Postseismic Displacements Following the 2007 Noto Peninsula Earthquake Detected by Dense GPS Observation
Total Page：16
File Type：pdf, Size：1020Kb
LETTER Earth Planets Space, 60, 139–144, 2008 Postseismic displacements following the 2007 Noto peninsula earthquake detected by dense GPS observation Manabu Hashimoto1, Hiroaki Takahashi2, Ryosuke Doke3, Minoru Kasahara2, Akira Takeuchi3, Kenusuke Onoue1, Yoshinobu Hoso1, Yo Fukushima1, Kajuro Nakamura1, Fumio Ohya1, Ryo Honda2, Masayoshi Ichiyanagi2, Teruhiro Yamaguchi2, Takahiro Maeda2, and Yoshihiro Hiramatsu4 1Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan 2Institute of Seismology and Volcanology, Graduate School, Hokkaido University, Sapporo 060-0810, Japan 3Graduate School of Science and Engineering, University of Toyama, Gofuku 3190, Toyama, Toyama 930-8555, Japan 4Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa, Ishikawa 920-1192, Japan (Received June 29, 2007; Revised November 3, 2007; Accepted November 13, 2007; Online published February 19, 2008) We have been conducting dense GPS observation in and around the epicentral region of the 2007 Noto peninsula earthquake since March 25, 2007, in order to detect postseismic displacements. Continuous observation has been underway at 12 sites to fill the gap of GEONET. Preliminary analysis of data up to early May shows that initial postseismic displacement rapidly decayed within 20 days after the occurrence of the mainshock. Horizontal displacements do not exceed 20 mm even at sites above the aftershock zone for this period. We also found a maximum uplift of about 20 mm there. Inversion of postseismic displacements with the variable slip model suggests a nearly right-lateral afterslip of less than 5 cm on the shallow portion of the source fault. Fitting a theoretical function to a time series of coordinate changes also suggests that the observed postseismic displacements might have been generated by afterslip. Key words: Noto peninsula earthquake, GPS, crustal deformation, postseismic deformation, afterslip. 1. Introduction a much denser network of GPS observation is required to On March 25, 2007, a Mjma = 6.9 earthquake hit the reveal the detailed distribution of postseismic displacement northwestern part of the Noto peninsula, central Japan, following the mainshock. With this aim, we started carry- claiming one casualty and causing serious structural dam- ing out dense continuous GPS observations in and around age, especially in the town of Monzen, Wajima city. This is the epicentral region following the occurrence of the Noto the fourth shallow inland earthquake of M 6.5 or larger in peninsula earthquake. the Japanese islands during the present decade. Postseismic displacements are often observed following large earthquakes, and these convey important information 2. GPS Observation and Data Processing on frictional properties of the source fault (e.g. Marone et Aftershocks were distributed close to the northwestern al., 1991; Perfettini et al., 2004; Montesi, 2004; Miyazaki coast of Noto peninsula, where only three GEONET sites et al., 2004), viscoelastic structure in the crust and upper are operated. Therefore, a much denser network is essen- mantle (e.g. Thatcher et al., 1980; Pollitz, 2005), pore pres- tial to reveal a detailed distribution of postseismic displace- sure changes (e.g. Peltzer et al., 1998; Jonsson´ et al., 2003), ment. Responding to the occurrence of the Noto peninsula among others. These factors may also control the strain ac- earthquake, three groups consisting of researchers from sev- cumulation and release process associated with earthquake eral universities started continuous GPS observation in and generation. Thus, it is essential to carry out such observa- around the epicentral area. During the period from 25 to tions as soon as possible to reveal the detailed history of the 29 March, 2007, we established 12 observation sites which evolution of postseismic deformations. are equipped with dual-frequency receivers (Fig. 1). We se- In Japan, the Geographical Survey Institute’s Earth Ob- lected apparently stable sites, such as reinforced concrete servation NETwork (hereafter GEONET) is under opera- buildings, basements of monuments etc, and fixed antenna tion and has revealed coseismic and postseismic displace- on the roofs, because of limited time available to select sites ments following large earthquakes (e.g. Sagiya et al., 2000; (Fig. 2). Phase data were sampled at intervals of 30 s, but Sagiya, 2004). However, average spacing is about 20– sampling rate was higher (0.1–1 Hz) at some sites in order 25 km for GEONET, which is as large as the size of af- to conduct a kinematic analysis of data. For the static anal- tershock distribution of the earthquakes of M 7. Therefore, ysis, however, all data were decimated to 30-s sampled data with the utility TEQC (Estey and Meertens, 1999). The el- ◦ Copyright c The Society of Geomagnetism and Earth, Planetary and Space Sci- evation mask was set at 5–10 . ences (SGEPSS); The Seismological Society of Japan; The Volcanological Society For the analysis of GPS data, we used the Bernese of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sci- ences; TERRAPUB. software (ver. 5.0) for static positioning of daily co- ordinates (Hugentobler et al., 2005) with IGS final 139 140 M. HASHIMOTO et al.: POSTSEISMIC DISPLACEMENTS DETECTED BY GPS CHHR UIRI UNOY Fig. 2. Photos of representative sites: Unoya (UNOY), Chihara (CHHR) and Uiri (UIRI) sites. Day from Mar.25 -5 0 5 10 15 20 25 30 35 40 45 50 20 20 020969:Mar.20 - May.5,2007 10 10 0 0 Fig. 1. Distribution of sites. Solid diamonds, solid and open squares indi- -10 -10 cate GPS sites deployed by Hokkaido University and Kanazawa Univer- Changes from Mar.21 in mm sity, University of Toyama, and Disaster Prevention Research Institute, Changes from Mar.21 in mm Kyoto University, respectively. Open diamonds show GEONET sites. -20 -20 E-W N-S U-D Large triangle indicates GEONET 020969, which is the reference point Mw6.9 for displacements. Gray circles are epicenters of aftershocks determined -5 0 5 10 15 20 25 30 35 40 45 50 by JMA and MRI (2007). Dark and light gray stars show the epicenter Day from Mar.25 of the mainshock and epicenters of aftershocks larger than M 4.5, re- spectively. Gray lines indicate surface traces of active faults (Research Fig. 3. Time series of changes of coordinates of GEONET site 020969 in Group of Active Faults of Japan, 1991). mm since March 21, 2007, in ITRF2005. Circles, diamonds and trian- gles show eastward, northward and vertical components, respectively. Open and solid symbols are original and spatial-filtered data, respec- tively. Error bars indicate 3-σ. ephemeredes, earth rotation parameters and satellite clock coefficients. All data from the IGS, GEONET and temporal sites were processed simultaneously to Amin (2002). We used the sites far away from the epicenter obtain their coordinates in ITRF2005 (http://itrf.ensg. of the mainshock for the calculation of common mode bias. ign.fr/ITRF solutions/2005/ITRF2005.php). We chose re- The common mode bias that was obtained was subtracted mote IGS sites, such as Tsukuba (TSKB), Daejong (DAEJ), from the original time series. Figure 3 shows the observed Yuzhno-Sakhalinsk (YSSK) and others, as fiducial sites and spatially-filtered time series of GEONET 020969 in and tried to strongly constrain to their predicted coordi- ITRF2005. Horizontal components are rather stable, while nates in ITRF2005 in order to align the site coordinates in vertical ones have as large a scatter as 10 mm. Judging ITRF2005. However, their preliminarily estimated coordi- from the stability of the horizontal components and epicen- nates sometimes largely differed from the ITRF2005 pre- tral distance, the choice of GEONET 020969 as a reference dictions. In such a case, Bernese automatically excludes site is adequate. Due to the large scatter in the vertical com- such sites from the list of fiducial sites and calculates coor- ponent, however, it is hard to discuss deformation smaller dinates. than 10 mm. Time series of coordinates usually have relatively large scatters. We suspect that the scatter may be common noise 3. Observed Postseismic Displacement that can be attributed to errors in ephemeredes and satellite Figures 4–6 show the time series of coordinate changes clock delays and other common effects, such as troposh- relative to 020969. Most GEONET sites do not show any phere/ionosphere disturbances. In order to reduce such dis- significant movements since they are located relatively far tubances, we fix GEONET 020969. Geographical Survey from the epicenter. However, several sites, such as 940053, Institute (2007) (hereafter GSI) selected 020969 as the ref- 020972, AKGM, CHHR, UNOY, WMII and SNDI, show erence point for the calculation of coseismic displacements. a different trend. SNDI in particular rapidly moved west- However, we must check the stability of this site. In the cal- ward during the first 3 days and seemed to decelerate after- culation of time series we often apply the spatial filter tech- ward. It is unfortunate that several data points are missing nique proposed by Wdowinski et al. (1997) and Tabei and due to power failure and we could not know its detailed M. HASHIMOTO et al.: POSTSEISMIC DISPLACEMENTS DETECTED BY GPS 141 Day from Mar.25, 2007 Day from Mar.25, 2007 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 55 55 110 110 50 50 100 100 940053 45 45 90 90 950248 940053 80 80 40 40 950253 950248 70 70