Double seismic zone within the ridge-shaped slab beneath southwest Japan
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2010
Received: 17 June 2010
Accepted: 1 November 2010
Published: 3 February 2011
We have investigated the intra-slab seismicity and focal mechanisms in the subducting Philippine Sea plate, beneath the region spanning Ise Bay and Lake Biwa, southwest Japan. Based on the hypocentral distribution of earthquakes, the slab geometry shows a gently dipping trend and ridge-shaped features, with the intra-slab seismicity forming a distinct double-plane seismic zone. The seismicity of the lower plane is much less active than that of the upper plane and is located about 10 km beneath the upper plane seismicity. Examination of the focal mechanism solutions revealed that the upper plane earthquakes occurred basically in a tension field, as is the case with most intra-slab events beneath southwest Japan. In contrast, we found that the lower plane earthquakes near the axis of the ridge-shaped slab occurred in a compression field. The seismogenic stress of the lower seismic plane is probably caused by the bending responsible for the formation of the ridge-shaped slab. Our analysis of earthquake waveforms confirmed that the upper and lower plane seismic zones correspond to the oceanic crust and mantle, respectively. Lateral stretching and bending of the slab could be the main driving factors that control the seismogenic stress field of the double seismic zone in this region.
A double-plane seismic zone (DSZ) structure for intra-slab earthquakes was first identified within the Pacific plate beneath northeast Japan (Hasegawa et al., 1978). Recently, the intra-slab seismicity within the PHS plate beneath the Kii Peninsula was also reported to have DSZ characteristics (e.g., Seno et al., 2001; Miyoshi and Ishibashi, 2004; Smith et al., 2004). Both the distribution of earthquake hypocenters in the region and arguments supporting slab dehydration suggest that the intra-slab earthquakes in the upper DSZ plane occur almost entirely within the oceanic crust, while the intra-slab events in the lower plane occur within the oceanic mantle (Seno et al., 2001; Yamasaki and Seno, 2003; Smith et al., 2004). Seno et al. (2001) also identified DSZ seismogenic layers by examining later phases associated with the slab structure. However, the seismogenic stress field of the DSZ has not been discussed to date even though its features are important for clarifying why intra-slab earthquakes occur beneath southwest Japan.
Beneath the region from Ise Bay to Lake Biwa, the shallow and gently dipping slab, with a NW-SE trend, has a convex shape (Fig. 1), as inferred from the distribution of intra-slab earthquakes (Miyoshi and Ishibashi, 2004, 2008), seismic tomography (e.g., Nakajima and Hasegawa, 2007), and receiver function analyses (e.g., Shiomi et al., 2008). Miyoshi and Ishibashi (2008) called this part of the slab the Isewan-Kohoku Slab (IKS). In the study reported here, we first determined the fine hypocenter and focal mechanism distributions in and around the IKS. Based on these results, we then verified the existence of a DSZ within the subducting slab. The origin of the seismogenic stress field of the DSZ within the IKS is discussed, taking into account the geometry of the subducting slab.
2. Data and Method
The data used in this study were obtained by the high-sensitivity seismograph network (Hi-net) (e.g., Obara et al., 2005) and the former Kanto-Tokai observation network of the National Research Institute for Earth Science and Disaster Prevention (NIED). Additional data were obtained from the Japan Meteorological Agency (JMA), Kyoto University, Nagoya University, and the University of Tokyo. About 7,000 earthquakes were selected for analysis from an earthquake catalog based on manual arrival time picking prepared by NIED Data Management Center. The analyzed earthquakes occurred between April 2002 and November 2009 beneath the region extending from Ise Bay to Lake Biwa. The focal depth of the selected earthquakes ranges from 0 to 100 km, and the minimum magnitude threshold is 2.5 and 0.0 for events shallower and deeper than 30 km, respectively.
The focal mechanism solutions for the intra-slab earthquakes were determined from initial P-wave motion polarities using the HASH (version 1.1) program (Hardebeck and Shearer, 2002). We analyzed only those events for which eight or more P-wave polarities were available. Although this technique can account for possible errors in the assumed earthquake locations and seismic velocity model, as well as in the polarity observations, we evaluated only possible errors associated with polarity observations. The takeoff angles and azimuths had been previously determined when the earthquakes were located. For each event, the preferred focal mechanism was taken to be the average of all acceptable solutions found by a grid search approach.
3. Hypocenter and Focal Mechanism Distributions
Examination of the cross-sections of Fig. 3(b) revealed the existence of a main seismic zone of intra-slab earthquakes. A number of isolated events, located about 10 km below the upper plane seismicity (e.g., cross-sections C-J of Fig. 3(b)), can also be recognized. Since the hypocenters of these earthquakes were determined with sufficient accuracy, they suggest the existence of a second, lower seismic zone. We define such isolated events as lower plane earthquakes of the DSZ if they are located 7 km deeper than the average focal depth of the intra-slab earthquakes. In this way, we identified 25 lower plane events in the vertical cross-sections C-E of Fig. 3(b). These events are shown by open circles in Fig. 3. The largest event was M 3.0.
On the other hand, two events (events d and e in Fig. 4) in the lower seismic plane are of strike-slip type, with the T -axis orientation in the E-W and NE-SW direction, respectively. These events were located near the southwestern end of the IKS. Such observations suggest that the stress field for the lower seismic zone is different from that of the upper zone near the axis of the IKS and is the same near the southwestern end of the IKS.
In this study, we located the hypocenters and determined the focal mechanisms for events that occurred beneath the region spanning Ise Bay to Lake Biwa. Our results reveal that the intra-slab seismicity within the PHS plate defined a distinct DSZ. The upper plane earthquakes were identified as being normal-fault or strike-slip types, with the T -axis orientation ranging from the E-W to NE-SW directions. The lower plane events could be classified into two main types: (1) thrust- or strike-slip-type events that occurred beneath the axis part of the IKS, with the P-axis oriented in directions ranging from E-W to NE-SW; (2) strike-slip-type earthquakes that occurred in the southwestern part of the IKS, with the T -axis orientation in the E-W and NE-SW direction. A study of intra-slab seismograms revealed that the events in the upper plane occurred within the oceanic crust and those in the lower plane within the mantle. These results strongly suggest the existence of a DSZ within the IKS. The seismogenic stress of the DSZ could be explained qualitatively by slab stretching and bending, both parallel to the trough axis.
We are grateful to NIED, Kyoto University, University of Tokyo, Nagoya University, and JMA for providing the seismic records used in this study. Eartquake magnitudes were provided by the JMA. We thank Drs. Tetsuzo Seno and Kelin Wang for valuable comments that improved our manuscript. We also thank Dr. Bogdan Enescu and researchers of the Hi-net group for their helpful comments and discussions. The GMT software (Wessel and Smith, 1998) was used to draw the figures.
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