Tsutomu Sasatani

Site-Specific Issues for Strong Ground Motions during the Kocaeli, Turkey, Earthquake of 17 August 1999, as Inferred from Array Observations of Microtremors and Aftershocks

Array observations of microtremors and aftershocks were carried out near the permanent strong-motion observation sites and the damaged areas, after the Kocaeli, Turkey, earthquake of 17 August 1999. The major objectives were to determine S -wave velocity structures at the sites and to understand the site effects on strong motion or damage. Array observation of microtremors is a useful method for determining the S -wave velocity structures in a sedimentary basin, with less practical restriction than the other geotechnical methods. The spatial autocorrelation method (SPAC) was applied to array data of microtremors for determining S -wave velocity structures. The SPAC method generally provides an equivalent result with that of the frequency-wavenumber method, using fewer array sites and a smaller array size. Most strong-motion sites near the fault are classified into stiff and/or very hard soils. The records cannot directly be used for interpreting damage to buildings in the sedimentary basin. Records of long durations of strong motion at ATS, near Avcilar, west of Istanbul, are closely related to the low velocity ( V s ∼ 200 m/sec) of the surface layers. The S -wave velocity structure at Avcilar, where there was severe damage during the mainshock, is similar to that of the lowland (ATS), and it differs significantly from that of CNA, located 4 km northeast of Avcilar, where the strong-motion record was obtained from the mainshock. The strong ground motion at Avcilar during the mainshock is estimated to be similar to that at ATS. Of the strong-motion sites, Sakarya (SKR) is located on very hard soil, whereas thick and soft sediments cover downtown Adapazari. It is plausible that strong ground motions during the mainshock in the damaged area, ADC, were significantly different from those of SKR. A large difference between the strong motions of a hillside and the lzmit Bay area in and around Golcuk is also indicated by a comparison of aftershock records.

Double seismic zone beneath the middle of Hokkaido, Japan, in the southwestern side of the Kurile Arc

Abstract The vertical section of microearthquakes, determined accurately by using the Hokkaido University network, shows two dipping zones (the double seismic zone) 25–30 km apart in the depth range of 80–150 km beneath the middle of Hokkaido in the southwestern side of the Kurile arc. Hypocentral distribution of large earthquakes ( m b > 4) based on the ISC (International Seismological Centre) bulletin also shows the double seismic zone beneath the same region. The hypocentral distribution indicates that the frequency of events occurring in the lower zone is four times greater than that in the upper zone. The difference in seismic activity between the two zones beneath Hokkaido is in contrast with the region beneath northeastern Honshu in the northeastern Japan arc. Composite focal mechanisms of microearthquakes and individual mechanisms of large events mainly characterize the down-dip extension for the lower zone as is observed beneath northeastern Honshu. For the upper zone, however, the stress field is rather complex and not necessarily similar to that beneath northeastern Honshu. This may be considered to indicate the influence of slab contortion or transformation in the Hokkaido corner between the Kurile and the northeastern Japan arcs.

Source Models of Two Large Intraslab Earthquakes from Broadband Strong Ground Motions

We construct source models of two large intraslab earthquakes (the 1993 Kushiro-oki and 1994 Hokkaido Toho-oki earthquakes) along the southern Kurile–Hokkaido arc based on strong motion records; the source models well explain observed broadband strong ground motions. We assume a distribution of rectangular-shaped asperities on a fault plane and apply the empirical Green9s function method to the synthesis of strong ground motions from the asperities. The asperity parameters (area and stress drop) are estimated by matching the synthetic waveforms (displacement, velocity, and acceleration time histories) and spectra to the observed ones. Our source models have the following characteristics compared with those of inland and plate-boundary earthquakes having a comparable seismic moment: (1) the total asperity area is much smaller, and (2) the stress drop is extremely high (about 200–400 MPa). These indicate that the large intraslab earthquakes radiate seismic energies from a small area in a short time. This rupture process results in extremely strong radiation of short-period seismic waves. Finally, we confirm a correlation between outer (entire fault size, seismic moment, and short-period level of the acceleration source spectrum) and inner fault parameters (areas and stress drops of asperities) for the two intraslab earthquakes.

Source spectral characteristics of two large intra-slab earthquakes along the southern Kurile-Hokkaido arc

Abstract In 1993 and 1994, two large earthquakes occurred along the southern Kurile-Hokkaido arc: the 1993 Kushiro-oki earthquake (15 January; Mw=7.6) and the 1994 Shikotan earthquake (4 October; Mw=8.2). These were intra-slab earthquakes that ruptured through a substantial part of the subducting oceanic lithosphere. We study source spectral characteristics of these large intra-slab earthquakes based on strong-motion records. For the Kushiro-oki earthquake, the S-wave spectra on three rock-site stations are directly compared with the theoretical spectra based on the omega-squared source model and the source parameters estimated from long-period seismic waves. The observed spectra are roughly consistent with the theoretical ones at low frequencies (

The 1994 Hokkaido Toho-oki earthquake sequence: the complex activity of intra-slab and plate-boundary earthquakes

Abstract We study the complex activity of the 1994 Hokkaido Toho-oki earthquake sequence based on strong motion records and teleseismic data. First we examine S-wave acceleration spectra of the aftershocks by comparison with that of the main shock; the main shock ( M w =8.2), an intra-slab earthquake, has been characterized by strong radiation of high-frequency seismic waves. From different shapes of the spectral ratios of the aftershocks to the main shock, the aftershocks can be classified into three types. Next we redetermine the source parameters (focal depth, seismic moment and source duration) for larger events by waveform modeling of teleseismic P-waves. The hypocentral locations and stress drops estimated from these parameters show that the three aftershock types fall into different categories of subduction zone earthquakes; intra-slab, relatively deep plate-boundary, and shallow plate-boundary earthquakes. Finally, we estimate source spectra for three different earthquake categories after correcting S-wave acceleration spectra for the attenuation effect. The source spectrum at high frequencies is modeled by using a high-cut filter whose parameters are a cut-off frequency, f max and a decay rate. The source spectrum of intra-slab earthquakes has high f max and small decay rate; due to this feature and the high stress drop, intra-slab earthquakes radiate strong high-frequency seismic waves. Through this study, we demonstrate that the 1994 Hokkaido Toho-oki earthquake sequence has the complex activity of intra-slab and plate-boundary earthquakes.

Velocity-Type Strong-Motion Seismometer Using a Coupled Pendulum: Design and Performance

We have developed a velocity-type strong-motion seismometer that can record strong ground motion over a wide range of periods with high resolution in the near-field of large earthquakes. The pendulum of this seismometer is a coupled pendulum with a natural period of several seconds. This pendulum is able to consistently cancel out the influence of transverse acceleration, that is, the acceleration perpendicular to the measuring direction. We immersed the coupled pendulum in a highly viscous silicon oil. The silicon oil overdamps the pendulum so that it becomes a wide-band, velocity-type seismometer. This seismometer can record ground velocity from 2 × 10 –7 m/sec to 1 m/sec in a period range of 0.01–1000 sec. Furthermore, the silicon oil has effects on maintaining the strength of the pendulum system and suppressing its parasitic oscillations. This article discusses the design principles and presents test results and observed seismograms from large earthquakes.

Source parameters of the 2007 Noto Hanto earthquake sequence derived from strong motion records at temporary and permanent stations

A large crustal earthquake, the 2007 Noto Hanto earthquake, occurred west off the Noto peninsula in Ishikawa prefecture, Japan, on March 25, 2007. We started temporary strong motion observation at five sites within the aftershock area from about 13 h after the main shock occurrence. We first applied the spectral inversion method to S-wave strong ground motion records at the temporary and permanent stations. We obtained source specta of the main shock and aftershocks, Qs (quality factor for S-wave) values (Qs = 34.5 f0.95) and site responses at 22 sites. We then estimated a source model of the main shock using the empirical Green’s function method. The source model consists of three strong motion generation areas and well explains the observed records. Finally, we examined the consistency of the main-shock source models estimated from the above two analyses. The high-frequency level of the acceleration source spectrum based on the main-shock source model is consistent with the source spectrum estimated from the spectral inversion. The combined area of strong motion generation areas is approximately half of the value expected by the empirical relationship, and the high-frequency level of acceleration source spectrum is approximately 2.5 times larger than the empirical relationship for shallow inland and inter-plate earthquakes.

Strong ground motions from an Mj 6.1 inland crustal earthquake in Hokkaido, Japan: the 2004 Rumoi earthquake

An inland crustal earthquake (Mj 6.1) occurred on December 14, 2004 in the northern part of Hokkaido, Japan. A large ground acceleration of 1127 cm/s2 and a large pseudo-velocity response of over 100 cm/s were recorded at the nearest strong-motion station, HKD020, about 10 km from the hypocenter. This large ground motion is considered to be attributable to the source effect and the site effect. The site effect is investigated using the traditional spectral ratio technique and the theoretical evaluation based on the subsurface structure model. The results imply that the site effect has an insignificant effect on the large ground motion at HKD020. The source effect is investigated by constructing the source model that explains the broad-band strong-motion records at four stations around the epicenter using the empirical Green’s function method. The estimated source model satisfies the empirical relationship between the strong motion generation areas and the seismic moment for inland crustal earthquakes. The high-frequency level of the acceleration source spectrum is also consistent with the empirical relationship. These results suggest that this earthquake is a normal crustal event and that the large ground motion at HKD020 is mainly attributable to the source effect, short distance from the strong motion generation area and the forward directivity effect. Finally, the temporal change of the site response at HKD020 is examined using long duration records including the main shock and several aftershocks. The site response based on the S-wave horizontal-to-vertical spectral ratio method shows the nonlinearity for the main shock and an aftershock occurring about 20 s after the main shock. However, the site response shows linearity for other later aftershocks. This site response change is attributed to the difference in ground motion amplitude.

Two-layer Q s structure of the slab near the southern Kurile trench

We estimate Qs values (quality factor for S-wave) of the slab near the southern Kurile trench by using seismic data observed at ocean bottom seismometer (OBS) stations from selected local earthquakes. The seismic rays pass mainly through the slab and this enable us to directly estimate the slab Qs values. The spectral inversion and coda normalization method are applied to these data. The estimated Qs values from the two methods are nearly the same and increase with frequencies. However, these Qs values are not high enough to explain the abnormal distribution of ground motion that has been recognized as abnormal distribution of seismic intensities at the Japanese arc. Thus we propose a two-layer Qs structure of the slab to explain the both facts, our slab Qs values and the abnormal distribution of ground motion. The two-layer Qs structure consists of the upper layer with not so high Qs values (39f1.0) and lower layer with very high Qs values (500f1.0); the upper layer has a thickness of about 50 km. Seismic activity is restricted only within the upper layer of the two-layer Qs structure. This may mean that the two-layer Qs structure reflects the different material property between the upper and lower layer of the slab.