IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3W_2A-P16

Date/Time: July 23 Poster

Two-decades-long broadband seismic observations at Aso volcano, Japan
Mare Yamamoto1 , Takahiro Ohkura2 , Satoshi Kaneshima3 , Hitoshi Kawakatsu41Geophysics, Graduate School of Science, Tohoku University, Japan, 2 Aso Volcanological Laboratory,
Kyoto University, Japan, 3 Graduate School of Science, Kyushu University, Japan, 4 Earthquake
Research Institute, University of Tokyo, Japan
E-mail: mare@zisin.gp.tohoku.ac.jp

Aso volcano is one of the most active volcanoes in Japan. It has erupted in Strombolian style repeatedly,and recent activities take place at the central cone composed of seven craters aligned NW-SE direction witha length of 1 km. At Aso volcano, various kinds of volcanic signals with broad frequency contents have been
observed since the pioneering work by Sassa, who installed Wiechert and Galitzin long-period seismometers in
1930s. Our observations using modern broadband seismometers have revealed the details of conduit system andphysical processes generating these seismic signals. In this presentation, we review the results of our 20-year-longbroadband observations at Aso volcano, and discuss future direction of our study.
In 1994, we started our broadband seismic observation close to the active crater of Aso volcano. At the first stageof our observation, we revealed that Sassa’s "second kind" of tremor has an even longer period component, andthose Sassa and others were observing are higher modes of the long period tremors (LPTs). The characteristicsof the LPTs are summarized as follows: (1) continually emitted regardless of surface activity; (2) spectra show
several common spectral peaks (15, 7.5, 5 s); (3) decay fairly fast and the duration is only a few cycles; and (4)often accompanied with short-period tremors. Analyzing the dense broadband network data, we have revealed thatthe kinematic source of the LPTs is an oscillation of a crack-like conduit beneath the crater. The strike and widthof the crack are almost the same as those of the chain of craters, and it extends nearly vertically from a depth of
300-400m below the surface to a depth of about 2.5 km.
We also studied the physical properties of the crack-like conduit. To qualitatively interpret the nature of the LPT
source, we developed an efficient method for modeling fluid-filled crack, and demonstrated that the LPTs can bemodeled as an oscillation of a thin crack filled with gas-ash mixture. The existence of such crack-like conduit isalso supported by other geophysical studies like seismic reflection and magnetotelluric surveys, and the crack is
considered as a subsurface path connecting a postulated magma chamber at a depth of around 5 km and thesurface craters.
In addition to obtaining such static image of volcanic system, our continuous broadband observation over years alsoenabled us to monitor the temporal changes in the system. For example, during our observation, we sometimes
observed fluctuations in the dominant periods of LPTs which seem to correlate with changes in SO2 emission rate,
and we could estimate the changes in thermal state of the volcanic fluid by seismological means based on ourconduit model.
These results suggest the importance of continuous observation, and at the same time, imply that constructing
quantitative physical model explaining multi-disciplinary observations is crucial to deepen our understanding ofvolcanic systems.

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