SSS32-P01

Room:Convention Hall

Time:May 26 18:15-19:30

Inference of a slip distribution from aftershock data and friction law: a Bayesian modelwith a prior of magnitude
IWATA, Takaki1∗
1

Tokiwa University

An statistical method to estimate a fault slip distribution of a mainshock with the spatial distribution of its aftershocks andrate- and state-dependent friction law [Dieterich, 1994] has been suggested [Iwata, 2008]. In this method, the fault plane of amainshock is divided into subfaults, and then the amplitudes of slip in each of the subfaults are optimized to fit the real spatial
distribution of the aftershock activity with the distribution expected from the rate- and state-friction law. Because we optimize alarge number of parameters simultaneously in this approach, a roughness penalty is imposed to stabilize the optimization; for thispurpose, a Bayesian model with a smoothness prior for the spatial slip distribution is constructed and the estimation is carriedout.
One of the problems in this method is how to determine the strength of the roughness penalty objectively. In many casesof seismological/geophysical studies, the strength is determined by the principle of the minimization of Akaike’s BayesianInformation Criterion [ABIC; Akaike, 1980] and ABIC is computed through the Laplace approximation [Tierny and Kadane,1986]. However, because of some technical reasons originated from the formula of the friction law, the Laplace approximation
is not applicable to this method and the computation of the value of ABIC is impractical.
This study proposes that the information on the magnitude of a mainshock is incorporated in the Bayesian model. This isbecause it has been empirically found that the amplitudes of the estimated slip in the subfaults or the corresponding magnitude
to the estimated slip distribution much depends on the strength of the roughness penalty; if we impose a constraint on themagnitude, then the appropriate strength could be chosen objectively. To implement this idea, a prior distribution of the magnitudeof a mainshock is constructed. It is supposed to be a normal distribution of which mean is retrieved from the Global CMT
catalogue and standard deviation is given from Kagan [2010]. Then, the posterior distributions of the strength and the spatial slip
distribution are computed simultaneously through the Markov chain Mote Carlo method. This framework provides the practicalcomputational method to estimate the spatial slip distribution of a mainshock inferred from its aftershock data.
References:- Akaike, in Bayesian Statistics (eds. Bernardo et al.), pp.143-166, 1980.
- Dieterich, J. Geophys. Res., 99, 2601-2618, 1994.
- Iwata, 2008 JPGU Meeting, S142-009, 2008.
- Kagan, Tectonophys., 490, 103-114, 2010.
- Tierney and Kadane, J. Am. Stat. Assoc., 81, 82-86, 1986.

Keywords: slip distribution, aftershocks, Bayesian estimation, prior distribution, Markov chain Monte Carlo method

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SSS32-P02

Room:Convention Hall

Time:May 26 18:15-19:30

Tidal triggering of earthquakes after the 2011 Tohoku earthquake
TANAKA, Sachiko1∗
1

NIED

I examined correlations between the Earth tides and earthquakes off the Pacific coast of eastern Japan for about four years after
the 2011 Tohoku earthquake (Mw 9.1). A previous study has reported a high correlation in the northern part of the Tohoku source
area, where the mainshock rupture initiated, in about ten years prior to the Tohoku earthquake (Tanaka, 2012). The data I used
are the Global Centroid Moment Tensor (CMT) solutions of shallow earthquakes (depths less than 70 km) with Mw 5.0 or larger
for the period from 1976 to 2014. For each event, I theoretically calculated tidal shear stresses on the fault plane consideringboth the solid Earth tides and ocean loading tides (Tanaka et al., 2002). Assigning the tidal phase angle at the origin time of each
event, I tested whether they concentrate near some particular angle or not by using the Schuster’s test. In this test, the result isevaluated by p-value, which represents the significance level to reject the null hypothesis that the earthquakes occur randomly
irrespective of the tidal phase angle. For about four years after the Tohoku earthquake, no significant correlation was found in
the area of high correlation before the Tohoku earthquake; p-values there are larger than 30% in the post-Tohoku earthquake
period. On the other hand, small p-values were observed off the Iwate prefecture, north of the Tohoku source area. The smallest(2.8%) is near the coast, where large postseismic afterslip has been identified by geodetic measurements (Ozawa et al., 2012). In
this region, no significant correlation was found for about 35 years prior to the Tohoku earthquake. Furthermore, the temporalvariation of p-value in the post-Tohoku earthquake period revealed that the p-value was smallest (1.4%) just after the Tohoku
earthquake, and gradually increased with time. This seems to be correlated with the time evolution of afterslip showing rapid
decay over time (Ozawa et al., 2012). These observations suggest that in addition to the precursory stage of a giant earthquake,tidal triggering could occur in the early acceleration stage of large postseismic slip.
Keywords: the 2011 Tohoku earthquake, Earth tides, earthquake triggering, postseismic afterslip

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SSS32-P03

Room:Convention Hall

Time:May 26 18:15-19:30

Seismic activity and attenuation structure aroud the Fukushima and Yamagata Prefectures’ border after Tohoku earthquake
MIYAGKI, Keiichiro1 ; TSUMURA, Noriko1∗
1

Graduate School of Science Chiba University

Around the border between Fukushima and Yamagata prefectures, seismicity was suddenly activated after off the Pacific coastof Tohoku earthquake. We estimated distribution and focal mechanisms of the earthquakes to clarify features of the seismic
activity. A three?dimensional seismic structure in the northeastern Japan derived in a previous study showed that fluid might
affect the seismic activity in this swarm. A Qp/Qs and Qs value greatly change by existence of fluid. Then, we estimated the Q
values to clarify physical properties in this region by taking velocity amplitude spectral ratio between P and S waves.
First, we found that hypocenters were concentrated into four clusters. We also observed hypocenter migration to lateral andvertical direction in some clusters. Most earthquakes have the thrust-type focal mechanisms. Average Qp/Qs and Qs values on
the ray paths from hypocenters to stations show high Qp/Qs and low Qs at relatively near stations from source region. Whilepaths from the hypocenters to far stations show low Qp/Qs and high Qs. This feature might indicate that high attenuation region
exists in nearby source region. Further, we estimated a detail Q structure of the swarm area by using combination of spectra ratio
data which have very similar ray paths. As a result, the blocks in which many earthquakes occurred have high Qp/Qs and low
Qs, whereas those for the region between the clusters show vice versa. When we compared with other geophysical data, this highQp/Qs and low Qs values seem to reflect the influence of fluid. Hypocenter migration would be explained by upward migrationof fluids due to difference of density.

Keywords: attenuation structure, Seismic activity, swarm, hypocenter migration, fluid

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SSS32-P04

Room:Convention Hall

Time:May 26 18:15-19:30

Changes in frequency ratio of inter-plate vs intra-plate earthquakes in the source area ofthe 2011 Tohoku earthquake
IMOTO, Masajiro1∗ ; MORIKAWA, Nobuyuki1 ; FUJIWARA, Hiroyuki1
1

NIED

In the current seismic hazard maps for Japan, contributions from earthquakes of non-specified fault parameters in the subduction area are considered either as interplate or intraplate earthquakes. Here, a ratio of the number of interplate earthquakes tothe total number of quakes is used as a given parameter obtained from the frequency distribution of earthquakes in depth. In thepresent study, we attempt to employ focal mechanism solutions for estimating the ratio. As a case study, ratios are examined from3663 mechanism solutions of earthquakes of magnitude 4.0 or greater occurring in the area of 36N-41N and 139E-144E, which
covers the source area of the 2011 Tohoku earthquake (M9.0). First, we calculate the minimum 3-D rotation angle (Kagan angle)
between the observed mechanism of an earthquake and expected one, which can be estimated on assumptions of configurationsof plate interfaces and relative motions between the plates. Inter plate events are assigned, of which Kagan angles are less thana certain threshold level. We estimate a ratio at every 0.1 by 0.1 spacing grids. After operating smoothing method with ABIC,
we obtain those ratios at every grids. The estimate is separated into that for the pre 2011 Tohoku earthquake period or for the
post-earthquake period. As a result, we have found that, in general, ratios of the inter-plate earthquakes for the post-earthquake
period are smaller than those for the pre-earthquake period. The ratios obtained in the present study are significantly smaller thanthe current ones.
Keywords: seismic hazard maps for Japan, Tohoku, interplate earthquake, Kagan angel, 2011 Tohoku earthquake

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SSS32-P05

Room:Convention Hall

Time:May 26 18:15-19:30

Focal mechanism of earthquake swarms in Gassan and Ooisawa, Yamagata Prefecture
OSHIMA, Atsushi1 ; IWATA, Naoyoshi1 ; HASEMI, Akiko1∗
1

Faculty of Science, Yamagata University

Seismic activity near Gassan Mountain was activated 6 days after the 2011 off the Pacific coast of Tohoku Earthquake. In
this area, the activity had been low before the Tohoku earthquake. About 15km south of this area, earthquake swarm activity
has repeated in Ooisawa area. Recent activities occurred in 2003 and 2006. Triggered activity was not occurred there. We
determined source mechanisms of earthquakes magnitude larger than 2.0 of Gassan and Ooisawa activities using P-wave first
motion to investigate a difference between two regions. The numbers of source mechanisms determined were 33, 14 and 12 for
Gassan, 2003 Ooisawa and 2006 ooisawa activities, respectively. In Gassan, 7 of 33 were strike slip faults, 20 reverse faults withstrike slip component, 2 reverse faults and 4 normal faults with strike slip component. Okada et al.(2011) calculated Coulomb
stress change on the fault plane of the strike slip event on Apr 4, and obtained a positive change of 0.29MPa. Since the mechanism
solutions determined in this study contained strike slip component similar to that used by Okada et al.(2011) , Coulomb stresschange may be the cause of the Gassan activity. Considering 6days delay and the low velocity lower crust beneath Gassan into
account, an increase of pore fluid pressure is supposed to be another cause of triggering. For Ooisawa swarm, we determined 26
mechanism solutions. All of them were reverse slip faults with E-W or NWN-SES P-axis. As reasons why activities were nottriggered in Ooisawa, we suppose that reverse faults, on which a slip was inhibited by the Tohoku earthquake, are predominant,and/or that fluid was not moved into the upper crust there.
References
Okada et al.(2011) EPS,63,749-754
Okada et al.(2015) Geofluids,15, 293?309

Keywords: 2011 Tohoku-Oki earthquake, induced seismicity, focal mechanism, Gassan, Oisawa

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SSS32-P06

Room:Convention Hall

Time:May 26 18:15-19:30

Active foreshocks of M>7.5 earthquakes in the northern Japan to Kuril Trenches
HARADA, Tomoya1∗ ; YOKOI, Sayoko1 ; SATAKE, Kenji1
1

Earthquake Research Institute, Univ. of Tokyo

Along the northern Japan to Kuril Trenches, active foreshock sequences preceded some M>7.5 earthquakes, providing a good
opportunity to understand the characteristics of foreshocks for large interplate events. Active foreshocks are identified in the M-T
diagrams and cumulative frequency curve of earthquakes before and after the mainshocks. The earthquakes preceded by active
foreshocks are: the 2006 (Mw8.3) and 2007 (Mw8.1) offshore Simushir earthquakes, the 1963 (Mw8.6), 1991 (Mw7.6), 1995
(Mw7.9) offshore Urup events, the 1978 (Mw7.8) offshore Iturup event, the 1969 (Mw8.2) offshore Shikotan event, the 1989
(Mw7.4) offshore northern Sanriku event. In contrast, M>7.5 interplate earthquakes offshore Hokkaido to northern Sanriku in
1952 (Mw8.1), 1968 (Mw8.3), 1973 (Mw7.8), 1994 (Mw7.8), 2003 (Mw8.1), and intraslab earthquakes in 1958 (Mw8.4), 1978
(Mw7.8), 1993 (Mw7.7), 1994 (Mw8.3) had few or no foreshocks.
Some results from our examination of the foreshock sequences are as follows. Fitting the ETAS model (Ogata, 1988, 1992)
to foreshock sequences show that the active foreshocks were composed of large foreshocks and their aftershocks. Foreshocksof the 2007 Kuril outer-rise earthquake were interpreted as aftershocks of the 2006 interplate earthquake. Relocated foreshocks
show that they migrate in various, not unique, directions. Distributions of foreshock do not overlap with the large coseismic slips(asperities) of the mainshocks of interplate earthquakes.
Relocation of foreshocks and maishocks were made by the modified JHD method and time-difference grid-search method(Hurukawa & Harada, 2014). The coseismic slip distributions were estimated by the teleseismic body-wave inversion (Kikuchi& Kanamori, 2003).

Keywords: northern Japan Trench to Kuril Trench, Remarkable foreshock activity, ETAS model, Modified Joint Hypocenter
Determination method, time-difference grid search method, teleseimic body-wave inversion

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SSS32-P07

Room:Convention Hall

Time:May 26 18:15-19:30

Precursory seismic activity surrounding the source region of the 1968 Tokachi-oki earthquake
HIRATSUKA, Shinya1∗
1

ISV, Faculty of Science, Hokkaido Univ.

The 1968 Tokachi-oki earthquake (Mw8.2) occurred off the Pacific coast of Aomori prefecture and ruptured northern andsouthern asperities. 26 years later, the 1994 Sanriku-haruka-oki earthquake (Mw7.7) occurred near the epicenter of 1968 eventand ruptured only its southern asperity [Nagai et al. (2001)]. According to Sato et al. (1996), the rupture process of the 1994
event was very similar to the earlier stage rupture process of the 1968 event. The question arises, ”Why did the 1994 event not
rupture the northern asperity of 1968 event”? In order to address this question, we investigated the long-term seismicity patternwith reference to the slip distribution of the 1968 Tokachi-oki (Mw8.2) and 1994 Sanriku-haruka-oki (Mw7.7) earthquakes. We
used the earthquake catalogue compiled by the Japan Meteorological Agency (JMA) for the past 90 years since 1923.
There are two major clusters that are considered to be important for characterizing the spatio-temporal seismicity pattern inand around the source region of the 1968 event. The one is a cluster of events located off the Pacific coast of Iwate prefecture
between the Japan Trench and the southern asperity of the 1968 event. We call this cluster the ”east-west trending seismic activity”, because it is distributed along the east-west direction. The other is a cluster of events located off the Pacific coast of Iwate
prefecture between the 10 and 20km depth contours of the upper interface of the subducted Pacific plate. We call this cluster the”southern seismic activity”, because it is located southern side of the southern asperity of the 1968 event. The epicentral area ofthe southern seismic activity include the rupture zone of the 1989 and 1992 Sanriku-oki earthquakes, which are regarded as theultra-slow earthquake by Kawasaki et al. (1995, 1998, 2001).
The 1931 Iwate-oki earthquake (M7.2) occurred off the Pacific coast of Sanriku and ruptured the southern asperity of the1968 event [Yamanaka and Kikuchi (2004)]. The 1931 event was preceded three years earlier by a M7.0 event that occurred
about 30km to the west. 4 years later, a M6.9 earthquake occurred very close to the epicenter of the M7.0 event in 1935. The
1933 activity in the zone of east-west trending seismic activity consists of nine large earthquakes (M>6.0) with the largest of
M7.1. Three of the events initially occurred in the eastern part of the zone, then expanded into the west. The 1933 Sanriku-oki
earthquake (M8.1) occurred in the outer-rise region off the Pacific coast of Iwate prefecture. The 1941 activity in the zone ofsouthern seismic activity consists of four large earthquakes (M>6.0). Two of the events occurred in the eastern part of the zone,
then the other two events occurred in the western part of the zone. A strong swarm activity (including eight M>6.0 earthquakes)
occurred in the zone in 1952. The 1960 Iwate-oki earthquake (M7.2) occurred off the Pacific coast of Sanriku and ruptured thesouthern periphery of the southern asperity of the 1968 event. The rupture propagated to the deep direction from the hypocenter[Yamanaka and Kikuchi (2004)]. A 1945 event (M7.1) occurred off the Pacific coast of Aomori prefecture and ruptured thenorthern asperity of the 1968 event. The 1945 event was preceded two years earlier by a M7.1 event that occurred about 40km to
the east. On March 22, 1944, M6.1 event occurred in the region sandwiched by the northern and southern asperities of the 1968
event. This event may contribute to weaken the strength of this region and allowed to propagate the rupture from the southern tonorthern asperities when main shock of the 1968 event occurred.
References:
Kawasaki et al., 1995, JPE, 43, 105-116.
Kawasaki et al., 1998, zisin 2, 50, 293-307.
Kawasaki et al., 2001, Tectonophysics, 330, 267-283.
Nagai et al., 2001, zisin 2, 52, 267-289.
Sato et al., 1996, GRL, 23, 33-36.
Yamanaka Y. and M. Kikuchi, 2004, JGR, 109,B07307,doi:10.1029/2003JB002683.

Keywords: 1968 Tokachi-oki earthquake, 1994 Sanriku-haruka-oki earthquake, 1933 Sanriku-oki earthquake, precursory seismic
activity, asperity

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SSS32-P08

Room:Convention Hall

Time:May 26 18:15-19:30

Seismic activity around the upper surface of the Pacific slab beneath Kanto
HIKITA, Masahiro1∗ ; NAKAJIMA, Junichi1
1

Tohoku University

Kanto is known as a unique region characterized by the subduction of two oceanic plates, the Philippine Sea plate and thePacific plate, and intensive seismicity occurs along the upper surfaces of the two subducting plates (e.g., Uchida et al., 2007).
In particular, many interplate earthquakes occur along the upper surface of the Pacific slab from the north of the Tokyo bay tosouthern Ibaraki prefecture, forming an N-S-trending marked seismic cluster at depths of 60-90 km, and the seismic cluster iscomposed of many isolated sub-clusters. In this study, we apply double-difference location method (Waldhauser and Ellsworth,
2000) to a large number of catalog-derived differential arrival-time data, and relocate sub-earthquakes in the N-S-trending cluster.
Delineation of detailed distribution hypocenters around the upper surface of the Pacific slab provides a clue to understand thecause of the marked seismic activity in this area.
The main results obtained in this study are as follows: (1) most of the sub-clusters have the thickness of 5-10 km. (2) small
repeating earthquakes and thrust-type earthquakes mainly occur at the middle and deeper part of the sub-clusters. (3) earthquakesthat occur at the shallow part of the sub-clusters have focal mechanisms different from low-angle thrust type. In the next step, we
will relocate hypocenters more precisely using waveform-derived differential arrival-time data, and investigate the seismogenesis
along the upper surface of the Pacific slab beneath Kanto.

Keywords: repeating earthquakes, Kanto, activity of earthquakes

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SSS32-P09

Room:Convention Hall

Time:May 26 18:15-19:30

Matched Filter Method implemented as an automatic hypocenter location system
OHMI, Shiro1∗
1

Disaster Prevention Research Institute, Kyoto University

Seismic activity near Mt. Hotaka in the Hida Mountains, central Japan was analyzed by using the Matched Filter Method(MFM). In this analysis, MFM was implemented as an automatic hypocenter relocation system. We selected about thirty (30)template earthquakes in the target region that enables us to detect more than 3,000 events and locate about 800 earthquakes inthe time period from April 2013 to October 2013. Comparison with manually inspected results indicates that location errors by
MFM system is within a couple of kilometers. The seismic activity in the target region started in April 2013 and most intense
activity occurred in October 2013. The largest event took place on October 8, 2013 at 19:28 (JST) whose magnitude was 3.9(JMA). Epicentral area extends about 4 km in EW direction with 1 km in NS direction at the eastern frank of Mt. Hotaka.
Although manually inspected catalogue data is essential to evaluate seismic activity, we suppose MFM is one of the powerfultools to automatically obtain preliminary results for the swarm activity that concentrated in a small area such like this study or
volcanic regions.
Keywords: Swarm activity, Hida Mountain range, Matched Filter Method

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SSS32-P10

Room:Convention Hall

Time:May 26 18:15-19:30

Relocation of the 1944 Tonankai earthquake and its aftershocks: The fault plane andcharacteristics of the seismicity
HURUKAWA, Nobuo1∗ ; HARADA, Tomoya2
1

Building Research Institute, 2 Earthquake Research Institute, University of Tokyo

We relocated the mainshock and its aftershocks of the 1944 Tonankai earthquake (M 7.9) using the modified joint hypocenter
determination (MJHD) method in order to obtain their accurate hypocenters and to identify the fault plane of the mainshock.
We used both P- and S-wave initial arrival times at stations worldwide reported by the Japan Meteorological Agency (JMA)
and International Seismological Summary (ISS). We confirmed by relocated hypocenters that the mainshock and many direct
aftershocks had occurred along the plate boundary between the Eurasian and Philippine Sea plates. We also confirmed thatthe eastern end of the aftershock area reached the Tenryu River, where induced shallow crustal earthquakes also occurred. The
aftershocks south of Shionomisaki and along the SE coast of the Kii Peninsula are crustal earthquakes induced by the mainshock.

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SSS32-P11

Room:Convention Hall

Time:May 26 18:15-19:30

Southern Extent of Seismicity in the Philippine Sea plate south of the Nankai Trough
NAKATA, Kenji1∗ ; KOBAYASHI, Akio1 ; HIRATA, Kenji2 ; YAMAZAKI, Akira3 ; TSUSHIMA, Hiroaki1 ;BABA, Hisatoshi4 ; USHIDA, Takashi4 ; ICHINOSE, Satomi4 ; ISHIHARA, Takanori4 ; INAMURA, Kazuya4 ;HASUZAWA, Tsuyoshi4 ; KATSUMATA, Akio1 ; MAEDA, Kenji1
1

Meteorological Research Institute, JMA, 2 National Research Institute for Earth Science and Disaster Prevention, 3 Kakioka
Magnetic Observatory, JMA, 4 Tokai University

Meteorological Research Institute (MRI) has deployed the pop-up type Ocean Bottom Seismometers (OBSs) in the area south
off Kii peninsula for several times to investigate seismicity in the area. Yamazaki et al. (2011, Tech. Rep. MRI) confirmed themicroearthquake activities at a depth of 10-25 km around the Nankai Trough by the four time operations since 2005. Hirata et al.
(2013 and 2012, JpGU meeting; 2012, SSJ meeting) carried out observations on the south of the Nankai Trough in 2010. They
found microearthquakes at a depth of around 10 km by using the 22 OBSs. It was considered that these microearthquakes wereclassified as seismic activities in the oceanic crust of the Philippine Sea Plate (PSP) (Obana et al., 2005, JGR).
Where is the southern limit of this seismic activity? To investigate the extent of the seismic activity, we conducted the OBS
observations in 2013 and 2014, in the area farther south of the 2010 network. The observation network was deployed in therange of 31.6-32.3N. We retrieved 10 OBSs in each 2013 and 2014 observations. First, clock time was corrected based on thetime differences at the deployment and the retrieval. Next, event waveform data were selected from the continuous data by anevent trigger. Finally, we picked the arrival times and amplitudes and determined the hypocenters with the method of Hirata andMatsu’ura, 1986. Here, we used the same velocity-structure model of P-wave velocity as Hirata et al. (2013) which was base on
Kodaira et al. 2000. Sediment layer correction was done using the arrival times of PS converted wave.
Magnitude and hypocenter of an earthquake were determined for the events inside the observation network of 2013 and 2014.
Magnitudes of events ranged in about M 0.0-0.5 and depths were estimated at about 10 km. This result shows that the same
type of microearthquakes occur as in the area of the 2010 observation. Numbers of events determined inside the observation
network of 2013 and 2014 are 36 and 23. Total number of the two operations is approximately half of the number of 112 in 2010.
The seismic activity of the microearthquakes in the southern area of the observation network is relatively lower than that in thenorthern area. Furthermore we investigated the hypocenters determined outside the observation network. As a result, there aresome hypocenters (M>=1) on the north of the network, however, there are not any hypocenters determined on the south of the
network. Therefore, it indicates that the southern extent of the seismicity of the inside PSP (M>=1) is around 31.6-31.9N in thearea of 135.3-136.3E.

We used short-period OBSs (4.5 Hz, 3-comp.). Deployment and retrieval of OBSs were done by the Keifu-maru and the
Ryofu-maru of JMA. Observation periods, area and numbers of retrieved OBSs were as follows,
2010: Period, from 12/June to 14/Sep./2010 (about 3 months); Number of OBSs, 22;
Area, south off Kii Peninsula (off Cape Shiono-misaki) (31.9-32.8N, 135.6-136.2E )2013: Period, from 12/July to 30/Sep./2013 (about 3 months); Number of OBSs, 10;Area, south off Kii Peninsula (far south of obs. in 2010) (31.8-32.3N, 135.8-136.3E )2014: Period, from 7/Aug. to 30/Oct./2010 (about 3 months); Number of OBSs, 10;Area, south off Kii Peninsula (west side of obs. in 2013) (31.6-32.3N, 135.3-135.8E )Keywords: seismicity, microearthquake, OBS, Nankai trough, outer rise

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SSS32-P12

Room:Convention Hall

Time:May 26 18:15-19:30

Imaging an active fault in the eastern Guadalquivir basin (Southern Spain) with highresolution seismic tomography
SERRANO, Inmaculada1∗ ; TORCAL, Federico2 ; MARTIN, Jose benito1
1Andalusian Institute of Geophysics, Granada University, Spain, 2 Pablo Olavide University, Seville, Spain

The Torreperogil seismic series took place in the Guadalquivir Basin (Southern Spain), a large flexural foreland basin with alinear ENE-WSW trending bounded to the north by the Iberian Massif and to the south by the Betic Cordillera and filled from a
middle Miocene to Plio-Quaternary sedimentary sequence characterized by a large number of low magnitude (below Mw 3.7 or
Md 3.9) and very shallow microearthquakes. We calculated the high resolution seismic velocity, Poisson’s ratio, crack density
and saturation ratio structures in and around the source areas of the Torreperogil seismic series (October 2012-April 2013).
In the upper layers of the crust, strong low-velocity anomalies are extensively distributed under the central zone, which together with high Poisson’s ratio and crack density values may correspond to rocks which are less likely to fracture, perhaps dueto the accumulation of tectonic and seismic stress. 93% of the earthquakes occurred at depths of up to 8 km, which could indicatethat the base of the seismogenic zone lies at this depth. The seismic series was concentrated in layers of strong structural heterogeneities (in the boundary area between low and high anomalies), which were likely to generate earthquakes due to differential
strain accumulation beneath the region. The high velocity areas are also considered to be strong yet brittle parts of the fault zone,
which are likely to generate earthquakes (at depths of between 5 km and 9 km). In contrast, low velocity areas are probably lesslikely to fracture, allowing seismic slippage to take place (from 2 to 4 km depth).
The best estimate of the depth of the main shock (mbLg: 3.9) is 7.6 km, which could tend to nucleate at the base of theseismogenic zone, at the ”fault end” on the boundary between a low velocity zone to the east and a high velocity zone to the west,
indicating the fault plane which separates both areas laterally. Assuming that this seismic contrast is one of the main Torreperogil
faults it could imply that stress has accumulated in an existing fault zone with lateral heterogeneity in velocity.

Keywords: Seismic tomography, Seismicity, Tectonic, Spain, Betic Cordilleras

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SSS32-P13

Room:Convention Hall

Time:May 26 18:15-19:30

Precursory seismicity change of the 2013 Nantou, Taiwan earthquake sequence revealedby ETAS, PI, and Z-value methods
KAWAMURA, Masashi1∗ ; CHEN, Chien-chih2 ; WU, Yih-min3
1

Dep. of Earth Sciences, National Taiwan Normal Univ., 2 Dep. of Earth Sciences, National Central Univ., Taiwan, 3 Dep. of
Geosciences, National Taiwan Univ.

ML 6.2 and ML 6.3 earthquakes occurred in the Nantou area of central Taiwan on Mar. 27, 2013 and June 2, 2013, respectively.
Because their epicenters are close to one another, we regard the March ML 6.2 and June ML 6.3 earthquakes as an event sequence.
To investigate precursory seismicity change of the Nantou earthquake sequence (or the March ML 6.2 earthquake), we applied
the Epidemic-Type Aftershock-Sequences model (ETAS model) to the earthquake catalog data of the Central Weather Bureau(CWB) covering broader Taiwan region. Application of more than one model to an earthquake catalog would be informative inelucidating the relationships between seismicity precursors and the preparatory processes of large earthquakes. Based on thismotivation, we further applied two different approaches: the pattern informatics (PI) method and the ZMAP method, which isa gridding technique based on the standard deviate (Z-value) test to the same earthquake catalog data of CWB. As a result, wefound that the epicenter of the 2013 ML 6.2 Nantou earthquake was surrounded by three main seismic quiescence regions prior
to its occurrence. The assumption that this is due to precursory slip (stress drop) on fault plane or its deeper extent of the ML 6.2
Nantou earthquake is supported by previous researches based on seismicity data, geodedic data, and numerical simulations using
rate- and state-dependent friction laws (Kawamura and Chen, 2013).
Keywords: Seismic quiescence, The Nantou earthquake, Stress accumulation, ETAS model, Pattern informatics, ZMAP

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