Tomotake Ueno

Aftershock distribution of the 2004 Mid Niigata Prefecture Earthquake derived from a combined analysis of temporary online observations and permanent observations

The 2004 Mid Niigata Prefecture Earthquake (Mj = 6.8) occurred on 23 October 2004 in the northeastern part of the Niigata-Kobe Tectonic Zone where large contraction rates were observed. The mainshock was followed by an anomalously intense aftershock activity that included nine Mj ≥5.5 aftershocks. We deployed three temporary online seismic stations in the aftershock area from 27 October, combined data from the temporary stations with those from permanent stations located around the aftershock area, and determined the hypocenters of the mainshock and aftershocks with a joint hypocenter determination (JHD) technique. The resulting aftershock distribution showed that major events such as the mainshock, the largest aftershock (Mj = 6.5), the aftershock on 27 October (Mj = 6.1), etc. occurred on different fault planes that were located nearly parallel or perpendicular to each other. This might be due to heterogeneous structure in the source region. The strain energy was considered to have been enough accumulated on the individual fault planes. These features are probably a cause of the anomalous intensity of the aftershock activity.

Improvement in the Extended-Time Multitaper Receiver Function Estimation Technique

Abstract Receiver functions are calculated by deconvolving the vertical component of teleseismic P waves from the corresponding horizontal components. In order to carry out stable spectral divisions in the deconvolutions, various techniques have been developed, such as the water level method, multitaper method, and extended-time multitaper method. In this study, we propose an improvement of the extended-time multitaper method to correctly estimate the amplitudes of the receiver functions with any time lengths.

Migration of low‐frequency tremors revealed from multiple‐array analyses in western Shikoku, Japan

[1] Multiple-array observation above a belt-like tremor zone was conducted to investigate the detailed location and migration of tremor activity in western Shikoku, Japan. In March 2007, an episodic tremor and slip event occurred, and highly coherent waveforms were recorded at three arrays. Multiple signal classification analysis for the data from each array enabled measuring precise arrival directions. The majority of tremor signals suggested relatively low slowness. The arrival directions of tremor signals were used to locate tremor sources by the grid search method. Tracking the tremor activity showed that the tremor migrated within several hours in the northeast-southwest direction over a distance of 12-15 km, and its migration velocity was 1-2.5 km h -1 . This migration velocity is more rapid than the mean velocity of 0.5 km h -1 over the whole tremor episode lasting several days. Such a short-timescale migration may represent fluctuation of slip acceleration during the slow slip event. Whenever a tremor migrates southwestward, very low frequency earthquakes occur in the vicinity of the tremor migration terminus. This indicates that the tremor migration is related to the occurrence of very low frequency earthquakes and slow slip events.

Aftershock seismicity and fault structure of the 2005 West Off Fukuoka Prefecture Earthquake (MJMA7.0) derived from urgent joint observations

On March 20, 2005, a large MJMA7.0 earthquake occurred in the offshore area, west of Fukuoka prefecture, northern Kyushu, Japan. A series of joint observations were carried out by teams from several universities in Japan with the aim of investigating the aftershock activity. Six online telemetered and 17 offline recording seismic stations were installed on land around the aftershock area immediately followed the occurrence of the mainshock. Because aftershocks were located mainly in offshore regions, we also installed 11 ocean bottom seismometers (OBSs) just above the aftershock region and its vicinity in order to obtain accurate locations of hypocenters. The OBS observation was carried out from March 27 to April 13, 2005. We further conducted temporary GPS observations in which ten GPS receivers were deployed around the aftershock region. The aftershocks were mainly aligned along an approximately 25-km-long NW-SE trend, and the hypocenters of the main aftershock region were distributed on a nearly vertical plane at depths of 2–16 km. The mainshock was located near the central part of the main aftershock region at a depth of approximately 10 km. The largest aftershock of MJMA5.8 occurred near the southeastern edge of the main aftershock region, and the aftershock region subsequently extended about 5 km in the SE direction as defined by secondary aftershock activity. Enlargement of the aftershock region did not occur after the peak in aftershock activity, and the aftershock activity gradually declined. The distribution of hypocenters and seismogenic stress as defined by aftershocks suggest that the 2005 West Off Fukuoka Prefecture Earthquake occurred on the fault that is the NW extension of the Kego fault, which extends NW-SE through the Fukuoka metropolitan area, and that the largest aftershock occurred at the northwestern tip of the Kego fault.

Spatial distribution of static stress drops for aftershocks of the 2005 West Off Fukuoka Prefecture earthquake

We investigated the spatial distribution of static stress drops of the aftershocks of the 2005 West Off Fukuoka Prefecture earthquake, with the aim of assessing the possibility that another earthquake will occur on the SE extension of the earthquake fault. The waveforms from six temporary online telemetry stations installed in and around the aftershock region were measured. Small stress drops were estimated for the aftershocks that occurred relatively distant from the SE and NE ends of the earthquake fault. Conversely, the aftershocks that occurred around the SE end of aftershock region are characterized by large stress drops. These results imply the possibility of a stress concentration at the SE edge of the main shock fault.

Subsurface structures derived from receiver function analysis and relation to hypocenter distributions in the region from the eastern Shikoku to the northern Chugoku districts, Southwest Japan

We carried out receiver function imaging to estimate detailed configurations of the Philippine Sea (PHS) slab beneath the Shikoku and Chugoku districts and crustal structures in the region. We used two temporary seismic arrays with an average station spacing of about 10 km in the Chugoku district. Beneath Shikoku a clear northward dipping discontinuity at about 20–40 km depth was found, with other discontinuities below. We present a new interpretation that the clear boundary corresponds to the under surface of a low velocity layer in the upper part of the PHS slab. We also found a discontinuity at about 60 km depth beneath the Chugoku district, which is thought to be the aseismic PHS slab. Small scale discontinuities were found in the crust beneath the source area of the 2000 western Tottori Earthquake and around the Median Tectonic Line. The discontinuities might relate to the occurrence of large inland earthquakes.

Crust and uppermost mantle structure beneath central Japan inferred from receiver function analysis

We apply the receiver function method to estimate the structure of the crust and the uppermost mantle at an area that traverses central Japan including the Niigata-Kobe Tectonic Zone (NKTZ). The resultant receiver function images show clear seismic discontinuities, such as the subducting Philippine Sea plate, the Moho in the overriding plate, and other discontinuities inside the crust around the NKTZ. We also address station corrections for shallow structures using a synthetic receiver function. Crustal discontinuities seem to be complicated at the south side from the northern limit of the NKTZ. The dip of the discontinuities changes around the Atotsugawa active fault located in the NKTZ. The Moho discontinuity in the overriding plate is continuous and gradually dips to the south. The depths of the Moho discontinuity in the receiver function image exceed 40 km at the southern part of the profile line, and are 5–10 km deeper than that indicated by an explosion analysis of the same profile line. It seems that the differences between the estimated depths obtained by the two methods indicate complicated structures around the Moho discontinuity.