ALBERT

Description: The Central Andes of southern Peru, Bolivia, Argentina and Chile (between 12°S and 42°S) comprise the largest orogenic plateau in the world associated with abundant arc volcanism, the Central Andean Plateau, as well as multiple segments of flat-slab subduction making this part of the Earth a unique place to study various aspects of active plate tectonics. The goal of this continental-scale ambient noise tomography study is to incorporate broad-band seismic data from 20 seismic networks deployed incrementally in the Central Andes from 1994 May to 2012 August, to image the vertically polarized shear wave velocity (Vsv) structure of the South American Cordillera. Using dispersion measurements calculated from the cross-correlation of 330 broad-band seismic stations, we construct Rayleigh wave phase velocity maps in the period range of 8–40 s and invert these for the shear wave velocity (Vsv) structure of the Andean crust. We provide a dispersion misfit map as well as uncertainty envelopes for our Vsv model and observe striking first-order correlations with our shallow results (∼5 km) and the morphotectonic provinces as well as subtler geological features indicating our results are robust. Our results reveal for the first time the full extent of the mid-crustal Andean low-velocity zone that we tentatively interpret as the signature of a very large volume Neogene batholith. This study demonstrates the efficacy of integrating seismic data from numerous regional broad-band seismic networks to approximate the high-resolution coverage previously only available though larger networks such as the EarthScope USArray Transportable Array in the United States.

Description: Plant-driven fungal weathering is a major pathway of soil formation, yet the precise mechanism by which mycorrhiza alter minerals is poorly understood. Here we report the first direct in situ observations of the effects of a soil fungus on the surface of a mineral over which it grew in a controlled experiment. An ectomycorrhizal fungus was grown in symbiosis with a tree seedling so that individual hyphae expanded across the surface of a biotite flake over a period of three months. Ultramicroscopic and spectroscopic analysis of the fungus-biotite interfaces revealed intimate fungal-mineral attachment, biomechanical forcing, altered interlayer spacings, substantial depletion of potassium (similar to 50 nm depth), oxidation of the biotite Fe(II), and the formation of vermiculite and clusters of Fe(III) oxides. Our study demonstrates the biomechanical-chemical alteration interplay at the fungus-biotite interface at the nanometer scale. Specifically, the weathering process is initiated by physical distortion of the lattice structure of biotite within 1 mu m of the attached fungal hypha. Only subsequently does the distorted volume become chemically altered through dissolution and oxidation reactions that lead to mineral neoformation.

Description: On 2012 May 20 and 29, two damaging earthquakes with magnitudes Mw 6.1 and 5.9, respectively, struck the Emilia-Romagna region in the sedimentary Po Plain, Northern Italy, causing 26 fatalities, significant damage to historical buildings and substantial impact to the economy of the region. The earthquake sequence included four more aftershocks with Mw ≥ 5.0, all at shallow depths (about 7–9 km), with similar WNW–ESE striking reverse mechanism. The timeline of the sequence suggests significant static stress interaction between the largest events. We perform here a detailed source inversion, first adopting a point source approximation and considering pure double couple and full moment tensor source models. We compare different extended source inversion approaches for the two largest events, and find that the rupture occurred in both cases along a subhorizontal plane, dipping towards SSW. Directivity is well detected for the May 20 main shock, indicating that the rupture propagated unilaterally towards SE. Based on the focal mechanism solution, we further estimate the co-seismic static stress change induced by the May 20 event. By using the rate-and-state model and a Poissonian earthquake occurrence, we infer that the second largest event of May 29 was induced with a probability in the range 0.2–0.4. This suggests that the segment of fault was already prone to rupture. Finally, we estimate peak ground accelerations for the two main events as occurred separately or simultaneously. For the scenario involving hypothetical rupture areas of both main events, we estimate Mw = 6.3 and an increase of ground acceleration by 50 per cent. The approach we propose may help to quantify rapidly which regions are invested by a significant increase of the hazard, bearing the potential for large aftershocks or even a second main shock.

Description: We use traveltime data of local earthquakes and controlled sources observed by a large, temporary, amphibious seismic network to reveal the anatomy of the southcentral Chilean subduction zone (37–39°S) between the trench and the magmatic arc. At this location the giant 1960 earthquake (M = 9.5) nucleated and ruptured almost 1000 km of the subduction megathrust. For the three-dimensional tomographic inversion we used 17,148 P wave and 10,049 S wave arrival time readings from 439 local earthquakes and 94 shots. The resolution of the tomographic images was explored by analyzing the model resolution matrix and conducting extensive numerical tests. The downgoing lithosphere is delineated by high seismic P wave velocities. High v p/v s ratio in the subducting slab reflects hydrated oceanic crust and serpentinized uppermost oceanic mantle. The subducting oceanic crust can be traced down to a depth of 80 km, as indicated by a low velocity channel. The continental crust extends to approximately a 50-km depth near the intersection with the subducting plate. This suggests a wide contact zone between continental and oceanic crust of about 150 km, potentially supporting the development of large asperities. Eastward the crustal thickness decreases again to a minimum of about a 30-km depth. Relatively low v p/v s at the base of the forearc does not support a large-scale serpentinization of the mantle wedge. Offshore, low v p and high v p/v s reflect young, fluid-saturated sediments of forearc basins and the accretionary prism.

Description: A unique and very interesting earthquake of magnitude Mw 7.2 occurred in the Van region of Turkey on October 23, 2011 that caused a heavy loss of human lives and properties. The earthquake occurred on a blind oblique thrust fault oriented towards the NE–SW direction and dipping towards NW as evidenced by focal mechanism solution and aftershock distribution. In this study, we analyzed the seismogenesis and earthquake triggering during this sequence with the help of estimated seismological parameters (b-value of frequency–magnitude relation, p-value of aftershocks temporal decay and D-value of fractal dimension), 2D mapping of b- and p-values, 3D mapping of b-value and coseismic Coulomb stress modeling. The estimated seismic b-value equal to 0.89 reveals that the mainshock occurred in a highly stressed region and sequence comprised larger magnitude aftershocks due to the presence of large size asperities within the rupture zone. The normal estimate of p-value (0.98) suggests a tectonic genesis of the aftershocks sequence. The estimated D-value equal to 1.80 reveals that rupture propagated in a two-dimensional plane filled up by fractures. The spatial 2D and 3D mapping of seismic b-value suggests that the Van earthquake originated in a highly heterogeneous fractured rock matrix with fluid intrusions into it at deeper depth beneath the mainshock hypocenter region. The estimated coseismic Coulomb stress using the variable slip model for depth range 0–30 km exhibits a ‘butterfly’ pattern and most of the aftershocks fall (90%) in the region of enhanced Coulomb stress. This suggests that most of the aftershock activities have been triggered by transfer of positive Coulomb stress due to coseismic slip of the mainshock. The results estimated in the present study have potential useful implications in future seismic hazard assessment and risk mitigation in Van and the surrounding regions.

Description: We investigate depth variations of the 410 and 520 km-discontinuities beneath Asia and the Pacific which serve as examples for a continental and an oceanic region, respectively. The depths are derived from travel-time differences between the PP-phase and its precursors that are reflected at the discontinuities. After accounting for differences in average crustal thickness, we find that the depth of the ‘410’ is rather uniform but larger than expected beneath both regions with a value of approximately 418 km. Signals from the ‘520’ are slightly less pronounced. However, while the average depth of the ‘520’ beneath Asia is about 519 km, we obtain a value of about 531.5 km for the Pacific. Here, the depression of the discontinuities can be explained in view of thermal anomalies in relation to mantle plumes. For Asia, however, the observations seem to require a more complex pattern of thermal anomalies possibly complemented by variations in chemical composition.

Description: The high elevation of the southern Puna plateau, the widespread melting of its crust, the gap in intermediate depth seismicity and the recent eruptions of ignimbrite complexes can be explained by delamination of the lithospheric mantle beneath it. To test this hypothesis, an array consisting of 73 broad band and short period seismic stations was deployed in the region for a period of 2 years starting in 2007. We inverted the data using the two plane wave approach and obtained 1-D and 3-D Rayleigh wave phase velocities. Our dispersion curve shows that at short periods (〈70 s) the phase velocities are slightly higher than those of the Tibetan plateau and lower than those of the Anatolian plateau. At periods of 100–140 s we observe a low velocity zone that might be remnant hot asthenosphere below a flat slab (7–10 Ma). We estimate the average continental lithosphere thickness for the region to be between 100 and 130 km. Our three-dimensional Rayleigh wave phase velocities show a high velocity anomaly at low frequencies (0.007, 0.008, and 0.009 Hz) slightly to the north of Cerro Galan. This would be consistent with the hypothesis of delamination in which a piece of lithosphere has detached and caused upwelling of hot asthenosphere, which in turn caused widespread alkaline-collision related volcanism. This interpretation is also corroborated by our shear wave velocity model, where a high velocity anomaly beneath the northern edge of Cerro Galan at 130 km depth is interpreted as the delaminated block on top of the subducting Nazca slab.

Description: The Laramide orogeny (80-50 Ma) resulted in thick-skinned deformation of the western United States, more than 700 km inboard of the plate boundary where the Farallon Plate was subducting below North America. Most studies conclude that this event was the result of low-angle or flat subduction of the Farallon plate, whereby horizontal compressive stress from the shallow slab produced inboard crustal compression and shortening. However, it is still not clear what factors caused the Farallon plate to shallow prior to Laramide time or how stress was transferred from the flat slab to the continental interior. Three hypotheses have been proposed for triggering flat subduction: (1) an increase in the westward velocity of the North American plate; (2) subduction of an buoyant oceanic plateau with abnormally thick crust and possibly a low density harzburgite mantle lithosphere layer; and (3) slab suction produced by subduction-induced mantle wedge flow and enhanced by the presence of thick Colorado Plateau lithosphere in the backarc. In this study, we use numerical models to study the development of low-angle subduction below a continental plate with a structure similar to that of the western US. The two-dimensional, plain strain models use the SOPALE code, in which Arbitrary Lagrangian-Eulerian finite element techniques are used to compute the coupled thermal-mechanical evolution of the lithosphere-upper mantle system. We first assess what factors are needed to dynamically develop low-angle subduction. We find that the main control is the continental velocity, with enhanced slab shallowing as the continental velocity increases. In order to create a section of horizontal (i.e., flat) subduction, a further requirement is the presence of an oceanic plateau with a low-density harzburgite layer. The slab suction force seems to be less capable of creating a flat slab than the other two factors. This may be due to the low viscosity in the mantle wedge in our models (10^19~10^20Pa s), which cannot produce a sufficiently large hydrodynamic force to flatten the oceanic plate. Future work will examine variations in the strength of both the continental plate and the interface between the continent and low-angle oceanic plate, in order to explore the relationship between flat subduction and Rocky Mountain foreland deformation during Laramide time.

Description: Frequency-domain helicopter-borne electromagnetic (HEM) data sets are commonly interpreted using one-dimensional (1-D) modelling and inversion tools. In many cases this approach is valid (e.g., horizontally layered targets and groundwater applications) but there are areas of higher dimension that are not recovered correctly applying 1-D methods. In recent years multi-dimensional forward modelling codes became available. As there is no analytic solution to verify (or falsify) the obtained numerical solutions, comparison with 1-D values as well as amongst various two-dimensional (2-D) and three-dimensional (3-D) codes is essential. Values obtained over the background structure and at the centre of a large (a few hundred metres edge length) structure should match 1-D values. At the edges of the structure – i.e., at higher dimensional parts – only a comparison of different codes is possible. The more codes – especially if based on different methods and/or written by different programmers – agree the more reliable is the obtained synthetic data set.

Description: The seismic signature of the Moho from which geologic and tectonic evolution hypotheses are derived is to a large degree a result of the seismic methodology which has been used to obtain the image. Seismic data of different types, passive source (earthquake) broad-band recordings, and controlled source seismic refraction, densely recorded wide-angle deep seismic reflection, and normal incidence reflection (using VibroseisTM, explosives, or airguns), have contributed to the description of the Moho as a relatively complex transition zone. Of critical importance for the quality and resolution of the seismic image are the acquisition parameters, used in the imaging experiments. A variety of signatures have been obtained for the Moho at different scales generally dependent upon bandwidth of the seismic source. This variety prevents the development of a single universally applicable interpretation. In this way source frequency content, and source and sensor spacing determine the vertical and lateral resolution of the images, respectively. In most cases the different seismic probes provide complementary data that gives a fuller picture of the physical structure of the Moho, and its relationship to a petrologic crust–mantle transition. In regional seismic studies carried out using passive source recordings the Moho is a relatively well defined structure with marked lateral continuity. The characteristics of this boundary change depending on the geology and tectonic evolution of the targeted area. Refraction and wide-angle studies suggest the Moho to be often a relatively sharp velocity contrast, whereas the Moho in coincident high quality seismic reflection images is often seen as the abrupt downward decrease in seismic reflectivity. The origin of the Moho and its relation to the crust–mantle boundary is probably better constrained by careful analysis of its internal details, which can be complex and geographically varied. Unlike the oceanic Moho which is formed in a relatively simple, well understood process, the continental Moho can be subject to an extensive variety of tectonic processes, making overarching conclusions about the continental Moho difficult. Speaking very broadly: 1) In orogenic belts still undergoing compression and active continental volcanic arcs, the Moho evolves with the mountain belt, 2) In collapsed Phanerozoic orogenic belts the Moho under the collapse structure was formed during the collapse, often by a combination of processes. 3) In regions having experienced widespread basaltic volcanism, the Moho can result from underplated basalt and basaltic residuum. In Precambrian terranes the Moho may be as ancient as the formation of the crust, in others Precambrian tectonic and magmatic processes have reset it. We note that seismic reflection data in Phanerosoic orogens as well as from Precambrian cratonic terranes often show thrust type structures extending as deep as the Moho, and suggest that even where crust and mantle xenoliths provide similar age of formation dates, the crust may be semi-allochothonous

Description: Providing quantitative microzonation results that can be taken into account in urban land-use plans is a challenging task that requires collaborative efforts between the seismological and engineering communities. In this study, starting from the results obtained by extensive geophysical and seismological investigations, we propose and apply an approach to the Gubbio basin (Italy) that can be easily implemented for cases of moderate-to-low ground motion and that takes into account not only simple 1D, but also more complicated 3D effects. With this method, the sites inside the basin are classified by their fundamental resonance frequencies, estimated from the horizontal-to-vertical spectral ratio applied to noise recordings (HVNSR). The correspondence between estimates of the fundamental frequency from this method and those derived from earthquake recordings was verified at several calibration sites. The amplification factors used to correct the response spectra are computed by the ratio between the response spectra at sites within the basin and the response spectra at a hard-rock site using data from two seismic transects. Empirical amplification functions are then assigned to the fundamental frequencies after applying an interpolation technique. The suitability of the estimated site-specific correction factors for response spectra was verified by computing synthetic response spectra for stations within the basin, starting from the synthetic recording at a nearby rock station, and comparing them with observed ones.

Description: Analysis of seismic anisotropy in the crust and the uppermost mantle gives lots of information about the ambient mantle flow, stress state, and the dynamic processes inside the Earth. Thus, seismic anisotropy and its main distinctive features beneath the southeastern Mediterranean region are studied through the analysis of teleseismic shear-wave splitting observed at six broadband seismic stations belonging to the GEOFON and the MedNet. Although the number of the recording stations is small; a total of 495 splitting parameters are obtained, which revealed significant variations in the observed fast polarization directions beneath the study area. The stations in northern Egypt and Cyprus show fast velocity directions oriented roughly N–S to NNE–SSW, coincident with many previous results. A slightly different splitting pattern comprising NE–SW fast polarization directions is observed in the stations located along the Dead Sea fault in the southeastern Mediterranean; which are consistent with the current strike-slip motion between Africa and Arabia. In addition, NW–SE fast polarization directions are recognized in the latter group. The observed delay times vary greatly but their averages lie between 0.35 and 1 s. Although large-scale mechanisms, such as the absolute plate motion of Africa and Arabia towards Eurasia and the differential motion between Arabia and Africa can be invoked to predominantly explain the origin of anisotropic features, we suggest that density-driven flow in the asthenosphere is a possible additional cause of the wide range of the splitting pattern observed beneath some stations.

Description: The intention of this article is to present the definitions of different functionals of the Earth's gravity field and possibilities for their approximative calculation from a mathematical representation of the outer potential. [...] More or less, what is compiled here is well-known in physical geodesy but distributed over a lot of articles and books which are not cited here. In the first instance this text is targeted at non-geodesists and it should be "stand-alone readable".

Description: The Iquique Local Network (ILN), a temporal network of broadband and short period seismic stations has been operating in Northern Chile since 2009. The aim of this installation was to locally densify the permanent seismic installation of the Integrated Plate Boundary Observatory in Chile (IPOC), with the main goal to decrease the magnitude of detected earthquake, to improve the hypocentral location accuracy, to allow a more accurate investigation of seismic source parameters, and to analyse proposed seismogenic structures of the Northern Chile seismic gap. The network setup evolved with time, with different geometries at different installation phases, aiming to study different seismicity features. In the first phase, started in 2009 and operational since 2010 until autumn 2013, the network had a sparse configuration, targeting a broad region extending from 19.5° S in the North to approximately 21.3° S South of Iquique. In the following stage, operational until fall 2017, most broadband stations were rearranged into a small aperture seismic array (PicArray) close to the village of Pica, to monitor with array techniques the shallow seismicity at the plate interfacer, intermediate and deep focus seismicity. These data are freely available under the Creative Commons Attribution 4.0 International Licence (CC BY 4.0) at the GEOFON data centre under network code IQ.

Description: In eastern Turkey, the ongoing convergence of the Arabian and African plates with Eurasia has resulted in the westward extrusion of the Anatolian Plate. To better understand the current state and the tectonic history of this region, we image crust and uppermost mantle structure with ambient noise tomography. Our study area extends from longitudes of 32° to 44°E. We use continuous data from two temporary seismic deployments, our 2006–2008 North Anatolian Fault Passive Seismic Experiment and the 1999–2001 Eastern Turkey Seismic Experiment, as well as from additional seismographs in the region. We compute daily cross-correlations of noise records between all station pairs and stack them over the entire time period for which they are available, as well as in seasonal subsets, to obtain interstation empirical Green's functions. After selecting interstation cross-correlations with high signal-to-noise ratios and measuring interstation phase velocities, we compute phase velocity maps at periods ranging from 8 to 40 s. At all periods, the phase velocity maps are similar for winter and summer subsets of the data, indicating that seasonal variations in noise sources do not bias our results. Across the study area, we invert the phase velocity estimates for shear velocity as a function of depth. The shear velocity model, which extends to 50 km depth, highlights tectonic features apparent at the surface: the Eastern Anatolian Plateau is a prominent low-velocity anomaly whereas the Kırşehir Massif has relatively fast velocities. There is a large velocity jump across the Inner Tauride Suture/Central Anataolian Fault Zone throughout the crust whereas the North Anatolian Fault does not have a consistent signature. In addition, in the southeastern part of our study area, we image a high velocity region below 20 km depth which may be the northern tip of the underthrusting Arabian Plate.

Description: The tables and graphs result from evaluation of Model 1 (see Veröffentlichungen des Zentralinstituts für Physik der Erde, Nr. 41, Potsdam 1976). They help to approximate values of the hydrostatic pressure P, the bulk modulus and its pressure derivative and the seismic parameter of a sample at any volume contraction x in the range between 0.5 and 1 if these quantities referred to the initial state (P = 0) are known. The considered functions are tabulated for a set of grid points of the rectangle 0.5 ≤ x ≤ 1, 2.1 ≤ ϰ_1 ≤ 10, where ϰ_1 is the initial value of the pressure derivative of the bulk modulus. The lattice pitchs are 0.01 in x and 0.1 in ϰ_1. The inverse problem can also be solved approximately with the aid of the tables and, above all, the plots presented here by using the "method of corresponding curves".

Description: Summary 1. Preface 2. Theoretical excursus 3. Some remarks on Model 1 4. Instructions for using tables and plots 5. References 6. Tables and figures

Description: We use SAR interferometry to measure the strain accumulation along the left-lateral Haiyuan fault system (HFS), that marks the north-eastern boundary of the tibetan plateau. The last major earthquakes that occured along the HFS are the M~8 1920 Haiyuan earthquake (strike-slip mechanism) and the Ml=8-8.3 1927 Gulang earthquake that ruptured a thrust fault system. No large earthquake is reported on the central section of the HFS, the "Tianzhu seismic gap", since ~1000 years. We first analyze the complete ENVISAT SAR data archive along 4 descending and 2 ascending tracks for the 2003-2009 period and construct an InSAR-based mean Line-Of-Sight (LOS) velocity map around the HFS from the eastern end of the Qilian shan (102° E), to the west, to the Liupan shan (106° E), to the east. Data are processed using a small baseline chain type. For each track, all radar images are coregistrated to a single master and interferograms are produced using a local adaptative range filtering. Residual orbital and atmospheric delays are jointly inverted and corrected for each unwrapped interferogram. Atmospheric corrections are validated using the ERA40 global atmospheric model (ECMWF). The interferograms series on each track are then inverted to obtain the increments of LOS radar delays between acquisition dates, adapting the Lopez-Quiroz et al. 2009 time series analysis. The obtained LOS mean velocity maps show a dominant left-lateral motion across the fault with along-strike variations: some fault sections are locked at shallow depth while others are creeping and local vertical movements are observed (subsidence in the "Jingtai" pull-apart basin). For various fault slip rates imposed below 20 km (4-10 mm/yr), we model the shallow velocity by inverting the mean LOS velocity maps for both strike-slip and dip-slip motion on vertical, 5km x 2.5km discretized patches, using a least-square method with an appropriate degree of smoothing. The fault geometry follows the surface trace of the fault from SPOT images, with two main segments, on both sides of the Jingtai basin: one along the M~8 1920 rupture to the east and one along the creeping section to the west. For a far-field velocity of 6 mm/yr, the creeping rate is estimated to be up to 4 mm/yr on the western segment. We assess the influence of atmospheric noise on our velocity solution. Finally we compare the ENVISAT time series analysis with that obtained using the ERS dataset spanning the 1993- 1998 period to investigate for potential time variations of the strain accumulation across the Haiyuan fault.

Description: The Van (Eastern Anatolia, Turkey) earthquake occurred on Sunday, October 23, 2011 with a moment magnitude of 7.2. The tectonics of this region is characterized by strike–slip faulting on the Bitlis Suture Zone, and thrusting in the Zagros fold and thrust belt. Using high-rate (1 second) GPS data from permanent GNSS stations from the CORS-TR network, co-seismic displacements of eleven stations were determined using precise point positioning during this earthquake. We used the time series of coordinate changes for fourteen CORS-TR stations, and calculated the crust movements before and after the earthquake. According to the PPP solutions computed using high frequency GPS data to determine the co-seismic motions of stations, we conclude for the Van earthquake an occurrence time of 10:41:22 (UTC). No pre-seismic horizontal movement of stations at the level more than 5 mm before the earthquake could be observed. That means that no kinematic warning or prediction before the earthquake exists. Along an east–west horizontal line north of the Van Sea with a length of about 100 km, the northern part of this line experienced extension of 0.2–1 ppm in a NW–SE direction. The southern part experienced N–S shortening of 0.5–1.5 ppm. The N–S shortening we estimated geodetically matches well with the N–S shortening and thrust focal mechanism derived independently using seismic data by the USGS. Co-seismic surface displacements derived from the GPS data are consistent with the teleseismic source model given by the USGS. The geodetic source model derived from the GPS data reproduces the same moment magnitude and centroid as the teleseismic model, but shows a higher spatial resolution of the slip distribution. We also analyzed the post-seismic surface displacements derived from the GPS data within the first two weeks after the mainshock. No reasonable slip distribution on the co-seismic fault plane could be found, indicating that the sources for the early post-seismic deformation might come from the widely scattered aftershocks.

Description: Deep-Earth convection can be understood by studying hotspot volcanoes that form where mantle plumes rise up and intersect the lithosphere, the Earth’s rigid outer layer. Hotspots characteristically leave age-progressive trails of volcanoes and seamounts on top of oceanic lithosphere, which in turn allow us to decipher the motion of these plates relative to “fixed” deep-mantle plumes, and their (isotope) geochemistry provides insights into the long-term evolution of mantle source regions. However, it is strongly suggested that the Hawaiian mantle plume moved ~15° south between 80 and 50 million years ago. This raises a fundamental ques- tion about other hotspot systems in the Pacific, whether or not their mantle plumes experienced a similar amount and direction of motion. Integrated Ocean Drilling Program (IODP) Expedition 330 to the Louisville Seamounts showed that the Louisville hotspot in the South Pacific behaved in a different manner, as its mantle plume remained more or less fixed around 48°S latitude during that same time period. Our findings demonstrate that the Pacific hotspots move independently and that their trajectories may be controlled by differ- ences in subduction zone geometry. Additionally, shipboard geochemistry data shows that, in contrast to Hawaiian volcanoes, the construction of the Louisville Seamounts doesn’t involve a shield-building phase dominated by tholeiitic lavas, and trace elements confirm the rather homoge- nous nature of the Louisville mantle source. Both observations set Louisville apart from the Hawaiian-Emperor seamount trail, whereby the latter has been erupting abundant tholeiites (char- acteristically up to 95% in volume) and which ex- hibit a large variability in (isotope) geochemistry and their mantle source components.

Description: We use local earthquake data observed by the amphibious, temporary seismic MERAMEX array to derive spatial variations of seismic attenuation (Qp) in the crust and upper mantle beneath Central Java. The path-averaged attenuation values (t∗) of a high quality subset of 84 local earthquakes were calculated by a spectral inversion technique. These 1929 t∗-values inverted by a least-squares tomographic inversion yield the 3D distribution of the specific attenuation (Qp). Analysis of the model resolution matrix and synthetic recovery tests were used to investigate the confidence of the Qp-model. We notice a prominent zone of increased attenuation beneath and north of the modern volcanic arc at depths down to 15 km. Most of this anomaly seems to be related to the Eocene–Miocene Kendeng Basin (mainly in the eastern part of the study area). Enhanced attenuation is also found in the upper crust in the direct vicinity of recent volcanoes pointing towards zones of partial melts, presence of fluids and increased temperatures in the middle to upper crust. The middle and lower crust seems not to be associated with strong heating and the presence of melts throughout the arc. Enhanced attenuation above the subducting slab beneath the marine forearc seems to be due to the presence of fluids.

Description: For a long time the root mean square (RMS) error has been used in the EM community: - to characterize data fit for a particular model; - as a criterion to compare several models obtained from inversion. The RMS error appears to be a natural choice since we usually tackle inverse problems in a least-squares sense. Over the years, RMS became a customary criterion and gained ultimate significance. However, on the hunt for low RMS values, one often needs to introduce subjectivity by arbitrarily adjusting error floors or masking “bad” data without referring to the assumptions behind RMS. In this contribution, we revisit basic assumptions behind RMS, demonstrate its deficiency and propose alternative ways, which may provide more insight into our data and allow a more comprehensive assessment of the quality of the modelling result/resistivity model.

Description: Waveforms of 50 teleseismic receiver functions and dispersion curves have been used to invert jointly for the 1D average crust and uppermost mantle seismic structure beneath GEOFON station ISP. The surface wave (Rayleigh and Love) dispersion curves were adopted from an earlier study and greatly help to stabilize the inversions by alleviating the deficiencies of inverting the receiver functions alone. The teleseismic receiver functions had a backazimuth distribution from which three stacks were created. All the stacks resulted in compatible inversions. Nine different influence factors ranging between 0.1 and 0.9 were employed and the influence factors around 0.5 were preferred for producing a reasonable seismic structure to represent the geology under station ISP. Three distinct crustal depth ranges are identified based on the inverted velocity-depth profile. The first corresponds to 0–5 km where the velocity-depth profile has a positive gradient with an increase from 2.5 km s−1 to 3.0 km s−1. The second depth range is 5–10 km and the velocity-depth profile shows a positive gradient between 3.0 km s−1 and 3.20 km s−1. A velocity jump from 3.20 km s−1 to 3.62 km s−1 tops the third depth range (10–20 km), which is characterized by a constant velocity. There is another velocity drop at 20 km depth from 3.62 km s−1 to 3.40 km s−1 and the low velocity therein is persistent down to 27.5 km. Below this depth range, the shear velocity increases until 40 km depth where the Moho discontinuity is estimated with Sn velocity of 4.4 km s−1. In general, seismic velocities under SW Anatolia are considerably slower than those of the Preliminary Reference Earth Model (PREM).

Description: The geometry, kinematics, and mode of back‐arc extension along the Andaman Sea plate boundary are refined using a new set of significantly improved hypocenters, global centroid moment tensor (CMT) solutions, and high‐resolution bathymetry. By applying cross‐correlation and double‐difference (DD) algorithms to regional and teleseismic waveforms and arrival times from International Seismological Centre and National Earthquake Information Center bulletins (1964–2009), we resolve the fine‐scale structure and spatiotemporal behavior of active faults in the Andaman Sea. The new data reveal that back‐arc extension is primarily accommodated at the Andaman Back‐Arc Spreading Center (ABSC) at ~10°, which hosted three major earthquake swarms in 1984, 2006, and 2009. Short‐term spreading rates estimated from extensional moment tensors account for less than 10% of the long‐term 3.0–3.8 cm/yr spreading rate, indicating that spreading by intrusion and the formation of new crust make up for the difference. A spatiotemporal analysis of the swarms and Coulomb‐stress modeling show that dike intrusions are the primary driver for brittle failure in the ABSC. While spreading direction is close to ridge normal, it is oblique to the adjacent transforms. The resulting component of E‐W extension across the transforms is expressed by deep basins on either side of the rift and a change to extensional faulting along the West Andaman fault system after the Mw = 9.2 Sumatra‐Andaman earthquake of 2004. A possible skew in slip vectors of earthquakes in the eastern part of the ABSC indicates an en‐echelon arrangement of extensional structures, suggesting that the present segment geometry is not in equilibrium with current plate‐motion demands, and thus the ridge experiences ongoing re‐adjustment.

Description: Magnetotellurics (MT) relies on natural electromagnetic field variations to investigate the electrical conductivity structure of the subsurface. Modern MT data are often multivariate due to simultaneous recordings of multiple-channel time series of two (horizontal) electric and three magnetic field components at multiple stations. Single site and remote reference processing only use a small portion of data to estimate the impedance tensor. The multiple-station approach, initially presented by Egbert (1997), uses all data information to improve the signal-to-noise ratios, which results in better estimations of the transfer functions. This is particularly import in industrialized regions, where the influence of man-made noise signals often exceeds the natural EM fields and hampers the estimation of MT impedance tensors. We have included the multiple-station data approach in our processing scheme EMERALD and tested it with different data sets. A non-robust calculation of the impedance tensor based on the multiple-station approach shows already slightly improved results compared to robust single site or even remote reference estimators. However, in case of high level man-made noise advances of the multiple-station algorithm are not observed. Tests with existing robust routines within EM, which are based on bivariate assumptions, do not reveal a significant improvement.

Description: Cratons with their thick lithospheric roots can influence the thermal structure, and thus the convective flow, in the surrounding mantle. As mantle temperatures are hard to measure directly, depth variations in the mantle transition zone (MTZ) discontinuities are often employed as a proxy. Here, we use a large new data set of P-receiver functions to map the 410 km and 660 km discontinuities beneath the western edge of the East European Craton and adjacent Phanerozoic Europe across the most fundamental lithospheric boundary in Europe, the Trans-European Suture Zone (TESZ). We observe significantly shorter travel times for conversions from both MTZ discontinuities within the craton, caused by the high velocities of the cratonic root. By contrast, the differential travel time across the MTZ is normal to only slightly raised. This implies that any insulating effect of the cratonic keel does not reach the MTZ. In contrast to earlier observations in Siberia, we do not find any trace of a discontinuity at 520 km depth, which indicates a rather dry MTZ beneath the western edge of the craton. Within most of covered Phanerozoic Europe, the MTZ differential travel time is remarkably uniform and in agreement with standard Earth models. No widespread thermal effects of the various episodes of Caledonian and Variscan subduction that took place during the amalgamation of the continent remain. Only more recent tectonic events, related to Alpine subduction and Quarternary volcanism in the Eifel area, can be traced. While the East European craton shows no distinct imprint into the MTZ, we discover the signature of the TESZ in the MTZ in the form of a linear region of about 350 km width with a 1.5 s increase in differential travel time, which could either be caused by high water content or decreased temperature. Taking into account results of recent S-wave tomographies, raised water content in the MTZ cannot be the main cause for this observation. Accordingly, we explain the increase, equivalent to a 15 km thicker MTZ, by a temperature decrease of about 80 K. We discuss two alternative models for this temperature reduction, either a remnant of subduction or an indication of downwelling due to small-scale, edge-driven convection caused by the contrast in lithospheric thickness across the TESZ. Any subducted lithosphere found in the MTZ at this location is unlikely to be related to Variscan subduction along the TESZ, though, as Eurasia has moved significantly northward since the Variscan orogeny.

Description: We constrain structural features and shear-velocity structure of a low-velocity anomaly in the lower mantle beneath the Pacific (we term it the “Pacific Anomaly”) on the basis of forward travel time and waveform modeling of the observed direct S, Sdiff, ScS, SKS, and SKKS phases sampling a great arc across the anomaly from eastern Eurasia to southern South America. After correction for the effects of earthquake mislocation and the seismic heterogeneities outside the Pacific Anomaly, seismic observations suggest that the Pacific Anomaly along the great arc consists of at least two separated portions with a 740-km-wide gap between them. The western portion of the anomaly is about 1050 km wide, extends at least 740 km above the core-mantle boundary (CMB), and exhibits a trapezoidal shape with lateral dimensions increasing slightly with depth. The velocity structure of the western portion varies from −3.0% at the top (740 km above the CMB) to −3.5% at 100 km above the CMB and an average shear-velocity reduction of −5% in the bottom 100 km of the mantle. The eastern portion of the anomaly reaches at least 340 km above the CMB beneath the mid-Pacific with an 1800-km-wide base and has a uniform velocity reduction of −3%. Waveform modeling further suggests a very low velocity layer with a shear-velocity reduction of −10% located at the edge of the western portion of the anomaly. Combining the latest results from others, we present a general picture of structural and velocity structures of the Pacific Anomaly. The structural and velocity features suggest that the anomaly represents a cluster of metastable thermo-chemical piles.

Description: Integrated Ocean Drilling Program (IODP) Expedition 340T returned to the 1.4-km-deep Hole U1309D at Atlantis Massif to carry out borehole logging including vertical seismic profiling (VSP). Seismic, resistivity, and temperature logs were obtained throughout the geologic section in the footwall of this oceanic core complex. Reliable downhole temperature measurements throughout and the first seismic coverage of the 800–1400 meters below seafloor (mbsf) portion of the section were obtained. Distinct changes in velocity, resistivity, and magnetic susceptibility characterize the boundaries of altered, olivine-rich troctolite intervals within the otherwise dominantly gabbroic se-quence. Some narrow fault zones also are associated with downhole resistivity or velocity excursions. Small deviations in temperature were measured in borehole fluid adjacent to known faults at 750 mbsf and 1100 mbsf. This suggests that flow of seawater remains active along these zones of faulting and rock alteration. Vertical seismic profile station coverage at zero offset now extends the full length of the hole, including the uppermost 150 mbsf, where detachment processes are expected to have left their strongest imprint. Analysis of wallrock properties, together with alteration and structural characteristics of the cores from Site U1309, highlights the likely interplay between lithology, structure, lithospheric hydration, and core complex evolution.

Description: We study the distribution of the aftershocks of Tocopilla Mw 7.7 earthquake of 2007 November 14 in northern Chile in detail. This earthquake broke the lower part of the seismogenic zone at the southern end of the Northern Chile gap, a region that had its last megathrust earthquake in 1877. The aftershocks of Tocopilla occurred in several steps: the first day they were located along the coast inside the co-seismic rupture zone. After the second day they extended ocean-wards near the Mejillones peninsula. Finally in December they concentrated in the South near the future rupture zone of the Michilla intermediate depth earthquake of 2007 December 16. The aftershock sequence was recorded by the permanent IPOC (Integrated Plate Boundary Observatory in Chile) network and the temporary task force network installed 2 weeks after the main event. A total of 1238 events were identified and the seismic arrival times were directly read from seismograms. Initially we located these events using a single event procedure and then we relocated them using the double-difference method and a cross-correlation technique to measure time differences for clusters of aftershocks. We tested a 1-D velocity model and a 2-D one that takes into account the presence of the subducted Nazca Plate. Relocation significantly reduced the width of the aftershock distribution: in the inland area, the plate interface imaged by the aftershocks is thinner than 2 km. The two velocity models give similar results for earthquakes under the coast and a larger difference for events closer to the trench. The surface imaged by the aftershocks had a length of 160 km. It extends from 30 to 50 km depth in the northern part of the rupture zone; and between 5 and 55 km depth near the Mejillones peninsula. We observed a change in the dip angle of the subduction interface from 18° to 24° at a depth of 30 km. We propose that this change in dip is closely associated with the upper limit of the rupture zone of the main event. We also studied the focal mechanisms of the aftershocks, most of them were thrust events like the mainshock. As the aftershock activity was significantly reduced, on 2007 December 13, an ML 6.1 event occurred offshore of the Mejillones peninsula reactivating the seismicity. Three days later the Michilla intraslab earthquake of Mw 6.8 ruptured an almost vertical fault with slab-push mechanism. The aftershocks locations of this event define a planar zone about 11 km in depth, situated right bellow the subduction interface.

Description: The paper consists of two main elements: (1) the creation of a harmonized seismicity data file for the eastern Mediterranean area and the generation of a classical timeindependent PSHA, and (2) the extension of the earthquake model of the timeindependent approach to a newly developed time‐dependent PSHA and the comparison with the results of (1).

Description: This study represents the first investigating potential kinematic boundary conditions for subduction erosion in a systematic manner. For the subduction erosional process, the aperture at the box's rear, which allowed rearward material loss, was shown to have the largest influence on obtained results. If the amount of material leaving the system was larger than the amount of material subducted at the wedge's toe, the margin evolved as erosional. We found the surface slope to be the second important parameter, which strongly controls the amount of basally eroded material.

Description: A sequence of large strike‐slip earthquakes occurred west of Sunda Trench beneath the Wharton Basin. First reports indicate that the main shock was extremely complex, involving three to four subevents (Mw 〉 8) with a maze of aftershocks. We investigate slip models of the two largest earthquakes by joint inversion of regional and teleseismic waveform data. Using the Mw7.2 foreshock, we developed hybrid Green's Functions for the regional stations to approximate the mixture of oceanic and continental paths. The main shock fault geometry is defined based on the back projection results, point‐source mechanisms, aftershock distribution, and fine tune of grid searches. The fault system contains three faults, labeled F1 (89°/289° for dip/strike), F2 (74°/20°), and F3 (60°/310°). The inversion indicates that the main rupture consisted of a cascade of high‐stress drop asperities (up to 30 MPa), extending as deep as 50 km. The rupture propagated smoothly from one fault to the next (F1, F2, and F3 in sequence) with rupture velocities of 2.0–2.5 km/s. The whole process lasted about 200 s, and the major moment release (〉70%) took place on the N‐S oriented F2. The Mw8.2 aftershock happened about 2 h later on a N‐S oriented fault with a relatively short duration (~60 s) and also ruptured as deep as 50 km. The slip distributions suggest that the earthquake sequence was part of a broad left‐lateral shear zone between the Australian and Indian plates and ruptured the whole lithosphere. These earthquakes apparently reactivated existing fracture zones and were probably triggered by unclamping of the great Sumatran earthquake of 2004.

Description: A long, complex aftershock sequence has followed an Mw 6.1 mainshock in February 2008, in Storfjorden, Svalbard. Relatively located aftershock epicenters and focal mechanisms from moment tensor inversion suggest that the series took place along a NE–SW trending and steeply SSE dipping oblique-normal fault, with the total aftershock extent exceeding the mainshock rupture. The magnitude of the mainshock and the duration of the aftershock activity render the Storfjorden sequence important in terms of earthquake hazard assessment in the Svalbard region. We have evaluated all involved uncertainties and combined seismological information with background knowledge for the area, in order to achieve as sound a seismotectonic interpretation as possible. Our results clearly indicate that the activity in Storfjorden should be attributed to a previously unknown tectonic structure. In addition, there are indications of activation of secondary structures and possibly of stress triggering. These new findings affect our understanding of the region's earthquake potential.

Description: Amplitude ratio of 30 short-period conspicuous P5KP and PKPab phases from five intermediate depth or deep events in Fiji-Tonga recorded at European stations around 150° distance shows a mean value two to three times the ratio of the synthetic amplitudes obtained by the normal-mode theory (and ak135 model) or by full-wave theory (and PREM). There is a large variance in the results, also observed in five amplitude ratios from one event in Argentina observed at temporary stations in China around 156°. Global recordings of three major deep earthquakes in Fiji, Bonin, and Western Brazil observed at ASAR, WRA, and ZRNK arrays, at 59 North America stations and at six South Pole stations displayed conspicuous P4KP and PcP (or ScP) phases. The amplitude ratio values of P4KP vs P(S)cP are sometimes almost one order of magnitude larger than the corresponding values of the synthetics. In both cases, arrival times and slowness values (corrected for ellipticity and station elevation) at the distances up to 23° beyond the A cutoff point predicted by ray theory match both the synthetics, suggesting the observations are the AB branch of PmKP (m = 4, 5) around 1 Hz. In disagreement to ray theory, no reliable BC branch is observed neither on the recordings nor on the normal-mode synthetics. The high amplitude ratio values cannot be explained by realistic perturbations of the velocity or attenuation values of the global models in the proximity of the core-to-mantle boundary (CMB). We speculate that the focusing effects and/or strong scattering most likely associated to some anomalous velocity areas of the lowermost mantle are responsible for that. The results suggest limitations of the previous evaluations of the short-period attenuation in the outer core from PmKP amplitudes (m ≥ 3), irrespective of the fact that they are obtained by using ray theory, normal-mode or full-wave synthetics. Attempts to use PmKP arrival times in order to refine velocity structure in the proximity of CMB should be also regarded with care if the propagation times have been computed with ray theory.