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We used recent seismic data and advanced techniques to investigate 3D fault geometry over the transition from the partially coupled to the fully coupled plate interface inboard of the Nankai Trough off the Kii Peninsula, Japan. We found that a gently dipping plate boundary decollement with a thick underthrust layer extends beneath the entire Kumano forearc basin. The 1 April 2016 Off-Mie earthquake (Mw6.0) and its aftershocks occurred, where the plate boundary decollement steps down close to the oceanic crust surface. This location also lies beneath the trenchward edge of an older accretionary prism (∼14 Ma) developed along the coast of the Kii peninsula. The strike of the 2016 rupture plane was similar to that of a formerly active splay fault system in the accretionary prism. Thus, the fault planes of the 2016 earthquake and its aftershocks were influenced by the geometry of the plate interface as well as splay faulting. The 2016 earthquake occurred within the rupture area of large interplate earthquakes such as the 1944 Tonankai earthquake (Mw8.1), although the 2016 rupture area was much smaller than that of the 1944 event. Whereas the hypocenter of the 2016 earthquake was around the underplating sequence beneath the younger accretionary prism (∼6 Ma), the 1944 great earthquake hypocenter was close to oceanic crust surface beneath the older accretionary prism. The variation of fault geometry and lithology may influence the degree of coupling along the plate interface, and such coupling variation could hinder slip propagation toward the deeper plate interface in the 2016 event.

The idea of the formation of deep-focus earthquakes does not fit into the framework of Mr. Reid's theory of Elastic recoil explaining the origin of earthquakes as the emission of the energy of elastic deformations by the rocks of the earth's interior due to the simple physical fact of the absence of elastic deformations at a depth of several tens of kilometers. More can be said, if humanity had not experienced the devastating catastrophes caused by deep-focus earthquakes, scientists, in the person of Cambridge University professor Mr. Jeffries, would still have argued that such events cannot take place in the bowels of the Earth. Indeed, it is a very, very dubious occupation to expect elastic deformations in hot, semi-molten rocks at depths of hundreds of kilometers. On the other hand, seismologists represented by Mr. C. Frohlich of the University of Austin, Texas continue to cling stubbornly to the idea of mythical elastic stresses of the rocks, allegedly occurring in the Wadati-Benioff subduction zones. As a result of the dispute between the two schools, deep-focus earthquakes turned out to be an established scientific fact, therefore, Mr. Jeffries and Co. were wrong. But Mr. Frohlich and Co turned out to be wrong, for the deep-focus earthquakes that occurred outside the subduction zones completely and without any hope destroyed the idea of elastic deformations at great depths. Is the intrigue preserved? Not! In fact, the process of forming a deep focus earthquake is simple and is a classic hydraulic shock. From hydrodynamics, it is known that where fluid flows through pipelines, in our case, magma flows through cracks in the Earth’s mantle, there have always been, are and will happen, hydraulic shocks.

At transition zone depths in subduction zones, deep-focus earthquakes (300–690 km depth) are thought to be associated with faulting that arises from phase transformations. In order to test the viability of this mechanism experimentally, an investigation was conducted on fayalite at high pressure, P, and high temperature, T, under deviatoric stress in order to initiate transformational faulting. Experiments were performed in a 3000-ton multi-anvil press using an 18/11 octahedral cell with 6 piezoelectric transducers mounted on the rear side of the anvils to monitor acoustic activity in situ. Acoustic emission (AE) signals were collected at a sampling rate of 40 MHz using a triggered system and a data buffer for capturing full waveforms of AE events. The use of multiple transducers distributed in a micro-seismic array allowed for events to be located within the sample based on the arrival time of signals and non-linear least squares inversion techniques. Uncertainty in location estimates were on the order of ~1 mm. The system was tested by comparing the contrasting mechanical properties of quartz beads and AgCl samples. The multi-anvil apparatus constitutes an inherently noisy environment both acoustically and electrically, therefore methods of noise reduction were developed. Results of AE experiments on Fe2SiO4 under high pressure (3.7–9.2 GPa) and high temperature (673–1273 K) conditions in the spinel stability field showed acoustic events that locate within or within 1σ of the sample in five experiments defined by the P,T envelope P = 3.9–8.4 GPa and T = 748–923 K. Optical and scanning electron microscopy of the recovered specimens displayed conjugated faulting associated with transition from fayalite (olivine structure) to ahrensite (spinel structure) and microstructural analysis revealed microstructural morphology identical to that found in similar faulting experiments on Mg2GeO4. This is the first time an olivine→spinel structured transition in a silicate mineral has demonstrated macroscopic faulting and associated microstructures, as well as acoustic activity, under conditions that nominally promote plastic deformation.

The 2014 M w 7.9 Rat Islands intermediate-depth (∼107 km) earthquake is studied using seismic observations at teleseismic distances. The nodal plane with a steep-dip angle (87°) and a strike of 308° inferred from the Global CMT solution is selected as the causative fault plane because of the consistency with the distribution of the aftershocks relocated using the Joint Hypocenter Determination method. Its slip history is constrained using broadband P and SH waveforms and long-period (3–6 mHz) seismic waves in vertical and transverse components. The rupture process associated with this plane shows a rupture propagating at ∼2 km/s. The rupture essentially breaks one large slip patch (peak slip amplitude of 3.5 m) located 30 km west of the hypocenter. The size of this slip patch extends from 80 to 140 km depth, which would be the largest depth extent of earthquake slip ever documented. We show that this earthquake breaks the entire cold core of the subducting slab defined by the depth of the 600–650 °C isotherm. In addition, the lateral extent of the rupture appears to be bounded by the heterogeneities of subducting slab. Our results further indicate that, along this segment of Aleutian Arc, the intermediate-depth earthquakes might contribute to the westward transporting motion of Rat block. Lastly, the slip distribution of this earthquake as well as that of two other well studied M w > 7.5 intermediate-depth earthquakes suggests that the coldest portion of the subducting slab, outlined by the double Wadati–Benioff zone, is seismic during large intermediate-depth earthquakes. Such a result is important for estimating the seismic hazard of future large intermediate-depth earthquakes.

The existence of earthquakes within continental lithospheric mantle remains a highly controversial topic. Here, we present a detailed set of seismological analyses confirming the occurrence of a mantle earthquake beneath the Wind River Range of central Wyoming. Combining regional waveform inversion with the analysis of the delay and relative amplitudes of teleseismically-observed depth phases, we demonstrate that the 2013 Wind River earthquake – a M W 4.7 highly-oblique thrust-faulting event – occurred at 75 ± 8 km , well beneath the base of the crust. The magnitude, mechanism, and location of this earthquake suggest that it represents simple brittle failure at relatively high temperatures within the mantle lithosphere, as a result of tectonic, rather than magmatic, processes.

We apply a multiple source inversion method to systematically study the source processes of 25 large deep-focus (depth >400 km) earthquakes with Mw > 7.0 from 1994 to 2012, based on waveform modeling of P, pP, SH and sSH wave data. The earthquakes are classified into three categories based on spatial distributions and focal mechanisms of the inferred sub-events: 1) category one, with non-planar distribution and variable focal mechanisms of sub-events, represented by the 1994 Mw 8.2 Bolivia earthquake and the 2013 Mw 8.3 Okhotsk earthquake; 2) category two, with planar distribution but focal mechanisms inconsistent with the plane, including eighteen earthquakes; and 3) category three, with planar distribution and focal mechanisms consistent with the plane, including six earthquakes. We discuss possible physical mechanisms for earthquakes in each category in the context of plane rupture, transformational faulting and shear thermal instability. We suggest that the inferred source processes of large deep-focus earthquakes can be best interpreted by cascading failure of shear thermal instabilities in pre-existing weak zones, with the perturbation of stress generated by a shear instability triggering another and focal mechanisms of the sub-events controlled by orientations of the pre-existing weak zones. The proposed mechanism can also explain the observed great variability of focal mechanisms, the presence of large values of CLVD (Compensated Linear Vector Dipole) and the super-shear rupture of deep-focus earthquakes in the previous studies. In addition, our studies suggest existence of relationships of seismic moment ∼ (source duration) 3 and moment ∼ (source dimension) 3 in large deep-focus earthquakes.

The great Sanhe-Pinggu M8 earthquake occurred in 1679 was the largest surface rupture event recorded in history in the northern part of North China plain. This study determines the fault geometry of this earthquake by inverting seismological data of present-day moderate-small earthquakes in the focal area. We relocated those earthquakes with the double-difference method. Based on the assumption that clustered small earthquakes often occur in the vicinity of fault plane of large earthquake, and referring to the morphology of the long axis of the isoseismal line obtained by the predecessors, we selected a strip-shaped zone from the relocated earthquake catalog in the period from 1980 to 2009 to invert fault plane parameters of this earthquake. The inversion results are as follows: the strike is 38.23°, the dip angle is 82.54°, the slip angle is −156.08°, the fault length is about 80 km, the lower-boundary depth is about 23 km and the buried depth of upper boundary is about 3 km. This shows that the seismogenic fault is a NNE-trending normal dip-slip fault, southeast wall downward and northwest wall uplift, with the right-lateral strike-slip component. Moreover, the surface rupture zone, intensity distribution of the earthquake and seismic-wave velocity profile in the focal area all verified our study result.

From 2011 to 2012, we conducted marine seismic surveys using a multichannel seismic reflection (MCS) system and ocean bottom seismometers (OBSs) in and around the hypocentral region of the 1983 Nihonkai–Chubu earthquake. The survey covered the area from the Japan coast of the Japan Sea to the Yamato Basin and the Japan Basin. We investigated the relationship between the Nihonkai–Chubu earthquake and the crustal structure obtained from seismic reflection imaging and the P-wave velocity structure near the hypocentral region. The focus in the present paper is on seismic data that were acquired using both the MCS system and OBSs. Based on the results, the crustal structure of the Japan Sea around the hypocentral region of the 1983 Nihonkai–Chubu earthquake is divided into three types: island arc crust, thick oceanic crust, and oceanic crust. The Nihonkai–Chubu earthquake occurred near the boundary between the thick oceanic crust and the island arc crust. There is a strong possibility that the east-dipping reflector detected in the crust was the source fault of the Nihonkai–Chubu earthquake. In addition, the location of anticlines formed by the east-dipping reverse fault occurring in the boundary of the crustal structure associated with the source fault of the Nihonkai–Chubu earthquake was confirmed in the range of latitude from 39 ° 20 ′ to 41 ° 15 ′ N , although they were not found south of this segment. There is also a concentrated area of reverse faults and folds approximately 100 km west of the source region of the Nihonkai–Chubu earthquake. This area is located near the boundary between the thick oceanic crust and the oceanic crust.

The physical mechanism of deep earthquakes (depth >300 km) remains enigmatic, partly because their rupture dimensions are difficult to estimate due to their low aftershock productivity and absence of geodetic or surface rupture observations. The two largest deep earthquakes, the recent Great 2013 Sea of Okhotsk earthquake (M 8.3, depth 607 km) and the Great 1994 Bolivia earthquake (M 8.3, depth 637 km), together provide a unique opportunity to compare their rupture patterns in detail. Here we extend a travel-time sub-event location method to perform full teleseismic P-waveform inversion. This new method allows us to explain the observed broadband records with a set of sub-events whose model parameters are robustly constrained without smoothing. We find that while the Okhotsk event is mostly unilateral, rupturing 90 km along strike with a velocity over 4 km/s, the Bolivia earthquake ruptured about half this distance at a slow velocity (about 1.5 km/s) and displayed a major change in rupture direction. We explain the observed differences between the two earthquakes as resulting from two fundamentally different faulting mechanisms in slabs with different thermal states. Phase transformational faulting is inferred to occur inside the metastable olivine wedge within cold slab cores whereas shear melting occurs inside warm slabs once triggered.

An earthquake of magnitude 7.3 Mw occurred on 18th January 2011 in Southwestern Pakistan, Baluchistan province (Dalbadin Region). The area has complex tectonics due to interaction of Indian, Eurasian and Arabian plates. Both thrust and strike slip earthquakes are dominant in this region with minor, localized normal faulting events. This earthquake under consideration (Dalbadin Earthquake) posed constraints in depth and focal parameters due to lack of data for evaluation of parameters from Pakistan, Iran or Afghanistan region. Normal faulting mechanism has been proposed by many researchers for this earthquake. In the present study the earthquake was relocated using the technique of travel time residuals. Relocated coordinates and depth were utilized to calculate the focal mechanism solution with outcome of a dominant strike slip mechanism, which is contrary to normal faulting. Relocated coordinates and resulting mechanism are more reliable than many reporting agencies as evaluation in this study is augmented by data from local seismic monitoring network of Pakistan. The tectonics in the area is governed by active subduction along the Makran Subduction Zone. This particular earthquake has strike slip mechanism due to breaking of subducting oceanic plate. This earthquake is located where oceanic lithosphere is subducting along with relative movements between Lut and Helmand blocks. Magnitude of this event i.e. Mw = 7.3, re evaluated depth and a previous study of mechanism of earthquake in same region ( Shafiq et al., 2011 ) also supports the strike slip movement.

The 2011 Tohoku earthquake occurred via subduction of an oceanic tectonic plate, where we had no historical record of this size of earthquake. We have examined shipboard geophysical observations conducted above the rupture area for any indications before the earthquake. The location of the largest slip along the fault surface coincides with gravity changes measured at the sea surface separated by three months all before the earthquake. This gravity change cannot be explained by the local gravity gradient due to bathymetry along the cruise tracks. The measured gravity changes can be interpreted either as an uplift of ocean bottom or a density increase along the fault surface of which the time scale of evolution is about 3 months. This observation may constrain the physical mechanism by which this large and slow slip can be generated along this fault.

A M s6.4 earthquake occurred at Linkou country, Heilongjiang Province (44.8°N, 129.9°E) on January 2, 2016 at a depth of 580 km. Pre-seismic gravity anomalies obtained at a 1 Hz sampling rate from Crustal Movement Observation Network of China (CMONOC) are analyzed after the earthquake. The results show that: (1) different from previous studies, both pre-seismic amplitude perturbation and co-seismic amplitude perturbation are not critical inversely proportional to epicentral distance; (2) unlike shallow earthquake, the pre-seismic and co-seismic amplitude perturbation of gravity illustrate synchronous spatial variation characters with decrease of epicentral distance for Linkou earthquake. This may because Linkou earthquake is a deep earthquake and occurred in Pacific Plate subduction zone; (3) compared to basement and semi-basement, cave can provide a better observation environment for gPhone gravimeter to detect pre-seismic gravity anomalies.

We integrated a regional tomography model, 20-year seismicity and earthquake focal mechanisms of the 4 March 2010 M L 6.4 Jiashian earthquake source region to delineate the seismogenic structure and mechanics of a relatively rare damaging inland earthquake that occurred in the southwestern fold-and-thrust belt of Taiwan. The main shock of the Jiashian earthquake occurred at a depth of 23 km beneath the slate belt of the southern Central Range with a sinistral thrust mechanism. Based on seismic tomography and seismicity, the Jiashian earthquake sequence occurred in the Chishan transfer fault zone, which we interpret to be located near the transition zone between the subducted Eurasian plate to the south and the arc–continent Taiwan collision to the north. The distribution of the Jiashian aftershocks define a WNW-ESE striking fault plane that dips 30–40° NNE which is consistent with the optimal fault plane of the GPS-derived coseismic slip model. Focal mechanisms and inverted stress results of the earthquake sequence indicate both thrust and strike–slip motions with the maximum compressive stress at nearly 90° to the regional compressive stress and thus sub-parallel to the strike of the Taiwan orogen. We propose that the 2010 Jiashian earthquake resulted from rupturing of a buried oblique fault within the Chishan transfer fault zone at the subduction–collision transition zone in southern Taiwan. The orogen-parallel P-axis of the 2010 Jiashian earthquake represents collision-related lateral compression, although a local stress deviation is also possible.

article The physical mechanism of intermediate-depth earthquakes is still under debate. In contrast to conditions in the crust and shallow lithosphere, at temperatures and pressures corresponding to depths >50 km, rocks ought to yield by creep or flow rather than brittle failure. Some physical process has to enable brittle or brittle-like failure for intermediate-depth earthquakes. The two leading candidates for that are dehydration embrittlement and ther- mal shear runaway. Given their great depth, intermediate-depth earthquake processes can't be observed direct- ly.Insteadwe must relyona combination ofseismologyand thestudy of laboratory analogs tounderstand them. Earthquake nests are regions of highly concentrated seismicity that are isolated from nearby activity. In this paper we focus on three intermediate-depth earthquake nests — Vrancea, Hindu Kush and Bucaramanga, and what they reveal about the mechanics of intermediate-depth earthquakes. We review published studies of tectonic setting, focal mechanisms, precise earthquake locations and earthquake source physics at these locations, with an emphasis on the Bucaramanga nest. All three nests are associated with subducting litho- sphere and at least two of the nests have consistently larger stress drops compared to shallow seismicity. In contrast, the Bucaramanga nest has a larger b-value, larger variability of focal mechanisms and shows no evidence of aftershock sequences unlike the other two. We also report for the first time finding a signifi- cant number of repeating earthquakes, some with reverse polarity.

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.

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.

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.

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.

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.

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.

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.

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.

Quantitative estimates of the crack density (ϵ), saturation rate (ξ) and porosity (ψ) parameters from seismic velocities (Vp, Vs) and Poisson’s ratio (σ) in the 2001 Bhuj earthquake area in western India indicate that the 2001 Bhuj earthquake hypocenter is associated with high-ϵ, high-ξ and high-ψ in the depth range of about 23–28 km, extending 15–30 km laterally. These anomalies may be due to a fluid-filled, fractured rock matrix, which might have contributed to trigger the 2001 Bhuj earthquake in the intraplate, stable continental region of the Indian Peninsula. This feature is similar to that of the 1995 Kobe earthquake [D. Zhao et al., Science 274 (1996) 1891–1894]. High-ψ areas are generally consistent with high-ϵ areas, but high-ξ areas have a wider distribution, indicating that micro-cracks exist in more localized areas within the crust, and that permeation of fluids in the hypocenter zone might have occurred extensively through the intergranular and fractured pores due to hidden intersecting fault geometry. Here we suggest the possibility that earthquake occurrence is closely related to in situ material heterogeneities, rather than stress conditions alone.

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.

The study addresses the depth distribution of stresses in the Kamchatka Wadati-Benioff zone, based on detailed geometry of the deep seismically active structures, homogeneous data of earthquake focal mechanisms and the inverse technique by Gephart and Forsyth (Gephart, J., Forsyth, D., 1984. An improved method for determining the regional stress tensor using earthquake focal mechanism data: application to the San Fernando earthquake sequence. J. Geophys. Res. 89, 9305–9320). The used data set includes 205 CMT Harvard solutions for earthquakes which belong to the Wadati–Benioff zone and 17 for a deep-seated subduction segment. The inverse technique used allows for determining the best fit principal stress directions σ1, σ2, σ3 and the ratio R=σ2−σ1/σ3−σ1 in several depth intervals in the Wadati–Benioff zone and for the deep-seated slab segment considered as a single body. The depth ranges 0–40 and 41–60 km are characterized by maximum compressive stress σ1 which is essentially horizontal and trends SE, indicating the NW–SE convergence between the Pacific and Eurasian lithospheric plates. The minimum compression σ3 in the above depth ranges is down-dipping due to the slab pull. The stress tensors obtained for the depth layer 61–80 km and K2, the lower part of the subduction zone, indicate a heterogeneous stress field. Further analysis of the above sub-volumes showed that the stress heterogeneity is attributed to different stress regimes in the frontal and in-slab parts of the Wadati–Benioff zone: the frontal part is under slab-parallel compression and extension which dips at approximately 40° to SE; the in-slab part is subjected to down-dipping to slab-parallel extension and shallow dipping to SE compression. This stress distribution suggests unbending of the subducting plate at depths greater than 61 km. The stresses in the depth range 81–90 km are of similar orientations to those in the in-slab parts of 61–80 and 91–230 km but of different dips. The deep-seated slab segment is characterized by compression which is parallel to the dip direction of this volume and by nearly horizontal extension of SE trend. These results show that the main geodynamic forces which drive the presently active processes at different depths in the Kamchatka and Northern Kurille region are the NW-SE convergence between the Eurasian and Pacific plates (ridge push) causing the observed almost horizontal compression of SE trend at 0–60 km depth, the slab pull, unbending forces at depths 61–230 km causing the observed slab-parallel compression in the frontal part of the Wadati–Benioff zone and down-dip to slab-parallel extension in the in-slab part.

The test and application of the time and magnitude predictable model in the subduction zones around the circum-Pacific belt is performed. Each subduction zone was separated in several seismogenic regions on the basis of tectonic and geometrical features of the subducting lithosphere and for three different depth ranges. The data derived from these seismogenic regions were used in order to check the suitability of the regional time and magnitude predictable model, as this model was defined for shallow events. The results obtained in the present study confirm that this model appropriately describes the behavior of deep seismicity in these regions, since a positive ( c =0.31±0.003) and a negative ( C =−0.13±0.047) dependence were found for the logarithm of the interevent times and the magnitude of following mainshock, respectively, on the magnitude of the preceding mainshock. Moreover, the slip-predictable model was rejected, since a negative dependence was found ( E =−0.19) between the interevent times and the magnitudes of the following mainshock. Probabilities of occurrence of the future strong (M≥7.0) events in each seismogenic region have been determined for the next decade conditioned on the previous occurrence of such events. The magnitudes of the expected events have also been estimated. Statistical tests for the estimated probabilities exhibit a good correlation with the occurrence rate of such events. Furthermore, the statistical significance of the model has been studied through additional tests. The results of this study demonstrate the validity of the regional time and magnitude predictable model in the areas examined.

Characteristics of two recent moderate earthquakes from India (1993 Killari, Mw 6.1 and 1997 Jabalpur, Mw 5.8) are presented in this paper to illustrate the influence of geologic and tectonic setting on the nature of seismicity in stable continental regions (SCRs). The Killari event, which occurred in the middle of a cratonic region of low background seismicity, long recurrence interval and poorly developed neotectonic features, may be qualified as a common type of SCR event. This earthquake is also characterized by a shallow focal depth and a long sequence of aftershocks. In contrast, the Jabalpur earthquake shows a spatial association with a well-defined tectonic structure with significant background seismicity. A remarkable feature of this earthquake is its deep focus, not commonly observed in SCR seismicity. This event was associated with fewer aftershocks unlike most other SCR earthquakes. Analogous events are found in other shield regions as well. Based on their general characteristics, a broad classification of moderate SCR earthquakes is attempted here. Two broad groups have been identified in this study: (1) a set of deep focus earthquakes that are related to intracratonic paleorifts, and (2) shallow-focus earthquakes associated with discrete faults in the less deformed Precambrian terrains. A synthesis of earthquake data from different geologic environments will improve our understanding of the seismogenesis in the shield regions. © 1999 Elsevier Science B.V. All rights reserved.

Both seismological observations and mineral physics experiments suggest that old and cold subducting slabs might contain a wedge of metastable olivine (`MO'). Dynamically consistent models of subducting slabs, in which the kinetics of the olivine to spinel (wadsleyite or ringwoodite) transition is treated in a simplified way, are carried out to evaluate the buoyancy effect of MO on the subduction velocity. Assuming slab thicknesses according to the cooling half-space model, we find that MO forms in slabs with ages greater than approximately 70 Myr, and that a significant slowing down of subduction velocity occurs for ages greater than 100 Myr. Because a decrease in the subduction velocity for old lithospheric ages is not observed, our models suggest that the amount of MO in slabs is less than approximately 5000 km2 in cross section, and that for old lithosphere the cooling half-space model is not applicable.

The 1932 deep Fiji earthquake is studied from the point of view of comparing it to the recent large events in the transition zone. Spectral analysis and body-wave modeling suggest a moment of 3.4 × 10 27 dyn cm (within a multiplicative or divisive factor of 1.5), which makes it comparable to, but not significantly larger than, the great 1994 earthquake, 770 km to the north. Its seismic moment remains well below that of the largest deep shocks (1994 in Bolivia or 1970 in Colombia). Relocation efforts show that the earthquake took place on the front edge of the Wadati-Benioff Zone, in an area where the latter features a complex geometry involving significant warping. The mechanism of the earthquake involves down-dip compression and can be readily explained in terms of the prevailing large-scale stresses in the slabs.

We investigate the role of grain-size reduction during the olivine-spinal transformation on rheological properties of subducting slabs on the basis of a scaling model for micro structural development during nucleation and growth. In this model, the size of spinal grains nucleating at olivine grain-boundaries is controlled by the relative rates of nucleation and growth, taking into account the impingement through the collision of grains due to growth. When the volume fraction of spinal reaches a certain threshold value (critical volume fraction ~ 1 to 10 %, depending on the P,T conditions in the slab), the new phase will form a continuous film and will significantly reduce the strength of the two-phase aggregate if spinal grain-size is small. The size of spinal grains delta 0 at this stage is calculated and is shown to be highly sensitive to temperature. At relatively high temperatures (T > 1000 K), delta_0 shows an Arrhenius-type dependence on temperature, that is delta_0 ~ exp(-E* /RT) with E* ~ 400 kJ/mol, whereas a more complicated temperature dependence is found at low temperatures (T < 900 K), where a grain-size reduction of up to 4 orders in magnitude is possible. Strength profiles of slabs due to combined effects of temperatures and of grain-size reduction are calculated. It is shown that: (1) the strength of slabs will have an unusual temperature dependence through the temperature dependence of grain-size; and (2) a subducting slab has a complicated rheological structure containing a weak region below the tip of a metastable olivine-bearing wedge in a cold slab. Possible implications of these anomalous rheological structures of slabs on the dynamics of subduction are discussed including the mechanisms of deep earthquakes.

Using Wigner-distribution estimation, the high-frequency fall-off of source spectra was estimated. The technique was applied to the analysis of the source spectra of the Vladivostok M = 6.4 deep-focus earthquake of 22 July 1994 and the Northern Japan Sea, M = 5.7 deep-focus earthquake of 22 July 1994. It is shown that the high-frequency fall-off of the source spectra of these two earthquakes may be characterized by two parts. Between the first corner frequency f1 and the second corner frequency f2, the high-frequency fall-off of the source spectra can be represented by f−5; above f2, the high-frequency fall-off can be represented by f−2.

The effects of a strong non-linear thermo-kinetic coupling in the olivine-spinel transition are investigated with a 2D model for fast-subducting slabs. The thermo-kinetic coupling problem is solved in the Lagrangian framework by the differential algebraic method and by using experimental input parameters. Narrow regions with metastable olivine are found in fast and cold slabs down to the 660 km discontinuity, while the thermo-kinetic coupling reduces the depth and width of the zone where olivine and spinel coexist. In contrast to recent kinetic models, both latent heat release and heat diffusion place important constraints on the structure of the metastable wedge. Both processes trigger the ongoing phase transformation in the cold slab interior. A thermal instability over a short timescale (10 4 –10 5 yr) can occur due to a positive thermo-kinetic feedback. This effect is enhanced at greater depths. The depth of the metastable wedge is found to be extremely sensitive to certain geophysical and thermodynamic parameters such as the lithospheric basal temperature, slab width and the activation energy for diffusion. Variations in these parameters within the experimental uncertainties can cause significant changes in the depth of the metastable wedge. Hence predictions about the exact position of the metastable olivine wedge are rather difficult. We propose that deep focus earthquakes may occur due to a thermal runaway caused by shear heating, while the thermal instability is enhanced by the latent heat release associated with the olivine-spinel phase transition.

Various types of energy can be stored in metastable materials at high pressure. If appropriate forms of energy released at a reasonable fast rate (⩽ 10 2 ) become available, some of the released energy must radiate elastic wave in a solid body. The energy efficiency in earthquake production for an earthquake having a magnitude of 7 or 8 was found to be 0.1%. “Earthquakes” caused by metastable phase transformations occurring under high pressure and temperature conditions have recently been confirmed in the laboratory by studies of the α → γ phase transformation in olivine. It is proposed that most earthquakes occurring at depths greater than ca 50 km inside the Earth represent the loci of the kinetic boundaries of the various phase transformations known to take place in the mantle silicates. Even the peculiar wedge-shaped double seismic zone observed between 70 and 180 km in the intermediate-focus earthquakes and the deep double seismic zone observed between 300 and 450 km can be interpreted in terms of this model.

To explore a possible mechanism of deep earthquakes, this paper analyzes the unstable propagation of a stress-induced phase transition which is initiated in a homogeneous stress field. This Stephen problem is formulated as an initial-value problem for the phase boundary, and the driving force of the boundary is computed by using the solution of the boundary-value problem for a partially transformed material. The propagation of the phase transition under uniform pressure is numerically simulated. It is shown that (1) under lower pressure, the transition is terminated at a certain size, but it can propagate unstably when an initially transformed region is sufficiently large; and (2) when the pressure attains a critical value, the propagation becomes unstable, and goes in a particular direction depending on the initial shape. These results confirm the possibility of the unstable propagation of phase transition, and provide a theoretical basis for the hypothesis that the phase transition of a mantle material can trigger a deep earthquake.

A waveform inversion scheme is presented for multiple shocks. An earthquake source is modeled as a sequence of point sources with different focal mechanisms. Each point source is defined by an onset time, relative spatial coordinates, and a moment tensor. All the point sources are allowed to occur at only grid points with the same interval on a single flat plane and to have the same trapezoidal time history. Unknown source parameters are obtained by an iterative procedure. Each iteration step consists of two substeps; first, onset times and relative spatial coordinates of subevents with respect to a reference point are determined. Second, seismic moment-tensors of the subevents are obtained by a conventional linear moment-tensor inversion. A statistical test of a change in the focal mechanism during the source process is made using confidence ellipses of principal stress axes for each subevent. We apply the inversion to WWSSN teleseismic long-period P waveforms from a deep focus earthquake of the 1972 Banda Sea (mb = 6.6). The result suggests that the earthquake consisted of two major subevents with a time interval of 7.5 s, and that the main rupture, connecting the two subevents, propagated westward by approximately 20 km. The best double-couple seismic moment is 5.7 × 1018 Nm and 2.0 × 1018 Nm for the first and the second subevents, respectively. Differences between directions of the P- and T-axes of the two major subevents may suggest that the main rupture of the earthquake propagaged along a northward convex fault surface.

The relevance of the anticrack faulting mechanism to the origin of deep-focus earthquakes depends critically on the physical processes operating during fault generation and movement. We show here that the resistance to sliding on fault zones produced by this mechanism in Mg2GeO4 depends only weakly on confining pressure, but strongly on sliding rate. Both of these characteristics are inconsistent with friction. The microstructures of the fault zones and the crystal-plastic rheology of the constituent phases indicate that sliding must occur primarily by grain-boundary sliding. Previously, we reported that anticrack faulting is accompanied by elastic radiation of energy (acoustic emissions), and that it can operate during the α → β transformation in (Mg,Fe)2SiO4 at the pressures and temperatures at which deep earthquakes occur. The present results and the recent demonstration that metastable olivine is present in the subducting slab beneath Japan provide additional support for the anticrack theory of deep-focus earthquakes.

Analysis of long-period seismic waves suggests that non-double couple components in source moment tensors of intermediate-depth and deep earthquakes are significant and appear to respond to the state of predominant strain release within slabs. The strain regime is partially or fully induced by the sources themselves or by slab structures near the sources. We observe consistency in the non-double couple components from three different seismic wave inversions. Among 21 earthquakes studied, 17 events have the same sign in the three non-double couple components, and the signs show a correlation with the state of strain release within the slabs. The effects of unmodeled propagation structure and instability of the inversion procedure are unlikely to be responsible for the consistent non-double couple components because the suites of seismic waves traverse different paths and the various inversion schemes have the different resolvabilities of the moment tensors.

The joint hypocentral determination method is used to relocate deep seismicity reported in the International Seismological Center catalog for earthquakes deeper than 400 km in the Honshu, Bonin, Mariannas, Java, Banda, and South America subduction zones. Each deep seismic zone is found to display planar features of seismicity parallel to the Harvard centroid-moment tensor nodal planes, which are identified as planes of shear failure. The sense of displacement on these planes is one of resistance to deeper penetration.

At a time when physical models for the occurrence of deep earthquakes are being proposed, capable of explaining their fault-like source process, it is important that seismology characterizes all elements relevant to the physical occurrence of shear at depth. To identify the characteristic modes of deformation of the deep subducted lithosphere in the Tonga region, families of events with similar faulting orientation were classified and mapped on the Benioff zone using 133 centroid-moment tensor solutions for all events with M 0 ⩾ 3 × 10 24 dyn cm since 1977. The existence of two principal modes of deformation acting on the subducted lithosphere is confirmed and detailed in this present study: (1) The down-dip compression associated with the subducting flow is a ubiquitous factor in the deformation of the whole lithosphere, with families of focal mechanisms whose orientation follows strictly the local geometry of the Benioff zone. (2) A lateral shear involves the whole deep lithosphere, associated with the relative motion between surface plates and deep mantle. In addition, observations are made of the presence of families of events with down-dip extensional or strike-slip mechanisms, mostly located along the western margin of the seismicity, where the lithosphere has already undergone the major compressional deformation and has rotated to a more horizontal posture. The transition from the down-dip compression to the new stress state takes place over a length of no more than 100 km, which corresponds to the local geometry of the Benioff zone. The overlapping occurrence of different modes of deformation, in space and time, indicates that the physical mechanisms proposed for the generation of deep earthquakes should incorporate the concept that deep faults or their physical equivalent at the 600-km depth conditions are stable features of a deep subducted lithosphere, whose failure behaviour sometimes obeys to a highly heterogeneous local state of stress.

Post-seismic behavior induced by a deep-focus earthquake in subduction zone is closely investigated numerically on realistic upper-mantle models, taking into account the pressure and temperature conditions. For this purpose, the post-seismic stress changes expected from the deep event are calculated by a two-dimensional finite element method (2D FEM), assuming newtonian and power-law creep behaviors as justified by high P – T experiments. Several cases are investigated in detail for various fault depths and thermal structures. Two cases are considered for the location of the fault: one when an earthquake occurs within oceanic crust overlying a subducting plate, and the other when an earthquake is located within the plate. We also examine the case when the temperature distribution within the plate is lower by about 200°C on average than that in the upper mantle at the same depth. In addition, the post-seismic behavior due to a deep-focus event in a petrological thermal model of a mantle wedge is investigated. The results show that the post-seismic behavior associated with the deep event occurring within the oceanic crust indicates a pattern of relaxation of the coseismic stress changes. Lower temperatures inside the plate would suppress the post-seismic displacements and stress changes effectively. The petrological model also has some influence on the post-seismic flow pattern in the mantle wedge. We also study a possible correlation between deep-focus earthquakes and a shallow large event which has been detected in the northwestern Pacific region. However, effective mechanisms to explain the phenomenon could not be identified from the viewpoint of post-seismic behavior caused by power-law creep.

This paper utilizes a statistical technique to investigate the global seismic energy distribution in latitude. The energy distribution in latitude has two peaks: one in each of the Northern and Southern Hemispheres with an axis of symmetry at about 10°N. The two peaks for shallow earthquakes have an interval of 90°, while those for intermediate and deep ones concentrate in low latitudes. The twin peak distribution in energy of intermediate and deep earthquakes becomes more apparent with increasing depth. Almost no energy is released in the North and South pole areas. The energy released in the Northern Hemisphere is about 30% greater than that in the Southern Hemisphere, except for the focal depths from 100 to 150 km. Through a study of the shear stress component caused by the corresponding long-period lunisolar potential, it is found that the shear stress distribution due to lunisolar constant flattering wave (M0S0) coincides with some of the above phenomena. We conclude that the long-period tidal shear stress may be one of the possible causes for the seismic energy release; or a triggering effect.

The spatial pattern of the epicentral distribution in the northwest Chubu district of Honshu Island, Japan changed drastically after the occurrence of the 1948 Fukui earthquake ( M = 7.1 ). The change can be explained by invoking that the WNW-ESE trending seismic belt which contains the focal region of the Fukui earthquake, was activated, and that the activity along another seismic belt trending NE-SW which had been conspicuous before the Fukui earthquake, disappeared. A notable feature is that large earthquakes occurred successively along the newly activated seismic belt migrating to the east. The 1984 Nagano-ken Seibu earthquake ( M = 6.8 ) occurred on the extension of that migrational activity. The focal region of the Nagano-ken Seibu earthquake is situated at a distinct inflection point in the seismic zone which was activated after the Fukui earthquake. The spatial pattern of the seismic zone is very similar to the distribution of Quaternary volcanoes in that region, and the same pattern is also recognized in the Bouguer anomaly. These features suggest that the seismic belt is a weak zone in the crust corresponding to the boundary of the tectonic blocks. The focal region of the Nagano-ken Seibu earthquake had been very quiet over a period of 10 years, with the seismic activity subsequently resuming several years before the Nagano-ken Seibu earthquake presenting two active centers in the seismic gap, near one of which a large destructive earthquake had occurred in 1906. It seems that the occurrence of the Nagano-ken Seibu earthquake could have been predicted if the characteristics of the seismic activity after the Fukui earthquake and the tectonic features in the region had been correctly interpreted.

A numerical study of stress within descending slabs beneath island arcs provides a general interpretation for the occurrence of deep- and intermediate-focus earthquakes. The computation of stress due to several factors, which have been proposed for the interpretation of the double-planed seismicity of intermediate-depth events, shows that thermal expansion and olivine-spinel phase change play an important role in earthquake generation. The stress field due to the combination of these two causes gives the following results; in the depth range 50–300 km, the compressional and tensional stress predominates in the upper and central part of the slab respectively. Maximum shear stress amounting to 1.6 GPa in compressional and 0.4 GPa in tensional fields occurs at a depth of 200 km. The stress is generally very small at around 300–350 km. Below 400 km, compressional stress exists at the central part of the slab and tensional stress appears near the upper and lower surfaces of the slab. Maximum shear stress is 2.8 GPa in compression and 1.6 GPa in tension at a depth of 500 km. The principal axes in these high stress fields align almost parallel to the dip direction of the slab. These stress features explain well such observed seismological facts beneath island arcs as the seismicity-depth relationship, planer configuration of seismic zone, existence or non-existence of double-planed seismicity and focal mechanisms of intermediate and deep focus earthquakes.

The small amplitudes of surface waves for intermediate and deep-focus earthquakes lead to too low values of the surface-wave magnitudes. On the basis of available material, average magnitude corrections are deduced, which increase with focal depth and amount to 0.1–0.5 for focal depths of 50 100 km and to 0.5–0.7 for depths of 100–700 km.

The Earth is generally as inhomogeneous as it is anisotropic. Nevertheless, one often comes across interpretations based on the assumption that the Earth is inhomogeneous and isotropic, or that the Earth is homogeneous and anisotropic. The present paper attempts to shed some light on the problem of model-type selection by using the linearization approach to compute the travel times in 2-D inhomogeneous and slightly anisotropic media. A computer program was developed for such media, with the linearization approach applied only to the anisotropic part. Some numerical results usable for typical Dss of models are presented. These are used to demonstrate how the effects of anisotropy and those of inhomogeneity influence the resulting travel times (1-D isotropic model, 1-D anisotropic model, 2-D inhomogeneous isotropic model, and 2-D inhomogeneous anisotropic model). Estimating their extent in realistic (but model) situations should help to distinguish the effects of inhomogeneity from those of anisotropy in practice.

We have used a data set of earthquakes whose mechanisms have been studied, here or elsewhere, by moment tensor inversion, to confirm some earlier ideas concerning the state of stress in subducting slabs. The moment tensor method offers the advantages of speed and objectivity over earlier methods, usually based exclusively on first-motion data. Deep earthquakes (below 300–400 km) tend to have compression axes aligned with the dip of the subducting slab. The behavior of intermediate earthquakes (from 70 to 300 km) is less clear. There is some evidence for down-dip tension, but the alignment is not as obvious as it is in the case of the compression axes of deep events. Tonga shows a tendency toward down-dip compression at intermediate depths, but other deep-extending slabs, as well as shallow-extending slabs (slabs with maximum depths shallower than 300 km), seem to be closer to down-dip tension.