Offshore double-planed shallow seismic zone in the NE Japan forearc region revealed by sP depth phases recorded by regional networks

Geophys. J. Int. (2009) 178, 195–214 doi: 10.1111/j.1365-246X.2009.04048.x Offshore double-planed shallow seismic zone in the NE Japan forearc region revealed by sP depth phases recorded by regional networks Shantha S. N. Gamage,1 Norihito Umino,1 Akira Hasegawa1 and Stephen H. Kirby2 1 Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan E-mail: 2 US Geological Survey, Menlo Park, CA 94025, USA Accepted 2008 November 10. Received 2008 November 6; in original form 2007 November 22 Key words: Seismicity and tectonics; Body waves; Wave propagation; Subduction zone processes; Intra-plate processes; Asia. 1 I N T RO D U C T I O N Knowledge of the accurate locations of earthquakes and their focal mechanisms in subduction zones constitutes important information in terms of gaining a better understanding of the subduction process of oceanic plates beneath continental plates. Some of the difficulties encountered in determining focal depths for small offshore shallow earthquakes using regional network arrival times recorded at onland stations arise from a lack of seismic network detectability and the inadequate distribution of seismic stations. The hypocentres of suboceanic earthquakes are typically mislocated when using conventional location methods based on a simple  C 2009 The Authors C 2009 RAS Journal compilation  one-dimensional seismic velocity structure model (e.g. Engdahl et al. 1982). The degree of mislocation depends on the velocity heterogeneity of the lithosphere and mantle in the subduction system, the location of the hypocentres in relation to the subducting plate and the configuration of the seismic network (Engdahl et al. 1982; McLaren & Frohlich 1985). Both relative location methods and methods that utilize seismic ray tracing through a laterally inhomogeneous Earth have been developed to improve the accuracy of solutions. Focal depths can be determined accurately even for earthquakes that occur outside an observation network, provided that the depth phases from the events can be detected. Teleseismic depth phases 195 GJI Seismology SUMMARY We detected the sP depth phase at small epicentral distances of about 150 km or more in the seismograms of shallow earthquakes in the NE Japan forearc region. The focal depths of 1078 M > 3 earthquakes that occurred from 2000 to 2006 were precisely determined using the time delay of the sP phase from the initial P-wave arrival. The distribution of relocated hypocentres clearly shows the configuration of a double-planed shallow seismic zone beneath the Pacific Ocean. The upper plane has a low dip angle near the Japan Trench, increasing gradually to ∼30◦ at approximately 100 km landward of the Japan Trench. The lower plane is approximately parallel to the upper plane, and appears to be the near-trench counterpart of the lower plane of the double-planed deep seismic zone beneath the land area. The distance between the upper and lower planes is 28–32 km, which is approximately the same as or slightly smaller than that of the double-planed deep seismic zone beneath the land area. Focal mechanism solutions of the relocated earthquakes are determined from P-wave initial motion data. Although P-wave initial motion data for these offshore events are not ideally distributed on the focal sphere, we found that the upper-plane events that occur near the Japan Trench are characterized by normal faulting, whereas lower-plane events are characterized by thrust faulting. This focal mechanism distribution is the opposite to that of the doubleplaned deep seismic zone beneath the land area. The characteristics of these focal mechanisms for the shallow and deep doubled-planed seismic zones can be explained by a bending– unbending model of the subducting Pacific plate. Some of relocated earthquakes took place in the source area of the 1933 Mw8.4 Sanriku earthquake at depths of 10–23 km. The available focal mechanisms for these events are characterized by normal faulting. Given that the 1933 event was a large normal-fault event that occurred along a fault plane dipping landward, the earthquakes that currently occur just beneath or oceanwards of the Japan Trench are probably its aftershocks, suggesting that aftershock activity continues to the present day, 70 years after the main shock. 196 Shantha S. N. Gamage et al. 140E 142E quake that occurred in the deeper portion (41 km) of the Pacific plate beneath the Japan Trench by long-period seismic wave form analysis. The focal mechanism of this event in the trench-outer rise region of the NE Japan subduction zone is a thrust fault type with an approximately horizontal P axis. Seno & Yamanaka (1996) pointed out that subduction zones with double-planed deep seismic zones, such as the NE Japan, are generally characterized by the occurrence of compressional deep events at the trench-outer rise region, as with the event described by Seno & Gonzalez (1987). In the present study, we relocated the focal depths of many offshore earthquakes in the NE Japan forearc region using the sP depth phase, and determined their focal mechanisms. This analysis was performed with the aim of understanding the detailed structure of seismic activity in the region. 2 DETECTION OF THE sP PHASE AND D E T E R M I N AT I O N O F F O C A L D E P T H The NE Japan subduction zone marks the zone along which the Pacific plate subducts beneath the North America/Okhotsk plate, generating not only interplate earthquakes but also intraplate and intraslab events. The detailed seismic structure of the NE Japan arc has been revealed by many studies based on seismic observations (e.g. Matsuzawa et al. 1986, 1990; Hasegawa et al. 1991, 1994; Zhao et al. 1992, 1994; Zhao & Hasegawa 1993). Fig. 1 shows the locations of seismic stations in NE Japan established and maintained by Tohoku University, Hirosaki University, Hokkaido University, University of Tokyo, The Japan Meteorological Agency (JMA) and Hi-net. We selected shallow offshore M > 3.0 earthquakes that occurred in NE Japan during the period from 2000 January to 2006 December 2006 and investigated their seismograms recorded by 144E 146E 130E 135E 140E 145E 45N NA plate 42N /Okhots k plate-100 0 00 0 A -3 40N -60 -2000 -4000 0 00 -5 00 B -6000 -6000 0 35N C D 40N 00 -70 30N late 00 -20 -100 00 -70 F I -500 0 G 38N Pacific p E -6 H 00 0 0 00 00 -60 J -5 00 K -50 L -5000 M 36N DK N 00 -70 Figure 1. Map of the study area and locations of cross-sections A to N shown in Fig. 8. The width of each cross-section is 50 km. Solid squares represent seismic stations operated by Tohoku University, Hokkaido University, Hirosaki University, University of Tokyo, The Japan Meteorological Agency (JMA) and the National Research Institute for Earth Science and Disaster Prevention (NIED). Contour lines indicate the depth of the ocean floor. Larg (...truncated)