the preferred orientation of known shallow (〈300 m) and deeper (~1000 m) fault systems in the area, as well as the maximum horizontal stress orientation. At lower frequencies corresponding to deeper parts of the basin, the anisotropic parameters exhibit higher magnitude in the November data. This may perhaps be caused by the higher pore-pressure changes in the gas reservoir in that depth range.

Description: [1]  Eruptive activity at the summit of Kilauea Volcano, Hawaii beginning in 2010 and continuing to the present time is characterized by transient outgassing bursts accompanied by very long period (VLP) seismic signals triggered by rockfalls from the vent walls impacting a lava lake in a pit within the Halemaumau pit crater. We use raw data recorded with a 11-station broadband network to model the source mechanism of signals accompanying two large rockfalls on August 29, 2012 and two smaller average rockfalls obtained by stacking over all events with similar waveforms to improve the signal-to-noise ratio. To determine the source centroid location and source mechanism, we minimize the residual error between data and synthetics calculated by the finite difference method for a point source embedded in a homogeneous medium that takes topography into account. We apply a new waveform inversion method that accounts for the contributions from both translation and tilt in horizontal seismograms through the use of Green's functions representing the seismometer response to translation and tilt ground motions. This method enables a robust description of the source mechanism over the period range 1-1000 s. The VLP signals associated with the rockfalls originate in a source region ~1 km below the eastern perimeter of the Halemaumau pit crater. The observed waveforms are well explained by a simple volumetric source with geometry composed of two intersecting cracks including an east striking crack (dike) dipping 80 ∘ to the north, intersecting a north striking crack (another dike) dipping 65 ∘ to the east. Each rockfall is marked by a similar step-like inflation trailed by decaying oscillations of the volumetric source, attributed to the efficient coupling at the source centroid location of the pressure and momentum changes induced by the rock mass impacting the top of the lava column. Assuming a simple lumped parameter representation of the shallow magmatic system, the observed pressure and volume variations can be modeled with the following attributes: rockfall volume (200 − 4500 m 3 ), length of magma column (120-210 m), diameter of pipe connecting the Halemaumau pit crater to the subjacent dike system (6 m), average thickness of the two underlying dikes (3 – 6 m), and effective magma viscosity (30–210 Pa s). Most rockfalls occur during episodes of sustained deflation of the Kilauea summit. The mass loss rate in the shallow magmatic system is estimated to be 1400  −  15, 000 kg s − 1 based on measurements of the temporal variation of VLP period in the two large rockfalls that occurred on August 29, 2012.

Description: [1]  On 5 September 2012, a large thrust earthquake (M w 7.6) ruptured a densely-instrumented seismic gap on the shallow-dipping plate boundary beneath the Nicoya Peninsula, Costa Rica. Ground motion recordings directly above the rupture zone provide a unique opportunity to study the detailed source process of a large shallow megathrust earthquake using very nearby land observations. Hypocenter relocation using local seismic network data indicates that the event initiated with small emergent seismic waves from a hypocenter ~10 km offshore, 13 km deep on the megathrust. A joint finite-fault inversion using high-rate GPS, strong-motion ground velocity recordings, GPS static offsets, and teleseismic P waves reveals that the primary slip zone (slip 〉 1 m) is located beneath the peninsula. The rupture propagated down-dip from the hypocenter with a rupture velocity of ~3.0 km/s. The primary slip zone extends ~70 km along strike and ~30 km along dip, with an average slip of ~2 m. The associated static stress drop is ~3 MPa. The seismic moment is 3.5 x 10 20  Nm, giving M w  = 7.6. The co-seismic large-slip patch directly overlaps an onshore inter-seismic locked region indicated by geodetic observations, and extends down-dip to the intersection with the upper plate Moho. At deeper depths, below the upper plate Moho, seismic tremor and low frequency earthquakes have been observed. Most tremor locates in adjacent areas of the megathrust that have little co-seismic slip; a region of prior slow slip deformation to the southeast also has no significant co-seismic slip or aftershocks. An offshore locked patch indicated by geodetic observations does not appear to have experienced co-seismic slip, and aftershocks do not overlap this region, allowing the potential for a comparable size rupture offshore in the future.

Description: [1]  Seismic shear waves emitted by earthquakes can be modeled as plane (transverse) waves. When entering an anisotropic medium they can be split into two orthogonal components moving at different speeds. This splitting occurs along an axis, the fast polarization, that is determined by geologic conditions. We present here a comprehensive analysis of the Silver and Chan (1991) method, used to obtain shear wave splitting parameters, comprising theoretical derivations and statistical tests of the assumptions used to construct the standard errors. We find discrepancies in the derivations of equations in their article, with the most important being a mistake in how the standard errors are calculated. Our simulations suggest that the degrees of freedom are being overestimated by this method and consequently the standard errors are too small. Using a set of S waveforms from very similar shallow earthquakes on Reunion Island, we perform a statistical analysis on the noise of these replicates and find that the assumption of Gaussian noise does not hold. Further, the properties of background noise differ substantially from the noise obtained from the shear wave splitting analysis. However, we find that the standard errors for the fast polarization are comparable to the spread in the fast polarization parameters. Delay time errors appear to be comparable to delay time estimates once cycle skipping is accounted for. Future work using synthetic seismograms with simulated noise should be conducted to confirm this is the case for earthquakes in general.

Description: [1]  The sliver strike-slip Great Sumatra Fault (GSF) traverses mainland Sumatra from the Sunda Strait in the southeast to Banda Aceh in the northwest, and defines the present day plate boundary between the Sunda Plate in the north and the Burmese Sliver Plate in the south. It has been well studied on mainland Sumatra but poorly north of Banda Aceh in the Andaman Sea. Here we present deep seismic reflection images along the northward extension of the GSF over 700 km until it joins the Andaman Sea Spreading Centre and we interpret these images in the light of earthquake, gravity, and bathymetry data. We find that the GSF has two strands between Banda Aceh and Nicobar Island: a transpression in the south and a deep narrow active rift system in the north dotted with volcanoes in the center, suggesting that the volcanic arc is coincident with rifting. Further north of Nicobar Island, an active strike-slip fault, the Andaman-Nicobar Fault, cuts through a rifted deep basin until its intersection with the Andaman Sea Spreading Centre. The volcanic arc lies just east of the rift basin. The western margin of this basin seems to be a rifted continental margin, tilted westward, and flooring the Andaman-Nicobar forearc basin. The Andaman-Nicobar forearc basin is bounded in the west by backthrusts similar to the West Andaman and Mentawai faults. The cluster of seismicity after the 2004 great Andaman-Sumatra earthquake just north of Nicobar Island coincides with the intersection of two strike-slip fault systems.

Description: [1]  The Japan Tohoku-Oki earthquake (9.0 Mw) of 11 March 2011 has left signatures in the Earth's gravity field that are detectable by data of the GRACE mission. Because ESA's satellite gravity mission GOCE – launched in 2009 – aims at high spatial resolution, its measurements could complement the GRACE information on coseismic gravity changes, although time-variable gravity was not foreseen as goal of the GOCE mission. We modeled the coseismic earthquake geoid signal and converted this signal to vertical gravity gradients at GOCE satellite altitude. We combined the single gradient observations in a novel way reducing the noise level, required to detect the coseismic gravity change, subtracted a global gravity model, and applied tailored outlier detection to the resulting gradient residuals. Furthermore, the measured gradients were along-track filtered using different gradient bandwidths where in the space domain Gaussian smoothing has been applied. One year periods before and after earthquake occurrence have been compared with the modeled gradients. The comparison reveals that the earthquake signal is well above the accuracy of the vertical gravity gradients at orbital height. Moreover, the obtained signal from GOCE shows a 1.3 times higher amplitude compared with the modeled signal. Besides the statistical significance of the obtained signal, it has a high spatial correlation of ~0.7 with the forward modeled signal. We conclude therefore that the coseismic gravity change of the Japan Tohoku-Oki earthquake left a statistically significant signal in the GOCE measured gravity gradients.

Description: [1]  The Ninetyeast Ridge (NER), one of the longest linear volcanic features on the Earth, offers an excellent opportunity of understanding the isostatic response to the interactions of mantle plume with the migrating mid-ocean ridge. Bathymetry, geoid and gravity (ship-borne and satellite) data along 72 closely spaced transects and 17 overlapping grids on the NER are analyzed and modeled to determine the effective elastic thickness ( Te ) beneath the entire ridge. The results of 2-D and 3-D flexural modeling of the NER show large spatial variations in Te values ranging from 4 to 35 km, suggesting that the ridge was compensated along its length by different isostatic mechanisms. The southern (south of 22°S latitude) and northern (north of 2°N latitude) parts of the NER have Te values of 〉10 and 〉23 km, respectively, revealing that the southern part was emplaced on a lithosphere of intermediate strength possibly on flank of the Indian plate, whereas the northern part was emplaced in an intraplate setting. In contrast, in the central part of the NER (between latitudes 22°S and 2°N), highly variable Te values (4–22 km) are estimated. The scattered Te values in the central NER suggest that this part may have evolved due to the occurrence of more frequent ridge jumps caused by the interaction of Kerguelen hot spot with rapid northward migration of the Wharton spreading ridge. Residual Mantle Bouguer Anomaly (RMBA) map of the NER and adjacent basins reveals that the entire length of the NER is associated with a significant negative anomaly up to 200 mGal, indicating the presence of thickened crust or less dense mantle beneath the ridge. 3-D crustal thickness map of the NER, generated by inversion of the RMBA data, shows a thick crust ranging from 15 to 19 km. The present study clearly shows that NER possesses a highly segmented isostatic pattern with the occurrence of sub-crustal underplating or sub-surface loading.

Description: [1]  We conducted deep-sea magnetic measurements using autonomous underwater vehicles in the Bayonnaise knoll caldera, the Izu-Ogasawara island arc, which hosts the large Hakurei hydrothermal field. We improved the conventional correction method applied for removing the effect of vehicle magnetization, thus greatly enhancing the precision of the resulting vector anomalies. The magnetization distribution obtained from the vector anomaly data shows an ∼ 2-km-wide belt of high magnetization, trending NNW–SSE going through the caldera, and a low magnetization zone ∼ 300 m by ∼ 500 m in area, extending over the Hakurei site. Comparison between the results obtained using the vector anomaly and the total intensity anomaly shows that the magnetic field is determined more accurately, especially in areas of sparse data distribution, when the vector anomaly rather than the total intensity anomaly is used. We suggest a geologically motivated model that basaltic volcanism associated with the backarc rifting occurred after the formation of the caldera, resulting in the formation of the high magnetization belt underneath the silicic caldera. The Hakurei hydrothermal field lies in the intersection of the basaltic volcanism belt and the caldera wall fault, suggesting a mechanism that hot water generated by the heat of the volcanic activity has been spouting out through the caldera wall fault. The deposit apparently extends beyond the low magnetization zone, climbing up the caldera wall. This may indicate that hot water rising from the deep through the alteration zone is transported laterally when it comes near the seafloor along fissures and fractures in the caldera wall.

Description: [1]  Using multiple ScS reverberations we examine mantle reflectivity structure beneath northeast China and the northwest Pacific. We find several upper mantle discontinuities, including a melt layer with a mean thickness of 64 km atop the 410-km discontinuity, present on both sides of the subducting slab near the Nankai trench. The transition zone contains a split 520-km discontinuity in several paths, and tomographic images show stagnant slabs at this depth. We believe this may be slab-related based on experimental work (Saikia, A., Frost, D. J., Rubie, D. C., 2008. Splitting of the 520-kilometer seismic discontinuity and chemical heterogeneity in the mantle. Science 319 (5869), 1515–1518). A negative reflector is found in one path beneath the northeast China craton at a depth of 598 km. Mid-mantle reflectors are found in all of our paths and are present throughout a wide depth range (~750 – 1600 km).

Description: [1]  Three-dimensional P - and S -wave velocity (V P , V S ) models and high-resolution earthquake relocations are determined for the New Madrid Seismic Zone using double-difference local earthquake tomography. The data set consists of arrival times and differential times recorded by the Cooperative New Madrid Seismic Network (CNMSN) from 2000-2007 and the 1989-1992 Portable Array Network and Data Acquisition deployment. Waveform cross-correlation derived differential times for the CNMSN data are also incorporated. The velocity solutions are compatible with previous solutions centered on the active arms of seismicity and cover a broader area including mafic intrusions along the margin of the Reelfoot Rift. Major features include elevated V P and V S associated with the mafic plutons and reduced V P and V S along and southeast of the Axial fault (AF), a major arm of seismicity trending along the rift axis. Low V P extends to a depth of at least 20 km along the portion of the AF that extends south of the Missouri bootheel. A locally high V P /V S anomaly imaged along the central portion of the Reelfoot fault is spatially correlated with a significant change in fault trend and is interpreted as a region containing high pore pressure and/or water-filled microcracks.

Description: [1]  We present a catalog of InSAR constraints on deformation that occurred during earthquake sequences in southern Iran between 1992-2011, and explore the implications on the accommodation of large-scale continental convergence between Saudi Arabia and Eurasia within the Zagros Mountains. The Zagros Mountains, a salt-laden fold-and-thrust-belt involving ~10 km of sedimentary rocks overlying Precambrian basement rocks, have formed as a result of ongoing continental collision since 10-20 Ma that is currently occurring at a rate of ~3 cm/yr. We first demonstrate that there is a biased misfit in earthquake locations in global catalogs that likely results from neglect of 3D velocity structure. Previous work involving two M ~ 6 earthquakes with well-recorded aftershocks has shown that the deformation observed with InSAR may represent triggered slip on faults much shallower than the primary earthquake, which likely occurred within the basement rocks (〉10 km depth). We explore the hypothesis that most of the deformation observed with InSAR spanning earthquake sequences is also due to shallow, triggered slip above a deeper earthquake, effectively doubling the moment release for each event. We quantify the effects that this extra moment release would have on the discrepancy between seismically and geodetically constrained moment rates in the region, finding that even with the extra triggered fault slip, significant aseismic deformation during the interseismic period is necessary to fully explain the convergence between Eurasia and Saudi Arabia.

Description: [1]  The identification and evaluation of trigger mechanisms for volcano flank instabilities and/or collapse represent a key issue for risk assessment in densely populated volcanic areas, as well as in long distance settings, particularly in case of island or coastal volcanoes. Here, we address quantitatively the effects of external (seismic) and inner (magmatic) forcing on the stress-strain state associated to flank instabilities at Mt. Etna (Sicily, southern Italy) by means of a 2-D Finite-Difference-Method numerical modelling. Modelled seismic actions include strong near-field, strong far-field and low-magnitude near-field earthquakes. Magmatic actions consider the inner presssure changes induced by energetic lava fountains in the summit crater area, sub-vertical and oblique dyke ascent below the summit area. Model results are validated in light of available monitoring data and recent eruptive activity. Numerical results show that the main strain effects are produced by high-magnitude near-field earthquakes (expected return time of ~10 3  yrs), and by vertical uprise of a magma dyke below the volcano summit area. Maximum displacements in the order of tens of centimetres may involve the summit area, up to some 10 6  m 3 /m over some km laterally. Stress releases up to 10 7  Pa may affect a limited portion of the magmatic conduit, thus favouring major effusive flank eruptions. Major catastrophic events, such as volcano flank collapse, should not be expected by applying, either individually or combined, the aforementioned actions.

Description: [1]  Seismicity closely related to hydrological impacts has been observed in several locations worldwide; particularly in intraplate areas where tectonic stressing rates are small. The triggering mechanism is usually explained by a poroelastic response of the seismogenic crust to surface water flux, leading to pore pressure changes at depth. To explain the earthquake triggering in response of those small stress changes, however, the crust has to be near a critical state in which other transient processes might be significant. One of the prominent examples is the Mt. Hochstaufen in SW Germany, where seismicity is known to vary seasonally. A previous analysis showed that the seismicity in 2002 was highly correlated with model forecasts based on fluid diffusion and rate- and state-dependent frictional nucleation. Here we revisit this case by accounting additionally for poroelastic effects, as well as for thermoelastic and tidal stresses. We also test whether the model can explain the observations of the subsequent eight years between 2003 and 2010. Our analysis confirms that rainfall is the dominant driving force in this region. The model not only fits the year 2002 activity very well, but provides with the same parameters a reasonable fit to the subsequent period, with a probability gain of about 4 per event in comparison to a time-independent Poisson model.

Description: ABSTRACT [1]  The absolute magnitude of stress in the crust and the shear strength of faults are poorly known, yet fundamental quantities, in lithospheric dynamics. While stress magnitude cannot be measured directly, deviatoric stress state can be inferred indirectly from focal mechanism solutions collected before and after an earthquake. We extend a standard stress inversion for normalized stresses to invert for the 3D spatial distribution of absolute deviatoric stress and variation of fault strength with depth using focal mechanism solutions and coseismic stress changes produced by large earthquakes. We apply the method to the 2011 M9 Tohoku-oki, Japan earthquake. The northern Japan forearc crust between 5 and 15 km depth appears to be weak with fault strength of 40–90 MPa, consistent with a coefficient of friction of 0.2-0.5. The M9 Tohoku-oki coseismic stress change was large enough, relative to the ambient stress, to rotate the principal stress directions typically ~20° in the upper 20 km of the crust. The data from Japan require a heterogeneous ambient deviatoric stress field with short wavelength (~20-50 km) fluctions in principal stress orientations.

Description: [1]  We review marine heat flow data along the Nankai Trough and show that observations 〉 30 km seaward of the deformation front are 20% below conductive predictions (129–94 mW m -2 ) but consistent with the global heat flow average for oceanic crust of the same age (16-28 Ma). Heat flow values  〈  30 km seaward of the deformation front are generally 20% higher than conductive predictions. This heat flow pattern is consistent with the advection of heat by fluid flow in the subducting oceanic crust and explains both the high heat flux in the vicinity of the trench, 〉 200 and 〉 140 mW m -2 , and steep landward declines to values of approximately 60 mW m -2 over distances of 65 and 50 km along the Muroto and Kumano transects, respectively. Along the Ashizuri transect the lack of heat flow data preclude a definitive interpretation. We conclude that fluid flow in the subducting oceanic crust leads to temperatures that are generally 25 ° C higher near the toe of the margin wedge and 50 - 100 ° C lower near the downdip limit of the seismogenic zone than estimated by purely conductive models.

Description: Observations have been obtained within an intense (precipitation rates 〉 50 mm hour -1 ) narrow cold-frontal rainband (NCFR) embedded within a broader region of stratiform precipitation. In-situ data were obtained from an aircraft which flew near a steerable dual-polarisation Doppler radar. The observations were obtained to characterise the microphysical properties of cold frontal clouds, with an emphasis on ice and precipitation formation and development. Primary ice nucleation near cloud top (−55°C) appeared to be enhanced by convective features. However, ice multiplication led to the largest ice particle number concentrations being observed at relatively high temperatures (〉 −10°C). The multiplication process (most likely rime-splintering) occurs when stratiform precipitation interacts with supercooled water generated in the NCFR. Graupel was notably absent in the data obtained. Ice multiplication processes are known to have a strong impact in glaciating isolated convective clouds, but have rarely been studied within larger organised convective systems such as NCFRs. Secondary ice particles will impact on precipitation formation and cloud dynamics due to their relatively small size and high number density. Further modelling studies are required to quantify the effects of rime splintering on precipitation and dynamics in frontal rainbands. Available parameterizations used to diagnose the particle size distributions do not account for the influence of ice multiplication. This deficiency in parameterizations is likely to be important in some cases for modelling the evolution of cloud systems and the precipitation formation. Ice multiplication has significant impact on artefact removal from in-situ particle imaging probes.

Description: This paper proposes a selective ensemble mean technique for tropical cyclone (TC) track forecast based on the errors of ensemble prediction system (EPS) members at short lead times (SLTs, 12 h in this study). The means (SEAV) and weighted means (SEWE) of selected EPS members are applied to EPS products from the European Centre for Medium-Range Weather Forecasts (ECMWF), Japan Meteorological Agency (JMA), National Centers for Environmental Prediction (NCEP), and China Meteorological Administration for 35 TCs in the Western North Pacific in 2010 and 2011. Verification results show that SEAV behaves better than SEWE, with a skill of 5% to 30% over relevant ensemble means of EPS within 72 h. The SEAV method is the most effective for the JMA EPS, with a skill of 10% even at 96 h. SEAV predictions are compared with the high-resolution deterministic model predictions of ECMWF and several official forecasts, with special consideration given to the time delay associated with numerical model products in operation. The SEAV for the ECMWF EPS can overcome the high-resolution ECMWF deterministic model at 24 h. Case analyses and sensitivity tests on the error thresholds of member selection and SLT lead times are also presented in this paper.

Description: The predictive quality of an ensemble model of cirrus ice crystals to model passive and active measurements of ice cloud, from the ultraviolet (UV) to the microwave, is tested. The ensemble model predicts ice mass ∝ D 2 (m-D), where D is the maximum dimension of the ice crystal, and m is the mass. This predicted m-D relationship is applied to a moment estimation parametrization of the particle size distribution (PSD), to estimate the PSD shape, given ice water content (IWC) and in-cloud temperature. The same microphysics is applied across the electromagnetic spectrum to model UV, infrared, microwave and radar observations. The short-wave measurements consist of airborne UV backscatter lidar estimates of the volume extinction coefficient, total solar optical depth, and space-based multi-directional spherical albedo measurements, at 0.865 µm, between the scattering angles 85 o and 125 o . The airborne long-wave measurements consist of high-resolution interferometer upwelling brightness temperatures, obtained between the wavelengths of about 3.45 µm and 4.1 µm, and 8.0 µm to 12.0 µm. The low frequency measurements consist of ground-based Chilbolton 35 GHz radar reflectivity measurements and space-based upwelling 190 GHz brightness temperature measurements. The predictive quality of the ensemble model is demonstrated to be generally within the experimental uncertainty of the lidar backscatter estimates of the volume extinction coefficient and total solar optical depth. The ensemble model prediction of the high-resolution brightness temperature measurements is generally within ±2 K and ±1K, at solar and infrared wavelengths, respectively. The 35 GHz radar reflectivity and 190 GHz brightness temperatures are generally simulated to within ±2 dBZ e , and ±2 K, respectively. The directional spherical albedo observations suggest that the scattering phase function of the most randomized ensemble model gives the best fit to the measurements (generally within ±3%). This paper demonstrates that the ensemble model, assuming the same microphysics , is physically consistent across the electromagnetic spectrum.

Description: This paper investigates two schemes that perturb sea-surface temperatures (SSTs) and soil moisture content (SMC) in the Met Office Global and Regional Ensemble Prediction System (MOGREPS), to address a known deficiency of a lack of ensemble spread near the surface. Results from a two-month long trial during the northern hemisphere summer show positive benefits from these schemes. These include a decrease in the spread deficit of surface temperature and improved probabilistic verification scores. SST perturbations exhibit a stronger impact than SMC perturbations, but when combined the increased spread from the two schemes is cumulative. A regional ensemble system driven by the global ensemble members largely reflects the same changes seen in the global ensemble but cycling fields, like SMC, between successive regional forecasts does show some benefit.

Description: [1]  We investigate whether predictions of mantle structure from tectonic reconstructions are in agreement with a detailed tomographic image of seismic P-wave velocity structure under the Caribbean region. In the upper mantle, positive seismic anomalies are imaged under the Lesser Antilles and Puerto Rico. These anomalies are interpreted as remnants of Atlantic lithosphere subduction and confirm tectonic reconstructions that suggest at least 1100 km of convergence at the Lesser Antilles island arc during the past ~45 Myr. The imaged Lesser-Antilles slab consists of a northern and southern anomaly, separated by a low velocity anomaly across most of the upper mantle, which we interpret as the subducted North America-South America plate boundary. The southern edge of the imaged Lesser Antilles slab agrees with vertical tearing of South America lithosphere. The northern Lesser Antilles slab is continuous with the Puerto Rico slab along the northeastern plate boundary. This results in an amphitheatre-shaped slab and it is interpreted as westward subducting North America lithosphere that remained attached to the surface along the northeastern boundary of the Caribbean plate. At the Muertos Trough, however, material is imaged until a depth of only 100 km, suggesting a small amount of subduction. The location and length of the imaged South Caribbean slab agrees with proposed subduction of Caribbean lithosphere under the northern South America plate. An anomaly related to proposed Oligocene subduction at the Nicaragua rise is absent in the tomographic model. Beneath Panama, a subduction window exists across the upper mantle, which is related to the cessation of subduction of the Nazca plate under Panama since 9.5 Ma and possibly the preceding subduction of the extinct Cocos-Nazca spreading center. In the lower mantle two large anomaly patterns are imaged. The westernmost anomaly agrees with the subduction of Farallon lithosphere. The second lower mantle anomaly is found east of the Farallon anomaly and is interpreted as a remnant of the late Mesozoic subduction of North and South America oceanic lithosphere at the Greater Antilles, Aves ridge and Leeward Antilles. The imaged mantle structure does not allow us to discriminate between an ‘Intra-Americas’ origin and a ‘Pacific origin’ of the Caribbean plate.

Description: [1]  We have measured interseismic deformation across the Ashkabad strike-slip fault using 13 Envisat interferograms covering a total effective timespan of ~30 years. Atmospheric contributions to phase delay are significant and variable due to the close proximity of the Caspian Sea. In order to retrieve the pattern of strain accumulation, we show it is necessary to use data from Envisat's Medium Resolution Imaging Spectrometer (MERIS) instrument, as well numerical weather model outputs from the European Centre for Medium-Range Weather Forecasting (ECMWF), to correct interferograms for differences in water vapour and atmospheric pressure respectively. This has enabled us to robustly estimate the slip rate and locking depth for the Ashkabad fault using a simple elastic dislocation model. Our data are consistent with a slip rate of 5–12 mm/yr below a locking depth of 5.5–17 km for the Ashkabad fault, and synthetic tests support the magnitude of the uncertainties on these estimates. Our estimate of slip rate is 1.25–6 times higher than some previous geodetic estimates, with implications for both seismic hazard and regional tectonics, in particular supporting fast relative motion between the South Caspian Block and Eurasia. This result reinforces the importance of correcting for atmospheric contributions to interferometric phase for small strain measurements. We also attempt to validate a recent method for atmospheric correction based on ECMWF ERA-Interim model outputs alone and find that this technique does not work satisfactorily for this region when compared to the independent MERIS estimates.

Description: [1]  The seafloor within the Perth Abyssal Plain (PAP), offshore Western Australia, is the only section of crust that directly records the early spreading history between India and Australia during the Mesozoic breakup of Gondwana. However, this early spreading has been poorly constrained due to an absence of data, including marine magnetic anomalies and data constraining the crustal nature of key tectonic features. Here, we present new magnetic anomaly data from the PAP that shows that the crust in the western part of the basin was part of the Indian Plate – the conjugate flank to the oceanic crust immediately offshore the Perth margin, Australia. We identify a sequence of M2 and older anomalies in the west PAP within crust that initially moved with the Indian Plate, formed at intermediate half-spreading rates (35 mm/yr) consistent with the conjugate sequence on the Australian Plate. More speculatively, we reinterpret the youngest anomalies in the east PAP, finding that the M0-age crust initially formed on the Indian Plate was transferred to the Australian Plate by a westward jump or propagation of the spreading ridge shortly after M0 time. Samples dredged from the Gulden Draak and Batavia Knolls (at the western edge of the PAP) reveal that these bathymetric features are continental fragments rather than igneous plateaus related to Broken Ridge. These microcontinents rifted away from Australia with Greater India during initial breakup at ~130 Ma, then rifted from India following the cessation of spreading in the PAP (~101-103 Ma).

Description: [1]  We develop a three-step Maximum-A-Posteriori probability (MAP) method for coseismic rupture inversion, which aims at maximizing the a posterior probability density function (PDF) of elastic deformation solutions of earthquake rupture. The method originates from the Fully Bayesian Inversion (FBI) and Mixed linear-nonlinear Bayesian inversion (MBI) methods, shares the same posterior PDF with them, while overcoming difficulties with convergence when large numbers of low-quality data are used and greatly improving the convergence rate using optimization procedures. A highly-efficient global optimization algorithm, Adaptive Simulated Annealing (ASA), is used to search for the maximum of a posterior PDF (" mode " in statistics) in the first step. The second step inversion approaches the " true" solution further using the Monte Carlo Inversion (MCI) technique with positivity constraints, with all parameters obtained from step one as the initial solution. Then slip artifacts are eliminated from slip models in the third step using the same procedure of the second step, with fixed fault geometry parameters. [2]  We first design a fault model with 45°-dip angle and oblique slip, and produce corresponding synthetic InSAR datasets to validate the reliability and efficiency of the new method. We then apply this method to InSAR data inversion for the coseismic slip-distribution of the April 14, 2010 Mw 6.9 Yushu, China earthquake. Our preferred slip model is composed of three segments with most of the slip occurring within 15 km depth and the maximum slip reaches 1.38 m at the surface. The seismic moment released is estimated to be 2.32e + 19 Nm, consistent with the seismic estimate of 2.50e + 19 Nm.

Description: Two statistical downscaling models were developed for downscaling monthly GCM outputs to precipitation at a site in north-western Victoria, Australia. The first downscaling model was calibrated and validated with the NCEP/NCAR reanalysis outputs over the periods of 1950–1989 and 1990–2010 respectively. The projections of precipitation into future were produced by introducing the outputs of HadCM3, ECHAM5, GFDL2.0 and GFDL2.1, pertaining to A2 and B1 greenhouse gas emission scenarios to this downscaling model. In this model, the input data used in the development and future projections are not homogeneous, as they originate from two different sources. As a solution to this issue, the second downscaling model was developed and precipitation projections into future were produced with a homogeneous set of inputs. To produce a homogeneous set of inputs to this model, regression relationships were formulated between the NCEP/NCAR reanalysis outputs and the 20 th century climate experiment outputs corresponding to the variables used in the first downscaling model obtained from the ensemble consisted of HadCM3, ECHAM5 and GFDL2.0. The outputs of these relationships pertaining to the periods of 1950–1989 and 1990–1999, were used for the calibration and validation of this downscaling model respectively. Using the outputs of HadCM3, ECHAM5 and GFDL2.0 pertaining to A2 and B1 emission scenarios on these relationships, inputs for the second downscaling model pertaining to the period of 2000–2099 were generated. The first downscaling model with NCEP/NCAR reanalysis outputs, showed a high Nash-Sutcliffe Efficiency (NSE) of 0.75 over the period 1950–1999. When this downscaling model was run with the 20 th century climate experiment outputs of HadCM3, ECHAM5, GFDL2.0 and GFDL2.1, it exhibited limited performances over the period 1950–1999, which was indicated by relatively low NSEs of −0.62, -2.54, -0.40 and −0.48 respectively. The second downscaling model displayed a NSE of 0.35 over the period 1950–1999.

Description: The U.S. Navy's relocatable (RELO) ensemble prediction system is fully described and is examined in the Gulf of Mexico for 2010. After briefly describing the Ensemble Transfer (ET) method for the initial perturbation generation, we introduce a new time-deformation technique to generate the surface forcing perturbations from the atmospheric model fields. The extended forecast time (EFT) is introduced to quantify the advantages of the ensemble mean forecasts over a single deterministic forecast. The ensemble spread and its growth are investigated together with their relations with the ensemble forecast accuracy, reliability and skill. Similar to many other operational ensemble forecast systems at Numerical Weather Prediction (NWP) centers, the initial analysis error is underestimated by the technique used in the data-assimilation (DA) system. Growth of the ocean ensemble spread is also found to lag the growth of the ensemble mean error, a tendency attributed to insufficiently accounting for model-related uncertainties. As an initial step, we randomly perturb the two most important parameters in the ocean model mixing parameterizations, namely the Smagorinsky horizontal and Mellor-Yamada vertical mixing schemes. We examine three different parameter perturbation schemes based on both uniform and Gaussian distributions. It is found that all three schemes improve the ensemble spread to a certain extent, particularly the scheme with Gaussian distribution of perturbations imposed on both the horizontal and vertical mixing parameters. The findings in this paper indicate that the RELO ensemble forecast demonstrates superior accuracy and skill relative to a single deterministic forecast for all the variables and over all the domains considered in this paper. The ensemble spread provides valuable estimate of forecast uncertainty. However, the RELO uncertainty forecast capability could be further improved by accounting for more model-related uncertainties, for example, by the development of an error parameterization that imposes stochastic forcing at each model grid point.

Description: The gamma family of probability densities has recently been used to model raindrop size. The traditional approach of using method of moments to estimate the gamma distribution parameters, however, is known to be biased and can have substantial errors. A recently-developed approach combining moment information and a weighted least squares analysis generally produces substantially better results. Other procedures superior to the method of moments approach include maximum likelihood. In particular, maximum likelihood estimates have been shown to outperform method of moments estimators both in the case in which the full range of drop sizes are observed as well as the case in which small drop sizes fail to be observed because of the inability of disdrometers to record observations below a threshold. The foregoing comments on maximum likelihood concern the situation in which drop sizes are measured on a continuous scale. In this work we consider drop sizes from gamma distributions which are classified into broad size bins, as would be the case with data obtained from many disdrometers; this requires some modification of the maximum likelihood procedure. We do also allow for the possibility of drop sizes below a threshold or above another threshold not being observed. Maximum likelihood performance in this case is investigated through simulation of volume sampling from gamma distributions with known parameters. We compare the performance of the maximum likelihood estimates with those of method of moments (only a truncated-data version is viable) and the recently developed weighted least squares procedure, and also apply the three estimation procedures to some experimental data. Since the experimental data are surface data we indicate how drop fall velocity may be incorporated to obtain parameters for the volume distributions from the surface data.

Description: Despite the availability of several atmospheric reanalyses (e.g. ERA-Interim) there exists both considerable uncertainty in surface forcing fields for ice/ocean modelling and sensitivity to the choice of product used. Here we introduce a relatively high-resolution alternative forcing data set for ice-ocean models derived from the Canadian Meteorological Centre's (CMC) Global Deterministic Prediction System (GDPS). A set of daily 30 hr reforecasts is produced using the GDPS 33 km resolution model providing hourly atmospheric forcing fields for the period 2002–2011. The CMC GDPS Reforecasts (CGRF) are compared to ERA-Interim and several observational datasets to evaluate their suitability for forcing ocean models. In particular, the CGRF surface temperature, humidity and winds show equivalent biases to those found in ERA-interim. Moreover, the higher resolution of the CGRFs permit a more detailed representation of atmospheric structures and topographic steering resulting in finer-scale coastal features and wind stress curl. While the CGRF dataset is not a reanalysis and thus is expected to be less well constrained by available observations, its higher resolution and small bias make it an attractive alternative for forcing ice/ocean models.

Description: [1]  We conducted total magnetic field and Bouguer gravity measurements to investigate the shallow structure beneath the summit caldera of Kīlauea Volcano, Hawai‘i. Two significant and distinctive magnetic anomalies were identified within the caldera. One is interpreted to be associated with a long-lived pre-historic eruptive centre, the Observatory vent, located ~1 km east of the Hawaiian Volcano Observatory. The second magnetic anomaly corresponds to a set of eruptive fissures that strike northeast from Halema‘uma‘u Crater, suggesting this is an important transport pathway for magma. The Bouguer gravity data were inverted to produce 3D models of density contrasts in the upper 2 km beneath Kīlauea. The models detect 3.0 km 3 of material, denser than 2800 kg m -3 , beneath the caldera that may represent an intrusive complex centred northeast of Halema‘uma‘u. Recent temporal gravity studies indicate continual addition of mass beneath the caldera during 1975–2008 centred west of Halema‘uma‘u and suggest this is due to filling of void space. The growth of a large intrusive complex, apparent cyclical caldera formation, and continual mass addition without inflation, however, can also be explained by extensional rifting caused by the continual southward movement of Kīlauea's unstable south flank.

Description: [1]  The 2006-2007 doublet of M W  〉 8 earthquakes in the Kuril subduction zone caused postseismic transient motion in the asthenosphere, which we observed on the Kuril GPS Array in 2007–2011. Here we show that the Maxwell asthenospheric viscosity that best fits the geodetic data increased by nearly an order of magnitude over the interval of four years, from 2 × 10 17 to 1 × 10 18  Pa s. These effective values of viscosity can be explained by a power-law rheology for which strain rate is proportional to stress raised to a power n  〉 1. The apparent change in viscosity can also be caused by other factors such as coupling between afterslip and viscoelastic flow. The open and intriguing question in connection with postseismic data after the Kuril earthquake doublet is the magnitude of the long-term asthenospheric viscosity, which shall be revealed by continued observations. An asthenosphere with viscosity of about 1 × 10 19  Pa s is favored by the postseismic deformation still observed several decades after the 1960 Chile and 1964 Alaska M W ~9 earthquakes. However, postseismic deformation associated with the 1952 southern Kamchatka M W ~9 earthquake currently is not observed in the northern Kurils, an indication that the long-term asthenospheric viscosity in the Kurils is lower than in Chile and Alaska.

Description: [1]  The region of central Chile offers a unique opportunity to study the links between the subducting Juan Fernandez Ridge, the flat slab, the Double Seismic Zone (DSZ) and the absence of modern volcanism. Here, we report the presence and characteristics of the first observed DSZ within the intermediate-depth Nazca slab using two temporary seismic catalogues (OVA99 and CHARSME). The lower plane of seismicity (LP) is located 20–25 km below the upper plane (UP), begins at 50 km depth and merges with the lower plane at 120 km depth, where the slab becomes horizontal. Focal mechanism analysis and stress tensor calculations indicate that the slab's state of stress is dominantly controlled by plate convergence and overriding crust thickness: Above 60–70 km depth, the slab is in horizontal compression, and below, it is in horizontal extension, parallel to plate convergence, which can be accounted for by vertical loading of the overriding lithosphere. Focal mechanisms below 60–70 km depth are strongly correlated with offshore outer rise bend faults, suggesting the reactivation of pre-existing faults below this depth. The large interplane distances for all Nazca DSZs can be related to the slab's unusually cold thermal structure with respect to its age. Since LPs globally seem to mimic mantle mineral dehydration paths, we suggest that fluid migration and dehydration embrittlement provide the mechanism necessary to weaken the rock and that the stress field determines the direction of rupture.

Description: [1]  Among the different types of waves embedded in seismic noise, body waves present appealing properties but are still challenging to extract. Here we first validate recent improvements in numerical modeling of microseismic compressional ( P ) body waves and then show how this tool allows fast detection and location of their sources. We compute sources at ~ 0.2 Hz within typical P teleseismic distances (30-90 degrees) from the South California Seismic Network (SCSN) and analyze the most significant discrete sources. The locations and relative strengths of the computed sources are validated by the good agreement with beam-forming analysis. These ~75 noise sources exhibit a highly heterogeneous distribution, and cluster along the usual storm tracks in the Pacific and Atlantic oceans. They are mostly induced in the open ocean, at or near water depths of 2800 and 5600 km, most likely within storms or where ocean waves propagating as swell meet another swell or wind sea. We then emphasize two particularly strong storms to describe how they generate noise sources in their wake. We also use these two specific noise bursts to illustrate the differences between microseismic body- and surface-waves in terms of source distribution and resulting recordable ground motion. The different patterns between body- and surface-waves result from distinctive amplification of ocean wave-induced pressure perturbation and different seismic attenuation. Our study demonstrates the potential of numerical modeling to provide fast and accurate constraints on where and when to expect microseismic body waves, with implications for seismic imaging and climate studies.

Description: [1]  Earthquakes that rupture across steps between faults can be larger than those predicted from individual fault lengths, making understanding multifault events critical to assessing earthquake hazard. Empirical data from earthquake surface ruptures suggest that the distances between faults that rupture together can range from 〈1 km to 5 km. Dynamic and quasi-static models of planar faults determine similar distances. However, studies of interactions between realistic, 3D non-planar faults are few. A general comparison of quasi-static stress perturbations and triggering potentials with mechanical models incorporating either planar or non-planar faults highlight the sensitivity of planar fault models to model parameters and reveal no clear relationship between mean fault slip and triggering potential. More specifically, planar fault models predict triggering across a 3 km extensional step, while models incorporating non-planar faults indicate that a connecting fault is necessary to transfer slip through a 3 km step along the 1992 Landers, California earthquake rupture. The mechanical approach taken captures the stress changes as well as the total stress following fault slip, improving the criterion used to determine triggered failure potential. This underscores the need for additional constraint on fault strength and cohesion. The focus on complex fault geometry restricts analyses to the quasi-static realm, limiting the application of results to fault interactions over the short distances and slow rupture velocities for which the quasi-static stress field is relevant or approximates the dynamic stress field.

Description: [1]  Batch and flow-through experiments were performed on quartz-feldspar granular aggregates and sandstone samples to investigate time-dependent effects of fluid-rock interactions on fluid and rock conductivity, respectively. The experiments were conducted at temperatures up to 164, at confining and pore pressures up to 10 and 5 MPa, respectively, and for up to 136 days. It showed that changes in rock conductivity were unequivocally related to changes in pore fluid conductivity. It is inferred that these changes were dependent on kinetically controlled dissolution reactions between the mineral grains and the fluid. The time-dependent signature of rock conductivity implied a detectable transition from initial dissolution towards some state of equilibrium. The response of rock conductivity to temperature changes followed an Arrhenius-type behavior. An exploratory kinetic evaluation of the conductivity data for sandstone samples yielded an apparent activation energy of approximately 32 kJ/mol. A concurrent chemical fluid analysis showed that this is an integrated value over all reactions occurring in parallel within a sample. These reactions namely concern silica and silicate dissolutionbut also the dissolution of accessory salt minerals. It is concluded that measuring the evolution of rock conductivity in combination with chemical pore fluid analysis constitutes a powerful and quantitative tool for monitoring time-dependent changesin pore fluid chemistry and thus fluid-rock interactions in real time.

Description: [1]  We describe a multi-parameter experiment at Erebus volcano, Antarctica, employing Doppler radar, video, acoustic, and seismic observations to estimate the detailed energy budget of large (up to 40-m-diameter) bubble bursts from a persistent phonolite lava lake. These explosions are readily studied from the crater rim at ranges of less than 500 m, and present an ideal opportunity to constrain the dynamics and mechanism of magmatic bubble bursts that can drive Strombolian and Hawaiian eruptions. We estimate the energy budget of the first second of a typical Erebus explosion as a function of time and energy type, and constrain gas pressures and forces using an analytic model for the expansion of a gas bubble above a conduit that incorporates conduit geometry and magma and gas parameters. The model, consistent with video and radar observations, invokes a spherical bulging surface with a base diameter equal to that of the lava lake. The model has no ad hoc free parameters, and geometrical calculations predict zenith height, velocity and acceleration during shell expansion. During explosions, the energy contained in hot over-pressured gas bubbles is freed and partitioned into other energy types, where by far the greatest non-thermal energy component is the kinetic and gravitational potential energy of the accelerated magma shell (〉10 9 J). Seismic source energy created by explosions is estimated from radar measurements and is consistent with source energy determined from seismic observations. For the generation of the infrasonic signal, a dual mechanism incorporating a terminally disrupted slug is proposed, which clarifies previous models and provides good fits to observed infrasonic pressures. A new and straightforward method is presented for determining gas volumes from slug explosions at volcanoes from remote infrasound recordings.

Description: [1]  We explore the application of GPS data to earthquake early warning and investigate whether the co-seismic ground deformation can be used to provide fast and reliable magnitude estimations and ground shaking predictions. We use an algorithm to extract the permanent static offset from GPS displacement time series and invert for the slip distribution on the fault plane, which is discretized into a small number of rectangular patches. We developed a completely “self-adapting” strategy in which the initial fault plane model is built based on a quick, approximate magnitude estimation, and is then allowed to increase in size based on the evolutionary magnitude estimation resulting from the slip inversion. Two main early warning outputs are delivered in real-time: magnitude and the along-strike extent of the rupture area. These are finally used to predict the expected ground shaking due to the finite source. We tested the proposed strategy by simulating real-time environments for three earthquakes. For the Mw 9.0, 2011 Tohoku-Oki earthquake our algorithm provides the first magnitude estimate of 8.2 at 39 sec after the origin time, and then gradually increases to 8.9 at 120 sec. The estimated rupture length remains constant from the outset at ~360 km. For the Mw 8.3, 2003 Tokachi-Oki earthquake the initial magnitude estimate is 8.5 at 24 sec and drops to 8.2 at 40 sec with a rupture length of 290 km. Finally, for the Mw 7.2, 2010 El Mayor-Cucapah earthquake the magnitude estimate is 7.0 from the outset with a rupture length of 140 km. The accuracy of the ground shaking prediction using the GPS-based magnitude and finite extent is significantly better than existing seismology-based point source approaches. This approach would also facilitate more rapid tsunami warnings

Description: ABSTRACT [1]  Analysis of Lake Bonneville shorelines using LIDAR digital elevation data challenges accepted models of Wasatch fault deformation since the late Pleistocene. While footwall deformation of the Weber segment of the Wasatch fault is consistent with back-rotation of the footwall block and greatest displacement rate towards the center of the segment, shorelines along the footwall of the Salt Lake City segment decrease in elevation towards the interior and are highest at the segment boundaries, an opposite pattern of footwall deformation than predicted for boundaries arresting or strongly inhibiting displacement during earthquakes. The spatial pattern of footwall rebound implies that some of the proposed persistent fault segment boundaries do not stop earthquake ruptures that originate on adjacent fault segments, nor constrain ruptures initiated within the Salt Lake City segment. Net vertical fault displacement at the boundary between the Salt Lake and Provo segments is 16—20 m over the past 16.3—18.5 ka, corresponding to a vertical displacement rate of 0.8—1.2 mm/yr, a net fault slip rate of 2.0—2.8 mm/yr and horizontal extension rate of 1.8—2.6 mm/yr on the 25 o west-southwest dipping fault that forms the southern Salt Lake City segment boundary. Shoreline analysis suggests isostatic rebound caused by a drop in lake level was concentrated during a relatively short (~2000 yr) time period following the Bonneville flood at ~16 ka. LIDAR-derived topography in conjunction with robust geomorphic datums improves our ability to map deformation associated with lithospheric flexure and faulting while demonstrating the limitation of lacustrine shorelines in this type of analysis.

Description: [1]  We use recent results on statistical analysis of seismicity to present a robust method for comprehensive detection and analysis of earthquake clusters. The method is based on nearest-neighbor distances of events in space-time-energy domain. The method is applied to a 1981–2011 relocated seismicity catalog of southern California having 111,981 events with magnitudes m  ≥ 2, and corresponding synthetic catalogs produced by the Epidemic Type Aftershock Sequence (ETAS) model. Analysis of the ETAS model demonstrates that the cluster detection results are accurate and stable with respect to (i) three numerical parameters of the method, (ii) variations of the minimal reported magnitude, (iii) catalog incompleteness, and (iv) location errors. Application of the method to the observed catalog separates the 111,981 examined earthquakes into 41,393 statistically significant clusters comprised of foreshocks , mainshocks and aftershocks . The results reproduce the essential known statistical properties of earthquake clusters, which provide overall support for the proposed technique. In addition, systematic analysis with our method allows us to detect several new features of seismicity that include (i) existence of a significant population of single-event clusters ; (ii) existence of foreshock activity in natural seismicity that exceeds expectation based on the ETAS model; and (iii) dependence of all cluster properties, except area, on the magnitude difference of events from mainshocks but not on their absolute values. The classification of detected clusters into several major types, generally corresponding to singles, burst-like and swarm-like sequences, and correlations between different cluster types and geographic locations is addressed in a companion paper.

Description: [1]  For the first time, we report the amplitude variation with angle (AVA) pattern of bottom simulating reflectors (BSRs) beneath fracture-filled gas hydrate deposits when the effective medium is anisotropic. The common depth point (CDP) gathers of two mutually perpendicular multi-channel seismic profiles, located in the vicinity of Site NGHP-01-10, are appropriately processed such that they are fit for AVA analysis. AVA analysis of the BSR shows normal-incidence reflection coefficients of -0.04 to -0.11 with positive gradients of 0.04 to 0.31 indicating class IV pattern. The acoustic properties from isotropic rock physics model predict class III AVA pattern which cannot explain the observed class IV AVA pattern in Krishna-Godavari basin due to the anisotropic nature of fracture-filled gas hydrate deposits. [2]  We modeled the observed class IV AVA of the BSR by assuming that the gas hydrate bearing sediment can be represented by horizontally transversely isotropic (HTI) medium after accounting for anisotropic wave propagation effects on BSR amplitudes. The effective medium properties are estimated using Backus averaging technique and the AVA pattern of BSRs is modeled using the properties of overlying HTI and underlying isotropy/HTI media with or without free gas. Anisotropic AVA analysis of the BSR from the inline seismic profile shows 5–30 % gas hydrate concentration (equivalent to fracture density) and the azimuth of fracture system (fracture orientation) with respect to the seismic profile is close to 45°. Free gas below the base of gas hydrate stability zone is interpreted in the vicinity of fault system (F1).

Description: [1]  Sequestration of large amounts of CO 2 within deep underground reservoirs has been proposed as a potential approach for reducing atmospheric emissions of greenhouse gases. A CO 2 sequestration project should address the associated environmental and safety issues and, in this respect, the importance of geomechanics has recently been widely recognized. Geomechanics is even more important when fluid injection is planned in faulted reservoirs. How much CO 2 can be safely injected into multi-compartment reservoirs? Are geomechanical constraints more restrictive than flow-dynamic constraints? These and other questions are addressed in the present study using a three-dimensional Finite Element - Interface Element geomechanical model. We simulate the possible mechanical failure in both the injected formation and the caprock, the fault/thrust reactivation, and the ground surface displacement in a faulted reservoir of the off-shore northern Italy, where seismic surveys provided an accurate characterization of the faulted geological structure. Based on reliable petrophysical/geomechanical properties from well-logs and pore overpressure as predicted by a fluid-dynamic model, the results show that the injection of 1 × 10 6  ton/a of CO 2 may be performed over a few years only. Thereafter part of the injected formation fails by shear stress. A number of parametric scenarios are investigated to address the major uncertainties on the geomechanical response to CO 2 injection. The modeling outcome suggests that shear failure and faults/thrusts reactivation can occur much before attaining the hydraulic fracturing pressure, hence representing two major constraints for a safe and permanent containment.

Description: [1]  We present numerical subduction models to investigate overriding plate deformation at subduction zones. All models show forearc shortening, resulting predominantly from shear stresses at the subduction zone interface and opposite-sense mantle shear stresses at the base of the forearc lithosphere. Models dominated by backarc extension show that it results from trench-normal positive velocity gradients in the mantle below the overriding plate. Such gradients result from toroidal mantle flow induced by slab rollback, with velocities below the leading part of the backarc faster than the overriding plate velocity. The velocity gradients induce basal shear stresses that increase trenchward and cause trenchward overriding plate motion at a velocity ( v OP⊥ ) whose spatial average is below the trench retreat velocity ( v T⊥ ). The combination of basal shear stresses and average v OP⊥ 〈 v T⊥ causes trench-normal deviatoric tension in the backarc and backarc extension. Models dominated by backarc shortening show that it results from a relatively immobile subduction hinge and trenchward overriding plate motion driven by poloidal mantle flow. The poloidal mantle flow is induced by downdip slab sinking and causes the average v OP⊥ 〉 v T⊥ . This results in trench-normal deviatoric compression and shortening in the leading part of the overriding plate as it collides with the subduction hinge. Ultimately, the geodynamic models demonstrate that backarc extension is favored for narrow slabs and near lateral slab edges, and is driven by rollback induced toroidal mantle flow, while backarc shortening is favored for the center of wide slabs, and is driven by poloidal mantle flow resulting from downdip slab motion.

Description: [1]  The Gamburtsev Subglacial Mountains (GSM), located near the center of East Antarctica, remain one of the most enigmatic mountain ranges on earth. A lack of direct geologic samples renders theirtectonic history almost totally unconstrained. We utilize teleseismic Rayleigh wave data from a two-year deployment of broadband seismic stations across the region to image shear velocity structure and analyze the lithospheric age of the GSM and surrounding regions. We solve for 2-D phase velocitiesand invert these results for 3-D shear velocity structure. We perform a Monte Carlo simulation to improve constraints of crustal thickness and shear velocity structure.Beneath the core of the GSM, we find crustal thickness in excess of 55 km.Mantle shear velocities remain faster than global average models to a depth of approximately 250 km, indicating a thick lithospheric root. Thinner crust and slower upper mantle velocities are observed beneath the Lambert Rift System and the Polar Subglacial Basin.When compared with phase velocity curves corresponding to specific tectonothermal ages elsewhere in the world, average phase velocity results for the GSM are consistent with regions of Archean – Paleoproterozoic origin. Combined with radiometric ages of detrital zircons found offshore, these results indicate a region of old crust that has undergone repeated periods of uplift and erosion, most recently during the Mesozoic breakup of Gondwana. Lower crustal seismic velocities imply a moderately dense lower crust beneath the core of the GSM, but with lower density than suggested by recent gravity models.

Description: [1]  This is a second paper in a study of statistical identification and classification of earthquake clusters using a relocated catalog of 1981–2011 seismicity in southern California and synthetic catalogs produced by the ETAS model. Here we focus on classification of event families – statistically significant clusters comprised of foreshocks , mainshocks and aftershocks – that are detected with the methodology discussed in part I of the study. The families are analyzed using their representation as time oriented tree graphs. The results (i) demonstrate that the clustering associated with the largest earthquakes, m  〉 7, is statistically different from that of small-to-medium earthquakes; (ii) establish the existence of two dominant types of small-to-medium magnitude earthquake families– burst-like and swarm-like sequences – and a variety of intermediate cluster forms obtained as a mixture of the two dominant types; (iii) suggest a simple new quantitative measure for identifying the cluster type based on its topological structure; (iv) demonstrate systematic spatial variability of the cluster characteristics on a scale of tens of kilometers in relation to heat flow and other properties governing the effective viscosity of a region; and (v) establish correlation between the family topological structure and a dozen of metric properties traditionally considered in the literature (number of aftershocks, duration, spatial properties, b -value, parameters of Omori-Utsu and Båth law, etc .). The burst-like clusters likely reflect highly-brittle failures in relatively cold regions, while the swarm-like clusters are likely associated with mixed brittle-ductile failures in regions with relatively high temperature and/or fluid content. The results of this and paper I may be used to develop improved region-specific hazard estimates and earthquake forecasts.

Description: We examine the El Niño/Southern Oscillation (ENSO) influence on the quasi-biennial oscillation (QBO) modulation of the cold-point tropopause (CPT) temperatures. An analysis of approximately 5 decades (in most cases 1950s to near-present) of radiosonde data from eleven near-equatorial stations, distributed along the Equator, shows that the ENSO influence on the QBO is quite zonally symmetric. At all stations analyzed, the QBO has larger amplitude and longer period during La Niña conditions than during El Niño over this total period. We also show that, as a consequence of the ENSO influences on QBO periods and amplitudes, the differences between the warmer CPT temperatures during QBO westerly shear conditions and colder temperatures during QBO easterly shear conditions, are larger during La Niña than during El Niño for all stations considered for the entire period considered here. This strengthens earlier findings that the greatest dehydration of air entering the stratosphere from the troposphere occurs during the winter under La Niña and easterly QBO conditions. Also, stratosphere /troposphere wind and temperature profiles are derived to establish the degree of QBO downward penetration to influence zonal winds and temperatures in the upper troposphere.

Description: [1]  Paleoseismologic data from the southern Panamint Valley fault (PVF) reveal evidence of at least four surface ruptures during late Holocene time (0.33-0.48 ka, 0.9-3.0 ka, 3.3-3.6 ka and 〉4.1 ka). These paleo-earthquake ages indicate that the southern PVF has ruptured at least once and possibly twice during the ongoing (≤1.5 ka) seismic cluster in the Mojave section of the eastern California shear zone (ECSZ). The most recent event (MRE) on the PVF is also similar in age to the 1872 Owens Valley earthquake and the geomorphically youthful MRE on the Death Valley fault. The timing of the three oldest events at our site shows that the PVF ruptured at least once and possibly thrice during the well-defined 2-5 ka seismic lull in the Mojave section of the ECSZ. Interestingly, the 3.3-3.6 ka age of Event 3 overlaps with the 3.3-3.8 ka age of the penultimate (i.e., pre-1872) rupture on the central Owens Valley fault. These new PVF data support the notion that earthquake occurrence in the ECSZ may be spatially and temporally complex, with earthquake clusters occurring in different regions at different times. Coulomb Failure Function modeling of the Panamint Valley and Garlock faults reveals significant stress interactions between these two faults that may influence future earthquake occurrence. Specifically, our models suggest a possible rupture sequence whereby an event on the southern Panamint Valley fault can lead to the potential triggering of an event on the Garlock fault, which in turn could trigger the Mojave section of the San Andreas Fault.

Description: [1]  The lithospheric structure of the Indian plate has been investigated using converted wave techniques (P and S receiver functions, RFs) and a novel stacking analysis technique (without using deconvolution) applied to a large seismological dataset from permanent and temporary broadband seismic stations. We observe coherent energy from at least two seismic discontinuities i.e. the crust-mantle (Moho) and lithosphere-asthenosphere boundary (LAB) in the uppermost mantle. Here, we provide a novel seismic image of the Indian lithosphere showing definitive evidence of its flexure, which is interpreted to be primarily caused by the hard collision at ~55My resulting in the world's highest mountain chain - the Himalayas and the Tibetan plateau. Results from geoidal and gravity studies do suggest post-collisional flexuring of the Indian plate; however, the flexure lacks observational constraints. The observed wavelength of the flex is ~1000 km with the thickness of the Indian plate varying from ~70 km to 140 km; such a low value for a continent implies that the Indian plate has been reworked in the past. The plate deepens in the Himalayan region to a depth of ~150 km. Further, the converted phases are interpreted to be resulting from the bottom of the lithosphere. We clearly demonstrate that these are distinct and different from the mid-lithospheric discontinuity. For a large number of stations, the MLD and LAB are clearly separated in depth. Our observations suggest that the Archaean lithosphere is no longer intact and is prone to deformation.

Description: The variability of turbulent momentum flux in neutral and unstable atmospheric boundary layers is characterised by analysing surface-layer measurements and data from large-eddy simulations (LES). The method involves multiresolution (MR) decomposition of vertical wind and advected variables into eddy fluctuations on different scales. It provides a measure of the amount of flux variability that stems from same-scale correlations and from combinations of different scale eddy fluctuations. Combining two analysis methods enabled MR component cospectra to be introduced in order to study the contribution of downward and upward flux on different scales. These component cospectra were used to investigate at which scales most of the upward and downward momentum flux occurs. By using MR spectra, cospectra, and flow visualisation this investigation provides insights into turbulence structure and fluxes in neutral and unstable stratification. It is shown that most of the flux variability in the lower part of the boundary layer can be characterised as a combination of larger scale streamwise elongated horizontal wind streaks and smaller scale vertical wind fluctuations. These streaks are found to account for a large part of downward momentum flux at relatively large, energy-containing scales. Most of the upward momentum flux is found to occur at smaller scales. This can be interpreted as showing that upward momentum flux in these conditions is caused by the generation of smaller scale secondary motions when larger scale turbulence elements break down and dissipate. Differences in the height dependence of turbulence structure and momentum flux for neutral and unstably stratified conditions are also investigated and related to the existence of wind streaks and horizontal rolls in these different conditions.

Description: Idealized simulations on an f -plane of tropical cyclone (TC) landfall under a quiescent environment in the Southern Hemisphere are performed to investigate the effects of land-sea surface contrast on precipitation. In the control simulation with realistic roughness and moisture over land which is to the south of the TC, the simulated vortex moves toward land due to a land-induced steering flow. The abrupt decrease (increase) of tangential wind at the surface leads to convergence (divergence) on the onshore (offshore) flow side. Enhanced convergence at the top of the planetary boundary layer is found both in the onshore and offshore side, which is caused by the advection from the surface and the enhanced offshore radial wind. The boundary layer top convergence pattern is consistent with the rainfall distribution. Vertical wind shear develops during the landfall process associated with the low- and upper-level asymmetric flows across the model domain. The wavenumber-1-like low-level asymmetric flow is introduced by an asymmetric geopotential height field that is generated by the large area of frictionally-induced convergence at the onshore side and divergence at the offshore side. The upper-level asymmetric flow is attributed to asymmetric convection and associated diabatic heating after landfall. Most rainfall is found in the down-shear right quadrant, which is consistent with previous studies that focused on the effect of environmental shear. Although the existence of feedback from the vertical wind shear to rainfall remains an open question, the relation between maximum rainfall and vertical wind shear is robust especially when the shear magnitude is large after landfall.

Description: [1]  Studying heat transfer processes in sedimentary crustal rocks requires the correct thermal conductivity of the respective rock type. Often a single value is used for a given rock type, obtained from the measurements on homogenous samples. We demonstrate how variations in rock layering and micro-fractures on the sub-centimeter scale may influence thermal conductivity values at much larger scale. We obtain thermal conductivity images from lab measurements on two different, heterogeneous samples performed with an optical thermal conductivity scanner in two directions. We study different spatial averaging methods for parameterizing the structural heterogeneities and the associated variation of thermal conductivity within the samples. For each of these structural simplifications we set up a numerical model for a numerical heat transfer experiment in order to determine effective thermal conductivity values in two directions. We compare these values and the mean thermal conductivities obtained from different mixing laws and find that, in heterogeneous rocks, effective and mean thermal conductivity may differ substantially. This may cause significant errors in reservoir-scale simulations of heat transfer with associated severe consequences for estimated heat flow and temperatures.

Description: [1]  We analyze teleseismic P-waves from four Mw ≥ 6.5 earthquakes recorded by a petroleum industry survey in Long Beach, California. The survey used a 2-D array with up to 5200 seismometers, 120 m mean spacing, and 7 – 10 km aperture. At frequencies near 1 Hz P-wave travel-times and amplitudes exhibit coherent lateral variations over scales as short as ~400 m, including locally delayed travel-times and increased amplitudes at the crest of the Long Beach anticline. Deeper heterogeneity is indicated by P-wave phase velocities that deviate from reference model predictions for events from southwestern azimuths. We postulate that a sharp northeastward increase in Moho depth from the Inner Borderland (IB) to mainland southern California causes the anomalous phase velocities. Elastic forward modeling finds the travel-times are fit well by a Moho that dips 65° to the northeast and flattens ~10 km southwest of the Newport-Inglewood fault zone. Constraining the felsic thickness of mainland crust to 28 km requires an 8 km thick layer with a P-velocity of 7 km/s beneath it, which could result from basal accretion of former Farallon ocean crust or magmatic underplating during Miocene volcanism. Forward models with a 65° Moho dip predict a P-to-s conversion with a phase velocity of ~5 km/s. Deconvolution of the array's mean P-wave signal isolates a similar later arriving phase. The steep crust thickness transition supports a locally abrupt boundary to IB rifting. Our results highlight the utility of dense short-period arrays for passive imaging at near surface to uppermost mantle depths.

Description: [1]  In this study we present new high-resolution, regional-scale, Vp and Vp/Vs models of the northern-central Apennines along with accurate 3D locations of a large set of local earthquakes. The main velocity anomalies are consistent with the surface geology in the shallow layers and present evidence for fluids stored within the basement at greater depths beneath the extensional belt. The Adria and Tyrrhenian mantle are defined by positive velocity anomalies below 30 km depth, while a low Vp, high Vp/Vs region in between indicates the existence of an hydrated wedge. The results yield new constraints on active processes in the Apennines and more generally envisage the evolution of a post-collisional belt. Velocity anomalies and earthquakes are consistent with a complex system of delamination and sinking of the Adria continental lithosphere, with the peeling of the crust identified by intermediate-depth seismicity. Change of seismicity and structural patterns along the belt indicates that this tectonic process is diachronous and that fluids, released by sunken lithosphere, are stored within the crust conditioning the occurrence of seismicity and the onset of extension.

Description: Since 2007 a large decline in Arctic sea ice has been observed. The large-scale atmospheric circulation response to this decline is investigated in ERA-Interim reanalyses and HadGEM3 climate model experiments. In winter, post-2007 observed circulation anomalies over the Arctic, North Atlantic and Eurasia are small compared to interannual variability. In summer, the post-2007 observed circulation is dominated by an anticyclonic anomaly over Greenland which has a large signal-to-noise ratio. Climate model experiments driven by observed SST and sea ice anomalies are able to capture the summertime pattern of observed circulation anomalies, although the magnitude is a third of that observed. The experiments suggest warm SSTs and reduced sea ice in the Labrador Sea lead to warm temperature anomalies in lower troposphere which weaken the westerlies over North America through thermal wind balance. The experiments also capture cyclonic anomalies over north-western Europe, which are consistent with downstream Rossby wave propagation.