Characteristics of the Short-Period S-Wave Attenuation Field in the Source Zone of the Strongest Tohoku Earthquake of March 11, 2011 (Mw = 9.0)

Аннотация

The characteristics of the short-period S-wave attenuation field in the source zone of the strongest Tohoku earthquake of March 11, 2011 in the northeast of Japan are considered (M_w = 9.0). The records of shallow local earthquakes obtained by the MAJO station at distances of 250 to 700 km are processed. A method based on the analysis of the ratio of the maximum amplitudes of the S_n and P_n waves (the S_n/P_n parameter) is used. The source zone is divided into four regions bounded by coordinates 36°–37°, 37°–38°, 38°–39°, and 39°–40°18′ N; and 140°30ʹ–145° E. It is established that all the regions contain segments of a rapid decrease in the S_n/P_n values, which are followed by segments of their sharp growth at small epicentral distances in all the areas. Another segment of rapid decrease in the S_n/P_n parameter is identified in all regions at relatively large distances. It is assumed that the first segments of rapid decrease in the S_n/P_n values are related to the gradual sinking of the S-wave rays in the mantle wedge. In this case, the minimum values of the parameter correspond to rays partly moving along the foot of this wedge. Such an effect is explained by the fact that the biggest content of the deep-seated fluids ascending due to the dehydration of the oceanic crust rocks correspond to the bottom of the mantle wedge. The segments of a sharp increase in the S_n/P_n values are likely to correspond to the propagation of rays within the upper part of the plate characterized by very weak attenuation. The second segments of the rapid decrease in the S_n/P_n parameter are related to the ray penetration into the waveguide formed in the bottom part of the plate formed as a result of the dehydration of the mantle rocks. The mean values S_n/P_n(Δ) in the four areas are much lower than in the source zone of the strongest Maule earthquake of February 27, 2010 (Chile, M_w = 8.8). This effect agrees with the earlier assumption about the larger fluid content in the subduction zones in the west of the Pacific Ocean compared to the east. Furthermore, this allows us to explain the features of the aftershock processes in these two enormous regions of the Pacific Ring.