Seafloor Displacement Research Articles

Resolving depth-dependent subduction zone viscosity and afterslip from postseismic displacements following the 2011 Tohoku-oki, Japan earthquake

We developed a 3-D, viscoelastic finite element model of the M9 2011 Tohoku-oki, Japan earthquake capable of predicting postseismic displacements due to viscoelastic relaxation and afterslip. We consider seismically inferred slab geometries associated with the Pacific and Philippine Sea Plate and a wide range of candidate viscoelastic rheologies. For each case, we invert for afterslip based on residual surface displacements (observed GPS minus that predicted due to viscoelastic relaxation) to develop combined viscoelastic relaxation and afterslip models. We are able to find a mechanical model that fully explains all observed geodetic on-land and seafloor horizontal and vertical postseismic displacements. We find that postseismic displacements are in about equal parts due to viscoelastic relaxation and afterslip, but their patterns are spatially distinct. Accurately predicting both horizontal and vertical on-land postseismic displacements requires a mantle wedge viscosity structure that is depth dependent, reflecting the manner in which temperature, pressure, and water content influence viscosity. No lateral heterogeneities within the mantle wedge viscosity structure beneath northern Honshu are required. Westward-directed postseismic seafloor displacements may be due flow via low-temperature, plastic creep within the lower half of a Pacific lithosphere weakened by plate bending. The distribution of afterslip is controlled by the location of coseismic slip from the Tohoku-oki and other regional historic earthquakes. The paradigm by which afterslip is thought of as the dominant postseismic mechanism immediately following earthquakes, with viscoelastic relaxation to follow in later years, is shown to no longer be valid.

Probabilistic imaging of tsunamigenic seafloor deformation during the 2011 Tohoku‐oki Earthquake

AbstractDiverse observations from the 2011 Mw 9.0 Tohoku‐oki earthquake pointed to large coseismic fault slip proximal to the Japan Trench. This seismic failure prompted a reevaluation of the conventional view that the outer forearc is generally aseismic. However, the nature of near‐trench fault slip during this event remains debated, without consensus on whether slip peaked at the trench or at greater depths. Here we develop a probabilistic approach to image the spatiotemporal evolution of coseismic seafloor displacement from near‐field tsunami observations. In a Bayesian framework, we sample ensembles of nonlinear source models parameterized to focus on near‐trench features, incorporating the uncertainty in modeling dispersive tsunami waves in addition to nominal observational errors. Our models indicate that seafloor in the region of the earthquake was broadly uplifted and tilted seaward approaching the deep‐ocean trench. Over length scales of ~40 km, seafloor uplift peaks at 5 ± 0.6 m near the inner‐outer forearc transition and decreases to 2 m at the trench axis over a distance of 50 km, corresponding to a seafloor tilt of 0.06 ± 0.02 m/km. Over length scales of ~20 km, peak uplift reaches 7 ± 2 m at the similar location, but uplift at the trench is less constrained. Elastic modeling that reproduces the observed tilt requires a coseismic slip deficit at the trench. Such a deficit is effectively consistent with a metastable frictional model for the shallowest megathrust. While large shallow earthquakes in the region cannot be completely ruled out, aseismic deformation is the most likely mode for satisfying the long‐term slip budget.

First direct observation of coseismic slip and seafloor rupture along a submarine normal fault and implications for fault slip history

Properly assessing the extent and magnitude of fault ruptures associated with large earthquakes is critical for understanding fault behavior and associated hazard. Submarine faults can trigger tsunamis, whose characteristics are defined by the geometry of seafloor displacement, studied primarily through indirect observations (e.g., seismic event parameters, seismic profiles, shipboard bathymetry, coring) rather than direct ones. Using deep-sea vehicles, we identify for the first time a marker of coseismic slip on a submarine fault plane along the Roseau Fault (Lesser Antilles), and measure its vertical displacement of ∼0.9 m in situ. We also map recent fissuring and faulting of sediments on the hangingwall, along ∼3 km of rupture in close proximity to the fault's base, and document the reactivation of erosion and sedimentation within and downslope of the scarp. These deformation structures were caused by the 2004 Mw 6.3 Les Saintes earthquake, which triggered a subsequent tsunami. Their characterization informs estimates of earthquake recurrence on this fault and provides new constraints on the geometry of fault rupture, which is both shorter and displays locally larger coseismic displacements than available model predictions that lack field constraints. This methodology of detailed field observations coupled with near-bottom geophysical surveying can be readily applied to numerous submarine fault systems, and should prove useful in evaluating seismic and tsunamigenic hazard in all geodynamic contexts.

Coral 13C/12C records of vertical seafloor displacement during megathrust earthquakes west of Sumatra

The recent surge of megathrust earthquakes and tsunami disasters has highlighted the need for a comprehensive understanding of earthquake cycles along convergent plate boundaries. Space geodesy has been used to document recent crustal deformation patterns with unprecedented precision, however the production of long paleogeodetic records of vertical seafloor motion is still a major challenge. Here we show that carbon isotope ratios (δC13) in the skeletons of massive Porites corals from west Sumatra record abrupt changes in light exposure resulting from coseismic seafloor displacements. Validation of the method is based on the coral δC13 response to uplift (and subsidence) produced by the March 2005 Mw 8.6 Nias–Simeulue earthquake, and uplift further south around Sipora Island during a M∼8.4 megathrust earthquake in February 1797. At Nias, the average step-change in coral δC13 was 0.6±0.1‰/m for coseismic displacements of +1.8 m and −0.4 m in 2005. At Sipora, a distinct change in Porites microatoll growth morphology marks coseismic uplift of 0.7 m in 1797. In this shallow water setting, with a steep light attenuation gradient, the step-change in microatoll δC13 is 2.3‰/m, nearly four times greater than for the Nias Porites. Considering the natural variability in coral skeletal δC13, we show that the lower detection limit of the method is around 0.2 m of vertical seafloor motion. Analysis of vertical displacement for well-documented earthquakes suggests this sensitivity equates to shallow events exceeding Mw∼7.2 in central megathrust and back-arc thrust fault settings. Our findings indicate that the coral C13/C12 paleogeodesy technique could be applied to convergent tectonic margins throughout the tropical western Pacific and eastern Indian oceans, which host prolific coral reefs, and some of the world's greatest earthquake catastrophes. While our focus here is the link between coral δC13, light exposure and coseismic crustal deformation, the same principles could be used to characterize interseismic strain during earthquake cycles over the last several millennia.

Dynamic models of an earthquake and tsunami offshore Ventura, California

AbstractThe Ventura basin in Southern California includes coastal dip‐slip faults that can likely produce earthquakes of magnitude 7 or greater and significant local tsunamis. We construct a 3‐D dynamic rupture model of an earthquake on the Pitas Point and Lower Red Mountain faults to model low‐frequency ground motion and the resulting tsunami, with a goal of elucidating the seismic and tsunami hazard in this area. Our model results in an average stress drop of 6 MPa, an average fault slip of 7.4 m, and a moment magnitude of 7.7, consistent with regional paleoseismic data. Our corresponding tsunami model uses final seafloor displacement from the rupture model as initial conditions to compute local propagation and inundation, resulting in large peak tsunami amplitudes northward and eastward due to site and path effects. Modeled inundation in the Ventura area is significantly greater than that indicated by state of California's current reference inundation line.

A new concept for an ocean bottom pressure meter capable of precision long‐term monitoring in marine geodesy and oceanography

AbstractLong‐term vertical seafloor displacements and geostrophic changes in the water column height could be easily monitored if pressure meters were less susceptible to drift. Currently, these signals, which have typical amplitudes from decimeters to less than 1 mm/yr, cannot be differentiated from instrumental drift. In this paper, we introduce and outline a new constructional concept for an ocean bottom pressure meter that aims for unequivocal detection and monitoring of long‐term trends. The concept is based on a differential pressure sensor that measures the pressure difference between the environment and a reference pressure within a sealed volume. This sealed volume conserves the instantaneous pressure at the moment of its closure at the monitoring location in a temperature‐compensated manner. Furthermore, the approach enables easy in situ calibration of the differential pressure gauge by simply opening the reference pressure chamber to the environment and checking the zero point offset.

Tsunami Generation Above a Sill

The generation of surface waves by seafloor displacement is a classic problem that arises in the study of tsunamis. The generation of waves in a two-dimensional domain of uniform depth by uplift or subsidence of a portion of a flat bottom boundary has been elegantly studied by Hammack (Tsunamis: a model of their generation and propagation, Ph.D. thesis, California Institute of Technology, 1972), for idealized motions. The physical problem of tsunami generation is more complex; even when the final displacement is known from seismic analysis, the deforming seafloor includes relief features such as mounts and trenches. Here, following Kajiura (J Oceanogr Soc Jpn 28:260–277, 1972), we investigate analytically the effect of bathymetry on the surface wave generation, by solving the forced linear shallow water equation. While Kajiura’s geometry consisted of a step-type bottom bathymetry with a rectangular uplift to understand the effect of the continental shelf on tsunami generation, our model bathymetry consists of an uplifting cylindrical sill initially resting on a flat bottom, a geometry which helps evaluate the effect of seamounts on tsunami generation. We find that as the sill height increases, partial wave trapping reduces the wave height in the far field, while amplifying it above the sill.

Evaluation of spatial resolution and estimation error of seafloor displacement observation from vessel-based bathymetric survey by use of AUV-based bathymetric data

A repeated bathymetric survey reveals seafloor displacement between before and after geodynamic events. We evaluated the less-known spatial resolution and estimation error of the seafloor displacement observation from a vessel-based multi-narrow beam bathymetric survey. In this evaluation, bathymetric data from vessel-based and near-seafloor high-resolution autonomous underwater vehicle (AUV)-based surveys in the same area were used. Simulated vessel-based bathymetric “before and after” data of the seafloor displacement were made using AUV-based bathymetric data. The displacement was verified by comparing these simulated data using the analysis conditions that no locational errors of beam sounding points exist, a footprint effect is uniform, depth accuracy is constant in the analysis area, and there are no depth offset between two data. As a result, we found that the smallest vertical seafloor displacement that can be detected occurs when the horizontal extent of the deformation is larger than several times the size of the footprint (area of the narrow sounding beam projected onto the seafloor) of the used vessel’s multi-narrow beam echo sounder, and in the situation that the amplitude of the depth difference is greater than the accuracy of the vessel-based depth measurement (standard deviation of measuring error). When local slopes of the bathymetry are gentler than those of the artificial variation appeared in the depth differences between two data, the horizontal seafloor displacement seems to be difficult to resolve accurately. The local slope of the artificial depth variation is derived from the wavelength and the amplitude which are equivalent to ~1–3 times of the footprint size and the accuracy of the depth measurement, respectively.

Tsunami source and its validation of the 2014 Iquique, Chile, earthquake

The slip distribution of the 1 April 2014 Iquique earthquake is obtained by using the least squares inversion of tsunami data at three Deep-Ocean Assessment and Reporting of Tsunamis stations. Most of the slip is concentrated along a 60 km by 40 km slip patch near the hypocenter, with magnitude ranging from 5 to 7 m and a depth of 23 km. The earthquake magnitude from the inversion is estimated as Mw 8.0. The slip distribution is converted into seafloor displacement based on Okada's formula. A nonlinear shallow water equation model is used to simulate tsunami wave propagation, and the simulated water surface elevations are compared with the measured data at 10 tide gauges along the Chilean coast. The agreement is excellent at gauges where the local bathymetry data are complete and the gauges are open to the ocean; otherwise, mismatches of up to 10 min in arrival time and 1.0 m in amplitude are seen.

The seismic cycle in the area of the 2011 Mw9.0 Tohoku‐Oki earthquake

AbstractWe model seismic and aseismic slip on the Japan megathrust in the area of the Mw9.0, 2011 Tohoku‐Oki earthquake based on daily time series from 400 GPS stations of the GEONET network and campaign measurements of six sea floor displacements. The coseismic and postseismic slip distributions are inverted simultaneously using principal component analysis‐based inversion method (PCAIM). Exploring a wide range of boundary conditions and regularization constraints, we found the coseismic slip distribution to be quite compact with a peak slip between 30 and 50 m near the trench. Our model shows deep afterslip fringing the downdip edge of the coseismic rupture but also a dominant zone of shallow afterslip. Afterslip over the first 279 days following the main shock represents about 40% of the coseismic moment. We compare the coseismic and postseismic models with an interseismic coupling model derived from inland and sea bottom measurements determined in a self‐consistent manner. Assuming that seismic and aseismic slip had to match the long‐term slip rate along the megathrust, the recurrence time of Mw9.0 earthquakes is estimated to 100–300 years, while historical and paleotsunami records suggest a return period more of the order of 1000 years. The discrepancy is smaller if the shallower portion of the megathrust is assumed to produce both aseismic slip, as the afterslip model suggests, and seismic slip during occasional large tsunamigenic earthquakes.

A first GPS measurement of vertical seafloor displacement in the Campi Flegrei caldera (Italy)

This study shows how the GPS technique can be utilized for seafloor displacement measurements and improved the survey control infrastructure in Campi Flegrei caldera, two thirds of which is submerged under the sea. In the Gulf of Pozzuoli, about 2.5km from the coast where the sea depth is 97m, a continuous GPS station (CFB1) has been installed since the end of 2011 on the top of a elastic-beacon buoy, rigidly connected by a steel cable to the ballast on the sea bottom. We investigate the use of GPS data to estimate the vertical displacement of the seafloor under the buoy. The GPS data were processed in kinematic mode and the vertical component of the measurements was corrected for the errors due to the horizontal motion of the buoy induced by wind and sea currents. We report here the results for approximately 17months of continuous GPS data acquisition, and we show, for the first time, a measure of vertical displacement of the seabed in the Gulf of Pozzuoli. From January 2012 to May 2013, the seafloor uplifted by about 3–4cm. The similarity of the pattern of the CFB1 time-series compared to the permanent GPS stations of the NeVoCGPS network located onshore is remarkable, evaluation of the Pearson's correlation coefficient between these stations and CFB1 indicates that the stations are measuring the same phenomenon. This result is important, because all models of evolution of bradyseism in the Campi Flegrei caldera are based on the interpretation of measures only on the emerged part of the caldera, without use of any measures to date in the Gulf of Pozzuoli.The methodology shown in this paper is reliable over time and economical, compared to other systems of measurement of marine geodesy. The major limitation is the depth of the sea, confining this technique to the shallow water, up to 100m depth. However, a large part of the submerged Campi Flegrei caldera is shallower than 100m, so geodetic monitoring by means of GPS buoys at several sites in the Gulf of Pozzuoli would allow to extend interpretative models to the entire caldera, submerged and emerged.

Rupture to the Trench: Dynamic Rupture Simulations of the 11 March 2011 Tohoku Earthquake

Research Article| May 01, 2013 Rupture to the Trench: Dynamic Rupture Simulations of the 11 March 2011 Tohoku Earthquake Jeremy E. Kozdon; Jeremy E. Kozdon Department of Geophysics, Stanford University, 397 Panama Mall, Stanford, California 94305jekozdon@nps.edu *Now at Department of Applied Mathematics, Naval Postgraduate School, 833 Dyer Road, Monterey, California 93943. Search for other works by this author on: GSW Google Scholar Eric M. Dunham Eric M. Dunham Department of Geophysics and Institute for Computational and Mathematical Engineering, Stanford University, 397 Panama Mall, Stanford, California 94305edunham@stanford.edu Search for other works by this author on: GSW Google Scholar Bulletin of the Seismological Society of America (2013) 103 (2B): 1275–1289. https://doi.org/10.1785/0120120136 Article history first online: 14 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jeremy E. Kozdon, Eric M. Dunham; Rupture to the Trench: Dynamic Rupture Simulations of the 11 March 2011 Tohoku Earthquake. Bulletin of the Seismological Society of America 2013;; 103 (2B): 1275–1289. doi: https://doi.org/10.1785/0120120136 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of the Seismological Society of America Search Advanced Search Abstract There is strong evidence that the 11 March 2011 Tohoku earthquake rupture reached the seafloor. This is surprising because the shallow portion of the plate interface in subduction zones is thought to be frictionally stable, leading to the widely held view that coseismic rupture would stop several tens of kilometers downdip of the seafloor. Various explanations have been proposed to reconcile this seeming inconsistency, including dynamic weakening (e.g., thermal pressurization) and extreme stress release around shallow subducted seamounts. We offer a simpler explanation supported by 2D dynamic rupture simulations of the Tohoku earthquake. Our models account for depth‐dependent material properties and the complex geometry of the fault, seafloor, and material interfaces, based on seismic surveys of the Japan Trench. The fault obeys rate‐and‐state friction with standard logarithmic dependence of shear strength on slip velocity in steady state. In our preferred model, the uppermost section of the fault is velocity strengthening. Rupture nucleates on a deeper, velocity‐weakening section. Waves released by deep slip reflect off the seafloor, transmitting large stress changes to the upper section of the fault driving the rupture through the velocity‐strengthening region to the trench. We validate the model against seafloor deformation and 1‐Hz Global Positioning System (GPS) data. The seafloor displacements constrain the seismogenic depth and overall amount of slip, particularly near the trench. Our simulations reproduce many features in the GPS data, thereby providing insight into the rupture process and seismic wave field. Sensitivity to parameters is explored through an extensive suite of simulations. Neither static seafloor deformation nor onshore 1‐HzGPS data can uniquely determine near‐trench frictional properties due to trade‐offs with average stress drop. While conducted specifically for the Japan Trench region, our simulations suggest that rupture to the trench in megathrust events is quite possible, even if velocity‐strengthening properties extend tens of kilometers landward from the trench.Online Material: Mp4 movies of particle velocities. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Evaluation of Seismic Rupture Models for the 2011 Tohoku-Oki Earthquake Using Tsunami Simulation

Developing a realistic, three-dimensional rupture model of the large offshore earthquake is difficult to accomplish directly through band-limited ground-motion observations. A potential indirect method is using a tsunami simulation to verify the rupture model in reverse because the initial conditions of the associated tsunamis are caused by a coseismic seafloor displacement correlating to the rupture pattern along the main faulting. In this study, five well-developed rupture models for the 2011 Tohoku-Oki earthquake were adopted to evaluate differences in simulated tsunamis and various rupture asperities. The leading wave of the simulated tsunamis triggered by the seafloor displacement in Yamazaki et al. (2011) model resulted in the smallest root-mean-squared difference (~0.082 m on average) from the records of the eight DART (Deep-ocean Assessment and Reporting of Tsunamis) stations. This indicates that the main seismic rupture during the 2011 Tohoku earthquake should occur in a large shallow slip in a narrow range adjacent to the Japan trench. This study also quantified the influences of ocean stratification and tides which are normally overlooked in tsunami simulations. The discrepancy between the simulations with and without stratification was less than 5% of the first peak wave height at the eight DART stations. The simulations, run with and without the presence of tides, resulted in a ~1% discrepancy in the height of the leading wave. Because simulations accounting for tides and stratification are time-consuming and their influences are negligible, particularly in the first tsunami wave, the two factors can be ignored in a tsunami prediction for practical purposes.

Importance of horizontal seafloor motion on tsunami height for the 2011 Mw=9.0 Tohoku-Oki earthquake

It is now clear that the 2011 Tohoku-Oki earthquake ruptured the subduction interface all the way to the Japan Trench. However, there is significant disagreement about just how much slip occurred at the trench, with most geodetic studies locating only a small fraction of the maximum slip there, whereas broadband seismic studies put the majority of the slip near the trench. Measurements of seafloor displacement near the trench also imply more slip there than is estimated by the geodetic studies. Here, by means of a joint inversion of displacement measurements and seafloor pressure data, we show that it is possible to reconcile geodetic and seismic studies and that a considerable amount of slip indeed occurred at the trench, with slip magnitudes reaching 57–74% of the maximum slip. The seafloor displacement predicted by our model agrees very well with independent measurements made close to the trench. We also find good agreement between our tsunami model and independent measurements of tsunami height in the ocean and on land. Re-running of the inversion without the near-field pressure gauge data, however, leads to underprediction of the flooding on land, even when the seafloor geodetic data are still included. Thus, even if the seafloor geodetic measurements had been available in real time, they would still not have allowed reliable prediction of near-field tsunami inundation.Close to the trench, the dip of the plate interface is shallow, which serves to decrease the vertical motion of the sea floor for any given slip. However, the horizontal motion is conversely larger. Because the ocean floor slopes steeply near the trench, it acted like a giant wedge, converting the horizontal motion of the ground into an uplift of the water column and causing the peak initial tsunami wave height to be twice as high as it would have been from vertical displacement alone.

Comparison of the seafloor displacement from uniform and non-uniform slip models on tsunami simulation of the 2011 Tohoku–Oki earthquake

The numerical simulations of recent tsunami caused by 11 March 2011 off-shore Pacific coast of Tohoku–Oki earthquake (Mw 9.0) using diverse co-seismic source models have been performed. Co-seismic source models proposed by various observational agencies and scholars are further used to elucidate the effects of uniform and non-uniform slip models on tsunami generation and propagation stages. Non-linear shallow water equations are solved with a finite difference scheme, using a computational grid with different cell sizes over GEBCO30 bathymetry data. Overall results obtained and reported by various tsunami simulation models are compared together with the available real-time kinematic global positioning system (RTK-GPS) buoys, cabled deep ocean-bottom pressure gauges (OBPG), and Deep-ocean Assessment and Reporting of Tsunami (DART) buoys. The purpose of this study is to provide a brief overview of major differences between point-source and finite-fault methodologies on generation and simulation of tsunamis. Tests of the assumptions of uniform and non-uniform slip models designate that the average uniform slip models may be used for the tsunami simulations off-shore, and far from the source region. Nevertheless, the heterogeneities of the slip distribution within the fault plane are substantial for the wave amplitude in the near field which should be investigated further.

Tsunami Early Warning Within Five Minutes

Tsunamis are most destructive at near to regional distances, arriving within 20–30 min after a causative earthquake; effective early warning at these distances requires notification within 15 min or less. The size and impact of a tsunami also depend on sea floor displacement, which is related to the length, L, width, W, mean slip, D, and depth, z, of the earthquake rupture. Currently, the primary seismic discriminant for tsunami potential is the centroid-moment tensor magnitude, M w CMT , representing the product LWD and estimated via an indirect inversion procedure. However, the obtained M w CMT and the implied LWD value vary with rupture depth, earth model, and other factors, and are only available 20–30 min or more after an earthquake. The use of more direct discriminants for tsunami potential could avoid these problems and aid in effective early warning, especially for near to regional distances. Previously, we presented a direct procedure for rapid assessment of earthquake tsunami potential using two, simple measurements on P-wave seismograms—the predominant period on velocity records, T d , and the likelihood, T 50 Ex , that the high-frequency, apparent rupture-duration, T 0, exceeds 50–55 s. We have shown that T d and T 0 are related to the critical rupture parameters L, W, D, and z, and that either of the period–duration products T d T 0 or T d T 50 Ex gives more information on tsunami impact and size than M w CMT , M wp, and other currently used discriminants. These results imply that tsunami potential is not directly related to the product LWD from the “seismic” faulting model, as is assumed with the use of the M w CMT discriminant. Instead, information on rupture length, L, and depth, z, as provided by T d T 0 or T d T 50 Ex , can constrain well the tsunami potential of an earthquake. We introduce here special treatment of the signal around the S arrival at close stations, a modified, real-time, M wpd(RT) magnitude, and other procedures to enable early estimation of event parameters and tsunami discriminants. We show that with real-time data currently available in most regions of tsunami hazard, event locations, m b and M wp magnitudes, and the direct, period–duration discriminant, T d T 50 Ex can be determined within 5 min after an earthquake occurs, and T 0, T d T 0, and M wpd(RT) within approximately 10 min. This processing is implemented and running continuously in real-time within the Early-est earthquake monitor at INGV-Rome ( http://early-est.rm.ingv.it ). We also show that the difference m b − log10(T d T 0) forms a rapid discriminant for slow, tsunami earthquakes. The rapid availability of these measurements can aid in faster and more reliable tsunami early warning for near to regional distances.

Dynamic rupture of the 2011 Mw 9.0 Tohoku‐Oki earthquake: Roles of a possible subducting seamount

Using a hybrid MPI/OpenMP parallel finite element method for spontaneous rupture and seismic wave propagation simulations, we investigate features in rupture propagation, slip distribution, seismic radiation, and seafloor deformation of the 2011 Mw 9.0 Tohoku‐Oki earthquake to gain physical insights into the event. With simplified shallow dipping (10°) planar fault geometry, 1D velocity structure, and a slip‐weakening friction law, we primarily investigate initial stress and strength conditions that can produce rupture and seismic radiation characteristics of the event revealed by kinematic inversions, and seafloor displacements observed near the epicenter. By a large suite of numerical experiments aided by parallel computing on modern supercomputers, we find that a seamount of a dimension of ∼70 km by 23 km just updip of the hypocenter on the subducting plane, parameterized by higher static friction, lower pore fluid pressure, and higher initial stress than surrounding regions, may play a dominant role in the 2011 event. Its high strength stalls updip rupture for tens of seconds, and its high stress drop generates large slip. Its failure drives the rupture to propagate into the shallow portion that is likely velocity‐strengthening, resulting in significant slip near the trench within a limited area. However, the preferred model suggests that the largest slip in the event occurs near the hypocenter. High‐strength patches along the downdip portion of the subducting plane are most effective among several possible factors in generating high‐frequency seismic radiations, suggesting the initial strength distribution there is very heterogeneous.

Clues from joint inversion of tsunami and geodetic data of the 2011 Tohoku-oki earthquake

The 2011 Tohoku-oki (Mw 9.1) earthquake is so far the best-observed megathrust rupture, which allowed the collection of unprecedented offshore data. The joint inversion of tsunami waveforms (DART buoys, bottom pressure sensors, coastal wave gauges, and GPS-buoys) and static geodetic data (onshore GPS, seafloor displacements obtained by a GPS/acoustic combination technique), allows us to retrieve the slip distribution on a non-planar fault. We show that the inclusion of near-source data is necessary to image the details of slip pattern (maximum slip ~48 m, up to ~35 m close to the Japan trench), which generated the large and shallow seafloor coseismic deformations and the devastating inundation of the Japanese coast. We investigate the relation between the spatial distribution of previously inferred interseismic coupling and coseismic slip and we highlight the importance of seafloor geodetic measurements to constrain the interseismic coupling, which is one of the key-elements for long-term earthquake and tsunami hazard assessment.

Evidence for active strike-slip faulting along the Eurasia-Africa convergence zone: Implications for seismic hazard in the southwest Iberian margin

New seismic imaging and seismotectonic data from the southwest Iberian margin, the site of the present-day boundary between the European and African plates, reveal that active strike slip is occurring along two prominent lineaments that have recently been mapped using multibeam bathymetry. Multichannel seismic and subbottom profiler images acquired across the lineaments show seafloor displacements and active faulting to depths of at least 10 km and of a minimum length of 150 km. Seismic moment tensors show predominantly WNW–ESE right-lateral strike-slip motion, i.e., oblique to the direction of plate convergence. Estimates of earthquake source depths close to the fault planes indicate upper mantle (i.e., depths of 40–60 km) seismogenesis, implying the presence of old, thick, and brittle lithosphere. The estimated fault seismic parameters indicate that the faults are capable of generating great magnitude (Mw ≥ 8.0) earthquakes. Such large events raise the concomitant possibility of slope failures that have the potential to trigger tsunamis. Consequently, our findings identify an unreported earthquake and tsunami hazard for the Iberian and north African coastal areas.

Disturbance of deep-sea environments induced by the M9.0 Tohoku Earthquake

The impacts of the M9.0 Tohoku Earthquake on deep-sea environment were investigated 36 and 98 days after the event. The light transmission anomaly in the deep-sea water after 36 days became atypically greater (∼35%) and more extensive (thickness ∼1500 m) near the trench axis owing to the turbulent diffusion of fresh seafloor sediment, coordinated with potential seafloor displacement. In addition to the chemical influx associated with sediment diffusion, an influx of 13C-enriched methane from the deep sub-seafloor reservoirs was estimated. This isotopically unusual methane influx was possibly triggered by the earthquake and its aftershocks that subsequently induced changes in the sub-seafloor hydrogeologic structures. The whole prokaryotic biomass and the development of specific phylotypes in the deep-sea microbial communities could rise and fall at 36 and 98 days, respectively, after the event. We may capture the snap shots of post-earthquake disturbance in deep-sea chemistry and microbial community responses.