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Volume 52, Issue 3
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Takeshi KUDO, Yoshiteru KONO1999 Volume 52 Issue 3 Pages 341-350
Published: December 25, 1999
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSRelationship between distributions of shallow earthquakes and gradients of gravity anomaly field in Southwest Japan is investigated. Zones with steep horizontal gradient of gravity anomaly field are presumably caused by faults bounding different density structures in the crust. On the other hand, most shallow earthquakes are supposed to be caused by dislocations at faults in the crust. From both presumptions, it follows that the distribution of epicenters should be overlapped with that of the steep gravity gradient zones. This study verified the overlap in Southwest Japan. However, very low seismicity along the steep gradient zone of gravity anomaly was followed by the mainshock of the 1995 Hyogo-ken Nanbu earthquake (the Kobe earthquake). We also assessed such kind of seismic gaps beneath the steep gradient zones of gravity anomaly field. Many steep gradient zones of gravity anomalies with very low seismicities can be extracted. Some of them might be due to density contrasts at geological boundaries or inactive faults and some can be areas of future seismic hazards like the case of the Kobe earthquake.View full abstractDownload PDF (3219K) -
Hiroaki YAMANAKA, Katsumi KURITA, Nobuyuki YAMADA, Kazuoh SEO1999 Volume 52 Issue 3 Pages 351-360
Published: December 25, 1999
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSTachikawa fault is an active fault located in the western part of the Tokyo Metropolitan area. A structure of deep sedimentary layers around the fault was explored by microtremor array measurements and an analysis of existing seismic refraction data for the purpose of estimating its effects on earthquake ground motion. It was found from the microtremor array measurements that a basement depth in S-wave profile at down-thrown side of the fault is larger than that at up-thrown side with depth difference of 1.6km. A two-dimensional model was constructed using the explosion data and the depth data on the sediments derived from the microtremor array measurements. Travel times of initial P-wave for the proposed model calculated by a finite difference ray tracing fit the observed travel times better than the model from a conventional time term analysis. We evaluated spatial variations of amplification of earthquake ground motion in the proposed model using a 2D finite difference numerical simulation. It was found that ground motion in the down-thrown side is amplified due to effects of the basement step.View full abstractDownload PDF (1238K) -
Tokutaro HATORI1999 Volume 52 Issue 3 Pages 361-368
Published: December 25, 1999
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSMany tsunamis have been actively generated along the Kurile-Kamchatka trench. Historical tsunamis in the Kamchatka region have been recorded since 1737. The 1952 Kamchatka tsunami, magnitude on the Imamura-Iida scale (m=4), is the largest, and hit North Japan, Hawaii and South America. In recent years, the South Kurile (East Hokkaido) tsunami on Dec. 4, 1994 (m=3) and the Kamchatka tsunami on Dec. 5, 1997 (m=1) were generated near the trench. In the present paper, the distribution of cumulative energy (square value of tsunami heights, H2, where H: the mean height in segment unit) for each 150km segment along the Kurile-Kamchatka trench is investigated for the recent 99-year (1900-1998) and historical (1737-1899) periods. The result shows that the cumulative value, ΣH2, from South Kamchatka to the North Kurile regions is the largest, and the 2nd highest value in the South Kurile Islands during 1737-1998. For the recent 99-year, the observed value in South Kamchatka reached the expected value which was calculated assuming the mean rate of energy accumlation. On the contrary, the observed value in the North Kamchatka and South Kurile regions are twice larger than the expected value because of the recent high seismic activities.View full abstractDownload PDF (716K) -
Katsuyuki ABE1999 Volume 52 Issue 3 Pages 369-377
Published: December 25, 1999
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSA method for determining the tsunami magnitude Mt from tsunami run-up heights is developed by using the Abe's (1989) relation which has been employed for estimating run-up heights from earthquake magnitudes. The magnitude Mt is estimated from the relation, Mt=2logHm+6.6, where Hm is the maximum value of the local-mean run-up height in meters and the logarithmic mean of heights is taken over a distance of about 40km along a coast. Except for unusual events, the moment magnitude can be estimated from Mw=Mt-2C, where C=0 for tsunamis in the fore arc and C=0.2 for tsunamis in the back arc. For 12 large tsunamis that occurred around Japan from 1894 to 1998, Mw estimated from Mt is found to be essentially equivalent to that estimated from seismic waves. The present method is applied to determining Mt for 21 historical tsunamis that occurred around Japan on and after the 15th century. There are 13 large tsunamis with Mt=8 or over for the period from 1498 to 1893.View full abstractDownload PDF (1230K) -
Youichi ASANO, Norihito UMINO, Ayako NAKAMURA, Tomomi OKADA, Shuichiro ...1999 Volume 52 Issue 3 Pages 379-394
Published: December 25, 1999
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSSeismic array observations were made for about 3 months from July 28 to November 4, 1998, to estimate inhomogeneous structure of the crust and uppermost mantle beneath the Ou backbone range, which is one of the most seismically active areas in northeastern Japan. We deployed L-shaped small aperture seismic arrays at 13 locations in and around the range. Each array is composed of 16 seismometer stations with a 50m separation and 3 sets of DAT recorders, which record 18-channel seismic signals continuously for about 4 weeks with a 100-Hz sampling. We recorded waveforms from 100kg explosions detonated at 11 locations for the seismic refraction/reflection experiment of the 1998 Joint Seismic Observation of Tohoku. We also observed many earthquakes, including aftershocks of M 6.1 earthquake that occurred on September 3, 1998, in the northern edge of our array observation network. Both explosion and natural earthquake data recorded by the arrays are used to estimate a spatial distribution of seismic scatterers in the crust and uppermost mantle. We evaluated semblance coefficients and powers of seismograms to detect coherent wave trains with large amplitudes. Then the azimuth and slowness of these wave trains are converted to their source locations by assuming a single scattering in the model velocity structure. Estimated distribution of scatterers shows that most of them are distributed within the crust beneath the Ou backbone range. Particularly, concentrations of scatterers are seen around the fault planes and their deeper extensions of active faults bounding both at eastern and western edges of the backbone range and the fault plane of the September 3, 1998, earthquake of M6.1. Some of the scatterers are also located around the S-wave reflectors (bright spots) detected by the previous study in the lower crust of this area.View full abstractDownload PDF (2175K) -
Sadaki HORI, Shigeki HORIUCHI, Mizuho ISHIDA, Masahiro OOI1999 Volume 52 Issue 3 Pages 395-405
Published: December 25, 1999
Released on J-STAGE: March 11, 2010
JOURNAL FREE ACCESSRecent advances in broadband seismic observation enable us to analyze high quality waveform data just after the earthquake occurrence. An automated system for rapid determination of earthquake mechanisms for local events occurring beneath central Japan is developed with this point as background. When an earthquake occurs, its hypocentral parameters are determined automatically in several tens of seconds. In a case when an event occurring in a given area has a magnitude larger than a certain value, its source process is automatically analyzed by the use of a broadband waveform inversion technique. Considering the importance to shorten the computer process time to obtain the result, we calculate the Green's functions immediately after hypocentral parameters are calculated and when waveform data to be used in the inversion are being retrieved. The data of the Earth's response parameters, which are used in the calculation of the Green's functions, are stored on the hard disk so as to reduce the total lapse time. As a result, the CMT solution and source time function can be obtained in a few minutes after the earthquake occurrence. Inadequacy of theoretical waveform calculated by the 1-D Earth's model is corrected by introduction of station corrections to the observed arrival time and overparameterization to the source time function, where a number of double couple small sub-events are located in a certain time range. The CMT solution is represented by superimpose of moment tensors of small sub-events with very short duration. All the sub-events are assumed to occur at the same location (i. e., at the hypocenter) but to have different onset times. The source time function is also estimated from the temporal distribution of the sub-events. To evaluate the reliability of the results obtained from the present study, we compare them with the following two data sets: i) focal mechanism solutions determined by P first motion analysis, and ii) CMT solutions obtained from the other method for the same events. Our results are basically consistent with these two sets of mechanism solutions. However the agreement of the solutions is obviously better among the CMT solutions. Possible explanation for the difference between CMT and focal mechanism solutions is that the latter may be inaccurate because of insufficient push and pull first motion data, or that the total rupture may occur along the fault different from the initial rupture.View full abstractDownload PDF (1338K)
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