Great Subduction Earthquakes Research Articles

How Long Can the Postseismic and Interseismic Phases of Great Subduction Earthquake Sustain? Toward an Integrated Earthquake‐Cycle Perspective

AbstractThe transition of subduction systems from postseismic to interseismic phase is an important indicator of their stress state and readiness for subsequent events. However, understanding the dynamics of this transition of megathrust earthquakes is hindered by the short instrumental time compared to the centuries‐long recurrence interval (T). Here, with an updated earthquake‐cycle perspective of global spatiotemporally limited geodetic observations, we use generic viscoelastic earthquake‐cycle models to explore the evolution of time‐varying surface deformation. Self‐consistently, our models reproduce key observed features of the deformation cycle, suggesting common processes activated at different margins. The relative postseismic and interseismic durations are found dependent mainly nonlinearly on the ratio of T to mantle viscosity, with a plausible range of ∼0.2–0.4T at representative margins and minimum >0.55T, respectively. Our results thus provide a useful tool for understanding the similarities and differences of earthquake‐cycle deformation and guiding monitoring of possible future signals at world‐wide subduction zones.

Finding Simplicity in the Complexity of Postseismic Coastal Uplift and Subsidence Following Great Subduction Earthquakes

AbstractFollowing great subduction earthquakes, postseismic deformation of coastal areas shows consistent seaward motion but complex vertical deformation. Understanding both the horizontal and vertical components in the same geodynamic framework presents challenges. Here, by modeling short‐term (a few years) postseismic viscoelastic relaxation (VER) and afterslip following synthetic and real subduction earthquakes, we demonstrate that the complexity of the vertical deformation can be explained in simple terms. Along a margin‐normal profile, VER results in an up‐down‐up trisegment, long‐wavelength pattern common to most megathrust earthquakes, including near‐trench uplift, midway subsidence, and near‐arc uplift, with locations controlled by coseismic fault slip. The magnitude of the first two segments is controlled mainly by oceanic mantle viscosity, and the third by mantle wedge viscosity. In contrast with VER, afterslip results in an up‐down bimodal pattern of variable wavelengths specific to individual earthquakes. Its site‐specific and heterogeneous nature is primarily responsible for the complexity in vertical deformation, but its effect can be adequately modeled using a simple elastic model. If the coast is near the megathrust rupture zone, variable combinations of the VER and afterslip effects lead to either uplift or subsidence. If the coast is in the near‐arc segment of VER deformation, uplift usually occurs. Modeling the common VER process enables the identification of site‐specific afterslip, which helps to understand the mechanism of afterslip in the context of the broad spectrum of fault slip behavior. Our results also have important implications to deciphering coastal paleoseismic records to constrain coseismic versus postseismic deformation of ancient earthquakes.

The 1877 megathrust earthquake of North Chile two times smaller than thought? A review of ancient articles

The 1877 North Chile megathrust earthquake is commonly considered as one of the largest historical earthquakes that occurred on the Chilean subduction zone. The literature generally attributes it a magnitude close to 9, associated with a rupture of about 500 km long. Because it occurred nearly a century and a half ago, the area is often described as a mature seismic gap, that is to say, a place where a great subduction earthquake similar to that of 1877 would be imminent. A careful study of historical articles describing the earthquake and subsequent tsunami shows that the size of the rupture has most likely been greatly overestimated. First of all, it seems that the area of severe shaking, corresponding to intensity VIII on the Mercalli scale, has been artificially increased. Secondly, it appears that once errors and exaggerations are corrected, nowhere was the subsequent tsunami actually higher than 10 m. It also appears that the initial exaggerations have been further amplified in the modern literature to reach today’s estimation of the earthquake size. In reality, all historical observations, i.e. the intensities of destruction on the continent due to the earthquake itself and the characteristics of the subsequent tsunami, concur to attribute to the earthquake a rupture of only ∼225 km long, located between 20.5°S and 22.5°S. Both this reduced length and its localization match very well the patchy coupling revealed by recent GPS measurement in the area and support the hypothesis of a rupture of the Loa segment alone. The estimation of an earthquake magnitude from the subsequent tsunami is always difficult because the amplitude of the tsunami at a given place depends on many more parameters than the seismic moment alone. The source localization which changes the azimuth and path towards a given receiving place, the source depth, the rupture velocity, all play a role. They remain largely unknown for the 1877 event. However, the comparison of the 1877 revised figures with recent tsunamis suggest the magnitude should also be lowered to around 8.5 : slightly larger than Illapel 2015, significantly smaller than Maule 2010. In all cases, the seismic hazard of the region should be revised downwards: the next expected earthquake in the region should be either significantly smaller than feared (magnitude ∼8.5 rather than ∼9), or occur much later than announced.

Seismic and Aseismic Cycle of the Ecuador–Colombia Subduction Zone

The Colombia–Ecuador subduction zone is an exceptional natural laboratory to study the seismic cycle associated with large and great subduction earthquakes. Since the great 1906 Mw = 8.6 Colombia–Ecuador earthquake, four large Mw > 7.5 megathrust earthquakes occurred within the 1906 rupture area, releasing altogether a cumulative seismic moment of ∼35% of the 1906 seismic moment. We take advantage of newly released seismic catalogs and global positioning system (GPS) data at the scale of the Colombia–Ecuador subduction zone to balance the moment deficit that is building up on the megathrust interface during the interseismic period with the seismic and aseismic moments released by transient slip episodes. Assuming a steady-state interseismic loading, we found that the seismic moment released by the 2016 Mw = 7.8 Pedernales earthquake is about half of the moment deficit buildup since 1942, suggesting that the Pedernales segment was mature to host that seismic event and its postseismic afterslip. In the aftermath of the 2016 event, the asperities that broke in 1958 and 1979 both appears to be mature to host a large Mw > 7.5 earthquakes if they break in two individual seismic events, or an Mw∼7.8–8.0 earthquake if they break simultaneously. The analysis of our interseismic-coupling map suggests that the great 1906 Colombia–Ecuador earthquake could have ruptured a segment of 400 km-long bounded by two 80 km wide creeping segments that coincide with the entrance into the subduction of the Carnegie ridge in Ecuador and the Yaquina Graben in Colombia. These creeping segments share similar frictional properties and may both behave as strong seismic barriers able to stop ruptures associated with great events like in 1906. Smaller creeping segments are imaged within the 1906 rupture area and are located at the extremities of the large 1942, 1958, 1979, and 2016 seismic ruptures. Finally, assuming that the frequency–magnitude distribution of megathrust seismicity follows the Gutenberg–Richter law and considering that 50% of the transient slip on the megathrust is aseismic, we found that the maximum magnitude subduction earthquake that can affect this subduction zone has a moment magnitude equivalent to Mw ∼8.8 with a recurrence time of 1,400 years. No similar magnitude event has yet been observed in that region.

Bayesian Inversion for a Stress‐Driven Model of Afterslip and Viscoelastic Relaxation: Method and Application to Postseismic Deformation Following the 2011 MW 9.0 Tohoku‐Oki Earthquake

AbstractPostseismic deformation following great subduction earthquakes is commonly modeled as the sum of the contributions from afterslip on the megathrust and viscoelastic relaxation of the coseismic stress changes in the mantle. However, it is generally difficult to separate the contributions of these mechanisms to geodetically observed postseismic deformation. Here we develop an inversion method for estimating the parameters of a stress‐driven postseismic deformation model that incorporates velocity‐strengthening afterslip and viscoelastic mantle relaxation governed by a biviscous Burgers rheology using coseismic and postseismic geodetic data. We assume that afterslip and viscoelastic relaxation are driven by coseismic stress changes and interact mechanically with each other. The unknown parameters to be estimated include the coseismic slip distribution that drives the postseismic processes, and the fault frictional and mantle viscosity parameters. We present a Bayesian formulation of the inverse problem and an algorithm for estimating the posterior probability density function. This approach allows us to quantify the uncertainties of the model parameters. We apply the method to the coseismic displacements and postseismic displacement time series of the 2011 Tohoku‐Oki earthquake. Our results show that the observations place tight constraints on the model parameters, with the estimated model effectively reproducing the first‐order spatial and temporal patterns of horizontal and vertical postseismic deformation. We find that the relative contributions of afterslip and viscoelastic relaxation to horizontal and vertical postseismic deformation vary in space. We suggest that our method may potentially provide a way to objectively separate the contributions of these two mechanisms to postseismic deformation.

Three-dimensional local earthquake tomography of pre-Cenozoic structures in the coastal margin of central Chile: Pichilemu fault system

Following the Mw 8.8 Maule megathrust earthquake that occurred on February 27, 2010, in central Chile, a sequence of normal faulting crustal earthquakes occurred close to the city of Pichilemu. This activity lasted several months and included two large events (Mw = 6.9 and Mw = 7.0) on March 11, 2010. An initial investigation of this activity analyzed a data set of about 630 earthquakes recorded locally by 8 short-period seismic stations and attributed this seismicity to the activation of the “Pichilemu fault system.” While there is no visible surface faulting associated with this system, it is inferred from discontinuities in lithofacies in the metamorphic complex of Pichilemu and from morphological breaks attributed to post-Pliocene neotectonic activity. There are no recorded shallow crustal earthquakes in the Pichilemu area prior to the 2010 events. In this study, we combine locally recorded earthquake data from the 2010 seismic deployment and data from 20 seismic stations (short period, three components, continuous recording) deployed in 2017 around Pichilemu, Chile, to create a more detailed characterization of the Pichilemu fault system through local earthquake tomography (LET). The combined data set composed of P- and S-wave arrival times from 3691 events was inverted to generate a 3D elastic wave speed model from the surface to about 50-km depth. One hundred twenty-two focal mechanisms for relocated earthquakes with M ≥ 1 were also generated. Relocated hypocenters show that most of the recorded seismicity is associated with the Pichilemu fault system; its main structure is oriented N145°E, and it is seismically active along about 50 km long. Normal faulting mechanisms predominate for events with M≥2, being similar to the mechanisms of the Mw 7.0 March 11, 2010, Pichilemu earthquakes. Low-velocity anomalies correlate with fracture zones associated with the Pichilemu fault, and a high contrast in Vp/Vs coincides with known structures of Paleozoic to Mesozoic age. A high Vp/Vs ratio is observed where a projection of the fault reaches the interplate contact, suggesting that this zone of the forearc crust is likely weakened by the presence of fluids from the slab. A high Vp anomaly is contiguous with the fault system and appears to be related to the presence of granitic rocks which belong to the Coastal Batholith within the Cordillera de la Costa. We suggest that the location of the Pichilemu fault system is governed by rheological contrasts inherited from the evolution of the subduction complex represented by the current Cordillera de la Costa, and we infer that the orientations of those structures, the possible hydration of them from the interplate contact and associated crustal blocks, play a key role in fault activation following the stressing by a great subduction earthquake.

Comparative Characteristics of Seismic and Deformation Effects for Three Great Subduction Megaearthquakes

The comparative characteristics of seismic and deformational effects of three great subduction megaearthquakes in Sumatra in 2004 (Mw = 9.2); in Maule, Chile, in 2010 (Mw = 8.8); and in Tohoku, Japan, in 2011 (Mw = 9.0) are considered. In all cases the main rupture at the time of the earthquake was located in the subduction zone on the surface of the oceanic lithospheric plate plunging gently beneath a continent or an island arc. The process of destruction in each focus is characterized by a megathrust, with a gentle inclination angle of ~8–18o, according to the definitions of its focal mechanism, in almost full compliance with a gentle dipping of the oceanic lithospheric plate. The displacement occurred in a locked area of the subducting plate extended for several kilometers below the ocean bottom to a depth of 30–40 km. In all three cases, the maximum coseismic slip, obtained according to the data of geodetic measurements, occurred in the upper 25 km of the locked area, while its lower part radiated coherent short-period seisms. The trace of the rupture on the surface, marked by the aftershock area, ranged from 400–600 km in the case of Tohoku and Maule earthquakes up to ~1500 km during the Sumatra earthquake. The rupture in the Maule and Tohoku earthquakes was bilateral, with its approximately symmetric propagation from the epicenter, while in the Sumatra earthquake the rupture spread unilaterally relative to the epicenter from southeast to northwest. The propagation time of the rupture also varied. If in the first two cases it was 140–160 s, for the Sumatra earthquake it lasted 500–600 s. The largest tsunami wave, up to 40–60 m in height, was recorded during the Tohoku earthquake, extending for more than 200 km along the coast of Sanriku province. The rupture during the megaearthquakes under discussion is not confined in depth to the locked seismogenic area, marked by the area of the aftershock nearest in time and the zone of maximum coseismic slips determined by the geodetic GPS measurements. The rupture continues aseismically (postseismic slip) in the transition zone from brittle to brittle-plastic slip to depths of ~60–80 km. In addition, there is evidence that the displacements on the megathrust can continue slightly deeper to the region of the brittle–plastic slip. Such episodic events of slow slip (“silent” or “slow” earthquakes) and “seismic tremor” were recorded in southwestern Japan and southern Chile. These differences in seismic and deformation effects can serve as evidence of the change in frictional properties with depth along the surface of the megathrust. It should also be recognized that the strength barriers and asperities on the megathrust surface expressed in certain geological structures or yet unclear nature of segments of high frequency radiation can manifest themselves in the character of distribution of the accompanying seismicity during the great subduction earthquakes.

Months-long thousand-kilometre-scale wobbling before great subduction earthquakes

Megathrust earthquakes are responsible for some of the most devastating natural disasters1. To better understand the physical mechanisms of earthquake generation, subduction zones worldwide are continuously monitored with geophysical instrumentation. One key strategy is to install stations that record signals from Global Navigation Satellite Systems2,3 (GNSS), enabling us to track the non-steady surface motion of the subducting and overriding plates before, during and after the largest events4-6. Here we use a recently developed trajectory modelling approach7 that is designed to isolate secular tectonic motions from the daily GNSS time series to show that the 2010 Maule, Chile (moment magnitude 8.8) and 2011 Tohoku-oki, Japan (moment magnitude 9.0) earthquakes were preceded by reversals of 4-8 millimetres in surface displacement that lasted several months and spanned thousands of kilometres. Modelling of the surface displacement reversal that occurred before the Tohoku-oki earthquake suggests an initial slow slip followed by a sudden pulldown of the Philippine Sea slab so rapid that it caused a viscoelastic rebound across the whole of Japan. Therefore, to understand better when large earthquakes are imminent, we must consider not only the evolution of plate interface frictional processes but also the dynamic boundary conditions from deeper subduction processes, such as sudden densification of metastable slab.

A recent increase in megathrust locking in the southernmost rupture area of the giant 1960 Chile earthquake

After a great subduction earthquake, viscoelastic stress relaxation causes prolonged seaward motion of inland areas of the upper plate, as was observed around the turn of the century in the area of the 1960 Mw 9.5 Chile earthquake with Global Navigation Satellite System (GNSS) measurements. However, recent GNSS observations during 2010–2019 indicate a systematic decrease in the velocity of the seaward motion over a region covering the latitudinal range of the southern half of the 1960 rupture. Data from the only long-lived continuous site in this region (COYQ since 1997), situated over 200 km away from the trench, suggest that the decrease in the seaward velocity (or increase in the landward velocity) occurred within a few years prior to 2010. This rapid and regional change cannot be explained by viscoelastic relaxation. We thus propose that the change was caused by a relatively sudden downdip widening of the zone of locking along the megathrust. Using three-dimensional finite element modelling, we find that the observed velocity change cannot be otherwise explained, although the amount of the increase in locking cannot be uniquely determined because of trade-offs between, and uncertainties in, the various parameters involved. For example, the degree of the increase in locking is affected by the value of coseismic slip in 1960 in the southernmost part of the rupture zone. A postseismic deformation model with greater coseismic slip in accordance with the most recent coseismic slip model in the literature better fits COYQ data prior to 2005 and requires greater locking increase afterwards. A model with less coseismic slip requires less locking increase but an additional long-term slow slip event prior to 2005. The rapid surface velocity change and the inferred increase in megathrust locking several decades after a great earthquake present new challenges to the understanding of fault mechanics and subduction zone dynamics.

Seismic loading of fault-controlled fluid seepage systems by great subduction earthquakes

Various types of fluid expulsion features (mud volcanoes, pockmarks, authigenic carbonate mounds and associated gas pipes, etc.) are often found above subduction zones, which have the highest seismic potential on Earth. Faults potentially control the liberation of deep-seated greenhouse gases into the feeder systems of seepage features located above subduction thrusts. These feeder systems could be stressed by large earthquakes, yet the mechanisms that can drive episodic mobilization of stored hydrocarbon gases remain poorly understood. Here I address the potential stress loading on fluid expulsion systems created by past earthquakes nucleated at both accretionary and erosive subduction margins. The most significant effects occur in the epicentral area where subduction earthquakes can produce normal stress changes as high as 20–100 bar, although these are generally restricted to relatively small regions. Coseismic normal stress changes and elastic strain relaxation upon a ruptured subduction thrust could increase crustal permeability by dilating fault-controlled conduits, and channelling fluids to the seafloor. Fluid pressure pulses released during subduction earthquakes can greatly contribute to the rupture of fluid pathways that have been brought closer to failure from coseismic static stress changes, although the inaccessible location of most submarine seepage systems has so far hampered probing these relationships.

Spatiotemporal Variation of Mantle Viscosity and the Presence of Cratonic Mantle Inferred From 8 Years of Postseismic Deformation Following the 2010 Maule, Chile, Earthquake

AbstractPostseismic displacements following great subduction earthquakes show significant long‐wavelength and time‐dependent patterns caused primarily by transient viscoelastic relaxation processes occurring broadly at depth. However, the Earth's viscosity structure and time‐dependent variations are still poorly understood, especially in the years immediately following a great earthquake. Here we investigate the spatiotemporal variation of mantle viscosity proximal and distal to the southern Andes using 8 years of continuous high‐resolution GPS observations following the 2010 Mw 8.8 Maule earthquake in south Central Chile. We remove the potential influences of relocking and afterslip on far‐field GPS displacements and estimate viscosities that can explain the 3‐D displacements. The optimal viscosity structure exhibits a low‐viscosity (~1018Pa s) mantle beneath the Andean volcanic arc and high‐viscosity (>1022Pa s) cratonic mantle, indicating a dependence of transient viscosity on temperature. Comparisons of the viscosity distributions at different times show that mantle viscosities increase with time throughout the study region. Viscosity increase is generally fastest in the mantle wedge beneath the Andes and slows down with increasing distance from the source region of the Maule earthquake. Such temporal viscosity evolution may indicate a stress dependence of the viscosity proximal to the rupture zone, while regions east of the Andes act as a relatively rigid body (i.e., cratonic mantle) with much higher viscosity. Our results thus suggest that both temperature structure and stress state contribute to spatiotemporal variations of the mantle viscosity. Heterogeneous spatiotemporal variations of viscosity seem to control the expansion and duration of the postseismic deformation and therefore the postseismic stress evolution.

Crustal Deformation Following Great Subduction Earthquakes Controlled by Earthquake Size and Mantle Rheology

AbstractAfter a great subduction earthquake, viscoelastic stress relaxation causes opposing motion of Earth's surface in the strike‐normal direction, with the dividing boundary located roughly above the downdip termination of the rupture. As the effect of the viscoelastic relaxation decays with time, the effect of the relocking of the megathrust becomes increasingly dominant to cause the dividing boundary to migrate away from the rupture zone, eventually leading to wholesale landward motion. The evolution of the postseismic deformation is controlled not only by mantle viscosity but also by the size of the earthquake. Large coseismic fault slip induces greater stress perturbation that takes a longer time to relax, and a greater rupture length along‐strike results in a pattern of postseismic viscous mantle flow less efficient for stress relaxation. Here we employ spherical‐Earth finite element models of Burgers rheology to quantify postseismic deformation processes for ten 8.0 ≤ Mw ≤ 9.5 subduction earthquakes. Using geodetic data as constraints, we reconstruct spatiotemporally continuous evolution of the postseismic deformation following each earthquake. We comparatively examine the “reference time” when the dividing boundary of the opposing motion passes through the map view location of the 50‐km depth contour of the subduction interface. Our results suggest a positive dependence of the reference time on earthquake size, although site‐ and/or event‐specific factors such as subduction rate, afterslip, and postseismic locking state of the megathrust also affect the evolution. Upper mantle viscosities constrained by available geodetic observations show somewhat different values between subduction zones located far from one another.

Engineering Applications Using Probabilistic Aftershock Hazard Analyses: Aftershock Hazard Map and Load Combination of Aftershocks and Tsunamis

After the Tohoku earthquake in 2011, we observed that aftershocks tended to occur in a wide region after such a large earthquake. These aftershocks resulted in secondary damage or delayed rescue and recovery activities. In addition, it has been reported that there are regions where the intensity of the vibrations owing to the aftershocks was much stronger than those associated with the main shock. Therefore, it is necessary to consider the seismic risk associated with aftershocks. We used the data regarding aftershocks that was obtained from the Tohoku earthquake and various other historically large earthquakes. We investigated the spatial and temporal distribution of the aftershocks using the Gutenberg–Richter law and the modified Omori law. Subsequently, we previously proposed a probabilistic aftershock occurrence model that is expected to be useful to develop plans for recovery activities after future large earthquakes. In this study, the probabilistic aftershock hazard analysis is used to create aftershock hazard maps. We propose a hazard map focusing on the probability of aftershocks on the scale of the main shock for use with a recovery activity plan. Following the lessons learned from the 2011 Tohoku earthquake, we focus on the simultaneous occurrence of tsunamis and aftershocks just after a great subduction earthquake. The probabilistic aftershock hazard analysis is used to derive load combination equations of the load and resistance factor design. This design is intended to simultaneously consider tsunamis and aftershocks for tsunami-resistant designs of tsunami evacuation buildings.

Postseismic uplift of the Andes following the 2010 Maule earthquake: Implications for mantle rheology

AbstractPostseismic surface deformation associated with great subduction earthquakes is controlled by asthenosphere rheology, frictional properties of the fault, and structural complexity. Here by modeling GPS displacements in the 6 years following the 2010 Mw 8.8 Maule earthquake in Chile, we investigate the impact of heterogeneous viscosity distribution in the South American subcontinental asthenosphere on the 3‐D postseismic deformation pattern. The observed postseismic deformation is characterized by flexure of the South America plate with peak uplift in the Andean mountain range and subsidence in the hinterland. We find that, at the time scale of observation, over 2 orders of magnitude gradual increase in asthenosphere viscosity from the arc area toward the cratonic hinterland is needed to jointly explain horizontal and vertical displacements. Our findings present an efficient method to estimate spatial variations of viscosity, which clearly improves the fitting to the vertical signal of deformation. Lateral changes in asthenosphere viscosity can be correlated with the thermomechanical transition from weak subvolcanic arc mantle to strong subcratonic mantle, thus suggesting a stationary heterogeneous viscosity structure. However, we cannot rule out a transient viscosity structure (e.g., power law rheology) with the short time span of observation.

Site response in a representative region of Manzanillo, Colima, Mexico, and a comparison between spectra from real records and spectra from normative

The October 1995 Mw=8.0 Colima-Jalisco earthquake triggered damage in buildings located in Manzanillo city (Tena, A, 1997) [1]. Great damages were reported in different regions of that city. It was also observed liquefaction in the Manzanillo port. In order to improve the knowledge of the subsoil of Manzanillo, Rayleigh waves were inverted applying spatial autocorrelation (SPAC) to array measurements of microtremors at five representative sites of the city. This procedure allowed us to obtain a shallow S-wave velocity profile at each site. We compute the transfer function for each profile and correlate the results with geology. We calculate theoretical surface records. For this purpose, we used a numerical simulation of wave propagation of an input signal (Green function) through the structure. In this case, records of two great subduction earthquakes were used as Green functions. We use these surface signals to estimate response spectra for each site. On the other hand, design spectra according to the manual of seismic design of the Federal Commission of Electricity 2008 edition (MOC-CFE) [2] were obtained and compared with previous results.We observed important differences between the seismic responses in different regions of Manzanillo. For instance, the response at site 5 (alluvial deposits) for a period of 0.25s is twice the response of site 4 (sand deposits) for the same period. Therefore, the site effects are relevant in Manzanillo.For the majority of the cases we observe a reasonable agreement between the values of the dominant periods computed according to the MOC-CFE [2] normative and those obtained directly from Transfer Function (TF). On the other hand, we found significant differences between design spectra and response spectra obtained from seismic records for the 0.1–0.6s interval. Amplitudes are underestimated for the first ones.

Broadband Strong Ground Motion Simulations of Large Subduction Earthquakes

The great subduction earthquakes that occurred recently in Peru, Chile, and Japan have provided unprecedented information about the ground motions generated by such earthquakes. The 23 June 2001 M 8.4 Peru earthquake was recorded at eight strong-motion stations; the 27 February 2010 M 8.8 Maule, Chile, earthquake was recorded at over 10 strong-motion stations; and the 11 March 2011 M 9.0 Tohoku, Japan, earthquake was recorded at more than a thousand stations and produced the most extensive dataset of recordings for any earthquake. For the first time, data are available to guide the generation of ground-motion simulations from great subduction earthquakes. Broadband ground-motion simulations can enhance the usefulness of the recordings of these earthquakes by providing a means of interpolating and extrapolating the recorded data. Once they have been validated, broadband ground-motion simulations can be used for forward predictions of the ground motions of great subduction events in regions such as Cascadia, in which there are no strong-motion recordings of large subduction earthquakes. In this study, we test our ability to use a hybrid method to simulate broadband strong-motion recordings of megathrust earthquakes by demonstrating that our simulations reproduce the amplitudes of the recorded ground motions without systematic bias. We use simulations to study the distribution of various intensity measures of ground motion caused by these earthquakes and to validate our ground-motion simulation method by comparing the simulated ground motions with recorded ground motions as well as with empirical ground-motion prediction models.

現地地震計を用いた地震防災システムの開発と性能評価

An earthquake disaster prevention system using onsite seismometers has been developed since 2009. However, ground motion intensities are underestimated for far-field earthquakes, such as great subduction earthquakes. In this paper, a general procedure to improve S-wave intensity prediction for far-field earthquakes is presented. The proposed method predicts S-wave intensity (peak ground acceleration) just before arrival of S-wave, using information of P-wave intensity as well as its duration. Finally, accuracy of the proposed method is validated with simulation using observed records. These results confirmed that S-wave intensity can be accurately estimated by the proposed method.

Interplate locking condition derived from seafloor geodetic data at the northernmost part of the Suruga Trough, Japan

AbstractWe observed seafloor crustal deformation at two observation sites on opposite sides of the Suruga Trough off Japan from 2005 to 2011 to investigate the interplate locking condition at the source region of the anticipated great subduction earthquake, named Tokai earthquake. We estimated the displacement velocity vectors relative to the Amurian Plate on the basis of repeated observations. Our results at the two points, Suruga northeast and Suruga northwest (SNW) were 42 ± 8 mm/yr toward N94 ± 3°W and 39 ± 11 mm/yr toward N84 ± 9°W, respectively. These directions are the same as those measured at on‐land GPS stations. The magnitudes of the velocity vectors indicate a significant shortening of approximately 4 mm/yr between SNW and on‐land GPS stations located to the west of the Suruga Trough. The results show that the plate interface is strongly locked (no slip) shallower than the source region of the anticipated Tokai earthquake.

Splay fault triggering by great subduction earthquakes inferred from finite element models

AbstractWe have investigated the influence that megathrust earthquake slip has on the activation of splay faults using a 2‐D finite element method (FEM), taking into account the effects of gravity and variations in the frictional strength properties of splay faults. We simulated both landward‐dipping and seaward‐dipping splay fault geometries, and imposed depth‐variable slip distributions of subduction events. Our results indicate that the two types of splay fault exhibit a similar behavior, with variations in frictional properties along the faults affecting only the seismic magnitude. The triggering process is controlled by a critical depth. Megathrust slip concentrated at depths shallower than the critical depth will favor normal displacement, while megathrust slip concentrated at depths deeper than the critical depth is likely to result in reverse motion. Our results thus provide a useful tool for predicting the activation of secondary faults and may have direct implications for tsunami hazard research.

Active tectonics and geomorphology of the Kamchatsky Bay coast in Kamchatka

Kamchatsky Bay is the northernmost bay at the Pacific Kamchatka coast. It is located at the junc� tion between the Kamchatka segment of the Pacific subduction zone and the dextral transform fault of the western Aleutians. The combination of the subduction and collision processes in this region results in the unique set of tectonic controls influencing its geological and geomorphological evolution. The Kamchatka River estuarine area is located on the northern coast of Kamchatsky Bay. The modern Kam� chatka River valley, its estuary, and an aggradation marine terrace some 30 km long and up to 5 km wide were formed in this area during the Holocene. A vast area in the rear part of the terrace and in the Stolbovskaya lowlands is now occupied by the peats deposited directly above lacustrine-lagoonal and fluvial facies. These aggradational landforms record traces of tsunamis and vertical coseismic deformations associated with great subduction earthquakes, as well as strikeslip an d thrust faulting associated with the collision. The results indicate that the average recurrence interval for major tsunamis in the Kamchatsky Bay is 300 years. The recurrence interval on individual fault zones associated with the collision between the western Aleutian and Kamchatka arcs is a few thousand years for earthquakes of magnitude between 7 and 7.5. For the entire region, the recurrence interval for major crustal earthquakes associated with motions along faults may be equal to a few hundred years, which is comparable with that for subductionzone earthquakes.