Precise aftershock distribution of the 2011 off the Pacific coast of Tohoku Earthquake revealed by an ocean-bottom seismometer network
- Masanao Shinohara1Email author,
- Yuya Machida1,
- Tomoaki Yamada1,
- Kazuo Nakahigashi1,
- Takashi Shinbo1,
- Kimihiro Mochizuki1,
- Yoshio Murai2,
- Ryota Hino3,
- Yoshihiro Ito3,
- Toshinori Sato4,
- Hajime Shiobara1,
- Kenji Uehira5, 9,
- Hiroshi Yakiwara6,
- Koichiro Obana7,
- Narumi Takahashi7,
- Shuichi Kodaira7,
- Kenji Hirata8,
- Hiroaki Tsushima8 and
- Takaya Iwasaki1
© 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. 2012
Received: 3 February 2012
Accepted: 5 September 2012
Published: 28 January 2013
The 2011 off the Pacific coast of Tohoku Earthquake occurred at the plate boundary between the Pacific plate and the landward plate on March 11, 2011, and had a magnitude of 9. Many aftershocks occurred following the mainshock. Obtaining a precise aftershock distribution is important for understanding the mechanism of earthquake generation. In order to study the aftershock activity of this event, we carried out extensive sea-floor aftershock observations using more than 100 ocean-bottom seismometers just after the mainshock. A precise aftershock distribution for approximately three months over the whole source area was obtained from the observations. The aftershocks form a plane dipping landward over the whole area, nevertheless the epicenter distribution is not uniform. Comparing seismic velocity structures, there is no aftershock along the plate boundary where a large slip during the mainshock is estimated. Activity of aftershocks in the landward plate in the source region was high and normal fault-type, and strike-slip-type, mechanisms are dominant. Within the subducting oceanic plate, most earthquakes have also a normal fault-type, or strike-slip-type, mechanism. The stress fields in and around the source region change as a result of the mainshock.
The 2011 off the Pacific coast of Tohoku Earthquake is one of the largest earthquakes to have occurred near the Japan islands in the historical record. The Japan Meteorological Agency (JMA) reported that the epicenter was positioned off Miyagi and the magnitude of the earthquake reached 9.0. Because the focal mechanism of the main-shock relates to the relative motion of the subducting Pacific plate and the overriding landward plate, the 2011 earthquake occurred at the plate boundary between the subducting Pacific plate and the landward plate (e.g. Nettles et al., 2011). Teleseismic data from the mainshock revealed that the source region of the earthquake spread over a width of approximately 500 km (e.g. Yagi and Fukahata, 2011). In the source region of the mainshock, several large earthquakes having a magnitude greater than 7 occurred, and the source regions of these earthquakes were obtained (Yamanaka and Kikuchi, 2004). However, the magnitude of the 2011 earthquake was larger than those of previous large earthquakes and the source region of the mainshock seems to contain those of the previous earthquakes.
Relating to the large magnitude of the mainshock, a large slip, greater than 20 m, is estimated near the hypocenter from various geophysical data (e.g. Ozawa et al., 2011; Yagi and Fukahata, 2011). A large tsunami, excited by the mainshock, damaged a wide coastal area of northwestern Japan, and was observed by ocean-bottom pressure meters, GPS wave gauges and many coastal tide gauges. From the tsunami data, a large slip (approximately 50 m) on the plate boundary near the Japan Trench was estimated (Fujii et al., 2011; Maeda et al., 2011). The technique of sea floor geodetic observations using a GPS/acoustic combination enables us to measure sea-floor movement directly, and the movement of the sea floor during the mainshock reached 24 m in the off-Miyagi region (Sato et al., 2011). After the occurrence of the mainshock, many aftershocks occurred. From the epicentral distribution of the aftershocks as measured by a land-based seismic network, the source region of the earthquake is also considered to spread over a large region 500 km wide. Several aftershocks have magnitudes greater than seven. The largest aftershock of magnitude (M = 7.7) occurred thirty minutes after the mainshock in the southernmost area of the source region. Obtaining a precise aftershock distribution is essential for understanding the generation mechanism of such a large earthquake. In addition, this kind of information provides useful constraints for studies of rupture over such a wide source region. The precise determination of aftershock distribution is difficult using only land seismic network data in the case that the source region is situated under an offshore area far from a coast line. It is widely known that an ocean-bottom seismometer (OBS) observation is useful for obtaining a high-resolution aftershock distribution of large earthquakes which have occurred under the sea floor (e.g., Shinohara et al., 2004; Sakai et al., 2005; Araki et al., 2006; Hino et al., 2006; Shinohara et al., 2008).
Magnitude 7 class earthquakes have repeatedly occurred beneath the landward slope of the Kuril and Japan Trenches considering the time intervals of large earthquakes, the probability of occurrence of an earthquake with a magnitude of 7.5 in the next 30 years is estimated to be 99% off Miyagi region. Based on this high probability, continuous OBS observation is being performed in the region off Miyagi. In addition, repeating large characteristic earthquakes with a magnitude of 7 have occurred at intervals of approximately 20 years in the off-Ibaraki region (Mochizuki et al., 2008). To investigate the mechanism of the repeating earthquakes, thirty-four long-term OBSs were deployed in the off-Ibaraki region following the occurrence of the 2011 earthquake. In 1997, a cabled ocean-bottom tsunami and seismic observation system was deployed in the region off Sanriku for real-time observations. This system has three seismometers, and two tsunami-meters and observed the mainshock. These data from the OBSs observing the mainshock and aftershocks should play an important role in the improvement of hypocenter location in the source region. However, the number of deployed OBSs is insufficient to obtain the precise hypocenter distribution in the whole source region of the 2011 Earthquake. To reveal the precise aftershock distribution, an OBS network covering the whole source region is needed.
Four days after the mainshock, we started an aftershock observation using pop-up-type OBSs in order to obtain the detailed aftershock activity of the 2011 Tohoku Earthquake (Shinohara et al., 2011). We repeated the deployment and recovery of the OBS four times. This paper focuses on the precise aftershock distribution in the whole source area, with an emphasis on the depths of events, using the OBS data from the first- and second-period observations, and we discuss the aftershock activity during the first three months after the mainshock.
We use various types of digital recording OBS systems. Most of the OBSs have three-component velocity sensitive electro-magnetic geophones with a natural frequency of 4.5 Hz, and employ a glass sphere as a pressure vessel. The maximum recording period is approximately three months. We also use long-term OBSs, which have 1-Hz 3-component geophones and a titanium pressure capsule. The longest recording period is 1 year for this type of OBS (Kanazawa et al., 2009). Broadband-type OBSs (BBOBSs), which have broadband seismometers with a large dynamic range, were also deployed. An observational band for the BBOBS is 360 s–100 Hz and the BBOBS have a precise absolute pressure gauge for the detection of long-period events and vertical crustal movement. The resolution of the A/D conversion is 24 bits. Accurate timing, estimated to be within 0.05 s, is provided by a precise crystal oscillator. All the OBSs are of a pop-up type with an acoustic release system. The OBS position at the sea floor was estimated by using acoustic ranging and ship GPS positions. The accuracy of the OBS positions at the sea floor is estimated to be a few tens of meters. We also determined the water depth of the OBSs by acoustic ranging.
3. Data and Hypocenter Determination
We used waveform data from the OBSs for 1st- and 2nd-period observations just after the mainshock to mid-June, 2011. Approximately three months data were processed for a location of aftershocks. Because the OBS network covered widely the whole source region, heterogeneity of seismic velocity should largely affect the hypocenter determination of aftershocks. To minimize any influence of the heterogeneity of seismic velocity in the study area, we divided the observation area into three regions and the locations of aftershocks was carried out for each region. The results from each region were combined to obtain the aftershock distribution of the whole region. The border zones of the three regions overlap. For a combination of the results from the three areas, we selected earthquakes with a small error of location for each area and individual results were merged. Although there is a possibility that some gap in the hypocenter location exists in the border zones, this error is estimated to be smaller than a few kilometers because of the high resolution of the locations and the similarity of the velocity structures for the three regions.
The JMA had determined event positions using data of the permanent telemetered land seismic network, operated by the National Research Institute for Earth Science and Disaster Prevention (NIED), JMA, and universities (the JMA unified hypocenter catalog). We selected 1908 events whose epicenter is located below the OBS network. Data from all OBSs in each region were combined into multistation waveform data files for each event. P- and S-wave arrival times were determined from a computer display (Urabe and Tsukada, 1991).
Using a precise velocity structure in the study region is important for an accurate hypocenter location. There are many seismic velocity surveys using OBSs in the study regions. Although the study area may have a large lateral heterogeneity in the seismic structure caused by the plate subduction, a simple one-dimensional velocity structure for the hypocenter location was modeled by introducing the results of a refraction study whose profile was laid in each study area. The velocity structures for the hypocenter location were derived from the results of Takahashi et al. (2004) for the northern region, Miura et al. (2003) for the middle region, and Nakahigashi et al. (2012) for the southern region. The profiles were selected to be situated in the regions for hypocenter location. Therefore, we extracted a one-dimensional velocity structure in the center of the region from the results of the refraction surveys to minimize the effect of heterogeneity.
4. Results and Discussion
The epicenter distribution is not uniform. In the epicenter distribution, the aftershocks may be divided into a number of clusters from a geometrical viewpoint. Most of the aftershocks have a depth shallower than 60 km. The aftershocks form a plane dipping landward over the whole area, which is consistent with the result that the mainshock is an interplate earthquake. Aftershocks beneath the slope on the island arc side are frequent, and there is relatively low seismic activity beneath the area within the trench region where the water depth is greater than about 3000 m. The landward slope close to the Japan Trench off Miyagi is estimated to be a source region of the large tsunami during the main-shock (Fujii et al., 2011; Maeda et al., 2011) Aftershocks were located near this tsunami source region in contrast to low seismicity in other trench regions. In the off-Miyagi region, aftershocks with depths around 30 km also occur in the center of the Japan Trench.
In the northern region, which is the off-Sanriku and off-Miyagi regions, the activity of the aftershocks is high beneath the landward slope. There is low seismicity in the region where a large slip during the mainshock is estimated (Yoshida et al., 2011), which is consistent with the result of Asano et al. (2011). Especially there was little seismicity at the plate boundary. The region where, after the mainshock, seismicity at the plate boundary became low had shown an ordinary seismic activity before the main-shock (Fig. 5). In the middle region, which is the off-Fukushima region, aftershocks mainly occurred along the plate boundary; however, there was also aftershock activity in the landward plate. The region with low seismicity is off Fukushima. Although this region had low seismicity before the mainshock, Yokota et al. (2011) reported, from using strong motion data, that large seismic energy was released from this area during the mainshock. In the southern region, which is the off-Ibaraki and off-Boso regions, there was a high seismic activity in the plate boundary region. In addition, aftershock activity was also high in the landward plate. A deep earthquake, whose depth was approximately 50 km, occurred below the landward slope having a water depth of about 3000 m off Ibaraki. On the other hand, aftershock activity is low in the region near the Japan Trench. There also seems to have been a low seismicity in the source region of the M 7.7 aftershock, which occurred 30 minutes after the mainshock. In the southern region, the Philippine
Sea plate is considered to be subducting below the landward plate (e.g. Nakahigashi et al., 2012). Before the main-shock, several seismic gaps near the edge of the Philippine Sea plate were found (Yamada et al., 2011). In the aftershock distribution of the 2011 mainshock, we also see low seismic activities in the region where seismic gaps existed before the mainshock. It is inferred that the seismicity in this region is related to heterogeneity.
We carried out aftershock observations using pop-up-type OBSs in order to obtain a detailed aftershock distribution of the 2011 Tohoku earthquake. Deployment and recovery of the OBS were repeated four times, and we have used data from more than 70 OBSs recorded just after the mainshock to the middle of June, 2011. We selected 1908 events whose epicenters were located below the OBS network from the JMA earthquake catalog, and P- and S-wave arrival times were picked from the OBS data. Hypocenters were estimated by a maximum-likelihood estimation technique and one-dimensional velocity structures were modeled using the results of previous refraction studies in the study region. Thickness changes of sedimentary layers at each OBS site were evaluated and estimated travel times by the location program were corrected. A precise aftershock distribution, for approximately three months over the whole source area, with an emphasis on the depths of events, using the OBS data, was obtained. The OBS networks located 1005 earthquakes with a high spatial resolution. The epicenter distribution is not uniform. In the epicenter distribution, the aftershocks may be divided into a number of clusters from a geometrical viewpoint. The aftershocks form a plane dipping landward over the whole area. Comparing our results with velocity structures from marine seismic surveys, there is no aftershock along the plate boundary in the region off Miyagi, where a large slip during the mainshock is estimated. A plate coupling in this region may change due to the occurrence of the mainshock. The activity of aftershocks within the landward plate above the source region is high, and many aftershocks within the landward plate had a normal fault type, or strike-slip type, mechanism. On the other hand, many events with a reverse fault (thrust) type mechanism occurred along the plate boundary. Within the subducting oceanic plate, most of earthquakes had a normal fault type, or strike-slip type, mechanism. The stress field in and around the source region of the 2011 mainshock changes as a result of the mainshock.
The work of the officers and crew of R/V Kairei, R/V Yokosuka, R/V Natsushima, R/V Ryofu-maru, R/V Keifu-maru, and M/V Shinyu-maru is appreciated. We express thanks to Ms. M. Oowada, Messrs. S. Hashimoto, T. Yagi, H. Abe, and K. Uchida for help in the preparation of the OBS observations. The support of Drs. K. Katsumata, R. Azuma, T. Isse, Y. Kaiho, H. Miyamachi, Y. Kaneda, and Mr. T. No for the observations, data processing and discussion have been essential for performing this study. We thank Ms. Y. Nihei for the OBS data processing. Drs. T. Kanazawa, N. Hirata, K. Obara, and S. Sakai gave us useful advice to carry out this study. We are also grateful to two anonymous reviewers and Dr. A. Hasegawa for their critical reviews for an improvement of the discussions. This study is partly supported by the Special Coordination Funds for the Promotion of Science and Technology (MEXT, Japan) titled as the integrated research for the 2011 off the Pacific coast of Tohoku Earthquake. Most of the figures were created using GMT (Wessel and Smith, 1991).
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