Preliminary Study of the Strong Ground Motions of the Tohoku, Japan, Earthquake of 11 March 2011: Assessing the Influence of Anelastic Attenuation and Rupture Directivity

Abstract

Online material : Vs30 values, various distances estimated for recording stations, and corner frequencies of the low-cut filter applied to data for present study. On 11 March 2011 a megathrust earthquake (M 9.0)—the largest recorded earthquake in the history of Japan—occurred off the Pacific coast of Tohoku-oki (Table 1). The earthquake ruptured the interplate boundary offshore of northeastern Honshu with several earthquake rupture models suggesting peak epicentral displacements up to 30–40 m and duration about 150 s. Several focal mechanism estimates ( e.g. , the weighted cumulative moment tensor [WCMT] with slab geometry from U.S. Geological Survey; http://earthquake.usgs.gov/earthquakes/eqinthenews/2011/usc0001xgp/finite_fault.php, last accessed August 2011) show a thrust fault striking at ∼195° and dipping at ∼10°, with a rupture model of about 250 km × 150 km. The tectonic setting of the broader Japan area corresponds to a subduction area with the Pacific plate subducting beneath the Eurasian plate. In general, attenuation in subduction zones is controlled by the low-velocity low- Q material of the mantle wedge dividing the zone into along-arc and back-arc regions ( e.g. , Ghofrani and Atkinson 2011; Skarlatoudis et al. 2011; Boore et al. 2009). This phenomenon affects ground motions in Japan in general ( e.g. , Hasegawa et al. 1994; Yoshimoto et al. 2006) and the Tohoku-oki earthquake is no exception, resulting in lower levels of ground motions at larger distances, mainly in back-arc areas. Moreover, this is a case of a megathrust event for which an extremely extensive set of local seismic, strong motion, and geodetic data is available, suggesting it may be one of the best recorded earthquakes in the history of seismology. In the present work we examine the peak and spectral characteristics of the strong motion records available from the Kyoshin network (K-NET; http://www.k-net.bosai.go.jp/, last accessed August 2011). Furthermore, we perform …