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Research on earthquake potential evaluation by active faults
Yuichi SUGIYAMA（Earthquake Research Department, GSJ）
2000
vol. 51 (9) P.377-378
2 figs
Keywords: active fault, earthquake potential, earthquake probability, seismogenic fault, behavioral segment, Kinki Triangle
Abstract : Research on earthquake potential evaluation by active faults has focused on the systematic investigation of major active faults in the Kinki Triangle where the 1995 Kobe earthquake occurred. We have studied 25 active faults in the region by the end of FY 1998. These faults were rearranged into 18 seismogenic faults, and each seismogenic fault was then divided into several behavioral segments on the basis of fault parameters obtained by comprehensive investigations. We have assessed earthquake potential of each segment based on the earthquake probability calculated from the average recurrence interval and elapsed time since the last rupture. The 100-year earthquake probability of the 24-km-long Aibano segment on the west coast of Lake Biwa is estimated at 12 to 38%. The 44-km-long Uemachi segment traversing Osaka City has a maximum 5% earthquake probability in the 21st century. On the other hand, at least 14 behavioral segments including the Hokudan segment that produced the Kobe earthquake have relatively low earthquake potential. Their probabilities of producing a characteristic earthquake in the next century are estimated to be less than 0.2%. We have also revealed the age of paleofaulting events and long-term slip rate of several major faults in central Japan, such as Itoigawa-Shizuoka Tectonic Line, Kozu-Matsuda Fault and Fujikawa river-mouth Fault zone. These results made a big contribution to the earthquake potential evaluation of the above three faults by the Earthquake Research Committee at the Headquarters of Earthquake Research Promotion.
Groundwater Monitoring Studies in the Tokai Intensified Observation Area and Other Special Observation Areas for Earthquake Prediction Research
Eikichi TSUKUDA(Director of Research Planning Office, GSJ)
2000
vol. 51 (9) P. 391-400
11 figs
Keywords: groundwater monitoring system, telemeter network, Tokai earthquake, earthquake prediction, active fault, the Kinki and Chubu areas
Abstract : In this paper, the author as the project leader introduces the groundwater monitoring system in the Tokai-Izu and Chubu-Kinki districts, which was developed during the research period from 1994 to 1999. The Tokai region, a central part of Honshu (Main Island), is considered to be the area where a destructive earthquake, the Tokai Earthquake, is very likely to occur. The Tokai Earthquake, however, is considered to be predictable since the nature of its earthquake mechanism is well understood than other earthquakes. Therefore, the instruments such as strainmeters, tiltmeters and extensometers are intensively deployed in the area. They are supposed to be able to identify precursory anomalies. The observation of groundwater is also believed to be one of the practical methods to predict the Tokai Earthquake because there are many records of groundwater changes before large earthquakes. Since 1976, the Geological Survey of Japan (GSJ) has been developing the telemeter groundwater observation system in the area according to the proposal of the Geodetic Council of Ministry of Education, Science and Culture (Tsukuda et al, 1980). Groundwater data are transferred to the Japan Meteorological Agency (JMA) and carefully monitored day to day and on a 24 hours basis. The meeting of the Earthquake Assessment Committee in JMA is held monthly to discuss whether there is any anomalous changes of the data. It is a major task for the research group of GSJ to prepare the charts of three-month and two-year groundwater data, and submit them to the committee with comments. One of our major objectives of this study is to develop algorithms to detect anomalous changes automatically before large earthquakes. As a milestone for this purpose, Matsumoto N. (1992; 1999) developed a data correction method using statistical analysis to make it possible to detect pre- and co-seismic anomalous changes of groundwater level by eliminating the effects of barometric pressure, earth tide, rain fall and other noise. After the 1995 Kobe earthquake, we concentrated our efforts to establish new comprehensive observation wells in the Kinki district in 1996 and then extended the area of observation to the Chubu district in 1998. It is well known that there are many active faults in the Kinki and Chubu districts. We located observation wells near active faults using a careful examination of subsurface geology. We also developed a comprehensive observation system for the analysis of groundwater, borehole strainmeter, seismometers and GPS data, in those areas for earthquake prediction studies at the end of this study. The data analysis of the groundwater level in Kinki shows that the system is able to detect earthquake related crustal deformations corresponding to the volumetric strain changes on the order of 10-9 (Koizumi et al, 1999). Almost all our groundwater related data and data correction methods are publicized at http://gxwell.aist.go.jp.
Laboratory studies on nucleation process of large earthquakes
Takashi SATOH(Earthquake Research Department, GSJ), Akio CHO(Earthquake Research Department, GSJ), Naoyuki KATO(Earthquake Research Department, GSJ), Hironori KAWAKATA(Earthquake Research Department, GSJ), Xinglin LEI(Earthquake Research Department, GSJ), Koji MASUDA(Earthquake Research Department, GSJ), Kinichiro KUSUNOSE(Earthquake Research Department, GSJ) and Mitsuhiko SIMADA((Disaster Prevention Research Institute, Kyoto University; Uji, Kyoto, 611-0011 Japan))
2000
vol. 51 (9) P. 401-410
11 figs
Keywords: earthquake nucleation process, fault bend, multiple unstable sliding, post failure, triaxial test
We report here the results of a 5-year research project on the nucleation process of large earthquakes, which had been conducted since FY 1994 to 1998. This project mainly consisted of two laboratory works on 1) the geometry effect on earthquake source process, and 2) the fault formation process under high confining pressures corresponding to hypocentral depth. The geometry effect was investigated by bi-axially compressing granite samples with an artificial bent fault. Multiple unstable sliding was observed on the faults. The first sub-event stopped at the bend, while the following sub-events propagated over the entire fault plane. The time interval between the first and the second sub-events decreases with an increase in the relative displacement of the first sub-event. This can be successfully explained by a numerical simulation based on a rate- and state-dependent friction law. The relation between the local stress drop of unstable sliding and time from the preceding event indicates that the frictional strength begins to recover at 10-4 to 10-3 seconds after a slip had stopped, and then increases in proportion with the logarithm of the duration of stationary contact. In order to investigate the fault formation process in detail, we conducted a series of triaxial compression experiments on Westerly granite under a confining pressure of 100 MPa. The fault formation process could be controlled quasi-statically by keeping the circumferential strain rate constant. After the peak stress, two stages of quasi-static stress drop with different rates were identified before dynamic failure. Based on the observation of fractures developed in the samples loaded up to various stages after the peak stress, the fault formation process can be summarized as follows. At first, a number of linear cracks occur on the sample surface, then a shear fracture nucleates locally near the mid-plane of the sample surface. The fracture plane grows toward the sample interior as well as along the sample surface. The critical size of the fracture plane just before the dynamic failure was 30 to 50 mm along the surface and 5 to 7 mm in depth. We could monitor the strain variation associated with the fault growth by reloading the sample after putting strain gauges around the fracture planes. From the results of this project, it is suggested that we could detect the quasi-static nucleation process of large earthquakes. It is also suggested the predictability of occurrence time of larger aftershocks that sometimes occur adjoining the source area of large earthquakes. We could detect the recovery of frictional strength after very short stationary contact, which has an implication for earthquakes with complex source process and/or behavior of earthquake fault just after the rupture front has passed. In order to apply these results to natural earthquakes, it is necessary to clarify the rock fracture mechanism and establish the friction constitutive law under hypocentral conditions.
Pleistocene sequence stratigraphy and tectonic movements in the Noda and Omiya districts, central Kanto Plain.
Tsutomu NAKAZAWA（Environmnetal Geology Department, GSJ） and Hidenori ENDO（Environmnetal Geology Department, GSJ）
2000
vol. 51 (9) P. 411-415
3 figs, 2 tables
Keywords: sequence stratigraphy, sedimentary facies, subsurface successions, tectonic subsidence, Pleistocene, Kanto Plain
Abstract : This paper outlines the tectonic movements inferred from the depositional sequences in the Pleistocene successions of the central Kanto Plain. Seven depositional sequences are recognized in the shallow subsurface Pleistocene successions by the observation of three cores at Noda, Kasukabe, and Kawaguchi, 85-110 meters in length. The difference of elevations of ravinement and bay ravinement surfaces in the depositional sequences reveals that Kasukabe was continuously subsiding relatively compared with the other two, and rapid aggradation occurred concurrently with the subsidence.
K-Ar age of the old-somma lava flow (OS2), southeastern foot of the Hakone Volcano
Jun'ichi ITOH(Environmental Geology Department, GSJ)
2000
vol. 51 (9) P. 417-420
4 figs, 1 table
Keywords: K-Ar age, the old somma lava, OS2, Hakone volcano
Abstract : To determine the eruption age of the old somma lava flow, Hakone volcano, I have performed K-Ar age dating for Cpx-Opx dacite lava flow, which is collected from the upper part of the old somma. The groundmass fraction yielded an age of 0.19±0.03 Ma and 0.22± 0.03 Ma.
Development of a numerical simulator of fault system activity analysis -Research on the numerical simulation of crustal deformation-
Yutaka MURAKAMI(Geophysics Department, GSJ)
2000
vol. 51 (9) P. 421-427
8 figs
Keywords: Numerical simulation, Fault system, Crustal deformation, Active fault, Finite element method
Abstract : Elasto-plastic finite element program Plasto-Slip is developed in order to analyze the activity of an active fault system. This software uses the same finite element mesh as the Bird simulator (Bird and Kong, 1994), which is well-known as the fault system analysis software, so that an exact comparison can be made between the results of both simulators. Numerical examples of both simulators for the case of Kinki fault system are shown.
Research on long-term earthquake forecasting based on active fault and paleoearthquake information.
Yuichi SUGIYAMA(Director of Research Planning Office, GSJ)
2000
vol. 51 (9) P. 429-433
4 figs, 1 table
Keywords: active fault, paleoseismology, multi-segment earthquake, tsunami (deposits), (seismic) hazard map
Abstract : esearch on long-term earthquake forecasting based on active fault and paleoearthquake information contains four research themes. The first theme is a comprehensive investigation of major active faults throughout Japan to evaluate earthquake potential of each fault. Main active faults in the Kinki Triangle have been mostly surveyed and evaluated by the end of FY 1998. Major faults in Chubu, Kanto and Shikoku districts are now under the subject of this investigation. The second theme is a basic study on multi-segment earthquakes. The purpose of this study is to clarify the mechanism of rupture propagation on large-scale fault systems and refine the estimation of rupture extent and earthquake magnitude. The study is conducted on the North Anatolian fault in Turkey and Chelungpu fault in Taiwan, both of which generated large multi-segment earthquakes in 1999. The third theme is to study off-fault paleoseismology to estimate the probability and size of earthquakes and/or seismic hazards on the basis of paleoearthquake information, for example from tsunami deposits and liquefaction traces. It is carried out chiefly in the eastern Hokkaido. The fourth theme specializes in public outreach. It includes a systematic compilation and publication of active fault strip maps and 1: 500,000-scale neotectonic maps. The public outreach brings about active fault data available in various digital formats through various media. We are now planning a new research theme to produce high-precision seismic hazard maps based on the synthesis of the newest data on active faults, paleoearthquake, basin structure and subsurface geology
ntegrated Groundwater Monitoring Studies in the Intensified Observation Area and near Major Active Faults
Eikichi TSUKUDA（Director of Research Planning Office, GSJ ）, Naoji KOIZUMI(Earthquake Research Department, GSJ) and Yasuto KUWAHARA(International Geology Office, GSJ)
2000
vol. 51 (9) P. 435-445
20 figs
Keywords: groundwater monitoring, earthquake prediction, intensified observation area, active fault, hydrological precursor, groundwater level, borehole strain meter, data correction system
Abstract : This project started in 1999 to succeed the previous project "Groundwater monitoring studies in the Tokai intensified observation area and other special observation areas for earthquake prediction research". The main purposes of this project are 1) to maintain the integrated monitoring well network system t is composed of 40 observation wells in Tokai, southern Kanto, Kinki and Chubu regions, and to develop more effective methods of data processing and analysis, 2) to clarify the mechanism of hydrological precursors of an earthquake, 3) to study changes of groundwater level, an borehole strain meter, radon concentration, etc. related to earthquakes. Some results of this project for FY 1999 are briefly introduced in this paper
tudies on physical and chemical processes in earthquake source region
Takashi SATOH(Earthquake Research Department, GSJ), Koichiro FUJIMOTO(Earthquake Research Department, GSJ) and Shinobu ITO(Geothermal Research Department, GSJ)
2000
vol. 51 (9) P. 447-450
2 figs
Keywords: healing process, water-rock interaction, mass transfer, Nojima fault, trapped wave, permeability, earthquake source process
Abstract : In order to investigate the physical and chemical processes, especially the healing process, in source regions of crustal earthquakes, we have started a 5-year research project since FY 1999. Laboratory friction experiments under crustal conditions are prepared to establish the friction constitutive law at very low slip rates where the healing process progresses. By using the GSJ Hirabayashi well, repeated hydraulic tests and observation of the fault trapped wave have been conducted to detect the healing process of the Nojima fault, which ruptured during the 1995 Hyogo-ken-nanbu earthquake. The modeling of mass transfer in and around the Nojima fault is also planned by integrating the results of the detailed analysis of the core samples, groundwater obtained from the Hirabayashi well and local hot spring water around the fault. Precise measurements of microearthquakes using borehole seismometers have been carried out at Ohtaki, Atotsugawa and Inagawa regions to investigate the earthquake source process in more detail.
A review: Project of Fuji volcano using quadrangle mapping of Fujinomiya
Akira TAKADA(Environmental Geology Department, GSJ), Takahiro YAMAMOTO(Geology Department, GSJ) and Naomichi MIYAJI(Shizuoka Prefectural Agricultural Experiment Station)
2000
vol. 51 (9) P. 451-457
4 figs
Keywords: Fujinomiya, Fuji volcano, Fujigawa faults system, basaltic volcano, lava flow, fissure eruption
Abstract : Fuji volcano has developed on a collision zone, so that the tectonic boundary conditions are important for the evolution of its volcano. Generally, a volcano more than 100 km3 accumulates excess stress generated by dike intrusions, and increases in gravitational instability. The volcano starts to acquire a stress relaxation system. Fuji volcano has evolved into this stage. If the stress relaxation system does not work well, the volcano accumulates magma beneath it, and results in a large-volume eruption. Unlike the normal, polygenetic, basaltic volcanoes, Fuji volcano has commenced eruptions more than 1 km3. After fissure eruption sites had converged toward the summit with oscillation during the period of 0-A.D.1000, the central eruption phase had started. A question remains whether the 1707 Hoei eruption means a start of flank eruption phase or just a fluctuation in the central eruption phase. It is an important problem how to release the accumulated stress in the case of Fuji volcano. We propose plans for a study of Fuji volcano using quadrangle mapping of Fujinomiya. (1) We will make clear the stratigraphy of lava flows from the Younger Fuji volcano in the Fujinomiya area in detail. (2) We will study the mode of emplacement of deposits from the Older Fuji volcano. (3) We will evaluate the degree of stress coupling between the volcanic activity of Fuji volcano and tectonic stress change such as the activity of the Fujigawa faults system.
Research plan on "Study of evaluation method of offshore active faults"
Yukinobu OKAMURA(Marine Geology Department, GSJ) and Ken IKEHARA(Marine Geology Department, GSJ)
2000
vol. 51 (9) P. 459-462
2 figs
Keywords: offshore active fault, seismic profile, fault-related fold, Nankai Trough, turbidite, paleoseismicity
Abstract : The research group on "Study of evaluation method of offshore active faults" was started in 1997, and reorganized in 1999 as a five-year project. This group has two main targets. The first target is to establish the methods of identification and evaluation of offshore active faults based on the idea of fault-related folding. The second target is to estimate the paleoseismicity by the analysis of seismogenic turbidites. The main target area is the offshore area of the Tokai district where a major seismic gap has been presumed along the Nankai Trough.
Seismic hazard and gravity basement structure
Hidenori ENDO(Environmental Geology Department, GSJ)
2000
vol. 51 (9) P. 463-469
8 figs
Keywords: 1995 Hyogo-ken Nanbu Earthquake, 1978 Miyagi-ken Oki Earthquake, 1994 Sanriku Harukaoki Earthquake, Kobe, Sendai, Hachinohe, seismic hazard, gravity, basement, environmental geology, urban geology
Abstract : The purpose of this paper is to introduce our research project, "Subsurface geology for the mitigation of the seismic hazard". The first argument concerns the similar relationship between the distribution of houses damaged by the earthquake and the gravity basement structure. Based on our analysis of 1995 Hyogo-ken Nanbu Earthquake (Kobe), 1978 Miyagi-ken Oki Earthquake (Sendai) and 1994 Sanriku Harukaoki Earthquake (Hachinohe), it was revealed that the damaged houses were mainly distributed in steep gradient zones of the gravity basement. The second point to detail particularity among these seismic hazards is that the damaged areas are mainly located on the foot part (Kobe), graven-like part or shoulder part (Sendai), and broad part (Hachinohe) of the steep gradient zones
tudies on the deep subsurface structure of the plains for strong motion estimation
Hisao ITO, Takanobu YKOKURA, Naomi KANO, Kazuo YAMAGUCHI, Masao KOMAZAWA, Yuichi SUGIYAMA, Kazutoshi IMANISHI, Tsutomu KIGUCHI, Kazuki KOKETSU and Yoshiaki HISADA
2000
vol. 51 (9) P. 471-475
5 figs
Keywords: strong motion, converted wave, gravity survey, reflection survey, velocity structure, S wave velocity
Abstract : In order to obtain the deep subsurface structure of the plains for strong motion estimation, we have started a new 5-year research project since FY1999. Although the three-dimensional structure is really necessary for strong motion estimation in urban plains, few systematic efforts have been made to establish an optimal method to study the three-dimensional structure, including S-wave velocity structure. Although S-wave velocity structure is one of the most important parameters for strong motion estimation, it is still difficult to obtain deep S wave velocity structure. We started a study on P-S converted waves, and the preliminary results show the P-S converted wave is useful for the estimation of S-wave velocity structure. We also studied the gravity structure in several areas to deduce the three-dimensional structure of the plains. We also plan to make strong motion estimation using the obtained structure, and study the optimum methods to estimate a three-dimensional structure. It is also necessary to study the source-receiver effect for high frequency strong motion generation.
