JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, B11313, doi:10.1029/2009JB006665, 2010 Slip distribution of the 2003 Tokachi‐oki Mw 8.1 earthquake from joint inversion of tsunami waveforms and geodetic data F. Romano,1 A. Piatanesi,1 S. Lorito,1 and K. Hirata2 Received 4 June 2009; revised 23 July 2010; accepted 24 August 2010; published 23 November 2010. [1] We study the 2003 Mw 8.1 Tokachi‐oki earthquake, a great interplate event that occurred along the southwestern Kuril Trench and generated a significant tsunami. To determine the earthquake slip distribution, we perform the first joint inversion of tsunami waveforms measured by tide gauges and of coseismic displacement measured both by GPS stations and three ocean bottom pressure gauges (PG) for this event. The resolution of the different data sets on the slip distribution is assessed by means of several checkerboard tests. Results show that tsunami data constrain the slip distribution offshore, whereas GPS data constrain the slip distribution in the onshore zone. The three PG data only coarsely constrain the offshore slip, indicating that denser networks should be installed close to subduction zones. Combining the three data sets significantly improves the inversion results. Joint inversion of the 2003 Tokachi‐oki earthquake data leads to maximum slip values (∼6 m) confined at depths greater than ∼25 km, between 30 and 80 km northwest of the hypocenter, with a patch of slip (3 m) in the deepest part of the source (∼50 km depth). Slip values are very low (≤1 m) updip from the hypocenter. Furthermore, the rupture does not extend on the plate interface off Akkeshi. As a significant back slip amount (∼4 m) has accumulated there since the last 1952 earthquake, this segment could rupture during the next large interplate event along the Kuril Trench. Citation: Romano, F., A. Piatanesi, S. Lorito, and K. Hirata (2010), Slip distribution of the 2003 Tokachi‐oki Mw 8.1 earthquake from joint inversion of tsunami waveforms and geodetic data, J. Geophys. Res., 115, B11313, doi:10.1029/2009JB006665. 1. Introduction et al., 2006]. Furthermore, the event generated a large tsu- nami along the southern coast of Hokkaido, with runup [2] The M 8.1 Tokachi‐oki earthquake of 25 September w heights higher than 4 m at some locations (Figure 1) [Tanioka 2003 (Figure 1) has been one of the strongest and better et al., 2004a]. The tsunami was recorded along the coasts of recorded seismic events in the last 50 years in Japan, and the both Hokkaido and Honshu islands by the Japanese tide first large interplate earthquake ever recorded by the Japanese gauge network (Figure 1). strong motion networks K‐NET and KiK‐net [Kinoshita, [4] In previous studies, the 2003 Tokachi‐oki earthquake 1998; Aoi et al., 2000]. Large earthquakes occur frequently has been analyzed following different approaches and using off Hokkaido Island (northern Japan) because the Pacific various geophysical data sets. The seismic source, for example, Plate subducts at the Kuril Trench just under the Hokkaido has been investigated inverting tsunami traveltimes [Hirata region (Figure 1). Several tsunamigenic earthquakes occurred et al., 2004], tsunami waveforms [Tanioka et al., 2004b], in the last 50 years in this region, as for example the 1952 teleseismic data [Yamanaka and Kikuchi, 2003; Horikawa, Tokachi‐oki (M8.2), the 1958 Etorofu (M8.1), the 1963 off 2004; Robinson and Cheung, 2010], strong motion data Urup (M8.1), the 1973 Nemuro‐oki (M7.7), and the 1994 [Honda et al., 2004, Aoi et al., 2008; Nozu and Irikura, 2008], Hokkaido Toho‐oki (M8.1) [Piatanesi et al., 1999; Utsu, GPS data [Miyazaki and Larson, 2008], teleseismic and 1999; Watanabe et al., 2006]. strong motion data jointly [Yagi, 2004], and finally combin- [3] Many GPS stations recorded the on land surface dis- ing GPS and strong motion data [Koketsu et al., 2004]. placement due to the 2003 Tokachi‐oki earthquake. The [5] In this study we perform, for the first time, as regards vertical component of the coseismic displacement was clearly this event, a joint inversion of geodetic and tsunami data. In captured also by three pressure gauges (PG) (two ocean ‐ general, fault planes of large subduction zone earthquakes lie bottom pressure gauges and one cable end station) installed beneath both land and ocean seafloor. Thus, tsunami wave- on the seafloor near the Kuril Trench (Figure 1) [Mikada forms and PG data can help constraining the slip distribution in the offshore zone, whereas geodetic data can mainly con- 1Department of Seismology and Tectonophysics, Istituto Nazionale di strain the slip onto the onshore zone [Satake, 1993]. There- Geofisica e Vulcanologia, Rome, Italy. 2 fore, inverting them jointly might lead to the retrieval of a Seismology and Volcanology Research Department, Meteorological better source model with respect to that estimated by inverting Research Institute, Japan Meteorological Agency, Tsukuba, Japan. data separately. Copyright 2010 by the American Geophysical Union. [6] A description of the data used, their preprocessing, the 0148‐0227/10/2009JB006665 fault parameterization, and the methods for Green’s functions B11313 1of12 B11313 ROMANO ET AL.: TOKACHI‐OKI 2003 JOINT SLIP INVERSION B11313 Figure 1. Location map of the 2003 Tokachi‐oki earthquake. Epicenter (white star) and focal mechanism are from Japan Meteorological Agency (JMA). Thin black lines are the surface projection of the subfaults used in this study. Blue triangles, green dots, and red dots are the positions of tide gauge stations, GPS stations, and ocean bottom pressure gauges (PG1, PG2, and PG3), respectively, used in the inversions (Tables S1 and S2 in the auxiliary material). In the inset, yellow squares are the aftershocks with Mw ≥ 4 in the 2 days after the main shock (U.S. Geological Survey); also indicated are the positions along the coast of Hokkaido where the runup measurements were higher than 4 m (Mabiro, Hamataiki, Horokayantou, and Oikamanai [Tanioka et al., 2004a]). calculation are provided first (sections 2, 3, and 4). Then, we recordings are provided by Japan Meteorological Agency describe the synthetic checkerboard tests used to assess data (JMA), the Hokkaido Regional Development Bureau (HRDB) resolving power for the slip distribution, both for single data of the Ministry of Land, Infrastructure and Transport, and the set inversions and for their joint inversion (section 5). Finally, Hydrographic and Oceanographic Department (HOD) of the we discuss the earthquake source characteristics of the Japan Coast Guard [Hirata et al., 2004]. In this study we use 2003 Tokachi‐oki earthquake retrieved from the inversion 16 tsunami waveforms (Table S1 in auxiliary material and (sections 6 and 7). Figure 1).1 Particular attention is dedicated in making the azimuthal coverage around the earthquake source as large as 2. Data [7] Tsunami waves from the 2003 Tokachi‐oki earth- 1Auxiliary materials are available in the HTML. doi:10.1029/ quake were recorded at Japanese coastal tide gauges. These 2009JB006665. 2of12 B11313 ROMANO ET AL.: TOKACHI‐OKI 2003 JOINT SLIP INVERSION B11313 possible, and in selecting only the recordings with a good [13] Green’s functions (horizontal and vertical coseismic signal‐to‐noise ratio. The selected records are sampled at displacements) at the GPS and PG stations, as well as the 1 min. Before using them in the inversion the tidal component initial condition for the tsunami propagation, are computed is removed by fitting it with a sum of three harmonics. using a layered Earth’s model by means of the PSGRN/ Moreover, for each waveform, we choose a time window PSCMP numerical code [Wang et al., 2006]. As input including only the first oscillations of the tsunami wave to parameters the PSGRN/PSCMP code needs P and S wave minimize the contribution of local effects on the tsunami velocities as well as density values for each of the layers. signal. Local effects, such as coastal reflections and resonance Here we use values from the vertical cross sections along of the bays could hide information on the seismic source and line F‐F′ as reported by Wang and Zhao [2005], adopting a are particularly difficult to model. model with four layers, whose parameters are listed in [8] The ground displacement associated to the earthquake Table S4 (see the auxiliary material). was recorded at the GPS stations of the GEONET network, operated by the Geographical Survey Institute of Japan (GSI). We use the GPS (Table S2 in auxiliary material and 5. Inversion Scheme and Checkerboard Figure 1) distributed over Hokkaido and northern Honshu Resolution Tests for which the coseismic offsets were estimated by Larson and Miyazaki [2008]. [14] We solve the inverse problem using a particular implementation of the simulated annealing technique called [9] The vertical component of the coseismic displacement “ ” has been recorded also on the seafloor by PGs, labeled as the heat bath algorithm [Rothman, 1986], following pre- PG1 (depth 2218 m), PG2 (depth 2210 m) as well as with a vious studies using tsunami, GPS, and strong motion data depth sensor, labeled PG3 (depth 2540 m), that is included [e.g., Piatanesi et al., 2007; Lorito et al., 2010]. For tsunami in a conductivity‐temperature‐depth meter (CTD) sensor time series, we use a cost function sensitive to both ampli- [Hirata et al., 2002]. In this work we use the vertical co- tude and phase matching [Spudich and Miller, 1990; Sen seismic static offsets at the PGs estimated by Mikada et al. and Stoffa, 1991]: 2 3 [2006] (Table S2 in the auxiliary material). Ptf 2 ðÞu ðÞt u ðÞt XN 6 O S 7 6 t 7 EmðÞ¼ 61 À i 7 ð1Þ 4 Ptf Ptf 5 3. Fault Parameterization k¼1 2 2 uOðÞþt uS ðÞt ti ti [10] The source area (Figure 1) is set on the basis of the k M ≥ 4 aftershock distribution in the 2 days after the main w In equation (1) u and u are the observed and synthetic shock.