Crustal deformation map for the 2011 off the Pacific coast of Tohoku Earthquake, detected by InSAR analysis combined with GEONET data
© 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: 23 June 2011
Published: 27 September 2011
Applying interferometric SAR (InSAR) analysis using ALOS/PALSAR data to a megathrust earthquake (Mw 9.0) that ruptured off-shore of Tohoku, Japan, on 11 March, 2011, we succeeded in mapping a ground displacement distributed over ~600 km range in the north-south orientation and ~200 km range in the east-west orientation. A relative slant range lengthening of ~4 m that is the largest displacement ever observed in Japan on InSAR records was detected at the tip of Oshika peninsula, consistent with GPS observations. The interferograms, calculated using quick but less accurate orbit data, are contaminated by long-wavelength noise, preventing us from measuring the correct ground displacement. We demonstrate that an InSAR analysis incorporating GEONET data effectively reduces noise and enables the prompt and accurate mapping of the ground displacement. Additionally, a clear coseismic signal due to an inland earthquake in northern Ibaraki on 19 March, 2011 (Mj 6.1) was also detected. Our preferred fault model assuming two segments shows a nearly pure normal fault motion with a west-dipping fault plane. The ACFF calculation suggests that the Mw 9.0 main shock promoted the Mj 6.1 event.
Key wordsThe 2011 Tohoku earthquake (Mw9.0) InSAR crustal deformation northern Ibaraki earthquake (Mj 6.1)
A megathrust earthquake with a moment magnitude (Mw) of 9.0 occurred on 11 March, 2011 (JST), rupturing the plate boundary to the east off northeastern Japan. Nationwide GPS continuous observation sites, termed GEONET (GPS Earth Observation Network System), recorded widely distributed ground displacements, locally in excess of 5 m (Geospatial Information Authority of Japan (GSI), 2011a).
Satellite synthetic aperture radar (SAR) data can provide detailed and spatially comprehensive ground information and an interferometric SAR (InSAR) enables us to measure ground deformation with high precision (e.g., Massonnet and Feigl, 1998; Bürgmann et al., 2000). One of the advantages of InSAR observation is that it can map vast ground displacement. One ALOS/PALSAR image used in this study covers an area approximately 70 by 70 km wide, which is helpful in grasping the spatial extent of crustal deformation. An additional advantage is that local crustal deformation can be monitored and detected. Although GPS sites (GEONET) are densely deployed with an average placement interval of ~20–30 km, it remains difficult to obtain high quality displacement data for small to moderate-sized earthquakes/volcanic activities, impeding detailed analysis of the source properties, such as fault modeling. Alternatively, the ground resolution of SAR data (PALSAR) is ~10 m, so the InSAR observation works well to monitor and detect small to moderate-sized ground deformation anomalies over a large area.
We conducted an InSAR analysis of the 2011 Tohoku earthquake using ALOS/PALSAR data and succeeded in mapping a wide spatial range ground displacement mainly due to the Mw 9.0 event. The SAR observation for the earthquake has not yet been completed. As of this writing, the SAR images obtained so far do not cover the whole crustal deformation area. However, the InSAR results provide us with some new findings and we believe that they contribute to the understanding of the Mw 9.0 event and other relevant events. The purpose of this paper is to rapidly report surface displacements detected from InSAR observation after the great earthquake. In this paper, we focus our discussion on the following two aspects of the interferogram: an extremely wide range crustal deformation caused by the Mw 9.0 main shock and a local change associated with the induced inland earthquake with a JMA (Japan Meteorological Agency) magnitude (Mj) of 6.1 in northern Ibaraki.
2. InSAR Analysis
To reveal the crustal deformation due to the 2011 Tohoku earthquake, we collected ALOS/PALSAR images acquired before and after the Mw 9.0 event. JAXA (The Japan Aerospace Exploration Agency) made urgent observations for the great earthquake on 15 March (path 401), 20 March (path 404), 21 March (path 55) and 1 April (path 402) with fine beam mode, and on 14 March (path 60) and 31 March (path 61) with ScanSAR mode, respectively. The ScanSAR mode can observe a wide range width of ~350 km, which is capable of monitoring the extremely vast crustal deformation field at once. However, the interferometric coherence of the obtained images was too low to detect ground displacements, which may have been the result of large perpendicular baselines (~2 km and ~5 km). Thus, we used the fine beam mode data for the interferometric analysis. Three ascending orbit data (paths 401, 402, and 404) and one descending orbit data (path 55) are available for the In-SAR analysis as of this writing. Six to eight scenes were concatenated for each path to cover the crustal deformation area. We processed the ALOS/PALSAR data using GSISAR software (Fujiwara and Tobita, 1999; Fujiwara et al., 1999; Tobita et al., 1999; Tobita, 2003).
3. Crustal Deformation Obtained from InSAR Analysis
There still remains a discrepancy of ~20 cm between the two. A possible cause of the remaining discrepancy is post-seismic deformation (GSI, 2011b), attributed to a difference of observation date between the GPS (960550) and the InSAR measurements. The 960550 was unavailable due to the devastating tsunamis, so the above-mentioned displacement is an almost pure coseismic displacement. To confirm an effect of post-seismic deformation, we investigated the data at 960549 site, which is located ~30 km west-northwest from 960550 and was still operable. At the site, a post-seismic displacement equivalent to ΔLOS of ~13 cm is measured until 31 March. Considering the effect of post-seismic deformation, the excess ΔLOS of InSAR compared to the GPS measurement seems to be comparable. We additionally confirmed whether or not the discrepancy results from the accuracy of InSAR measurement. A difference between the GPS-observed ΔLOS and the corresponding ΔLOS in the interferogram, which is evaluated using the data during the same observation period, is ~8.2 cm at the maximum (RMS (root mean square) residual: ~2.2 cm) for path 402. Thus, the discrepancy significantly exceeds the uncertainty of InSAR measurement.
Several GEONET sites along the coast ceased operation because of devastating tsunamis, so we have not obtained the spatially detailed ground deformation along the coast due to the sparseness of observation density (GSI, 2011a). However, the InSAR results successfully detected the spatially comprehensive ground deformation with high displacement gradients, showing that the coastal zone widely deformed with displacements of meters.
There are some reports that seismic and volcanic activities are enhanced after the great earthquake maybe due to static/dynamic stress perturbations (e.g., Koyama, 2002). Northeastern Japan is one of the most tectonically active regions, having many active faults and volcanoes. Active fault traces and active volcanoes are plotted on the inter-ferograms in Fig. 1, represented by thick black lines and red triangles, respectively. Comparing the locations and the corresponding ΔLOS fields, no remarkable anomalies have been recognized along the faults or in close proximity to the volcanoes.
4. GPS-combined InSAR Analysis
Using the precise orbit data, the unnatural fringes are considerably reduced. However, the unnatural deformation pattern showing that the eastern part moves more towards the satellite than the western part still slightly remained in the northernmost part of Honshu, which may be attributed to an ionosphere disturbance. We further made a GPS-correction to the interferograms using precise orbit data (Fig. 1) to suppress the noises and to map the ground displacement more correctly, although there is no crucial difference between the result with GPS correction and that without correction.
5. Inland Earthquake of Northern Ibaraki (Mj 6.1)
Fault parameters of our preferred model. We define the location of each fault as the center. The unit of length, width, and depth is in kilometers. Dip, strike and rake are in degrees, and slip is in meters. In the lowest row, the fault parameters from the CMT solution (NIED Earthquake Mechanism Search (http://www.fnet.bosai.go.jp/event/search.php?LANG=en)) are listed for comparison.
The Mj 6.1 earthquake occurred at the east of the southern Tanakura tectonic line. The region had been seismically inactive but after the Mw 9.0 event the seismicity became active. It is striking that many of the earthquakes are of a normal fault type, which is not a typical type for the region of reverse/strike-slip motions, reflecting the plate loading in an east-west direction. The timing of the seismicity activation suggests a possible causal relationship with the Mw 9.0 event. The crust along the coastal northeastern zone of Japan is considered to be subjected to a strong extensional stress change due to the reverse fault slip of the Mw 9.0 event and the subsequent afterslip (GSI, 2011b), which presumably culminates in the promotion of a normal-fault slip. To test the idea quantitatively, we evaluated ΔCFF at a depth of 5 km with an effective coefficient of friction of 0.4 and a rigidity of 30 GPa (Stein et al., 1994). Assuming fault #1 (Table 1) is a receiver fault, the ΔCFF calculated using the fault model proposed by GSI (GSI, 2011c) has a positive sign with a value of 0.68 MPa. We evaluated the stability of our stress change pattern for an effective coefficient of friction from 0.2 to 0.8, and confirmed that in all cases the ΔCFF values have a positive value (0.56 to 0.92 MPa). The ΔCFF agrees that the static stress change caused by the Mw 9.0 event facilitated the occurrence of the Mj 6.1 event.
6. Concluding Remarks
We successfully mapped an extremely wide range crustal deformation caused by the Mw 9.0 main shock, with a relative slant range lengthening of ~4 m at the tip of Oshika peninsula.
We demonstrated that an InSAR analysis incorporating GEONET data effectively works to reduce the noise, and enables prompt and accurate mapping of ground displacement.
We detected a coseismic signal due to an inland earthquake at northern Ibaraki (Mj 6.1). Our fault model assuming two segments shows a nearly pure normal fault motion with west-dipping fault plane.
The ΔCFF calculation suggests that the Mw 9.0 event promoted the occurrence of the Mj 6.1 event.
PALSAR data was provided from Earthquake Working Group under a cooperative research contract with JAXA (Japan Aerospace Exploration Agency). The ownership of PALSAR data belongs to JAXA and METI (Ministry of Economy, Trade and Industry). We used GMT (Generic Mapping Tools) provided by Wessel and Smith (1998) for constructing the figures. We thank two anonymous reviewers and the editor (Prof. Yomogida) for their helpful comments to improve our manuscript.
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