A tentative study on determination of earthquake magnitude at Unzen volcano

Magnitudes of earthquakes around Unzen volcano, Japan were estimated. We applied Tsuboi's formula using data from a long period seismograph (T sub(o)=10s) installed at Unzendake Weather Station. For small earthquakes we developed the formula M=logAz+1.73log Delta -0.46, where Az is the vertical amplitude obtained by an A74 short period seismograph (T sub(o)=1s) at point A of Unzendake Weather Station. Applying the formula to earthquakes beneath the summit of Unzen volcano, we found that earthquakes with a magnitude of 2 occurred in the period of the emergence of the first lava dome in 1991 and earthquakes with a magnitude of about 1 occurred in another period of 1991. The earthquakes in the former period had two peaks (around M0.2 and M1.1) in their magnitude-frequency distribution. The earthquakes with a peak around M1.1 in the magnitude-frequency distribution are considered to be associated with depression of the summit area due to intrusion of lava.