We have derived preliminary results for the source process of the March 11, 2011 off the Pacific coast of Tohoku Earthquake (the 2011 Tohoku Earthquake; M w = 9.0) from two types of seismic waveform data: teleseismic P waves and regional strong motion data. The common features of these two analyses are as follows: (a) The main rupture is located to the east of the initial break point (the shallower side of the hypocenter), and maximum slip amounts were more than 25 m. (b) The size of the main fault was about 450 km in length and 200 km in width; the duration of rupture was more than 150 s; and M w was 9.0. (c) The initial rupture gradually expanded near the hypocenter (0-40 s) and subsequently propagated both southwards and northwards.
It is important to grasp ground motion distributions right after a major earthquake. Ground motion is very sensitive to subsurface structure, but because seismic stations are sparsely distributed, it is necessary to estimate ground motion distributions at sites with no stations from subsurface structure data at those sites and ground motions data recorded only at surrounding stations. In this study, we investigated relationship between ground motion and subsurface structure to estimate ground motion distribution applying corrections according to the subsurface structure differences. Maximum velocity responses with a period of 3 s or longer were correlated with the first natural period of the deep subsurface structure, but maximum velocity responses with a shorter period correlated more strongly with the average S-wave velocity in the upper 30 m (AVS30) than with the first natural period. However, the ratios of maximum velocity responses at one station to those at a nearby station often differed for different earthquakes, indicating that there is limitation in estimating the ratios of maximum velocity responses only from the subsurface structures. Moreover, we did not detect any notable correlations between the subsurface structures and the durations of the velocity responses. Although these results were obtained by using relative velocity responses, similar results were obtained when pseudo-velocity responses were used. 顕著な地震の発生直後に地動の分布を把握することは重要である。地動は浅部地盤構造の影響を強く受けるが、地震計の分 布は疎らであるため、地震計が存在しない地点においては、浅部地盤構造データを用いて、周辺の地震計の観測値から地動を 推定することが望まれる。我々は、浅部地盤構造を考慮した補正を取り入れて様々な周期帯の地震動分布を推定するため、浅 部地盤構造と地動の関係について調査した。その結果周期 3 秒以上の最大速度応答は深部地盤構造の一次固有周期と相関があ ることを見出した。また、それより短周期では、深さ 30 m の S 波速度(AVS30)との相関が強いことが分かった。しかし近 接する 2 観測点における最大速度応答の比は、しばしば地震によって異なる値を取る。これは、構造のみに頼った推定には限 界があることを表す。他方、速度応答の継続時間と構造との間には顕著な相関は認められなかった。これらの結果は相対速度 応答から得られたものであるが、擬似速度応答を用いても結果は同様である。
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