Wave properties and focal mechanisms of N-type earthquakes at Asama volcano

N-type earthquakes sometimes occur before the eruption of andesitic volcano es, but their source mechanism has net been understood well. Their waveform s have stationary periods and decay slowly resembling to damped oscillation . They have common characteristics: of spectra among the different stations ; these spectral peaks appear at almost equal intervals of 1.0 Hz with comm on sharpness. We made detailed analysis on the N-type earthquakes observed at Asama volcano in July 1995. During 10 days activity, the duration of eac h earthquake increased gradually from 40 s to 3 min, while the peak frequen cy decreased from 2.7 to 0.8 Hz. Hypocenters are distributed slightly west under the summit cl ater with 0.4-1.7 km above sea-level. Source mechanism of the N-type earthquake is determined using 3 waveform inversion technique . Synthetic waveforms are calculated using 2D finite difference method. Sur face topography is included in the calculation to incorporate the case that the source region is higher than the stations. Since the optimum solution has a large volumetric component, we approximated the volumetric part as a volume change of fluid filled sphere or cylinder or plane crack, and decomp osed the solution into volumetric and non-volumetric part. Consequently, it is revealed that the optimum solution can be expressed as "crack expansion (contraction) + small fault slip", because the spectral peak distribution advocates the crack expansion model among these three candidates. The frequ ency change during this activity may be explained by the time variation of the sound speed of inner fluid from 300 to 100 m/s. The change of the void fraction of water-steam phase might have caused the decrease of the sound s peed. (C) 2001 Elsevier Science B.V. All rights reserved.