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: [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: [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.