十勝岳直下のモホ面付近で発生した低周波微小地震 : 震源とスペクトル

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A series of small and moderate explosive eruptions occurred at Tokachi-dake volcano from December 16, 1988 to March 5, 1989. Hokkaido University installed six temporal stations with high sensitive seismographs around Mt. Tokachi on January 1989, in addition to the permanent stations. We located six volcanic earthquakes near Mt. Tokachi and more than 600 events of the 1989 earthquake swarm in the nortern Tokachi district about 25 km east of Mt. Tokachi. All of these earthquakes were originated at the depth shallower than 15 km. On August 7, 1989, five low-frequency microearthquakes with the maximum magnitude 1.6 and a short tremor were observed for three minutes. To know accurate locations of the low-frequency events we calculated simultaneously both hypocenters of 21 earthquakes including the largest low-frequency event and values of velocity of four layered structure using an inversion method of damped least squares. Hypocenter of the largest event was determined to be 43.438°N, 142,659°E, and 28.8 km deep. It is located near the Moho boundary, 4 km northwest of the summit of Mt. Tokachi. P-wave spectral analysis of the largest low-frequency event presented lower predominant frequency of 3 to 4 Hz in comparison with those of shallow earthquakes. The unusual character of the low-frequency event may be attributed to source process and not to high attenuation of seismic waves. The occurrence of the low-frequency microearthquakes may suggest release of magma from the mantle diapir staying near the Moho boundary under Tokachi-dake volcano.
1992-04-01

十勝岳直下のモホ面付近で発生した低周波微小地震 : 震源とスペクトル

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