GEOPHYSICAL RESEARCH LETTERS, VOL. 37, L09304, doi: /2010gl042935, 2010

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1 Click Here for Full Article GEOPHYSICAL RESEARCH LETTERS, VOL. 37,, doi: /2010gl042935, 2010 Seismic characteristics around the fault segment boundary of historical great earthquakes along the Nankai Trough revealed by repeated long term OBS observations Kimihiro Mochizuki, 1 Kazuo Nakahigashi, 1 Asako Kuwano, 1 Tomoaki Yamada, 1 Masanao Shinohara, 1 Shin ichi Sakai, 1 Toshihiko Kanazawa, 1 Kenji Uehira, 2 and Hiroshi Shimizu 2 Received 17 February 2010; accepted 30 March 2010; published 12 May [1] The existence of a static fault segment boundary has been proposed for segmentation of the historical great earthquakes along the Nankai Trough, southwest of Japan. Due to the extremely low seismicity, the seismic characteristics around the boundary have remained too uncertain to allow detailed discussion of the cause of the fault segmentation. We collected four years of continuous seismic data around the segment boundary through repeated marine observations and determined accurate hypocenters, magnitudes and focal mechanisms of the observed earthquakes. The Tokai segment to the east of the boundary shows particularly low seismicity. An abrupt change in the P axis orientation of intra slab earthquakes coincides with heterogeneous structure within the subducting Philippine Sea Plate. The boundaries between regions of different seismic character are parallel to the magnetic anomaly lineation over the Shikoku Basin of the subducting plate, implying that they are determined by the formation process of the basin. Citation: Mochizuki, K., K. Nakahigashi, A. Kuwano, T. Yamada, M. Shinohara, S. Sakai, T. Kanazawa, K. Uehira, and H. Shimizu (2010), Seismic characteristics around the fault segment boundary of historical great earthquakes along the Nankai Trough revealed by repeated long term OBS observations, Geophys. Res. Lett., 37,, doi: / 2010GL Introduction [2] The Philippine Sea (PHS) Plate subducts beneath southwest Japan along the Nankai Trough towards the northwest direction at a rate of 4 cm/year [DeMets et al., 1994]. Geodetic observations by the dense GPS network throughout Japan suggest a high rate of interplate mechanical coupling ( 100%) along the entire Nankai Trough [e.g., Mazzotti et al., 2000], while regular seismicity is extremely low. In fact, great earthquakes of magnitudes (M) 8 have occurred repeatedly at intervals of years [Ando, 1975], and the amount of coseismic slip ( 4 m) during the latest 1944 M7.9 Tonankai and 1946 M8.0 Nankai earthquakes match the accumulation of strain energy during the inter seismic period. The occurrence modes of these repeating great earthquakes have remained systematic through 1 Earthquake Research Institute, University of Tokyo, Tokyo, Japan. 2 Faculty of Sciences, Kyushu University, Fukuoka, Japan. Copyright 2010 by the American Geophysical Union /10/2010GL history such that along trough seismogenic segments slipped all at once or a great earthquake occurred in the eastern segment with respect to a static prominent segment boundary followed by another great earthquake in the western segment after a short period ranging from days to several years. The epicenters of both of the most recent great earthquakes were located adjacent to the fault segment boundary, and their ruptures propagated away from the boundary without crossing it (Figure 1). [3] In order to investigate possible structural factors that could produce this fault segment boundary, a number of seismic surveys have been conducted [e.g., Kodaira et al., 2006; Mochizuki et al., 1998]. They revealed strong heterogeneity within the crust of the subducting PHS Plate. The fault segment boundary appeared coincident with a major structural heterogeneity represented by an abrupt change in thickness of the subducting oceanic crust; the crust is thicker to the east ( 10 km) than to the west ( 7 km). Accordingly, the plate interface is located at a shallower depth in the east by 1.5 km. A proposed explanation for the seismic fault segmentation is that the abrupt stepwise depth change of the plate interface may prevent a rupture that initiates in the east from immediately propagating across it. 2. Observations and Data Analysis [4] Regular seismicity and focal mechanisms may indicate in situ tectonic activities and the state of stress. Therefore, they provide information about the mechanisms of earthquake generation and rupture propagation. However, due to the extremely low seismicity and the large distance offshore from the onshore seismic network, the detailed nature of the seismic activity around the segment boundary is not well understood and the data is limited to that obtained by temporary seismic observations of local activity using ocean bottom seismometers (OBSs) [e.g., Obana et al., 2005]. Long term monitoring by a large scale marine seismic network is necessary to observe a sufficient number of earthquakes. For this purpose, we conducted observations across the boundary using long term OBSs (LTOBSs) (Figure 1). We started the series of observations with nine LTOBSs around the boundary off Cape Shionomisaki in November, 2003, and completed the series by recovering 27 LTOBSs at the end of November, Hence, we successfully collected 4 years of continuous seismic data (Figure 1c). During the cruises for LTOBS deployment/ recovery operations between observation terms, we conducted active source seismic surveys over newly added sta- 1of5

2 Figure 1. Tectonic setting around Japan, and station locations for marine seismic observations along the Nankai Trough. (a) Tectonic setting around Japan. The PHS Plate subducts along the NW direction (black arrow, 310 ) beneath the Eurasia (or Amurian) Plate along the Nankai Trough, southwest of Japan. The yellow rectangle corresponds to the region shown in Figure 1b. KSC is the Kinan Seamount Chain on the extinct spreading center. (b) Station locations of our marine seismic observations. Yellow circles mark the LTOBS stations. White and black stars indicate the epicenters of the most recent 1944 Tonankai (M7.9) and 1946 Nankai (M8.0) earthquakes, respectively. Magenta curves depict expected fault boundaries for future great earthquakes. The dashed yellow curve outlines the observation region of the LTOBS seismic network. Shikoku Basin is a back ark basin on the PHS Plate. (c) Observation periods at the stations from December, 2003 through December, tions using a small volume (9 liter) airgun to derive a shallow ( 5 km) one dimensional (1 D) P wave velocity (Vp) structure at every station. We constructed a full 1 D structure to depths of 30 km by referring to existing nearby seismic profiles [Kodaira et al., 2006; Mochizuki et al., 1998; Nakanishi et al., 2008; Sato et al., 1998]. Initial S wave velocities (Vs) were calculated by assuming a Poisson s ratio of [5] We extracted from the continuous LTOBS data 7116 relatively large earthquakes by referring to the Japan Meteorological Agency (JMA) earthquake catalog listing earthquakes observed by the onshore seismic network (JMA earthquakes). We manually picked their arrival times at both onshore and offshore stations. We also visually identified smaller earthquakes from continuous LTOBS data that are not listed in the JMA catalog (OBS earthquakes). We classified the JMA earthquakes into two classes by the number of arrival time picks. JMA earthquakes with more than four P wave and more than one S wave arrival picks, amounting to a total of 6012, were classified as Class 1 earthquakes. We classified the remaining 1104 JMA earthquakes together with OBS earthquakes as Class 2 earthquakes. The larger number of arrival time picks for Class 1 earthquakes places more constraints on their hypocentral locations, which makes the hypocenters of Class 1 earthquakes more reliable. [6] Because of the extremely low seismicity along the Nankai Trough, the relative importance of each earthquake for clarifying the regional seismic characteristics is quite high. Therefore, we used a two stage procedure in an effort to determine as many hypocenters as possible. During the first stage, we employed a straightforward hypocenter determination method, hypomh [Hirata and Matsuura, 1987], that stably provides optimal estimates of the hypocenter and origin time of each earthquake based on the given 1 D velocity structures at the stations, a constant Poisson s ratio of 0.25, and a priori information about observational errors and estimate uncertainties. We incorporated the actual three dimensionality of the structure, as well as thick sediments with large Vp/Vs values, into station corrections. Earthquakes within a certain range should have similar raypaths to the receivers so that a single set of station corrections can be applied. We grouped the observed earthquakes using a vertical cylinder with a 30 km radius centered at each station, and a set of station corrections for each group was determined by a statistical method using the Class 1 earthquakes. The range of 30 km was arbitrary chosen based on a rough estimation of the wavelength of local structural heterogeneity. Thus, the total range of the LTOBS network is outlined by the outer most segments of the 30 km radius circles (Figure 1b). We iteratively applied the hypocenter determination method to each group while updating the station corrections at each iteration loop by the amount of travel time residuals. The number of converged hypocenters increased with the number of updates until they were reasonably estimated. We selected final sets of station 2 of 5

3 Figure 2. Hypocenter and magnitude distributions around the fault segment boundary, and sectional hypocenter projections over the along strike Vp structures. (a) Total hypocenter distribution color coded by depth. Circles indicate hypocenters of 2340 well determined earthquakes, and triangles those of 2335 other earthquakes. The solid red line defines a clear seismicity boundary at the fault segment boundary, and dashed red lines show boundaries between sub segments (A, B, and C) in the Nankai segment. Yellow lines show locations of along strike sections shown in Figure 2c. Magenta and dashed yellow curves are the same as in Figure 1b. (b) Magnitude distribution. Among 2340 well determined hypocenters, hypocenters of 2111 earthquakes whose magnitudes were also determined are plotted. (c) Sectional along strike Vp structures. Hypocenters within 10 km distance are plotted. Dashed black curves depict the plate interface [Baba et al., 2006], and shaded regions beneath the lines indicate the subducting oceanic crust with a thickness of 10 km. Solid and dashed red lines on the top axes show projected locations of the boundaries. corrections that minimize the travel time residuals after 30 iterations, and we applied them to the corresponding groups of the Class 2 earthquakes to obtain their hypocenters. At the end of the first stage, we obtained hypocenters for 1490 Class 1 and 3235 Class 2 earthquakes. We also determined the magnitudes of the Class 2 earthquakes based on a statistical linear relation between the OBS magnitudes of the Class 1 earthquakes determined from the observed maximum amplitudes on the OBS records and JMA magnitudes listed in the catalog. [7] In the second stage, we applied a joint tomographic inversion method, tomodd [Zhang and Thurber, 2003], to the Class 1 earthquakes with their initial hypocenters and origin times adopted from the results of the first stage. The initial 3 D velocity structures were constructed by smoothly combining the 1 D structures at the stations. A vertical grid interval of 5 km is too large to incorporate the sediment layers, so the station corrections for a group of earthquakes directly beneath each station (therefore, independent of the structural three dimensionality) were applied. After obtaining the final hypocenters of the Class 1 earthquakes as well as 3 D Vp and Vs structures, the hypocenters of the Class 2 earthquakes were relocated using these structures. Using this process of precise hypocenter determination, we generated 3of5

4 2340 hypocenters with more than three P and one S arrival picks that were more precisely determined than the remaining 2335 hypocenters (Figure 2a). Figure 3. Hypocenter distribution of earthquakes in the subducting oceanic crust and upper most mantle. (a) An example along dip section of Vp structure and projected hypocenters within 10 km distance (location shown as a black rectangle in Figures 3b and 3c). Dashed black curves and shaded region are the same as in Figure 2c. (b) Hypocenter distribution of earthquakes in the subducting oceanic crust. The active region is outlined by the solid yellow curve. Legends are the same as in Figure 2. (c) Hypocenter distribution of earthquakes in the oceanic upper most mantle. The active regions are outlined by the solid green curves. 3. Results and Discussion [8] The four years of continuous data accumulated through the marine seismic observations reveals an alongstrike non uniform distribution of earthquakes around the fault segment boundary, across which there exists a significant change in seismicity. In contrast to the particularly low seismicity in the Tonankai segment, slightly higher seismicity was observed in the Nankai segment where the hypocenter distribution shows regional variations in the intensity and depth distribution (Figure 2). The western Nankai segment can accordingly be divided into three sub segments from west to east (Figure 2a); A) a sub segment of very low seismicity in the western most area off Cape Muroto, B) an 80 km wide sub segment characterized by high seismicity in the uppermost mantle and oceanic crust in the landward direction from around the up dip limit of the fault, and C) a band of shallow seismicity in the oceanic crust extending from the trough axis with its sharp eastern side coincident with the fault segment boundary. The magnitude distribution shows no noticeable heterogeneity (Figure 2b). In order to clearly illuminate the variation in the hypocenter distribution, we individually plotted earthquakes occurring in the oceanic crust (Figure 3b) and in the mantle (Figure 3c) based on the plate interface depth [Baba et al., 2006] and by assuming a crustal thickness of 10 km (Figure 3a). The active regions of crustal and mantle earthquakes appear mutually independent, as these activities are clearly isolated in space (Figures 2c and 3). Therefore, each of these activities may have individual generation mechanisms. To further investigate the stress regimes in the subducting slab, we determined focal mechanisms from the first motion polarities (FPFIT [Reasenberg and Oppenheimer, 1985]) while raypath take off angles were precisely calculated by incorporating 3 D seismic velocity structures (SIMUL2000 [Thurber and Eberhart Phillips, 1999]). No systematic difference was found to exist between the focal mechanisms of the crustal and mantle earthquakes. We grouped the obtained reliable mechanism solutions into 20 km square cells based on their epicenters, and plotted a sector diagram for each cell. Most of the focal mechanisms are of the strikeslip type. Vectors of crustal deformation around the network can be well explained by subduction of the PHS Plate [Miyazaki and Heki, 2001]. However, P axis orientations are normal to the trough axis and oblique to the direction of plate convergence except for those off Cape Shionomisaki in the eastern half of sub segment C. A systematic clockwise landward P axis rotation is consistent with the results of previous studies using the on land seismic network [e.g., Xu and Kono, 2002]. Wang et al. [2004] interpreted the E W tension as the result of the slab pull force acting on the subducting PHS Plate further to the west along the Ryukyu Trench (Figure 1). We confirm that the same mechanism can be applied to the offshore intra slab earthquakes. The abrupt change in the P axis orientation in sub segment C coincides with a major structural heterogeneity in the oceanic crust and mantle. Its linear continuation to a shallow depth toward the trough axis implies a lateral stress discontinuity possibly caused by structural detachment. One of the nodal planes of 4of5

5 Tonankai by the Ministry of Education, Culture, Sports, Science and Technology of Japan. Figure 4. Seismic characteristics around the fault segment boundary compared with the structure of the subducting PHS Plate. The P axis distribution is shown by 20 width sector diagrams with a normalized diameter. The maximum cumulative number of sampled P axes in each sector within a 20 km square cell is indicated by the color at the center of each diagram (color code shown on top). Blue focal mechanisms are examples. The black arrow indicates the direction of plate convergence. Solid brown curves depict magnetic anomaly peaks illustrating the back arc opening history of the Shikoku Basin, and the brown figures are the assigned anomaly numbers. Thick dasehd brown lines limit the range of the Kinan Seamount Chain over the extinct spreading center. Other legends are the same as in Figure 3. earthquakes in the eastern most sub segment C, as well as the Tonankai Nankai segment boundary, is parallel to the magnetic anomaly lineation of the Shikoku Basin [Kido and Fujiwara, 2004] (Figure 4). Thus, we consider the focal planes of these earthquakes to be on pre existing structural fractures along isochronal crust produced during the backarc opening of the basin (Figures 1 and 4). Such structural fractures have been identified by active seismic surveys [Kodaira et al., 2006]. The spreading center of the basin rotated counterclockwise from N S to NW SE (Figure 4), and the PHS Plate fabric formed during this rotation is now subducting beneath the Nankai segment. The asymmetry of seismicity between sub segments A and C surrounding the extinct spreading center may be attributable to the rotation direction. 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