Seismicity around the seaward updip limit of the Nankai Trough seismogenic zone revealed by repeated OBS observations

FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 1 Seismicity around the seaward updip limit of the Nankai Trough seismogenic zone revealed by repeated OBS observations Koichiro Obana 1, Shuichi Kodaira 1, Yoshiyuki Kaneda 1, Kimihiro Mochizuki 2 and Masanao Shinohara 2 1 Research Program for Plate Dynamics, Institute for Frontier Research on Earth Evolution (IFREE) 2 Earthquake Research Institute, University of Tokyo, Tokyo, Japan. Introduction The seismogenic zone at a subduction zone does not normally extend to the trench axis and the shallowest part of the plate interface is thought to be seismically decoupled (Byrne et al., 1988). The seaward updip limit of the seismogenic zone is limited by a transition from the aseismic to seismogenic plate interface. The location of the updip limit of the seismogenic zone and its relation to the crustal structure are important to understand a transition process and the seismic rupture at the plate interface. The Nankai Trough seismogenic zone, southwestern Japan, is one of the most well-studied subduction seismogenic zones in the world (Fig. 1). Along the Nankai Trough, the Philippine Sea plate is subducting beneath the overriding Eurasian plate with a convergence rate of about 4.6cm/year off cape Muroto (Seno et al., 1993). Periodic great interplate earthquakes have been documented since the seventh century and the recurrence interval is about 100-200 years (Ando, 1975). The fault region of great earthquakes along the Nankai Trough is divided into four segments. The co-seismic rupture of the 1946 Nankai earthquake occurred on the two western segments, A and B (Ando, 1975). Many seismic surveys using a controlled source have been conducted around the Nankai Trough (e.g., Kodaira et al., 2000; Takahashi et al., 2002). Off Cape Muroto, two offshore and onshore seismic surveys were done (Kodaira et al., 2000; 2002) (MO104 and KY9903 in Fig. 1). According to their results, the updip limit of the coseismic slip zone extends beneath the young accretionary prism. A splay fault system consisting of several out-of-sequence thrust (OST) faults around the seaward limit of the 1946 Nankai earthquake dislocation area was imaged by multi-channel seismic (MCS) reflection surveys (Park et al., 2000). They concluded that the OSTs were related to the large interplate earthquake and may generate a tsunami by a deformation of forearc accretionary prism. Interplate coupling between subducting and overriding plates during the interseismic period would cause the crustal deformation observed by geodetic surveys. Mazzotti et al. (2000) estimated a current interplate coupling between subducting Philippine Sea plate and overriding plate along the Nankai Trough using a dense Global Positioning System (GPS) network. They concluded that the Nankai Trough subduction zone is fully coupled on the plate boundary. The minimum extent of the locked zone is from 15km to 24km depth of the plate boundary. However, an uncertainty of the seaward extent of the locked zone remains. Seismicity during the interseismic period could show the location of the updip limit of the seismogenic. The updip limit of the seismicity during the interseismic period between large events is comparable with the aftershock area of large interplate earthquakes (Byrne et al., 1988). However, the offshore seismicity around the Nankai Trough is very low and hypocenters are not determined well by on-land observations. An ocean bottom seismograph (OBS) observation is effective to observe more earthquakes below the seafloor and obtain accurate hypocenters of them. In this article, we observe seismicity around the updip limit of the seismogenic zone obtained by OBS observations off cape Muroto. A purpose of this study is to examine the updip limit of the seismogenic zone and aseismic-seismic transition at the plate boundary. Observation and analysis We began a micro-seismicity observation off Cape Muroto in 1998 using free-fall and pop-up type digital recording OBSs. The recording period of our OBS is limited to about two months. It is not long enough to observe low seismicity around the Nankai Trough. We have repeated installing and retrieving OBSs to elongate the observation period. We have carried out an OBS observation for nine months in total from 1998 to 2000. The data recorded by two JAMSTEC submarine cable seismic stations off cape Muroto were also used in the analysis. The seismic velocity structure has a large lateral variation in a subduction zone. In this study, we determined hypocenter locations using 3-D P- and S-wave velocity structures based on OBS seismic surveys. We referred to four OBS seismic surveys around the Nankai Trough (KR9810, MO104, KY9903, and KR9806 in Fig. 1) (Kodaira et al., 2000; 2002; Nakanishi et al., 2002; Takahashi et al., 2002). A three-dimensional velocity structure was derived from interpolating them. P-wave velocities (Vp) were defined at the top and bottom of each layer. Velocities within the layer were obtained by linear interpolation of the velocities at the top and bottom of each layer. S-wave velocities (Vs) were estimated from assuming Vp/Vs structure. In this model, we assumed that Vp/Vs is 3.32 and 2.14, in the sedimentary layer and accretionary prism, respectively. In other parts of the structure, Vp/Vs is assumed to be 1.73. These values are based on the results of seismic survey KR9810 off Cape Ashizuri (Takahashi et al., 2002). Finally, we constructed 3-D Vp and Vs models, which extend to 200km in the horizontal directions and 50km in depth with a 1km grid interval. For the first step of the hypocenter determination, P- and S- wave travel times between OBSs and 1km spacing grids in the 3- D model were calculated by solving the eikonal equation using finite differences (Zelt and Barton, 1998). At the second step, hypocenters were obtained by using these P- and S-wave travel 149

FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 1 time tables. Hypocenters are relocated iteratively from an initial guess of the hypocenter by a linearized inversion to minimize root-mean-square (RMS) travel time residuals weighted by the inverse of their picking errors. We examined several locations as initial guesses for each event. Because our 3-D model includes large velocity contrasts, there would be several local minimums of the travel time residual. Examined initial guesses were distributed in a space of 100km in horizontal directions with a 10km interval and 5 to 30km in depth with a 5km interval. As a final result, a hypocenter was determined at a location with a minimum weighted RMS residual from all inversion results. Results We tried to locate 582 events through our OBS observation period and 328 events converged within the 3-D model. However, many events were too small to pick up sufficient numbers of phase arrivals. We selected only 176 events with three or more P-arrivals and two or more S-arrivals as reliable hypocenters (Fig. 2). Seaward extension of the seismicity seems to be limited by the 4000m isobath contour which was sub-parallel to the 150 C isotherm contour on the top of the subducting oceanic crust (Hyndman et al., 1995). Cross sections along the seismic survey lines, MO104 and KY9903, with projected hypocenters, show two groups of seismicity (Fig. 3). One is a group corresponding to earthquakes that occurred at a depth shallower than the subducting oceanic crust, the other is a group corresponding to earthquakes that occurred in the uppermost mantle of the subducting oceanic plate. Shallow earthquakes near the top of the subducting oceanic crust make several clusters on both seismic survey lines. Almost all the shallow hypocenters were shallower than 10km in depth. Seismicity in the uppermost mantle is located from 20km to 30km in depth. The hypocenters in the uppermost mantle show a scattered distribution. Earthquakes of the shallow group show some clustering around the seaward limit of the co-seismic slip area of the 1946 Nankai earthquake (Ando, 1975). Some of these earthquakes were characterized by very similar waveforms. These similar waveforms imply that events occurred at the almost the same locations with the same mechanisms. We calculated cross-correlation coefficients of waveforms between pairs of events on a vertical component including both P- and S-wave arrivals. We treat pairs of events with correlation coefficients which were larger than 0.90 at more than two OBSs as similar earthquakes. The similar earthquakes were located at the seaward limit of the shallow earthquakes, which occurred at the top of the subducting oceanic crust (Fig. 4). Discussion Seismicity around the updip limit of the seismogenic zone A study of micro earthquakes at Parkfield along the San Andreas fault show seismicity clusters characterized by earthquakes regularly occurring (Nadeau et al., 1995). They were identified by high cross-correlation coefficients of waveforms. These earthquakes are interpreted as a repeated slip on a given asperity driven by a steady slip (Nadeau and Johnson, 1998). Similar earthquakes observed off cape Muroto were not regularly occurring. However, those earthquake clusters can be interpreted as earthquakes occurring at a locally coupled area in the aseismic-seismic transition zone. Existence of some clusters indicates several locked patches surrounded by steady slip zone. Similar earthquakes projected on a poststack depth migrated section of MCS profile along KY9903 locate around the top of the subducting oceanic crust (Fig. 5). It is still unknown whether earthquakes occurred in the subducting oceanic crust, in the overriding accretionary prism, or on the boundary between them because of limitation of the accuracy of the hypocenter depth. The friction on the plate interface between subducting oceanic crust and overriding accretionary prism seems to increase landward from the earthquake cluster. The decollement steps down from the sedimentary layer on the subducting crust to the top of oceanic crust around the similar earthquake cluster. In addition, the seafloor slope shows a steepening to landward from the similar earthquake cluster. Where the friction at the bottom boundary of the accretionary prism increases, the prism taper becomes steeper (Davis et al., 1983). Along the line KY9903, a subducting seamount was imaged by seismic survey (Kodaira et al., 2002). The steepening of the seafloor slope could be caused by the subduction of a seamount (Yamazaki and Okamura, 1989; Dominguez et al., 1998). However, steepening of the seafloor slope and stepping down of the decollement also coincide with the similar earthquake cluster along the other survey line MS105 (Fig. 6). The shallow earthquake clusters are likely associated with the change of a friction between the accretionary prism and a subducting oceanic plate. Along the line MS105, out-of-sequence thrusts (OST) cutting through the accretionary prism from top of the subducting oceanic crust to the seafloor were identified by MCS survey (Park et al., 2000). These OSTs cutting the topmost cover sequence were interpreted as seismic thrust during large interplate earthquakes including the 1946 Nankai earthquake. The earthquake cluster around the top of oceanic crust layer 2 is located where the OST converges to the top of the subducting oceanic crust. The landward plate interface from the OST is a seismogenic plate interface during large thrusts earthquakes with the subduction of an oceanic plate. The seaward limit of the shallow earthquakes coincides with the 150 C isotherm on the top of oceanic crust (Hyndman et al., 1995). This temperature is associated with the dehydration temperature for stable-sliding clays at the subduction zone and may correspond to the updip seismogenic limit (Hyndman et al., 1997). At this temperature, smectite to illite clay-mineral transition occur and the physical properties of them change from stable-sliding to unstable-sliding. However, smectite is not observed in large quantities at subduction zones everywhere (Hyndman et al., 1997). The fraction of smectite in a sediments entering the subduction zone off cape Muroto is less than 10-20% at the toe of the accretionary prism (Underwood et al., 1993). The smectite to illite transition is not enough to explain the onset of the seismogenic behavior (Moore and Saffer, 2001). A suite of diagenetic to low-grade metamorphic processes causes stick-slip behavior in fault zones and also alter the upper plate of the subduction zone to allow a stress drop sufficient to create a recordable earthquake (Moore and Saffer, 2001). Thus, the updip limit of the seismogenic zone coincides with a 150 C 150

FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 1 isotherm on the top of oceanic crust off cape Muroto lacking enough fraction of smectite in the downgoing oceanic sediment. From the above mentioned points, the shallow earthquake clusters including similar waveform earthquakes occurred in a transition zone from aseismic to seismogenic plate interface. However, the plate interface does not couple perfectly in the transition zone. Earthquakes characterized by similar waveforms occur in several locally locked patches. Co-seismic slip on the most seaward part of the 1946 Nankai earthquake estimated from tsunami (Tanioka and Satake, 2001) is smaller than cumulative relative motions between subducting and overriding plates during the interseismic period. Some of relative motions may be consumed by steady slip in the unlocked region of the transition zone. Earthquakes in the uppermost mantle Earthquakes also occurred in the uppermost mantle below the subducting oceanic crust (Fig. 3). These earthquakes may be caused by a dehydration embrittlement of serpentinized mantle. Earthquakes in the mantle beneath southwestern Japan were considered to be caused by the dehydration embrittlement of serpentinized mantle with back-arc igneous activity in the Izu-Bonin arc (Seno et al., 2001). Kodaira et al. (2002) explained the seismicity in the mantle beneath the Shikoku Island with a similar mechanism. A low P-wave velocity zone was imaged beneath the subducted seamount along the seismic survey line KY9903 (Kodaira et al., 2002). They thought that this low Vp (=7.5km/sec) is associated with serpentinized mantle related to the past plume activity along the Kinan seamount chain. If the mantle portion of the subducting slab is hydrated at the trench, dehydration may begin at a depth of about 20km on the basis of thermal modeling and a dehydration loci of serpentine (Seno et al., 2001). Earthquakes in the uppermost mantle observed our OBS observations were deeper than 20km in depth. Although the exact area of the serpentinized mantle could not be estimated, the earthquake in the uppermost mantle beneath the OBS array is explained by the dehydration embrittlement of hydrated mantle. Conclusions We observed seismicity off cape Muroto around the updip limit of the seismogenic zone using OBSs. The obtained hypocenters seem to be classified into two activities. One is seismicity around the top of the subducting oceanic crust and the other is in the uppermost mantle of the subducting Philippine Sea plate. The earthquakes around the top of the subducting oceanic crust make several clusters characterized by pairs of earthquakes with very similar waveforms. These earthquake clusters are located in the transition zone of the interplate coupling, which changes from aseismic stable sliding to seismogenic locked. 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FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 1 Figure 1. Map around the Nankai Trough. Open circles indicate OBS positions. Rectangular areas labeled A to D are coseismic rupture area of great interplate earthquakes (Ando, 1975). A solid star indicates the epicenter of 1946 Nankai earthquake. Area A and B were ruptured in 1946 Nankai earthquake. Solid lines, KR9810, MO104, KY9903, and KR9806, show profiles of OBS-Airgun seismic structure surveys. Figure 2. Epicentral distribution based on the 3-D velocity model. Solid circles indicate epicenters observed by OBS. Open circles indicate epicenter determined by Japan Meteorological Agency (JMA) from October 1998 to September 2001. Radius of each circle is taken to be proportional to magnitude. Open diamonds indicate location of the OBSs. The rectangle indicates the coseismic slip area of the 1946 Nankai Earthquake (Ando, 1975). Broken line is 150 C isotherm on the top of the subducting oceanic crust (Hyndman et al., 1995). Two thick lines, MO104 and KY9903, are Airgun-OBS seismic survey lines. Figure 4. (a) Epicenter of shallow earthquakes. Crosses indicate epicenters, which are shallower than 10km in depth. Open circles indicate earthquakes with similar waveforms. Each similar earthquake cluster is enclosed by a solid line. Dotted lines indicate the seismic survey lines KY9903 (Kodaira et al., 2002) and MS105 (Park et al., 2000). The rectangle and broken line are the same as in Fig. 2. Figure 3. Cross sections along two Airgun-OBS seismic survey lines in Fig. 2 (Kodaira et al., 2000; 2002). Solid circles indicate projected locations of hypocenters. Only hypocenters within 25km of either side of the survey line have been projected. The radius of each circle is taken to be proportional to the magnitude. Isovelocity contours of Vp are drawn with interval of 0.5km/sec. 152

FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 1 Figure 5. Poststack depth migrated section of MCS profile along KY9903 (upper panel) and its interpretation with projected hypocenters (lower panel) (after Park et al., 1999b). Co-seismic slip zone of 1946 Nankai earthquake (Ando, 1975) and interplate locked zone (Hyndman et al., 1995) are indicated at the top of the figure. Hypocenters located within 10km from the MCS line are projected on the lower panel. Open circles indicate earthquakes characterize by high cross correlation coefficient of the waveforms. Crosses indicate other earthquakes. Figure 6. A poststack depth migrated seismic profile along MS105 (upper panel) and its interpretation with projected hypocenters (lower panel) (after Park et al., 2000). Symbols are the same as in Fig. 5. 153