Masayoshi Ichiyanagi

Aftershock distribution of the October 4, 1994 Mw8.3 Kurile Islands Earthquake determined by a local seismic network in Hokkaido, Japan

On October 4, 1994, an earthquake with magnitude Mw8.3 occurred in the western part of Kurile Islands at 43.42°N, 146.81°E and 33 km in depth. The hypocenter parameters were determined by Hokkaido University in Japan. Aftershocks following this remarkable event were located using data from a local seismic network operated by Hokkaido University. We found that most of the aftershocks occurred (1) on the fault plane of the mainshock, (2) in the subducting plate around the fault plane of the mainshock, and (3) in the focal area of the largest aftershock, which occurred on October 9 with Mw7.3. Both (2) and (3) were not active immediately after the mainshock. Considering the time sequence of the aftershock activity, we identified one of the nodal planes of the Harvard quick CMT solutions as the fault plane of the mainshock; the strike is almost parallel to the trench axis and the dip angle is near vertical. It is obvious that this event is different from a low-angle thrust-type interplate earthquake. The distribution of aftershocks strongly suggests that it is an intraplate event.

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.

Postseismic displacements following the 2007 Noto peninsula earthquake detected by dense GPS observation

We have been conducting dense GPS observation in and around the epicentral region of the 2007 Noto peninsula earthquake since March 25, 2007, in order to detect postseismic displacements. Continuous observation has been underway at 12 sites to fill the gap of GEONET. Preliminary analysis of data up to early May shows that initial postseismic displacement rapidly decayed within 20 days after the occurrence of the mainshock. Horizontal displacements do not exceed 20 mm even at sites above the aftershock zone for this period. We also found a maximum uplift of about 20 mm there. Inversion of postseismic displacements with the variable slip model suggests a nearly right-lateral afterslip of less than 5 cm on the shallow portion of the source fault. Fitting a theoretical function to a time series of coordinate changes also suggests that the observed postseismic displacements might have been generated by afterslip.

Postseismic deformation following the 2005 West Off Fukuoka Prefecture Earthquake (M7.0) derived by GPS observation

We constructed a temporary GPS network around the aftershock area of the 2005 West Off Fukuoka Prefecture Earthquake (M7.0) in order to investigate the characteristics of its postseismic deformation. Our GPS network data, as well as the GEONET data, were analyzed using Bernese GPS software. We detected notable postseismic deformation in horizontal components close to the fault plane. The observed maximum displacement was 5.6 cm at the GNKI site on Genkaijima Island. A logarithmic law was adapted to the coordinate time series data, revealing decay times from 2 to 23 days, similar to those obtained for the 2003 Tokachi-oki Earthquake (M8.0). The amount of postseismic slips on the fault was assessed using the coseismic fault model proposed by the Geographical Survey Institute (GSI). We derived an optimum fault model of postseismic slip on the shallow (less than 3 km depth) portion of the fault. Our findings indicate that postseismic slip occurred only in shallow parts of the coseismic fault.

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.

Volcanic inflation of Mount Hokkaido‐Komagatake, Japan, determined from a dense GPS array

We installed and have been operating a dense GPS array with low power consumption single frequency GPS receivers around Mount Hokkaido-Komagatake volcano, southwestern Hokkaido, Japan, since 2000. Data were collected continuously and analyzed together with the Geographical Survey Institute of Japans nationwide continuous GPS network data. The analysis of daily baseline length changes for two years indicate a steady inflation of the volcanic body with 10 -7 /yr extensional strain. A pressure source was estimated at 2.4 km depth beneath the summit area with a volumetric increase rate of 4.86 × 10 5 m 3 /yr. The depth of the pressure source is in good agreement with the one expected from petrological study. Integrated magma volume has reached up to 2.6 x 10 7 m 3 if the inflation rate is possibly extrapolated back to the last magmatic eruption. Our results strongly suggest a steady magma accumulation at the reservoir beneath this volcano and an increasing potential to the next eruption near future.

Seismological monitoring on the 2003 Tokachi-oki earthquake derived from permanent OBSs and land-based observation - a challenge in monitoring M8 earthquake on the ocean floor

In July 1999, Japan Agency for Marine-Earth Science and Technology (JAMSTEC) installed a cabled geophysical observatory system off Kushiro, southeastern Hokkaido Island, Japan. This observatory system comprises three ocean bottom seismographs (OBSs), two tsunami gauges, and a cable-end environmental monitoring system, connected with a 240 km long fiber optical cable. Processing OBSs and land-based data together, and comparing magnitude common recorded with that determined by Japan Meteorological Agency (JMA), we found event detection level was improved down to magnitude 1.5, which is much lower than the previously designed as down to magnitude around 2. We compared detection level before and after installing OBSs, and found dramatic improvement on the earthquake detection level after installation of the cabled system. Four years and two months after the installation, a megathrust earthquake (The 2003 Tokachi-oki earthquake, MJMA 8.0) occurred just beneath the system. The system recorded clear unsaturated seismograms just at 28.6 km from the epicenter, which is the first observation on the ocean floor recording an earthquake with magnitude eight in the world. This paper reports hypocenter distribution derived from permanent cabled OBSs and land-based observation in the period from mainshock of the 2003 Tokachi-oki earthquake, to middle of May 2004. In the large slip area of the mainshock, a planar, with ten-degree dip, hypocenter distribution is obtained. Another deep planar seismic zone is found about 20 km depth from the plate interface. We think that the geophysical observations helps to understand the initiation process of the rupture of the 2003 Tokachi-oki earthquake and that observations including seismological, geodynamic, hydrogeological, and the other multidisciplinary observations would provide a clue to future understanding of seismogenic processes at southern end of the Kurile subduction zones