The 2011 off the Pacific coast of Tohoku Earthquake and its aftershocks observed by GEONET
© 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: 12 April 2011
Accepted: 16 June 2011
Published: 27 September 2011
The large displacement induced by the 2011 M 9.0 off the Pacific coast of Tohoku Earthquake was observed by GPS stations of the permanent GPS Earth Observation Network system (GEONET) in northeastern Japan. The displacement was characterized by the eastward displacement and subsidence in the Pacific coastal area. The horizontal displacement exceeded 5.3 m, which is the largest ever detected by GEONET. The mainshock was followed by a sequence of aftershocks. We processed the GPS data through a kinematic positioning strategy to clarify the deformation, including the deformation caused by the mainshock, with high temporal resolutions. The offsets calculated from the kinematic coordinates separately depict the coseismic displacements of the mainshock, the largest aftershock, and the third largest. The fault model for these earthquakes suggests that the largest (M 7.7) and third largest (M 7.3) aftershocks ruptured the southern and northern extensions of the mainshock fault, respectively.
Because the Japanese Islands are covered by a dense network of geodetic and seismic networks (i.e. GEONET, Hi-net, K-net, and KIK-net), a vast amount of data was recorded for the 2011 Tohoku earthquake. The GPS Earth Observation Network System (GEONET) is a permanent nationwide GPS array operated by the Geospatial Information Authority of Japan (formerly the Geographical Survey Institute, or GSI). It consists of more than 1300 continuous sites with a real-time transmission of 1-s data sampling (Hatanaka et al., 2003; Yamagiwa et al., 2006). GEONET has recorded coseismic and postseismic displacements for many earthquakes that have occurred since 1994, including the earthquake off Tohoku. It has also revealed plate motions and interseismic deformation along the plate boundaries (e.g. Sagiya, 2004). Before the 2011 Tohoku earthquake, a contraction in the east-west direction was observed in northeastern Japan (Sagiya et al., 2000) and the plate interface was estimated to be coupled along the Japan Trench (e.g. Nishimura et al., 2004).
On a routine basis, the GEONET GPS data are processed with Bernese 5.0 software to estimate the daily coordinates of the stations (Nakagawa et al., 2009). Moreover, the static coordinates are estimated at 3-h intervals using 6-h data to rapidly detect signals related to episodic events. However, the site coordinates estimated from the static positioning by the GEONET routine do not have enough temporal resolution to capture any rapid evolution of the deformation (e.g. Larson and Miyazaki, 2008). In the sequence of the 2011 Tohoku earthquakes, several M > 7 aftershocks occurred within 1 h after the mainshock. These aftershocks have the potential to cause displacements of more than several centimeters at the GEONET stations.
In this paper, we focus on the coseismic and postseismic deformation revealed by GEONET. We also show the results of kinematic positioning of GPS data to distinguish between the displacements caused by the mainshock and those caused by the aftershocks. Finally, we present simple models that assume rectangular faults for the mainshock and aftershocks. A detailed analysis of the distribution of coseis-mic and postseismic slip as estimated from the GEONET data is summarized in another paper (Ozawa et al., 2011).
2. The 2011 Tohoku Earthquake As Revealed by GEONET
The earthquake caused the suspension of electric power and data transmission at most GEONET stations along the Pacific coast of northeastern Japan. Hundreds of stations were temporarily unable to record data because of lengthy power failures that lasted for several days or even weeks. The number of suspended GEONET stations decreased to 18 by April 6, 2011. In this study, we have used 30-s sampled GPS data which were automatically downloaded, as well as those directly collected from the GPS receiver memory on site by GSI staff.
Figure 1 shows the coseismic displacement caused by the 2011 Tohoku earthquake. This was estimated based on the routine GEONET analysis by differencing the daily coordinates for March 10 through 12. The observed coseis-mic displacement includes not only the displacement by the mainshock, but also that by the aftershocks and transient postseismic signals. The horizontal displacement reached 5.3 m in the Oshika Peninsula near the epicenter, which is the largest displacement ever observed by GEONET. The area where the horizontal displacement exceeded 1 m stretched for approximately 420 km along the Pacific coast and reached the Japan Sea coast. The vertical deformation was characterized by subsidence as great as 1.2 m along the Pacific coast. However, the coastal area facing the Japan Sea was uplifted by a few centimeters, which is consistent with the predicted deformation by reverse faulting in an elastic half-space (Okada et al., 1985). A postseismic deformation following the mainshock was also observed. The maximum postseismic horizontal displacement reached 41 cm on the 25th day after the mainshock. Several sites in the Pacific coastal area were uplifted significantly in the postseismic period, which contrasts with the coseismic subsidence.
3. Kinematic Analysis of GPS Data
A displacement based on static daily coordinates has the advantage of precision but is affected by the many events that occur in a day. We analyzed GPS data by kinematic positioning to overcome this disadvantage. GPS data with 30-s sampling were processed to estimate 300-s coordinates using GIPSY 6.0, which uses the precise-point-positioning (PPP) strategy (Zumberge et al., 1997). Precise orbits and clocks determined by the Jet Propulsion Laboratory (JPL) were used for processing the GPS data. The GIPSY software provides two models (i.e., white noise and random walk) to estimate site coordinates. We chose the white-noise model, which is appropriate to detect a coseismic offset (Larson and Miyazaki, 2008). In addition, to follow highly-rapid deformation, we estimated 30-s coordinates using the final orbits and high-rate clocks of the International GNSS Service (IGS).
4. Coseismic Displacements and Their Fault Model
Estimated fault parameters for earthquakes.
Third largest aftershock
The 2011 M 9.0 Tohoku earthquake produced a great deformation of the Japanese Islands in terms of both extent and magnitude. GEONET presents a detailed pattern of the coseismic displacements of the foreshock, mainshock, and aftershocks. These displacements, which were integrated with daily coordinates from a traditional GPS analysis, were discriminated by kinematic positioning of the GPS data. The simple fault models for these earthquakes suggest that the largest aftershocks ruptured a region left unruptured by the mainshock, whereas the source region of the foreshock was included by that of the mainshock.
The destructive Tohoku earthquake led to tragic consequences for the Japanese people. Nevertheless, this event has left us with abundant geophysical data for a great (M 9-class) earthquake, such as has never been recorded anywhere before. These data must be used to mitigate the impact of future disasters.
The authors would like to acknowledge Prof. Shin ’ichi Miyazaki and an anonymous referee for their comment. The authors thank the Japan Meteorological Agency for providing the earthquake catalogue. The authors are indebted to GSI colleagues for maintaining and restoring GEONET.
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