Vulcanian eruptions with dominant single force components observed during the Asama 2004 volcanic activity in Japan

Takao Ohminato,Hiroyuki Kumagai,Hiroshi Tsuji,Jun Oikawa,Minoru Takeo,Etsuro Koyama,Taku Urabe,Tadashi Yamashina

doi:10.1186/bf03351955

Abstract

AbstractOn September 1, 2004, Mt. Asama in central Japan erupted for the first time in 21 years. Between this moderate eruption and mid-November of the same year, 4 additional moderate eruptions occurred. We installed 8 broadband seismic stations in addition to the short period seismic network around the volcano and succeeded in recording the near-field seismic signals associated with the summit eruptions. The results of the waveform inversions clearly show that the force system exerted at the source region is dominated by vertical single force components. The source depths of the single force are shallower than 200 m from the bottom of the summit crater, and the order of magnitude of the single force is 1010–1011N. The source time history of each vertical single force component consists of two downward forces and one upward force. The initial downward force probably corresponds to the sudden removal of a lid capping the pressurized conduit. The drag force due to viscous magma moving upward in the conduit can explain the upward force. The correlation between the single force amplitudes and the amounts of volcanic deposits emitted from the summit crater are not necessarily positive, suggesting that the amount of deposits remaining within the summit crater may have played an important role in the excitation of the single force.

Highlights

The most general representation of seismic point sources is a combination of 6 moment tensor components and 3 single force components (Aki and Richards, 1980)
In order to confirm the existence of single force components, we examined the following five cases: Case 1, 6 moment tensor components and no single force component; Case 2, 3 single force components and no moment tensor component; Case 3, 6 moment tensor components and 2 horizontal single force components; Case 4, isotropic moment tensor components and 3 single force components; Case 5, 6 moment tensor components and 3 single force components
The results of the waveform analyses indicate that a vertical single force whose depth range was 0–200 m from the bottom of the summit crater dominated the force system at the source region

Summary

Introduction

The most general representation of seismic point sources is a combination of 6 moment tensor components and 3 single force components (Aki and Richards, 1980). The distribution of the small earthquakes observed just before and after the September 1 eruption determined by the double-difference technique were nearly vertical from just beneath the summit crater to the depth of −1000 m, suggesting the vertical geometry of the conduit (Yamamoto et al, 2005) Keeping these hypocenter distributions in mind, we fixed the horizontal source location of the explosion earthquakes at the center of the summit crater. It was observed that the variance reductions were not significantly affected by the inclusion of Fz. When we analyzed the real data of the explosion earthquakes, the waveforms were not explained by the source mechanisms without Fz (Cases 1 and 3). For longer source lengths between 2800 m and 3200 m, the best-fit solu-

Origin of the vertical single force

Conclusions