A travel time study from a "source" of the Usu volcano earthquake swarm.

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.There had been few studies of explosion seismology in Hokkaido, one of which was conducted in 1968 and 1969 by the Japanese Research Group for Explosion Seismology (RGES) (OKADA et al., 1973). In 1968, seventeen explosions, which were up to 600kg, were made in the sea off Cape Erimo, and 22 observation sites were temporarily set up. About fifty scientists participated the experiment. In 1969, thirteen explosions, whose sizes were 300 or 600kg, were detonated in the sea off the Shakotan Peninsula, and 25 observation sites were deployed. A 1.5-ton quarry blast which was made near Sapporo was also used for the analysis. About seventy scientists participated the experiment.

A travel-time study made in the Hidaka region,Hokkaido,Japan,on the "source" of series of earthquakes of the Usu volcano.

“…Strong later phases with an apparent velocity of 6.6 km/sec can also be traced up to at least 200km. The same apparent velocity was also found in some single-event travel-time studies (NAGAMUNE, 1970;SHIMAMURA, 1981). Both phases with apparent velocities of 5.8 and 6.6km/sec are found clearly in the seismograms even at the distant stations ERM and MYR of RCEP.…”

“…Figure 2 shows that first arrivals with an apparent velocity of 5.8km/sec are traced up to about 140km. This value probably corresponds to the velocities of 5.8-6.2km/sec that have already been obtained by several travel-time studies in and near Hokkaido (NAGAMUNE, , 1970UTSU, 1969;DEN et al, 1971;DEN and HOTTA, 1973;ASANO et al, 1979;MORIYA and OKADA, 1980;MORIYA et al, 1981;SHIMAMURA, 1981). Strong later phases with an apparent velocity of 6.6 km/sec can also be traced up to at least 200km.…”

“…The mechanism by which they were generated is not clear because of a lack of information on the fine crustal structure as well as on detailed source processes. A further branch has an apparent velocity of 7.9km/sec, which probably corresponds to those of 7.7-7.8km/sec obtained from the phases of natural earthquakes used in the single-event travel-time studies (NAGAMUNE, , 1970SHIMAMURA, 1981). The arrivals with this apparent velocity may be those of refractions from the Moho-discontinuity.…”

“…In addition to this problem, there exists a thick sedimentary zone named the Ishikari-Yufutsu Lowland (or the Ishikari-Depression) with a low-gravity anomaly between the source area of the Usu volcano and the Hidaka Orogenic Belt. According to SHIMAMURA (1981), the seismic waves penetrating through the thick sedimentary zone were attenuated seriously and there was an offset of first arrival times of nearly 2sec. (1981).…”

A method of ray tracing in a complicated structure.

“…6(a)) mainly on the basis of explosion seismic studies (HOTTA et al, 1964;YOSHII et al, 1973YOSHII et al, , 1974OKADA et al, 1973OKADA et al, , 1979ONO et al, 1978;KANEDA et al, 1979;ITO et al, 1982). Supplementary information is obtained from studies on natural earthquakes (KIMURA and OKANO, 1980;SHIMAMURA, 1981). Gravity may also supply additional data.…”