Search

The Tonga subduction zone in the south-west Pacific is the fastest convergent plate boundary in the world with the most active mantle seismicity. This zone shows unique tectonic features including Samoan volcanic lineament of plume-driven origin near the northern rim of the Tonga subducting slab. The proximity of the Samoa hotspot to the slab is enigmatic and invokes debates on interactions between the Samoa plume and the Tonga subduction. Based on long-term observations of intermediate and deep-focus Tonga earthquakes reported in the Global Centroid Moment Tensor (CMT) catalog, we provide novel detailed imaging of this region. Accurate traveltime residua of the P- and S-waves recorded at two nearby seismic stations of the Global Seismographic Network are inverted for the P- and S-wave velocities and their ratio and reveal their pronounced lateral variations. In particular, they differ for the southern and northern parts of the Tonga subduction region. While no distinct anomalies are detected in the southern Tonga segment, striking low-velocity anomalies associated with a high Vp/Vs ratio are observed in the northern Tonga segment close to the Samoa plume. These anomalies spread through the whole upper mantle down to depths of ~ 600 km. Together with the fast extension of the northern back-arc Lau Basin, slab deformation and geochemical enrichment in the northern Tonga region, they trace deep-seated magmatic processes and evidence an interaction of the Tonga subduction with the Samoa plume.

We described the 2014 January 17 earthquake (M w = 4.3) occurred on the Siberian platform in the area of sublongitudinal part of the Angara river in the zone of possible influence of two large reservoirs—Ust-Ilimsk and Boguchan. This is the first event of such magnitude recorded in this previously aseismic area during the whole period of instrumental observations. A seismic moment, a moment magnitude, a hypocentral depth, and a focal mechanism of the event were calculated on the basis of surface wave amplitude spectra. Analysis of the geological and geophysical data showed that the earthquake origin is connected with high-velocity gradient zone located at the border of the Late Proterozoic cover and the Precambrian basement of the Siberian platform. Some evidences for a natural character of the earthquake were considered.

Constraints on the recurrence times of subduction zone earthquakes are important for seismic hazard assessment and mitigation. Models of such megathrust earthquakes often assume that subduction zones are segmented and earthquakes occur quasi-periodically owing to constant tectonic loading. Here we analyse the occurrence of small earthquakes compared to larger ones—the b-values—on a 1,000-km-long section of the subducting Pacific Plate beneath central and northern Japan since 1998. We find that the b-values vary spatially and mirror the tectonic regime. For example, high b-values, indicative of low stress, occur in locations characterized by deep magma chambers and low b-values, or high stress, occur where the subducting and overriding plates are strongly coupled. There is no significant variation in the low b-values to suggest the plate interface is segmented in a way that might limit potential ruptures. Parts of the plate interface that ruptured during the 2011 Tohoku-oki earthquake were highly stressed in the years leading up to the earthquake. Although the stress was largely released during the 2011 rupture, we find that the stress levels quickly recovered to pre-quake levels within just a few years. We conclude that large earthquakes may not have a characteristic location, size or recurrence interval, and might therefore occur more randomly distributed in time. The 2011 Tohoku-oki earthquake released stress within a subduction zone. Analysis of seismic data shows that stresses returned to pre-quake levels within a few years, implying that large quakes could occur more often than previously thought.

An M 6.8 (Mw 6.5) deep-focus earthquake occurred beneath the Bonin Islands at 21:18 (JST) on June 23, 2015. Observed high-frequency (>1 Hz) seismograms across Japan, which contain several sets of P- and S-wave arrivals for the 10 min after the origin time, indicate that moderate-to-large earthquakes occurred sequentially around Japan. Snapshots of the seismic energy propagation illustrate that after one deep-focus earthquake occurred beneath the Sea of Japan, two deep-focus earthquakes occurred sequentially after the first (Mw 6.5) event beneath the Bonin Islands in the next 4 min. The United States Geological Survey catalog includes three Bonin deep-focus earthquakes with similar hypocenter locations, but their estimated magnitudes are inconsistent with seismograms from across Japan. The maximum-amplitude patterns of the latter two earthquakes were similar to that of the first Bonin earthquake, which indicates similar locations and mechanisms. Furthermore, based on the ratios of the S-wave amplitudes to that of the first event, the magnitudes of the latter events are estimated as M 6.5 ± 0.02 and M 5.8 ± 0.02, respectively. Three magnitude-6-class earthquakes occurred sequentially within 4 min in the Pacific slab at 480 km depth, where complex heterogeneities exist within the slab.

A sequence of 98 teleseismically recorded earthquakes occurred off the east coast of Kamchatka at depths between 10-90 km around latitude 52.5°N and longitude 160°E on May 16–23, 2013. The swarm occurred along the northern limit of the rupture area of the 1952 Mw 9.0 great Kamchatka earthquake, the fifth largest earthquake in the history of seismic observations. On May 24, 2013 the strongest deep earthquake ever recorded of Mw 8.3 occurred beneath the Sea of Okhotsk at a depth of 610 km in the Pacific slab of the Kamchatka subduction zone, becoming the northernmost deep earthquake in the region. The deep Mw 8.3 earthquake occurred down-dip of the shallow swarm in a transition zone between the southern deep and northern shallow segments of the Pacific slab. Several deep aftershocks followed, covering a large, laterally elongated part of the slab. We suppose that the two described earthquake sequences, the May 16–23 shallow earthquake swarm and the May 24–28 deep mainshock-aftershock series, represent a single tectonic event in the Pacific slab having distinct properties at different depth levels. A low-angle underthrusting of the shallow part of the slab recorded by the shallow earthquake swarm activated the deep part; this process induced the deep mainshock-aftershock series only three days after the swarm. The domain of the subducting slab activated by the May 2013 earthquake occurrence was extraordinarily large both down-dip and along-strike.

Following the Mw7.9 Wenchuan earthquake, the Mw6.6 Lushan earthquake is another devastating earthquake that struck the Longmenshan Fault Zone (LFZ) and caused severe damages. In this study, we collected continuous broadband ambient noise seismic data and earthquake event data from Chinese provincial digital seismic network, and then utilized ambient noise tomography method and receiver function method to obtain high resolution shear wave velocity structure, crustal thickness, and Poisson ratio in the earthquake source region and its surroundings. Based on the tomography images and the receiver function results, we further analyzed the deep seismogenic environment of the LFZ and its neighborhood. We reveal three main findings: (1) There is big contrast of the shear wave velocities across the LFZ. (2) Both the Lushan earthquake and the Wenchuan earthquake occurred in the regions where crustal shear wave velocity and crustal thickness change dramatically. The rupture faults and the aftershock zones are also concentrated in the areas where the lateral gradients of crustal seismic wave speed and crustal thickness change significantly, and the focal depths of the earthquakes are concentrated in the transitional depths where shear wave velocities change dramatically from laterally uniform to laterally non-uniform. (3) The Wenchuan earthquake and its aftershocks occurred in low Poisson ratio region, while the Lushan earthquake sequences are located in high Poisson ratio zone. We proposed that the effect of the dramatic lateral variation of shear wave velocity, and the gravity potential energy differences caused by the big contrast in the topography and the crustal thickness across the LFZ may constitute the seismogenic environment for the strong earthquakes in the LFZ, and the Poisson ratio difference between the rocks in the south and north segments of the Longmenshan Fault zone may explain the 5 years delay of the occurrence of the Lushan earthquake than the Wenchuan earthquake.

We have done seismic tomography in and around the focal area of the 2008 Iwate-Miyagi Nairiku Earthquake (M 7.2) occurred on June 14, 2008 in NE Japan. We used data from temporary aftershock observation network deployed just after the occurrence of the present earthquake. Based on the distribution of aftershocks, the fault plane of the mainshock is inferred to dip to the west. Small immediate foreshocks and preceding seismic activity in 1999–2000 on the fault plane in the vicinity of the hypocenter of the mainshock of this earthquake were observed. Lower-seismic-velocity hanging wall can be imaged in the central and the northern part of the focal area. This possibly suggests the present earthquake is a compressional inversion earthquake. The low-velocity zone in the lower crust extends upward to the upper crust, branches into three portions and reaches each active volcano. This low-velocity region can be seen just beneath the mainshock hypocenter and the whole focal area, suggesting that crustal fluid possibly promote the occurrence of the 2008 earthquake.

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.

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.

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.

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.

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.

Earthquakes are observed to occur in subduction zones to depths of approximately 680 km, even though unassisted brittle failure is inhibited at depths greater than about 50 km, owing to the high pressures and temperatures. It is thought that such earthquakes (particularly those at intermediate depths of 50-300 km) may instead be triggered by embrittlement accompanying dehydration of hydrous minerals, principally serpentine. A problem with failure by serpentine dehydration is that the volume change accompanying dehydration becomes negative at pressures of 2-4 GPa (60-120 km depth), above which brittle fracture mechanics predicts that the instability should be quenched. Here we show that dehydration of antigorite serpentinite under stress results in faults delineated by ultrafine-grained solid reaction products formed during dehydration. This phenomenon was observed under all conditions tested (pressures of 1-6 GPa; temperatures of 650-820 degrees C), independent of the sign of the volume change of reaction. Although this result contradicts expectations from fracture mechanics, it can be explained by separation of fluid from solid residue before and during faulting, a hypothesis supported by our observations. These observations confirm that dehydration embrittlement is a viable mechanism for nucleating earthquakes independent of depth, as long as there are hydrous minerals breaking down under a differential stress.

The 2003 Tokachi-oki earthquake of M 8.0, which occurred off the south-east coast of Hokkaido, Japan, was preceded by noticeable deep seismicity in the subducting slab, including a deep-focus earthquake of M 7.1. The 1952 Tokachi-oki earthquake of M 8.2 and the 1993 Kushiro-oki earthquake of M 7.5, which occurred also off the south-east of Hokkaido along the Kurile Trench, were also preceded by high deep seismicity including large deep-focus earthquakes of the M 7 or over (focal depths 200–600 km). The relations between the deep seismicity and the ‘shallow’ large earthquake (focal depth ≤ 100 km) in these three cases are quite similar. This result strongly supports the author’s view (Mogi, 1973, 1988) that large ‘shallow’ earthquakes along the subduction zones are sometimes preceded by the occurrence of large deep-focus earthquakes.

On September 26, 2003, a large earthquake with a magnitude of 8.0 occurred along the Kuril trench off Tokachi, Hokkaido, Japan. We investigated the source process by using teleseismic P- and SH-wave data. The main source parameters are as follows: the seismic moment 1.0 × 1021 Nm (Mw= 8.0); (strike, dip, rake) = (230°, 20°, 109°); the depth of initial break point 25 km; source duration 40 sec; and the maximum slip 5.8 m. We estimated the fault area to be 90 × 70 km2, the average slip 2.6 m, and the stress drop 5.0 MPa. This earthquake was an interplate earthquake associated with the subduction of the Pacific plate. The rupture propagated northward from a shallow to a deep region. In this area, a great earthquake (magnitude 8.2) occurred in 1952. We also made limited inversion of nearfield records of the 1952 event and found that the 2003 asperity was also ruptured in 1952. Our result suggests that the 2003 Tokachi-oki earthquake was a recurrent event of the 1952 Tokachi-oki earthquake.

It is well established that an earthquake in the Earth's crust can trigger subsequent earthquakes, but such triggering has not been documented for deeper earthquakes. Models for shallow fault interactions suggest that static (permanent) stress changes can trigger nearby earthquakes, within a few fault lengths from the causative earthquake1,2,3, whereas dynamic (transient) stresses carried by seismic waves may trigger earthquakes both nearby and at remote distances4,5,6,7,8. Here we present a detailed analysis of the 19 August 2002 Tonga deep earthquake sequences and show evidence for both static and dynamic triggering. Seven minutes after a magnitude 7.6 earthquake occurred at a depth of 598 km, a magnitude 7.7 earthquake (664 km depth) occurred 300 km away, in a previously aseismic region. We found that nearby aftershocks of the first mainshock are preferentially located in regions where static stresses are predicted to have been enhanced by the mainshock. But the second mainshock and other triggered events are located at larger distances where static stress increases should be negligible, thus suggesting dynamic triggering. The origin times of the triggered events do not correspond to arrival times of the main seismic waves from the mainshocks and the dynamically triggered earthquakes frequently occur in aseismic regions below or adjacent to the seismic zone. We propose that these events are triggered by transient effects in regions near criticality, but where earthquakes have difficulty nucleating without external influences.

Stress changes in the crust due to an earthquake can hasten the failure of neighbouring faults and induce earthquake sequences in some cases1,2,3,4,5. The 1999 Hector Mine earthquake in southern California (magnitude 7.1) occurred only 20 km from, and 7 years after, the 1992 Landers earthquake (magnitude 7.3). This suggests that the Hector Mine earthquake was triggered in some fashion by the earlier event. But uncertainties in the slip distribution and rock friction properties associated with the Landers earthquake have led to widely varying estimates of both the magnitude and sign of the resulting stress change that would be induced at the location of the Hector Mine hypocentre—with estimates varying from -1.4 bar (ref. 6) to +0.5 bar (ref. 7). More importantly, coseismic stress changes alone cannot satisfactorily explain the delay of 7 years between the two events. Here we present the results of a three-dimensional viscoelastic model that simulates stress transfer from the ductile lower crust and upper mantle to the brittle upper crust in the 7 years following the Landers earthquake. Using viscoelastic parameters that can reproduce the observed horizontal surface deformation following the Landers earthquake, our calculations suggest that lower-crustal or upper-mantle flow can lead to postseismic stress increases of up to 1–2 bar at the location of the Hector Mine hypocentre during this time period, contributing to the eventual occurrence of the 1999 Hector Mine earthquake. These results attest to the importance of considering viscoelastic processes in the assessment of seismic hazard8,9,10,11.

The kinematics and dynamics of plate tectonics are frontal subjects in geosciences and the strong earthquake occurred along the plate boundary result directly from plate movement. By analyzing Ibaraki earthquake sequence, it has been found that the focal fault plane shows a special image of grading expansion along the direction of strike and adjustment along the dip direction respectively. With the consideration of strike, dip and slip directions of focal mechanism, we have confirmed that Ibaraki earthquake belongs to a thrust fault earthquake occurred under the Japan Trench. The cause of the earthquake sequence is discussed in the paper. The study on the temporal-spatial distribution of the earthquake sequence with a time-scale between the year-scale spatial geodetic data and the second-scale moment tensor of the strong earthquake has indicated the dynamic process of Pacific Plate subduction under the Eurasia Plate. According to the average slip distance of earthquake and the velocity of plate movement, it is predicted that a strong earthquake might occur in recent years.

Following a quasi-dynamic scheme proposed by Minear and Toksoz (1970), thermal structures of subduction zones for different models by finite element method (FEM) were calculated. Density distribution and P-wave anomaly of subduction zones were calculated at the same time. Comparing with seismological evidences and results of laboratories, it is proposed that earthquakes occurred below 400 km depth are probably controlled by anti-crack mechanism.

—We analyze the seismic moment-frequency relation in various depth ranges and for different seismic regions, using Flinn-Engdahl's regionalization of global seismicity. Three earthquake lists of centroid-moment tensor data have been used the Harvard catalog, the USGS catalog, and the Huang et al. (1997) catalog of deep earthquakes. The results confirm the universality of the β-values and the maximum moment for shallow earthquakes in continental regions, as well as at and near continental boundaries. Moreover, we show that although fluctuations in earthquake size distribution increase with depth, the β-values for earthquakes in the depth range of 0–500 km exhibit no statistically significant regional variations. The regional variations are significant only for deep events near the 660 km boundary. For declustered shallow earthquake catalogs and deeper events, we show that the worldwide β-values have the same value of 0.60 ± 0.02. This finding suggests that the β-value is a universal constant. We investigate the statistical correlations between the numbers of seismic events in different depth ranges and the correlation of the tectonic deformation rate and seismic activity (the number of earthquakes above a certain threshold level per year). The high level of these correlations suggests that seismic activity indicates tectonic deformation rate in subduction zones. Combined with the universality of the β-value, this finding implies little if any variation in maximum earthquake seismic moment among various subduction zones. If we assume that earthquakes of maximum size are similar in different depth ranges and the seismic efficiency coefficient, χ, is close to 100% for shallow seismicity, then we can estimate χ for deeper earthquakes for intermediate earthquakes χ≈ 5%, and χ≈ 1% for deep events. These results may lead to new theoretical understanding of the earthquake process and better estimates of seismic hazard.

According to the feature of Hindukush-Pamirs intermediate focus earthquake belt with a S-shaped pattern of dip direction reversion by using the data of earthquake catalogues obtained by seismic networks in Xinjiang and Mid-Asia area of the former Soviet Union, by means of focal mechanism solution and tectonic stress analysis, it is considered that the intermediate focus earthquake belt is possibly formed by the compression rupture which is caused by the collision between Indian and European Plates in the lithosphere of the upper mantle. Under the action of torsion moment, the continuous torsional break of reverse part of the earthquake belt might be the reason why the intermediate focus strong earthquakes occur repeatedly in the same place. In this paper, the boundary line between the intermediate focus earthquake belt and the shallow focus earthquake region of the western part of south Tianshan is also defined from the angle of seismicity division.

It is usually assumed that earthquakes in intraplate regions occur in the upper crust, and northwestern Italy is generally assigned to this kind of ‘normal’ seismicity. In this work, the depth distribution of the events localized in this area by the Istituto Geofisico Geodetico (IGG) seismic network in the period 1991–1997 is analyzed in detail. In particular, the location capability of the network is discussed, adopting as reference quarry blasts (for the epicentral position) and the locations obtained from a dense temporary network (for the depth estimate). Within the so-obtained error limits, the depth distribution of events show a characteristic pattern: while for most of the area covered by the network the well-located seismicity lies within the first 20 km of depth, in a band following the inner arc of the Western Alps, numerous events have anomalously large focal depths, reaching a maximum of 114 km. These depth determinations cannot be attributed to instabilities of the location procedure: different choices of the propagation models used for the hypocentral determination led to very similar depth values, always significantly larger than the standard values for the surrounding areas. A strong correlation has been found between the 3-dimensional distribution of these foci and the P-wave propagation anomalies obtained from tomographic studies, suggesting a direct link between elastic and rheological properties of lower crust and upper mantle in this area.

Latent heat release by equilibrium mineralogical transformations in an adiabatically subducting slab reversibly perturbs temperatures and pressures so as to conserve entropy. However, latent heats of metastable transformations in such a slab yield irreversible isobaric temperature changes which increase entropy despite adiabatic constraints. As a result, latent heat release by metastable exothermic transformations can yield local superheating above the background adiabat, with the degree of potential superheating increasing with extent of metastable overstep. In real slabs, however, regions of metastably persisting low-pressure phases should undergo conductive warming from surrounding transformed material. Such warming should proceed more rapidly than warming of the bulk slab from the surrounding mantle, and the resulting decrease in metastable transition pressures will slightly decrease the degree of local superheating. Nonetheless, such local temperature increases may trigger seismic release of accumulated strain energy via a number of proposed mechanisms of shear instability. Adiabatic instability, in the form of shear localization in material with temperature-dependent rheology, is one mechanism which may be triggered by such latent heat release in metastable regions yet produce rupture that extends beyond the boundaries of such regions.

Information concerning a total number of 13700 instrumentally recorded earthquakes is used to study the geographical and the vertical distribution of the Earth's seismicity. From these earthquakes, which form four complete samples of data (M ≥ 7.0, 1894–1992; M ≥ 6.5, 1930–1992; M ≥ 6.0, 1953–1992; M ≥ 5.5, 1966–1992), 11511 are shallow (h ≤ 60 km), 2085 are of intermediate focal depth (61 ≤ h ≤ 300 km) and 564 are deep focus earthquakes (301 ≤ h ≤ 720 km). The parameters a and b of the frequency-magnitude relationship were calculated in a grid of equally spaced points at 1° by using the data of earthquakes located inside circles centered at each point. The radius of the circles increased from 30 km with a step of 10 km until the information for the earthquakes located inside the circle fulfil three criteria which concern the size of the sample used to compute these parameters at each point of the grid. The results are given in a qualitative way (epicenter maps) as well as in a quantitative way (mean return periods).

DEEP earthquakes generally show fewer aftershocks and a larger variability in magnitude–frequency relations than shallow earthquakes1–5. These characteristics and the factors that control them may place constraints on the mechanism of deep earthquakes, which is still uncertain6–8. Here we show that systematic variations in aftershock productivity and magnitude–frequency relations are related to the temperature of the downgoing slab, as inferred from the vertical velocity and age of the subducting lithosphere. Deep earthquakes with substantial aftershock sequences are found only in colder slabs, and earthquakes in warmer slabs consistently show few aftershocks. Magnitude–frequency relations for deep earthquakes also show systematic variations as a function of slab thermal structure, with warmer slabs showing fewer small earthquakes (lower 'b-values'). The statistical properties of deep earthquakes thus appear to be temperature sensitive to an extent not observed for shallow earthquakes, and not adequately explained by simple geometrical limitations on fault width.

The sources for thirteen deep focus earthquakes ofm b≥5.5 andh>400 km in Northwest Pacific region were studied using waveform fitting and shear fracture source model. The source parameters were obtained as follows: focal depth, faulting plane, slip direction, rupture velocity, rupture length, rupture direction and scalar moment tensor. It was found that all these earthquake sources can be interpreted as shear faulting and have simple source time functions. The strike direction of faulting plane for most deep focus earthquakes coincides with that of the subduction zone, especially in the deep part of the subduction zone, it results in the tendency of reducing the dip angle of the subduction zone. The multiple point source model was also used to study the source process. The waveform fitting is better than the shear fracture model, but the general rupture direction which coincides with that from unilateral shear source model can not be obtained from the multiple point source model.

According to the 3-D velocity images of the crust and upper mantle in Xinjiang and its adjacent areas and the projective distribution of epicenters at different depths, this paper reveals the relationship between the velocity structure feature of the crust and upper mantle and seismicity in the studied areas, and also reveals the deep seismogenic environment in the Tianshan earthquake belt, intermediate and deep focus earthquake region in Hindu Kush, intermediate earthquake belts (regions) in west Kunlun and Kalakorum Mountain and so on different earthquake belts. The seismogenesis is also discussed.

Because of the viscoelastic behaviour of the earth, accumulation of elastic strain energy by tectonic loading and release of such energy by earthquake fault slips at subduction zones may take place on different spatial scales. If the lithospheric plate is acted upon by distant tectonic forces, strain accumulation must occur in a broad region. However, an earthquake releases strain only in a region comparable to the size of the rupture area. A two-dimensional finite-element model of a subduction zone with viscoelastic rheology has been used to investigate the coupling of tectonic loading and earthquake fault slips. A fault lock-and-unlock technique is employed so that the amount of fault slip in an earthquake is not prescribed, but determined by the accumulated stress. The amount of earthquake fault slip as a fraction of the total relative plate motion depends on the relative sizes of the earthquake rupture area and the region of tectonic strain accumulation, as well as the rheology of the rock material. The larger the region of strain accumulation is compared to the earthquake rupture, the smaller is the earthquake fault slip. The reason for the limited earthquake fault slip is that the elastic shear stress in the asthenosphere induced by the earthquake resists the elastic rebound of the overlying plate. Since rapid permanent plate shortening is not observed at subduction zones, there must be either strain release over a large region or strain accumulation over a small region over earthquake cycles. The former can be achieved only by significant aseismic fault slip between large subduction earthquakes. The most likely mechanism for the latter is the accumulation of elastic strain around isolated locked asperities of the fault, which requires significant aseismic fault slip between asperities.

Correct representation of seismic waveforms propagating through the mantle from a 600 km deep earthquake is presented using graphic interpolation between synthetic seismograms computed across a grid of mantle depths and distances. All torsional normal modes with periods above 12 s are summed to create 72,846 seismograms at depths between the surface and the core-mantle boundary. The resulting time snapshots show the manner by which seismic shear energy propagates around the core away from the source.

The 9 March 1957 Aleutian earthquake has been estimated as the third largest earthquake this century and has the longest aftershock zone of any earthquake ever recorded—1200 km. However, due to a lack of high-quality seismic data, the actual source parameters for this earthquake have been poorly determined. We have examined all the available waveform data to determine the seismic moment, rupture area, and slip distribution. These data include body, surface and tsunami waves. Using body waves, we have estimated the duration of significant moment release as 4 min. From surface wave analysis, we have determined that significant moment release occurred only in the western half of the aftershock zone and that the best estimate for the seismic moment is 50–100×1020 Nm. Using the tsunami waveforms, we estimated the source area of the 1957 tsunami by backward propagation. The tsunami source area is smaller than the aftershock zone and is about 850 km long. This does not include the Unalaska Island area in the eastern end of the aftershock zone, making this area a possible seismic gap and a possible site of a future large or great earthquake. We also inverted the tsunami waveforms for the slip distribution. Slip on the 1957 rupture zone was highest in the western half near the epicenter. Little slip occurred in the eastern half. The moment is estimated as 88×1020 Nm, orM w =8.6, making it the seventh largest earthquake during the period 1900 to 1993. We also compare the 1957 earthquake to the 1986 Andreanof Islands earthquake, which occurred within a segment of the 1957 rupture area. The 1986 earthquake represents a rerupturing of the major 1957 asperity.

The estimation of the theoretical magnitude for deep foci based on the vertica component of medium-period PKIKP waves, Earth model PREM and source spectra is presented. The given graphs are valid for period T ranging from 1·6 s to 10 s, source depth of 25 km to 600 km and distance range of 16 500 km to 19 800 km. A simplified method of representing calibration curves σ is used.

We extend to the case of intermediate and deep earthquakes our application of the mantle magnitude M m to historical events. Because of the general lack of quantitative studies of deep earthquakes before the initiation of the Centroid Moment Tensor databank in 1977, we regard as historical all non-shallow earthquakes up to and including 1976. An analysis of 57 records from 41 events, using the Uppsala Wiechert seismometer and various long-period instruments at Pasadena, yields new moment estimates for 28 events whose moments had not previously been published. Our results correlate poorly with available traditional magnitudes, as would be expected from early saturation effects for magnitude scales measured at relatively high frequencies

THE mechanism responsible for deep-focus earthquakes, which occur at depths of 300–700 km in subducting slabs, has been a long-standing problem in geophysics. Unlike shallow earthquakes, deep earthquakes cannot be attributed to frictional instabilities across a fault plane, because of high frictional resistance to sliding at depth. A volumetric change associated with a phase transition, expected to occur at depth1–3, is often invoked as the physical mechanism; if so, the resulting source mechanism should contain a major isotropic component. Although many researchers have attempted to observe such an isotropic component4–10, no one has yet convincingly proved or disproved its presence. There exists a portion of the seismogram which is well suited to resolve the isotropic component of deep earthquakes but which has not been analysed by previous workers. Here I use this component in a systematic analysis of 19 large deep earthquakes, and show that no significant isotropic component (< 10% of the seismic moment) exists. A sudden implosive phase change can thus be ruled out as the primary physical mechanism for deep earthquakes.

Recent improvements in the seismological networks on the Ibero-Maghrebian region have permitted estimation of hypocentral location and focal mechanisms for earthquakes which occurred at South Spain, Alboran Sea and northern Morocco of deep and intermediate depth, with magnitudes between 3.5 and 4.5. Intermediate depth shocks, range from 60 to 100 km, with greater concentration located between Granada and Malaga. Fault-plane solutions of 5 intermediate shocks have been determined; they present a vertical plane in NE-SW or E-W direction. Seismic moments of about 1015 Nm and dimensions of about 1 km have been determined from digital records of Spanish stations.P-wave forms are complex. This may be explained by the crustal structure near the station, discontinuities in the upper mantle and inhomogeneities near the source. Deep activity at about 650 km has only 3 shocks since 1954 (1954, 1973, 1990). Shocks are located at a very small region. Fault-plane solutions show a consistent direction of the pressure axis dipping 45° in E direction. For the 1990 shock seismic moment is 1016 Nm and dimensions 2.6 km. TheP-waves are of simpler form with a single pulse. The intermediate and deep activities are not connected and no activity has been detected between 100 and 650 km. The intermediate shocks may be explained in terms of a recent subduction from Africa under Iberia in SE direction. The very deep activity must be related to a sunk detached block of lithospheric material still sufficiently cold and rigid to generate earthquakes.

We extend to the case of intermediate and deep earthquakes the mantle magnitude developed for shallow shocks byokal andTalandier (1989). Specifically, from the measurement of the spectral amplitude of Rayleigh waves at a single station, we obtain a mantle magnitude,Mm, theoretically related to the seismic moment of the event through $$M_m = \log _{10} M_0 - 20.$$ The computation ofMminvolves two corrections. The distance correction is the same as for shallow shocks. For the purpose of computing the frequency-dependent source correction, we define three depth windows: Intermediate (A) (75 to 200 km); Intermediate (B) (200–400 km) and Deep (over 400 km). In each window, the source correctionCS is modeled by a cubic spline of log10T.

We present a study of the lateral structure and mode of deformation in the transition between the Kuril and Honshu subduction zones. We begin by examining the source characteristics of the January 19, 1969, intermediate depth earthquake north of Hokkaido in the framework of slab-tearing, which for the December 6, 1978 event has been well documented by previous studies. We use a least-squares body wave inversion technique, and find that its focal mechanism is comparable to the 1978 event. To understand the cause of these earthquakes, which in the case of the 1978 event occurred on a vertical tear fault but does not represent “hinge” faulting, we examine the available International Seismological Centre [ISC] hypocenters and Harvard centroid-moment tensor [CMT] solutions to determine the state of stress, and lateral structure and segmentation in the Kuril and northern Honshu slabs. These data are evaluated in the framework of two models. Model (A) requires the subducting slab at the Hokkaido corner to maintain surface area. Model (B) requires slab subduction to be dominated by gravity, with material subducting in the down-dip direction. The distribution of ICS hypocenters shows a gap in deep seismicity down-dip of the Hokkaido corner, supporting model (B). From the CMT data set we find that three types of earthquake focal mechanisms occur. The first (type A) represents dip-slip mechanisms consistent with down-dip tension or compression in the slab in a direction normal to the strike of the trench. These events occur throughout the Honshu and Kuril slabs with focal mechanisms beneath Hokkaido showing NNW plungingP andT axes consistent with the local slab geometry. The second (type B) occurs primarily at depths over 300 km in the southern part of the Kuril slab with a few events in the northern end of the Honshu deep seismicity. These earthquakes have focal mechanisms with P axes oriented roughly E-W, highly oblique to the direction of compression found in the type A events, with which they are spatially interspersed. The third (type C) group of earthquakes are those events which do not fit in either of the first two groups and consist of either strike-slip focal mechanisms, such as the tearing events, or oddly oriented focal mechanisms. Examination of the stress axes orientations for these three types reveals that the compressional axes of the type C events are consistent with those of type B. The slab tearing events are just differential motion reflecting the E-W compressive states of stress which is responsible for the type B family of events. There is no need to invoke down-dip extension which does not fit the slab geometry. We conclude that these two states of stress can be explained as follows: 1) The type A events and the seismicity distribution support model (B). 2) The type B and C events upport model (A). The solution is that the slab subducts according to model (B), but the flow in the mantle maintains a different trajectory, possibly induced by the plate motions, which produces the second state of E-W compressive stress.

The 1992 Nicaragua earthquake was a ‘tsunami earthquake‘; that is, it generated tsunamis disproportionately large for its surface-wave magnitude, M_s. The moment magnitude, M_w, determined from long-period (~250-s) surface waves, was 7.6, significantly larger than the 20-s M_s of 7; this M_s–M_w disparity is also characteristic of tsunami earthquakes. The Nicaragua earthquake is the first tsunami earthquake to be captured by modern broadband seismic networks, allowing us to present here seismograms of sufficiently high quality to make inferences about the rupture mechanisms. We conclude that the Nicaragua earthquake was a slow thrust earthquake which occurred on the subduction interface between the Cocos and North American plates, and because of the absence of sediments on the trench floor offshore of Nicaragua, the slip propagated up-dip all the way to the ocean bottom, exciting large tsunamis. The occurrence of slip on a plate interface filled with soft subducted sediments caused the rupture process to be slower than in ordinary subduction-zone thrust earthquakes. Our results reinforce the idea that tsunami warning systems using long-period (≥100-s) waves are necessary to reduce the hazard from this type of earthquake.

SHALLOW earthquakes are produced by brittle shear fracture of rock and/or fictional sliding on pre-existing fault surfaces1. At very high pressures, however, brittle fracture and frictional sliding are impossible because frictional resistance to movement on any potential fault surface is so great that ductile stress-relief processes accomodate strain at lower shear stresses than those necessary to activate faulting. Nevertheless, more than 20% of earthquakes with magnitudes greater than five occur at depths greater than 300km, where the pressure exceeds l0 GPa (ref. 2). Possible explanations for this paradox have been offered by several recent experimental studies of shearing instabilities associated with phase transformations at high 3–8. One of these mechanisms is associated with anticrack development during the olivine → spinel (α → γ) phase transformation in Mg2GeO4 at pressures of 1–2 GPa (refs 4–6); it is particularly attractive because it operates between mineral structures known to occur in the mantle. We show here that this mechanism also can operate in natural silicate olivine, (Mg,Fe)2SiO4, during onset of the α → β transformation at the much higher pressures at which deep earthquakes occur. These results lend strong support to the hypothesis that the anticrack mechanism is responsible for such earthquakes4.

IT has been suggested1–7 that deep earthquakes in subduct ion zones may be caused by mineralogical transformations in the subducting slab. These transformations, from olivine to modified spinel to spinel, are also thought to affect the rheology of the descending slab2–4. Experiments in the system Mg2SiO4–Fe2SiO4 (ref. 8) have shown that at still higher pressure, coinciding with the 670-km seismic discontinuity between the upper and lower mantle, spinel dissociates to a fine-grained mixture of perovskite plus magnesiowiistite. Here we argue that the dissociation product in the mantle will also be very fine-grained, and that its eutectoid texture, combined with the low temperature in the slab, will prevent grain growth. We note (as have others9,3) that fine-grained materials at moderately high temperatures and low strain rates exhibit super-plastic behaviour; the resulting viscous behaviour of the slab below the spinel dissociation boundary may therefore account for the observed absence of seismicity below ∼700 km depth, even if slabs do penetrate into the lower mantle.

THE mechanism of deep-focus earthquakes has been a puzzle since their discovery almost 70 years ago1, 2, because brittle fracture and frictional sliding at depths in excess of 100–200 km would require unrealistic rock strengths3, 4. Rock strength does increase with pressure, but a few hundred MPa (equivalent to 10–20 km depth) suffices to inhibit most fracture, and elevated temperature activates ductile mechanisms that operate at stresses less than the fracture stength. A range of mechanisms has been proposed for deep earthquakes, including plastic instabilities5–7, shear-induced melting8–11 and instabilities accompanying recrystallization12, 13 or polymorphic phase transformation14–23. Each of these proposed mechanisms has exhibited certain inherent weaknesses (see Kirby22 for review and discussion). Here we report experimental observations of high-pressure faulting of metastable Mg2GeO4 olivine as it undergoes incipient transformation to a spinel 24, 25 structure. We present a model in which this faulting arises from the generation, propagation and linking-up of spinel-filled anticracks26. When applied to the olivine → spinel transformation in the Earth's mantle, the anticrack model27 satisfactorily accounts for the similarities and differences between shallow and deep earthquakes and removes the problems associated with frictional sliding.

The initial motion of primary body waves and polarization directions of secondary body waves have been applied successfully to the study of the mechanism operating at the earthquake focus. Equal area plots of these body wave characteristics resulted in radiation patterns that were compared to patterns due to theoretical focal mechanism sources. Such an approach indicated that a double couple force is the source mechanism operating at earthquake foci. This can be physically represented by faulting at the focus. This seems likely because of the relation between earthquakes and fault motion in shallow earthquakes. The possibility of other mechanisms operating in deep focus earthquakes has not been ruled out.

During the earthquake preparation a zone of cracked rocks is formed in the region of a future earthquake focal zone under the influence of tectonic stresses. In the study of the surrounding medium this region may be considered as a solid inclusion with altered moduli. The inclusion appearance causes a redistribution of the stresses accompanied by corresponding deformations. This paper deals with the study of deformations at the Earth's surface, resulting from the appearance of a soft inclusion. The Appendix contains an approximate solution of the problem for a soft elastic inclusion in an elastic half-space. It is assumed that the moduli of the inclusion differ slightly from those of the surrounding medium (by no more than 30%). The solution permits us to calculate the deformations at the Earth's surface for the inclusion with an arbitrary heterogeneity and anisotropy. The problem is solved by the small perturbation method. The calculation is made for a special case of a homogeneous isotropic inclusion where only the shear modulus decreases. The shear stresses act at infinity. The equations are deduced for the estimation of deformations and tilts at the Earth's surface as a function of the magnitude of the preparing earthquake and the distance from the epicentre. Comparison has shown a satisfactory agreement between the theoretical and field results. Let us assume that the zone of effective manifestation of the precursor deformations is a circle with the centre in the epicentre of the preparing earthquake. The radius of this circle called ‘strain radius’ may be calculated from the equation $$\rho = 10^{0.43M} km,$$ where M is the magnitude. It was shown that the precursors of other physical nature fall into this circle.

The earthquakes of central coastal Peru occur principally in two distinct zones of shallow earthquake activity that are inland of and parallel to the axis of the Peru Trench. The interface-thrust (IT) zone includes the great thrust-fault earthquakes of 17 October 1966 and 3 October 1974. The coastal-plate interior (CPI) zone includes the great earthquake of 31 May 1970, and is located about 50 km inland of and 30 km deeper than the interface thrust zone. The occurrence of a large earthquake in one zone may not relieve elastic strain in the adjoining zone, thus complicating the application of the seismic gap concept to central coastal Peru. However, recognition of two seismic zones may facilitate detection of seismicity precursory to a large earthquake in a given zone ; removal of probable CPI-zone earthquakes from plots of seismicity prior to the 1974 main shock dramatically emphasizes the high seismic activity near the rupture zone of that earthquake in the five years preceding the main shock. Other conclusions on the seismicity of coastal Peru that affect the application of the seismic gap concept to this region are: (1) Aftershocks of the great earthquakes of 1966, 1970, and 1974 occurred in spatially separated clusters. Some clusters may represent distinct small source regions triggered by the main shock rather than delimiting the total extent of main-shock rupture. The uncertainty in the interpretation of aftershock clusters results in corresponding uncertainties in estimates of stress drop and estimates of the dimensions of the seismic gap that has been filled by a major earthquake. (2) Aftershocks of the great thrust-fault earthquakes of 1966 and 1974 generally did not extend seaward as far as the Peru Trench. (3) None of the three great earthquakes produced significant teleseismic activity in the following month in the source regions of the other two earthquakes. The earthquake hypocenters that form the basis of this study were relocated using station adjustments computed by the method of joint hypocenter determination.

A study of stress accumulation in seismic gaps and of stress transfer along linear plate boundaries is presented. Time-dependent reloading of plate boundaries following seismic ruptures is modeled by a modified Elsasser model of a coupled lithosphere/asthenosphere plate system. This model is applied to study a series of large earthquakes in the Aleutian Islands and the Alaska peninsula in 1938–1965. It is found that the Rat Island earthquake and the 1948 earthquake in the central Aleutians are likely to have been triggered by adjacent ruptures, in the sense that their occurrence would have come at a later time had their neighboring segments not been ruptured. Stresses in the Unalaska Gap and the Shumagin gap are at a relatively high level and these segments of the plate boundary may be expected to rupture in the near future. In general, in the ten years (about 16% of the earthquake cycle for the Aleutians) following an earthquake, the stress recovery in the rupture zone is highly nonlinear, resulting in a much more rapid stress accumulation than the linear case. Even at a later stage of an earthquake cycle, adjacent ruptures can cause an acceleration of loading rate in addition to the coseismic stress jump. A good example is the influence of the 1964 Alaska earthquake on the 1938 rupture zone. A general conclusion of this work is that long term earthquake prediction models must take into account the nonlinear stress accumulation behavior in seismic gaps. Also, we have shown the interaction of adjacent plate boundary segments, which suggests that some large earthquakes may have been triggered by nearby ruptures.

Large earthquakes often occur as multiple ruptures reflecting strong variations of stress level along faults. Dense instrument networks with which the volcano Kilauea is monitored provided detailed data on changes of seismic velocity, strain accumulation and earthquake occurrence rate before the 1975 Hawaii 7.2-mag earthquake. During the ∼4 yr of preparation time the mainshock source volume had separated into crustal volumes of high stress levels embedded in a larger low-stress volume, showing respectively high- and low-stress precursory anomalies.

The geometry of deep ( greater than 400 km) earthquakes in the subducting western edge of the Pacific plate, when related to the local distribution of seismic activity, reveals a complex pattern of planes of shear failure, enabling the modes of deformation by which the descending plate shortens and thickens to be identified. The deepest part of the slab is displaced laterally, suggesting that it fails to penetrate the discontinuity at 670 km.

The hypothesis has been advanced that major portions of the annual pole motion are caused by the seasonal transport of air masses. These pole motions in turn can act as trigger forces on earthquakes. This is tested by observations of northern hemisphere pressures, pole motion, and occurrence of deep focus earthquakes during the interval 1922 to 1935.