Kuril Trench Research Articles

Re-examination of aftershocks of the 1952 Tokachi-oki earthquake and a comparison with those of the 2003 Tokachi-oki earthquake

We relocated hypocenters of the March 4, 1952 Tokachi-oki (M j 8.2) earthquake and its aftershocks by reassessing data of the Central Meteorological Station network at the time and found that the distribution of relocated aftershocks is clearly bounded on its eastern extension by the Kushiro canyon, which extends south-east from the coast of Hokkaido near Kushiro to the Kuril trench. The reevaluated aftershock pattern is quite similar to the pattern of the September 26, 2003 Tokachi-oki earthquake (M j 8.0). Detailed seismic intensity maps based on the report of human perceptions in 1952 and the seismic intensity meter network in 2003 also show resemblance. Similar aftershock pattern and seismic intensity distribution imply that both earthquakes are a pair of characteristic earthquakes of the same size sharing the same source area and the same focal process.

Source process of the recurrent Tokachi-oki earthquake on September 26, 2003, inferred from teleseismic body waves

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.

Joint bulk-sound and shear tomography for Western Pacific subduction zones

Detailed regional body wave tomographic inversion of the Western Pacific region has been performed using P and S travel times from common sources and receivers, with a joint inversion in terms of bulk-sound and shear wave-speed variations in the mantle. This technique allows the separation of the influence of bulk and shear moduli, and hence a more direct comparison with mineral physics information. The study region is parameterized with cells of side 0.5° to 2° and 19 layers to a depth of 1500 km, while the rest of the mantle was parameterized with 5×5° cells with 16 layers between the surface and the core–mantle boundary. A simultaneous inversion is made for regional and global structures to minimize the influence of surrounding structures on the regional image. A nested iterative inversion scheme is employed with local linearization and three-dimensional ray tracing through the successive model updates. The results of the regional tomographic inversion reveal the penetration of a subducted slab below the 660 km discontinuity at the Kurile–Kamchatka trench, while flattening of slabs above this depth is observed in the Japan and Izu–Bonin subduction zones on both the bulk-sound and shear wave-speed images. The penetration of a subducted slab down to a depth of at least 1200 km is seen below the southern part of the Bonin trench, Mariana, Philippine, and Java subduction zones. Fast shear wave-speed perturbations associated with the subducted slabs, down to the 410 km transition zone, are larger than the comparable bulk-sound perturbations for all these subduction zones except the Philippines. The bulk-sound signature for the subducted slab is more pronounced than for shear in the Philippines, Talaud, New Guinea, Solomon, and Tonga subduction zones, where penetration of the slab into the middle mantle is observed. Variation in the amplitude ratio between bulk-sound and shear wave-speed anomalies correlates well with the subduction parameters of the descending slab. Slabs younger than 90 Ma at the trench show bulk-sound dominance in the upper mantle, while older slabs have a stronger shear wave-speed signature. Spreading of the fast shear wave-speed zone between 800 and 1000 km is observed in the areas of deep subducted slab penetration, but has no comparable expression in the bulk-sound images. This high-velocity feature may reflect physical or chemical disequilibria introduced to the lower mantle by subducted slabs.

Slip distribution of the 1952 Tokachi‐Oki earthquake (M 8.1) along the Kuril Trench deduced from tsunami waveform inversion

We inverted 13 tsunami waveforms recorded in Japan to estimate the slip distribution of the 1952 Tokachi‐Oki earthquake (M 8.1), which occurred southeast off Hokkaido along the southern Kuril subduction zone. The previously estimated source area determined from tsunami travel times [Hatori, 1973] did not coincide with the observed aftershock distribution. Our results show that a large amount of slip occurred in the aftershock area east of Hatori's tsunami source area, suggesting that a portion of the interplate thrust near the trench was ruptured by the main shock. We also found more than 5 m of slip along the deeper part of the seismogenic interface, just below the central part of Hatori's tsunami source area. This region, which also has the largest stress drop during the main shock, had few aftershocks. Large tsunami heights on the eastern Hokkaido coast are better explained by the heterogeneous slip model than previous uniform‐slip fault models. The total seismic moment is estimated to be 1.87 × 1021 N m, giving a moment magnitude of Mw = 8.1. The revised tsunami source area is estimated to be 25.2 × 103 km2, ∼3 times larger than the previous tsunami source area. Out of four large earthquakes with M ∼ 7 that subsequently occurred in and around the rupture area of the 1952 event, three were at the edges of regions with relatively small amount of slip. We also found that a subducted seamount near the edge of the rupture area possibly impeded slip along the plate interface.

Unusually large earthquakes inferred from tsunami deposits along the Kuril trench.

The Pacific plate converges with northeastern Eurasia at a rate of 8-9 m per century along the Kamchatka, Kuril and Japan trenches. Along the southern Kuril trench, which faces the Japanese island of Hokkaido, this fast subduction has recurrently generated earthquakes with magnitudes of up to approximately 8 over the past two centuries. These historical events, on rupture segments 100-200 km long, have been considered characteristic of Hokkaido's plate-boundary earthquakes. But here we use deposits of prehistoric tsunamis to infer the infrequent occurrence of larger earthquakes generated from longer ruptures. Many of these tsunami deposits form sheets of sand that extend kilometres inland from the deposits of historical tsunamis. Stratigraphic series of extensive sand sheets, intercalated with dated volcanic-ash layers, show that such unusually large tsunamis occurred about every 500 years on average over the past 2,000-7,000 years, most recently approximately 350 years ago. Numerical simulations of these tsunamis are best explained by earthquakes that individually rupture multiple segments along the southern Kuril trench. We infer that such multi-segment earthquakes persistently recur among a larger number of single-segment events.

Real-time geophysical measurements on the deep seafloor using submarine cable in the southern Kurile subduction zone

A permanent real-time geophysical observatory using a submarine cable was developed and deployed to monitor seismicity, tsunamis, and other geophysical phenomena in the southern Kurile subduction zone. The geophysical observatory comprises six bottom sensor units, two branching units, a main electro-optical cable with a length of 240 km and two land stations. The bottom sensor units are: 1) three ocean bottom broadband seismometers with hydrophone; 2) two pressure gauges (PGs); 3) a cable end station with environmental measurement sensors. Real-time data from all the undersea sensors are transmitted through the main electro-optical cable to the land station. The geophysical observatory was installed on the continental slope of the southern Kurile trench, southeast Hokkaido, Japan in July 1999. Examples of observed data are presented. Sensor noises and resolution are mentioned for the ocean bottom broadband seismometers and the PGs, respectively. An adaptable observation system including very broadband seismometers is scheduled to be connected to the branching unit in late 2001. The real-time geophysical observatory is expected to greatly advance the understanding of geophysical phenomena in the southern Kurile subduction zone.

Drifter Observations of Anticyclonic Eddies near Bussol' Strait, the Kuril Islands

Hydrographic surveys and satellite imaging reveal that mesoscale anticyclonic (AC) eddies are common features of the area south of Bussol' Strait, the deepest of the Kuril straits connecting the western North Pacific and Sea of Okhotsk. To examine the velocity structure of these eddies, we deployed groups of 15-m drogued satellite-tracked surface drifters over the Kuril-Kamchatka Trench in the fall of 1990 and late summer of 1993. Drifters in both groups entered large AC eddies centered over the axis of the trench seaward of Bussol' Strait and subsequently underwent a slow northeastward translation. One drifter (Drifter 1315) deployed near the center of the “Bussol' eddy” in 1990, remained in the eddy for roughly 45 days and made five loops at successively greater distances from the eddy center. Large-amplitude (80–100 cm/s) storm-generated inertial oscillations were observed during the first two loops. The vorticity field associated with the eddy resulted in a Doppler “red-shift” of inertial frequency motions such that the “effective” inertial period of 21 hours was roughly 4 hours greater than the nominal inertial period for the drifter latitude (45°N). In 1993, a second drifter (Drifter 15371) was retained in the Bussol' eddy for about 40 days. This eddy had characteristics similar to those of the 1990 eddy but was devoid of significant high-frequency motions until the drifter's final half loop. The observed spatial scales, persistence, and slow poleward translation of the eddies suggests that they play an important role in the dynamics of the East Kamchatka and Oyashio current systems.

Deep circulation in the northwest corner of the Pacific Ocean

We deployed for two years a line of nine current-meter moorings bearing instruments at depths of 2, 3, and 4 km running southeast of Hokkaido, to measure currents above the continental slope, Kuril Trench, and Hokkaido Rise. The mean flow was directed southwestward above the continental slope, northeastward above the trench and upper rise (except at one mooring), and westward onto the lower rise. The mean currents were highly barotropic, except above the continental slope, and unexpectedly swift (8 cm s −1 in the trench). The velocity pattern above the Hokkaido Rise is like that observed earlier above the Aleutian Rise at Long. 175°W, and may be due, as suggested for the latter, to varying topographic beta associated with the curvature of the bottom profile of the rise. Thermal-wind fields from three CTD sections along the mooring line, while consistent among themselves, were unlike the observed mean shear, and therefore useless for estimating mean transports. Estimates based on the direct current measurements alone, for depths greater than 2000 m, are 4×10 6 m 3 s −1 southwestward above the continental slope and 20×10 6 m 3 s −1 northeastward in the trench; but the former might be too small, the latter too large, by as much as 10×10 6 m 3 s −1 because of the relatively broad mooring spacing. These measurements, in combination with many others reported earlier, unequivocally describe swift deep southward flow along the inshore sides of the Izu-Ogasawara, Japan, and Kuril Trenches, and opposed flow along their offshore sides, as well as above their axes (except in the Izu-Ogasawara Trench). The southward flow may be, at least in part, the recirculation western-boundary current predicted for the northern North Pacific, although the oceanic geometry is different from, and more complicated than, that of the classic analytical predictive models. Reasons for the strong opposed flow are obscure. Water properties reveal that deep water spreads into the Izu-Ogasawara Trench, and probably into the Japan Trench as well, flows northward through the Kuril Trench, and, at least at some levels, around the northern end of the Emperor Seamount Chain to fill the long Aleutian Trench. This plus eastward flows through the Main Gap in the Emperor Seamount Chain and the passage between the Hess Rise and the Hawaiian Ridge seem to be the principal deep outflows from the Northwest Pacific Basin into the Northeast Pacific Basin. The Meiji Sediment Drift, lying along the eastern flank of the far northern Emperor Seamount Chain, is composed of material from the Bering Sea. To account for its deposition, we conjecture that the deep Kamchatka Current, presently carrying this material southward, splits at the latitude of the northernmost Emperors, one branch flowing eastward as a zonal jet, and continuing southward along the eastern flank of the Seamount Chain as a deep western-boundary current. Descriptive ambiguities and dynamical puzzles are considered.

A block-fault model for deformation of the Japanese Islands derived from continuous GPS observation

We interpret the continuous GPS data for the Japanese islands from 1996 to 1999 using the model of Hashimoto and Jackson (1993). In this model crustal deformation is represented by a combination of rigid block motions and deformation due to slip deficits along the fault-bounding blocks. Hashimoto and Jackson used 19 blocks, 104 faults, and geodetic data spanning 100 years. In the present work we assume the same fault and block geometry, and we use only the continuous GPS data. Compared to the previous study, the motions of the major blocks are a bit larger: 107 ± 8 mm/yr for the Pacific plate, 47 ± 2 mm/yr for the Philippine Sea plate, and 24 ± 2 mm/yr for the Izu block, all relative to the Amurian plate. The estimated slip directions on active inland faults are now more consistent with geological estimates. Slip deficit rates exceed 10 mm/yr along the Itoigawa-Shizuoka Tectonic Line, Shinanogawa Seismic Zone, Atotsugawa fault, Hanaori fault, Arima-Takatsuki Tectonic Line, Rokko faults, Median Tectonic Line, and southern boundary of the Beppu-Shimabara graben. Some interplate faults along the Japan and Kurile trenches have slip deficit rates larger than 100 mm/yr, although postseismic deformation from the 1994 Far Off Sanriku earthquake may contaminate these estimates. An interplate fault off Kyushu has a negative slip deficit, possibly due to post-seismic movements from the 1996 events in Hyuganada.

Full interseismic locking of the Nankai and Japan‐west Kurile subduction zones: An analysis of uniform elastic strain accumulation in Japan constrained by permanent GPS

We analyze permanent Global Positioning System (GPS) data obtained over Japan between 1995 and 1997 to estimate the instantaneous interseismic coupling ratio of the seismogenic zones due to the subduction of the Pacific and Philippine Sea plates below the Japanese islands. We first derive the GPS strain rate fields that characterize the crustal deformation of southern and northern Japan and invert them to determine the effective subduction velocity along the central Nankai trough on one side and the Japan‐west Kurile trench on the other. These “reference free” velocities are close to those predicted by plate motion models with respect to Eurasia. We conclude that the Eurasian reference frame gives a good approximation to the subduction motion and that to first order, both subduction zones were fully locked during the period of measurements. We then test whether the coupling ratio shows local variations within the seismogenic zones. To do this, we divide the subduction interface into 35 km×30 km elements that we model by point source groups, and we invert the GPS velocity field referenced to Eurasia to derive the coupling ratio (between 0 and 1) on each fault element. The results are coherent over the 3 years and confirm that both the central Nankai and the Japan‐west Kurile seismogenic zones are homogeneously fully locked. Most of the coupling ratios are close to 1 and a few are close to 0; intermediate values are rare. The zones of decoupling correspond either to strong postseismic afterslip associated with the 1994 Sanriku‐Oki interplate earthquake (Japan trench) or to a small overestimation of the actual lower limit of the locked zone. We conclude that within the resolution of the GPS data and the model, (1) partial coupling did not exist during these 3 years along the Nankai and Japan‐west Kurile trenches; (2) the small seismic coupling ratio previously derived from earthquakes analysis for the Japan and Kurile trenches may indicate that a significant part of the elastic energy is dissipated silently through slow earthquakes and postseismic afterslip; and (3) the heterogeneous coseismic slip pattern observed for the large and great earthquakes that rupture both subduction zones is in great contrast to the homogeneous loading. Finally, we discuss the nonelastic residual deformation within the frame of the long‐term deformation of the Japanese islands.

K–Ar ages of the Akan‐Shiretoko volcanic chain lying oblique to the Kurile trench: Implications for tectonic control of volcanism

AbstractThe Akan‐Shiretoko volcanic chain, situated in the Southwestern Kurile arc, consists mainly of nine subaerial andesitic stratovolcanoes and three calderas. The chain extends in a SW–NE direction for 200 km, situated oblique to the Kurile trench at an angle of 25 degrees. Thirty‐seven new K–Ar ages, plus previous data, suggest that volcanic activity along the Akan‐Shiretoko volcanic chain began at ca 4 Ma at Akan, at the southwestern end of the chain, and systematically progressed northeastward, resulting in the southwest‐northeast‐trending volcanic chain. This spatial and temporal distribution of volcanoes can be explained by anticline development advancing northeastward from the Akan area, accompanied by magma rising through northeast‐trending fractures that developed along the anticlinal axis. The northeastward development of the anticline caused uplifting of the Akan‐Shiretoko area and changed the area from submarine to subaerial conditions. Anticline formation was likely due to deformation of the southwestern Kurile arc, with southwestward migration of the Kurile forearc sliver caused by oblique subduction of the Pacific plate. The echelon topographic arrangement of the Shiretoko, Kunashiri, Etorofu and Urup was formed at ca 1 Ma.

Different stress directions in the aftershock focal mechanisms of the Kushiro-Oki earthquake of Jan. 15, 1993, SE Hokkaido, Japan, and horizontal rupture in the double seismic zone

On 15 January, 1993, a large intermediate-depth earthquake with a magnitude of M=7.8, shook the eastern part of the island of Hokkaido, Japan, close to the northeastern Japan trench and to the Kuril trench; this earthquake is now referred to as the Kushiro-Oki earthquake. The main shock of the Kushiro-Oki earthquake showed a down-dip extension mechanism having horizontal and vertical nodal planes. The hypocenter was located at a depth of 107 km in the lower plane of the double seismic zone in the subducting Pacific plate. After the mainshock, more than 300 aftershocks were recorded by three seismograph networks. From these data we were able to determine the aftershock hypocenters and 63 focal mechanism solutions for the aftershocks. The aftershock region extended horizontally in a northerly direction from the lower plane towards the upper plane of the double seismic zone in the subducting plate; it did not, however, reach the upper plane. We estimated the maximum and minimum compressional stress directions along the deep seismic zone by converting the mechanism solutions from horizontal plane projections to an oblique plane parallel to the deep seismic zone. Thirty-eight mechanisms were found to have maximum and minimum compressional stress directions very similar to those of the mainshock, but 25 aftershock mechanisms had almost opposite directions to those of the mainshock. The hypocenters of these aftershocks were spatially mixed. This suggests that the stress state is strongly inhomogeneous in the aftershock region, and that horizontal movement occurs by the down-dip extensional stress which is stored in the region between two planes of the deep seismic zone. The remanent down-dip extensional stress was released as the consistent group of aftershocks. High aftershock activity may be attributed to the inhomogeneity of the material in the region. The previous four intermediate-depth earthquakes beneath Hokkaido Island as well as aftershock distributions of two of them have been examined for comparison with the Kushiro-Oki earthquake. The mainshocks of the four large intermediate-depth earthquakes are also located on the lower plane with a similar mechanism to that of the Kushiro-Oki earthquake. The aftershock regions of two of these earthquakes are also distributed horizontally. They indicate dominantly horizontal movement, with down-dip extension mechanisms at an intermediate depth in the subducting Pacific plate beneath Hokkaido Island.

Polarization anomaly of Love waves caused by lateral heterogeneity

SUMMARY We calculate surface waves propagating in a laterally heterogeneous structure beneath the Kuril trench, where significant Love-wave polarization anomalies, called quasiLove waves, are generated. Since 3-D wave propagation in the two-dimensionally heterogeneous structure can be assumed, we apply the 2.5-D finite diVerence method to the surface-wave calculations. The calculations show that a velocity contrast of 7 per cent at depths of less than 210 km beneath the Kuril trench cannot generate quasiLove waves, and that an unlikely contrast of 20 per cent is required to generate clear quasi-Love waves. The possible cause of the quasi-Love waves inferred from previous studies on coupled free oscillations is a lateral variation in azimuthal anisotropy. The lateral variation in azimuthal anisotropy beneath the Kuril trench suggests a change in the mantle flow induced by the subducting slab.

Refined coseismic displacement modeling for the 1994 Shikotan and Sanriku‐oki earthquakes

We consider refined modeling of GPS measurements of coseismic displacement for two large earthquakes occurring in the Japan and Kurile Trenches. Calculations are performed for a layered sphere and the homogeneous half‐space more commonly used to model such data, using fault geometry and slip distributions from previously published studies. We show that the difference between the layered sphere and homogeneous half‐space calculations can be significant, as expected from previous theoretical studies. However, it does not appear large enough to seriously affect results from previously published studies of these earthquakes. Nevertheless, the fact that the effects of sphericity and especially layering are measurable in coseismic observations of great earthquakes at moderate distances indicates that these effects should be taken into account in order to fully exploit the precision of modern GPS measurements.

Outer slope faulting associated with the western Kuril and Japan trenches

SUMMARY Elongated fault escarpments on the outer slopes of the western Kuril and Japan trenches have been investigated through detailed swath bathymetric mapping. Numerous horsts and grabens formed by these escarpments were identified. Distinct N70°E linear alignment of the escarpments, parallel to the magnetic anomaly lineations, was revealed on the outer slope of the western Kuril Trench. In the Japan Trench north of 39°00aeN, most of the escarpments are parallel to the trench axis and oblique to the magnetic lineations. A zig-zag pattern of faulting exists south of 39°00aeN. Each topographic profile was decomposed by computer analysis into two curves representing (1) the smoothed long-wavelength slope of the subducting ocean-crust surface and (2) the short-wavelength (<10 km) roughness of plateaus and valleys edged by outwardand inward-facing fault escarpments. Throughout the surveyed areas, escarpment heights increase from the crest of the trench outer swell down to a depth of about 6000 m on the slope of the outer trench wall, but with no distinct increase below that depth. No significant diVerence is recognized in fault throws towards and away from the trench. It can be concluded that these elongated escarpments originate from normal faults on the upper layer of the oceanic crust under extensional stress in a direction perpendicular to the trench axis, which is caused by downward bending of the subducting lithosphere. The relationship of escarpment height to escarpment length is similar to that obtained from normal fault escarpments in the East Pacific Rise crest. The maximum length and height of escarpments are small in the Kuril Trench compared with those in the Japan Trench, implying a diVerence in mechanical strength depending on the fault orientation. The crust is weakest along the inherited spreading fabric, second weakest probably along the non-transform oVset direction and strongest in directions very oblique to these orientations. Seamounts appear to be more rigid than normal ocean crust, with no particular weak orientations, resulting in fewer but larger faults along the axis of plate bending, as most clearly represented in the subducting Daiichi‐Kashima Seamount.

Location of Love‐to‐Rayleigh conversion due to lateral heterogeneity or azimuthal anisotropy in the upper mantle

Love waves can be converted to Rayleigh waves, which are sometimes called quasi‐Love waves, in laterally heterogeneous or azimuthally anisotropic mantle. A local four‐station network of broadband seismometers in Hokkaido, Japan, records the quasi‐Love waves from the earthquakes located in and around Oregon and California, USA. We infer that these quasi‐Love waves are generated by lateral heterogeneity or azimuthal anisotropy beneath the Aleutian or Kuril trench. We attempt to locate the Love‐to‐Rayleigh conversion areas for the August 17, 1991, off coast of northern California earthquake by using the group velocities of the observed surface waves. The locations of the Love‐to‐Rayleigh conversion areas concentrate near the Kuril trench. The concentration is the most clear at a low frequency of 8 mHz. This successful location of the conversion areas provides us with an important constraint on the generation of the quasi‐Love waves.

Paleotectonic structures and their influence on recent seismo‐tectonics in the south Kuril subduction zone

Abstract Bathymetric data from south of Hokkaido obtained during a cruise of R/V Hakuho‐Maru are summarized, and their correlation with earthquake occurrence is discussed. There are structural lineations on the seaward slope of the Kuril Trench, oblique to the Kuril Trench axis and parallel to the magnetic lineations in the Pacific plate. The structural lineations comprise horst‐grabens generated by normal faulting. This suggests that Cretaceous tectonic structures originating at the spreading centre affect present seismotectonics around the trench axis. The structural‐magnetic relation is compared to the case of the Japan Trench. North‐east of the surveyed area, there are two major fracture zones (Nosappu Fracture Zone and Iturup Fracture Zone) that divide the oceanic plate into three segments. If the fracture zones (FZ) and the zone of paleo‐mechanical weakness, represented by magnetic lineations, can control the direction of normal faults at a trench, the extent of the resulting topographic roughness on the seaward slope of the trench would be different across an FZ because of the differences in ages. By studying recent large earthquakes occurring in the south Kuril region, it is shown that several main‐aftershock distributions for large earthquakes in this region are bounded by the Nosappu FZ and the Iturup FZ. Two models (Barrier model and Rebound model) are presented to interpret earthquake occurrence near the south Kuril Islands. The Barrier model explains seismic boundaries seen in several examples for earthquake occurrence in the south Kuril regions. The fracture zone forming the boundary of two segments with different magnetic lineations is also the boundary of two different normal fault systems on their ocean bottom, and the difference in sea‐bottom roughness between two normal fault systems should affect the seismic coupling at a plate interface. Due to the difference of seismic coupling, earthquake occurrence is controlled by an FZ and then the FZ acts as a seismic boundary (Barrier model). Existing normal faults created by plate bending of subducting oceanic plate should rebound after its subduction (Rebound model). This rebound of normal faults may cause intraplate earthquakes with a high‐angle reverse‐fault mechanism such as the 1994 Shikotan Earthquake. The energy released by an intraplate earthquake generated by normal‐fault rebounding is not directly related to that of interplate earthquakes such as low‐angle thrust earthquakes. It is a reason why large earthquakes occurred in the same region during a relatively short period.

Polarization Anomalies of Surface Waves Recorded by a Broadband Seismometer Network in Hokkaido, Japan.

Polarization anomalies of surface waves suggest the existence of lateral variations of isotropic and azimuthally anisotropic velocity structures in the upper mantle. We investigate the polarization anomalies of fundamental-mode Rayleigh and Love waves (37 earthquakes, 128 paths) at periods of 5-30 s as recorded by a local four-station network of broadband seismometers in Hokkaido, Japan. The network has been operated by the Research Center for Earthquake Prediction of Hokkaido University since December 1988. Rayleigh waves coming from many back-azimuthal ranges show three types of particle motion anomalies, which are usually called inclined, tilted, and sloping motions. The Rayleigh anomalies observed in the data for the Vanuatu region are mainly caused by the azimuthally anisotropic structure beneath the northwestern Pacific, because the effects of the lateral eterogeneities on the inclined motions are considered to be negligible. The Love waves coming from the earthquakes located near Oregon and California, USA, show anomalous waves in the vertical and radial components. It was expected that the waves were higher-mode Rayleigh waves. We calculate synthetic waveforms with normal modes for an oceanic spherically symmetric Earth model for the August 17, 1991, earthquake off the coast of northern California, which shows significant anomalous Love waves. A comparison of the synthetic and observed waveforms suggests that the anomalous waves are not higher-mode Rayleigh waves and require the Love to Rayleigh conversion. The conversion locations concentrate in and around the Kuril trench region. The Love wave anomalies may be caused by lateral variation in the isotropic or anisotropic structures beneath the Kuril trench region.

The use of foreshocks in probabilistic prediction along the Japan and Kuril trenches

Abstract This article is intended to evaluate the probabilities of occurrence of mainshocks immediately after the occurrence of possible foreshocks and propose an optimal prediction algorithm based on possible foreshocks along the Japan and Kuril trenches and also to investigate the regional variation of foreshock activity in that region. Every earthquake or earthquake cluster defined specifically, except for small aftershocks, is treated as a possible foreshock. The parameters for defining possible foreshocks are magnitude (Mf), the size of square segment [D° (latitude) × D° (longitude)], time interval (Tf), and the number of earthquakes (Nf) having occurred in that space-time. However, Tf is fixed at 10 days in this study. The expected space of mainshocks to occur is defined as the segment where proposed foreshocks have occurred, and the expected time interval (Ta) is varied as a parameter. To estimate the probabilities of occurrence of mainshocks with magnitude ≧Mm0 after the occurrence of possible foreshocks, the following three indices, alarm rate (AR), truth rate (TR), and probability gain (PG) are adopted. The parameters of Nf, Mf, D, and Ta have a significant effect on evaluating the probabilities. The optimal values of parameters for possible foreshocks, which may provide us with a totally performed prediction algorithm, are estimated on the basis of the Precursor Information Criterion (PIC) by using the Japan Meteorological Agency (JMA) hypocenter catalog data from 1980 to 1993. The estimated values are Mf ≧ 5, Nf = 3, Ta = 5 days, and D = 0.5° for mainshocks with magnitude ≧6, which give a set of values of 13%, 25%, 617, and 75 to AR, TR, PG, and PIC, respectively; and Mf ≧ 4, Nf = 1, Ta = 1 day, and D = 0.25° for mainshocks with magnitude ≧5, which give values of 9%, 0.8%, 56, and 220 to the same indices as above. Moreover, it is found by using the data from 1926 to 1993 that there is a strong regional variation of foreshock activity; that is, four regions exhibit high activity of foreshocks, and the deeper the hypocenter of the mainshock, the smaller the percentage of mainshocks preceded by foreshocks.

Quick aftershock relocation of the 1994 Shikotan Earthquake and its fault planes

The 1994 Shikotan earthquake of Mw 8.2 occurred near the western Kurile trench where the Pacific plate is subducting beneath the North American plate. Although its mechanism is thrust‐type, it is not a typical low‐angle thrust event with a nodal plane parallel to the plate boundary that occurs at the plate boundary. In order to determine which of the two nodal planes of the main shock was the fault plane, we relocated the main shock and all aftershocks with M &gt; 5.3 using a modified joint hypocenter determination method. Earthquakes on the northwestern side of the aftershock area including the main shock are located along a plane dipping east‐southeastward with a dip of 70°. This plane is about 80 to 150 km long and 70 km wide. The main shock occurred at the north‐northeastern bottom of this plane, which is almost parallel to one of the two nodal planes of the main shock determined by Harvard University, which has a strike of N51°E and a dip of 76°. Thus we conclude that this plane is the fault plane, that the fault broke at the northeastern bottom of the fault plane and that the rupture propagated to the surface and also to the southeast. In addition to the aftershocks aligned with this fault plane, there is a secondary alignment, which is subparallel to the main fault plane and about 50 km east, where the largest aftershock of Mw 7.3 occurred on Oct. 9. Since several aftershocks occurred on this secondary fault plane on the same day as the main shock, it seems reasonable to suppose that this secondary fault plane was generated together with the main fault plane at the time of the main shock.