Tadafumi Ochi

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.

Strain partitioning and interplate coupling along the northern margin of the Philippine Sea plate, estimated from Global Navigation Satellite System and Global Positioning System-Acoustic data

Southwest Japan is located in the subduction margin between the continental Amurian and oceanic Philippine Sea plates. Recent land GNSS (Global Navigation Satellite System) and offshore Global Positioning SystemAcoustic geodetic measurements were used to clarify the deformation in and around these plate margins. We examined strain partitioning and interplate coupling using a block modeling approach on the observed velocities. Although the main plate boundaries are the Nankai Trough and Sagami trough, our results suggest that one-third of the relative plate motion between the two plates is accommodated by several block boundaries in the southeastern margin of the Amurian plate. The most active boundaries, with a slip rate of ≥8 mm/yr, cross southwest Japan from the Okinawa Trough through the Median Tectonic Line and Niigata Kobe tectonic zone, to the eastern margin of the Japan Sea. A subparallel boundary with a slip rate of 4–5 mm/yr is along the coastline of Japan. These two boundaries have a right-lateral shear motion that accommodates part of the interplate motion, with a boundary across the Korean Peninsula and Japan Sea. The slip partitioning results in an eastward decrease of relative block motion from 78 to 4 mm/yr along the Nankai Trough and Suruga trough. Interplate coupling is moderate to strong at 10–25 km depth along the Nankai Trough, but it is lower at ~132°E, ~136°E, and ~137°E than in the surrounding regions, corresponding to the segment boundaries of past megathrust earthquakes, suggesting that regions of insufficient strain accumulation act as a barrier for earthquake rupture. INTRODUCTION Southwest Japan is situated in a subduction zone, where the northern margin of the Philippine Sea plate subducts beneath the southeastern margin of the continental Amurian plate. Large M ≥ 8 megathrust earthquakes have ruptured shallow parts of the plate interface with a recurrence interval of 100–200 yr along the Nankai Trough, the main boundary between these plates (Ando, 1975; Ishibashi, 2004). Many slow earthquakes, including slow slip events (SSEs), low-frequency tremors, and very low frequency earthquakes (VLFEs), also occur around the rupture area of the megathrust earthquakes on the plate interface (cf. Obara, 2010). However, previous studies suggest that the relative plate motion between the Amurian and Philippine Sea plates is not accommodated by a single boundary. Numerous active faults and high seismicity, including M ~7 earthquakes on the islands of Japan, indicate strain partitioning in the continental margin (e.g., Shen-Tu et al., 1995). The most famous feature of the strain partitioning is the fault system of the Median Tectonic Line (MTL), which accommodates part of the margin-parallel component of the oblique relative motion between the Amurian and Philippine Sea plates (i.e., Fitch, 1972). Its right-lateral strike-slip motion was proposed based on geological studies (Kanaori, 1990; Tsutsumi et al., 1991), and has been confirmed by geodetic observations (Miyazaki and Heki, 2001; Tabei et al., 2003). Another dextral strike-slip fault system subparallel to the MTL has been proposed 400 km from the Nankai Trough (Gutscher and Lallemand, 1999; Gutscher, 2001). Contemporary deformation in southwest Japan has been monitored by dense, continuous GNSS (Global Navigation Satellite System) networks since the mid-1990s. The largest network is the GEONET (GNSS Earth Observation Network System), operated by the Geospatial Information Authority of Japan (Sagiya, 2004). The GEONET showed that high strain rates are found not only GEOSPHERE GEOSPHERE; v. 14, no. 2 doi:10.1130/GES01529.1 11 figures; 3 tables; 1 supplemental file CORRESPONDENCE: nishimura .takuya .4s@ kyoto -u .ac.jp CITATION: Nishimura, T., Yokota, Y., Tadokoro, K., and Ochi, T., 2018, Strain partitioning and interplate coupling along the northern margin of the Philippine Sea plate, estimated from Global Navigation Satellite System and Global Positioning System-Acoustic data: Geosphere, v. 14, no. 2, p. 535–551, doi: 10 .1130 /GES01529.1. Science Editor: Shanaka de Silva Guest Associate Editor: Laura M. Wallace Received 15 March 2017 Revision received 16 November 2017 Accepted 19 January 2018 Published online 16 February 2018

Relation between the coupling and tremor rates in the transition zone around the Shikoku region

We examine the spatiotemporal distributions of the interplate couplings and the tremor count rate in the Shikoku region, which is located in the western part of the Nankai subduction zone in Japan, from July 2007 to June 2015 by comparing the results of static geodetic inversion using onshore global navigation satellite system data and the published tremor catalog. We observe that the temporal fluctuations in these two rates correlated well at a few spot-like points in the tremor-occurrence zone at a deeper location next to the strongly coupled area. The well-correlated points are densely distributed around the long-term slow slip area in the Bungo Channel and are randomly dispersed in the central and eastern portions. The correlation coefficients of the well-correlated points differ from point to point; however, at some points, zero tremor counts can be expected when the coupling rates reach the plate convergence rate. This indicates that tremors occur on the plate interface and that the tremor count rates can be used as proxies for monitoring the small fluctuations in interplate coupling.