HY.SUetal.:DynamicTriggeringSemi-VolcanicTremorinJapaneseVolcanicRegion
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Dynamic Triggering Semi-Volcanic Tremor in Japanese Volcanic Region by The 2016 Mw 7.0 Kumamoto Earthquake
Heng-Yi Su1*, Aitaro Kato2
1Department of Earth Sciences, National Central University, Taoyuan City, Taiwan, R.O.C
2Earthquake Research Institute, University of Tokyo, Tokyo, Japan
Abstract Semi-Volcanic Tremor is a non-impulsive deep seismic signal detected in the volcanic region.
It has been identified by previous studies that the 2004 Sumatra Earthquake and the 2011
Tohoku-Oki Earthquake triggered the semi-volcanic tremors in the Hokkaido. These trig-
gered tremors are roughly located at epicenters of ambient tremors Therefore, we download-
ed the seismic data from Hi-net and V-net and examined whether the 2016 Kumamoto earth-
quake triggered the semi-volcanic tremor. After data analysis, we detected a burst of tremors
triggered during the passage of surface waves at the central Hokkaido, which is slightly
western and deeper than the location determined by the previous study. Furthermore, the
triggered tremor is more relative to the Love wave and we infer the temporal shear stress
loading caused by Love wave deformation prompted fractures of some pre-existing faults
those might be weakened by infiltration of fluids expelled from the underneath magmatic
body. Repetitive stress loading leaded to an excitation of a burst of low-frequency earth-
quakes.
1. Introduction Recently, the research of the triggered
tremor is a good way to help us to better
understand the physical mechanism of the
earthquake. Knowing the mechanism be-
tween earthquakes can help us to improve
the early warning system and the seismic
hazard analysis. The dynamic triggering
earthquake is well known in the volcanic
and geothermal regions, which have the
high background earthquake activities or the
fault on the boundary of the plates (Hill et
al., 1993). Recently, some studies found that
we can still detect the signal of triggered
earthquake in the relatively stable intraplate
region (Gomberg et al., 2004). Moreover,
long-period non-volcanic tremor was found
and located along the subduction zone in
southwest Japan (Obara, 2002). We can also
identify the similar non-volcanic signal in
the volcanic region (Obara, 2012; Chao and
Obara, 2016). Based on the previous study,
there are some triggered regular earthquakes
have been detected after the Kumamoto
HY.SUetal.:DynamicTriggeringSemi-VolcanicTremorinJapaneseVolcanicRegion
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Figure 1. The map of Japan. The High Sensitivity Seismograph Network (Hi-net) and the
V-net operated by the National Research Institute for Earth Science and Disaster Prevention�NIED�of Japan are shown as black crosses and blue circles. The red triangles represent the
active volcano near the V-net.The inset shows an enlarged view of the rectangle with epi-
center and focal mechanism of the 2016 Kumamoto earthquake.
earthquake and we can find that there are
some high frequency components in the
waveform from the study (Enescu et al.,
2016). Therefore, we are going to investi-
gate that whether we can identify the trig-
gered tremor by the Kumamoto earthquake
or not and explain the mechanism of the
triggered tremor if we find the
semi-volcanic tremor. Furthermore, people
inferred the triggered events are more cor-
related to Rayleigh wave in western of japan
(Miyazawa et al., 2008). However, the
tremor found in Taiwan was triggered by
Love wave (Peng and Chao, 2008). There-
fore, we will examine the correlation be-
tween the triggered tremor we found in
Hokkaido and the surface wave.
2. Observation In this study, we used the seismic data rec-
orded by the V-Net, and by the High Sensi-tivity Seismograph Network�Hi-Net�which
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Figure 2. We can see the clearly Love wave on transverse component. After applying the fil-
ter, the tremor triggering by the Love wave is clearly visible during the 800 seconds to 1200
seconds. The bottom picture is to plot the frequency component of the transverse wave with
0.5 Hz low-pass filters.
are both operated by the National Research
Institute for Earth Science and Disaster Prevention�NIED�of Japan. We have
processed the seismic data from the short
period seismometer of V-Net. To identify
triggered tremor, we utilize the Butterworth
band-pass filter in the frequency range of 2
to 8 Hz, which is the dominant frequency
components of triggered tremor, and search
for the bursts of non-impulsive seismic sig-
nal. In order to correlate the triggered trem-
or with the surface waves, we first removed
the instrument response, then using the filter
under the frequency range of 0.01 to 0.2 Hz
to the broadband seismic data. The triggered
tremor is often well correlated to the surface
waves passing. That is to say that after we
remove the dominant frequency component
of the surface waves, we can see the
low-frequency earthquake phenomena af-
terwards. Therefore, we might see the peak
of the triggered tremor that is relative to the
phase of surface waves. Figure 2 shows the
surface wave compared with the seismo-
gram filtered by band-pass filter in 2 to 8 Hz
recorded by the V-net station TKOV.In the
spectrogram we can clearly see the fre-
quency components of triggered tremor
during the 800s to 1200s, of which ampli-
tudes extend up to around 10 Hz.
3. Tremor Location After detecting the triggered tremor, we
picked the S wave arrival time of each burst
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Figure 3. Observations of triggered tremor and ambient tremors in the volcanic region of
central Hokkaido. The green circle is the triggered tremor detected in this study. The white
circle is the triggered tremor detected in the previous study. The diamonds are the location of
different bursts in the tremor. The yellow circles are the ambient low-frequency earthquakes
determined by the JMA. The red triangle is the active volcano. The inverted triangles are the
V-net station and the purple inverted triangles are the station we used to locate the triggered
tremor. The crosses are the stations of Hi-net and the red crosses are the station we used to
locate the triggered tremor. Color bar shows the time sequences of different bursts of earth-
quake.
of the low-frequency earthquake and uti-
lized the different travel time between dif-
ferent seismic stations to locate the triggered
event. Figure 3 shows triggered tremor hy-
pocentral distribution and the error of depth
is within 10 km. The result was calculated
from five Hi-net stations and two V-net sta-
tions. The triggered tremor source is located
at 142.6863°E, 43.1753°N and 26 km depth.
Comparing with previous study (Chao and
Obara, 2016), ours is deeper. Also, we shift
the bursts of the triggered tremor back to the
source region. We use the record of stations
TKOV and shift the time with 10.72s, which
is computed by Taup (Crotwell et al., 1999).
In Figure 4, we want to determine the rela-
tionship between the surface wave and trig-
gered tremor. If the triggered tremor is sen-
sitive to the change of the stress field, it will
be well correlate to each surface wave phase.
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Figure 4. Correlating the Rayleigh wave and Love wave with triggered tremor. The upper and
middle waveforms are under 0.01 to 0.2 Hz filter and represent Rayleigh wave and Love
wave, respectively. The lower waveform is filter by band-pass 2 to 8 Hz and we can clearly
see the tremor signal. We shifted each burst of low-frequency earthquake back to the tremor
source region and correlated the peak of both Rayleigh wave and Love wave to triggered
tremor.
Figure 5. Depth section of the P-wave velocity structure superimposed with triggered and
ambient tremors. A low-velocity body is imaged beneath these tremors.
-2
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4
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N.TKOV.BR.sac.new
-4
-2
0
2
4
X 10+
11
N.TKOV.BT.sac.new
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Bandpass2-8Hz.new
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Here the Love wave is more in phase with
tremor bursts and the amplitude of Love
wave is also larger than Rayleigh wave,
suggesting that the tremor might be trig-
gered by the Love wave.
4. Discussions and Conclusion We verify the triggered tremor source in the
region of the deep volcanic low-frequency
earthquake in central Hokkaido (Obara,
2012; Chao and Obara, 2016). Nevertheless,
the depth of the triggered tremor source in
this study is greater than previous one (Chao
and Obara, 2016). Also the average depth of
the triggered tremor is greater than most of
the ambient tremors. Yet, the determination
of depth still need more evidences (different
method) to support it. The deep
low-frequency earthquakes occur near the
low-velocity zone (Hasegawa et al., 2009;
Obara, 2012). In this study, our research
focuses on the volcanic region, and the
low-frequency tremor we detected might be
adjacent to the magma body. In addition, the
tremor is more correlative to the Love wave.
This correlation result indicates that the
temporal shear stress loading caused by
Love wave deformation prompted fractures
of some pre-existing faults those might be
weakened by infiltration of fluids expelled
from the underneath magmatic body. Repet-
itive stress loading leaded to an excitation of
a burst of low-frequency earthquakes. Fig-
ure 5 shows that there is a low-velocity zone
beneath the ambient tremors and the trig-
gered tremors, which is more likely a mag-
matic body or other fluid body.
5. Acknowledgement We thank the NIED provides the great and
high quality seismic network data and the
JMA provides the seismic catalog. The fig-
ures in this study were created by using the
Generic Mapping Tools, SAC and
MATLAB. The data processing was done in
the Earthquake Research Institute and su-
pervises by Prof. Aitaro Kato. Also, I ap-
preciate to the help from Dr. Kevin Chao
when he was in his short-term visiting in
ERI.
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