Kanto Earthquakes Research Articles

Advancements in Numa Terrace Investigations for Understanding the History of the Great Kanto Earthquakes

Advancements in Numa Terrace Investigations for Understanding the History of the Great Kanto Earthquakes

Recurrence and Long-Term Evaluation of Kanto Earthquakes

ABSTRACT The 1923 Taisho Kanto earthquake—an interplate event along the Sagami trough where the Philippine Sea plate subducts beneath the Kanto region—produced ground shaking, crustal deformation, landslides, and tsunamis, and caused the worst earthquake disaster in Japan. Based on seismological and geodetic data, many fault models have been proposed, extending ∼100 km from the epicenter, with a moment magnitude (Mw) of 7.8–8.2, and large slips of ∼8 m located near the epicenter and beneath the Miura Peninsula. The penultimate 1703 Genroku Kanto earthquake produced similar macroseismic effects around Sagami Bay and the Miura Peninsula, but larger coastal uplift and tsunami in the Boso Peninsula. The proposed fault models extend off the Boso Peninsula with Mw of 8.1–8.5. In 2004, the Earthquake Research Committee (ERC) classified the Kanto earthquakes as “Taisho type” and “Genroku type” with recurrence intervals of 200–400 yr and 2300 yr, respectively. In 2014, the ERC revised the long-term evaluation to a recurrence interval of 180–590 yr and a 30 yr probability of 0%–5% based on the Brownian passage time model. With the Cabinet Office, the ERC considered the source area of the maximum possible earthquake of Mw 8.6–8.7. The recent historiographical and paleoseismological studies have identified other candidates for the past Kanto earthquakes in 1495, 1433, 1293, and 878. Various combinations of these candidates give a mean recurrence interval of 210–315 yr, an aperiodicity parameter of 0.04–0.76, and a 30 yr probability of 0.0%–19%. The Cabinet Office has calculated the seismic intensity and tsunami heights of various types of Kanto earthquakes. National and local governments estimate the damage from these hazards. For the Tokyo metropolitan area, the estimated damage and occurrence probability are more significant for M ∼7 earthquakes with various types and depths, and most mitigation efforts are directed at such events.

A Bayesian approach to age estimation of marine terraces and implications for the history of the great Kanto earthquakes, central Japan

We quantitatively evaluated the emergence ages of the tectonically uplifted marine terraces, called the Numa terraces, in the southernmost part of the Boso Peninsula, central Japan. The combination of the complete dataset of the geological dating survey and the model inversion method is newly proposed. Along the Sagami Trough, the subduction boundary of the Philippine Sea Plate, M8 class interplate earthquakes are known to have repeatedly occurred and caused huge crustal deformation in adjacent regions, leaving tectonic landforms behind, such as the Numa terraces. Although many geological studies have investigated the Numa terraces, the estimation of their emergence ages contains ambiguity due to employed qualitative judgments and separate evaluations at different survey points. In this study, we first construct a unified dataset for dating throughout the southern Boso area by compiling the existing data and our new data. We also examin the lateral continuity of the terraces using a new technique to amplify subtle topographic changes in digital elevation data. Second, we propose a new method to estimate the emergence ages of terraces using the temporal distribution of the sample ages via a model of the sedimentation process and Bayesian statistic. The emergence ages of the three levels of the Numa terraces, uplifted in the prehistorical era, are determined as 5855–5455 yBP, 3345–3025 yBP, and 2125–1415 yBP, in descending order. This result robustly shows that the recurrence intervals of terrace-forming great Kanto earthquakes can vary by more than a factor of two. The robustness of the result is supported by the quantitative evaluations of the formation ages and the errors in the sedimentation process, made possible by our method for the first time. This result will provide inevitable information for further subduction zone researches and future hazard assessments.

Active deformation of Sagami Bay triggered by approach of the Izu island arc

Sagami Bay in central Japan is known as the seismogenic and tsunamigenic zone of the 1703 and 1923 Kanto earthquakes that caused devastating damage in the Tokyo metropolitan area. However, the tsunamigenic structure within the frontal wedges of the subduction zone remains unresolved after the last disastrous events. In the present study, the detailed structural features of the shallowest structure of the Sagami Tectonic Line in Sagami Bay were revealed by seismic reflection, a deep-towed high-resolution sub-bottom profiler, and swath bathymetry data. Based on our data, the middle segments of the Sagami Tectonic Line, which is distributed along the southwestern rim of the Miura Knoll, are considered highly likely to be an active fault that has been a repeatedly active feature. The trough-fill sediments of the south Sagami Bay are currently thought to be actively deformed by the compressive stress accompanying northwestward subduction of the tip of Izu-Ogasawara (Bonin) Arc crust on the Philippine Sea plate.

Ancient and Medieval Events and Recurrence Interval of Great Kanto Earthquakes along the Sagami Trough, Central Japan, as Inferred from Historiographical Seismology

Abstract To know the Ancient and Medieval events and the recurrence interval of the great Kanto earthquakes such as the 1923 and 1703 ones generated by the subduction of the Philippine Sea plate along the Sagami trough, central Japan, I examined the latest dataset of historical records. I used only rank-A (contemporary) materials in the Online Database of Historical Documents on Japanese Earthquakes and Eruptions in the Ancient and Medieval Ages. Among destructive earthquakes in the Kanto district before the sixteenth century, the 878 Gangyo earthquake, which has been suspected an inland event generated from the Isehara fault, is considered the oldest candidate of the Kanto earthquake based on the large-scale disaster and intense aftershock activity, though tsunami is not mentioned. The 1293 Showo (Einin) Kamakura earthquake can be regarded as a great Kanto earthquake, because of severe damage in the Kanto district and remarkable aftershock activity. During the fifteenth century, both of the 1433 Eikyo and the 1495 Meio earthquakes are regarded as candidates of the Kanto earthquake; the former caused severe damage around Kamakura on the northern coast of Sagami Bay with high-aftershock activity and presumably caused tsunami, and the latter seems to have caused large tsunamis at Kamakura and the west coast of Sagami Bay. Although further investigation is necessary to clarify which one is the Kanto earthquake, we can say that an interplate earthquake probably occurred in the fifteenth century. Intervals between successive events, in 878, 1293, 1433 or 1495, 1703, and 1923, range from 140 to 270 yr except for the first interval of 415 yr. It is very difficult to confirm whether a great interplate earthquake occurred or not during this interval by means of historiographical seismology, because the Kanto district was in the worst situation of poor historical records in those days.

Geohazards in coastal areas near the northernmost Sagami Trough, central Japan: review of neotectonic activity in onshore and offshore areas of the Izu island arc collision–subduction zone

Abstract The northernmost coast of Sagami Bay, central Japan, is situated in the eastern part of the Izu island arc collision–subduction zone where the Philippine Sea Plate has been subducted beneath the Eurasian Plate or North American Plate since the middle Miocene. It is an area at high risk of geological disaster because it is a developed suburban area located just 50–80 km SW of the megalopolis of Tokyo. We have used our own onshore and offshore neotectonic field data to create a summary of the geological and topographical information related to geohazards over various spatial and temporal ranges, and we provide additional information from archaeological and historical disaster records. The geological hazards and disasters are reviewed on a logarithmic timescale from 10 Ma to modern times. We examine this information with respect to the Great Kanto earthquakes that have repeatedly affected the present area, as they provide a typical example of hazard history on a collision–subduction zone that is useful not only for assessing the local risk but also for providing generic risk. The results can assist in preparedness for various geological hazards, particularly earthquakes, tsunamis, and crustal movements such as uplift and subsidence, as well as active faults and folds.

History of the great Kanto earthquakes inferred from the ages of Holocene marine terraces revealed by a comprehensive drilling survey

We measured the emergence ages of four marine terraces in the Chikura lowland, which lies to the southeast of the Boso Peninsula, in eastern Japan, to reevaluate the history of the great earthquake occurrences along the Sagami Trough over the past 10,000 years. The dates of the marine terraces are measured via radiocarbon dating of shell fossils obtained from the marine deposits. The sampling method employed in this study collects core samples using a dense and systematic drilling survey, which increased the reliability when correlating shell fossils with marine terraces. In addition, radiocarbon dating was performed with accelerator mass spectrometry, which produces more highly accurate measurements than those measured in previous studies. Moreover, we explored the surface profiles of the terraces with detailed digital elevation model (DEM) data obtained using LiDAR. The maximum emergence ages of the marine terraces were dated at 6300 cal yBP, 3000 cal yBP, and 2200 cal yBP from the top terrace excepting the lowest terrace (which was estimated at AD1703). In addition, another previously unrecognized terrace was detected between the highest and the second terrace in both the dating and the geomorphological analyses and was dated at 5800 cal yBP. The newly obtained ages are nearly a thousand of years younger than previously estimated ages; consequently, the intervals of the great earthquakes that occurred along the Sagami Trough are estimated to be much shorter and more varied than those of previous estimations. This result revises the data used in the current assessment of the probabilities of earthquakes along the Sagami Trough, which could devastate the Tokyo metropolitan area. Furthermore, it demonstrates that the current approach could be a powerful tool to increase the accuracy of assessments of the other areas with depositional marine terraces.

History of the Kanto earthquakes recorded in marine terraces and tsunami deposits in the southern coast of the Boso Peninsula, central Japan

フィリピン海プレートが沈み込む相模トラフ沿いでは，過去からプレート間地震（いわゆる関東地震）がくり返し生じてきた．歴史上に記録されている1703年元禄関東地震と1923年大正関東地震では，南関東沿岸に地殻変動を伴い，大きな津波が襲ったことが知られている．地殻変動は海岸段丘などの離水海岸地形や隆起生物遺骸として，また津波は津波堆積物として，それぞれ地形や地層に記録されている．房総半島南部沿岸では，地殻変動や津波の影響を特に大きく受け，複数のレベルに海岸段丘が発達していたり，縄文海進期の内湾堆積物中に複数枚の津波堆積物が挟まれていたりする．これらの記録を解読することで，過去7000年から8000年以上に渡る地震や津波の履歴を復元することができる．

The source model and recurrence interval of Genroku-type Kanto earthquakes estimated from paleo-shoreline data

In the southern Kanto region of Japan, where the Philippine Sea plate is descending at the Sagami trough, two different types of large interplate earthquakes have occurred repeatedly. The 1923 (Taisho) and 1703 (Genroku) Kanto earthquakes characterize the first and second types, respectively. A reliable source model has been obtained for the 1923 event from seismological and geodetical data, but not for the 1703 event because we have only historical records and paleo-shoreline data about it. We developed an inversion method to estimate fault slip distribution of interplate repeating earthquakes from paleo-shoreline data on the idea of crustal deformation cycles associated with subduction-zone earthquakes. By applying the inversion method to the present heights of the Genroku and Holocene marine terraces developed along the coasts of the southern Boso and Miura peninsulas, we estimated the fault slip distribution of the 1703 Genroku earthquake as follows. The source region extends along the Sagami trough from the Miura peninsula to the offing of the southern Boso peninsula, which covers the southern two thirds of the source region of the 1923 Kanto earthquake. The coseismic slip takes the maximum of 20 m at the southern tip of the Boso peninsula, and the moment magnitude (Mw) is calculated as 8.2. From the interseismic slip-deficit rates at the plate interface obtained by GPS data inversion, assuming that the total slip deficit is compensated by coseismic slip, we can roughly estimate the average recurrence interval as 350 years for large interplate events of any type and 1400 years for the Genroku-type events.

Geological and historical evidence of irregular recurrent earthquakes in Japan.

Great (M∼8) earthquakes repeatedly occur along the subduction zones around Japan and cause fault slip of a few to several metres releasing strains accumulated from decades to centuries of plate motions. Assuming a simple 'characteristic earthquake' model that similar earthquakes repeat at regular intervals, probabilities of future earthquake occurrence have been calculated by a government committee. However, recent studies on past earthquakes including geological traces from giant (M∼9) earthquakes indicate a variety of size and recurrence interval of interplate earthquakes. Along the Kuril Trench off Hokkaido, limited historical records indicate that average recurrence interval of great earthquakes is approximately 100 years, but the tsunami deposits show that giant earthquakes occurred at a much longer interval of approximately 400 years. Along the Japan Trench off northern Honshu, recurrence of giant earthquakes similar to the 2011 Tohoku earthquake with an interval of approximately 600 years is inferred from historical records and tsunami deposits. Along the Sagami Trough near Tokyo, two types of Kanto earthquakes with recurrence interval of a few hundred years and a few thousand years had been recognized, but studies show that the recent three Kanto earthquakes had different source extents. Along the Nankai Trough off western Japan, recurrence of great earthquakes with an interval of approximately 100 years has been identified from historical literature, but tsunami deposits indicate that the sizes of the recurrent earthquakes are variable. Such variability makes it difficult to apply a simple 'characteristic earthquake' model for the long-term forecast, and several attempts such as use of geological data for the evaluation of future earthquake probabilities or the estimation of maximum earthquake size in each subduction zone are being conducted by government committees.

History of the paleo-earthquakes along the Sagami Trough, central Japan: Review of coastal paleoseismological studies in the Kanto region

Two subduction zone interplate earthquakes have been recorded along the Sagami Trough, the first in AD 1703 (Genroku Earthquake) and the second in AD 1923 (Taisho Earthquake). While the source areas of these two events overlapped within and around the Sagami Bay, the 1703 Genroku Earthquake had a larger rupture area, which propagated to off the Boso Peninsula. Currently, our understanding of prehistorical earthquakes has been facilitated by Holocene marine terraces and tsunami deposits, through which we have come to the understanding that past Kanto earthquakes can be divided into two types - the Taisho-type and the Genrokutype. Taisho-type earthquakes are thought to be more common, occurring approximately every 400 years on average, but after several occurrences (at 2,000-2,700 year intervals) a Genroku-type earthquake would take place and propagate to off the Boso Peninsula. These less frequent coseismic Genroku-type ruptures are not consistent with global navigation satellite system (GNSS) geodetic data, which show strong plate interface coupling in this area due to a high slip-deficit rate. To resolve this discrepancy, it is necessary to consider hypotheses such as changes to the long-term slip-deficit rate, the occurrence of aseismic fault slips, or even the existence of another earthquake type. Furthermore, recent marine terrace survey reports in the Boso Peninsula have shown that the emergent ages between the eastern coast and western coast do not necessarily correlate, which lends credence to the possibility that another earthquake type occasionally occurs in the region off the Boso Peninsula.

Finite-Difference Simulation of Long-Period Ground Motion for the Sagami Trough Megathrust Earthquakes

We perform long-period ground motion simulations for Sagami Trough earthquakes by a three dimensional finite-difference method. The Sagami Trough has been the site of two well-known megathrust earthquakes, the 1923 Taisho- and the 1703 Genroku-type Kanto earthquakes. However, a lack of accumulated historical earthquake records prevents us fromobtaining knowledge of the source model of the next anticipated event for long-period ground motion hazard evaluation. Therefore, it is important to consider numerous possibilities for the unknown source parameters. We compare ground motions for several scenarios with different source area, and with magnitudes ranging from Mw7.9 to 8.6. Peak ground velocity (PGV) within the Kanto basin, including the Tokyo metropolitan area, differs by several times depending on the choice of the source area. The effects of the variety in fault parameters, such as rupture starting points and asperity patterns, are also studied. They can greatly vary the ground motion within the Kanto area, especially in the direction of rupture propagation, suggesting the severe impact of directivity effects. Source models with different rupture starting points produce PGV and 5% damped velocity response (Sv) that vary from each other by as much as 10-20 times. PGV and Sv vary by up to five times depending on the asperity pattern. Our simulation results show that the predicted ground motion for the earthquake scenarios strongly depends on both the source size and other fault parameters of the source models. It is suggested that the seismic hazard assessment requires statistical evaluation of ground motions from as many source models as possible in order to overcome the uncertainties of the source.

Geological evidence of recurrent great Kanto earthquakes at the Miura Peninsula, Japan

[1] The Tokyo metropolitan area's well-documented earthquake history is dominated by the 1703 and 1923 great Kanto earthquakes produced by slip on the boundary between the subducting Philippine Sea plate and the overlying plate. Both earthquakes caused ∼1.5 m of uplift at the Miura Peninsula directly above the inferred fault rupture, and both were followed by tsunamis with heights of ∼5 m. We examined cores ∼2 m long from 8 tidal flat sites at the head of a small bay on the peninsula. The cores penetrated two to four layers of shelly gravel, as much as 0.5 m thick, with abundant shell fragments and mud clasts. The presence of gravel indicates strong tractive currents. Muddy bay deposits that bound the gravel layers show vertical changes in grain size and diatom assemblages consistent with abrupt shoaling at the times of the currents. The changes may further suggest gradual deepening of the bay during the intervals between the strong currents. We infer, based on 137Cs, 14C, and 210Pb dating, that the top two shelly gravel layers represent tsunamis associated with the 1703 and 1923 great Kanto earthquakes, and that the third layer was deposited by a tsunami during an earlier earthquake. The age range of this layer, AD 1060–1400, includes the time of an earthquake that occurred in 1293 according to a historical document. If so, the recurrence interval before the 1703 earthquake was almost twice as long as the interval between the 1703 and 1923 earthquakes.

Japan's Tohoku Earthquake and Tsunami

The magnitude 9.0 Tohoku or Sendai Earthquake (Fig. 1) struck just off the northeast coast of Honshu, Japan on 11 March 2011 making it the fourth largest earthquake to be recorded since 1900, and the largest Japanese earthquake since modern seismometers were developed 130 years ago. Despite the earthquake being much more powerful than had been expected from the subduction zone east of Honshu, the earthquake preparedness of Japan resulted in relatively little earthquake damage—despite the protracted shaking with ground accelerations up to three times that of gravity. However, it was the resulting 10–15 metre high tsunami waves that wreaked havoc along the coastal plain, resulting in a death toll in the tens of thousands and an on‐going drama at the Fukushima I nuclear power plant. Modern seismology has its origins in the analyses of the 1906 San Francisco and 1923 Great Kanto earthquakes. The 2011 Tohoku (or ‘northeast’) earthquake looks set to similarly significantly advance our understanding of earthquakes and tsunamis due to the unprecedented volume of seismic, GPS, tide gauge and video data available. There is much information to be gained on how large earthquakes rupture, how buildings behave under prolonged severe shaking and how tsunamis propagate.Tohoku earthquake global displacement wavefield from IRIS. http://www.iris.edu/hq/files/iris_news/images/Sendai_RS.jpg image

三浦半島南部，小網代湾の津波堆積物調査から推定された過去三回の関東地震:--1703年元禄関東地震の一つ前の関東地震について--

三浦半島南部，小網代湾の津波堆積物調査から推定された過去三回の関東地震:--1703年元禄関東地震の一つ前の関東地震について--

Crustal block kinematics and seismic potential of the northernmost Philippine Sea plate and Izu microplate, central Japan, inferred from GPS and leveling data

We clarify the contemporary deformation observed by GPS and leveling for the greater Tokyo (Kanto, eastern Tokai, and the Izu islands) region, where the Izu‐Ogasawara (Bonin) arc is subducting and colliding with the central part of the Japan arc. From these data, we develop a kinematic model of fault sources with variable components of seismic and aseismic slip. Under the assumption that the contemporary deformation during 1995–2000 is representative of the long‐term interseismic strain field, the geodetic data are inverted to estimate the rotation poles of the crustal blocks, the degree of elastic strain accumulation on faults, and the volumetric inflation sources beneath volcanoes. The present crustal movements are explained by four crustal blocks: the Izu microplate, the central Japan block, the Pacific plate, and the Philippine Sea plate. Along the Suruga and Sagami margins of the Philippine Sea plate lie strongly coupled faults, which include sites of the 1854 M = 8.4 Tokai, 1923 M = 7.9 Kanto, and 1703 M ∼ 8.2 Genroku Kanto earthquakes. In contrast, the Philippine Sea plate boundary immediately north of the Izu peninsula, site of M ≤ 7.5 collisional earthquakes, is only weakly coupled. The boundary between the Izu microplate and the Philippine Sea plate experiences left‐lateral motion with the rate of ∼30 mm/yr. Most of this boundary is locked and thus has a large potential of future earthquakes. The Izu microplate is found to rotate rapidly clockwise at 10°/Myr, with a rotation pole relative to the central Japan block located just north of its northern boundary. The pole and angular velocity of the Izu microplate depends, however, on the assumed location of the boundary. With the exception of the site of the 1938 M ≥ 7 earthquake swarm, which we infer has the current potential to produce a M ∼ 8.1 earthquake, the Pacific megathrust has a very low seismic potential, although the geodetic data have little resolving power for much of the Pacific plate interface east of Tokyo.

首都圏直下型地震の危険性の検証―本当に危険は迫っているのか？―

I discuss the danger of earthquakes occurring directly beneath the metropolitan area from temporal and geographical viewpoints. Temporally, large (M>7) earthquakes in Kanto occur 70 years before and a few years after great interplate Kanto earthquakes. The recurrence times of such great earthquakes are more than 220 years. Because 80 years have passed since the last one, at least60 years remain before reaching the active period. It is not legitimate to calculate the probability of large earthquakes occurring in the near future, using the rate of occurrence during the active period before a great earthquake. From a geographical viewpoint, S. Kanto is located in the outer zone south of the Median Tectonic Line, where few active faults are distributed. However, in S. Kanto, exceptionally, the Tachikawa fault and the 1855 Ansei-Edo earthquake are located in the outer zone. This zone is specified by a low-velocity zone in the mantle wedge of the upper plate. Dehydration from the subducting slab may weaken the upper plate in this zone, producing anomalous intraplate earthquakes. The upper crust above this low-velocity area should be marked especially as an area having a potential for large earthquakes in the future. The probability of M7 earthquakes being generated at the interface between the Philippine Sea and the upper plates is small. Temporally, the danger of large earthquakes occurring in the near future beneath the metropolitan area does not seem to be large.

Seismotectonic modeling of the repeating M 7-class disastrous Odawara earthquake in the Izu collision zone, central Japan

Odawara City in central Japan, in the northernmost margin of the Philippine Sea (PHS) plate, suffered from severe earthquake disasters five times during the last 400 years with a mean repeat time of 73 years; in 1633, 1703, 1782, 1853 and 1923. In this region, non-volcanic Izu outer arc (IOA), the easternmost part of the PHS plate, has been subducted beneath Honshu (Japanese main island), and volcanic Izu inner arc (IIA) on the west of IOA has made multiple collision against Honshu. I hypothesize ‘West-Sagami-Bay Fracture’ (WSBF) beneath Odawara, a north-south striking tear fault within the PHS plate that has separated the descending IOA crust from the buoyant IIA crust, through examinations of multiple collision process and the PHS plate configuration. WSBF is considered a blind causative fault of the 1633, 1782 and 1853 M 7 Odawara earthquakes, and is inferred to have ruptured also during the 1703 and 1923 great Kanto earthquakes simultaneously with the interplate main fault. A presumable asperity on WSBF just beneath Odawara seems to control the temporal regularity of earthquake occurrence. Though WSBF has not yet been detected directly, it is considered an essential tectonic element in this region, which might be a fracture zone with a few or several kilometer thickness actually. The WSBF hypothesis is the only conceptual model to explain the earthquake recurrence beneath Odawara.

Source Time Function and Duration of Mexican Earthquakes

We have compiled source-time functions (STFs) of 54 Mexican earthquakes (3.0 ≤ M ≤ 8). The depth, H , of 48 of these earthquakes lies between 10 and 50 km. The STFs of 43 earthquakes were determined from local and regional strong-motion and/or broadband recordings. For the rest of the events, the STFs were taken from published analysis of teleseismic body waves, or from the University of Michigan Source-Time Function catalog. If available, the seismic moments were taken from the analysis of teleseismic body waves; if not, as was often the case for earthquakes with M ≤ 5.5, they were estimated from the local recordings themselves. From the STF of each earthquake we have measured pulse duration of each subevent (τp), total rupture duration (τt), and rise time (τr), along with their respective seismic moments. The data fit the relation \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(M\_{0}{/}{\tau}\_{\mathrm{p}}^{3}{\sim}6.7{\times}10^{23}\) \end{document} dyne cm/s3. This relation is similar to that reported for deeper earthquakes (mostly in the 50 to 125 km depth range) in the Kanto district of Japan. The pulse duration of Mexican and Kanto earthquakes, scaled to a seismic moment of 1026 dyne cm, τps, is about 5 s, nearly independent of depth for 10 ≤ H ≤ 125 km. We compliment these two data sets with those for California (H ≤ 20 km) and for deep earthquakes ( H ≥ 100 km). When viewed over the entire depth range, the total rupture duration (τt) is related to M by log τt = (0.363 ± 0.008) log M - 8.58 (±0.190), and the duration, within the scatter of the data, appears to be independent of depth. When the data are examined in smaller subsets, as has been done by several authors, certain trends manifest themselves. For example, the pulse and the total duration of deep earthquakes ( H ≥ 100 km) scales as \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(M_{0}^{0.16}\) \end{document} and \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(M_{0}^{0.25}\) \end{document}, respectively, and the moment-scaled pulse and total duration decreases by about 20% between 100 and 600 km depth. This decrease is much less than that reported previously. Furthermore, an earlier finding of fast rise times for very deep earthquakes is not substantiated by the recent data. Thus, the anomaly in the duration of the deep earthquakes, as compared to the shallow ones, appears to be smaller than previously suspected. Manuscript received 15 June 1999.

Three-dimensional finite-difference simulations of long-period strong motions in the Tokyo metropolitan area during the 1990 Odawara earthquake (MJ5.1) and the great 1923 Kanto earthquake (MS8.2) in Japan

AbstractUtilizing a crustal velocity model that includes the complexity of the irregular subsurface structure of the Kanto basin, we have performed three-dimensional (3D) finite-difference (FD) simulations of near-source long-period strong ground motions in the Tokyo metropolitan area for the 1990 Odawara (MJ 5.1) and the 1923 Kanto (MS 8.2) earthquakes. Constraints on the development of the 3D velocity model come from available geological and geophysical data, as well as our previous 1D waveform modeling results (Sato et al., 1998a). The simulation of the moderate-sized Odawara earthquake demonstrates that the 3D velocity model works quite well at reproducing the recorded long-period (T > 3.33 sec) strong motions, including basin-generated surface waves, for a number of sites located throughout the Kanto basin region. Using this validated 3D model along with the variable-slip rupture model of Wald and Somerville (1995), we then simulate the long-period (T > 4 sec) ground motions in this region for the 1923 Kanto earthquake. The simulation results for the 1923 event show that the largest ground motions occur east of the epicenter along the central and southern part of the Boso Peninsula. These large motions arise from strong rupture directivity effects and are comprised of relatively simple, source-controlled pulses with a dominant period of about 10 sec. North of the epicentral region, in the Tokyo area, 3D basin-generated phases are quite significant, and these phases produce large-amplitude late-arriving pulses in the ground motions. At station Hongo (HNG), which is the only site having digitized and restored near-fault strong-motion records for this event, our 3D simulations compare quite well with the ground motions of the restored Imamura seismogram. For the restored Ewing record, our 3D simulations reproduce the phase and amplitude of the initial pulses of motion; however, the dominant period of the large-amplitude later phases is noticeably shorter in the simulations (about 5 to 6 sec) than in the observation (13 sec). These results suggest that the restored Imamura seismogram may be a better representation of the gross features of the actual motion than the restored Ewing seismogram, although the first (clipped and restored) part of the Imamura seismogram may still underestimate the strength of the actual motion.