ALBERT

Description: On 12 September 2007, an Mw 8.4 earthquake occurred within the southern section of the Mentawai segment of the Sumatra subduction zone, where the subduction thrust had previously ruptured in 1833 and 1797. Traveltime data obtained from a temporary local seismic network, deployed between December 2007 and October 2008 to record the aftershocks of the 2007 event, was used to determine two-dimensional (2-D) and three-dimensional (3-D) velocity models of the Mentawai segment. The seismicity distribution reveals significant activity along the subduction interface and within two clusters in the overriding plate either side of the forearc basin. The downgoing slab is clearly distinguished by a dipping region of high Vp (8.0 km/s), which can be a traced to ∼50 km depth, with an increased Vp/Vs ratio (1.75 to 1.90) beneath the islands and the western side of the forearc basin, suggesting hydrated oceanic crust. Above the slab, a shallow continental Moho of less than 30 km depth can be inferred, suggesting that the intersection of the continental mantle with the subducting slab is much shallower than the downdip limit of the seismogenic zone despite localized serpentinization being present at the toe of the mantle wedge. The outer arc islands are characterized by low Vp (4.5–5.8 km/s) and high Vp/Vs (greater than 2.0), suggesting that they consist of fluid saturated sediments. The very low rigidity of the outer forearc contributed to the slow rupture of the Mw 7.7 Mentawai tsunami earthquake on 25 October 2010.

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: In the framework of the Dead Sea Integrated Research project (DESIRE), 59 seismological stations were deployed in the region of the Dead Sea Basin. Twenty of these stations recorded data of sufficiently high quality between May and September 2007 to be used for ambient seismic noise analysis. Empirical Green’s functions are extracted from cross-correlations of long term recordings. These functions are dominated by Rayleigh waves, whose group velocities can be measured in the frequency range from 0.1 to 0.5 Hz. Analysis of positive and negative correlation lags of the Green’s functions makes it possible to identify the direction of the source of the incoming energy. Signals with frequencies higher than 0.2 Hz originate from the Mediterranean Sea, while low frequencies arrive from the direction of the Red Sea. Travel times of the extracted Rayleigh waves were measured between station pairs for different frequencies, and tomographically inverted to provide independent velocity models. Four such 2D models were computed for a set of frequencies, all corresponding to different sampling depths, and thus together giving an indication of the velocity variations in 3D extending to a depth of 10 km. The results show low velocities in the Dead Sea Basin, consistent with previous studies suggesting up to 8 km of recent sedimentary infill in the Basin. The complex structure of the western margin of the Basin is also observed, with sedimentary infill present to depths not exceeding 5 km west of the southern part of the Dead Sea. The high velocities associated with the Lisan salt diapir are also observed down to a depth of ~5 km. The reliability of the results is confirmed by checkerboard recovery tests.

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: Multibeam bathymetry revealed the occurrence of numerous craterlike depressions, so-called pockmarks, on the sea floor of the Hammerfest Basin and the Loppa High, south-western Barents Sea. To investigate whether these pockmarks are related to ongoing gas seepage, microbial processes associated with methane metabolism were analyzed using geochemical, biogeochemical and microbiological techniques. Gravity cores were collected along transects crossing individual pockmarks, allowing a direct comparison between different locations inside (assumed activity center), on the rim, and outside of a pockmark (reference sites). Concentrations of hydrocarbons in the sediment, particularly methane, were measured as headspace (free) gas, and in the occluded and adsorbed gas fraction. Down to a depth of 2.6 m below sea floor (mbsf) sulfate reduction rates were quantified by radiotracer incubations. Concentrations of dissolved sulfate in the porewater were determined as well. Neither the sulfate profiles nor the gas measurements show any evidence of microbial activity or active fluid venting. Methane concentrations and sulfate reduction rates were extremely low or even below the detection limit. The results show that the observed sediment structures are most likely paleo-pockmarks, their formation probably occurred during the last deglaciation.

Description: In 2004 and 2005 a passive seismic experiment was carried out in the northern and northeastern part of the Bohemian Massif (Sudetes) to study the lithospheric structure. We present results from Ps and Sp receiver function analyses. With one exception, Moho depth at stations in the northwestern part of the study area varies between 28 and 32 km. Thicker crust up to 35 km was mapped toward the south (Moldanubian unit) and toward the east (Moravo–Silesian and Brunovistulian units) confirming results from previous active seismic measurements. There exists a relatively sharp step in Moho depth between units of the central Sudetes (~ 30 km) and the Moravo–Silesian unit (~ 35 km). The vp/vs ratios inverted from primary and multiple Moho Ps conversions hint for different crustal compositions of the units. Toward the Carpathian thrust we have no clear indications for any crustal root or slab beneath the western Carpathians. However, our data suggests a deepening of the Moho or at least a complicated crust–mantle transition in this area. Additional Ps phases were observed between 6 and 10 s delay time in the Sudetes. These phases cannot be explained by Moho reverberations, but are most probably caused by low velocity zones in the middle crust or lithospheric mantle as shown by modeling of theoretical receiver functions. The stations showing these abnormal phases are located in the area of Permo-Carboniferous basins on probably Teplá–Barrandian crust. Therefore we assume that the phases hint at a mid-crustal low velocity zone between 16 and 20 km depth, which is interpreted as a felsic solidified magma reservoir of the Permo-Carboniferous volcanism beneath the Sudetic Basins. Sp receiver functions show phases with negative polarity at 9 to 12 s lead time on average, which we interpret as lithosphere–asthenosphere boundary at about 80 to 110 km depth.

Description: The transition between the lithosphere and the asthenosphere is subject to numerous contemporary studies as its nature is still poorly understood. The thickest lithosphere is associated with old cratons and platforms and it has been shown that seismic investigations may fail to image the lithosphere-asthenosphere boundary in these areas. Instead, several recent studies have proposed a mid-lithospheric discontinuity of unknown origin existing under several cratons. In this study we investigate the Tanzania craton in East Africa which is enclosed by the eastern and western branches of the East African Rift System. We present evidence from S receiver functions for two consecutive discontinuities at depths of 50–100 km and 140–200 km, which correspond to significant S wave velocity reductions under the Tanzania craton and the Albert and Edward rift segments. By comparison with synthetic waveforms we show that the lower discontinuity coincides with the LAB exhibiting velocity reductions of 6–9%. The shallower interface reveals a velocity drop that varies from 12% beneath the craton to 24% below the Albert-Edward rift. It is interpreted as an infiltration front marking the upper boundary of altered lithosphere due to ascending asthenospheric melts. This is corroborated by computing S velocity variations based on xenolith samples which exhibit a dense system of crystallized veins acting as pathways of the infiltrating melt. Mineral assemblages in these veins are rich in phlogopite and pyroxenite which can explain the reduced shear wave velocities. Melt infiltration represents a suitable mechanism to form a mid-lithospheric discontinuity within cratonic lithosphere that is underlain by anomalously hot mantle.

Description: The passive margin of the South Atlantic shows typical features of a rifted volcanic continental margin, encompassing seaward dipping reflectors, continental flood basalts and high-velocity/density lower crust at the continent–ocean transition, probably emplaced during initial seafloor spreading in the Early Cretaceous. The Springbok profile offshore western South Africa is a combined transect of reflection and refraction seismic data. This paper addresses the analysis of the seismic velocity structure in combination with gravity modelling and isostatic modelling to unravel the crustal structure of the passive continental margin from different perspectives. The velocity modelling revealed a segmentation of the margin into three distinct parts of continental, transitional and oceanic crust. As observed at many volcanic margins, the lower crust is characterised by a zone of high velocities with up to 7.4 km/s. The conjunction with gravity modelling affirms the existence of this body and at the same time substantiated its high densities, found to be 3100 kg/m3. Both approaches identified the body to have a thickness of about 10 km. Yet, the gravity modelling predicted the transition between the high-density body towards less dense material farther west than initially anticipated from velocity modelling and confirmed this density gradient to be a prerequisite to reproduce the observed gravity signal. Finally, isostatic modelling was applied to predict average crustal densities if the margin was isostatically balanced. The results imply isostatic equilibrium over large parts of the profile, smaller deviations are supposed to be compensated regionally. The calculated load distribution along the profile implies that all pressures are hydrostatic beneath a depth of 45 km.

Description: In this study we present the new tomographic code ANITA which provides 3-D anisotropic P and isotropic S velocity distribution based on P and S traveltimes from local seismicity. For the P anisotropic model, we determine four parameters for each parameterization cell. This represents an orthorhombic anisotropy with one predefined direction oriented vertically. Three of the parameters describe slowness variations along three horizontal orientations with azimuths of 0°, 60°, and 120°, and one is a perturbation along the vertical axis. The nonlinear iterative inversion procedure is similar to that used in the LOTOS code. We have implemented this algorithm for the updated data set of central Java, part of which was previously used for the isotropic inversion. It was obtained that the crustal and uppermost mantle velocity structure beneath central Java is strongly anisotropic with 7–10% of maximal difference between slow and fast velocity in different directions. In the forearc (area between southern coast and volcanoes), the structure of both isotropic and anisotropic structure is strongly heterogeneous. Variety of anisotropy orientations and highly contrasted velocity patterns can be explained by a complex block structure of the crust. Beneath volcanoes we observe faster velocities in vertical direction, which is probably an indicator for vertically oriented structures (channels, dykes). In the crust beneath the middle part of central Java, north to Merapi and Lawu volcanoes, we observe a large and very intense anomaly with a velocity decrease of up to 30% and 35% for P and S models, respectively. Inside this anomaly E-W orientation of fast velocity takes place, probably caused by regional extension stress regime. In a vertical section we observe faster horizontal velocities inside this anomaly that might be explained by layering of sediments and/or penetration of quasi-horizontal lenses with molten magma. In the mantle, trench parallel anisotropy is observed throughout the study area. Such anisotropy in the slab entrained corner flow may be due to presence of B-type olivine having predominant axis parallel to the shear direction, which appears in conditions of high water or/and melting content.

Description: Local earthquake data from a dense temporary seismological network in the southern Dead Sea area have been analysed within the project DESIRE (Dead Sea Integrated Research Project). Local earthquakes are used for the first precise image of the distribution of the P-wave velocity and the vP/vS ratios. 65 stations registered 655 local events within 18 months of observation time. A subset of 530 well-locatable events with 26 730 P- and S-arrival times was used to calculate a tomographic model for the vP and vP/vS distribution. Since the study area is at first-order 2-D, a gradual approach was chosen, which compromised a 2-D inversion followed by a 3-D inversion. The sedimentary basin fill is clearly imaged through high vP/vS ratios and low vP. The basin fill shows an asymmetric structure with average depth of 7 km at the western boundary and depth between 10 and 14 km at the eastern boundary. This asymmetry is reflected by the vertical strike-slip eastern border fault, and the normal faulting at the western boundary, caused by the transtensional deformation within the last 5 Myr. Within the basin fill the Lisan salt diapir is imaged through low vP/vS ratios, reflecting its low fluid content. The extensions were determined to 12 km in E–W and 17 km in N–S direction while its depth is 5–6 km. The thickness of the pre-basin sediments below the basin fill cannot be derived from the tomography data—it is estimated to less than 3 km from former investigations. Below the basin, down to 18 km depth very low P-wave velocities and low vP/vS ratios are observed—most likely caused by fluids from the surrounding crust or the upper mantle.

Description: The North Anatolian Fault Zone (NAFZ) below the Sea of Marmara forms a “seismic gap” where a major earthquake is expected to occur in the near future. This segment of the fault lies between the 1912 Ganos and 1999 İzmit ruptures and is the only NAFZ segment that has not ruptured since 1766. To monitor the microseismic activity at the main fault branch offshore of Istanbul below the Çınarcık Basin, a permanent seismic array (PIRES) was installed on the two outermost Prince Islands, Yassiada and Sivriada, at a few kilometers distance to the fault. In addition, a temporary network of ocean bottom seismometers was deployed throughout the Çınarcık Basin. Slowness vectors are determined combining waveform cross correlation and P wave polarization. We jointly invert azimuth and traveltime observations for hypocenter determination and apply a bootstrap resampling technique to quantify the location precision. We observe seismicity rates of 20 events per month for M 〈 2.5 along the basin. The spatial distribution of hypocenters suggests that the two major fault branches bounding the depocenter below the Çınarcık Basin merge to one single master fault below ∼17 km depth. On the basis of a cross-correlation technique we group closely spaced earthquakes and determine composite focal mechanisms implementing recordings of surrounding permanent land stations. Fault plane solutions have a predominant right-lateral strike-slip mechanism, indicating that normal faulting along this part of the NAFZ plays a minor role. Toward the west we observe increasing components of thrust faulting. This supports the model of NW trending, dextral strike-slip motion along the northern and main branch of the NAFZ below the eastern Sea of Marmara.

Description: Here we present the results of local source tomographic inversion beneath central Java. The data set was collected by a temporary seismic network. More than 100 stations were operated for almost half a year. About 13,000 P and S arrival times from 292 events were used to obtain three-dimensional (3-D) Vp, Vs, and Vp/Vs models of the crust and the mantle wedge beneath central Java. Source location and determination of the 3-D velocity models were performed simultaneously based on a new iterative tomographic algorithm, LOTOS-06. Final event locations clearly image the shape of the subduction zone beneath central Java. The dipping angle of the slab increases gradually from almost horizontal to about 70°. A double seismic zone is observed in the slab between 80 and 150 km depth. The most striking feature of the resulting P and S models is a pronounced low-velocity anomaly in the crust, just north of the volcanic arc (Merapi-Lawu anomaly (MLA)). An algorithm for estimation of the amplitude value, which is presented in the paper, shows that the difference between the fore arc and MLA velocities at a depth of 10 km reaches 30% and 36% in P and S models, respectively. The value of the Vp/Vs ratio inside the MLA is more than 1.9. This shows a probable high content of fluids and partial melts within the crust. In the upper mantle we observe an inclined low-velocity anomaly which links the cluster of seismicity at 100 km depth with MLA. This anomaly might reflect ascending paths of fluids released from the slab. The reliability of all these patterns was tested thoroughly.

Description: We present a series of 1-D shear velocity models for the Sumatran Forearc and Arc derived from Rayleigh wave group dispersion in noise correlation functions from vertical and pressure records from an onshore–offshore seismic deployment. The 1-D models represent the crustal structure of the downgoing Indian Plate, the accretionary prism and the arc. There is a progression in shear velocity across the forearc to the arc associated with thickening of the accretionary prism and the development of an arc crust. The velocity structure inferred for the upper 20 km based on path averages between stations on the accretionary prism has velocities consistent with a thick sediment package in agreement with estimates of depth to the plate boundary determined from active source experiments. We also find low Indian Plate shear velocities, 〈4 km s−1 to 25 km depth beneath our station locations on the downgoing plate. These low seismic velocities are consistent with at least 14–24 per cent serpentinization of the oceanic crust and upper mantle of the downgoing plate. This high degree of serpentinization, may weaken the plate interface and explain the segmentation observed in the great Sumatran thrust earthquakes if the serpentinization is localized. The success of this study suggests that future onshore–offshore seismic deployments will be able to utilize this method.

Description: Earthquake early warning systems should provide reliable warnings as quickly as possible with a minimum number of false and missed alarms. Wireless meshed networks, coupled with low-cost seismometers for monitoring, evaluation, and information about seismic vibrations in space and time are introducing a new generation of warning infrastructures for mega-cities. The use of a cooperative method for signal analysis makes it possible to distinguish earthquakes (with a certain minimal magnitude) from other ground shaking in a city. The paper gives a short overview of our approach for developing decentralized early warning systems and an evaluation based on experiences gained from model investigations, testbeds in Berlin, and prototype installations in Istanbul.

Description: It has recently been shown that correlations of seismic noise can contain significant information about the Green's function along the station profile. Using an array of 38 temporary broad-band stations located in Finland between 1998 September and 1999 March, we study the resulting 703 noise correlations to understand how they are influenced by the directivity of the noise field. The latter information is obtained through f-k analysis of data from two permanent seismic arrays in Germany and Norway and from a subset of stations of the array in Finland. Both types of analysis confirm that the characteristic of the seismic noise is strongly frequency-dependent. At low frequencies (0.02-0.04 Hz), we observe diffuse noise and/or randomly distributed sources. In contrast, the noise is strongly direction-dependent and not fully diffuse in the intermediate period ranges (0.04-0.25 Hz) which correspond to the first and second micro seismic peak, created at the Irish and Scottish coast and the western coast of Norway In this frequency interval the noise is sufficiently close to a plane wave to introduce systematic errors in group velocities for station pairs which are not parallel to the direction of the dominant incident noise. Phase velocities calculated by slant stack over many traces are however correct, independently of profile direction. In the high-frequency band (0.25-1.0 Hz), the situation is a mix between the low-frequency and the intermediate frequency cases. Average phase velocities and individual group velocities from well-oriented profiles are in excellent agreement with results from Rayleigh wave studies of the area.

Description: Determining the relative orientation of the horizontal components of seismic sensors is a common problem that limits data analysis and interpretation for several acquisition setups, including linear arrays of geophones deployed in borehole installations or ocean bottom seismometers deployed at the seafloor. To solve this problem we propose a new inversion method based on a complex linear algebra approach. Relative orientation angles are retrieved by minimizing, in a least-squares sense, the l2-norm between the complex traces (hodograms) of adjacent pairs of sensors. This methodology can be applied without restrictions only if the wavefield recorded by each pair of sensors is very similar. In most cases, it is possible to satisfy this condition by low-pass filtering the recorded waveforms. The main advantage of our methodology is that, in the complex domain, the relative orientations of seismic sensors can be viewed as a linear inverse problem, which ensures that the preferred solution corresponds to the global minimum of a misfit function. It is also possible to use simultaneously more than one independent data set (other seismic events) to better constrain the solution of the inverse problem. Furthermore, by a computational point of view, our method results faster than the relative orientation methods based on waveform cross-correlation. After several tests on synthetic data sets we applied successfully our methodology to different types of real data. These applications include the alignment of borehole sensors relative to a Vertical Seismic Profiling (VSP) acquisition and the orientation of Ocean Bottom Seismometers (OBS) relative to a neighbouring land station of known orientation. Using land stations, the absolute orientation of OBS can be retrieved. Finally, as a last application, we checked the correct orientation for land stations of a seismological array in Germany.

Description: TOPO-EUROPE addresses the 4-D topographic evolution of the orogens and intra-plate regions of Europe through a multidisciplinary approach linking geology, geophysics, geodesy and geotechnology. TOPO-EUROPE integrates monitoring, imaging, reconstruction and modelling of the interplay between processes controlling continental topography and related natural hazards. Until now, research on neotectonics and related topography development of orogens and intra-plate regions has received little attention. TOPO-EUROPE initiates a number of novel studies on the quantification of rates of vertical motions, related tectonically controlled river evolution and land subsidence in carefully selected natural laboratories in Europe. From orogen through platform to continental margin, these natural laboratories include the Alps/Carpathians–Pannonian Basin System, the West and Central European Platform, the Apennines–Aegean–Anatolian region, the Iberian Peninsula, the Scandinavian Continental Margin, the East-European Platform, and the Caucasus–Levant area. TOPO-EUROPE integrates European research facilities and know-how essential to advance the understanding of the role of topography in Environmental Earth System Dynamics. The principal objective of the network is twofold. Namely, to integrate national research programs into a common European network and, furthermore, to integrate activities among TOPO-EUROPE institutes and participants. Key objectives are to provide an interdisciplinary forum to share knowledge and information in the field of the neotectonic and topographic evolution of Europe, to promote and encourage multidisciplinary research on a truly European scale, to increase mobility of scientists and to train young scientists. This paper provides an overview of the state-of-the-art of continental topography research, and of the challenges to TOPO-EUROPE researchers in the targeted natural laboratories.

Description: We examine a 24-hour period of active San Andreas Fault (SAF) tremor and show that this tremor is largely composed of repeated similar events. Utilizing this similarity, we locate the subset of the tremor with waveforms similar to an identified low frequency earthquake (LFE) “master template,” located using P and S wave arrivals to be ∼26 km deep. To compensate for low signal-to-noise, we estimate event-pair differential times at “clusters” of nearby stations rather than at single stations. We find that the locations form a near-linear structure in map view, striking parallel to the SAF and near the surface trace. Therefore, we suggest that at least a portion of the tremor occurs on the deep extension of the fault, likely reflecting shear slip, similar to subduction zone tremor. If so, the SAF may extend to the base of the crust, ∼10 km below the deepest regular earthquakes on the fault.

Description: While the Dead Sea basin has been studied for a long time, the available knowledge about the detailed seismicity distribution in the area, as well as the deeper structure of the basin, is limited. Therefore, within the framework of the international project DESIRE (DEad Sea Integrated REsearch project), a dense temporary local seismological network was operated in the southern Dead Sea area. We use 530 local earthquakes, having all together 26 730 P- and S-arrival times for a simultaneous inversion of 1-D velocity models, station corrections and precise earthquake locations. Jackknife tests suggest an accuracy of the derived hypocentre locations of about 1 km. Thus, the result is the first clear image of the absolute distribution of the microseismicity of the area, especially in depth. The seismicity is concentrated in the upper crust down to 20 km depth while the lower limit of the seismicity is reached at 31 km depth. The seismic events at the eastern boundary fault (EBF) in the southern part of the study area represent the northward transform motion of the Arabian Plate along the Dead Sea Transform. North of the Boqeq fault the seismic activity represents the transfer of the motion in the pull-apart basin from the eastern to the western boundary. We find that from the surface downward the seismic events are tracing the boundary faults of the basin. The western boundary is mapped down to 12 km depth while the EBF reaches about 17 km depth, forming an asymmetric basin. One fifth of the data set is related to a specific cluster in time and space, which occurred in 2007 February at the western border fault. This cluster is aligned vertically, that is, it is perpendicular to the direction of the dominating left-lateral strike-slip movement at the main transform fault.

Description: The Maule earthquake of 27th February 2010 (Mw=8.8) affected ~500 km of the Nazca-South America plate boundary in south-central Chile producing spectacular crustal deformation. Here, we present a detailed estimate of static coseismic surface offsets as measured by survey and continuous GPS, both in near- and farfield regions. Earthquake slip along the megathrust has been inferred from a joint inversion of our new data together with published GPS, InSAR, and land-level changes data using Green's functions generated by a spherical finite-element model with realistic subduction zone geometry. The combination of the data sets provided a good resolution, indicating that most of the slip was well resolved. Coseismic slip was concentrated north of the epicenter with up to 16 m of slip, whereas to the south it reached over 10m within two minor patches. A comparison of coseismic slip with the slip deficit accumulated since the last great earthquake in 1835 suggests that the 2010 event closed a mature seismic gap. Slip deficit distribution shows an apparent local overshoot that highlight cycle-to-cycle variability,which has to be taken into accountwhen anticipating future events from interseismic observations. Rupture propagation was obviously not affected by bathymetric features of the incoming plate. Instead, splay faults in the upper plate seem to have limited rupture propagation in the updip and along-strike directions. Additionally, we found that along-strike gradients in slip are spatially correlated with geometrical inflections of the megathrust. Our study suggests that persistent tectonic features may control strain accumulation and release along subduction megathrusts.

Description: [1] Long-term monitoring of geoelectromagnetic fields, recorded with a network of nine stations covering an area of approximately 25,000 km2at low geomagnetic latitudes in northern Chile, reveals systematic and seasonally modulated variations of geomagnetic field properties. The observed seasonal variation affects almost exclusively the east-west magnetic field component for periods between 100 and 3000 s. The ground-based measurements of magnetic and electric fields show statistically significant coherences with the interplanetary electric field derived from solar wind and interplanetary magnetic field data of the Advanced Composition Explorer satellite. The interplanetary electric field (IEF) penetrates the polar ionosphere from where it propagates toward equatorial latitudes by waveguide transmission, with ionosphere and solid Earth acting as conducting boundaries. Signal coherence between IEF and ground data peaks at periods of approximately 90 min and up to the four harmonics. Coherence values reach 0.4 at these periods and depend on the electromagnetic field component. They vary with season and local time. Transfer functions computed between IEF and ground-based electric and magnetic fields show local maxima at similar periods (90 min and harmonics). The coupling between the east-west magnetic field component and the IEF shows significant seasonal variability, much larger than the other electromagnetic field components. We conclude that the IEF drives primarily a global circuit of Pedersen currents in the ionosphere. Resulting time-varying magnetic fields induce electric currents in the ground. Related ground-based magnetic (primarily north-south) and electric (primarily east-west) signals vary coherently at all local times and seasons. Conversely, magnetic signals caused by the IEF-driven Hall currents depend much on local time and season. We show for the first time that these ionospheric Hall currents cause no induction in the ground, but they generate magnetic signatures that are confined to the waveguide between ionosphere and Earth's surface.

Description: On 8 January 2006, an intermediate-depth earthquake occurred at the western part of the Hellenic trench close to the island of Kythera (southern Greece). This is the first intermediate-depth earthquake in the broader Aegean area that has produced such an extensive set of useful recordings, as it was recorded by the main permanent seismological networks and numerous acceleration sensors operating in Greece, as well as by EGELADOS, a large-scale temporary amphibian broadband seismological network deployed in the southern Aegean area. An effort to combine all the available data (broadband velocity and acceleration sensor) was made to study the properties of ground-motion attenuation of this earthquake. The combination of both types of data revealed interesting properties of the earthquake wave field, which would remain hidden if only one type of data was used. Moreover, the data have been used for a validation of existing peak ground-motion empirical prediction relations and the preliminary study of the very inhomogeneous attenuation pattern of the southern Aegean intermediate-depth events at both near- and far-source distances

Description: Splay faults are thrusts that emerge from the plate boundaries of subduction zones. Such structures have been mapped at several convergent margins and their activity commonly ascribed to large megathrust earthquakes. However, the behavior of splay faults during the earthquake cycle is poorly constrained because typically these structures are located offshore and are difficult to access. Here we use geologic mapping combined with space and land geodesy, as well as offshore sonar data, to document surface-fault ruptures and coastal uplift at Isla Santa María in south-central Chile (37°S) caused by the 27 February 2010 Maule earthquake (Mw 8.8). During the earthquake, the island was tilted parallel to the margin, and normal faults ruptured the surface and adjacent ocean bottom. We associate tilt and crestal normal faulting with growth of an anticline above a blind reverse fault rooted in the Nazca–South America plate boundary, which slipped during the Maule earthquake. The splay fault system has formed in an area of reduced coseismic plate-boundary slip, suggesting that anelastic deformation in the upper plate may have restrained the 2010 megathrust rupture. Surface fault breaks were accompanied by prominent discharge of fluids. Our field observations support the notion that splay faulting may frequently complement and influence the rupture of subduction-zone earthquakes.

Description: We used data from both permanent and temporary seismic networks on Iceland and Greenland to investigate the crustal thickness by partly reinterpreting earlier data (P receiver functions) and adding S receiver functions to better constrain the results. We obtained good conversions from the Moho and also crustal multiples in both Iceland and Greenland. The central ice covered part of Greenland has an average crustal thickness of 40 km, typical for a craton. At the edges of Greenland the crustal thickness decreases to 30–40 km. On the east coast of Greenland, where the track of the Iceland plume is thought to have affected the lithosphere, the crustal thickness is only 24–26 km. In contrast to previous studies, we find that the crustal thickness in the east and the northwest coastal regions of Iceland is more than 40 km, similar to beneath the active volcanic region. In the southwest region of Iceland and along the mid-ocean ridge, the crustal thickness is only 25 km or less. Also in contrast to earlier receiver function interpretations, which deduced a broad crust-mantle transition zone for Iceland, we find indications for a normal, sharper Moho beneath a number of sites

Description: Teleseismic data recorded during one and a half years are investigated with the receiver function technique to determine the crustal and upper-mantle structures underneath the highly elevated Altiplano and Puna plateaus in the central Andes. A series of converting interfaces are determined along two profiles at 21°S and 25.5°S, respectively, with a station spacing of approximately 10 km. The data provide the highest resolution gained from a passive project in this area, so far. The oceanic Nazca plate is detected down to 120 km beneath the Altiplano whereas beneath the Puna, the slab can unexpectedly be traced down to 200 km depth at longer periods. A shallow crustal low-velocity zone is determined beneath both plateaus exhibiting segmentation. In the case of the Altiplano, the segments present vertical offsets and are separated by inclined interfaces, which coincide with major fault systems at the surface. An average depth to Moho of about 70 km is determined for the Altiplano plateau. A strong negative velocity anomaly located directly below the Moho along with local crustal thinning is interpreted beneath the volcanic arc of the Altiplano plateau between 67°W and 68.5°W. A deep section of the Puna profile reveals thinning of the mantle transition zone. Although poorly resolved, the detected anomaly may suggest the presence of a mantle plume, which may constitute the origin of the anomalous temperatures at the depth of the upper-mantle discontinuities.

Description: Rapid estimation of earthquake rupture propagation is essential to declare an early warning for tsunami-generating earthquakes. An increasing number of seismological methods have been developed to determine rupture parameters, such as length, velocity and propagation direction, especially since the occurrence of the Sumatra–Andaman earthquake that resulted in a devastating tsunami in the Indian Ocean region. Here, we present a new method to follow the rupture process in near real time by a polarization analysis of local and regional P phases that permits a faster determination of rupture properties than using teleseismic records. The new technique has the capability to provide detailed information in less than 10 min. Originally, the method stems from a single-station earthquake location method and is expanded here to monitor P-phase polarization variations through time. As the earthquake source moves away from the hypocentre, the backazimuth of an incoming P phase is expected to change accordingly. With polarization analysis we may be able to monitor the temporal change in Pwave backazimuth to follow the rupture process in near real time. Three component P phases are scanned to determine the azimuthal variation as a function of time. The backazimuth of a moving rupture front is determined by the first eigenvector of the covariance matrix. The linearity of the particle motion is used as a measure of the quality of the data. Seismic stations at local and regional distances (〉∼ 30◦) are used. We tested the new method with a theoretical simulation and observed seismograms of the Sumatra–Andaman earthquake (2004 December 26, Mw = 9.3), and we were able to follow the rupture for the first 200 s. For larger ruptures, stations at more than 30◦ epicentral distances would be required. The method is also successfully applied to the Wenchuan earthquake (2008 May 12, Mw = 8.0).

Description: To study the applicability of the passive seismic interferometry technique to near-surface geological studies, seismic noise recordings from a small scale 2-D array of seismic stations were performed in the test site of Nauen (Germany). Rayleigh wave Green's functions were estimated for different frequencies. A tomographic inversion of the traveltimes estimated for each frequency from the Green's functions is then performed, allowing the laterally varying 3-D surface wave velocity structure below the array to be retrieved at engineering–geotechnical scales. Furthermore, a 2-D S-wave velocity cross-section is obtained by combining 1-D velocity structures derived from the inversion of the dispersion curves extracted at several points along a profile where other geophysical analyses were performed. It is shown that the cross-section from passive seismic interferometry provides a clear image of the local structural heterogeneities that are in excellent agreement with georadar and geoelectrical results. Such findings indicate that the interferometry analysis of seismic noise is potentially of great interest for deriving the shallow 3-D velocity structure in urban areas.

Description: As part of the DEad Sea Integrated REsearch project (DESIRE) a 235 km long seismic wide-angle reflection/refraction (WRR) profile was completed in spring 2006 across the Dead Sea Transform (DST) in the region of the southern Dead Sea basin (DSB). The DST with a total of about 107 km multi-stage left-lateral shear since about 18 Ma ago, accommodates the movement between the Arabian and African plates. It connects the spreading centre in the Red Sea with the Taurus collision zone in Turkey over a length of about 1 100 km. With a sedimentary infill of about 10 km in places, the southern DSB is the largest pull-apart basin along the DST and one of the largest pull-apart basins on Earth. The WRR measurements comprised 11 shots recorded by 200 three-component and 400 one-component instruments spaced 300 m to 1.2 km apart along the whole length of the E–W trending profile. Models of the P-wave velocity structure derived from the WRR data show that the sedimentary infill associated with the formation of the southern DSB is about 8.5 km thick beneath the profile. With around an additional 2 km of older sediments, the depth to the seismic basement beneath the southern DSB is about 11 km below sea level beneath the profile. Seismic refraction data from an earlier experiment suggest that the seismic basement continues to deepen to a maximum depth of about 14 km, about 10 km south of the DESIRE profile. In contrast, the interfaces below about 20 km depth, including the top of the lower crust and the Moho, probably show less than 3 km variation in depth beneath the profile as it crosses the southern DSB. Thus the Dead Sea pull-apart basin may be essentially an upper crustal feature with upper crustal extension associated with the left-lateral motion along the DST. The boundary between the upper and lower crust at about 20 km depth might act as a decoupling zone. Below this boundary the two plates move past each other in what is essentially a shearing motion. Thermo-mechanical modelling of the DSB supports such a scenario. As the DESIRE seismic profile crosses the DST about 100 km north of where the DESERT seismic profile crosses the DST, it has been possible to construct a crustal cross-section of the region before the 107 km left-lateral shear on the DST occurred.

Description: Lithium elemental and isotopic compositions of 33 glass and whole-rock samples from nine oceanic island regions were determined to characterize the Li inventory of the deep mantle. The Li contents of the investigated lavas range from 1·5 to 13·3 μg g−1, whereas δ7Li ranges from 2·4 to 4·8‰. There are weak co-variations between the Li/Y, δ7Li, and Sr–Nd–Pb isotope compositions of the lavas, indicating that the Li elemental and isotopic characteristics of ocean island basalt to some extent reflect mantle source heterogeneity. In detail, HIMU-type lavas are characterized by δ7Li values (up to 4·8‰) slightly heavier than those for average normal mid-ocean ridge basalt (3·4 ± 1·4‰) and by comparatively low Li contents; EM1-type lavas are characterized by isotopically light Li (average 3·2‰) and relative Li enrichment, whereas EM2-type lavas tend to heavier δ7Li values (up to 4·4‰) with high Li concentrations. The Li contents and isotope characteristics of HIMU-type lavas are consistent with recycling of altered and dehydrated oceanic crust, whereas those of the EM1-type lavas can be attributed to sediment recycling. The Li characteristics of EM2-type lavas may reflect reworking of mantle wedge material that has been infiltrated by fluids derived from the subducting plate.

Description: Mt. Merapi is one of the most dangerous volcanoes in Indonesia, located within the tectonically active region of south-central Java. This study investigates how Mt. Merapi affected - and was affected by - nearby tectonic earthquakes. In 2001, a Mw6.3 earthquake occurred in conjunction with an increase in fumarole temperature at Mt. Merapi. In 2006, another Mw6.3 earthquake took place, concomitant with an increase of magma extrusion and pyroclastic flows. Here, we develop theoretical models to study the amount of stress transfer between the earthquakes and the volcano, showing that dynamic, rather than static, stress changes are likely responsible for the temporal and spatial proximity of these events. Our examination of the 2001 and 2006 events implies that volcanic activity at Mt. Merapi is influenced by stress changes related to remote tectonic earthquakes, a finding that is important for volcano hazard assessment in this densely inhabited area.

Description: We report on a rare example of aseismic response of a creeping fault to the earthquake cycle of a nearby megathrust. Interferometric synthetic aperture radar (InSAR) is used to detect and analyze shallow creep of two crustal faults at Mejilones Peninsula, Northern Chile, located in the hanging wall of the 2007 Mw 7.7 Tocopilla subduction earthquake. We generate two independent time series of surface deformation spanning ∼3.5 yr of late interseismic and ∼1.5 yr early postseismic deformation associated with this event. The analysis reveals creep on the Mejillones fault as well as on a previously unmapped fault to the west of the Mejillones fault. The InSAR deformation maps and distributed slip models obtained from the data reveal that fault creep reversed between the interseismic and postseismic periods. Given the regional stress field perturbations due to interseismic and coseismic deformation, we argue that the observed shallow creep and its slip reversal are directly linked to the megathrust seismic cycle. Moreover, from similar eastward dips but opposite slip directions of the two faults, we infer that fault strength must be very low and that the kinematics is controlled by crustal flexure associated with the seismic cycle on the underlying megathrust. ⺠New evidence of crustal fault creep in Northern Chile from InSAR. ⺠New evidence that sense of fault creep is governed by megathrust earthquake cycles. ⺠New evidence that stress induced by megathrust earthquake alters the sense of creep.

Description: The Bohemian Massif is part of the Variscan belt of central Europe. We carried out a high resolution mapping of lithospheric thickness beneath central Europe by investigating 264 teleseismic events recorded at 80 broad band stations in the western Bohemian Massif with the method of S receiver function analysis. A negative phase beneath the Saxothuringian and north-eastern Teplá-Barrandian units at about 9-10 s before the S onset is interpreted as caused by the lithosphere-asthenosphere boundary (LAB) at 80 - 90 km depth. In the Moldanubian unit, the negative phase occurs at 13-15 s before the S onset, corresponding to lithospheric thickness of 120 - 130 km. The boundary between the domains is oriented E-W and probably marks the northern extension of Moldanubian Saxothuringian to the Moldanubian unit. The observed crustal/lithospheric domains could represent two distinct microplates with a relatively sharp boundary cutting through the whole lithosphere.

Description: The lithosphere-asthenosphere boundary corresponds to the base of the “rigid” plates – the depth at which heat transport changes from advection in the convecting deeper upper mantle to conduction in the shallow upper mantle. Although this boundary is a fundamental feature of the Earth, mapping it has been difficult because it does not correspond to a sharp change in temperature or composition. Various definitions of the lithosphere and asthenosphere are based on the analysis of different types of geophysical and geological observations. The depth to the lithosphere-asthenosphere boundary determined from these different observations often shows little agreement when they are applied to the same region because the geophysical and geological observations (i.e., seismic velocity, strain rate, electrical resistivity, chemical depletion, etc.) are proxies for the change in rheological properties rather than a direct measure of the rheological properties. In this paper, we focus on the seismic mapping of the upper mantle high velocity lid and low velocity zone and its relationship to the lithosphere and asthenosphere. We have two goals: (a) to examine the differences in how teleseismic body-wave travel-time tomography and surface-wave tomography image upper mantle seismic structure; and (b) to summarise how upper mantle seismic velocity structure can be related to the structure of the lithosphere and asthenosphere. Surface-wave tomography provides reasonably good depth resolution, especially when higher modes are included in the analysis, but lateral resolution is limited by the horizontal wavelength of the long-period surface waves used to constrain upper mantle velocity structure. Teleseismic body-wave tomography has poor depth resolution in the upper mantle, particularly when no strong lateral contrasts are present. If station terms are used, features with large lateral extent and gradual boundaries are attenuated in the tomographic image. Body-wave models are not useful in mapping the thickness of the high velocity upper mantle lid because this type of analysis often determines wave speed perturbations from an unknown horizontal average and not absolute velocities. Thus, any feature which extends laterally across the whole region beneath a seismic network becomes invisible in the Teleseismic body-wave tomographic image. We compare surface-wave and body-wave tomographic results using southern Africa as an example. Surface-wave tomographic images for southern Africa show a strong, high velocity upper mantle lid confined to depths shallower than ~200 km, whereas body-wave tomographic images show weak high velocity in the upper mantle extending to depths of ~300 km or more. However, synthetic tests show that these results are not contradictory. The absolute seismic velocity structure of the upper mantle provided by surface wave analysis can be used to map the thermal lithosphere. Priestley and McKenzie (Priestley, K., McKenzie, D., 2006. The thermal structure of the lithosphere from shear wave velocities. Earth and Planetary Science Letters 244, 285–301.) derive an empirical relationship between shear wave velocity and temperature. This relationship is used to obtain temperature profiles from the surfacewave tomographic models of the continental mantle. The base of the lithosphere is shown by a change in the gradient of the temperature profiles indicative of the depth where the mode of heat transport changes from conduction to advection. Comparisons of the geotherms determined from the conversion of surface-wave wave speeds to temperatures with upper mantle nodule-derived geotherms demonstrate that estimates of lithospheric thickness from Vs and from the nodule mineralogy agree to within about 25 km. The Lithospheric thickness map for Africa derived from the surface-wave tomographic results shows that thick lithosphere underlies most of the Archean crust in Africa. The distribution of diamondiferous kimberlites provides an independent estimate of where thick lithosphere exists. Diamondiferous kimberlites generally occur where the lower part of the thermal lithosphere as indicated by seismology is in the diamond stability field.

Description: The combination of the Sunda megathrust and the (strike-slip) Sumatran Fault (SF) represents a type example of slip-partitioning. However, superimposed on the SF are geometrical irregularities that disrupt the local strain field. The largest such feature is in central Sumatra where the SF splits into two fault strands up to 35 km apart. A dense local network was installed along a 350 km section around this bifurcation, registering 1016 crustal events between April 2008 and February 2009. 528 of these events, with magnitudes between 1.1 and 6.0, were located using the double-difference relative location method. These relative hypocentre locations reveal several new features about the crustal structure of the SF. Northwest and southeast of the bifurcation, where the SF has only one fault strand, seismicity is strongly focused below the surface trace, indicating a vertical fault that is seismogenic to ~15 km depth. By contrast intense seismicity is observed within the bifurcation, displaying streaks in plan and cross-section that indicate a complex system of faults bisecting the bifurcation. In combination with analysis of topography and focal mechanisms, we propose that the bifurcation is a strike-slip duplex system with complex faulting between the two main fault branches.

Description: In 2007 a M7.7 earthquake occurred near the town of Tocopilla within the northern Chile seismic gap. Mainshock slip, derived from coseismic surface deformation, was confined to the depth range between 30-55 km. We relocated ~1100 events during six months before and one week after the mainshock. Aftershock seismicity is first congruent to the mainshock slip and then it spreads offshore west and northwest of Mejillones Peninsula (MP). Waveform modeling for 38 aftershocks reveals source mechanisms that are in the majority similar to the mainshock. However, a few events appear to occur in the upper plate, some with extensional mechanisms. Juxtaposing the Tocopilla aftershocks with those following the neighboring 1995 Antofagasta earthquake produces a striking symmetry across an EW axis in the center of MP. Events seem to skirt around MP, probably due to a shallower Moho there. We suggest that the seismogenic coupling zone in northern Chile changes its frictional behavior in the down-dip direction from unstable to mostly conditionally stable. For both earthquake sequences, aftershocks agglomerate in the conditionally stable region, whereas maximum inter-seismic slip deficit and co-seismic slip occurs in the unstable region. The boundary between the unstable and conditionally stable zones parallels the coastline. We identify a similar segmentation for other earthquakes in Chile and Peru, where the offshore segments break in great M〉8 earthquakes, and the onshore segments in smaller M〈8 earthquakes. Using critical taper analysis, we demonstrate a causal relationship between varying slip behavior on the interface and forearc wedge anatomy that can be attributed to spatial variations in the rate-dependency of friction.