Abstract

A single mobile observation was made in Hokkaido to complement its crustal structure which had been poorly known. A swarm of Usu volcanic earthquakes, whose source region was concentrated within a diameter of 1km, was used as a source. Because of the small size of the source region and the good time keeping, errors in travel times are as little as 0.2sec, which is enough for travel time studies. Thirty-nine stations were temporarily operated on two profiles, whose length was 320km and 100km, respectively.The seismometer was basically an ocean bottom seismometer which was slightly modified for the land use. The small size, the wide dynamic range, the low power drain and the sturdiness were helpful for the efficient observations.The obtained crustal structure illustrates: (1) The granitic layer, with the velocity of 5.8km/sec, is shallow at the top and the thickness is about 20km. (2) The basaltic layer, with the velocity of 6.6km/sec, is immediately beneath the granitic layer. Its thickness is also about 20km. Hence the crust is thicker than usual compared to other regions in Japan. (3) The upper mantle velocity is 7.8km/sec.The profiles were inherently not reversed, however one of them is so planned as to be reversed by possible future earthquake swarms of Teshikaga region, which have occurred recurrently in the past.

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