Fault extent of the largest aftershock of the 1968 Tokachi-Oki, Japan, earthquake and an interpretation of the normal faulting focal mechanism

The focal mechanism of the 1968 Tokachi-Oki earthquake (MJ = 7.9) was thrust faulting but that of the largest aftershock (MJ = 7.5) was normal faulting. The tectonic implication of the reversal of the focal mechanisms of the two events has not been clarified yet. In order to investigate this, we ought to take into account the relative location of the two fault planes. In the present study, the length and the direction of rupture propagation for the largest aftershock is derived from the azimuth dependence of the duration of observed strong ground motion. If we assume that the rupture initiation point is the hypocenter location determined by the Japan Meteorological Agency (JMA), the location of the fault plane is considerably further north of the tsunami source area as estimated previously, and at the northern end of the aftershock area within 24 hours after the main shock. The fault plane of the largest aftershock is deeper than that of the main shock and the two fault planes are nearly parallel to each other. The thrust faulting of the main shock and the normal faulting of the largest aftershock indicate that the part of the Pacific plate between the two fault planes moved relatively further northwestward due to the two events than the deeper part beneath the fault plane of the largest aftershock.ArticleEARTH PLANETS AND SPACE. 63(12):1213-1216 (2011)journal articl

Nonlinear response of surface layers at KiK-net stations in Japan

13th World Conference on Earthquake Engineering, August 1-6, 2004, Vancouver, B.C., CanadaArticleProceedings of the 13th World Conference on Earthquake Engineering. : 2690 (2004)conference pape

Scale-dependence of seismic energy-to-moment ratio for strike-slip earthquakes in Japan

We analyzed four pairs of a large (M_w ≈ 6) and a small (M_w ≈ 3.5 to 4) shallow strike-slip earthquakes to investigate the scale-dependence of the seismic energy-to-moment ratio, an important macroscopic parameter which reflects the basic physical process of seismic slip. These earthquakes occurred in the south-western part of Japan, and high-quality close-in records (epicentral distance < 50 km) are available for both the small and large earthquakes. The paired events have almost the same focal mechanism and hypocenter location. We used the spectral ratio of the paired events in order to remove the effects of attenuation along the wave propagation path and the station site response. We then estimated the seismic energy from the source spectra estimated from the spectral ratio. The energy-to-moment ratio increases with the earthquake size. This scale-dependence is very similar to that found earlier for earthquakes in Southern California

Source Characteristics of the Largest Three Aftershocks of the 1983 Japan Sea Earthquake

Intermediate-period P-wave records at far-field stations are analyzed to derive the focal depth and the source characteristics of the largest three aftershocks of the Japan Sea earthquake of May 26, 1983. Event A (June 9, 21 : 49, MJMA=6 .1) and Event B (June 9, 22 : 04, MJMA=6 . 0) occurred near the south edge of the source region of the main shock. Their focal depths are determined as 13 km and 12 km, respectively. Event C (June 21, 15 : 25, MJMA=A7 . 1) took place near the north edge of the source region. Its focal depth is estimated at about 8 km. The essential part of moment rate function for Event A is found to be expressed by a single pulse with a width of about 1 sec, while that for Event B is composed of two pulses separated in time. The moment rate function obtaine d for Event C consists of two pulses with nearly the same width of about 3.5 sec, and the time lag between them is estimated at 2.5 sec.ArticleTohoku Geophysical Journal. 33(1):83-95 (1990)departmental bulletin pape

SOURCE PARAMETERS RELEVANT TO HETERO-GENEITY OF A FAULT PLANE

Accelerograms due to 16 earthquakes with JMA magnitude from 5.5 to 7.4 are analyzed. Second corner frequencies for the earthquakes are in a range from 1 to 8 Hz, and ratios of the rms stress-drop to the global stress-drop are from 2 to 10. The second corner frequency increases with the ratio of the rms stress-drop to the global stress-drop, and it is almost independent of the characteristic length of the fault plane. Based on stochastic source models, the second corner frequency is inversely proportional to an average size of element faults constituting a fault plane, so that the present result suggests that the average size decreases with increase in the heterogeneity of stress-drop on the fault plane. Also, the result implies that the average size of element faults is almost independent of the entire fault size for the earthquakes analyzed in this study.ArticleJournal of Physics of the Earth. 32(6):511-529 (1984)journal articl

A STATISTICAL MODEL FOR PREDICTION OF QUASIREALISTIC STRONG GROUND MOTION

The total power, the duration, and the starting time of band-pass filtered accelerograms are chosen as model parameters to express amplitude variations of ground acceleration in both the time and frequency domains. In order to estimate model parameters of certain earthquakes and site conditions, a probabilistic source model is used and accelerograms recorded during the Matsushiro earthquake swarm are analyzed statistically. The duration and the starting time of the strong ground motion are found to have close relations to the amplification characteristics of surface layers at the sites. As an example, model parameters are estimated by assuming the source parameters and the site conditions of the Miyagi-Oki earthquake of June 12, 1978. Wave-forms, running spectra, and velocity response spectra of synthetic accelerograms resemble those of observed accelerograms.ArticleJournal of Physics of the Earth. 29(6):537-557 (1981)journal articl

RAPID ESTIMATION OF FAULT PARAMETERS FOR NEAR-FIELD TSUNAMI WARNING

ArticleNatural disaster science. 9(1):99-113 (1987)journal articl