On the Volcanic Earthquakes with Slowly Decaying Coda Wave

Abstract

Among the various type of volcanic earthquake and volcanic tremor, there are few type of volcanic earthquakes which including the coda wave vibration. The coda waves show slowly decaying for amplitude and have stationary period that the forms of them are resembled the damping free motion of seismometer pendulum. In this paper, writers call them convincingly "T-type earthquake". There are about sixty active volcanoes in Japan, but T-type earthquake has only been observed in the following six volcanoes namely Mt. Asamayama, Sakurajima, Mt. Tarumaeyama, Mt. Kusatsu-Shiranesan and Kuchinoerabujima. T-type earthquakes have a tendency to swarm, but its occurence is very rare. For example at Mt. Asamayama number of T-type earthquakes don’t exceed 0.1% of the total number of volcanic earthquakes in recent 10 years. The form of T-type earthquake differs a little from volcano to volcano but it generally resembles to the B-type earthquake except the part of coda waves. Common feartures of coda wave of T-type earthquake are as follows: 1) Periods of coda waves are distributed among 0.2～1.5 sec. 2)Damping coefficients “h” of coda waves are distributed among 0.003～0.03. Coda wave has predominant amplitude in the horizontal component of perpendicular direction from the crater. Attenuation of amplitude of the coda wave with distance from the crater is larger than the case of the B-type earthquakes which is mainly composed of the seismic surface wave. From the above mentioned result, writers suggest that the coda wave of T-type earthquake has a character of seismic SH wave. Comparison of maximum amplitude of T-type earthquake which recorded at different point on the same volcano suggests that origin of these earthquakes are located at shallow beneath the crater, similar to the case of the B-type volcanic earthquake. The coda waves of T-type earthquake are considered to generate by the free oscillation of some source beneath the crater. It is also a worthy of note that T-type earthquakes is only observed at the andesitic volcano which has erupted the Phreatic type eruption or the Vulcanian type eruption in recent years and T-type earthquakes sometime swarmed when abnormal volcanic activity was observed.