Variations in Regional Phase Propagation in the Area around Japan

The high level of seismicity and dense network of short-period stations in Japan allows a detailed characterization of regional phase propagation. There are substantial differences depending on the location of the source and stations. Despite the structural complexity, Lg is clearly seen in many parts of Japan. Lg is particularly well developed in the western part of the main island, Honshu, and will propagates through some volcanic zones with loss of high-frequency components. There are also zones of Lg blockage in northeastern Honshu and for subduction zone events with significant oceanic paths for which the mantle phases Pn and Sn are particularly clear. The oceanic region in the Sea of Japan blocks Lg propagation paths for events in mainland Asia at many stations, but there are clear Lg propagation corridors through Korea to stations in the west (Kyushu, western Honshu) and in the north into Hokkaido. The Lg phase carries substantial energy for large events, and the differences in efficiency of propagation influences the intensity of ground shaking. Thus the 1995 Hyogo-ken Nanbu (Kobe) earthquake shows intensity contours extended to the west in the region of efficient Lg-wave propagation. The Rg phase and the fundamental mode of Love waves are very significant for the shallow 2000 Tottori-ken Seibu event, and the effect of the combination of Lg with these slightly lower frequency surface waves would help to explain the discrepancy between the Japan Meteorological Agency magnitude from regional stations (Mj 7.3) and the moment magnitude from distant observations (Mw 6.6).