TY - JOUR

T1 - Source mechanisms of volcanic explosion earthquakes

T2 - Single force and implosive sources

AU - Nishimura, Takeshi

N1 - Funding Information:
I am grateful to Bernard Chouet for his useful and helpful suggestions and comments on this study. I also thank Jim Mori and Yu'ich Nishimura who kindly provided me information on Mt. Langila and Mt. Tokachi. The manuscript benefited from reviews by Vyacheslav M. Zobin. Careful reviews of two anonymous reviewers were helpful for improving the manuscript. I express appreciation to Phil Dawson and Sharon Kedar for their kind assistance and to all staffs and students in the school of Solid Earth Physics, Tohoku University, and Kimmy K. Nishimura for their supports from Japan during my stay at the U.S. Geological Survey. This study was partially supported by the Grant for Scientific Research and the Program for Overseas Researcher of the Ministry of Education Science, Sports and Culture, Japan.

PY - 1998/11

Y1 - 1998/11

N2 - Volcanic explosion earthquakes accompanying explosive eruptions are viewed as representing an abrupt release of pressurized magma beneath a volcano through a vent. I examine the source mechanisms of explosion earthquakes assuming two extreme cases of magma property. When magma is assumed to be a perfect gas which migrates upward isentropically through a vent acting as a nozzle, the seismic source is expressed as a single force. On the other hand, when magma flow is assumed to be incompressible, an implosive source is dominant. Both source models predict that the seismic magnitude is proportional to the cross-sectional area of the vent. I investigate observed seismic magnitudes for different volcanoes based on data in published papers and reports. The results show that the seismic magnitude of the largest event for each volcano is essentially proportional to the cross-sectional area of the vent for vent radii ranging from 10 to 600 m. The consistency found between the theoretical prediction and the observed relation suggests that the vent area plays a substantial role in the magnitude of explosion earthquakes. I further estimate that initial pressures in the reservoir are of the order of a few MPa, with a one order of magnitude uncertainty based on the single force model. I also apply the implosive source model to the observed seismic magnitudes and estimate the product of bulk modulus of magma and flow velocity to be 1010-1011 J/m2 s.

AB - Volcanic explosion earthquakes accompanying explosive eruptions are viewed as representing an abrupt release of pressurized magma beneath a volcano through a vent. I examine the source mechanisms of explosion earthquakes assuming two extreme cases of magma property. When magma is assumed to be a perfect gas which migrates upward isentropically through a vent acting as a nozzle, the seismic source is expressed as a single force. On the other hand, when magma flow is assumed to be incompressible, an implosive source is dominant. Both source models predict that the seismic magnitude is proportional to the cross-sectional area of the vent. I investigate observed seismic magnitudes for different volcanoes based on data in published papers and reports. The results show that the seismic magnitude of the largest event for each volcano is essentially proportional to the cross-sectional area of the vent for vent radii ranging from 10 to 600 m. The consistency found between the theoretical prediction and the observed relation suggests that the vent area plays a substantial role in the magnitude of explosion earthquakes. I further estimate that initial pressures in the reservoir are of the order of a few MPa, with a one order of magnitude uncertainty based on the single force model. I also apply the implosive source model to the observed seismic magnitudes and estimate the product of bulk modulus of magma and flow velocity to be 1010-1011 J/m2 s.

KW - Compressible flow

KW - Implosive source

KW - Incompressible flow

KW - Magma

KW - Single force model

KW - Volcanic explosion earthquakes

UR - http://www.scopus.com/inward/record.url?scp=0032417769&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=0032417769&partnerID=8YFLogxK

U2 - 10.1016/S0377-0273(98)00088-2

DO - 10.1016/S0377-0273(98)00088-2

M3 - Article

AN - SCOPUS:0032417769

SN - 0377-0273

VL - 86

SP - 97

EP - 106

JO - Journal of Volcanology and Geothermal Research

JF - Journal of Volcanology and Geothermal Research

IS - 1-4

ER -