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The Southern Patagonian Icefield (SPI) lies above an area of slow convergence between Antarctic and South-America plates, where limited seismicity is recorded by global and regional seismic networks. To understand the seismic behavior of this zone, we analyze two years of continuous broad-band data recorded by 27 seismometers, deployed around the SPI. Substantial ice loss coupled with the unusually low viscosity of the underlying mantle is causing a rapid uplift. Our findings indicate that most of the seismicity occurs in the upper crust , likely associated with the (re)activation of regional compressive structures. However, earthquakes immediately beneath the SPI generally are shallower and show normal or strike-slip faulting. We suggest that this activity is promoted as a response to the crustal relaxation after rapid ice removal of SPI. The almost complete absence of interplate and intraslab events is consistent with a locked megathrust fault interface, highlighting the similarity of this region with the Cascadia subduction zone. • Shallow seismicity is observed in the majority in the area of southern Patagonia Icefield. • The inferred stress suggests the solid earth response to mass loss from the icefield as the major cause. • The almost complete absence of interplate earthquakes suggests that the megathrust may be locked. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Seismogenic characteristics near the NE rifted margin of the South China Sea are revealed. • NW-SE tensional stress controls the Taiwan collision-Manila subduction transition. • Intraslab pre-existing faults affect the seismogenic behavior of a convergent system. Abstract Based on the distribution of an earthquake swarm determined from an ocean bottom seismometer (OBS) network deployed from 20 August to 5 September 2015 in the northernmost part of the South China Sea (SCS) and data from inland seismic stations in Taiwan, we resolved a M L 4.1 earthquake occurring on 1 September 2015 as a NE-SW-trending left-lateral strike-slip event that ruptured along the pre-existing normal faults generated during the SCS opening phases. The direction of the T-axes derived from the M L 4.1 earthquake and the background seismicity off SW Taiwan present high consistency, indicating a stable dominant NW-SE tensional stress for the subducted Eurasian Plate (EUP). The distinct stress variations on the two sides of these reactivated NE-SW trending features suggest that the presence of pre-existing normal faults and other related processes may lessen the lateral resistance between the Taiwan collision and Manila subduction system, and facilitate the slab-pull process for the subducting portion, which may explain the NW-SE tensional stress environment near the transition boundary in the northernmost part of the SCS. [ABSTRACT FROM AUTHOR]

Mass-balancing voltages from four buried broadband seismometers connected to the NEPTUNE Canada seafloor cable are being recorded at 24-bit resolution. Sites are located on the Vancouver Island continental shelf, the nearby Cascadia accretionary prism, the eastern flank of the Juan de Fuca Ridge, and the western flank close to the Juan de Fuca Ridge axis. Tidal variations are present throughout the records. Variations in vertical acceleration at three of the sites match predicted gravitational attraction variations very well; those at the fourth site show a small residual that is probably caused by sensitivity to tilt resulting from sensor inclination. Horizontal accelerations, which at tidal periods are sensitive primarily to tilt, are anomalously large relative to standard-earth model results. After removal of predicted tidal body and ocean attraction and loading terms, the residuals are seen to follow ocean pressure variations. Responses range from 0.4 μrad dbar −1 (0.04 μrad kPa −1 ) at 10° true (down under positive load) at the continental shelf site, to 0.6 μrad dbar −1 at 243° at the Cascadia prism, 0.4 μrad dbar −1 at 90° at the eastern Juan de Fuca Ridge flank, and 0.2 μrad dbar −1 at 116° true on the western ridge flank. Except at the continental shelf site, tilts are roughly perpendicular to structural strike. The tilt observations can be explained by loading-induced deformation in the presence of local lithologic gradients or by the influence of faults or structurally controlled anisotropic elastic properties. The observations highlight the utility of using mass position data from force-feedback broad-band seismometers for geodynamic studies. [ABSTRACT FROM AUTHOR]

Located at the arc-continental collision region between the Eurasian (EP) and Philippine Sea Plates (PSP), Taiwan is usually considered to have a complex tectonic environment, particularly along the eastern coast of the island. To gain a better understanding of the geological evolution of the east Taiwan area, the data from 8 Ocean Bottom Seismometers (OBS) acquired during the Across Taiwan Strait Explosion Experiment in 2012 and 14 inland seismic stations were used to determine a more detailed and accurate distribution of marine earthquakes. Based on the 333 relocated earthquakes and available geophysical data, we suggest two main tectonic boundaries for eastern Taiwan. South of 23.25°N, the homogeneous distribution of earthquakes in the crustal portion for both the inland and offshore areas suggests an ongoing collisional process. North of this location, between approximately 23.25°N and 23.8°N, the abrupt increasing of seismicity depth infers that the underthrusted arc/fore-arc material is deforming due to the collisional compression at depth. In this segment, the subsidence of the arc/fore-arc area determines the transition from collision to subduction. North of 23.8°N, the northwestern dipping PSP is well illustrated by the seismicity both onshore and offshore, indicating a dominant subduction process. [ABSTRACT FROM AUTHOR]

The Tonga-Lau-Fiji region is important to study plate-plume and subduction-ridge interactions, but its deep mantle structure is still not very clear. Here we present high-resolution tomography of 3-D P-wave azimuthal anisotropy down to 400 km depth of the Tonga subduction zone derived from arrival-time data of local earthquakes recorded at seafloor and land seismometers. The subducting Tong slab is imaged as high-velocity anomalies at depths of 100-400 km, whereas large-scale low-velocity anomalies down to 400 km depth are revealed in the mantle wedge beneath the backarc basin and volcanic arc. Trench-parallel anisotropy beneath the Lau Basin extends southwards to ∼140 km depth at ∼20.5°S, representing the extent of both southward flow of the Samoan plume and toroidal flow by the slab rollback. At depths of 140-400 km, the Lau Basin and Fiji Plateau mainly exhibit plate-parallel fast-velocity directions (FVDs) north of ∼20.5°S, indicating strong corner flow in the mantle wedge driven by the slab subduction and dehydration. The Tonga slab exhibits trench-parallel FVDs at depths of <200 km, reflecting fossil fabric formed during the plate spreading stage, whereas, at greater depths, the slab mainly exhibits trench-normal FVDs, which may reflect complicated deformations within the slab. These results suggest that the Samoan plume has a significant impact on the Tonga-Lau-Fiji region, leading to variations in the scale and depth extent of mantle flows. • 3-D Vp azimuthal anisotropy of the Tonga subduction zone is investigated. • Trench-parallel anisotropy occurs down to ∼140 km depth under the Lau Basin. • The Samoan plume significantly affects corner flow in the mantle wedge. [ABSTRACT FROM AUTHOR]

Couplings between large magnitude subduction zone earthquakes and the subsequent response of their respective volcanic arcs are generally accepted, but the mechanisms driving this coupling are not known. The 2010 Maule earthquake (Mw8.8) ruptured approximately 500 km along strike in the south-central part of Chile, and provides an opportunity to investigate earthquake-volcano interactions. In an exploratory campaign, we deployed four broadband seismometers atop and surrounding the Nevados de Chillán volcanic complex because it is located directly behind one of the two primary slip patches of the Maule Earthquake. The data recorded (from December 2011 to April 2012) shows significant seismic activity, characterized by numerous volcano tectonic events and tremor episodes occurring within the volcanic complex. We recorded two strong aftershocks of the Maule earthquake (Mw 6.1 in January 2012, and Mw 7.1 in April 2012) and investigated the response of the volcano to the incoming seismic energy. We find that volcanic tremor increased significantly upon arrival of the seismic waves from the Mw 6.1 event, which was then followed a few hours later by a significant increase in volcano-tectonic events when tremors subsided. This delay between tremor and volcano-tectonic events suggests that fluid and/or magma migration (manifested as tremor) readjusted the local stress state that then induced the volcano-tectonic events. The increased activity persisted during the subsequent two weeks. In contrast, the Mw 7.1 event, which occurred at a similar epicentral distance from the volcano, did not produce any significant seismic response of the volcanic complex. Analysis of the particle velocity records of the two events shows that the volcanic system was perturbed in different ways because of the incidence angle of the incoming energy, inducing a back-elliptical vertical and fault parallel motion for the Mw 6.1 and no clear directional dependence for the Mw 7.1. This suggests dilatation-induced fluid migration within the complex, and a kinematic mechanism of the perturbation rather than the perturbation amplitude. Our results demonstrate the importance for continued monitoring of the arc behind Maule, with an increased seismometer and GPS array density to determine the style of deformation currently occurring in the arc. [ABSTRACT FROM AUTHOR]

Abstract: This work focuses on the analysis of a unique set of seismological data recorded by two temporary networks of seismometers deployed onshore and offshore in the Central Lesser Antilles Island Arc from Martinique to Guadeloupe islands. During the whole recording period, extending from January to the end of August 2007, more than 1300 local seismic events were detected in this area. A subset of 769 earthquakes was located precisely by using HypoEllipse. We also computed focal mechanisms using P-wave polarities of the best azimuthally constrained earthquakes. We detected earthquakes beneath the Caribbean forearc and in the Atlantic oceanic plate as well. At depth seismicity delineates the Wadati–Benioff Zone down to 170km depth. The main seismic activity is concentrated in the lower crust and in the mantle wedge, close to the island arc beneath an inner forearc domain in comparison to an outer forearc domain where little seismicity is observed. We propose that the difference of the seismicity beneath the inner and the outer forearc is related to a difference of crustal structure between the inner forearc interpreted as a dense, thick and rigid crustal block and the lighter and more flexible outer forearc. Seismicity is enhanced beneath the inner forearc because it likely increases the vertical stress applied to the subducting plate. [Copyright &y& Elsevier]

Abstract: Detecting unconventional seismic signals related to subduction zone processes at depth in continuous ocean bottom seismometer (OBS) records requires the analysis and identification of noise due to instrumental problems, deployment sites or sea state conditions. The temporary OBS deployment at the Lesser Antilles subduction zone provides new insights into the feasibility of detecting unconventional signals such as non volcanic tremor (NVT), long-period (LP) or ultra-long period (ULP) events. Analysis of noise at an array comprising several sites and types of instruments and comparison with recordings on land shows transients in the noise. Episodes can be identified considering the diversity of sites and instrument types and comparing the seismic signals with meteorological and oceanographic data. In order to reliably detect NVT (1–10Hz) originating from inside the solid Earth, one must first characterize noise induced by the activity of the atmosphere and hydrosphere at the sea-bottom as well as on land. The semidiurnal modulation of noise amplitude can be shown here not to be due to that of the NVT from a seismic source at depth which is related to the subduction interplate and whose activity is modulated by the tidal stresses as inferred for other megathrusts on emerged forearcs. Here, the semidiurnal modulation is rather due to the effect of the tides themselves, such as tidal currents, since they do not affect all types and all components of the unique multi-station array of OBS that could be deployed on this submerged forearc. The short period cut-off of the strong noise due to ocean surface infragravity waves increases to longer periods with OBS depth, thereby increasing the observational window with low noise to lower frequencies, and deep OBS sites may be advantageous for detecting LP events. [Copyright &y& Elsevier]

The Mw 7.0 Constitución earthquake of March 2012 is one of the largest interplate aftershocks of the Maule 2010 Mw 8.8 mega-thrust earthquake. This event was recorded by high-rate GPS stations, local seismometers and accelerometers, the Global Seismographic Network and SAR acquisitions by the ENVISAT satellite. We have used these data to perform a kinematic inversion and back projection to identify the principal characteristics of this event. The Constitución earthquake nucleated at 39 km depth and then propagated up-dip at subshear speed towards its centroid, with an unusually long initiation phase that lasted almost 6 s. The largest slip of this event was located in the deeper part of the subduction interface, between the region of maximum co-seismic slip of the 2010 Maule earthquake, and the area where rapid afterslip occurred following that event. Features of the Constitución earthquake may suggest that larger interplate aftershocks of the Maule event preferentially occur in the deeper part of the plate interface where ruptures are complex, produce high frequencies and involve numerous asperities. [ABSTRACT FROM AUTHOR]

Abstract: New marine seismic reflection and coincident wide-angle ocean-bottom seismometer data acquired offshore Taiwan provide high-resolution constraints on the crustal structure of an incipient mountain belt during the earliest stage of arc–continent collision. The new seismic reflection image and travel-time tomography velocity model show evidence for crust of the distal southern Chinese continental margin being subducted eastward beneath the Manila trench and underplated to the accretionary wedge before collision with the southern Chinese continental shelf. The distal margin crust consists of highly extended continental crust interspersed with volcanic bodies and a high-velocity lower crustal layer of likely magmatic intrusions. The distal margin crust is 10–14km thick outboard of the trench, but thins to 6km thick beneath the lower slope of the Manila trench accretionary wedge. Along the lower slope of the accretionary prism, we image westward-verging imbricate thrusts and folded strata up to 10km thick. A sharp decrease in bathymetry marks the transition from lower to upper slope, where we observe a fast (>6.0km/s) seismic velocity anomaly at the base of the wedge that we interpret as structurally underplated crust from the distal continental margin. Our results support a model of arc–continent collision in Taiwan where the accretionary wedge is first thickened by structural underplating of distal margin crust prior to collision with the continental shelf. The crustal rocks exposed throughout the Central Range in Taiwan may be similarly derived from subducted and structurally underplated crust from the highly extended distal continental margin. [Copyright &y& Elsevier]

Abstract: Kueishantao is a volcanic island located offshore the northeastern coast of Taiwan and lies at the tip of the southern Okinawa Trough which is the back-arc basin of the Ryukyu subduction zone. Its last eruption occurred during the Holocene (~7ka), hence Kueishantao can be considered as an active volcano. In an effort to better understand how magmatic processes may interact with the regional tectonics, a seismic network was installed in the area during early January 2008. This network consisted of 16 three-component seismometers located both on Kueishantao and the coast of northeastern Taiwan. One year of data was analyzed yielding 425 earthquakes whose P and S arrival times were manually picked and each event was located using a nonlinear probabilistic location method. In order to improve the location accuracy, the minimum 1-D velocity model for this dataset was derived and all earthquakes were relocated using this model. The results show a tight cluster of events near Kueishantao while the remaining earthquakes are scattered between the island and mainland Taiwan. The majority of hypocentral depths range between 2.5 and 10km where the former depth coincides with the bottom of the shallow sedimentary layer and the latter with the ductile lower crust. Waveforms of the three largest events were also inverted for the determination of their deviatoric and full moment tensor. No statistically significant isotropic component was found, while two of the events can be explained by a double-couple source. The third event exhibited a low frequency content (<10Hz) and a large non-double-couple component suggesting fluid involvement at its source. A stress inversion of all available focal mechanisms in the area shows that fluid circulation in the upper crust generates a local stress field around Kueishantao facilitating the opening of cracks along the NW–SE direction of regional extension. [Copyright &y& Elsevier]

Abstract: The southernmost part of the Ryukyu subduction, where the Philippine Sea Plate is subducting under the Eurasian Plate, is known to be a very seismically active region of transition from a north-dipping subduction along the Ryukyu subduction to an ~SE–NW collision along the Taiwanese orogenic wedge. In this paper, we will focus on the Ryukyu forearc area close to Taiwan where the deformation is paroxysmal. In order to decipher the nature of the seismic deformation in this region, a three month passive experiment, combining 22 Ocean Bottom Seismometers and 51 onland stations, has been led. Starting from an a-priori heterogeneous model, we have obtained 801 well-located earthquake hypocenters, a precise P-wave tomography model and 14 focal mechanisms. The seismicity along the Ryukyu forearc is mainly located not only in the vicinity of the Interplate Seismogenic Zone (ISZ) but also within both the subducting PSP and the overriding plate. Seismicity within the upper-plate is essentially localized east of Nanao basin where NW–SE extension occurs, and northwest of the Hoping basin where strike-slip dominates. As revealed by both the P-wave velocity structure and the newly derived seismicity, we argue that a sub-vertical step offsetting the subducting PSP around 10km may support the presence of a trench-parallel tear. The PSP also undergoes extension in its upper part that is probably caused by buckling and slab pull. The P-wave velocity structure reveals three other major features: (1) a continuity between the Central Range and the Ryukyu Arc with a shallower Moho (~30km depth) between ~122.3°N and ~122.5°N along the Ryukyu Arc, (2) high P-wave velocities along the eastern side of the Central Range and, (3) two bodies with similar high crustal velocities (6.5–7.0km/s) at 12–18km depths, embedded within the Ryukyu arc basement, just north of Hoping Basin and north of the Nanao Basin. [Copyright &y& Elsevier]

Abstract: Intraslab events recorded by ocean-bottom seismometers in the Okinawa trough provide an extended depiction of shear wave splitting in the southwest section of the Ryukyu subduction zone. At 100–200km from the western edge of the subduction system, we observed trench-normal fast polarization direction in the back arc compatible with 2D slab- or rifting-driven corner flow. Towards the edge, the fast directions are sub-parallel to the trench in the arc—back arc region, and rotate to trench-normal within 50km of the edge. Splitting constrained by land stations with paths mostly in the mantle wedge exhibits similar trench-normal fast directions in the subduction edge zone. Further inland, the dominant component of fast directions becomes roughly parallel to the Taiwan orogenic fabric. Splitting of P-to-S phases converted at the Moho of the Okinawa trough and of S phases from shallow events suggest that crustal anisotropy may affect the measured splitting, but the observed pattern reflects predominantly mantle anisotropy. The variation in splitting along the Okinawa trough cannot be explained by a B-type–A-type olivine fabric transition in the mantle wedge. It may indicate the presence of an along-arc flow in the mantle wedge towards the edge where it is blocked and deflected by the Eurasian lithosphere. This scenario bolsters previous studies suggesting a significant impact of Eurasian lithosphere on the dynamics of the Ryukyu subduction system. [Copyright &y& Elsevier]

Abstract: This paper investigates velocity structure of the active plate boundary in southern Taiwan by joint analysis of gravity anomaly and seismic arrival time data. P and S–P arrival time data from 3238 earthquakes were used. In addition to CWBSN permanent networks, seismic data include the Central Weather Bureau permanent networks and a temporary network consisting of 11 ocean bottom seismometers (OBSs) that was deployed to detect the aftershocks of the 2006 (ML7.1) Henchun earthquake occurred beneath southern Taiwan. The total available OBS data set consists of ~700 detected earthquakes, from which around 500 could be well located where about 450 events have been used in simultaneous inversion for hypocenters, three-dimensional Vp and Vp/Vs models for the study area. The main objective of incorporating gravity analysis is used to improve the velocity model for the offshore area, where it is poorly sampled by local earthquakes. This study found a low-velocity zone existing above the subducting South China Sea slab in the mantle wedge. Based on gravity modeling and our resulting velocity and Poisson''s ratio models suggest that the subduction complex, which is characterized with a low P-wave velocity and low Poisson''s ratios beneath the southern Taiwan. This duplex structure is characterized by a zone of low P-wave velocities in the range of 6.2–6.8km/s between 25 and 40km depth. It also shows that earthquake hypocenters do not fall within this low velocity zone. We have also used seismic tomography velocities to estimate the volume percentage of serpentinite and silica concentrations in southern Taiwan. The calculated serpentinite is about 30% and the volume percentage of quartz estimated is about 20% at the base of the forearc lower crust. [Copyright &y& Elsevier]

Abstract: The Sumatran margin suffered three great earthquakes in recent years (Aceh-Andaman 26 December 2004 Mw=9.1, Nias 28 March 2005 Mw=8.7, Bengkulu 12 September 2007 Mw=8.5). Here we present local earthquake data from a dense, amphibious local seismic network covering a segment of the Sumatran margin that last ruptured in 1797. The occurrence of forearc islands along this part of the Sumatran margin allows the deployment of seismic land-stations above the shallow part of the thrust fault. In combination with ocean bottom seismometers this station geometry provides high quality hypocentre location for the updip end of the seismogenic zone in an area where geodetic data are also available. In this region, the Investigator Fracture Zone (IFZ), which consists of 4 sub-ridges, is subducted below the Sunda plate. This topography appears to influence seismicity at all depth intervals. A well-defined linear streak of seismicity extending from 80 to 200 km depth lies along the prolongation of closely spaced IFZ sub-ridges. More intermediate depth seismicity is located to the southeast of this string of seismicity and is related to subducted rough oceanic seafloor. The plate interface beneath Siberut Island which ruptured last in 1797 is characterised by almost complete absence of seismicity. [ABSTRACT FROM AUTHOR]

Abstract: Interplate coupling in the southernmost extent of the northeast Japan subduction zone (east off-Kanto, Japan) is investigated in detail using a large data set of small repeating earthquakes. In this region of the trench-trench-trench type triple junction, the subducting Pacific plate is in contact with two different overlying plates (the North American and Philippine Sea plates). The border between the two overlying plates extending northwestward from the triple junction is identified along the upper surface of the Pacific plate based on the slip vectors of many interplate events including small repeating earthquakes. The interplate coupling coefficient estimated from the cumulative slip of small repeating earthquakes for the period from 1993 to 2007 reveals a distinct change from south (ca. 0.3) to north (ca. 0.7) across the border. This border also corresponds to the southern limit of the source areas of M>7 interplate earthquakes in the last 80 years along the Japan trench, again indicating the stronger coupling to the north. We also investigate hypocenter distribution and seismic velocity structure along the plate boundary from a large number of travel-time data obtained from the nationwide seismograph network. The results reveal a distinct low-velocity zone just above the Pacific plate and low seismicity along the plate boundary in the region overlain by the Philippine Sea plate, whereas there are no low-velocity zones and the seismicity is high in the region overlain by the North American plate. These observations imply that the geological difference of overlying plate controls large-scale coupling at the plate interface. [Copyright &y& Elsevier]

Abstract: Large earthquakes have repeatedly occurred in the area off southeastern Hokkaido Island, Japan, as the Pacific Plate subducts beneath the island, which is on the North American Plate. The most recent large earthquake in this area, the 2003 Tokachi-oki earthquake (Mw=8.0), occurred on September 26, 2003. In order to investigate aftershock activity in the rupture area, 47 Ocean Bottom Seismometers (OBSs) were quickly deployed after the main shock. In the present study, we simultaneously estimate the hypocenters and 3-D seismic velocity models from the P- and S-wave arrivals of the aftershocks recorded by OBSs. The subducting plate is clearly imaged as a northwest dipping zone in which Vp is greater than 7 km/s, and the relocated hypocenters also show the subducting Pacific Plate. The aftershock distribution reveals that the dip angle of the plate boundary increases abruptly around 90 km from the Kuril Trench. The bending of the subducting plate corresponds to the southeastern edge of the rupture area. The island arc crust on the overriding plate has P-wave velocities of 6–7 km/s and a Vp/Vs of 1.73. A region of Vp/Vs greater than 1.88 was found north of the epicenter of the main shock. The depth of the high Vp/Vs region extends about 10 km upward from the plate interface. The plate boundary just below the high Vp/Vs region has the largest slip at the main rupture. A high Vp anomaly (~7.5 km/s) is found in the island arc crust in northeast part of the study area, which we interpret as a structural boundary related to the arc–arc collisional tectonics of the Hokkaido region, as the rupture of the main shock terminated at this high Vp region. We suggest that the plate interface geometry and the trench-parallel velocity heterogeneity in the landward plate are principal factors in controlling the rupture area of the main shock. [Copyright &y& Elsevier]

Abstract: We have determined the three-dimensional shear wave speed structure of the upper mantle in and around the Philippine Sea region using seismograms recorded by dense land-based and long-term broadband ocean bottom seismographic stations. We used a surface wave tomography technique in which multimode phase speeds are measured and inverted for a 3-D shear wave speed structure by incorporating the effects of a finite frequency and ray bending. The new ocean bottom data provided us with improved spatial resolution (~300 km) in the Philippine Sea region. In the upper 120 km, the shear wave speed structure is well correlated with seafloor age. At depths greater than 160 km, fast anomalies of the subducting Pacific Plate are clearly defined. We also found slow speed anomalies beneath the Kyushu–Palau ridge at depths greater than 120 km. Along the Izu–Bonin(Ogasawara)–Mariana arc, we have detected three separate slow anomalies in the mantle wedge at depths shallower than 100 km beneath the back arc. Each anomaly has a width of ~500 km. Moreover, these three anomalies have a close relationship with the three groups of frontal and rear arc volcanoes having distinct Sr, Nd, and Pb isotope ratios. We suggest that each of the anomalies is a site of large-scale flow of upper mantle into the mantle wedge, and that each already contains a component from the adjacent subducting slab. [Copyright &y& Elsevier]

Along the south-western offshore Hellenic subduction zone, the overriding Aegean upper plate above the Mediterranean oceanic lithosphere generates uncommon large earthquakes on the offshore megathrust fault. The largest subduction thrust event, for half a century, has been the 14 February 2008 Methoni earthquake (Mw = 6.8) that occurred offshore of the southwest coast of Peloponnesus. We conducted micro-seismicity experiments around the rupture area and forearc domain -between Peloponnesus and Crete- using ocean bottom seismometers (OBS) jointly with land-based seismological stations. Our first experiment in 2006, had revealed an association of the Matapan Trough, a 400-km-long forearc basin, with local seismicity clustering and a possible gap in activity over the later Methoni rupture area. Here we present new data of post-Methoni seismic activity, recorded during a time-span of 11 months, beginning in October 2008 within the period of proposed afterslip on the megathrust, by an extended and dense seismic array consisting of up to 33 OBS. A minimum 1D velocity model was constructed for the region to provide better constraints on absolute locations and double-difference relocation was applied to produce an enhanced image of the spatial distribution of hypocenters. The high resolution earthquake locations confirm correlation of the Matapan Trough with local seismicity as a regional feature, also filling up the previously observed gap. Over the Methoni rupture area, we constrain seismicity to be located mainly within the upper plate. Hypocenters are also resolved above the updip and downdip edges of the rupture area, respectively. Seismic activity provides hints of upper plate structures which were activated in response to post-seismic deformation spreading within the forearc crust. Our findings highlight the characteristics of a megathrust domain which is related with a highly deformable overriding plate and controlled by a segmented lower plate topography. • Spatio-temporal distribution of the Mw 6.8 Methoni aftershocks by OBS • Aftershocks driven by afterslip • Post-seismic deformation spread within the forearc crust • Bending-related normal faulting of the subducting crust [ABSTRACT FROM AUTHOR]

• Detected and located 116 earthquakes in 40 days, 4× the amount in the regional catalog. • Imaged the Central Oregon forearc down to 40 km depth using local earthquake tomography. • 2D Vp and Vp/Vs models show a compositional difference between the upper and lower Siletzia Terrane. • The western lower crust has a low Vp zone representing the presence of slab derived fluids. • The eastern lower crust has a high Vp zone showing a strong, unfractured Siletzia Terrane. The crustal structure of the overriding plate in a subduction zone can have significant effects on subduction processes and therefore hazard along the margin. To investigate the controls of the overriding North American crust on the Cascadia Subduction Zone, a dense, trench perpendicular line of nodal seismometers was deployed in the Central Oregon forearc. The location of this deployment was chosen to fulfill two main goals (1) to examine the utility of large-N, short duration, nodal deployments in passive source seismic imaging (2) and to gain a better understanding of the structure of the Central Oregon forearc. With ∼40 days of continuous data recorded on 174, 3-component, 5 Hz instruments in a 130 km line with 500 m station spacing, 116 earthquakes and explosions are detected and located, a ∼3.5-fold increase from the number reported by the regional Pacific Northwest Seismic Network (PNSN) catalog. P, S, and PmP travel times from earthquakes located near the line of stations are used in a local earthquake tomography study to model Vp/Vs in the upper/middle crust and Vp throughout the crust of central Oregon down to 40 km depth. In conjunction, these models show a clear compositional discontinuity between the upper and lower Siletzia Terrane at ∼10 km depth as well as a zone of low Vp (6.5–6.8 km/s) in the lower crust that is attributed to slab derived fluids. This region of low Vp is juxtaposed with high Vp (6.9–7.2 km/s) to the east, interpreted as a strong, unfractured, block of Siletzia. High pore fluid pressures in the lower crust have been imaged to varying degrees across the Cascadia subduction zone. These features may be related to both the occurrence of tremor near the plate interface and the reduction of upper plate crustal earthquakes. [ABSTRACT FROM AUTHOR]

On April 16th 2016 a Mw 7.8 earthquake ruptured the central coastal segment of the Ecuadorian subduction zone. Shortly after the earthquake, the Instituto Geofisico de la Escuela Politecnica Nacional of Ecuador, together with several international institutions deployed a dense, temporary seismic network to accurately categorize the post-seismic aftershock sequence. Instrumentation included short-period and broadband sensors, along with Ocean Bottom Seismometers. This deployment complemented the permanent Ecuadorian seismic network and recorded the developing aftershock sequence for a period of one year following the main-shock. A subset of 345 events with M L > 3.5, were manually picked in the period of May to August 2016, providing highly accurate P- and S-onset times. From this catalogue, a high-quality dataset of 227 events, with an azimuthal gap <200°, are simultaneously inverted for, obtaining the minimum 1D velocity model for the rupture region, along with hypocentral locations and station corrections. We observe an average Vp/Vs of 1.82 throughout the study region, with relatively higher Vp/Vs values of 1.95 and 2.18 observed for the shallowest layers down to 7.5 km. The high relative Vp/Vs ratio (1.93) of the deeper section, between 30 km and 40 km, is attributed to dehydration and serpentinization processes. For the relocated seismicity distribution, clusters of events align perpendicular to the trench, and crustal seismicity is also evidenced, along with earthquakes located close to the trench axis. We also compute Regional Moment Tensors to analyze the different sources of seismicity after the mainshock. Aside from thrust events related to the subduction process, normal and strike-slip mechanisms are detected. We suggest that the presence of subducting seamounts coming from the Carnegie Ridge act as erosional agents, helping to create a scenario which promotes locking and allows seismicity to extend up to the trench, along zones of weakness activated after large earthquakes. Unlabelled Image • 1D velocity model, station corrections and hypocentral locations were obtained based on the aftershocks of the 2016 Pedernales earthquake. • Our minimum 1D velocity model allows us to observe and discuss depth ranges and to first order the velocity structure with depth. • Station correction terms are consistent with areas affected by site effects during the earthquake. • A portion of the relocated seismicity occurs very close to the trench (~5 – 10 km). • Regional moment tensors suggest the presence of a strike-slip fault in the marine forearc. [ABSTRACT FROM AUTHOR]