Seismic activity around the northern neighbor of the 2011 off the Pacific coast of Tohoku Earthquake with emphasis on a potentially large aftershock in the area
© 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. 2011
Received: 7 April 2011
Accepted: 3 June 2011
Published: 27 September 2011
We investigated the seismic activity around the northern neighbor of the 2011 off the Pacific coast of Tohoku Earthquake (MW 9.0) with special attention to a potential large aftershock in the area. We obtained a combined data set by adding our manually-picked locations to the catalog locations by the Japan Meteorological Agency. The hypocenter distribution delineates active and inactive bands of seismicity. The band of low seismicity corresponds to a zone of a large seismic slip, indicating that aftershocks occurred in peripheral neighbors of the mainshock asperity. The broad band of active seismicity along the coast corresponds to the zone of a large postseismic slip, suggesting the enhancement of the aftershock activity by the slip. Although the northern neighbor of the mainshock fault is a favored region of increased seismicity, as shown from a Coulomb stress calculation, no significant seismic activity is observed within the potential source area except along the Japan Trench and the SW corner. This implies that the zone of interplate moment release by previous large earthquakes and the subsequent slow slip acted as a barrier to the migration of both the mainshock rupture and aftershock activity. However, an aftershock area in the zone may reflect inhomogeneous moment release by past seismic and aseismic sequences.
2. Hypocenter Location
Figure 2(a) compares the hypocenter distribution of our data and the JMA catalog for a two-day period from March 16, 2011. For inland earthquakes, the JMA catalog includes more events than our location, but the situation is reversed for the oceanic events. The box in the inset map of Fig. 3(a) shows a comparison of magnitude-frequency distribution for the events. Our catalog contains more events with magnitudes smaller than 3.5. The total number of events is 421 in our catalog and 235 in the JMA catalog. The b-value from our data, 0.68, is significantly lower than those obtained in inland areas but consistent with those from events far east off the Tohoku district (Hirose et al., 2002). The average nominal location errors from events shown in the box in Fig. 3(a) are 0.030° for latitude and 0.058° for longitude. Figures 3(b) and (c) show the deviation of location relative to the JMA catalog. Although there is a systematic bias in our data probably because of the effect of the subducting slab with high seismic velocity, the epicenter deviation is generally smaller than 0.1°, provided the location is limited to the north of 39°N. Thus, our data are suitable for discussing the seismicity of oceanic events in the northern part of the aftershock zone. Because our catalog does not cover the period before March 15, 2011, we combined the JMA data during this period with our data of up to April 01, 2011. We think this combined data provides the best manually-located hypocenters for the northern part of the aftershock zone.
3. Aftershock Distribution around the Northern Neighbor of Tohoku-oki Earthquake
Several authors have evaluated the Coulomb stress change due to the rupture of the 2011 Tohoku Earthquake (Hiratsuka and Sato, 2011; Lay et al., 2011; Toda et al., 2011). For a thrust fault with the same geometry as the mainshock, the expected areas of increased seismicity are located around both edges in the strike direction of the mainshock fault and the area of outer rise. The southern part of the potential seismic source area of the Northern Sanriku earthquake (the polygon in Fig. 4(a)) is in the favored region of increased seismicity. However, there is no significant increase in this part except on the eastern edge and the SW corner. This may be related to the past history of seismic moment release by the three major interplate earthquakes in 1989 (MW 7.4), 1992 (MW 6.9), and 1994 (MW 7.7), i.e., the Sanriku-haruka-oki earthquake. Kawasaki et al. (1998, 2001) pointed out that slow slips followed the 1989 and 1992 events. The postseismic slip of the 1994 event was clearly detected by GPS and lasted more than one year (Heki et al., 1997; Nishimura et al., 2000). The areas of postseismic slip are illustrated in Fig. 3(b). Kawasaki et al. (1998, 2001) argued that the accumulated seismic moment in a region from 39.0°N to 40.6°N along the Japan Trench was considerably released by the seismic and aseismic sequences in the period between 1989 and 1995. This zone possibly acted as a barrier to both the rupture of the 2011 Tohoku Earthquake and the aftershock migration.
Figure 4(a) shows broad zones of seismicity far east off the northern Iwate prefecture (~39.6°–40.5°N, 143°–144°E) and near the trench axis. A similar increase in seismicity to the west of the trench axis was observed in the aftershock distribution of the 1994 Sanriku-haruka-oki earthquake (Fig. 4(b)). Uchida et al. (2004) found an increased activity of small repeating earthquakes prior to the occurrence of the 1989, 1992, and 1994 events. They interpreted that accelerated quasi-static slips caused stress concentrations at the asperities of mainshocks. If the same mechanism holds for the aftershock activity of the 2011 Tohoku Earthquake, increased seismicity far east off the northern Iwate prefecture is caused by the migration of the postseismic slip. The contour lines of the postseismic slip reach the area of increased aftershock activity (Fig. 4(a)). However, this activated area partly overlaps the zone of the aseismic slip of the 1989 earthquake (Fig. 4(b)). This discrepancy may be explained by either the spatially inhomogeneous interplate moment release by past seismic and aseismic sequences or by an inhomogeneous postseismic slip distribution of the 2011 Tohoku Earthquake.
Another example of triggering by a postseismic slip is the 2003 Tokachi-oki earthquake (MW 8.3) that took place at the SW corner of the Kuril arc (Fig. 4(b)). An earthquake doublet with M 7.0 and M 6.9 was triggered by the slow slip that migrated more than 100 km to NE from the source area of the Tokachi-oki event (Murakami et al., 2006). The Geospatial Information Authority of Japan is monitoring postseismic slip distribution using the data of the nationwide GPS network (GEONET). As of May 12, 2011, the northernmost boundary of the postseismic slip is found to migrate to the southern part of the potential source area (Fig. 4(a)). The monitoring of postseismic slip together with seismicity change around the potential source area is important in the prediction of future large earthquakes.
The aftershock distribution of the 2011 Tohoku Earthquake clearly shows the existence of parallel zones of active and inactive seismicity along the Japan Trench off the northern part of the Tohoku district. The location of the inactive zone seems to correspond to the asperity. The active band of seismicity along the coast correlates well with the zone of the postseismic slip, suggesting that the slip enhances the aftershock activity. The aftershocks migrated to the north but seismicity in the potential source area of the Northern Sanriku earthquake is low except on the eastern edge and the SW corner. The zone around the southern neighbor of the potential source has released seismic moment by the seismic and aseismic sequences in the period between 1989 and 1995. This zone possibly acted as a barrier to both the rupture of the 2011 Tohoku Earthquake and the aftershock migration. Active aftershock seismicity in the zone may represent spatially inhomogeneous moment release by the past sequence or heterogeneous postseismic slip distribution of the 2011 Tohoku Earthquake.
We used hypocentral parameters of the JMA catalog. The catalog was prepared by the JMA and the Ministry of Education, Culture, Sports, Science and Technology in Japan. The hypocenters of the catalog were determined by the data of JMA, NIED, Hirosaki University, Tohoku University, Hokkaido University, University of Tokyo, and Aomori Prefecture. We are very grateful to members of the above organizations. Constructive comments from two anonymous reviewers were helpful in revising the manuscript. All figures were drawn with the Generic Mapping Tools (GMT) developed by Wessel and Smith (1998).
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