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

4P1_2C-O18

Room A2

Date/Time: July 24 15:45-16:00

Hypocenter determination of B-type earthquakes using the envelope correlation method:
an application to Miyakejima volcano, Japan
Higashi Uchida1 , Takeshi Nishimura1 , Hisashi Nakahara1 , Hitoshi Yamasato2 , Eisuke Fujita3
1

Geophysics, Science, Tohoku University, Japan, 2 JMA, Japan, 3 NIED, Japan
E-mail: higashi@zisin.gp.tohoku.ac.jp

B-type earthquakes with emergent onsets of P and S phases are frequently observed in active volcanoes, and it
is difficult to locate them by traditional phase picking. Recently, we analyzed 1,049 B-type earthquakes occurring
at Miyakejima volcano, Japan from August 2010 to April 2011, and succeeded in locating 18 % of the observed
1,049 B-type earthquakes by stacking the waveforms of earthquake families to read the P- and S-wave onset times(Uchida et al., 2012). However, the hypocenters of the rest of B-type earthquakes still remain unknown.
In this study, aiming to locate all of the B-type earthquakes at Miyakejima, we apply the method of Obara (2002),
which was used for non-volcanic deep tremors on the subducting plate, to the seismograms observed at ninestations located within 4 km of the summit.

At each station, we compute a RMS envelope from 4 - 8 Hz

band-pass-filtered three-component seismograms, and measure the differential travel times between stations bytaking cross-correlations of the envelopes. We assume that the envelope is composed of S-wave traveling with thevelocity of 1250 m/s, as in Uchida et al. (2012).
To assess the applicability of the method to B-type earthquakes, we determine the hypocenters of individual eventsin each of the four earthquake families (L1, L2, H1, and H2) by applying the envelope correlation method, andcompare the resultant hypocenters with those obtained from phase picking of the stacked waveforms (hereinafterreferred to as "stack-and-pick hypocenters"). As a result, we found that the epicenters located by the envelope
correlation almost coincide with the stack-and-pick epicenters, and the differences are about 0.5 km. On the otherhand, the method sometimes yields large error in depth. For example, the hypocenters of family L1 and L2 clusterin the depth range of 1 km, respectively. While the cluster of L1 includes the stack-and-pick hypocenter, that ofL2 shifts 1 km shallower than the stack-and-pick hypocenter. In the case of H1 and H2, their hypocenters spreadover the depth range of 2 km. The inconsistency in the depths is probably due to the difficulties in measuring theS-wave travel times from the envelopes. To improve the accuracy of the depths, we need to consider the envelope
broadening by scattered waves, surface waves and/or reflection phases.
We then apply the envelope correlation method to all of the observed 1,049 B-type earthquakes, of which about 80% had never been located, and successfully locate them in an automated way. As a result, we found that 97 % ofthem are located within a 1.5 km diameter centered on the southern part of the summit caldera, where continuous
fumarolic gas emission occurs. Those hypocenters are distributed at the depths shallower than 3 km. Our studyshows the usefulness of the method as a tool to monitor volcanic earthquakes which are difficult to locate by phasepicking.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

322