Occurrence Of Great Earthquakes Research Articles

Incoming plate structure at the Japan Trench subduction zone revealed in densely spaced reflection seismic profiles

The structure of the incoming plate is an important element that is often considered to be related to the occurrence of great earthquakes in subduction zones. In the Japan Trench, where the 2011 Tohoku earthquake occurred, we collected seismic profiles along survey lines separated by 2–8 km to examine the structural characteristics of the incoming Pacific plate in detail. The average thickness of the incoming sediments was < 500 m along most of the Japan Trench, and it was < 300 m at ~ 38° N, where the large shallow megathrust slip occurred during the 2011 Tohoku earthquake. We mapped bending-related normal faults, including their dip direction and amount of throw. The numbers of eastward (oceanward) and westward (trenchward) dipping normal faults were generally comparable in the Japan Trench. Eastward dipping normal faults were dominant in the northern and southern parts of the Japan Trench, whereas westward dipping normal faults were more numerous in the central part. Graben-fill sediments deposited at the landward edge of the graben were bounded by eastward dipping normal faults. Trench-fill sediments were also observed along the trench axis. The sediment fills locally increased the thickness of the input sediments where they were deposited. The along-axis variation in input sediment and sediment fill distribution, and the variations in normal fault dip direction between the central and other parts of the Japan Trench may correspond to different slip styles along the plate boundary.

Simultaneous inference of plate boundary stresses and mantle rheology using adjoints: large-scale 2-D models

SUMMARY Plate motions are a primary surface constraint on plate and mantle dynamics and rheology, plate boundary stresses and the occurrence of great earthquakes. Within an optimization method, we use plate motion data to better constrain uncertain mantle parameters. For the optimization problem characterizing the maximum a posteriori rheological parameters we derive gradients using adjoints and expressions to approximate the posterior distributions for stresses within plate boundaries. We apply these methods to a 2-D cross section from the western to eastern Pacific, with temperature distributions and fault zone geometries developed primarily from seismic and plate motion data. We find that the best-fitting stress exponent, n, is about 2.8 and the yield stress about 100 MPa or less. The normal stress on the interplate fault zones is about 100 MPa and the shear stresses about 10 MPa or less.

Evolution of Seismic Site Classification According to the Criteria in Chilean Design Codes

Design codes establish seismic site classifications to determine the seismic demand of a structure according to the response of the soil foundation under the action of earthquake ground motions; the site classification can even condition the feasibility of a project. The occurrence of great earthquakes in Chile has tested its design codes, generating much information and experience regarding the seismic design of structures that have allowed researchers to identify variations in seismic demands according to the kind of ground foundation and to propose seismic site classification methods in Chilean regulations since the 1930s; countries in the vanguard of seismic design, such as the USA, Japan, and New Zealand, proposed methods even earlier. In this document, the evolution of methodologies for seismic site classification according to the criteria in Chilean codes is analysed from their implementation in the 1930s to the most recently proposed design code NCh 433, 2018–2021. Although the distinctive features of each country shape the criteria in their design codes, clear knowledge of the evolution of established criteria from their origins is considered an important tool that contributes to the better understanding, interpretation and application of the seismic site classification methodologies contained in a design code with better criteria. Likewise, the review indicates a distinct need to conduct a continuous evaluation of the classification criteria supported by records of new earthquakes, as well as by physical and numerical models that allow incorporating variables which condition the response of the terrain such as topography, lateral heterogeneities, and basic effects.

Seismological evidence of a dehydration reaction in the subducting oceanic crust beneath western Shikoku in southwest Japan

SUMMARYThe Philippine Sea plate (PHS) is subducting beneath southwest Japan from the Nankai Trough. In this region, deep low-frequency earthquakes/tremors (LFEs) are active and their epicentres are distributed along the downdip of the source region of M8-class earthquakes that have occurred every 100–150 yr. The LFE activity may potentially be strongly related to the occurrence of great earthquakes between tectonic plates. To investigate the structural features around the LFEs, we applied teleseismic receiver function (RF) analysis to the seismograms observed at permanent and temporal seismograph stations in western Shikoku and we detected seismological evidence of a slab dehydration reaction linked to LFE activity. Based on the first-order harmonic decomposition of RFs, we first estimated the average plunge azimuth of the PHS beneath western Shikoku. Considering the backazimuth dependence of converted phase amplitude, we constructed the cross-section of the radial component RFs, excluding the incoming waveform data from the updip directions of the dipping slab. In this profile, the parallel negative and positive P-to-S converted phase alignment within a distance of 10 km were imaged. These phase alignments corresponded to the top of the slab and the slab Moho discontinuity, respectively. At the northern side of the profile, the landward (continental) Moho was also detected. In the region where LFEs were actively distributed, both the landward Moho and slab surface were unclear. The second-order harmonic decomposition of RFs for several kilometres above the slab Moho indicated that the anisotropic symmetry axes suddenly changed at the southern limit of the LFE active region; the fast axes in the region were normal to the trench though axes in the southern area were parallel. According to the thermal and pressure condition, a phase transition with a dehydration reaction can occur in the oceanic crust near the southern edge of the LFE active region. Once the dehydration process advances, released water causes the serpentinization of the mantle material near the slab surface and the corresponding seismic velocity decreases. The impedance contrast decreases at the boundary between the lower crust and the mantle wedge, as well as that between the mantle wedge and oceanic crust; therefore, the amplitudes of the converted phases at these boundaries also become small. Considering that serpentinite ordinarily has strong anisotropy with a seismic fast axis direct to the shear direction, all features observed are evidence of the dehydration process in the flat slab.

A Tectonophysical Analysis of Earthquake Frequency—Size Relationship Types for Catastrophic Earthquakes in Central Asia

We performed a tectonophysical analysis of earthquake frequency–size relationship types for large Central Asian earthquakes in the regions of dynamical influence due to major earthquake-generating faults based on data for the last 100 years. We identified four types of frequency–size curves, depending on the presence/absence of characteristic earthquakes and the presence or absence of a downward bend in the tail of the curve. This classification by the shape of the tail in frequency–size relationships correlates well with the values of the maximum observed magnitude. Thus, faults of the first type (there are characteristic earthquakes, but no downward bend) with Mmax ≥ 8.0 are classified as posing the highest seismic hazard; faults with characteristic earthquakes and a bend, and with Mmax = 7.5–7.9, are treated as rather hazardous; faults of the third type with Mmax = 7.1–7.5 are treated as posing potential hazard; and lastly, faults with a bend, without characteristic earthquakes, and with a typical magnitude Mmax ≤ 7.0, are classified as involving little hazard. The tail types in frequency–size curves are interpreted using the model of a nonlinear multiplicative cascade. The model can be used to treat different tail types as corresponding to the occurrence/nonoccurrence of nonlinear positive and negative feedback in earthquake rupture zones, with this feedback being responsible for the occurrence of earthquakes with different magnitudes. This interpretation and clustering of earthquake-generating faults by the behavior the tail of the relevant frequency–size plot shows raises the question about the physical mechanisms that underlie this behavior. We think that the occurrence of great earthquakes is related to a decrease in effective strength (viscosity) in the interblock space of faults at a scale appropriate to the rupture zone size.

Reply to “Comment on ‘Statistical Analyses of Great Earthquake Recurrence along the Cascadia Subduction Zone’ by Ram Kulkarni, Ivan Wong, Judith Zachariasen, Chris Goldfinger, and Martin Lawrence” by Allan Goddard Lindh

In our article Kulkarni et al. (2013), we favored a clustered model for great Cascadia subduction zone (CSZ) earthquakes and our statistical analysis resulted in a probability of 0.65 that clustering was present in the turbidite record. The CSZ clustering analysis was originally motivated by the need to develop a CSZ logic tree for use in probabilistic seismic‐hazard analysis. In recognition of the two principal models that could explain the pattern of great earthquake occurrence observed in the turbidite record, we gave weights of 0.65 and 0.35 to clustered versus quasi‐periodic behavior in the logic tree, respectively (Wong et al. , 2014). In Lindh (2016), Al Lindh questioned the validity of the turbidite data and our analysis of temporal clustering and states that the CSZ actually behaves in a quasi‐periodic behavior. He summarizes his argument in four points, and we wish to respond to each point in the following. We confine our responses to the technical issues raised by Lindh. With regards to his discussion on seismic risk in the Pacific Northwest, we support his views. Certainly, understanding the behavior of the CSZ, particularly its recurrence, is a critical element in earthquake‐risk reduction in the Pacific Northwest. We appreciate Lindh for pointing out the more recent studies that indicate that for a few of the examples we cited, clustering of full‐rupture earthquakes may not be characteristic of the behavior that was previously suggested. However, the point we were trying to make is that earthquake clustering, regardless of scale or magnitude, is a behavior that is observed along plate boundaries and crustal faults worldwide. There are physical models that can explain temporal clustering, and the quasi‐periodic behavior favored by Lindh is unusual over long time spans. Many factors can come into play that can alter the long‐term behavior of a …

The Great 2006 and 2007 Kuril Earthquakes, Forearc Segmentation and Seismic Activity of the Central Kuril Islands Region

We present a structural study of the Central Kuril Islands forearc region, where the great megathrust tsunamigenic earthquake (M w 8.3) occurred on November 15, 2006. Based on new bathymetry and seismic profiles obtained during two research cruises of R/V Akademik Lavrentiev in 2005 and 2006, ten crustal segments with along-arc length ranging from 30 to 100 km, separated by NS– and NW–trending transcurrent faults were identified within the forearc region. The transcurrent faults may serve as barriers impeding stress transfer between the neighboring segments, so that stress accumulated within separate forearc segments is usually released by earthquakes of moderate-to-strong magnitudes. However, the great November 15, 2006 earthquake ruptured seven of the crustal segments probably following a 226-year gap since the last great earthquake in 1780. The geographic extent of earthquake rupture zones, aftershock areas and earthquake clusters correlate well with forearc crustal segments identified using the geophysical data. Based on segmented structure of the Central Kuril Islands forearc region, we consider and discuss three scenarios of a great earthquake occurrence within this area. Although the margin is segmented, we suggest that a rupture could occupy the entire seismic gap with a total length of about 500 km. In such a case, the earthquake magnitude M w might exceed 8.5, and such an event might generate tsunami waves significantly exceeding in height to those produced by the great 2006–2007 Kuril earthquakes.

A study of the relationship between volcanic activity and great earthquakes: The Kuril-Kamchatka region

We used the data on the activity of volcanoes in Kamchatka and the North Kuril Islands for the period from 1840 to early 2013 to identify the most significant cyclic components. The resulting periodicities were compared with the recurrence spectrum for great (M ≥ 7.7) earthquakes in the Kuril-Kamchatka region for 1841–2012. We detected 52.8–54.0, 8.58, and 5.72-year cycles, which are common both to seismicity and to volcanic activity. The first interval is close to the three times the value of the 18.613-year lunar rhythm (55.84 years). The 8.58 and 5.72-year periodicities seem to be controlled by solar activity variations and are the second and third harmonics in the 17.15-year cycle. This cycle and its harmonics are used for long-term prediction of great (M ≥ 7.7) earthquakes in the Kuril-Kamchatka region as a whole. It was concluded that the existing increased hazard of great earthquake occurrence in the Kuril-Kamchatka region will last until February 2016 (a 40% probability of a great earthquake during that period). In addition, the long-period phase of increased seismic hazard will last until 2027 with the probability of great earthquakes being 1.6 times the long-term average value.

Re-Estimated Effects of Deep Episodic Slip on the Occurrence and Probability of Great Earthquakes in Cascadia

Research Article| November 26, 2013 Re‐Estimated Effects of Deep Episodic Slip on the Occurrence and Probability of Great Earthquakes in Cascadia N. M. Beeler; N. M. Beeler U.S. Geological Survey, Cascades Observatory, 1300 Cardinal Court, Bldg. 10 Suite 100, Vancouver, Washington 98683 Search for other works by this author on: GSW Google Scholar Evelyn Roeloffs; Evelyn Roeloffs U.S. Geological Survey, Cascades Observatory, 1300 Cardinal Court, Bldg. 10 Suite 100, Vancouver, Washington 98683 Search for other works by this author on: GSW Google Scholar Wendy McCausland Wendy McCausland U.S. Geological Survey, Cascades Observatory, 1300 Cardinal Court, Bldg. 10 Suite 100, Vancouver, Washington 98683 Search for other works by this author on: GSW Google Scholar Author and Article Information N. M. Beeler U.S. Geological Survey, Cascades Observatory, 1300 Cardinal Court, Bldg. 10 Suite 100, Vancouver, Washington 98683 Evelyn Roeloffs U.S. Geological Survey, Cascades Observatory, 1300 Cardinal Court, Bldg. 10 Suite 100, Vancouver, Washington 98683 Wendy McCausland U.S. Geological Survey, Cascades Observatory, 1300 Cardinal Court, Bldg. 10 Suite 100, Vancouver, Washington 98683 Publisher: Seismological Society of America First Online: 14 Jul 2017 Online ISSN: 1943-3573 Print ISSN: 0037-1106 Bulletin of the Seismological Society of America (2014) 104 (1): 128–144. https://doi.org/10.1785/0120120022 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 N. M. Beeler, Evelyn Roeloffs, Wendy McCausland; Re‐Estimated Effects of Deep Episodic Slip on the Occurrence and Probability of Great Earthquakes in Cascadia. Bulletin of the Seismological Society of America 2013;; 104 (1): 128–144. doi: https://doi.org/10.1785/0120120022 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 Mazzotti and Adams (2004) estimated that rapid deep slip during typically two week long episodes beneath northern Washington and southern British Columbia increases the probability of a great Cascadia earthquake by 30–100 times relative to the probability during the ∼58 weeks between slip events. Because the corresponding absolute probability remains very low at ∼0.03% per week, their conclusion is that though it is more likely that a great earthquake will occur during a rapid slip event than during other times, a great earthquake is unlikely to occur during any particular rapid slip event. This previous estimate used a failure model in which great earthquakes initiate instantaneously at a stress threshold. We refine the estimate, assuming a delayed failure model that is based on laboratory‐observed earthquake initiation. Laboratory tests show that failure of intact rock in shear and the onset of rapid slip on pre‐existing faults do not occur at a threshold stress. Instead, slip onset is gradual and shows a damped response to stress and loading rate changes. The characteristic time of failure depends on loading rate and effective normal stress. Using this model, the probability enhancement during the period of rapid slip in Cascadia is negligible (<10%) for effective normal stresses of 10 MPa or more and only increases by 1.5 times for an effective normal stress of 1 MPa. We present arguments that the hypocentral effective normal stress exceeds 1 MPa. In addition, the probability enhancement due to rapid slip extends into the interevent period. With this delayed failure model for effective normal stresses greater than or equal to 50 kPa, it is more likely that a great earthquake will occur between the periods of rapid deep slip than during them. Our conclusion is that great earthquake occurrence is not significantly enhanced by episodic deep slip events. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Were Global M&gt;=8.3 Earthquake Time Intervals Random between 1900 and 2011?

The pattern of great earthquakes during the past ∼100 yr raises questions whether large earthquake occurrence is linked across global distances, or whether temporal clustering can be attributed to random chance. Great-earthquake frequency during the past decade in particular has engendered media speculation of heightened global hazard. We therefore examine interevent distributions of Earth's largest earth- quakes at one-year resolution, and calculate how compatible they are with a random- in-time Poisson process. We show, using synthetic catalogs, that the probability of any specific global interevent distribution happening is low, and that short-term clusters are the least repeatable features of a Poisson process. We examine the real catalog and find, just as expected from synthetic catalogs, that the least probable M ≥8:3 earth- quake intervals during the past 111 yr were the shortest ( t< 1 yr) if a Poisson process is active (mean rate of 3.2%). When we study an M ≥8:3 catalog with locally trig- gered events removed, we find a higher mean rate of 9.5% for 0-1 yr intervals, com- parable to the value (11.1%) obtained for simulated catalogs drawn from random- in-time exponential distributions. We emphasize short interevent times here because they are the most obvious and have led to speculation about physical links among global earthquakes. We also find that comparison of the whole 111-yr observed M ≥8:3 interevent distribution (including long quiescent periods) to a Poisson process is not significantly different than the same comparison made with synthetic catalogs. We therefore find no evidence that global great-earthquake occurrence is not a ran- dom-in-time Poisson process.

Indian Strong Motion Instrumentation Network

The Indian subcontinent is prone to earthquakes in both interplate and intraplate regions. Areas that have been identified as severe seismic regions include the Himalayan belt in the north from Kashmir to Manipur; Gujarat in the west; and the Andaman and Nicobar Islands in the southeast. The faults in these areas are capable of generating large-magnitude earthquakes that would subject neighboring areas to significant ground shaking. Thus, seismic hazard assessment is of prime importance in India. The relative velocity of the Indian plate with respect to the Eurasian plate near Delhi is about 5 cm/yr in the direction of N13° E (NUVEL-1A model of De Mets et al. 1994). The collision of these continental plates results in crustal shortening along the northern edge of the Indian plate. This process has given rise to three major thrust planes: the main central thrust (MCT), the main boundary thrust (MBT), and the main frontal thrust (MFT) (see, for example, Gansser 1964; Molnar and Chen 1982). The region has experienced several great earthquakes in the past hundred years or so (1897 Assam, 1905 Kangra, 1934 Bihar-Nepal, 1950 Assam). The Himalayan geodynamics and the occurrence of great earthquakes are well summarized by Seeber and Armbruster (1981), Khattri (1999), and Bilham and Gaur (2000). During the last episode of strain release, a 750-km-long segment, which lies between the eastern edge of the 1905 rupture zone and the western edge of the 1934 earthquake, remained unbroken (Figure 1). This segment, called the central seismic gap, continues to be under high strain ( e.g. , Singh et al. 2002). Large earthquakes occurred in this seismic gap in 1803 and 1833, but the magnitudes of these earthquakes were less than 8, and, hence, they were not gap-filling events (Khattri 1999; Bilham 1995). Based on these considerations and on …

Earthquake occurrence processes in the Indo-Burmese wedge and Sagaing fault region

Earthquakes in the Indo-Burmese wedge and Sagaing fault regions occur in response to the partitioning of the India–Sunda motion along these two distinct boundaries. Under the accretionary wedge of the Indo-Burmese arc, majority of the earthquakes occur in the depth range of 30–60km and define an eastward gently dipping seismicity trend surface that coincides with the Indian slab. The dip of the slab steepens in the east direction and earthquakes occur down to a depth of 150km, though the slab can be traced up to the 660km discontinuity. Although these features are similar to a subduction zone, the nature of the earthquakes and our analysis of their focal mechanisms suggest that these earthquakes are of intra-slab type which occur on steep plane within the Indian plate and the sense of motion implies a northward relative motion with respect to the Sunda plate. Thus these earthquakes and the stress state do not support active subduction across the Indo-Burmese arc which is also consistent with the relative motion of India–Sunda plates. The absence of inter-plate earthquakes, lack of evidence of the occurrence of great earthquakes in the historical records and non-seismogenic nature of the plate interface under the accretionary wedge suggest that seismic hazard due to earthquakes along the plate boundary may be relatively low. However, major intra-slab earthquakes at shallow and intermediate depths may still cause damage in the sediment filled valley regions of Manipur and Cachar in India and Chittagong and Sylhet regions of Bangladesh. In the Sagaing fault region, earthquakes occur through dextral strike slip motion along the north–south oriented plane and the stress state is consistent with the plate motion across the Sagaing fault.

A shallow strong patch model for the 2011 great Tohoku-oki earthquake: A numerical simulation

[1] A numerical simulation is conducted for understanding the mechanics of the 2011 great Tohoku-oki earthquake (Mw = 9.0), which widely broke the plate interface at the Pacific plate subducting beneath northern Honshu (Tohoku), Japan. In the model, frictional stress on the plate interface is assumed to obey a rate- and state-dependent friction law. A strong patch (asperity) with higher effective normal stress and a large value of characteristic slip distance is assumed at a shallower part of the plate interface. This strong patch controls the occurrence of great earthquakes that broke the entire seismogenic plate interface with recurrence intervals of several hundred years. The present model explains large coseismic slip at a shallower part of the 2011 great earthquake and accumulation of slip deficit at deeper parts, where smaller M7 class earthquakes repeatedly occurred before the great earthquake.

Effects of climate change on geo-disasters in coastal zones and their adaptation

Results of recent investigations suggest that climate change tends to exacerbate geo-disasters. Therefore, it is understood clearly that adaptation to climate change has rapidly become the most important and urgent issue for the future existence of human beings on Earth. These inferences form the background of this research. In comparison to those examining water disasters, few studies have examined climate-change-induced geo-disasters. This study aims at upgrading the methodology for estimating effects on geo-disasters of combined events, e.g., global warming with increased typhoon and rainfall severity or occurrence of great earthquakes. Such a methodology is expected to contribute to progress in the fields of natural disaster mitigation and land preservation, particularly near seacoasts and rivers.

The 2007 Bengkulu earthquake, its rupture model and implications for seismic hazard

The 12 September 2007 great Bengkulu earthquake (M w 8.4) occurred on the west coast of Sumatra about 130 km SW of Bengkulu. The earthquake was followed by two strong aftershocks of M w 7.9 and 7.0. We estimate coseismic offsets due to the mainshock, derived from near-field Global Positioning System (GPS) measurements from nine continuous SuGAr sites operated by the California Institute of Technology (Caltech) group. Using a forward modelling approach, we estimated slip distribution on the causative rupture of the 2007 Bengkulu earthquake and found two patches of large slip, one located north of the mainshock epicenter and the other, under the Pagai Islands. Both patches of large slip on the rupture occurred under the island belt and shallow water. Thus, despite its great magnitude, this earthquake did not generate a major tsunami. Further, we suggest that the occurrence of great earthquakes in the subduction zone on either side of the Siberut Island region, might have led to the increase in static stress in the region, where the last great earthquake occurred in 1797 and where there is evidence of strain accumulation.

The Mechanism and Dynamics of the Generation and Occurrence for Wenchuan MS8.0 earthquake

AbstractOn 12 May 2008, an MS8.0 earthquake occurred in Wenchuan, Sichuan Province. Its causative fault is the Yingxiu–Beichuan fault. Analysis of the velocity field before the earthquake discovers that the shortening rate along the direction of N103°E from west Bayan Har block (BHB) to the Longmen Shan fault zone (LFZ) is 13.0 mm/a, the right lateral slipping rate of LFZ is 1.1 mm/a, and the faults are locked. The coseismic displacement field shows that the earthquake maybe was caused by the thrust of BHB in direction of SE and the underthrust of Sichuan Basin (SB) in the direction at NW simultaneously. Analysis of the strain field shows that the principal compressive and the principal tensile strain rates in the epicenter area are –30.840×10–9/a and 13.956×10–9/a, respectively before the earthquake, and the principal compressive strain axis is in a direction of N105.4°E which is consistent with the direction of N103°E of the principal compressive stress axis from focal mechanism results. The earthquake fault trend and slipping direction obtained by the mechanism proposed in the paper are also consistent with the results of the focal mechanism and earthquake field investigations. The interactions of the Indian, the Philippine Sea and the Pacific plates with the Eurasian plate are long‐term acting forces responsible for the Wenchuan great earthquake occurrence. The Sumatra‐Andaman huge earthquake on December 2004 promoted occurrence of the Wenchuan earthquake.

Source analysis of the February 12th 2007, Mw 6.0 Horseshoe earthquake: Implications for the 1755 Lisbon earthquake

While very large earthquakes are generally confined to subduction zones, the SW Iberian margin –setting of the famous Mw 8.5–8.7, 1755 Lisbon tsunami earthquake‐ may be an exception to this rule. Evidence for active subduction is not conclusive here, but instead plate convergence in old oceanic lithosphere with large brittle layer thickness can account for the occurrence of great earthquakes along moderate‐length faults. We estimate the source parameters of the February 12th 2007, Horseshoe earthquake. Regional moment tensor inversion yields an Mw 6.0, reverse to strike‐slip faulting source in the upper mantle. Modelling teleseismic, surface‐reflected body waves (pP, pwP, sP) indicates a source depth of 40 km beneath the seafloor. Analysing apparent source time functions allows identifying the preferred fault plane (strike N245°E/ dip 55°/ rake 50°), and estimating rupture area (53 km2) and average slip (0.27 m). Scaling the source characteristics to the size of the 1755 earthquake suggests a fault length of 230–315 km, being compatible with the length of mapped faults in the area.

Deep electrical anomaly in the M7.5 Qiongzhou earthquake region and its relation with future seismicity

From the magnetotelluric detection in the epicentral region and the adjacent areas of the 1605 M7.5 Qiongzhou earthquake, we have discovered there is a low resistive body in the deep crust of the epicentral region. The low resistive body extends straightly from the depth of about 13 km to the upper mantle, which is supposed as an uprising mantle pole. We therefore consider it is just the existing mantle pole and its upwelling thermal material that result in the faulting and stick-slipping activities of the upper crust, which is an important factor for the M7.5 Qiongzhou great earthquake occurrence. The postseismic faulting activity is characterized by creep, which shows that the risk is greatly decreased for the occurrence of a great earthquake with similar intensity in the future.

Low‐frequency tremors, intraslab and interplate earthquakes in Southwest Japan—from a viewpoint of slab dehydration

Low‐frequency tremors (LFT) recently found in the Southwest Japan fore‐arc likely occur due to hydro‐fracturing from fluids leaving the dehydrating Philippine Sea slab. However, there are some places without such LFT; they are N. Izu‐Kanto, E. Shikoku, and S. Kyushu. These are the places where island‐arc type crust is subducting. We propose that dehydration of the subducted crust does not occur beneath these regions, because it is composed mainly of tonalite, lacking the quantity of hydrous minerals seen in normal subducting oceanic crust. We show that almost no earthquake occurs within the subducted crust in such regions, consistent with dehydration embrittlement hypothesis for intraslab seismicity. The lack of dehydration from the crust would also affect the mode of occurrence of interplate earthquakes, because pore fluid pressure is more difficult to rise at the thrust zone. This might be reflected in the irregular occurrence of great earthquakes in these regions.

A three-dimensional simulation of crustal deformation accompanied by subduction in the Tokai region, central Japan

Abstract A great earthquake is expected to occur along the Suruga Trough where the Philippine Sea plate subducts beneath the Eurasian plate. We estimate crustal deformation prior to this hypothetical earthquake, called the Tokai earthquake, based on a simulation model for plate subduction, a three-dimensional extension of the models by Stuart (1988) and Kato and Hirasawa (1996). In this model, the time evolution of a dislocation field in an elastic half-space is governed by a quasi-static interaction induced by the dislocation field itself as well as by a rate- and state-dependent friction law acting on the plate interface. Observed geophysical constraints taken into account are the shapes of the curved subducting plate interface, the recurrence interval of past great earthquakes (≃ 90–150 years), the average seismic coupling coefficient (≃0.5), the annual rate of level change (≃5 mm/year) between the two bench marks at Hamaoka and Kakegawa, and the backslip distribution on the plate interface estimated from the result of GPS observation. The simulation shows the following. The occurrence of great earthquakes is approximately periodic. During interseismic times, a strongly coupled region is formed in a seismic zone, and it shrinks as the next great earthquake approaches. The strongly coupled region is confined to an area where the plate inclination is gentle. The rupture of the earthquake starts at a point close to the maximum shear stress acting on the plate boundary. Subsidence of Hamaoka relative to Kakegawa changes to uplift several years before the earthquake. Volumetric strain undergoes a change from compression to extension, or vice versa, at certain observation sites several years before the earthquake. Changes in volumetric strain accelerate as the occurrence of the earthquake impends. The maximum strain change for 1 day observed on the ground surface just before the earthquake is 10 −8 to 10 −7 , a value about one-tenth of the value predicted by a related two-dimensional model. The pattern of shear stress accumulation and the location of the preslip are affected by the shape of the plate interface.