Yuzo Ishikawa

GPS seismometers with up to 20 Hz sampling rate

The large near-field displacements before and during an earthquake are invaluable information for earthquake source study and for the detection of slow/silent quakes or pre-seismic crustal deformation events. However due to bandwidth limitations and saturation current seismometers cannot measure many of these displacements directly. In a joint experiment between the University of New South Wales (UNSW) and the Meteorological Research Institute (MRI), two Trimble MS750 GPS receivers were used in the Real-Time Kinematic (RTK) mode with a fast sampling rate of up to 20 Hz to test the feasibility of a “GPS seismometer” in measuring displacements directly. The GPS antenna, an accelerometer, and a velocimeter were installed on the roof of an earthquake shake-simulator truck. The simulated seismic waveforms resolved from the RTK time series are in very good agreement with the results from the accelerometer and the velocimeter, after integrating twice and once respectively. Moreover, more displacement information are revealed in the GPS RTK results although they are noisier.

Temperature distribution and focal depth in the crust of the northeastern Japan

The thickness of seismogenic crust layer correlates with surface heat flow in most interplate seismic areas of the world (e.g., Sibson, 1982). Although the inverse relationship between heat flow and the base of seismogenic zone is obvious, the quantitative relationships are less certain and there should be variability of the focal depths among different tectonic settings. Comparisons of the heat flow (Yamano et al., 1997), thermal gradient (Tanaka et al., 1999) and earthquake (Japan Meteorological Agency, JMA) databases for the northeastern Japan provide detailed geologic and geophysical information about the earthquake process of island arc. Temperatures in the crust were calculated using a steady-state, one-dimensional, heat conductive transport model with heat generation as a function of heat flow and thermal gradient. The evaluated temperatures for D90, the depth above which 90% of earthquakes occur, range between 200°C and 500°C except for high heat flow and thermal gradient data. The consistency of temperature for D90 over a large depth interval supports that the temperature is the dominant factor governing the focal depth in the crust.

Stress field in the continental part of China derived from temporal variations of seismic activity

Abstract The seismic activity in East Asia during the last 3000 years and more has systematically been analyzed. A quantitative estimate has been attempted for the seismicity during the last 500 years, employing a statistical approach. The earthquake-generating stresses in and around China have been investigated by analyzing the relationships of the temporal variations of seismicity in various regions of East Asia. Seismicities in the region from the northern part of the North-South Seismic Belt of China (NSB) (using events with M ≧ 7 ) to the Japan trench (using events with M ≧ 7.5 ) through North China (using events with M ≧ 7 ), South China (using events with M ≧ 6 ) and the Korean peninsula (using events with M ≧ 6.5 ) were investigated. They all show high activity from about the 16th century to the beginning of the 18th century. The activity then subsided until the beginning of the 20th century. From the beginning of the 20th century, the activity of each region has risen again and remains high now. The short term variations of seismicity since 1900 in North China, the Korean peninsula and Japan also exhibit the same pattern. Synchronous variations of seismicity in these regions imply that regional earthquake-generating stress fields between the Japan trench and the northern part of the NSB of China originate under common tectonic conditions and that there is a transmission of the tectonic force from the subduction of the Pacific Ocean plate along the Japan trench to the northern part of the NSB through the Korean peninsula and North China. The seismic activities since 1900 in South China (using events with M ≧ 5), the Taiwan (using events M ≧ 7.3 ) and the Ryukyu regions (using events with M ≧ 6.7 ) were found to be high between 1900 and 1940. This high activity was followed by a quiet period starting from around 1945. Since 1960, the active period has continued up to the present time. These results suggest that the earthquake-generating stress fields between Taiwan and South China seem to be formed under common tectonic conditions i.e., due to the transmission of the tectonic force resulting from the subduction of the Philippine Sea plate along the Ryukyu trench and the collision between the Philippine Sea plate and the Eurasian plate in the Taiwan region. During this century, the Tibetan plateau (using events with M ≧ 6.8 ) and the Xinjiang region (using events with M ≧ 7 ) were seismically active from 1900 to 1916, then from around 1931 to 1955, and lately from around 1970 to the present. These active periods correspond to those (using events with M ≧ 7.7 ) at the boundary regions between the Indo-Australian and the Eurasian plate. This implies that the earthquake-generating stress fields in and around West China result from the transmission of the tectonic forces originating from the collision of the Indo-Australian plate and the Eurasian plate along the Himalayan mountain range.

An improved double-difference earthquake location algorithm using sP phases: application to the foreshock and aftershock sequences of the 2004 earthquake offshore of the Kii peninsula, Japan (Mw=7.5)

We have used s P phases to improve the hypocentral locations of the earthquakes that occurred offshore southeast of the Kii peninsula in association with the Mw=7.5 mainshock on September 5, 2004. The earthquakes were more than 100 km from the onshore seismic network and, thus, their focal depths were poorly constrained. The s P phases were recorded about 7–11 s after the initial P phases, with both phases having almost the same apparent velocities. The computation of ray-paths revealed that the arrival times of the later s P phases are sensitive to focal depths. We have recalculated the hypocenters and origin times for 36 events with more than six s P phase identifications on seismograms recorded at the High Sensitivity Seismograph Network by including s P phases in a double-difference earthquake location algorithm, which eliminates the errors introduced by crustal heterogeneity. The relocation results were then compared with those from the Japan Meteorological Agency based on traditional absolute location techniques.

Seismic observations at a seismic gap in the eastern margin of the Japan Sea using ocean bottom seismometers

Abstract The eastern margin of the Japan Sea is a nascent convergent plate boundary. Previous studies proposed the existence of a seismic gap along this boundary between 39°N and 40°N. The trend of this gap is reported by Ohtake (Island Arc 4, 156–165, 1995) to be north-northwest to south-southeast, but by Ishikawa (Gekkan Kaiyo, Suppl. 7, 102–107, 1994) and Matsuzawa (Prog. Abstr., Seismol. Soc. Jpn. 2, B92, 1995) to be north-northeast to south-southwest. During one month ocean bottom seismic observations were conducted using nine ocean bottom seismometers to investigate seismicity in and around the seismic gap area in detail. The observations revealed that the earthquake epicentral distribution had an echelon shape and could be divided into three groups. These groups have a north-northeast to south-southwest trend. This trend is consistent with the fault system in this area, which was formed by the back-arc spreading in the Early to Middle Miocene. This suggests that previously formed tectonic structures affect the present seismo-tectonics and that this area has weak planes with a north-northeast to south-southwest trend.