doi: 10.3319/TAO.2011.07.11.01(T) * Corresponding author E-mail: sjlee@earth.sinica.edu.tw Terr. Atmos. Ocean. Sci., Vol. 23, No. 1, 1-7, February 2012 Rupture Process of the 2011 Tohoku-Oki Earthquake Based upon Joint Source Inversion of Teleseismic and GPS Data Shiann-Jong Lee * Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan Received 23 June 2011, accepted 11 July 2011 ABSTRAcT This study investigated 18 broadband teleseismic records and 451 near field GPS coseismic deformation data to deter- mine the spatial and temporal slip distribution of the 2011 Tohoku-Oki earthquake (M 9.0). The results show a large trian- gular shaped slip zone with several asperities. The largest asperity centered above the hypocenter at about 5 - 30 km depth. A secondary large asperity was found in the deeper subduction zone beneath the hypocenter. The average slip on the fault is ~15 m and the maximum displacement on the biggest asperity is > 30 m. The temporal rupture process shows that the slip nucleated near the hypocenter at the beginning, and then ruptured to the shallow fault plane forming the largest asperity. The slip developed in the deeper subduction zone in the second stage. Finally, the rupture propagated toward the north and south of the fault along the Japan Trench. The source time function shows three segments of energy releases with two large peaks related to the development of the asperities. The overall rupture process is ~180 seconds. This source model coincides well with the aftershock distribution and provides a first-order information on the source complexity of the earthquake which is crucial for further studies. Key words: 2011 Tohoku-Oki earthquake, Rupture process, Joint source inversion Citation: Lee, S. J., 2012: Rupture process of the 2011 Tohoku-Oki earthquake based upon joint source inversion of teleseismic and GPS data. Terr. Atmos. Ocean. Sci., 23, 1-7, doi: 10.3319/TAO.2011.07.11.01(T) 1. InTRODucTIOn On 11 March 2011, a giant earthquake struck Japan with magnitude of M 9.0. The earthquake reported by the Japan Meteorological Agency (JMA) and US Geological Survey (USGS) both show that the hypocenter of the main- shock was located off the Pacific coast of northeast Honshu (Fig. 1). The initial report from the JMA suggested that the earthquake was a magnitude of 7.9 which was then modi- fied to 8.9 after an hour into the event. Further analysis of the seismic data resulted in an upgrade to a magnitude of 9.0. According to the USGS W-phase moment tensor inver- sion result, faulting during the earthquake has been inter- preted as reverse faulting on a low-angle plane along the Japan Trench. The studies of the rupture process of the Tohoku-Oki earthquake have been carried out intensively (e.g., Ide et al. 2011; Ozawa et al. 2011; Simons et al. 2011). However, in most of the studies only one kind of data or few data sets were used to invert the source model, thus the results might not be able to explain other coseismic observations. For ex- ample, using only GPS coseismic displacement data (i.e., Ozawa et al. 2011) cannot provide temporal rupture process information in the inversion and might not be able to explain the waveform characteristics observed at regional seismic stations. In order to obtain first-order information concern- ing the origin of this earthquake, especially how the rupture develops into the megathrust event, this study performs a joint source inversion using teleseismic body wave and GPS coseismic deformation data. This large earthquake was well recorded by the teleseismic stations worldwide providing a good opportunity to arrive at a quick determination of fault rupture behavior. The teleseismic data have good data qual- ity and azimuthal coverage to the source. Furthermore, the near field GPS coseismic deformation data also provide a good constraint on the total slip pattern of this giant event.