Masamitsu Takada

Crust and upper mantle resistivity structure in the southwestern end of the Kuril Arc as revealed by the joint analysis of conventional MT and network MT data

A joint analysis of data obtained by conventional magnetotellurics and network magnetotellurics (band-width, 0.003–7,680 sec) revealed detailed resistivity structure from the shallow crust to the upper mantle in the eastern part of Hokkaido, Japan, situated in the southwestern end of the Kuril Arc. The results are summarized as follows: (1) A conductive layer (a few to 10 Ωm), having a basin structure, is distributed widely to a maximum depth of about 6 km in the upper crust. Considering other independent studies, such as seismic reflection, gravity and drill core analyses, the bottom of this layer coincides with the boundary between the Tertiary and the Cretaceous formations. (2) A conductive layer (10–40 Ωm) situated in the lower crust extends from the volcanic front toward the backarc side, and is similar to feature with the Northeastern Japan Arc. (3) A highly resistive layer (5,000–10,000 Ωm) is analyzed in the upper to middle crust of the forearc side. Since the distribution of this layer is consistent with the high positive gravity anomaly region (227 mgal in maximum), the causative material may be common. A collisional tectonic event between the Eurasia plate and the Okhotsk Paleoland in the Cretaceous period may possibly be related with the existence of the layer, although the detailed tectonic implications are left to be solved. (4) The resistivity of the upper mantle is 40–100 Ωm. (5) The resistivity of the Pacific plate is estimated as 700–1,000 Ωm, which is almost consistent with that of the Northeastern Japan Arc (500 Ωm).

Coseismic and postseismic deformation related to the 2007 Chuetsu-oki, Niigata Earthquake

An intermediate-strength earthquake of magnitude Mj 6.8 occurred on July 16, 2007, centered beneath the Japan Sea a few kilometers offshore of Niigata Prefecture in central Japan. We constructed a dense GPS network to investigate postseismic deformation after this event, choosing our GPS sites carefully so as to complement the nationwide GPS GEONET array. Coseismic displacements caused by the mainshock detected at some GEONET sites were used to estimate coseismic fault parameters. The results indicate that the geodetic data can be explained by a combination of two rectangular faults dipping northwest and southeast. Minor but definite postseismic deformation was detected largely in the southern part of the dense network. The time series of site coordinates can be characterized by a logarithmic decay function, and the estimated time constant seems to be almost similar in range to that of the 2004 Mid-Niigata Prefecture Earthquake. We also found a possible site instability at 960566 (Izumo-zaki, GEONET) caused by a small, local landslide associated with the mainshock and therefore concluded that the data obtained at this site should not be used for coseismic or postseismic analysis.

Postseismic slip associated with the 2007 Chuetsu-oki, Niigata, Japan, Earthquake (M 6.8 on 16 July 2007) as inferred from GPS data

Postseismic crustal deformation associated with the 2007 Chuetsu-oki Earthquake, which occurred on 16 July 2007 with a magnitude of 6.8 at the southeastern rim of the Sea of Japan, near the coast of Mid-Niigata Prefecture, Central Japan, are detected by GPS observations. We analyzed continuous GPS data from the sites of the Geographical Survey Institute of Japan (GSI) and another dense temporary network, which we established just after the main shock to reveal spatio-temporal evolution of postseismic slip for 50 days after the main shock by geodetic inversion methods. Four models of faults are configured following Ohta et al. (2008, this issue), and these are optimized based on ABIC (Akaike’s Bayesian Information Criterion). The results of the inversion analysis show that the postseismic slip on the faults occurred at a downdip and updip extension of the coseismically slipped portion. The slip in the shallower portion decayed to be negligible within 2 weeks, and the slip in the deeper portion was still large after the slip in the shallower portion had almost terminated.

Deep Reflection Imaging beneath the Mizuho Plateau, East Antarctica, by SEAL-2002 Seismic Experiment

A seismic exploration was conducted on the Mizuho Plateau, East Antarctica, during the 2001/2002 austral summer season as the “Structure and Evolution of the East Antarctic Lithosphere (SEAL)” project by the 43rd Japanese Antarctic Research Expedition (JARE-43). The survey line of this exploration (SEAL-2002 profile) was almost perpendicular to the Mizuho inland traverse routes (JARE-41 refraction survey line; SEAL-2000) and was almost parallel to the coastal line along the Lutzow-Holm Bay. Several seismic shot records were obtained with clear arrivals of phases until a distance of 150 km in length. We have analyzed two shot data of both ends of the SEAL-2002 profile by using the conventional reflection method. Interval velocities were estimated by applying the normal-move-out (NMO) correction, then the obtained single-fold section obtained explicitly presents the horizontal reflectors originated from the middle crust, the lower crust and the Moho discontinuity. First, the reflector from the top of the middle crust was located at the depth of 23–24 km, which was corresponding to 8–9 s of two way travel time (TWT) in the single-fold section. Next, the reflector from the top of the lower crust was located at a depth of 31–34 km, corresponding to 11–12 s of TWT. The Moho reflector was observed in 13–14 s of TWT and the depth was estimated to be approximately 41–42 km.

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