GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L19604, doi: /2004gl020366, 2004

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1 GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L19604, doi: /2004gl020366, 2004 Characteristic seismic activity in the subducting plate boundary zone off Kamaishi, northeastern Japan, revealed by precise hypocenter distribution analysis using ocean-bottom seismometers T. Okada, K. Sakoda, T. Matsuzawa, R. Hino, and A. Hasegawa Research Center for Prediction of Earthquake and Volcanic Eruption, Graduate School of Science, Tohoku University, Sendai, Japan S. Sakai and T. Kanazawa Earthquake Research Institute, University of Tokyo, Tokyo, Japan Received 26 April 2004; revised 6 September 2004; accepted 15 September 2004; published 8 October [1] High seismic activity prevails along the plate boundary to the east of northeastern Japan. To understand how this seismic activity is related to subduction process, hypocenter locations are re-determined using data obtained over 6 years by fiber-cabled permanent ocean bottom seismometers off Kamaishi. The double-difference method is adopted to obtain the relative location in more detail. As a result of this analysis, a number of seismic clusters including small repeating earthquakes are identified along the plate boundary. This observation supports the hypothesis that seismic coupling is very low in this region and there only small asperities are distributed along the plate boundary off Kamaishi. A number of events with downdip compressional focal mechanisms are identified, corresponding to the upper plane seismicity of the double seismic zone in this area. Although most of these events are located in the crust of the subducting Pacific Plate, some are located in the mantle of the slab. INDEX TERMS: 7209 Seismology: Earthquake dynamics and mechanics; 7218 Seismology: Lithosphere and upper mantle; 7230 Seismology: Seismicity and seismotectonics; 8123 Tectonophysics: Dynamics, seismotectonics. Citation: Okada, T., K. Sakoda, T. Matsuzawa, R. Hino, A. Hasegawa, S. Sakai, and T. Kanazawa (2004), Characteristic seismic activity in the subducting plate boundary zone off Kamaishi, northeastern Japan, revealed by precise hypocenter distribution analysis using ocean-bottom seismometers, Geophys. Res. Lett., 31, L19604, doi: / 2004GL Copyright 2004 by the American Geophysical Union /04/2004GL Introduction [2] Many large earthquakes occur along the plate boundary east off northeastern Japan. However, there have been no earthquakes of magnitude greater than 6 in the off- Kamaishi area (N39 40, E ) since 1926 when the Japan Meteorological Agency (JMA) started to locate the hypocenters, whereas small- and micro-earthquakes are common. The amount of back-slip estimated by inverting GPS data for the last 5 years is also small off Kamaishi compared to that in southern areas such as the focal area of the 1978 off-miyagi earthquake [Suwa et al., 2003]. These features suggest that the majority of the plate boundary off Kamaishi is in steady-slip (creep), with only small asperities. Matsuzawa et al. [2002] have detected a characteristic earthquake sequence with M4.8 ± 0.1 regularly occurring at the same location off Kamaishi, Iwate Prefecture, since 1957, with a recurrence interval of 5.52 ± 0.68 yr. This has been interpreted as representing the repeating rupture of an asperity with a dimension of 1 km. The very regular occurrence of this rupture is perhaps due to the repeating slips of the isolated asperity surrounded by a stable sliding area that is slipping at a nearly constant rate. By comparing the source area of the two recent M4.8 earthquakes (1995 and 2001) obtained by waveform inversions, the two events can be clearly related to the repeated rupture of the same asperity area, supporting the hypothesis of persistent asperities [Okada et al., 2003]. Many similar earthquake groups (repeating small earthquakes) have also been discovered in this region [Igarashi et al., 2003; Uchida et al., 2003]. These repeating small earthquakes are also interpreted as representing the repeated rupture of small asperities along the plate boundary in the same manner as the M4.8 characteristic earthquake sequence. [3] It has proven difficult to obtain reliable hypocenter locations under the sea because seismic stations are usually only deployed on land. To assist with the accurate location of events, three ocean-bottom accelerometers and two pressure-meters were deployed off Kamaishi in 1997 (Figure 1) [Kanazawa et al., 1996; Hino et al., 2001]. The devices are fiber-cabled and have operated continuously. Using data from these ocean-bottom accelerometers, microearthquakes including the repeating small earthquakes in the off-kamaishi region are re-located in this study and employed to discuss the seismicity off Kamaishi, where the seismic coupling is expected to be low. 2. Hypocenter Determination Using Ocean Bottom Seismographs [4] When using data from ocean-bottom seismographs (OBSs) to determine the location of hypocenters, it is necessary to eliminate the effect of the sedimentary layer beneath the sea bottom by a station-correction method. In the present study the station corrections are estimated as follows. First, the hypocenters of microearthquakes of cluster B shown in Figure 2, which includes the M4.8 interplate characteristic earthquake sequence, are relocated using data from inland stations alone. Arrival time data from the catalogs of Tohoku University and the Japan Meteoro- L of5

2 Figure 1. Station distribution. Squares: Ocean-bottom stations used in this study. Triangles: Seismic stations routinely operated by Tohoku University, JMA and Hi-net. Circle: Kamaishi City. Outlined area represents the region in which hypocenters were relocated in this study. logical Agency (JMA) are used in this step. The velocity model employed is that proposed by Hasegawa et al. [1978]. The hypocenters for these events are thought to be located reliably without OBS data because they are located near the coast line. The station correction is then estimated by averaging sign-inverted travel-time residuals observed at each station for those reference events. Twenty-one welllocated events for the period from 1997 to 2002 are selected in this step for calculation of station corrections. Note that the station corrections obtained in this study are also effected by lateral heterogeneity of the crust. [5] The hypocenters of 1864 events occurring in the period in the area off Kamaishi (N , E ) are determined using OBS data with the station corrections. Figure 2 shows the hypocenter distribution without OBS data (Figure 2a) in comparison with the relocated hypocenter distribution using OBS data (Figure 2b). Without OBS data, the hypocenters far to the east off Kamaishi cannot be well determined, and are constrained to be located on the surface or around the Moho (at 31 km depth) in the velocity model. Using OBS data and the station corrections, on the other hand, most events are located at depths of 10 to 20 km along a low-angle westward dipping plane corresponding to the plate boundary (c.f. Bold line in Figure 2 [Zhao et al., 1997]). Note that the events are located at inadequately deeper region without the station corrections. We therefore infer that the station corrections used in this study would be adequate even for the event far to east off the Kamaishi to some extent. To the west of the boundary, the distribution of hypocenters becomes complex, although some earthquakes still occur near the plate boundary. 3. Relocation by the Double-Difference Location Method [6] The double-difference location method (DDLM) developed by Waldhauser and Ellsworth [2000] is adopted for re-relocation of microearthquakes in the western region of the off-kamaishi area where the station coverage is well. The initial locations of hypocenters are those determined using the station corrections, as discussed in the previous section. [7] For events with magnitudes of 1.0 or greater, differential arrival times are obtained using cross spectra calculated for waveform data from events with epicentral separation of less than 10 km. Waveforms were originally digitized at a sampling frequency of 100 Hz, with a time window of 2.56 s for both P- and S-waves. Time differences Figure 2. Hypocenter distribution off Kamaishi. Epicenter distribution (upper) and vertical cross sections along the line X-Y (lower) showing hypocenters relocated without (a) and with (b) OBS data. Triangles: OBS locations. Bold gray line: Plate boundary location estimated by Zhao et al. [1997]. 2of5

3 Figure 3. Hypocenter distribution relocated using (a) OBS data, and (b) OBS data and the double-difference method. Epicenter distribution (upper), and vertical cross section along the line X-Y (lower). Stars: Repeating small earthquakes [Uchida et al., 2003]. are estimated from the data in the frequency range of 3 to 12 Hz with squared coherency of greater than 0.8. [8] A total of 1217 events are re-relocated using DDLM in the western region (N , E ) off Kamaishi. A total of arrival time differences are obtained from catalog data for P-waves, and are obtained for S-waves. From the cross spectra, arrival time differences are obtained for P-waves and are obtained for S-waves. The LSQR method [Paige and Saunders, 1982] is employed to solve the equation. The average root mean square value of DDs for each event was reduced by this procedure from 1.42 s to 0.45 s. Here after, the re-relocation using DDLM will be referred to as DDLM-relocation. [9] Figure 3b shows the DDLM-relocated hypocenter distribution. The relocated hypocenters cluster together more densely than the hypocenters before the relocation (Figure 3a). The events belonging to a large cluster located beneath station OB3 at depths of 5 to 30 km fall into two groups (A1, A2). The events in cluster B, which includes the M4.8 characteristic earthquake sequence, are also clustered, particularly in terms of depth. The repeating small earthquakes [Uchida et al., 2003] are indicated by stars. After the DDLM-relocation the events in each group of repeating small earthquakes can be seen to occur very close to each other. [10] The location error is determined by relocating five repeating small earthquake events of cluster R by the singular value decomposition (SVD) method [Waldhauser and Ellsworth, 2000], which allows the location error to be estimated directly. They are located within a radius of about 500 m. The average of the standard deviation is 80.8 m in the horizontal direction and m in the vertical direction. Note that the error estimate in this scheme may have to be understood as a minimum estimate of the possible errors. We also compared the rupture areas of events, assuming a circular fault [Brune, 1970] and a stress drop of bar [Matsuzawa et al., 2002], to the separations among the relocated events. Averaged value of separation is about a few hundred meters and averaged diameter of the circular rupture areas is about 100 m. Although the estimated rupture areas of these events do not overlap strictly, the relatively large location errors and ambiguous estimations of source sizes do not allow a definitive answer whether they overlap or not. [11] The fault plane solution was also obtained using the DDLM-relocated hypocenters and polarity data for P-wave initial motions from the catalogs of Tohoku University and the JMA. The distribution of the P-axis based on these fault plane solutions are shown in Figure 4. In group B, which includes the M4.8 characteristic earthquake sequence, the events have low-angle thrust (LT) solutions. These LT events can be interpreted as occurred along the plate boundary because the fault plane dipping to the west is expected to correspond to the plate boundary between the Pacific Plate and the overriding plate (the North American 3of5

4 upper plane seismicity occurs within the oceanic crust of the subducting Pacific Plate [Matsuzawa et al., 1986; Hasegawa et al., 1994]. The present analysis clearly highlights the spatial separation between the LT events (interplate events) and the DC events (intraplate events). The DC events of group C are expected to be located within the oceanic crust, which is estimated to be 5 to 10 km thick. One possible cause of such DC events is dehydration embrittlement due to the dehydration of basalt/gabbro [e.g., Kirby et al., 1996]. [15] In contrast, the deeper DC events of group D would occur near the Moho or within the mantle of the subducting Pacific Plate. These deeper DC events would therefore occur by dehydration embrittlement in the mantle of the slab [e.g., Hacker et al., 2003]. In the 2003 M7.0 off-miyagi earthquake, an intra-slab event that occurred on May 26, 2003 in a nearby region [Okada and Hasegawa, 2003; Sakoda et al., 2004], the rupture extended into both the crust and the mantle of the subducting Pacific Plate. Upper plane seismicity also occurred near the hypocenter of this event within both the crust and the mantle of the slab prior to the 2003 event. High background seismicity in the mantle may therefore delineate areas in which the rupture zone of the mainshock can be more readily extended into the mantle. Figure 4. Summary of study results. Gray circles: Hypocenter distribution. Bold lines: P-axis orientation of the fault plane solution. Gray broken lines: Zone of LT events, corresponding to the location of the probable plate boundary. Stars: Repeating small earthquakes. Gray line: Location of the plate boundary estimated by Zhao et al. [1997]. or the Okhotsk plate [e.g., Seno et al., 1996]). These LT events are distributed along the zone between regions B and A2. Thus, we infer that distribution of these LT events corresponds with the plate boundary. The location of the plate boundary obtained in this study is almost coincident with that estimated by Zhao et al. [1997]. [12] In area C, at depths 5 to 10 km below the zone of the LT events, events with down-dip compression (DC) solutions are distributed parallel to the zone of the LT events. This zone of DC events forms part of the upper plane seismicity of the double seismic zone in NE Japan [Igarashi et al., 2001]. Note that DC events also occur in deeper parts of region D. 4. Discussion [13] As shown in Figure 4, the distribution of repeating small earthquakes is similar to that of LT events. This suggests that the repeating small earthquakes are distributed along the plate boundary. Seismic activity along this plate boundary seems to be low, and to be limited to certain clusters that include repeating small earthquakes. This observation supports the hypothesis that most of the plate boundary is in steady-slip (creep), and that only small asperities occur off Kamaishi [Matsuzawa et al., 2002]. [14] The DC events corresponding to upper plane seismicity occur 5 to 10 km beneath the plate boundary. This 5. Conclusion [16] A plausible hypocenter distribution for the off- Kamaishi area in northeastern Japan was derived using data from ocean-bottom accelerometers in conjunction with the double-difference method. The results reveal a number of repeating small earthquakes to be located along the plate boundary, with seismicity limited to small clusters that include these repeating earthquakes. Upper plane seismicity occurring 5 to 15 km beneath the plate boundary is also identified, most of which is distributed in the crust of the subducting Pacific Plate, while some is distributed into the mantle of the slab. [17] Acknowledgments. The authors would like to thank A. Zollo, and two anonymous reviewers for helpful comments, Dr. N. Uchida for providing us the information on the repeating small earthquakes, and Dr. F. Waldhauser for use of his program hypodd. The data analyzed was provided by the JMA and Hi-net. This work was conducted as part of the 21st COE program, Advanced Science and Technology Center for the Dynamic Earth of Tohoku University. This work was also partially supported by MEXT.KAKENHI ( ) and JSPS.KAKENHI ( ), Japan. References Brune, J. N. (1970), Tectonic stress and the spectra of seismic shear waves from earthquakes, J. Geophys. Res., 75, Hacker, B. R., S. M. Peacock, G. A. Abers, and S. D. Holloway (2003), Subduction factory: 2. Are intermediate-depth earthquakes in subducting slabs linked to metamorphic dehydration reactions?, J. Geophys. Res., 108(B1), 2030, doi: /2001jb Hasegawa, A., N. Umino, and A. 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