Great Shallow Earthquakes Research Articles

Heterogeneities of Short-Period S-Waves Attenuation Field in the Kuril-Kamchatka Region and their Relation to Large and Great Earthquakes

We study characteristics of short-period shear wave attenuation field in the lithosphere to pick out areas of possible preparation for large and great shallow earthquakes. We have processed more than 360 seismograms of events with source depths of 0–33 km recorded by station PET from two areas limited by coordinates of 45.0˚–50.5˚ N and 54.0˚–56.5˚ N, respectively (for brevity, we will call the areas as the southern and the northern ones). Besides, for the purposes of comparison we have analyzed seismograms by station KGB that recorded earthquakes from the area located between 52˚ и 54˚ N. We used the method based on analysis of Sn and Pn maximum amplitude ratio. We find that attenuation in the lithosphere of the northern area is generally much higher than in the southern one. At the same time attenuation in both areas is weaker than in the region of north-eastern Japan. Relatively lower attenuation corresponds to rupture zones of the great earthquakes of 1952 (Mw = 9.0) and 1963 (Mw = 8.6) occurred in the southern area more than 50 years ago. Higher attenuation is observed in the rupture zones of the recent events dated 1997 (Mw = 7.8), 2006 (Mw = 8.3) and 2018 (Mw = 7.3). The obtained data are in agreement with earlier conclusions stating that typical large subduction type earthquakes occur in the areas characterized by higher fluid content in the uppermost mantle; and large and great earthquakes are followed by deep fluids ascent during a few decades, which leads to attenuation decrease in the uppermost mantle. We also pick out the high attenuation zones where no large and great earthquakes (Mw ≥7.8) have occurred for quite a long time. We suggest that active processes of preparation for large earthquakes can be observed in these zones (first of all in the area of the Avacha Bay and to the east of it).

The long-term earthquake forecast for the Kuril-Kamchatka island arc for the September 2013 to August 2018 period; the seismicity of the arc during preceding deep-focus earthquakes in the sea of Okhotsk (in 2008, 2012, and 2013 at M = 7.7, 7.7, and 8.3)

Work was performed in 2008–2014 on the long-term prediction of great earthquakes at the Kuril-Kamchatka arc based on patterns that are observable in the locations of the rupture zones of great earthquakes (seismic gaps) and in the seismic cycle for such earthquakes. The prediction method has been successfully developed and applied to the region since 1965 (the LTEP method). This paper presents a long-term seismic forecast for the September 2013 to August 2018 period for the Kuril-Kamchatka arc earthquake-generating zone. The forecast predicts the following quantities for 20 constituent segments: phases of the seismic cycle, the normalized rate of small earthquakes A10, the magnitudes of moderate-size earthquakes to be expected with probabilities 0.8, 0.5, and 0.15, the maximum possible magnitudes and probabilities of occurrence for great (M ≥ 7.7) earthquakes in the most active depth interval, viz., 0–80 km. We also discuss the seismic process in this region for the 2008–2013 period, namely, the patterns that are observable in the locations of earthquake swarms, the values of and changes in several characteristics of the seismic process in the Kuril-Kamchatka seismic zone during the period 2008–2013, which occurred in relation with great shallow earthquakes beneath the Sea of Okhotsk (July 5, 2008, MW = 7.7; August 14, 2012, MW = 7.7; May 24, 2013, MW = 8.3, and the giant Tohoku earthquake of March 11, 2011 with MW = 9.0). The results corroborate the high seismic hazard for the area of Petropavlovsk-Kamchatskii and the urgent necessity of continuing and intensifying the ongoing work on seismic retrofitting and seismic safety.

How lithospheric subduction changes along the Calabrian Arc in southern Italy: geophysical evidences

Recent tomographic investigations performed down to ~300 km depth in the Calabrian Arc region gave insight in favor of the hypothesis that the Ionian subducting slab is continuous in depth beneath the central part of the Arc, while detachment of the deep portion of the subducting structure may have already taken place beneath the edges of the Arc itself. In the present study, we perform new geophysical analyses to further explore the structure of the subduction system and the structure and kinematics of the crustal units in the study area for a more comprehensive view of the local geodynamic scenario. Local earthquake tomography that we address to the exploration of the upper 40 km in the whole region of southern Italy furnishes P-wave velocity domains, suggesting southeast-ward long-term drifting of the southern Tyrrhenian unit with an advancement front matching well with the segment of Calabrian Arc where the subducting slab was found continuous and trench retreat can be presumed to have been active in the most recent times. This scenario of retreating subduction trench inducing drifting of the lithospheric unit overriding the subducting slab is further supported by the analysis of gravity anomalies, allowing us to better constrain the transitional zones between different subduction modes (continuous vs. detached slab) along the Arc. Also, the relocation of recent crustal seismicity, associated with geostructural data taken from the literature, provides evidence for NW-trending seismogenic structures in northeastern Sicily and northern Calabria that we interpret as Subduction-Transform Edge Propagator (STEP) faults guiding the southeast-ward drifting process of the southern Tyrrhenian unit. Crustal earthquake relocations show also seismolineaments in southern Calabria corresponding to the NE-trending longitudinal structures of the Arc where the great shallow earthquakes of 28 December 1908, and 5 and 7 February 1783 occurred. Seismicity and the extensional stress regime detected in these structures find also reasonable location in the proposed scenario, being interpretable in terms of shallow response of the central segment of the Arc to slab rollback and trench retreat.

Seismogenic characteristics of the Northern Mariana shallow thrust zone from local array data

[1] The Northern Mariana seismogenic zone has no shallow thrust earthquakes larger than Ms 7.4 in the historical seismological record and is traditionally considered ‘decoupled’ or ‘aseismic’. During the 2003–2004 Mariana Subduction Factory Imaging Experiment, we recorded local shallow earthquakes throughout the central and northern regions of the Mariana forearc using an array of terrestrial broadband and ocean bottom seismographs. Accurate locations for both the 2003–2004 local seismicity as well as earthquakes with Global Centroid Moment Tensor (GCMT) solutions from 1976 to 2008 were obtained using the hypocentroidal decomposition relocation method and a local velocity model. Additionally, focal mechanisms for the largest 2003–2004 earthquakes were determined using regional waveform inversion. Thrust faulting earthquakes occur along the Mariana megathrust between depths of 20–60 km, showing that the lack of great shallow thrust earthquakes does not result from a narrow seismogenic zone and that most seismicity occurs where the downgoing plate contacts the overriding mantle wedge. Clusters of small plate interface earthquakes with Ml 1.6–4.7 occur within patches 100–120 km west of the trench at depths of 30–45 km. Furthermore, the larger GCMT earthquakes (Mw 4.9–5.8) occur mostly updip and downdip of the patches of smaller earthquakes recorded by our local array and is suggestive of changes in the fault properties with depth. Clusters of small, forearc earthquakes occur discontinuously along the length of the Mariana subduction zone, showing that Northern Mariana is variable both along the strike of the margin and with depth along the seismogenic zone. We propose that the lack of great (Mw > 8) thrust faulting earthquakes is due in part to the variable frictional heterogeneity along the megathrust.

Rapid determination of Me for strong to great shallow earthquakes

We propose a new rapid procedure for determining the energy magnitude Me for shallow events from broadband teleseismic P‐wave signals within the distance range 20°–98°. To accomplish this task, we compute spectral amplitude decay functions for different periods using numerical simulations based on the reference Earth model AK135Q. By means of these functions, we correct the spectra of the teleseismic recordings for the propagation path effects, and calculate the radiated seismic energy ES, and hence Me. We use cumulative P‐wave windows for simulating a real‐ or near real‐time procedure and test it for 61 shallow earthquakes. The results show that our approach is able to provide a rapid and reliable Me determination within 7–15 minutes after the earthquake origin time, and is therefore suitable for implementation in rapid response systems.

Evidence for a Global Seismic-Moment Release Sequence

Temporal clustering of the larger earthquakes (foreshock-mainshock-aftershock) followed by relative quiescence (stress shadow) are characteristic of seismic cycles along plate boundaries. A global seismic-moment release history, based on a little more than 100 years of instrumental earthquake data in an extended version of the catalog of Pacheco and Sykes (1992), illustrates similar behavior for Earth as a whole. Although the largest earthquakes have occurred in the circum-Pacific region, an analysis of moment release in the hemisphere antipodal to the Pacific plate shows a very similar pattern. Monte Carlo simulations confirm that the global temporal clustering of great shallow earthquakes during 1952–1964 at M ≥ 9.0 is highly significant (4% random probability) as is the clustering of the events of M ≥ 8.6 (0.2% random probability) during 1950–1965. We have extended the Pacheco and Sykes (1992) catalog from 1989 through 2001 using Harvard moment centroid data. Immediately after the 1950–1965 cluster, significant quiescence at and above M 8.4 begins and continues until 2001 (0.5% random probability). In alternative catalogs derived by correcting for possible random errors in magnitude estimates in the extended Pacheco–Sykes catalog, the clustering of M ≥ 9 persists at a significant level. These observations indicate that, for great earthquakes, Earth behaves as a coherent seismotectonic system. A very-large-scale mechanism for global earthquake triggering and/or stress transfer is implied. There are several candidates, but so far only viscoelastic relaxation has been modeled on a global scale.

Kuril Islands Arc: Two Seismic Cycles of Great Earthquakes during which the Complete History of Seismicity (Ms >= 6) Is Observed

Two adjacent segments of Pacific-Eurasia plate boundary along the Kuril Islands arc were broken during two great shallow thrust earthquakes that occurred in 1963 (moment magnitude, M w = 8.5) and 1969 ( M w = 8.2), respectively. In 1994 a great shock ( M w = 8.3) rebroke the ∼200-km-long plate boundary segment previously broken by the 1969 event, and a series of shocks in 1991 ( M w = 7.6), 1995 ( M w = 7.9) and 1996 ( M w = 7.2) rebroke major portions of the ∼300-km-long segment previously ruptured by the 1963 event. Due to the heterogeneities and incompleteness of the instrumental seismicity catalog for shallow (focal depth, h M s ≥ 6, particularly for the first half of the century, this is one of the first times that a complete history of instrumental seismicity at an energy level as low as M s = 6, occurring in a focal region of great earthquakes during a full seismic cycle, has been observed. My findings indicate that significant activity at the M s ≥ 6 and M s ≥ 7 levels took place both within, and as well as along the downdip edges, of the forthcoming 19909s ruptures during most of the seismic cycle, whereas the adjacent regions of the plate boundary were relatively less active. The average displacements and repeat times (28.5 yr ± 3.5) observed for these great earthquakes along this section of the plate boundary as a whole, are consistent with the strain energy accumulated during that time by subduction of the Pacific plate under the Eurasian plate, at a rate of ∼10 cm/yr.

The large-scale distribution of great shallow intraplate earthquakes in mainland China and adjacent areas and seismic coupling along plate margins

In mainland China and its surroundings the large-scale distribution of great, shallow, intraplate earthquakes shows that there are four main areas of high intraplate seismicity which are (a) the Northern China Seismic Area in east China (30°–42°N); (b) the Southeast-coast Seismic Area in eastern China (19°–25°N); (c) the North-south trending Seismic Area in western China and its surroundings (Burma–China–Mongolia); (d) the Central Asian Seismic Area in west China and its surroundings (Pamirs–Tianshan Mts–Baikal). These four intraplate seismic areas are approximately perpendicular to those sections of the Eurasian plate boundary where the Eurasian plate has a strong seismic coupling with the North American–Pacific Ocean–Philippine Sea plates, and with the Indian plate. The large-scale uneven distribution of intraplate seismicity in China and its surroundings may be controlled by heterogeneity in the stress state on different sections of the plate boundary.

Long‐term seismic behavior of the focal and adjacent regions of great earthquakes during the time between two successive shocks

We analyze the spatial‐temporal‐energy distribution of shallow (depth, h<70 km) seismicity (surface wave magnitude, Ms≥7) occurring in the focal and adjacent regions of very great shallow earthquakes (rupture lengths of the order of hundreds of kilometers) in the circum‐Pacific seismic belt for as many decades as possible before and after the occurrence of the main shock. For this purpose we have revised the catalog of data for the world instrumental seismicity (1900–1995) by removing a series of heterogeneities in the earthquake reporting and magnitude determination. This revised catalog is fairly complete for earthquakes with M≥∼7 since early in the century. Our findings indicate that once a main shock‐aftershock sequence occurs the focal region enters a period of relative quiescence for large (Ms≥7) earthquakes. Significant seismic activity (Ms≥7) occurs during many decades prior to the next main shock. This activity is fairly continuous or random in time and lasts until the onset of the main event but clusters in space around the ends of the forthcoming rupture and/or in the vicinity of the principal epicenter (except in the case of rare bilateral breaks the main epicenter is located at one of these ends). Thus major portions of the rupture zone experience no events with Ms≥7 during the seismic cycle. Larger aftershocks also cluster near the edges of the rupture and near the principal epicenter. Preshock activity, minutes to weeks before the main shock (if any), occurs, in general, near the main epicenter. We also find that the nucleation and ends of the rupture often coincide in space with recognizable geometrical and/or geological inhomogeneities, “barriers” existing in the lithosphere. It appears that while strain energy accumulates, high stress concentrations often occur at the ends of the future rupture zone, which results in the long‐term foreshock activity and in the subsequent initiation of the main rupture from within those regions. We suggest a simple mechanical model that explains the observed pattern of earthquake occurrence for both unilateral and bilateral ruptures. In all cases of great earthquakes that have occurred this century for which a long and complete (to a level of 2 to 3 orders of magnitude smaller than the moment of the main shock) catalog of prior seismicity exists, we can not recognize any statistically significant “quiescent” or “doughnut” patterns in the preceding seismicity at the Ms≥7 level.

Stress relaxation at the lower dislocation edge of great shallow earthquakes

The problem of post-seismic stress evolution at the lower dislocation edge of great shallow earthquakes is investigated by a model reproducing a transcurrent boundary zone subject to a constant strain rate. Dislocation models with frictional stress threshold for slip arrest must be considered in order to study post-seismic stress evolution in the proximity of dislocation edges. It is found that the boundary zone is vertically divided in two regions: an upper region, where stress increases with time and a lower region, where stress decreases. Dislocations which are nucleated in the seismogenic region can propagate downward into the aseismic region: in this case earthquakes are followed by relaxation of the stress concentration at the lower dislocation edge. This may have effects on the space-time distribution of aftershocks.

The origin of S- C mylonites and a new fault-zone model

This paper presents a synthesis of experimental data on mechanical behaviour and deformation textures of simulated halite shear zones with special regard to the internal structures of S- C mylonites and their mechanical implications. Halite is the only mineral so far for which a complete transition between brittle shearing deformation to fully ductile shearing flow in the dislocation glide regime has been studied experimentally under large shear strains that may be encountered along faults or plate boundaries. Moreover, deformation textures in the shear zones are similar to those of mylonites and cataclasites. Thus the experimental results provide an insight into the petrogenesis of fault rocks, mylonites in particular, and the establishment of a realistic fault model. Between the brittle regime and the fully ductile regime (i.e. pressure-insensitive inelastic regime), there exists a wide and distinct regime called ‘semiductile’ in which deformation textures are nearly identical to those developed in the ductile regime, yet the shear resistance is pressure-dependent, and potentially unstable fault motion can occur in the low pressure part. In cooking salt shear-zone experiments, the characteristic texture in the semiductile and ductile regimes is foliation resembling mylonitic foliation, which formed in the direction of maximum elongation. In the semiductile regime, nearly homogeneous shearing deformation, characterized by the formation of uniform and pervasive foliation, changes into heterogeneous deformation after a critical shear strain and this strain localization induces potentially unstable fault motion. The primary mode of internal slip after the strain localization is slip along Y Riedel shears, parallel to the shear-zone boundary, interconnected with R 1 and some P Riedel shears. These shear surfaces of concentrated deformation are very similar to the internal structures of so-called ‘ S- C mylonites”. Hence, the semiductile regime is the primary candidate for the formation of S- C mylonites, and at least some S- C mylonites must have formed at seismogenic depths. This interpretation is consistent with recently reported interlaced pseudotachylytes and mylonites. It is also shown that the size of recrystallized halite grains (presumably mostly postdeformational) decreases with increasing shear strains, but not with the shear stress, and this is consistent with the grain-size reduction with strain concentration which has been confirmed for many mylonites in the world. Thus, estimation of flow stress from recrystallized grains needs to be done with great care and might often be erroneous. Existing fault models are critically examined from the viewpoint of the halite experiments, and a new fault model is proposed whereby fault zones are divided into brittle, semibrittle, semiductile and ductile regimes with increasing depth. The shear resistance of faults is highest in the semiductile regime, and the seismogenic depth is likely to extend down to the upper part of this regime. The semiductile regime thus emerges as an area of primary interest with regard to the petrogenesis of many S- C mylonites, modelling of large and great shallow earthquakes along pre-existing faults and plate boundaries, and quantitative evaluation of the mechanical interactions of plates across their boundaries.

Downward migration of seismic activity prior to some great shallow earthquakes in Japanese subduction zones?a possible intermediateterm precursor

Before the 1944 Tonankai earthquake along the Nankai Trough, seismic activity increased in the shallow depths, and then the activity gradually migrated downwards. When it reached its limit (a depth of approximatelty 70 km), the main shock occurred. Several deep earthquakes, including one ofM5.3, occurred several months prior to the Tonankai earthquake. A similar downward migration pattern also can be recognized regarding the 1952 Tokachi-oki earthquake. In this case the deepest earthquakes reached about 400 km. This may be one of the intermediate-term precursory phenomena of great thrusttype earthquakes in subduction zones. Recent observations in the Tokai district along the Suruga Trough, where a large earthquake is expected to occur in the future, suggest a similar downward migration pattern in the land area.

Coastline uplift in Oregon and Washington and the nature of Cascadia subduction-zone tectonics

Coastline deformation resulting from great shallow thrust earthquakes can provide information concerning the paleoseismicity of a subduction zone and thus information on the nature of potential seismicity. The Cascadia subduction zone is different from most other subduction zones in that it has been quiescent with respect to great earthquakes for at least the past 200 yr. The Washington-Oregon coastline also differs from most other coastlines associated with subduction zones in its lack of uplifted Holocene shoreline features and low overall rate of late Quaternary uplift (0.2-0.6 mm/yr). The uplift differences suggest that repeated great earthquakes have not occurred along the Cascadia subduction zone at least during the late Holocene. Alternatively, if the plate interface has generated earthquakes, the differences may be explained by longer recurrence intervals for great earthquakes, smaller magnitude earthquakes, or a mechanism that does not result in uplift of the coastline where expected.

Magnitudes of great shallow earthquakes from 1953 to 1977

The surface-wavemagnitudes M s are determined for 30 great shallow earthquakes that occurred during the period from 1953 to 1977. The determination is based on the amplitude and period data from all available station bulletins, and the same procedure as that employed in Gutenberg and Richter's “Seismicity of the Earth” is used. During this period, the Chilean earthquake of 1960 has the largest M s , 8.5. The surface-wave magnitudes listed in “Earthquake Data Reports” are found to be higher than M s on the average. By using the same method as that used by Gutenberg, the broad-band body-wave magnitudes m B are determined for great shallow shocks for the period from 1953 to 1974. m B is based on the amplitudes of P, PP and S waves which are measured on broadband instruments at periods of about 4–20 s. The 1-s body-wave magnitudes listed in “Bulletin of International Seismological Center” and “Earthquake Data Reports” are found to be much smaller than m B on the average. Through the examination of Gutenberg and Richter's original worksheets, the relation between m B and M s is revised to m B = 0.65 M s+ 2.5 which well satisfies the mg and M s data for M s between 5.2 and

Global variation of seismic activity

The global variation in the space—time distribution of great shallow earthquakes is examined using the surface-wave magnitude M and the magnitude M ω obtained from the seismic moment. In the past several decades, the seismic activity was complementary in the regions of low latitude (<40° ) and high latitude (>40° ). During the roughly twentyyear period around 1960, the seismic activity was very high in the high-latitude regions and appreciably low in the low-latitude regions. In particular, it is pointed out that the greatest earthquakes occurred roughly simultaneously in the north and south regions of high latitude around 1960. However, the present time seems to be an active period in the low-latitude regions. These regions account for the larger part of the earth's surface, and sustain a degree of destruction and loss of life that is far out of proportion with the relative energy release in the low and high latitude regions.

Two kinds of seismic gaps

The same term ‘seismic gaps’ has been used for different kinds of seismic gaps, resulting in some confusion. It is shown that there are two kinds of seismic gaps which are defined by two different features of seismic activity. One is a gap in the spatial distribution of rupture zones of the largest earthquakes in a seismic belt. This is termed a seismic gap of the first kind. A seismic gap of the first kind could be identified not only for great shallow earthquakes along plate boundaries, but also probably for smaller intra-plate earthquakes. The other is a gap in seismicity of smaller-magnitude earthquakes before larger earthquakes. This premonitory phenomenon is termed a seismic gap of the second kind. Focal regions of the largest earthquakes in an active seismic belt are frequently seismic gaps of both the first and the second kind. Some earthquakes, however, are not preceded by any appreciable premonitory gap (the second kind). This different feature in different cases may depend on the structural states of the earth’s crust, such as heterogeneity.

Addenda and corrections to “Magnitude of great shallow earthquakes from 1904 to 1952”

Letter| December 01, 1978 Addenda and corrections to “Magnitude of great shallow earthquakes from 1904 to 1952” Robert J. Geller; Robert J. Geller Search for other works by this author on: GSW Google Scholar Hiroo Kanamori; Hiroo Kanamori Seismological Laboratory California Institute of TechnologyPasadena, California 91125 Search for other works by this author on: GSW Google Scholar Katsuyuki Abe Katsuyuki Abe Seismological Laboratory California Institute of TechnologyPasadena, California 91125 Search for other works by this author on: GSW Google Scholar Bulletin of the Seismological Society of America (1978) 68 (6): 1763–1764. https://doi.org/10.1785/BSSA0680061763 Article history received: 11 Aug 1978 first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Tools Icon Tools Get Permissions Search Site Citation Robert J. Geller, Hiroo Kanamori, Katsuyuki Abe; Addenda and corrections to “Magnitude of great shallow earthquakes from 1904 to 1952”. Bulletin of the Seismological Society of America 1978;; 68 (6): 1763–1764. doi: https://doi.org/10.1785/BSSA0680061763 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyBulletin of the Seismological Society of America Search Advanced Search This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not currently have access to this article.

Magnitudes of great shallow earthquakes from 1904 to 1952

abstract The “revised magnitudes”, M, converted from Gutenberg's unified magnitude, m, and listed by Richter (1958) and Duda (1965) are systematically higher than the magnitudes listed by Gutenberg and Richter (1954) in Seismicity of the Earth. This difference is examined on the basis of Gutenberg and Richter's unpublished original worksheets for Seismicity of the Earth. It is concluded that (1) the magnitudes of most shallow “class a” earthquakes in Seismicity of the Earth are essentially equivalent to the 20-sec surface-wave magnitude, Ms; (2) the revised magnitudes, M, of most great shallow (less than 40 km) earthquakes listed in Richter (1958) (also used in Duda, 1965) heavily emphasize body-wave magnitudes, mb, and are given by M=14Ms+34(1.59mb−3.97). For earthquakes at depths of 40 to 60 km, M is given by M = (1.59 mb − 3.97). M and Ms are thus distinct and should not be confused. Because of the saturation of the surface-wave magnitude scale at Ms ≃ 8.0, use of empirical moment versus magnitude relations for estimating the seismic moment results in large errors. Use of the fault area, S, is suggested for estimating the moment.

Pre-seismic crustal deformation caused by an underthrusting oceanic plate, in eastern Hokkaido, Japan

Eastern Hokkaido is being considered as one of the prime candidates for a future great earthquake in Japan. Geodetic work and seismological data suggest that the continental plate is being compressed and dragged down into the asthenosphere by the underthrusting Pacific plate. A quantitative examination of this idea was carried out by an application of the finite-element method to a static-field problem of crustal deformation. The effect of heterogeneous medium with Poisson's ratio differing from 0.25 is significant. An inverse problem was solved by applying a quadratic programming method to the observed vertical displacement. A model, which assumes a nearly uniform tangential displacement along the interface of the oceanic and continental lithospheres down to a depth of 100 km, explains a gravity-change as well as the general features of crustal deformation observed in eastern Hokkaido. The displacement rate is estimated to be 2.7 cm/year. It indicates that a rebound of the continental lithosphere now brings about a thrust-type earthquake whose reverse slip is comparable to that estimated for great shallow earthquakes along the Kuril trench. A possibility of aseismic rebound is suggested at the lower part of the interface.

Relationship between shallow and deep seismicity in the western Pacific region

The two types of relationship between the shallow and deep seismic activities in island-arc regions are described: (1) A great shallow earthquake in a region is preceded and sometimes followed by the marked increase of deep seismic activity in the same down-dip seismic zone perpendicular to the trend of an arc structure. The increase of the deep seismic activity before the great shallow earthquake cannot be understood as an accidental event nor as a mere trigger of the great shallow earthquake, but as an essential forerunning phenomenon of it. This regularity is found in the Kurile-Kamchatka and northern Japan island-arc regions, particularly near the ends of these two arcs and their junctions, where great shallow earthquakes frequently occur. (2) The seismic activity tends to progress in time from a shallow to a deep region within a descending lithosphere. This regularity is found in the Mariana and Tonga arcs, where the seismic activity is nearly continuous from the surface to the deep region and no very great shallow earthquake occurred in the considered period. The rate of migration in these earthquakes along the descending lithosphere is estimated at about 50 km per year, nearly similar to that in shallow earthquakes. These regularities in the space-time distribution of shallow and deep earthquakes in island-arc regions can be explained from the viewpoint that the oceanic lithosphere is pushed downward under the continental lithosphere by external forces, probably due to the mantle convection. The present discussion suggests that the descending lithosphere does not act as a rigid plate but as a visco-elastic body under gradually increasing loading.