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The 2001 Bhuj earthquake took place in a region away from the active plate boundaries, which qualifies this earthquake to be a typical intraplate event that claimed a death toll of 20,000 people. The aftershock sequence of this earthquake is still continuing for the last eleven years, which makes this aftershock sequence as a unique intraplate earthquake sequence in the world. This sequence is being monitored by a close digital seismic network consisting of 5–12 three-component broadband seismographs and 10–20 accelerographs that led to a homogeneous earthquake catalog consisting of precise and accurate estimates of hypocentral and source parameters. This catalog enabled us to examine the evolution of this earthquake sequence. Our study reveals a few interconnected regularities in the rate of earthquake occurrence, b-values, fractal dimensions, stress drops, and the mean earthquake depth values. We observe that a slow decrease in background seismic activity and a number of bursts in seismic activity characterize the 2001 Bhuj earthquake sequence. We also notice that the background seismic activity is associated with a monotonic decrease in the mean earthquake depth values while the bursts in seismic activity are found to be associated with a decrease in fractal dimension, b-value and mean earthquake depth estimates, and an increase in stress drop values. We propose that these bursts in seismic activity are related to the episodes of break in deep fluid circulation in the direction of the Earth's surface. The effective background permeability of the mid-crust in the 2001 Bhuj earthquake region is evaluated to be about k ≈ 10 − 13 m 2 based on the estimates of changes in mean earthquake depth. Thus, we infer that seismogenesis of the 2001 Bhuj earthquake sequence is probably connected with active deep fluid circulation underlying the Kachchh seismic zone.

Rock is weak to shear-stress and the energy released by the 26th December, 2004, M ≈ 9.2 Sumatra–Andaman Earthquake, the largest earthquake for four decades, must have accumulated over enormous volumes of crust and mantle, certainly plate-wide, possibly world-wide. Here we report evidence for plate-wide stress accumulation. Changes in seismic shear-wave splitting monitor stress-induced changes in the geometry of the microcrack distributions in almost all rocks in the Earth’s crust. Such changes observed in Iceland show stress-accumulation beginning at least four years before the Sumatra–Andaman Earthquake. These changes were recognised as monitoring stress-accumulation before an impending large earthquake and 10 ‘stress-forecasts’ were emailed to Iceland Meteorological Office for some 29 months forecasting an impending large earthquake. The remarkable sensitivity of critical-systems of microcrack geometry to miniscule changes of stress had not been recognised at that time and the stress-accumulation was expected to lead to a M ⩾ 7 earthquake somewhere in Iceland. Only now is it recognised that the changes in shear-wave splitting were monitoring stress-accumulation which would eventually lead to the Sumatra–Andaman Earthquake at a distance of some 10,500 km on the opposite side of the Eurasian Plate. This extreme sensitivity confirms the critical nature of fluid-saturated stress-aligned microcracks in the Earth’s crust.

We simulate the propagation of earthquake waves in the continental margin of Antarctica using the elastic wave modeling algorithm, which is modified to be suitable for acoustic-elastic coupled media and earthquake source. To simulate the various types of earthquake source, the staggered-grid finite-difference method, which is composed of velocity-stress formulae, can be more appropriate to use than the conventional, displacement-based, finite-difference method. We simulate the elastic wave propagation generated by earthquakes combining 3D staggered-grid finite-difference algorithm composed of displacement-velocity-stress formulae with double couple mechanisms for earthquake source. Through numerical tests for left-lateral strike-slip fault, normal fault and reverse fault, we could confirm that the first arrival of P waves at the surface is in a good agreement with the theoretically-predicted results based on the focal mechanism of an earthquake. Numerical results for a model made after the subduction zone in the continental margin of Antarctica showed that earthquake waves, generated by the reverse fault and propagating through the continental crust, the oceanic crust and the ocean, are accurately described.

Shallow seismic activity in the crust of the overriding plate west of the source area increased significantly after the 2011 Mw 9.0 Tohoku earthquake which ruptured the plate boundary to the east off northern Japan beneath the Pacific Ocean. In order to understand the cause of this distinctive change in seismicity, we have precisely relocated earthquake hypocenters for several earthquake sequences that occurred during the period March 11–April 6, 2011, following the Tohoku earthquake by the double-difference method. Hypocenter distributions were used to discriminate the fault plane from the auxiliary plane of the focal mechanisms for those earthquake sequences. We then calculated the Coulomb stress change on those fault planes caused by the 2011 Mw 9 earthquake. In all cases, the estimated Coulomb stress changes at the plausible fault planes for those post-mainshock sequences are positive. The positive Coulomb stress change is mainly due to the reduction of normal stress on the fault plane of the earthquake sequences by the large, low-angle thrust fault responsible for the 2011 Mw 9 earthquake. The present observations suggest that static stress transfer possibly triggered those post-mainshock earthquake sequences.

The recent 10 August 2009 Coco earthquake (Mw 7.5), the largest aftershock of the giant 2004 Sumatra Andaman earthquake, occurred within the subducting India plate under the Burma plate. The Coco earthquake nucleated near the northwestern edge of the 2004 Sumatra-Andaman earthquake rupture under the unruptured updip segment of the plate boundary interface. The earthquake with predominant normal motion on approximately north-south to northeast-southwest oriented plane is very similar to the 27 June 2008 Little Andaman earthquake which occurred in the South Andaman region near the trench. We provide the only available estimate of coseismic offset due to the 2009 Coco earthquake at a survey-mode GPS site in the north Andaman, located about 60 km south of the Coco earthquake epicentre. The not so large coseismic displacement of about 2 cm in the ESE direction is consistent with the earthquake focal mechanism and its magnitude. We suggest that, like the 2008 Little Andaman earthquake, this earthquake too occurred on one of the approximately north-south to northeast-southwest oriented steep planes of the obliquely subducting 90°E ridge which was reactivated in normal motion after subduction, under the favourable influence of coseismic and ongoing postseismic deformation due to the 2004 Sumatra-Andaman earthquake. Another notable feature of this earthquake is its relatively low aftershock productivity. We suggest that the earthquake occurred very close to the aseismic region of the Irrawaddy frontal arc of very low seismicity where pre-existing faults are not so critically stressed and because of which the earthquake could trigger only a few aftershocks in its immediate vicinity.

We use the movement velocities of GPS stations in western Sichuan province, China, to determine the distribution of horizontal strain accumulation before the 2008 Wenchuan Ms8.0 earthquake with a piecewise approximation approach and the coseismic displacements to determine the fault slips of the earthquake with an inversion analysis method. The results show that the distribution of the principal strain rates is strongly related to the active faults in the region, but along Longmenshan fault where the earthquake occurred, the strain rates are much lower than the others. The fault slip distribution shows mainly a thrusting with dextral striking, and the fault slips in the upper parts of the fault plane are in general bigger. Using the current strain accumulation rate and the released energy by the earthquake, we predict such a big earthquake in Longmenshan fault zone will happen in 460 to 1380 years.

The 1914 Akita-Senboku earthquake was investigated using seismic intensity data estimated from damage records to clarify their magnitudes and source areas related to short-period seismic waves. We evaluated the most appropriate fault model of the Akita-Senboku earthquake on the basis of grid search analysis and other existing research works such as hypocenter distribution of micro earthquakes and S-wave velocity perturbations [Okada et al. (2010)]. The seismic intensity inversion analysis for the Akita-Senboku earthquake assuming an east-dipping fault model (depth: 6-13 km) indicated that the short-period radiation zone was located in the north and deep part of the fault plane and there was a low S-wave velocity zone in the lower crust beneath it. The magnitude of 6.5.6.6 yielded the least evaluation error. The magnitude was also estimated from the area of seismic intensity .ve-lower or more using empirical relationship [Muramatsu (1969)]. It resulted in M=6.6.∼6.7 and reinforced the accuracy of the intensity inversion result. It is concluded that the magnitude of the Akita-Senboku earthquake should be considerably less than M=7.1 after Utsu (1979) even if considering the estimation error. The 1896 Rikuu earthquake was also analyzed to be compared with the Akita-Senboku’s results and to consider their dieffrences. The fault location of the Rikuu earthquake was already identi.ed from surface faults. We assumed two conjugate faults for the northern fault zone of the eastern margin of the Mahiru mountains and the northern fault zone of the eastern margin of the Yokote basin. The most appropriate magnitude was estimated to be M=7.1, which is equivalent to that after Utsu (1979). Therefore, our results indicate that the Rikuu earthquake was considerably larger than the Akita-Senboku earthquake. The short-period radiation zone of the Rikuu earthquake, which might be less accurate due to the lack of intensity data compared to the Akita-Senboku earthquake, was located in shallow zone at the intersection of two conjugate faults. This location contrasts with that of the Akita-Senboku earthquake without surface faults.

SUMMARY Earthquake source parameter determination is of great importance for hazard assessment, as well as for a variety of scientific studies concerning regional stress and strain release and volcano-tectonic interaction. This is especially true for poorly instrumented, densely populated regions such as encountered in Africa, where even the distribution of seismicity remains poorly documented. In this paper, we combine data from satellite radar interferometry (InSAR) and teleseismic waveforms to determine the source parameters of the Mw 5.9 earthquake that occurred on 2008 February 3 near the cities of Bukavu (DR Congo) and Cyangugu (Rwanda). This was the second largest earthquake ever to be recorded in the Kivu basin, a section of the western branch of the East African Rift (EAR). This earthquake is of particular interest due to its shallow depth and proximity to active volcanoes and Lake Kivu, which contains high concentrations of dissolved carbon dioxide and methane. The shallow depth and possible similarity with dyking events recognized in other parts of EAR suggested the potential association of the earthquake with a magmatic intrusion, emphasizing the necessity of accurate source parameter determination. In general, we find that estimates of fault plane geometry, depth and scalar moment are highly consistent between teleseismic and InSAR studies. Centroid-moment-tensor (CMT) solutions locate the earthquake near the southern part of Lake Kivu, while InSAR studies place it under the lake itself. CMT solutions characterize the event as a nearly pure double-couple, normal faulting earthquake occurring on a fault plane striking 350° and dipping 52° east, with a rake of –101°. This is consistent with locally mapped faults, as well as InSAR data, which place the earthquake on a fault striking 355° and dipping 55° east, with a rake of –98°. The depth of the earthquake was constrained by a joint analysis of teleseismic P and SH waves and the CMT data set, showing that the earthquake occurred in the shallow crust, at approximately 8 km depth. Inversions of ENVISAT (Environment Satellite) and ALOS (Advanced Land Observation Satellite) data place the earthquake at 9 km. A comparison of the scalar moment (9.43 ± 0.06 × 1017 Nm from seismology and 8.99 ± 0.010 × 1017 Nm from the joint InSAR solution) shows good agreement between the two data sets. Such an agreement is in contrast to the large discrepancies observed (up to an order of magnitude) in other places along the EAR where similar earthquake sequences are associated with magmatic intrusion. From this, we infer that the rupture was brittle and occurred with little aseismic deformation as might be expected from magma injection. Our results provide insights into the style of rifting occurring in the South Kivu Volcanic Province and hence will aid future studies on seismic risk in the context of Lake Kivu and underline the importance of systematic monitoring of the EAR area.

The 1963 great Kurile earthquake was an underthrust earthquake occurred in the Kurile–Kamchatka subduction zone. The slip distribution of the 1963 earthquake was estimated using 21 tsunami waveforms recorded at tide gauges along the Pacific and Okhotsk Sea coasts. The extended rupture area was divided into 24 subfaults, and the slip on each subfault was determined by the tsunami waveform inversion. The result shows that the largest slip amount of 2.8 m was found at the shallow part and intermediate depth of the rupture area. Large slip amounts were found at the shallow part of the rupture area. The total seismic moment was estimated to be 3.9 × 1021 Nm (Mw 8.3). The 2006 Kurile earthquake occurred right next to the location of the 1963 earthquake, and no seismic gap exists between the source areas of the 1963 and 2006 earthquakes.

The nature of the deep water cycle, in which water moves from Earth's surface down into the mantle and then back to the surface continues to be debated. It is widely held that only part of water transported from the surface in subduction zones returns to the surface by direct expulsion at oceanic trenches and volcanism, and that a significant amount of water is carried into the mantle transition zone (410–660 kilometres) or even the lower mantle. To test this 'wet lithosphere' model, Harry Green II and colleagues compare patterns of seismicity and the stabilities of potentially relevant hydrous phases determined from experimental and seismic evidence. They conclude that the evidence requires a very dry lithosphere below 400 kilometres, suggesting that subducting slabs do not provide a pathway for significant amounts of water to enter the deep mantle. These authors test whether patterns of seismicity and the stabilities of potentially relevant hydrous phases are consistent with a wet lithosphere. They show that there is nearly a one-to-one correlation between the dehydration of minerals and seismicity at depths less than ∼250 km, but no correlation at greater depths. They conclude that subducting slabs must be essentially dry by 400-km depth and thus do not provide a pathway for significant amounts of water to enter the mantle transition zone or the lower mantle. Strong evidence exists that water is carried from the surface into the upper mantle by hydrous minerals in the uppermost 10–12 km of subducting lithosphere, and more water may be added as the lithosphere bends and goes downwards. Significant amounts of that water are released as the lithosphere heats up, triggering earthquakes and fluxing arc volcanism1. In addition, there is experimental evidence for high solubility of water in olivine, the most abundant mineral in the upper mantle, for even higher solubility in olivine’s high-pressure polymorphs, wadsleyite and ringwoodite2, and for the existence of dense hydrous magnesium silicates that potentially could carry water well into the lower mantle3 (deeper than 1,000 km). Here we compare experimental and seismic evidence to test whether patterns of seismicity and the stabilities of these potentially relevant hydrous phases are consistent with a wet lithosphere. We show that there is nearly a one-to-one correlation between dehydration of minerals and seismicity at depths less than about 250 km, and conclude that the dehydration of minerals is the trigger of instability that leads to seismicity. At greater depths, however, we find no correlation between occurrences of earthquakes and depths where breakdown of hydrous phases is expected. Lastly, we note that there is compelling evidence for the existence of metastable olivine (which, if present, can explain the distribution of deep-focus earthquakes4,5,6,7) west of and within the subducting Tonga slab8 and also in three other subduction zones, despite metastable olivine being incompatible with even extremely small amounts of water (of the order of 100 p.p.m. by weight7). We conclude that subducting slabs are essentially dry at depths below 400 km and thus do not provide a pathway for significant amounts of water to enter the mantle transition zone or the lower mantle.

SUMMARY We study a large M w = 7.6 earthquake that occurred on 2007 November 14 in the Northern Chile seismic gap near the city of Tocopilla. Using a variety of seismic data we show that this earthquake ruptured only the lower part of the interplate seismic zone and generated a series of plate interface aftershocks. Two large aftershocks on 2007 November 15 ruptured the interplate zone oceanwards of the Mejillones Peninsula, a major geographical feature in the Antofagasta region. On 2007 December 16, a large M w = 6.8 aftershock, that occurred near the southern bottom of the fault plane of the main event, is shown to be a slab-push earthquake located inside the subducted Nazca Plate and triggered by along slab compression. Aftershocks of this event demonstrate that it occurred on an almost vertical fault. The Tocopilla earthquake took place just after the installation of a new seismological network by Chilean, German and French researchers. The accelerometric data combined with far field seismic data provide a quite complete and consistent view of the rupture process. The earthquake broke a long (130 km) and narrow (about 30–50 km) zone of the plate interface just above the transition zone. Using a non-linear kinematic inversion method, we determined that rupture occurred on two well-defined patches of roughly elliptical shape. We discuss the consequences of this event for models of gap filling earthquakes in Chile proposed in the 1970s.

SUMMARY The 2005 June 13, an intraplate, intermediate depth earthquake (M w = 7.8, 98 km depth) occurred in northern Chile. Previous studies show that this earthquake is a slab-pull event with down dip extensional source mechanism. However, the physical origin and the role in seismic cycle of this kind of event are still under debate. We present a seismological source study from strong motion data, based on the simplified slip patch kinematic parametrization. We find two distinct sources. The main source is almost centred around the hypocentre and triggers the secondary one, situated deeper in the slab. The weaknesses and uncertainties of the kinematic models are partly resolved and discussed by constructing dynamic spontaneous rupture models with two radiative asperities and one embedding zone of non-radiative slip process, a required feature to fulfil the seismic moment constraint. Two kinds of models are compatible with observations: a crack mode model (combined with fast rupture velocity and small zone of non-radiative slip) and a pulse mode model (combined with slower rupture velocity and large zone of non-radiative slip). The fast rupture model is preferred for its physical simplicity. Replacing our result in the geometry and thermal contexts of the subduction, we conclude that the rupture has probably broken the whole lithosphere by encountering little resistance and without being very sensitive to the presence of the double seismic zone. This suggests that the rupture happens along a weak plane, on which large slab instabilities can take place.

SUMMARY investigation of the SH wave seismograms indicates that rupture occurred on a shallowly dipping fault, propagating in all directions from the hypocentre. The average rupture velocity in the downdip and updip directions are ∼2.0-3 and ∼1.0 km s −1 , respectively. The maximum slip is found to be ∼12 m and is located downdip of the earthquake's hypocentre. Taken over the fault as a whole, the average stress drop is ∼0.55 MPa. Used in conjunction with relocated aftershocks, we find that two barriers constrain the faulting extent of slip, creating a rupture area of ∼165 km × 140 km. These barriers coincide with submarine topographic features, are domain boundaries for the seismic gaps along the arc, and have high concentrations of large aftershocks near them. The high slip regions align with the orientation of the subducting plate, and resemble in size and shape the many seamounts about to subduct. The 2003 Tokachi-Oki earthquake was the largest earthquake to occur globally in that year and reruptured the portion of the Kuril- Japan Trench that last broke in the great Tokachi-Oki earthquake of 1952 (Hamada & Suzuki 2004). The abundance of local data in this region coupled with the large magnitude of the event has led to a range of models for the rupture being proposed. These solutions are summarised in Table 1. All of these solutions have the total moment for the earthquake significantly below that obtained from the long-period seismological data by the global CMT (GCMT) solution, with some solutions having less than one third of the GCMT moment. This study aims to address this anomaly, by first investigating the long-period moment tensor solution and then by finding a rupture model which is consistent with both broad-band teleseismic data and the long-period solution.

To recognize the seismogenic process, mechanism and behavior of seismic precursors, as well as look for the ways of earthquake prediction the granular physics principle is applied based on the basic structure of the crust and mantle and existing experimental observations, where we treat the earth crust and mantle as large scale discrete matters. Main outcomes are: A granular model of the structure and movement of the earth crust and mantle is established. A formation mechanism of the tectonic forces, which causes the earthquakes, and a model of propagation for precursive signals are proposed. Properties of the seismic precursive signal and its relevance with the earthquake occurrence are illustrated, and principle of ways to detect the effective seismic precursor is elaborated. In the paper the mechanism of palintectic earthquake is also explained by the jamming-unjamming transition of the granular flow. Some earthquake phenomena which were previously difficult to understand are explained, and the predictability of the earthquake is discussed. Due to the discrete nature of the earth crust and mantle, the continuum theory no longer applies during the quasi-static seismogenic process. In all in this paper, based on granular physics, we study the causes of earthquakes, earthquake precursors and earthquake prediction, and a new understanding, different from the traditional seismological point of view, is obtained.

Relationships between relative plate velocity and seismicity differ by plate-boundary class. We test the null hypothesis of linearity of earthquake rates with velocity in each of seven classes. A linear relationship is expected if earthquake rate is proportional toseismicmoment rate, whichis proportional torelative platevelocity. To reducebiasbyaftershocksandswarms,weestimateindependenceprobabilitiesofearth- quakes and use them as weights. We assign shallow earthquakes to boundary steps and classes,thensortboundarystepswithineachclassbyvelocityandplotcumulativeearth- quakesagainstcumulativemodelmomentrate.Weusetwomeasuresofnonlinearityand 10 4 stochastic simulations to assess significance. In subduction zones the relationship betweenseismicityandvelocityisnonlinearwith99.9%confidence.Slowersubduction at

article A survey of published scientific literature was undertaken to identify and catalog observed earthquake pre- cursors. The earthquake precursors selected for analysis included electric and magnetic fields, gas emissions, groundwater level changes, temperature changes, surface deformations, and seismicity. For each of these precursors, the published scientific literature was searched to document the statistics of each reported earth- quake precursor (spatial extent, time, duration, amplitude, signal/noise ratio), to analyze dependence of the observable for each precursor on earthquake magnitude, and to explore proposed physical models to explain eachearthquakeprecursor.Somegeneralcharacteristics wereobservedfortheseprecursoryphenomena.First, the largest amplitude precursory anomalies tend to occur before the largest magnitude earthquakes. Also, the number of precursory anomalies tends to increase the closer in time to the occurrence of the earthquake. Finally,theprecursoryanomaliestendtooccurclosetotheeventualepicenterof theearthquake.Ingeneral,the physical models indicate that all of the precursory phenomena are related to deformation that occurs near the fault prior to the main earthquake. While the models provide plausible physical explanations for the pre- cursors, there are many free parameters in the models that are poorly resolved.

The measurements of the parameters of split shear (S) waves from local deep-focus earthquakes recorded in 2005–2007 by a network of 12 seismic stations in Southern Sakhalin are presented. The results revealed the heterogeneous distribution of the anisotropic properties beneath Southern Sakhalin. The azimuths of the fast S-wave polarization beneath the stations in the central part of the peninsula are oriented along the NNW and NNE-NE directions normal to and along the Kuril Trench. Beneath the stations located along the western and eastern coasts, the azimuths of the fast S-wave polarization change their direction from NNW in the northern area to E-SE in the southern area. The highest anisotropy degree (up to 0.9–1.5%) is recorded beneath the central part of Southern Sakhalin. The maximum values of the discrepancy in the arrival time of the split S-waves are observed when the azimuth of the fast S-wave is oriented along the NNE beneath the active fault zones. The analysis of the variations of the S-wave lag time shows their weak depth dependence. The highest anisotropy is assumed in the upper layers of the medium (down to a depth of about 250 km). The results obtained for the dominating wave frequency of 1–5 Hz represent mainly the medium-scale anisotropy of the top of the studied region.

Summary The thermal evolution of the lithospheric slab at subduction zones and its geophysical effects are numerically calculated. An alternating-direction, implicit, finite-difference scheme is used to compute the thermal models taking into account all heating sources and phase boundaries. These models, with the appropriate spreading rates and dip angles, are compared with different island arc systems. Temperatures inside the slab are strongly controlled by the conductivity and by the time elapsed since the initiation of descent. The depth to which temperature anomalies persist is generally about 700 km or less. The thermal results are used to construct the seismic velocities and ray paths, density anomalies, and the resulting stress distribution. Comparing the theoretical stress distribution and the focal mechanism studies of intermediate- and deep-focus earthquakes indicates the importance of both the mantle's rheology and the temperature dependence of the slab's elastic properties. The intermediate and deep focus earthquakes are located along the coolest region of the slab. The theoretical results explain the source mechanisms and the orientation of principal stresses under major island arcs.

Summary Recent confirmation of the relationship between seismology and tectonic plate movement has focused attention on the deformation and instability of lithospheric plates. In this paper models of the underthrusting litho-sphere for the calculation of displacement and deflection are constructed. The first of the three contributions is a mathematical theory that rigorously demonstrates the elastic instability in the descending lithosphere. The theory states that lithospheric thrust beneath island arcs becomes unstable and suffers deflection as the compression increases. Thus, in the neighbour-hood of the edges where the lithospheric plate plunges into the asthenosphere and mesosphere its shape will be contorted. The second contribution is the calculation of the lateral displacement. It is found that, before contortion, the plate will thicken and contract at different positions with the variation in thickness following a parabolic profile. Finally, the depth distribution and the origin of the intermediate and deep focus earthquakes are explained in terms of plate buckling and contortion.

Strong anomalies in the concentration of helium, radon and gamma were observed in gases at the geochemical monitoring station, Bakreswar, West Bengal, India, about two weeks before the 7.9 M earthquake at Sichuan, China. The distance between the epicenter of the earthquake and the monitoring site is about 1800 km. This long distance preseismic observation indicates that the radius of influence of large magnitude earthquakes may be substantially large and may cut across plate boundaries. This paper presents the observed geochemical anomalies for the Sichuan earthquake and discusses empirical postulates between earthquake magnitude and its radius of influence.

The Ms7.4 earthquake of 18 July, 1969 is the only strong earthquake in the Bohai Sea that has been recorded by seismometers. Arguments about this large earthquake are focused on its causative fault. To tackle this problem, three cruises in the epicenter region from 2005 to 2008 were executed and 3 kinds of acoustic data with high-resolution, including shallow penetrated single channel seismic data, sidescan sonar data and CHIRP data were collected. This paper presents the results based on these new acoustic data. A small depression zone 2~3 m beneath the seabed is first found in the Holocene sediments in the epicenter region. The zone with an NE30- strike is 20 km long and 3 km wide. CHIRP data reveal that depositional border formed at about 5000 a B.P. has a throw of 1.5 m in the zone. The good match of the locations of the depression zone and the BZ28 fault and aftershocks distribution of the 1969 earthquake suggests that the depression zone is the result of tectonic activities of the BZ28 fault and the bottom deformation of sequence A which was formed since 150 a B.P. is caused by 1969 earthquake. The BZ28 fault with a NE30° strike, a branch fault of the Tan-Lu Fault Zone, is the causative fault of the Ms7.4 earthquake. CHIRP data show that the latest faulting age is the late Holocene. Based on the forming age of stratigraphic sequences and vertical displacement of the fault, the vertical slip rate of the BZ28 fault since late Pleistocene is calculated. The results show that the rate during the Holocene is 0.3 mm/a, which is larger than 0.05 mm/a during the late Pleistocene-Holocene, reflecting a trend of aggrandizement.

New Zealand is one of the more seismically active countries in the world, with more than 15,000 earthquakes located each year. Routine moment tensor analysis of regional seismic data for earthquakes with moment magnitude M w>∼3.5–4.0 has recently been implemented in New Zealand. Nearly 330 regional moment tensor (RMT) solutions have been calculated for earthquakes in the New Zealand region dating back to late 2003. This complements local magnitude ( M L), the primary magnitude calculated by GeoNet in New Zealand. The RMT catalog, along with 155 Global Centroid Moment Tensor (Global CMT) catalog solutions, is used to compare M w with M L for New Zealand earthquakes. In addition to M w and M L, there are more than 330 teleseismic body-wave magnitude ( m b) values available from the United States Geological Survey and International Seismological Center catalogs for events that also have an M w. These are used to examine the relationship between M w, M L, and m b for New Zealand earthquakes. There is a clear distinction in the relationship of M L to M w for shallow focus (≤33 km depth) and deep focus (>33 km depth) earthquakes. Shallow focus earthquakes show M L to be fairly consistent with M w, particularly for events with M w>∼4.5 and with ![Graphic][1] . Deep focus earthquakes have M L consistently larger than M w (more than a full magnitude unit for some events) with ![Graphic][2] . M w and m b are in fairly good agreement regardless of the depth, whereas m b estimates are consistently smaller than M L for deep events. This suggests that M L is overestimated for deep focus events and that the large M Ls are not the result of physical properties of the source. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif

Seismogenic zones of interplate megathrust earthquakes along the Nankai Trough can be subdivided into several segments. At each segment, seismic rupture has occurred at a recurrence interval of about one century. In many cases, some neighboring segments ruptured simultaneously or sequentially after a short interval. One of the factors that controls the properties of such seismic ruptures is the underground structure, including the plate configuration and heterogeneity around the subducting plate (slab) interface. To clarify the mechanism of megathrust earthquakes, detailed surveys and analyses of the underground structures are required. Moreover, the detection of seismic phenomena on the plate interface is important in discussing interplate coupling and interactions between such events and megathrust earthquakes. More and better knowledge of the underground structure around the plate interface has been accumulated by analyses of high-quality data from high-density seismograph networks in inland areas and from joint seismic explorations of the sea and land. Moreover, knowledge regarding a wide variety of newly detected slow earthquakes has contributed toward our understanding of the subducting plate interface. Additional information about underground structures, such as slab segmentations, is expected to provide a better understanding of the occurrence of megathrust earthquakes.

SUMMARY We investigate the 2007 Pisco, Peru, earthquake (M8.0) using seismological and geodetic data. Analysis of teleseismic records indicates that the earthquake is characterized by a steadily increasing moment rate, with the maximum occurring between about 55 and 70 s and the associated slip occurring about 40 km south of the epicentre. We combine InSAR data from L-band (ALOS) and C-band (Envisat) satellites to produce a 3-D image of the ground displacements caused by the earthquake. Distributed slip inversions of the interferograms identify a single large patch of slip in the location of the maximum moment rate identified by seismology. Despite a clear spatial coincidence between the location of the coseismic slip and the anomalously high topography of the Paracas Peninsula, we find no evidence that the earthquake was directly responsible for motion on any upper plate faults.

This paper discusses studies of the migration over time of hypocenters from great depths to the Earth’s surface. The calculations are based on the notion that surface fracturing of the external lithosphere corresponds to brittle fracturing, while internal lithosphere fracturing, accompanied by a decrease of earthquake hypocenter depth, corresponds to ductile fracturing. A procedure for calculating periods of the rising of earthquake sources in an elastic-plastic environment is suggested. Finally, it was revealed that the rising of hypocenters to the surface takes approximately 8–10 years.

Transformation-induced faulting, which has been proposed as a mechanism for deep-focus (>300 km) earthquakes, requires a thermal runaway associated with an exothermic chemical reaction. This shearing instability has been demonstrated for several polymorphic phase transformations including the olivine → wadsleyite and olivine → ringwoodite reactions. However, it has not been experimentally tested whether a strongly exothermic decomposition reaction can also lead to this instability, as would be the case if olivine were to be carried metastably into the lower mantle and break down to magnesium silicate perovskite + magnesiowustite. Here we report experiments designed to test this possibility using the proxy reaction albite → jadeite + coesite, a highly exothermic reaction with a large ΔV. We find no mechanical or microstructural evidence for transformation-induced faulting during this reaction. Applying these results to the reaction olivine (ol) → perovskite (pv) + mw potentially occurring in subducting oceanic lithosphere at the top of the lower mantle suggests that this breakdown reaction cannot trigger earthquakes. Thus, transformation-induced faulting as a mechanism of deep earthquakes is consistent with the abrupt cessation of earthquakes at the base of the mantle transition zone, but only if subduction zones are colder than currently proposed. We discuss other reasons for termination of earthquakes at ∼700 km and find them also difficult to reconcile with experimental evidence or natural observations.

On 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.

In this study a new tomographic method is applied to over 1500 high-quality PmP (Moho reflected wave) travel-time data as well as over 38,500 high-quality first P-wave arrivals to determine a detailed 3D crustal velocity structure under Beijing and adjacent areas. Results of detailed resolution analyses show that the PmP data can significantly improve the resolution of the model in the middle and lower crust. After the PmP data are included in the tomographic inversion, our new model not only displays the tectonic feature appeared in the previous studies, but also reveals some new features. The Zhangjiakou-Bohai Sea fault zone (Zhang-Bo zone) is imaged as prominent and continuous low-velocity (low-V) anomalies in the shallower crust, while in the middle and lower crust it shows intermittent low-V anomalies extending down to the uppermost mantle. Furthermore, the pattern of low-V anomalies is different along the Zhang-Bo zone from the southeast to the northwest, indicating that there exist large differences in the dynamic evolution of Taihangshan and Yanshan uplifts and North China depression basin. Prominent low-V anomalies are visible under the source area of the 4 July 2006 Wen-An earthquake (M 5.1), suggesting that the occurrence of the Wen-An earthquake is possibly related to the effect of the crustal fluids probably caused by the upwelling of the hot and wet asthenospheric materials due to the deep dehydration of the stagnant Pacific slab in the mantle transition zone. The fluids in the lower crust may cause the weakening of the seismogenic layer in the upper and middle crust and thus contribute to the initiation of the Wen-An earthquake. This is somewhat similar to the cause of the 1695 Sanhe-Pinggu earthquake and the 1976 Tangshan earthquake in the region, as well as the 1995 Kobe earthquake in Japan and the 2001 Bhuj earthquake in India.

A compiled gravity anomaly map of the Western Himalayan Syntaxis is analysed to understand the tectonics of the region around the epicentre of Kashmir earthquake of October 8, 2005 (Mw = 7.6). Isostatic gravity anomalies and effective elastic thickness (EET) of lithosphere are assessed from coherence analysis between Bouguer anomaly and topography. The isostatic residual gravity high and gravity low correspond to the two main seismic zones in this region, viz. Indus–Kohistan Seismic Zone (IKSZ) and Hindu Kush Seismic Zones (HKSZ), respectively, suggesting a connection between siesmicity and gravity anomalies. The gravity high originates from the high-density thrusted rocks along the syntaxial bend of the Main Boundary Thrust and coincides with the region of the crustal thrust earthquakes, including the Kashmir earthquake of 2005. The gravity low of HKSZ coincides with the region of intermediate–deep-focus earthquakes, where crustal rocks are underthrusting with a higher speed to create low density cold mantle. Comparable EET (∼55 km) to the focal depth of crustal earthquakes suggests that whole crust is seismogenic and brittle. An integrated lithospheric model along a profile provides the crustal structure of the boundary zones with crustal thickness of about 60 km under the Karakoram–Pamir regions and suggests continental subduction from either sides (Indian and Eurasian) leading to a complex compressional environment for large earthquakes.

Numerical simulation of earthquake cycles is a powerful tool to list the possible future earthquake scenarios, and to examine subsequent matters associated with scenarios. In the previous scenario of the Nankai Trough earthquake, ocean bottom observation networks deployed off the Kii peninsula are expected to detect the pre-seismic signals of the next earthquake. While, as a new Nankai Trough earthquake scenario recently proposed has a different hypocenter and occurrence pattern to those previously proposed, the expansion of the current ocean bottom observation network is indispensable for the disaster mitigation from the new scenario earthquake. To check the efficiency of the expansion of ocean bottom observation network against the new scenario, variations in interplate coupling and crustal deformation just before the occurrence of the scenario earthquake are carefully examined. The result shows that the significant crustal deformation is detectable enough earlier than the occurrence of scenario earthquake, if the pressure gauge sensor is installed on the ocean bottom just above the possible hypocenter.

We present an acoustic emission (AE) monitoring technique to study high-pressure (P > 1 GPa) microseismicity in multi-anvil rock deformation experiments. The application of this technique is aimed at studying fault mechanisms of deep-focus earthquakes that occur during subduction at depths up to 650 km. AE monitoring in multi-anvil experiments is challenging because source locations need to be resolved to a submillimetre scale due to the small size of the experimental assembly. AEs were collected using an 8-receiver array, located on the back truncations of the tungsten carbide anvils. Each receiver consists of a 150-1000 kHz bandwidth PZT transducer assembly. Data were recorded and processed using a high-speed AMSY-5 acquisition system from Vallen-Systems, allowing waveform collection at a 10 MHz sampling rate for each event signal. 3-D hypocentre locations in the assembly are calculated using standard seismological algorithms. The technique was used to monitor fault development in 3 mm long x 1.5 mm diameter olivine cores during axisymmetric compression and extension. The faults were generated during cold compression to similar to 2 GPa confining pressure. Subsequent AEs at 2-6 GPa and 900 degrees C were found to locate near these pre-existing faults and exhibit high pressure stick-slip behaviour.

The 1988 Udayapur Earthquake of Nepal–India border occurred at longitude 86.6°E and latitude 26.7°N in the vicinity of the rupture area of the great earthquake of 1934. We estimated the focal parameters and slip model of this earthquake by waveform modeling of the teleseismic P-waves using a multiple event deconvolution method. We analyzed 10 waveform data retrieved from IRIS, GDSN and GEOSCOPE. We found this earthquake to be a double-event earthquake with a time lag of about 8 s between the two successive subevents. Focal mechanism solution obtained in this study is consistent with already published solutions. Depth constraints obtained in this study indicates this earthquake to rupture the lower crust of the subducting Indian slab. The slip model with a fault dimension of 39 km × 21 km estimated in this study reveals one main rupture area (asperity) at the main hypocentral region with a maximum slip of 3.38 m.

The Kuril-Kamchatka seismofocal zone was thought to be a single plate approximately 90 km wide and dipping to a depth of 700 km at an angle of 40°–45°. This concept reflects primarily the physical differences (elastic wave velocities, density, temperature, etc.) between the seismofocal zone and the mantle hosting it. Detailed investigations show that the seismofocal zone proper is also heterogeneous with earthquake hypocenters variably concentrated and clustered within this zone, where both seismogenic and aseismic strata, as well as subvertical zones, can be identified. The latter are reflected in the structure and faults of the Earth’s crust and upper mantle.

Phase transformation of (Mg0.89Fe0.11)2SiO4 olivine, the proposed major composition of upper mantle, is related to the mechanism of deep focus earthquakes and the interaction between subduction zones and their ambient mantle. Nucleation rate and growth rate are two important parameters in the computation of the phase transformation. However, the nucleation rate has not been derived from the experimental data partly for technical difficulties. We modify the dimensionless parameter describing the impingement under grain boundary nucleation through physical analysis on the image of the impingement. Based on this modification and the experimental data, we first evaluate the nucleation of olivine. By fitting the new nucleation rates, we get the parameters K0 and γo1/3. The best fitting results are K0 = 6.9 × 1024s−1·m−2·K−1, γo 1/3=0.16 J·m−2.

Observation of low-frequency radio signals may be used as a tool of remote monitoring of seismic disturbances in the lower ionosphere. In the case of using a low frequency radio signal the most informative parameter is a signal phase. When using natural low-frequency radio emissions - electromagnetic signals of lightning discharges (atmospherics), the variation of the signal amplitude is analyzed. For strong (magnitude greater than 5) and not deep-focus (focal depth is less than 50 km) earthquakes it is found that the effects of earthquakes and their precursors in the signals of atmospherics appeared in the form of increasing amplitude, whose duration is generally a few hours. In this work peculiar properties of the effects of manifestations in the signals of atmospherics of deeper earthquakes are considered on the example of the events in the Malaysian-Indonesian region.

We use the double-difference earthquake location algorithm to relocate approximately 1,000 earthquakes in the eastern Tennessee seismic zone (ETSZ). We examine the earthquake hypocenter relocations in an effort to resolve fault orientations and thereby gain insights into the tectonics of the seismic zone. The analysis involves visual comparison of three-dimensional perspective plots of the hypocenter relocations oriented accord ing to focal mechanism nodal planes derived from events within several clusters of earthquakes. Relocations in the central, most seismically active, portion of the ETSZ indicate a diffuse west-striking, north-dipping zone of hypocenters. The orientation of this zone of hypocen ters is consistent with structure imaged by a previous seismic reflection profile that shows a reflective mid-to-upper crust with apparent dips of approximately 35° to the north. Relocation of a smaller cluster of earthquakes near the Tennessee-North Carolina border suggests a steeply dipping northwest trending fault in that area.

SUMMARY We present results from forward modelling to study the feasibility of using S-to-P converted phases to image the seismic discontinuity structure of the crust and upper mantle. We show that a significant level of P-wave energy arriving before the direct S-wave arrival can interfere with the S-to-P converted phases of interest and may result in Sp receiver function phases that do not represent true earth structure. The source of this P-wave energy is attributable to a number of phases, including those that have undergone multiple reflections off the Earth’s surface. For deep focus earthquakes (300‐600 km deep), a significant amount of P-wave energy is observed from pPPP, pPPPP and sPPPP phases, and arrives within the same time window as predicted for S-to-P converted phases from the direct S phase arrival. Furthermore, for earthquakes at all depths, interfering P-wave energy arrives within the same time window as predicted for S-to-P converted phases from the SKS phase arrival, limiting the usefulness of SKSp receiver functions for upper mantle imaging. To isolate true Sp receiver function phases from contamination due to other P-wave phases, we find it necessary to stack receiver functions from a range of epicentral distances and depths in order to aid the suppression of noise and other unwanted phases. We provide constraints on the noise levels to be expected as a function of epicentral distance and earthquake depth. We find that the lowest noise levels are achievable by restricting epicentral distance to less than 75 degrees and the depth of earthquakes used to less than 300 km.

High-resolution 3-D seismic velocity (Vp and Vs) and Poisson's ratio (σ) structures in the epicentral area of the 2005 west off Fukuoka prefecture earthquake were determined by inverting a large number of high-quality P- and S-wave arrival times and sP depth phase data to better understand the genesis of the main-shock and its aftershock sequence. The hypocenters of the earthquakes used in this study were relocated with a combined method which enables to accurately relocate the earthquakes occurred outside the seismic network by using sP depth phase data together with double-difference travel time data. This new approach of earthquake location can be applied to other offshore regions of subduction zones where few seismic stations exist while numerous suboceanic earthquakes occur. Strong crustal velocity heterogeneities were revealed in the hypocentral area, which might reflect the existence of fluids, geometrical irregularities of the fault zone, and geothermal anomalies in the region. The main-shock hypocenter was located near a distinct zone characterized by low-velocity, high-Poisson's ratio and high-conductivity anomalies, being considered to be associated with the existence of fluids in and around the source area. Such fluids might have reduced the seismic velocity of the crustal materials around the main-shock hypocenter and weakened the mechanic strength of the fractured rock matrix in the source area, and thus triggered the 2005 west off Fukuoka prefecture earthquake.

The deep part of the Tonga subduction zone consists of two differently oriented slab segments: the northern segment within latitudes 17–19°S, and the southern segment within latitudes 19.5–27°S. The orientation of the slab is (strike/dip): 110°/57° in its northern part and 210°/46° in its southern part. Both segments are seismically active at depths from 500 to 700 km. The mechanisms of deep-focus earthquakes reported in the Harvard moment tensor catalogue contain compensated linear vector dipole (CLVD) components that behave differently in both segments. The mean value of the CLVD is 3% for the northern segment but −10% for the southern segment. The mean absolute value of the CLVD is 12% for the northern segment and 16% for the southern segment. The complex behaviour of the CLVD is explained by spatially dependent seismic anisotropy in the slab. The inversion for anisotropy from the non-double-couple components of moment tensors points to orthorhombic anisotropy in the both slab segments. The anisotropy seems to have a uniform strength of 5–7% for P-waves and 9–12% for S-waves, and is oriented according to the orientation of each segment and the stress acting in it. The spatial variation of velocities is roughly similar in both segments. The retrieved anisotropy might have several possible origins. It can be: (1) intrinsic, caused by preferentially aligned anisotropic minerals such as wadsleyite, ringwoodite, ilmenite or others, (2) effective, caused, for example, by intra-slab layering, or (3) partly apparent, produced by systematic errors in the moment tensors due to neglecting 3D slab geometry and the slab/mantle velocity contrast when calculating the Green functions in the moment tensor inversion.

Using the teleseismic waveform data recorded by 19 PASSCAL broadband mobile seismic stations which worked nearly one year in the Changbaishan volcanic area and other 14 stations with portable broadband digital seismographs deployed about three months in the Jingpohu volcanic region, totally 423 high quality receiver functions were extracted. Through stacking these receiver functions in terms of midpoint stacks of P converted to S waves, the discontinuity distributions along with their patterns were obtained. The results show that beneath the studied area, 410km, 520km and 660km discontinuities exist. Both the 410km and the 660km discontinuities are relatively continuous and present positively correlative characteristics. The 410km discontinuity is a local uplift beneath the Changbaishan volcano, and 660km discontinuity is of the complex multi-interface. The thickness of the transition zone between 410km and 660km is about 250km and approaches the whole world average value. The 660km discontinuity descends beneath the Huichun deep earthquake area and above the 660km discontinuity in the stack profiles, where there are some phases which are not continuous on the two sides of the Huichun deep earthquake area. We infer that the west Pacific plate has already subducted into the upper mantle transition zone beneath the Eurasia plate. We consider that the local uplift of 410km discontinuity and the complex multi-interface characteristics of the 600km discontinuity as well as the occurrence of the Huichun deep earthquakes are all due to the slab activities in the transition zone. Because of the strong resistance from the 660km discontinuity, the subduction slab may become nearly horizontal and continue to subduct toward west. And beneath the Eurasia plate, the front part of the west Pacific plate possibly has been split into several segments rather than remaining a whole.

Data of the intermediate depth (the Geioyo and the Shizuoka) earthquakes in Japan recorded in a dense network is analysed and compared with various available attenuation relations. The approach of Midorikawa (Midorikawa S., 1993, Technophysics 218, 287–295) based on the empirical Green’s function technique of Irikura (Irikura, K., 1986, Proceedings of the 7th Japan Earthquake Engineering Symposium, pp. 151–156.) has been used to model the rupture responsible for these earthquakes and peak ground acceleration are simulated at selected observation points. The method presented in this work includes the transmission effect in a multiple layer crustal model for a finite fault earthquake source model. Sharp attenuation rate is seen for such intermediate depth earthquakes which is difficult to explain through conventional attenuation relations. Detail study of the methodology and comparison of results shows that the transmission factor plays an important role for the sharp attenuation rate for intermediate-to deep-focus earthquakes.

Between October 12, 1989 and December 31, 2002, the Alaska Volcano Observatory (AVO) located 162 deep long-period (DLP) events beneath 11 volcanic centers in the Aleutian arc. These events generally occur at mid- to lower-crustal depths (10–45 km) and are characterized by emergent phases, extended codas, and a strong spectral peak between 1.0 and 3.0 Hz. Observed wave velocities and particle motions indicate that the dominant phases are P- and S-waves. DLP epicenters often extend over broad areas (5–20 km) surrounding the active volcanoes. The average reduced displacement of Aleutian DLPs is 26.5 cm2 and the largest event has a reduced displacement of 589 cm2 (or ML 2.5). Aleutian DLP events occur both as solitary events and as sequences of events with several occurring over a period of 1–30 min. Within the sequences, individual DLPs are often separated by lower-amplitude volcanic tremor with a similar spectral character. Occasionally, volcano-tectonic earthquakes that locate at similar depths are contained within the DLP sequences. At most, Aleutian volcanoes DLPs appear to loosely surround the main volcanic vent and occur as part of background seismicity. A likely explanation is that they reflect a relatively steady-state process of magma ascent over broad areas in the lower and middle portions of the crust. At Mount Spurr, DLP seismicity was initiated by the 1992 eruptions and then slowly declined until 1997. At Shishaldin Volcano, a short-lived increase in DLP seismicity occurred about 10 months prior to the April 19, 1999 eruption. These observations suggest a link between eruptive activity and magma flux in the mid- to lower-crust and uppermost mantle.

Based on the displacement velocity of GPS sites before and after MS8.1 Kunlunshan earthquake, the crustal strain parameters are calculated with the elastic model expression. The variation of strain parameters indicates that the seismic area and its surroundings were dominated by compressive strain with a larger velocity before the earthquake. The vector of displacement velocity shows that the epicentral area before the earthquake was “stable” with weaker seismicity relative to the surroundings. The crustal deformation and strain reveals that the seismogenic body has accumulated higher stain potential energy under compression. In the period as MS8.1 earthquake occurred, strain velocity varies significantly due to releasing large amounts of strain energy. The calculated result indicates that the strain generated by the earthquake is up to 4.5×10−6 and the average left-lateral strike slip on the seismogenic fault is 3.31m (89°E~96°E).

In this study, a close link between the depth-distribution of subduction-zone earthquakes and dehydration events in the hydrated slab-peridotite is established. A phase diagram up to 27 GPa in the MgO–Al2O3–SiO2–H2O (MASH) system was constructed using a combination of thermodynamic calculation and Schreinemakers analysis on previous experimental data. At intermediate-depths (

We installed five broadband seismometers around Mt. Usu, Hokkaido, Japan, just before the first surface eruption on March 31, 2000. By using these broadband data with short-period seismograms recorded by Japan Meteorological Agency and Institute of Seismology and Volcanology of Hokkaido University, we located 590 earthquakes associated with the 2000 eruption of Mt. Usu from March 31, 12:58 to April 5, 2000, including 41 low-frequency earthquakes. Low-frequency earthquakes have clear predominant frequencies of about 1.0–1.5 Hz. While the seismicity of tectonic earthquakes was still active on April 5, there were almost no low-frequency events after April 2. All the low-frequency earthquakes occurred within a vertical and narrow zone near the craters of eruption, and they are shallower than 4 km. In contrast, the active area of tectonic earthquakes spreads out in a large area, particularly deep in the south of the craters. From these temporal and spatial patterns, together with GPS and gravity measurements that support the termination of magma activities before April 2, we suggest that low-frequency earthquakes were caused by magma or hydrothermal activities beneath the volcano while tectonic ones were caused by the regional tectonic stress affected by the intrusion of magma body near the craters. Related to long-term magma activities of this eruption, four deep crustal low-frequency events were identified on October 17, 1998 just in the south of Mt. Usu in the depth range of 20–30 km. As proposed for other active volcanoes, these deep crustal events may represent a part of deep magma activities beneath Mt. Usu.

Dehydration embrittlement has been proposed to explain both intermediate- and deep-focus earthquakes in subduction zones1,2,3,4,5. Because such earthquakes primarily occur at shallow depths or within the core of the subducting plate, dehydration at relatively low temperatures has been emphasized6,7,8. However, recent careful relocation of subduction-zone earthquakes9,10 shows that at depths of 100–250 km, earthquakes continue in the uppermost part of the slab (probably the former oceanic crust that has been converted to eclogite) where temperatures are higher. Here we show that at such pressures and temperatures, eclogite lacking hydrous phases but with significant hydroxyl incorporated as defects in pyroxene and garnet develops a faulting instability associated with precipitation of water at grain boundaries and the production of very small amounts of melt. This new faulting mechanism satisfactorily explains high-temperature earthquakes in subducting oceanic crust and could potentially be involved in much deeper earthquakes in connection with similar precipitation of water in the mantle transition zone (400–700 km depth). Of potential importance for all proposed high-pressure earthquake mechanisms is the very small amount of fluid required to trigger this instability.