Earthquakes can be classified into three groups geophysically : (a) Interplate earthquakes, which occur at the boundary between two plates ; (b) intraslab earthquakes, which occur within the subducting plate ; and, (c) intraplate earthquakes, which occur within the overriding plate. Directly beneath earthquakes are defined as earthquakes that occur just beneath people or artificial structures, causing casualties and damage. All three types of earthquake can be directly beneath earthquakes if they occur beneath or close to a land area. Earthquakes occurring beneath the Tokyo Metropolitan Area are those within : (1) the Okhotsk plate (type c); (2) at the interface between the Philippine Sea and Okhotsk plates (type a); (3) within the Philippine Sea slab (type b); (4) at the interface between the Philippine Sea and Pacific slabs (type a); and, (5) within the Pacific slab (type b). Although all of these types of event could be directly beneath earthquakes, damage caused by (1) or (2) seems to be larger than that caused by the others. There does not seem to be a consensus yet on rating the probabilities of the future occurrence of these events. It is important to conduct future studies on what types of earthquake pose a future threat beneath the Tokyo Metropolitan Area. An earthquake as a geophysical phenomena is not a disaster. When an earthquake causes various human casualties, it is called a disaster. If a huge scale of damage is caused by an intermediate scale of earthquake, even if the earthquake is not so severe the result might be a largescale disaster. When the next earthquake of M7 class hits central Tokyo, it is imaginable that it will cause a large-scale disaster. It is called the next Tokyo Earthquake which is an earthquake occurring directly beneath the Tokyo Metropolitan Area, in this volume. We have to prepare various measures against the next Tokyo Earthquake, which require us to understand the general nature of the disaster caused, and to estimate the extent of damage. In 2005, the Cabinet Office, Government of Japan conducted damage estimation research on earthquakes occurring below the Tokyo Capital Region. Eighteen types of earthquake were evaluated under various conditions including season, time of occurrence, and wind speed. In 2006, Tokyo Metropolitan Government (TMG) also conducted damage estimation research on the next Tokyo Earthquake to understand the damage that would occur under the jurisdiction of each government- Special ward. of central Tokyo, City, Town, and Village- because each local government has to revise its earthquake disaster management plan. According to these damage estimations, the Northern Tokyo Bay Earthquake of M7.3 causes most damage under conditions of : occurring on a weekday evening in winter with strong winds of 15m/s. In the case of Cabinet Office, Government of Japan research, 850, 000 houses collapse or are destroyed by fire, and there are 11, 000 fatalities. This scale of damage to housing is eight times that of the Hanshin-awaji Earthquake in 1995. As a result, this must be called a super urban disaster, compared to the Hanshin-awaji Earthquake, which was an urban disaster. Cabinet Office, Government of Japan published “General Policy Principles Relating to Countermeasures Policies for Tokyo Earthquakes” in 2005 and the “Tokyo Earthquake Disaster Mitigation Strategy” in 2006. TMG revised the “Earthquake Disaster Management Local Plan” in 2007. TMG plans to reduce damage by half over the next decade.
We present a new view of the morphology of slab (s) subducted just beneath the Tokyo Metropolitan Area in Japan. Previously, several different models of the surface geometry of the subducted Philippine sea plate slab (PH slab) have been published mainly using seismicity data (e.g., Nakamura and Shimazaki, 1981; Maki, 1984; Kasahara, 1985; Ishida, 1992; Noguchi, 1998; Hori, 2006). In this study, first we discriminate a previously unknown seismic slab (called slab SG, or seismic slab SG) above the downgoing Pacific plate slab (PC slab), second identify the possible internal structure of slab SG, and third demonstrate tectonic evolution models. It is clear that the currently known surface contours of PH slab indicate the shallowest part of slab SG as well. Most previous studies assumed a PH slab with a constant thickness, and paid little attention to the tectonic characteristics of the vertical extent and/or the bottom geometry of slab SG with variable thickness. The bottom extent of the seismic slab SG beneath the Metropolitan area reaches 36.5N at least. The horizontal extent of seismic slab SG covers most of the lowland Kanto Plain. The bottom depth of slab SG is approximately 120 km near 36.5N and 139.0E, being the same as the surface depth of the PC slab there. Below the Sagami Trough axis near 34.5N and 140.0E, the lowest portion of slab SG is located at a depth of 80 to 90 km. The western bottom end zone of slab SG generally strikes in the NNW-SSE direction, being approximately parallel to the volcanic front. We suggest four basic morphology models of slab SG as follows. (1) Slab SG consists of both the PH slab at a shallower depth and a deeper underlain slab (slab SL). (2) A bookshelf-like configuration of northwardly inclined multi-slabs on the PC slab due to the intermittent southward shift of accumulation sites of short slab tips with episodic subduction at just south of the previously active paleo-Sagami Trough (s). We emphasize that the above evolutional bookshelf model is, to some extent, similar to the sediment layer accretion process near the deep trench system, but the dynamic situation is not the same. (3) A structure combining models (1) and (2). (4) Slab SG is merely the eastern part, having been cooled by the downgoing PC slab, of 65-70 km thick lithosphere of the Izu Outer Block (JOB) without any other slab components. To clarify the structure of the SG slab in more detail, we should incorporate high-gain seismic data from the on-line operating dense seismic observation network deployed at both marine and land areas covering the metropolitan area. In addition, we must study the effects of 3D mantle wedge circulation with dehydration process due to the subduction of both PC and PH slabs from different directions, as well as the space-time evolution history of accretion tectonics at the northern end zone of the Philippine sea plate.
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.
The Tachikawa fault, which is the only active fault in the Tokyo Metropolis, is expected to be the source of shallow intra-plate earthquakes in the future. We performed additional trenching surveys at the northwestern part of the fault to obtain the paleoseismological parameters for evaluating earthquake potential caused by the fault. Trench wall observation, radiocarbon dating, and tephra analyses constrained the last surface-rupturing event at the Tachikawa fault between about 13, 700 cal yBP and 12, 800 cal yBP. The vertical separation of the event is estimated to be more than 2.6 meters.
The cabinet office of the Japanese government demonstrated the prospects of future seismic hazards associated with a working model for possible earthquakes in the capital area of Japan. If this assumption is not unrealistic, it is reasonable to use this working model. However, it has already been reported that several active faults may exist in this area. This discrepancy can lead the assessment into unreal issue. I reveal the nature of the Ayasegawa fault located close to the capital area on the basis of geomorphic features. The fault extends in the NWSE direction for at least over 30 km, and the fault trace is linear, which is indicative of lateral movement. There is a graben structure delineated by the fault in the Minuma ward, Saitama City. The vertical component of the Ayasegawa fault is upthrown to the southwest and the average vertical slip rate is 0.05 to 0.1 m/ky. The netslip rate should be much larger than the vertical one, taking lateral movement into account. Although the single vertical offset is assumed to be 0.8 to 4 m, the rupture history of the fault remains unknown. The Ayasegawa fault is an southeastern extension of the Fukaya fault, and is composed of an active fault extending more than 120km through the Kanto Plain across the capital area of Japan. To prepare for a real seismic hazard and to try to reduce damage, we should check the properties and clarify the rupture history of these active faults as an urgent task. Precise local information on these active faults is necessary for motivating people to develop an awareness of disaster mitigation.
The northern Ayasegawa fault is a part of the Fukaya fault system, which is the longest active fault in the Kanto district. The paleoseismology of the northern Ayasegawa fault was revealed by a combination of arrayed boring and ground penetrating radar (GPR) survey. The northern Ayasegawa fault produced a fold scarp with the NW-SE direction running along the boundary between the Oomiya 2 (O2) surface and fluvial lowland. The O2 was formed in Marine Isotope Stage 5a, and was slightly deformed with a wide warping zone. Sixteen sediment cores arrayed across the warping zone contain a series of tephra layers such as Hk-TP (ca. 60-65 ka), KMP, AT (26-29 ka), As-BP group (20-25 ka), and As-YP (15-16.5 ka). These key beds except Hk-TP were deposited and deformed parallel to each other, suggesting that no faulting events occurred between KMP and As-VP fall. The timing of the last faulting event is after the As-YP fall, and is probably younger than 10 ka based on an interpretation of GPR profiles and 14C ages. KMP should be deposited horizontally because it intervened in the peaty silt layer, which accumulated conformably on lacustrine deposits overlapping the fold scarp. Thus, the KMP horizon roughly indicates the vertical offset produced by the events occurred after the As YP fall. The events were probably singular, and the last one formed a vertical offset of more than 4 m. The older event occurred at around 70 ka between Hk-TP fall and O2 formation. Vertical deformation of the O2 was at least 7 m, indicating the possibility that the vertical offset caused by the penultimate event is at least 3 m. The vertical slip per event might reach 5 m, and the average vertical slip rate is nearly 0.1 mm/yr because the warping zone detected by the arrayed boring above is within the flexure zone shown by the P-wave seismic profile. The northern Ayasegawa fault is considered to be a single behavioral segment because of its longer recurrence interval and lower slip rate of 0.1 mm/yr in comparison with those of the other part of the Fukaya fault system.
The Central Disaster Management Council presents several scenarios of hazardous earthquakes that might strike the Tokyo metropolitan area in the near future. After the Great Hanshin Earthquake (January 17, 1995, M7.3 on the JMA scale) which caused devastating damage to both human lives and economic activities, studies on major active faults throughout Japan were accelerated to evaluate their potential for producing serious earthquakes. Within the past decade, three inland earthquakes of M6.9 to 7.3 occurred in Japan without obvious surface ruptures. The faults that caused those earthquakes, had not been identified previously by active fault researchers. Since historic times, the Tokyo metropolitan area has been heavily inhabited and artificially modified by various constructions ; therefore its original geomorphologies, with which active faults are deciphered, have been almost lost to date. The authors summarized data on Quaternary faults found at three construction sites and twelve records of seismic profiles, and reexamined borehole data on restricted places in the metropolitan area. This revealed four concealed faults displacing middle to late Pleistocene deposits in Chuo Ward and one in Koto Ward. These concealed Quaternary faults are classified as Class C active faults with an average slip rate of 0.1 to 0.01m/1000 years. Active faults, however, have not been studied in the central metropolitan area for some reasons. The authors would like to call for an immediate full-scale active fault study to prepare for earthquake disasters in the heart of Tokyo.
The Tokyo metropolitan area is known to have been struck by large earthquakes due to the subduction of the Philippine Sea Plate and the Pacific Plate beneath the North American plate. Recent damaging earthquakes that occurred beneath Tokyo include the 1855 Ansei Edo earthquake, the 1894 Meiji Tokyo earthquake, and the 1923 Kanto earthquake. Whereas the Kanto earthquake is known to have occurred at the top of the subducting Philippine Sea Plate, the other events are considered to have occurred in Tokyo bay, but their source depths are unknown. Many researchers have attempted to determine the source mechanisms of these earthquakes through analyses of patterns of seismic intensity distribution in the Kanto area, but the intensity pattern at the center of Tokyo would be considerably affected by the site amplification effect of the shallow, localized structure rather than be related directly to the source itself. In the present paper, we summarize the characteristics of strong ground motions and damage caused by the earthquakes. We then compare the pattern of intensities on local and regional scales with those of recent earthquakes occurring in Tokyo and corresponding computer simulations using heterogeneous crust and upper-mantle structure models below Tokyo to find referable source models for the Ansei Edo and Meiji Tokyo earthquakes.
The Headquarters for Earthquake Research Promotion of Japan published national seismic hazard maps for Japan in March 2005, at the initiation of the Earthquake Research Committee of Japan (ERCJ), on the basis of a long-term evaluation of seismic activity and a strong-motion evaluation. The National Research Institute for Earth Science and Disaster Prevention also promoted a special research project titled “National Seismic Hazard Mapping Project of Japan” to support the preparation of seismic hazard maps. Under the guidance of ERCJ, we carried out a study of hazard maps. There are two types of hazard map : one is a probabilistic seismic hazard map (PSHM), which shows the relation between seismic intensity value and its probability of exceedance within a certain time, the other is a scenario earthquake shaking map. In this paper, we focus on the PSHM for the Kanto region. We explain the modeling of seismic activity in the Kanto region and the methodology for the probabilistic seismic hazard analysis. Probabilistic seismic hazard maps for the Kanto region show the probability of ground motions equal to or larger than JMA seismic intensity 6-within 30 years. We separate contributions of various types of earthquake and make a PSHM for each. We can see that interplate and intraplate earthquakes without specified source faults make large contributions in the Kanto region.
Liquefaction and damage caused by anticipated near-field earthquakes in the Tokyo metropolitan area are outlined on the basis of official reports released by Central Disaster Management Council of Cabinet Office, Japan, and Tokyo Metropolitan Disaster Management Council. The history of liquefaction during past earthquakes in the area is introduced and correlation between past occurrences of liquefaction and seismic intensity is discussed. Finally, comments are presented on some aspects of liquefaction-induced damage and points of concern during future earthquakes.
The Central Disaster Management Council conducted damage estimation research on the next Tokyo Metropolitan Earthquake. The estimation clarified that the earthquake would bring not only a huge amount of physical damage, but also serious influence on the metropolitan functions such as politics, administration, and economy. The “Policy Framework for Tokyo Metropolitan Earthquakes” was set to secure the continuity of metropolitan functions as well to reduce damage. The “Tokyo Metropolitan Earthquake Disaster Reduction Strategy” was also shown by setting quantitative disaster reduction objectives with definite deadlines and concrete plans to execute disaster reduction measures effectively and properly. Furthermore, the “Guidelines for Tokyo Metropolitan Earthquakes Emergency Response Plan”, which provides the contents of emergency activities in a large area, and procedures and roles of government in the event of a disaster, was regulated. Measures for evacuees and people stranded without bearable means of returning to their homes, and contingency plans for central governments are under discussion.
The Tokyo Metropolitan Disaster Management Council, summarized a report on “Damage estimation for an earthquake directly underneath Tokyo” in May, 2006. Such earthquakes are supposed to imminently occur as M7.3 earthquake together with M6.9 one beneath the northern Tokyo Bay, because an earthquake of magnitude level 6 is more frequently to take place than the M7 one. In addition to the traditional estimation of human damage, it newly includes the number of stranded persons at large terminal stations and locked-in people in elevators as some typical features of disasters in modern cities. The northern Tokyo Bay earthquake produces ground shake of the Japan Meteorological Agency seismic intensity scale 6+ in the eastern wards area owing to bad ground structure. The number of houses collapsed by shock and/or liquefaction as well as those burnt down by fire increases in the ring-shaped zone of wooden houses in the wards area. In case of M6.9 earthquake, human damage amounts to about 2, 800 people dead, 51% of which due to fire, about 75, 000 people injured, 43% of which due to house collapse, 32% due to fall down of furniture etc. The number of refugees amounts to 2.71 million people, while stranded persons to 4.48 million people. About 7, 500 elevators stop to cause locked-in troubles. Tokyo Metropolitan Government revised the earthquake disaster countermeasure plan based on the new damage estimation report in May 2007.
The Mid-Niigata Earthquake caused a lot of landslides and subsidence of reclaimed land used for residential housing. Landslide damage has occurred historically, and was notable following the Miyagi-ken offshore Earthquake in 1978 with widespread damage to reclaimed land in urban areas. In 2006, Ministry of Land, Infrastructure and Transport revised the “Land Reclamation Act, ” which was promulgated in 1961, to retrofit and reinforce reclaimed districts against earthquakes. According to the revised Act, local governments have to implement risk assessment research on reclaimed land, and publish a risk-map of reclaimed lands. High-risk districts are designated as retrofitting zones. A new subsidy system involving both national and local governments promotes retrofitting and reinforcement projects for reclaimed land in these zones.
The Building Standards Act was enacted in 1950 to promote safe buildings in Japan. In 1981, the seismic code was largely revised, requiring that buildings would not collapse, causing casualties, under a JMA seismic intensity of scale 6 upper. Buildings built before 1981 adopting the old seismic code but not the new one are classed as “existing non-conforming buildings”. These buildings do not conform to the new seismic code, however, damage to buildings built before 1981 caused by the Great Hanshin-Awaji Earthquake in 1995 was much greater than that to buildings constructed after 1981. This resulted in the enactment of Building Retrofitting Promotion Act in 1995, which aims to promote anti-seismic performance checks and seismic retrofits. This paper shows the situation of current retrofitting and policy development. It indicates the situation of administrative guidance to owners of about 90, 000 specific buildings not constructed to an anti-seismic design. It is significant to promote seismic retrofits for houses that constitute the majority of buildings. In 2003, the total number of houses was estimated to be about 47, 000, 000, of which about 18, 500, 000 were built before 1980. 11, 500, 000 of these are estimated to be vulnerable to earthquakes. As a policy development, the target is to increase the current seismic retrofit rate of 75% to 90% by 2015. To achieve this, the Building Retrofitting Promotion Act has been amended, and retrofitting is being promoted by local governments, strengthening administrative guidance to owners of specific buildings, providing support and financing for seismic retrofits, as well as support through the tax system.
Active fault information has been more widely and closely researched, and extensive data have been collected. With the progress, earthquake disaster reduction measures based on active fault information should be examined. The purpose of this paper is to arrange disaster reduction measure cases that have been considered with active fault information, and to indicate problems and directions to promote disaster reduction measures based on a questionnaire survey. Disaster reduction measures based on active fault information are distinguished into three types : building restrictions, non-building restrictions, and risk communication. The questionnaire survey was carried out with residents of Yokosuka city. The questionnaire items concerned the recognition and perception of regional earthquake risk, the perception of general earthquake risk, the awareness of general disaster reduction measures, and the awareness of disaster reduction measures based on active fault information. The results show that there are also three factors in the perceptions of residents, which are building restrictions, non-building restrictions, and risk communication, and their measures may be implemented with the recognition of residents on the need for land use controls based on active fault information. Other analysis results indicate that earthquake risk perception is high, but there are no significant correlations between individual earthquake risk perceptions and the awareness of disaster reduction measures. To promote disaster reduction measures based on active fault information and earthquake risk, it is important that such awareness is connected.
In 2006, the Tokyo Metropolitan Disaster Management Council revised the estimation of major damage in the eastern wards of Tokyo metropolis for an earthquake beneath northern Tokyo Bay (M7.3) with a JMA seismic intensity of scale 6 upper. It is assumed that 12, 337 houses in Sumida City would collapse completely. The number of wooden dwelling houses built before 1981 adopting the old seismic code totals 21, 898 in Sumida City, of which 6, 863 are located in the most dangerous area with a high density of wooden dwelling houses. Seismic retrofitting of these houses is urgently needed. This paper reports on the countermeasures planned by Sumida City. Sumida City has designed a support program for safe living spaces, which promotes antiseismic performance checks and seismic retrofits. A financial support system for retrofitting provided by Sumida City began in January 2006, and this system is characterized by dividing seismic retrofits into easy retrofits and regular ones, with application to residents of rental housing. The Sumida Anti-seismic Reinforcement Forum 2006 was held in February 2006 to announce this system. Moreover, the Sumida Quakeproof Reinforcement Promotion Conference, which exercises governance involving inhabitants, companies, and local government, was organized in June 2006. This conference is now campaigning for seismic retrofits through meetings held at every community center of each community in the city.
Itabashi City, one the of 23 special wards of Tokyo, established “The Basic Ordinance for Disaster Mitigation” in 2002. The ordinance was discussed by a civic committee at four evening meetings. The ordinance is necessary to implement various disaster mitigation measures, not only by the local government but also by residents and companies. It clarifies three points of an important strategy for disaster mitigation : (1) fundamental concept of disaster mitigation and its communication to government and residents, (2) importance of disaster mitigation drills and education, and (3) importance of pre-event measures for damage reduction. The important issues for pre-event measures are a total education system for people ranging from children to the elderly, community improvements and retro-fitting of houses and facilities, and a support and subsidy system for disabled persons in the event of a disaster.
Considering the immense scale of damage caused by the next Tokyo Earthquake, occurring beneath a mega-city that is the capital of Japan, a quick response requires pre-disaster measures and post-disaster recovery planning to mitigate economic losses and damage to central functions. Tokyo Metropolitan Government (TMG) learned from the Great Hanshin-awaji Earthquake of 1995, and has prepared such recovery measures. Because a huge amount of damage is expected on a scale approximately ten times as great as that caused by the Great Hanshin-awaji Earthquake, restoration from the next Tokyo Earthquake cannot be completed without the participation of citizens and enterprises. In particular, it is important for the recovery of damaged districts to be implemented through community power. Accordingly, TMG published the “Planning Manual for Post-Disaster Urban Restoration Planning” in 1997, and “Administrative Manual for Post-Disaster recovery Measures for Living” in 1998. At the same time, a new type of drill for urban recovery after an earthquake, rather than disaster response activities, was developed by TMG and Tokyo Metropolitan University. This paper summarizes pre-disaster measures for post-disaster recovery planning prepared. by TMG. Two drill programs are introduced : (1) community restoration drill program for citizens and (2) administrative staff training for post-disaster urban reconstruction planning. Finally, the positioning and roles of these two programs among disaster reduction measures are discussed.
Large and prolonged shaking with long-period ground motions having periods of about 7 sec were observed in central Tokyo during the Off Niigata-ken Chuetsu, Japan, M6.8 earthquake on 16 July, 2007. The observed ground motions from a dense nationwide strong motion network (KNET and KiK-net) demonstrate clearly that the long-period ground motions consist of Rayleigh waves, which developed at the northern edge of the Kanto Basin and were induced by conversion from the S waves radiating from the earthquake source. The amplitude and the duration of the long-period surface waves were enhanced dramatically as they propagated in the Kanto Basin, which has a thick cover of sedimentary rocks overlaying rigid bedrock. Observed ground motions of long-period signals at the center of Tokyo from the 2007 Off Niigata-ken Chuetsu event correlated well with observations from the Chuetsu earthquake on 23 Oct. 2004 (M 6.8). By analyzing waveform data from the main shock and aftershocks of the 2007 Off Niigata-ken Chuetsu earthquake and the Chuetsu earthquake in 2004, it is found that the long-period surface wave having a dominant period of about 7 sec at the center of Tokyo is developed efficiently by a large earthquake with a magnitude greater than about M6.5-7, but it is not developed by small earthquakes of less than about M6.5.