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Migration of low frequency tremors revealed from multiple array analyses in western
Shikoku, Japan
Tomotake Ueno, Takuto Maeda*, Kazushige Obara, Youichi Asano, and Tetsuya Takeda
National Research Institute for Earth Science and Disaster Prevention
*Now at the Center for Integrated Disaster Information Research, Interfaculty Initiative in
Information Studies, The University of Tokyo
Short title: MIGRATION OF TREMORS BY MULTIPLE ARRAYS
———————————————
Tomotake Ueno
National Research Institute for Earth Science and Disaster Prevention
3-1 Tennodai, Tsukuba, Ibaraki 305-0006, Japan
1
Abstract
Multiple array observation above a belt-like tremor zone was conducted to
investigate the detailed location and migration of tremor activity in western Shikoku, Japan.
In March 2007, an episodic tremor and slip event occurred, and highly coherent waveforms
were recorded at three arrays. Multiple signal classification analysis for the data from each
array enabled measuring precise arrival directions. The majority of tremor signals
suggested relatively low slowness. The arrival directions of tremor signals were used to
locate tremor sources by the grid search method. Tracking the tremor activity showed that
the tremor migrated within several hours in the northeast-southwest direction over a
distance of 12–15 km, and its migration velocity was 1–2.5 km/h. This migration velocity is
more rapid than the mean velocity of 0.5 km/h over the whole tremor episode lasting
several days. Such a short timescale migration may represent fluctuation of slip
acceleration during the slow slip event. Whenever a tremor migrates southwestward,
very-low-frequency earthquakes occur in the vicinity of the tremor migration terminus.
This indicates that the tremor migration is related to the occurrence of very-low-frequency
earthquakes and slow slip events.
2
1. Introduction
In the Nankai subduction zone in southwest Japan, low-frequency tremors have
been detected [Obara, 2002] with high sensitivity seismograph networks (Hi-net) operated
by the National Research Institute for Earth Science and Disaster Prevention (NIED)
[Okada et al., 2004; Obara et al., 2005]. The tremors are located approximately 30–40 km
deep along the strike of the subducting Philippine Sea plate (PHS). In particular, in the
western Shikoku district, significant tremor episodes recur at six-month intervals, and they
are accompanied by slow slip events (SSEs) [Obara et al., 2004; Hirose and Obara, 2005].
Such episodic tremor and slip are also observed on the Cascadia margin in the northwestern
part of the North American plate [Rogers and Dragert, 2003]. Moreover, similar
phenomena have been detected in some subduction zones, including Costa Rica, Mexico,
Alaska, and New Zealand [e.g., Schwartz and Rokosky, 2007]. In southwest Japan,
very-low-frequency (VLF) earthquakes accompanied by tremors and SSEs have also been
detected [Ito et al., 2007], and they reflect intermediate frictional properties at the deeper
part of the megathrust seismogenic asperity on the subducting plate interface.
During tremor and other coincident phenomena that have a characteristic period of
several days, the tremor sources usually migrate along the strike of the subducting plate
3
with a mean velocity of approximately 10 km/day, which was inferred from the source
locations by the envelope correlation method (ECM) [Obara, 2002]. A similar tremor
migration velocity was estimated at the Cascadia subduction zone [Kao et al., 2006]. On
the other hand, on a very short timescale, low-frequency earthquakes, which are distinct
tremors composed of impulsive signals, migrated along the dip direction of the slab with a
velocity of ~45 km/h during a tremor burst, as measured by waveform correlation analysis
[Shelly et al., 2007]. Such a variation of the estimated velocity suggests that there may exist
a scale dependency of the migration velocity associated with the duration of slow events
[e.g., Ide et al., 2007; Matsuzawa et al., 2009].
The detailed migration of tremor activity is not well understood because the ECM
is based on the smoothed envelope pattern obtained at Hi-net stations separated by a mean
distance of approximately 25 km, and it is spatially limited for the hypocentral location of
tremors. To locate tremor sources with high accuracy, dense seismic array observation is
effective because wavefield analysis can be performed. In Cascadia, La Rocca et al. [2005]
observed well-correlated tremor waveforms with three array observations deployed in the
northern Washington and British Columbia region. La Rocca et al. [2008] pointed out that
small-aperture array analysis is useful to analyze signals from a single tremor source, and
4
more complex and detailed analyses are required to analyze multiple-source tremors. To
analyze the multiple signals in tremor wavetrains, the Multiple Signal Classification
(MUSIC) method [Schmidt, 1986] is suitable, as it is for studying volcanic tremors [e.g.,
Saccorotti et al., 2004].
In the present study, we carried out a dense seismic array observation in western
Shikoku, Japan. We located epicenters of tremor sources using arrival directions estimated
from MUSIC during the episode of March 2007. From the distribution of the tremor
sources, the migration of tremors and the relationship among tremors, VLF earthquakes,
and SSEs are discussed.
2. Data and analysis
2-1. Dense array observation
Western Shikoku is the most active tremor region and SSE area on the belt-like
tremor zone in southwest Japan, and tremor and SSE episodes occur approximately every
six months. To record signals from the tremor sources, dense seismic arrays were deployed
at three sites in western Shikoku separated by approximately 20 km, as shown in Figure 1.
The observation period was from February to May 2007 because a previous tremor episode
5
occurred in September 2006, and we anticipated the next tremor would occur during this
observation period. As expected, a tremor and SSE episode occurred in March 2007. Each
array was composed of 23 vertical-component seismometers with a natural frequency of 2
Hz and 3 three-component seismometers with a natural frequency of 1 Hz. The receiver
configuration of each array is illustrated in Figure 2. The mean spacing between the
receivers was approximately 30 m. The continuous waveform data were recorded at a
sampling interval of 0.01 s.
2-2. Array processing
To detect a coherent signal and arrival direction from dense seismic arrays, various
methods have been developed to estimate the slowness vector [e.g., Niedel and Tarner,
1971; Rost and Thomas, 2002]. Here we adopted a two-step approach to detect the tremor
signal. First, we seek a coherent wave propagating to the array as a candidate for the tremor
signal. Then, the MUSIC method [Schmidt, 1986] is applied to estimate an accurate
backazimuth of the propagation for each tremor signal, which is used to determine the
epicentral location of the tremor. The method of tremor epicenter determination is
described in the section 2-3.
6
2-2-1. Preprocessing
The tremor signal is dominant in the horizontal records compared to the vertical
ones. However, as shown in Figure 3, the tremor signal is clearly coherent and the tremor
amplitude is two or more times larger than the amplitude of background noise in the
vertical records, perhaps because the shear-vertical component of the tremor signals might
be recorded. Therefore, in our analysis, we used only vertical seismometers at each array.
The continuous waveform data observed at the three arrays were processed by a
bandpass filter with a frequency range of 2–8 Hz to eliminate background noise and to
enhance the tremor signal with a dominant frequency of around 2 Hz [Obara and Hirose,
2006]. To detect a coherent wavefield containing the tremor signal, we evaluated the
cross-correlation coefficients of the waveforms recorded at neighboring receivers for each
10-s time window every 2 s. By comparing the waveforms with and without tremor signals,
the averaged cross-correlation coefficients for the tremor signals are much higher than
those without tremors (Figure 3). For the processing described herein, we adopt array
records whose averaged cross-correlation coefficient is higher than 0.75 as candidates for a
tremor signal.
7
2-2-2. Estimation of arrival direction by the MUSIC method
Tremor often occurs at some source locations simultaneously during the time
period of SSE episode. In order to estimate an arrival direction of the coherent waveform
from the strongest tremor source in a given time window, we used the MUSIC method
[Schmidt, 1986] because this method can decompose arrival directions for multiple sources
with high accuracy from a contaminated wavefield [e.g., Goldstein and Archuleta, 1987]. If
we were using other traditional methods in the array analysis, the detection of the most
dominant arrival direction might have been less accurate.
For well-correlated wavetrains in the preprocessing step, we calculated a
cross-covariance matrix for each array’s data in a 10-s time window at a frequency range of
2–8 Hz. Then, eigenvalue decomposition was performed on the cross-covariance matrix. To
decompose the covariance matrix into signal and noise subspaces, the MUSIC method
requires a priori indication of the number of eigenvalues that correspond to signals. For the
case of tremor with ambiguous arrivals, because the difference of eigenvalues between
signal and background noise was small in comparison to a regular earthquake, it was
difficult to determine the number of eigenvalues that correspond to tremor signals. Hence,
8
we have an expedient criterion as follows: the sum of selected eigenvalues from the largest
signal must exceed 95% of the total sum of eigenvalues. Then, we search the MUSIC
spectrum in the slowness space for the most dominant backazimuth and slowness.
An example of the MUSIC spectra from each array at which the coherent signals
were obtained (Figure 3b) are plotted in Figure 4. Regardless of the complexity of the
tremor waveforms, clear peaks in the slowness space appear at each array. In order to
evaluate the error of the MUSIC spectrum, we adapt the delete-1 Jackknife method
[Tichelaar and Ruff, 1989] for the dataset, by removing a waveform recorded at a single
station in each array. Examples of estimated peaks and errors of the MUSIC spectra are
shown with the white circle and bars in the right panel of Figure 4. From this method, we
measure the stability of backazimuth and slowness for incoming waves.
In order to exclude distant signals and coherent noises originating near the surface,
we eliminated the backazimuth and slowness for which the MUSIC spectrum peak is
greater than 0.4 s/km. The uncertainty in the backazimuth of the MUSIC spectrum peak
estimated by the delete-1 Jackknife method tends to increase with decreasing slowness.
Therefore, we choose a well-determined slowness that satisfies the following threshold: the
error in slowness and backazimuth are smaller than 0.2 s/km and 60°, respectively, for peak
9
slowness of more than 0.25 s/km; and these values must be smaller than 0.4 s/km and 140°,
respectively, for peak slowness of less than 0.25 s/km.
2-3. Tremor source detection with multiple array data
We determine the focus point of the three array backazimuths as the epicenter of
the tremor source based on an assumption that tremors occur at the plate interface [Shelly et
al., 2006; Ghosh et al., 2009]. For each time window, the epicenters of tremor sources are
determined by the grid search method. The backazimuths for each grid point are evaluated
using the following equations to estimate the epicentral location:
⎪⎩
⎪⎨⎧
−−
−=Δ
caln
obsn
caln
obsn
nφφ
φφ
360
)180(
)180(
>−
≤−caln
obsn
caln
obsn
φφ
φφ, (1)
∑Δ=Δn
nr2 , (2)
( )∑ Δ−Δ−
=Δn
nsd n2
11 ⎟
⎠
⎞⎜⎝
⎛Δ=Δ ∑
nnn
1, (3)
10
where and denote the observed and calculated backazimuths for the th array,
respectively. denotes the difference between the observed and calculated values.
obsnφ
calnφ n
nΔ rΔ ,
Δ , and denote the sum of squares, the mean value, and the standard deviation of sdΔ nΔ ,
respectively. A point where the minimum value of rΔ and sdΔ are less than 30° and 10°
is treated as an epicenter of a tremor source.
The reliability of the epicentral location decreases as the distance from the three
arrays increases using only backazimuths. Furthermore, some sources are located at the
edge of the search area. Because these sources were focused over 100 km in epicentral
distance, we chose tremor sources less than 100 km from the nearest array determined by
the above location process. Then, to obtain a stable estimate for tracing the tremor
migration, we calculate the centroid of each tremor per hour by averaging individual
epicentral locations with more than 15 events within an hour. The error of this mean
location is evaluated by a bootstrap simulation [Efron and Tibshirani, 1986] using the
following equations:
∑∑= =⋅
=B
b
N
nb nL
nBB
1 1)(1ˆ , (4)
11
1
ˆ)(12
1 1
−⎭⎬⎫
⎩⎨⎧
−=∑ ∑= =
B
BnLN
B
b
N
nb
Bσ , (5)
where is the number of data. denotes the b th bootstrap sample consisting of a
random sample selected from the detected latitudes or longitudes of tremor sources each
hour.
N )(nLb
B denotes the bootstrap frequency, which is 2,000 trials in the present study.
3. Results
The backazimuths of coherent signals estimated well by the MUSIC method at
each array from 9–20 March 2007 are shown in Figure 5. As an indicator of epicentral
locations of tremors and their migration, we adopt a statistical "mode" that is the most
frequent value in the hourly backazimuth dataset within a 10 degree bin. In Figure 5, the
mode is shown by a square, and frequency magnitude indicates tremor activity. Here, we
detect coherent tremor signals continuing to arrive at the three arrays from 13–18 March. In
this period, several tremor signals frequently appeared per minute, as shown in Figure 3b,
even if they were recorded by vertical component seismometers. Out of this tremor period,
many cross-correlation coefficients of incoming waves were less than 0.75, as shown in
12
Figure 3a.
3-1. Characteristics of array estimation
The tremors were clearly detected as coherent signals with wide ranges of
backazimuths and low slowness, indicated by the dark gray region in Figure 5. Before the
afternoon of 13 March, a small number of coherent signals were detected as a weak tremor.
Then coherent signals were clearly detected from the afternoon of the 13th. This indicates
that the active tremor started. In the active tremor period, there were two stages because of
halting of tremor in the evening of the 15th; the first is from the 13th to the 15th and the
second is from the 16th to the 18th.
From the afternoon of the 13th to the morning of the 14th, the mode of
backazimuths clearly migrated from the north (approximately 340°) to the west
(approximately 240°) until the evening of the 14th in array C (Figure 5c). Then, the
migration of backazimuths in Figure 5c repeated in this region until the evening of the 15th.
In the same time period, although scattered backazimuths were obtained in array A, the
mode of backazimuths changed around 200° from 240° on the morning of the 14th, and
scattered between 0° and 60° from the 14th to the 15th (Figure 5a). In array B, tremor
13
signals are mainly distributed around a backazimuth of 0° with a very large mode size on
the 13th, and the signals started to have various backazimuths between 0° and 180° from
the beginning of the 14th (Figure 5b). The wide distribution of backazimuths means that the
sources are located underneath the array, because backazimuth estimation is often unstable
for a near-vertical incident wave. Therefore, the area of deep tremor activity migrated from
the vicinity of array A to array B from the 13th to the 14th. This migration was consistent
with the backazimuth change in array C.
On the evening of the 15th, the tremor activity became weak for a few hours as
shown in Figure 5. On the 16th, active tremor began again. After the evening of the 16th,
many tremor signals were suddenly detected at the three arrays. The tremor activity was
concentrated around a backazimuth of 200° in array B, whereas in arrays A and C the
backazimuth also clustered around 240°. This indicates that the tremor direction of each
array jumped southwestward.
3-2. Epicentral location of tremors determined by multiple arrays
The distributions of polar histograms of backazimuths are illustrated in Figure 6.
The histograms indicate that many signals propagated from between arrays A and B. The
14
centroids of tremor activity were distributed along the strike of the PHS slab [Ueno et al.,
2008], which is 60 km long and 20 km wide in the vicinity of 40 km deep in the depth
contour map of PHS (Figure 7a). The tremor epicenters concentrated between arrays A and
B were well located in our analysis; however other tremors, scattered far from the arrays,
were less accurately located. A similar distribution was also obtained by ECM as shown in
Figure 7b; however, tremor epicenters were relatively more scattered in the vicinity of the
arrays.
The time history of the spatial location of tremors projected on longitude and
latitude axes is presented in Figure 8. The vertical bar shows the error distribution based on
the bootstrap method of equation (5). The tremor activity gradually migrated
southwestward from the northern part of western Shikoku during the episode. The tremor
migrating southwest seems to include a repeated movement along the same route. Such a
migration pattern is reliable because the error in the epicentral locations of tremor during
the period from the 13th to the 16th is half as much as during the other period as shown in
Figure 8. In particular, from the 13th to the 14th and the 14th to the 15th, two
southwestward migrations through a similar route were clearly recognized because the
migration distance exceeds the error estimation. The first migration has a 1 km/h velocity
15
over a distance of 12 km for 12 hours, from the evening of the 13th to the morning of the
14th. The second migration has a 2.5 km/h velocity for six hours over a distance of 15 km
from the evening of the 14th to the morning of the 15th in the same region. And then, on
the evening of the 16th, the tremor started to migrate steeply southwestward, far from the
array stations, as shown in Figures 7 and 8. The steep migration appeared as a jump in the
tremor direction (Figure 5).
4. Discussion
4-1. Scale variation of migrating tremor
The tremor episode in March 2007 is characterized by a horizontal migration of
the epicenters with a mean velocity of roughly 0.5 km/h estimated from the traditional
ECM shown by broken lines in Figure 8b, as in the other tremor episodes in western
Shikoku [Obara, 2009]. We detected a short timescale tremor migration pattern with a
faster velocity of 1–2.5 km/h over a distance of 12–15 km for several hours. The sharp
onset of tremor at the migration front and persisting tremor activity behind the front are
interpreted as the rupture initiation of the SSE [Obara, 2009] and continuation of the slip,
respectively [Obara, 2009; Maeda and Obara, 2009]. In the present study, we found that
16
the tremor activity migrated more than once along the same path within one episode, as
shown in Figures 8. The repeating tremor migration in the short timescale might be a
seismic response to the local stress change caused by the small-scale fluctuation in the
spatio-temporal distribution of the slip velocity at the rupture front of the SSE and/or by the
continued slip after the passage of the rupture front.
Various timescales of tremor migration exist within the same region. An increased
migration velocity for a short time was predicted by a scaling relationship [Ide et al., 2007].
The migration velocity in the short timescale in this study also supports the scaling
relationship between characteristic duration and velocity [Matsuzawa et al., 2009]. The
faster migration of the tremor burst in the same region [Shelly et al., 2007] might provide
an endmember for this scaling relationship.
4-2. Relationship between the tremor and other coincident phenomena
In March 2007, the tremor activity, VLF earthquakes, and the SSE occurred
simultaneously at the western Shikoku region like in other episodes. The spatio-temporal
locations of the VLF earthquakes determined by Ito et al. [2009] and Matsuzawa et al.
[2009] during the SSE are indicated by diamonds in Figures 7 and 8. The VLF earthquake
17
locations appeared to be around the southwestern edge of the tremor activity in the early
mornings of the 14th and the 15th. Comparing the spatio-temporal locations of the tremor
and the VLF earthquakes, the tremor seems to migrate toward the VLF earthquake location,
which is the western edge of the tremor migration shown in Figure 8a. The VLF earthquake
is considered as a seismic slip with slow rupture velocity caused by the stress concentration
at a VLF patch surrounded by a slow slip region [Ito et al., 2007]. Consequently, the stress
was concentrated in a patch at the western edge because of the fluctuation of the SSE
reflected by the repeating tremor migration on 14–15 March, which triggered the VLF
earthquakes at this western edge. A similar situation was observed on the 17th. On the
evening of the 16th, after the tremor abruptly migrated southwestward, VLF earthquakes
occurred at the end of the tremor migration. Our results suggest that the occurrence of VLF
earthquakes is related to the stress concentration caused by small-scale fluctuations of SSE
slips that can be detected by short timescale tremor migration.
Here, we compare the tremor migration and the daily slip rate of the SSE obtained
by time-dependent inversion [Hirose and Obara, 2010] as shown in Figure 9. The
cumulative slip distribution, VLF earthquakes, and tremor activity that occurred in March
2007 are presented in Figure 9a. All of the events were concentrated in the northwest
18
portion of western Shikoku. This result was also consistent with the area exhibiting a
high-energy radiation of tremors [Maeda and Obara, 2009]. Prior to the SSE occurrence
and before the 13th, weak and scattered tremor activity appeared around the northern
portion of array B (Figure 9b–d). Once the slip began and its SSE area became large, the
number of tremor sources also increased (Figure 9d–f). The tremor activity appears to be
located repeatedly along the slow slip contours, and it migrates toward the source area of
the VLF earthquakes, as shown in Figure 9f–i.
On the evening of the 15th, the centroid of the tremor activity could not be
obtained for Figure 8a because the tremor activity abruptly became very weak for a few
hours, and then the tremor became active again as shown in Figure 5. On the evening of the
16th, the tremor activity location abruptly migrated southwestward as if by jumping, as
shown in Figures 5 and 8, and the slip rate contour of the SSE also simultaneously moved
to the southwestern area (Figure 9h and i). These phenomena of halting and jumping were
also found in Cascadia [Kao et al., 2006, 2007].
Since the active area of the VLF earthquakes and the SSE were located far from
our arrays, especially after the 17th as shown by ECM in Figure 8b, the centroid locations
of tremor activity had large uncertainties, as shown in Figures 7a, 8a, and 9i. While the
19
locations are not well-resolved, the migration pattern is still discernible above the errors.
Finally, the tremor activities scattered and gradually ceased after the conclusion of the SSE
(Figures 9j–l). Based on the above discussion, we conclude that the short timescale of
tremor migration is a small-scale fluctuation within the overall slow slips, and it is related
to the occurrence of VLF earthquakes. This type of rupture fluctuation and the relationships
among detailed spatio-temporal distributions of each slow event may be keys to disclosing
a series of slow events.
5. Conclusions
We succeeded in detecting the migration of tremors to investigate the arrival
directions of tremor signals by conducting a multiple array analysis on continuous wave
data. This array experiment was the first such observation and analysis of tremor activity in
the western Shikoku region. Combining the results of the array analyses, we obtained the
following information regarding the March 2007 episode:
1. Highly coherent tremor signals were obtained, and tremor sources were detected
with the multiple array analyses.
2. The majority of the apparent slowness values detected by the three arrays during
20
the active tremor period were relatively low. The incident angles of wavetrains from the
tremor source to the arrays were nearly vertical. The distributions of backazimuths
indicated the direction of the tremor sources.
3. A number of tremor sources were located in northwestern Shikoku. Furthermore,
we obtained a highly reliable distribution of tremor epicenters for each hour. These results
permitted repeating tremor migrations to be detected in a short time. The velocity of the
short timescale tremor migration was approximately 1–2.5 km/h over a distance of 12–15
km for this episode. The migration velocity for the whole episode was roughly 0.5 km/h.
4. The spatio-temporal distribution of the tremor sources was correlated to the VLF
earthquakes and the SSE that occurred in the same region. The short timescale migration of
the tremor suggests a small-scale fluctuation within the overall slow slip that was related to
the occurrences of the VLF earthquake.
21
Acknowledgments
We thank Dr. H. Hirose for providing figures and advice regarding the short term
SSE that occurred in 2007. We also thank Dr. T. Matsuzawa for his advice concerning a
very-low-frequency event and the scaling relationship between characteristic duration and
velocity of a migrating tremor. Dr. Y. Ito helped us prepare and deploy the three arrays and
discussed the relationships among slow events with us. Dr. B. Enescu discussed our work
and provided us with helpful comments. We drew all figures using Generic Mapping Tools
[Wessel and Smith, 1998]. Our study was supported by all Hi-net members. Professor J.
Vidale, an anonymous reviewer, and the associate editor provided useful comments to
improve our manuscript.
22
References
Efron, B., and R. Tibshirani (1986), Bootstrap Method for Standard Errors, Confidence
Intervals, and Other Measures of Statistical Accuracy, Statistical Science, 1, 54–77.
Ghosh, A., J. E. Vidale, J. R. Sweet, K. C. Creager, and A. G. Wech (2009), Tremor patches
in Cascadia revealed by seismic array analysis, Geophys. Res. Lett., 36, L17316,
doi:10.1029/2009GL039080.
Goldstein, P., and R. Archuleta (1987), Array analysis of seismic signals, Geophys. Res.
Lett., 14, 13–16.
Hirose, H., and K. Obara (2005), Repeating short- and long-term slow slip events with deep
tremor activity around the Bungo channel region, southwest Japan, Earth Planets Space,
57, 961–972.
Hirose, H., and K. Obara (2010), Recurrence behavior of short-term slow slip and
correlated non-volcanic tremor episodes in western Shikoku, southwest Japan, J.
Geophys. Res., doi: 10.1029/2008JB006050, in press.
Ide, S., G. C. Beroza, D. R. Shelly, and T. Uchide (2007), A scaling law for slow
earthquakes, Nature, 447, 76–79, doi:10.1038/nature05780.
Ito, Y., K. Obara, K. Shiomi, S. Sekine, and H. Hirose (2007), Slow earthquakes coincident
23
with episodic tremors and slow slip events, Science, 315, 503–506.
Ito, Y., K. Obara, T. Matsuzawa, and T. Maeda (2009), Very-low-frequency earthquake as
small asperity on plate boundary in transition zone from locked to aseismic slip, J.
Geophys. Res., 114, B00A13, doi:10.1029/2008JB006036.
Kao, H., S. Shan, G. Rogers, and H. Dragert (2007), Migration characteristics of seismic
tremors in the northern Cascadia margin, Geophys. Res. Lett., 34, L03304,
doi:10.1029/2006GL028430.
Kao, H., S. Shan, H. Dragert, G. Rogers, J. F. Cassidy, K. Wang, T. S. James, and K.
Ramachandran (2006), Spatialtemporal patterns of seismic tremors in northern Cascadia,
J. Geophys. Res., 111, B03309, doi:10.1029/2005JB003727.
La Rocca, M., D. Galluzzo, S. Malone, W. McCausland, G. Saccorotti, and E. Del Pezzo
(2008), Testing Small-Aperture Array Analysis on Well-Located Earthquakes, and
Application to the Location of Deep Tremor, Bull. Seism. Soc. Am., 98, 620–635,
doi:10.1785/0120060185.
La Rocca, M., W. McCausland, D. Galluzzo, S. Malone, G. Saccorotti, and E. Del Pezzo
(2005), Array measurements of deep tremor signals in the Cascadia subduction zone,
Geophys. Res. Lett., 32, L21319, doi:10.1029/2005GL023974.
24
Maeda, T., and K. Obara (2009), Spatio-temporal distribution of seismic energy radiation
from low-frequency tremor in western Shikoku, Japan, J. Geophys. Res., 114, B00A09,
doi:10.1029/2008JB006043.
Matsuzawa, T., K. Obara, and T. Maeda (2009), Source duration of deep
very-low-frequency earthquakes in western Shikoku, Japan, J. Geophys. Res., 114,
B00A11, doi:10.1029/2008JB006044.
Niedel and Tarner (1971), Semblance and other coherency measures for multichannel data,
Geophysics, 36, 482–497.
Obara, K. (2002), Nonvolcanic deep tremor associated with subduction in southwest Japan,
Science, 296, 1679–1681.
Obara, K. (2009), Phenomenology of deep slow earthquake family in southwest Japan
--Spatio-temporal characteristics and segmentation--, J. Geophys. Res., submitted to this
special section.
Obara, K., and H. Hirose (2006), Non-volcanic deep low-frequency tremors accompanying
slow slips in the southwest Japan subduction zone, Tectonophys., 417, 33–51.
Obara, K., H. Hirose, F. Yamamizu, and K. Kasahara (2004), Episodic slow slip events
accompanied by non-volcanic tremors in southwest Japan subduction zone, Geophys.
25
Res. Lett., 31, L23602, doi:10.1029/2004GL020848.
Obara, K., K. Kasahara, S. Hori, and Y. Okada (2005), A densely distributed
high-sensitivity seismograph network in Japan: Hi-net by National Research Institute
for Earth Science and Disaster Prevention, Rev. Sci. Instrum., 76, 021301,
doi:10.1063/1.1854197.
Okada, Y., K. Kasahara, S. Hori, K. Obara, S. Sekiguchi, H. Fujiwara, and A. Yamamoto
(2004), Recent progress of seismic observation networks in Japan –Hi-net, F-net,
K-NET and KiK-net–, Earth Planets Space, 56, xv–xxviii.
Rogers, G., and H. Dragert (2003), Episodic tremor and slip on the Cascadia subduction
zone: The chatter of silent slip, Science, 300, 1942–1943.
Rost and Thomas (2002), Array seismology: Methods and applications, Rev. Geophys., 40
(3), 1008, doi:10.1029/2000RG000100.
Saccorotti, G., L. Zuccarello, E. Del Pezzo, J. Ibanez, and S. Gresta (2004), Quantitative
analysis of the tremor wavefield at Etna Volcano, Italy, J. Volcanol. Geotherm. Res., 136,
223–245.
Schmidt, R. (1986), Multiple emitter location and signal parameter estimation, IEEE Trans.
Antennas Propag., 34, 276–280.
26
Shelly, D. R., G. C. Beroza, and S. Ide (2007), Non-volcanic tremor and low-frequency
earthquake swarms, Nature, 446, 305–307, doi:10.1038/nature05666.
Shelly, D. R., G. C. Beroza, S. Ide, and S. Nakamula (2006), Low-frequency earthquakes in
Shikoku, Japan and their relationship to episodic tremor and slip, Nature, 442, 188–191,
doi:10.1038/nature04931.
Schwartz, Y. S., and J. M. Rokosky (2007), Slow slip events and seismic tremor at
circum-pacific subduction zones, Rev. Geophys., 45, RG3004,
doi:10.1029/2006RG000208.
Tichelaar, B. W., and L. J. Ruff (1989), How good are our best models? Jackknifing,
Bootstrapping, and Earthquake Depth, EOS Trans. AGU, 70(20), 593.
Ueno, T., T. Shibutani, and K. Ito (2008), Configuration of the continental Moho and
Philippine Sea slab in Southwest Japan derived from receiver function analysis:
Relation to subcrustal earthquakes, Bull. Seism. Soc. Am., 98, 2416–2427.
Wessel, P., and W. H. F. Smith (1998), New, improved version of Generic Mapping Tools
released, EOS Trans. AGU, 79(47), 579.
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Figure captions
Figure 1. Array map and distribution of tremors in southwest Japan. Triangles indicate the
locations of each array, and inverse triangles denote Hi-net stations. Circles indicate the
epicenters of tremors detected using ECM [Obara, 2002] from 9–20 March 2007. The
depth contour map of the upper part of the PHS slab derived from receiver functions [Ueno
et al., 2008] is indicated by dashed lines.
Figure 2. Receiver configurations of each array. An open triangle indicates a vertical sensor.
A filled triangle indicates a 3-component sensor.
Figure 3. Examples of waveform records (a) without and (b) with tremor waveforms
recorded by array A at 22:10 on the 13th and the 14th of March 2007 (JST), respectively, in
upper panels. Lower panels illustrate the averaged correlation coefficient (A.C.), indicated
by triangles. Horizontal axis represents time (s).
Figure 4. Examples of waveforms recorded by arrays A (a), B (b), and C (c). Traces were
recorded at the same time as the upper panel in Figure 3b. Horizontal axes represent time
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(s). Right panels illustrate the MUSIC spectra for unshaded areas above the heavy bars.
Vertical and horizontal axes represent slowness Px (s/km) and Py (s/km), respectively.
White circles and bars show the mean peak of the normalized MUSIC spectrum and its
error estimated by the delete-1 Jackknife method.
Figure 5. Time series of arrival direction for arrays A (a), B (b), and C (c). Slowness values
for each backazimuth are shown in gray scale. Vertical axes represent the backazimuth
(degree). Horizontal axes represent time (day). Dots indicate peak values of the MUSIC
spectrum. Open squares show the backazimuth mode per hour and their size depends on
frequency.
Figure 6. Distribution of tremor sources and polar histograms for each array. Triangles
indicate locations of arrays. Gray dots denote tremor sources that were located using
multiple array analysis. The sector width of the diagram is 10°. The radiated axis is
normalized to unity.
Figure 7. Distribution of tremor activity per hour (circle colored to indicate day) of (a) the
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array analysis in this study, and (b) ECM [Obara, 2002]. VLF earthquakes (diamonds
denote day) and the depth contour map of the upper part of the PHS slab (dashed lines)
[Ueno et al., 2008] are indicated. Black triangles denote locations of arrays.
Figure 8. Changes in tremor sources for longitude (upper panel) and latitude (lower panel)
of (a) the array analysis in this study and (b) ECM [Obara, 2002]. Dots indicate the
centroid location of tremor activity for each hour. White diamonds denote VLF earthquakes.
The error bar was estimated using the bootstrap method. The two velocities of 1 and 2.5
km/h shown in (a) indicate a migrating tremor velocity from the evening of the 13th to the
morning of the 14th and from the evening of the 14th to the morning of the 15th,
respectively. Broken lines in (b) show the migration velocity of 0.5 km/h.
Figure 9. Mean tremor activity per hour and VLF earthquakes for the total study (a) and
each day (b)–(l). For the SSE, (a) illustrates the cumulative slip distribution with contours
in mm and (d)–(j) illustrate the daily slip rate distributions with contours in mm/y [Hirose
and Obara, 2010]. Black triangles indicate array locations. Diamonds and circles denote
VLF earthquakes and tremor activity, respectively. Note that the color scale is different for
30
31
(a) (days) and the others (hours).