IAES-19 Kyoto December 9-12(In press)

ESTIMATION OF CRUSTAL STRUCTURE IN HORONOBE AREA,HOKKAIDO, JAPAN, BY USING MULTIPLET-CLUSTERING ANALYSISHIROKAZU MORIYA1), KOICHI ASAMORI2), ITARU KITAMURA3), HIKARU HOTTA3),
HIDEFUMI OHARA2*) AND TADAFUMI NIIZATO2)
1) Graduate School of Environmental Studies, Tohoku University, Sendai 980-8579, Japan2) Horonobe Underground Research Unit, Japan Atomic Energy Agency,
432-2 Hokushin, Horonobe-cho, Hokkaido, 098-3224, Japan
3) Construction Project Consultants, Inc., 3-23-1 Takadanobaba, Tokyo, 169-0075, Japan* Now at Kumagai Gumi Co., Ltd., 2-1,Tsukudo-cho, Shinjuku-ku, Tokyo 162-8557, JapanABSTRACT
Hypocenter locations of shallow earthquakes in Horonobe area, northern Hokkaido, Japan, which
is near the convergent boundary between Okhotsk and Amuria plates, were determined to reveal thesubsurface structure and the mechanisms of earthquake occurrence. The absolute source locations of211 earthquakes which occurred in the period from December 2002 to September 2005 weredetermined; then, those earthquakes with similar waveforms were identified, and the source locationsof 26 multiplet groups were relocated by using cross-spectrum and clustering analyses. The relocatedhypocenters allowed two seismically active areas to be identified, at 10–20 km and 25–30 km depth.
The earthquake locations indicate structures trending nearly N-S direction, and the structures causing
repeated stick-slips at asperities, thus generating similar earthquakes. The relationships between
magnitude and the distance to the next event, and between magnitude and the time interval of event
occurrences were investigated. The relationships between them suggest that earthquake swarms would
be induced by continuous strain accumulation at asperities along faults and its subsequent release as aresult of plate tectonic movements. A cutoff line could be also seen in the relationship between
magnitude and the distance to the next event, suggesting that the distance between the source locations
depends on the event magnitude. These results have given us the knowledge that the asperities on thedelineated faults intermittently release strain energy as similar earthquakes with magnitudes of lessthan about 3.0.
KEY WORDSSubsurface measurement, Multiplet-clustering analysis, Repeating similar earthquake

IAES-19 Kyoto December 9-12
(In press)

INTRODUCTION
The eastern edge of the Japan Sea approximately coincides with the convergent boundary between
Okhotsk plate and Amurian plates[1], and earthquakes are probably associated with stressaccumulation and its release by the fault system in the crust owing to the movement of the plates. GPSand microearthquake data suggest that the plate boundary lies under the land surface in northern
Hokkaido, although it is difficult to trace the plate boundary. The Hokkaido Nansei-Oki earthquake(M7.8), which occurred inside the mobile belt, suggests a complicated fault structure, in which the fault
planes incline to the east in its northern part and to the west in its southern part. Therefore, it isreasonable to suppose the existence of multiple boundaries in a wide fault zone [e.g. 2].
On the other hand, Horonobe area, northern Hokkaido, is in an area that a recently installed seismic
network has shown to be characterized by high seismicity [3]. Although the location accuracy in depth
is not still confident, the estimated source depth of the earthquakes is usually shallower than 25 km,
and the earthquakes are considered to occur along hidden faults in a stress-concentrated zone.
Knowledge of the structures causing these seismic clusters and understanding of their mechanisms are
expected to provide information on the stability of the crust and the relationships between plate
tectonics and earthquakes in this region.
Similar earthquake events and high-resolution mapping techniques can be used to reveal thesestructures and earthquake mechanisms [4, 5, 6, 7, 8, 9, 10, 11, 12]. In this study, similar earthquakes in
Horonobe area were relocated by using a multiplet-clustering analysis, and the subsurface structures
and the mechanisms of seismic clusters were evaluated.
EARTHQUAKE DATA COLLECTION
The earthquake events studied in this research were collected by the Hi-net system of the National
Research Institute for Earth Science and Disaster Prevention (NIED) and the seismic network of theJapan Atomic Energy Agency (JAEA) from 20 December 2002 to 30 September 2005 (Figure 1).
Most of the events had magnitudes of less than 3.0. Among the total of 4217 events located by Hi-net,
we searched for those events whose P- and S-wave arrival times were detected at more than twoseismic stations in the JAEA system (the JAEA system consists of four stations). A total of 221 eventswere selected for analysis in this study. Similar earthquakes were identified within the selectedearthquakes for application of multiplet-clustering analysis (MCA) (Figure 2) [13]. Our aim was toinvestigate whether seismic clusters and structures could be identified after improvement of thelocation accuracy. The vertical component signal at the JAEA downhole station, –97 m below sea level,
was used to identify similar waveforms, because the P-wave waveforms at this station were clearer thanthose at other stations. The waveforms of all events were inspected by eye, and finally, 188 similar
events were classified into 26 groups.
RELOCATION OF SIMILAR EARTHQUAKES USINGMULTIPLET-CLUSTERING ANALYSISFirst, the absolute source locations of the selected similar earthquakes were determined using thehypocenter determination software Hypomh, based on a three-layered velocity structure, where thefocal mechanism were also estimated [3, 14, 15] (Figures 3 and 4). The source locations were widelydispersed because of location errors of more than 1 km, and no structures could be identified.

IAES-19 Kyoto December 9-12
(In press)

Figure 1. Locations of seismic stations
around Horonobe, showing both those of
JAEA and Hi-net. The dotted box
denotes the area of the source locations
shown in Figure 3.

Figure 2. Example of the waveforms of amultiplet.

After determination of the absolute source locations, MCA was applied to relocate the source locations[8, 13]. In MCA, the source locations of events in each group are first determined relative to a masterevent in the group by a cross-spectrum analysis. The time-window length for the cross-spectrum
estimation was 1 s and that for the Fast Fourier transform was 2.56 s, where the frequency resolution
was 0.039 Hz. The frequency range for the time-delay estimation was 5–15 Hz, and the mean value ofthe time delays estimated for 10 different moving time windows was used to estimate differential times.
After the relocation of the events in each group, typical events were selected from each group, and thesimilarity of the waveforms was evaluated within the selected representative events. Events withsimilar waveforms, but with insufficient similarity to have been classified into the same group by the
initial classification, were selected, and a clustering analysis was applied to determine the relative
positions of the clusters.
DISCUSSION
According to the estimated source locations, the depths are less than 30 km (Figure 2), and two
seismically active areas (areas A and B) were identified. The seismic sources in area A were located ataround 30 km depth, and those in area B were shallower, at 10–25 km depth.
Tamura et al. (2003) [3] imply the existence of earthquakes deeper than 20 km. However, we thinkthat the absolute source location, especially the depth, should not be discussed in figure 2, because theIAES-19 Kyoto December 9-12
(In press)

station correction is not considered and the coverage of seismic network is not enough to determine theconfident velocity structure for the event locations. We doubted the low-frequency events for thereason of deep events. We have checked the source locations of low-frequency events, which areobserved since 2000 and recorded on Japan Meteorological Agency (JMA) earthquake catalogue in thisarea, and confirmed that the analyzed events in this study are different from the low-frequency events.
On the other hand, the result of MCA is confident because the source location is the relative locations,and the influence of velocity structure in determination of source locations is smaller. The average ofRMS in the relative locations is from 0.16 ms to 0.09 ms, and which corresponds to 0.8 km and 0.45
km in distance if the wave velocity is assumed to be 5 km/s. Therefore, we do not discuss the absolute
location of the sources and the structures.
Because similar events were observed, there must exist unstable structures with asperities causing
repeated stick-slips and continuous stress loading enforcing the repeated slips. The nodal planes (Figure4) suggest steeply inclined structures oriented NNW to NNE, and the P-axes are oriented nearly NNWSSE and NNE-SSW. These results indicate that compressive stresses oriented nearly E-W are acting
on faults with steep inclinations. Considering that Horonobe area is at the convergent boundary
between the North American and Eurasian plates, we infer that the district is under continuous stressloading in a nearly West to East direction, and strain energy accumulates on the fault planes as a resultof the loading. This interpretation is supported by movements of the land surface detected by GPS [e.g.
16]. Therefore, it is reasonable to infer that the groups of similar earthquakes are expressions of therelease of strain energy at faults oriented nearly N-S, and that the faults are subjected to compressive
stress in an E-W direction.
The relationship between the magnitude of each event and the distance to next event (Figure 5)
shows a cutoff magnitude (shown by a broken line), although the range of magnitude is limited (0 to3.0). The distance to the next source location of similar earthquakes is more than several hundreds ofmeters, and the distance between event locations is larger than the expected source radius of theearthquakes, suggesting that the distance between source locations depends on the event magnitudes
and that the source areas of similar earthquakes do not overlap. It is remarkable that a similar
phenomenon has been observed along faults in California, where multiplets have been observed andstreaks of earthquakes identified [6]. In the case of creeping faults, lock and release at asperities
generates repeated earthquakes. Our results suggest that asperities release strain energy, and no moreshear slip is induced at the same asperities. This means that strain energy accumulates at several
asperities distributed along the fault because of the regional stress loading, and that the strain energy isreleased at the different asperities one after the other.
Figure 6 shows the relationship between magnitude and the time interval between the occurrence ofsimilar events for the earthquakes in areas A and B. In area A, the maximum magnitude was nearly 2.0,
and it did not vary in relation to time intervals of more than 10 hours. On the other hand, in area B, themaximum magnitude increased with the time interval up to magnitude 3.0.
These results suggest that the maximum values of the strain energy release rate and magnitude arelimited in area A, where the size of asperities and the frictional strength of asperities are limited. On
the other hand, the asperities in area B seem to have different size of asperities or different frictional
strength, and the range of released energy is extensive than that of the asperities in area A.
This difference between the two areas could not be definitively explained, but is assumed to berelated to size and strength of asperities and also conditions around the faults such as temperature androck properties changing with depth. These results give us the key knowledge that the asperities on thedelineated faults intermittently release strain energy as similar earthquakes, and that the feature ofsimilar earthquake occurrence such as relationship between magnitude and time interval is different
depending on the seismic clusters.

IAES-19 Kyoto December 9-12
(In press)

Figure 3. Source locations of analyzed
earthquakes, where the locations are determinedby “hypomh”

Figure 4. Source locations of analyzed
earthquakes, where the cross denotes thelocations by “hypomh” and the closed circle
and the hexagon denote the source locationsafter relocation.

IAES-19 Kyoto December 9-12(In press)
4

Events in Area A
Events in Area B

Distance to next event (km)2108642186420.1

860

123

Magnitude

Figure 5. Plot of the distance to the next earthquake as a function of themagnitude of the previous event for each consecutive pair of similar earthquakes.
44Time interval vs. Magnitude
Events for16 groups are analyzed

Time interval vs. Magnitude
Events for16 groups are analyzed

Area B

3322

Magnitude

Magnitude

Area A

100-1-4
10110-2100102Time interval (hours)

104-1-4
1010-2100102Time interval (hours)(a)(b)
Figure 6. Magnitude and time interval of event occurrence for each consecutive pairs ofsimilar earthquakes. (a) at Area A and (b) at Area B.

104

IAES-19 Kyoto December 9-12(In press)

SUMMARY
In this paper, similar earthquakes in Horonobe area were relocated by multiplet-clustering analysis
to reveal the mechanisms of earthquake swarms in this district. The seismic clusters revealed after therelocation allowed two high seismicity areas to be identified, at 10–20 km and around 25–30 km depth.
The relationships between magnitude and the distance to the next event, and magnitude and the timeinterval between event occurrence were investigated. A cutoff line was seen in the relationship between
magnitude and distance to next event, suggesting that the distance between the source locations
depends on the event magnitude and that the source areas of similar earthquakes do not overlap. The
relationships between them have suggested that earthquake seismic clusters would be induced bycontinuous strain accumulation along faults and its subsequent release as a result of plate tectonic
movements. These results have given us the key knowledge that the asperities on the delineated faults
intermittently release strain energy as similar earthquakes, and that the feature of similar earthquake
occurrence such as relationship between magnitude and time interval is different depending on theseismic clusters.
ACKKOWLEDGEMENT
The earthquake waveform data was downloaded from the web site of Hi-net owned by National
Research Institute for Earth Science and Disaster Pretension (NIED), Japan, and the information on thesource location was used from the data base of Japan Meteorological Agency (JMA). We thank NIEDand JMA for providing important waveform and source location data.
REFERENCES
1) G. F. Sella, T. H. Dixon and A. Mao, REVEL: A model for Recent plate velocities from space
geodesy, J. Geophys. Res., 107(B4)(2002), 10.1029/2000JB000033.
2) M. Kasahara, Chikyu Monthly, 20(1998), 16-21. (in Japanese)
3) S. Tamura, M. Kasahara and T. Moriya, The micro-seismicity and crustal structure in the northernpart of Hokkaido, inferred from temporal observation, J. Seism. Soc. Japan, 55(2003), 185-193. (inJapanese)
4) G. Poupinet, W. L. Ellsworth and J. Fréchet, Monitoring velocity variations in crust using
earthquake doublets: an application to the Calaveras fault, California, J. Geophys. Res., 89(1984),5719–5731.
5) H. Moriya, K. Nagano and H. Niitsuma, Precise source location of AE doublets by spectral matrix
analysis of triaxial hodogram, Geophysics, 59(1994), 36–45.
6) D. Schaff, G. C. Beroza and B. E. Shaw, Postseismic response of repeating aftershock, Geophys.
Res. Lett., 25(1998), 4549–4552.
7) A. M. Rubin, G. Dominique and J. Got, Streaks of microseismicities along creeping faults, Nature,400(1999), 635–641.

IAES-19 Kyoto December 9-12
(In press)

8) W. S. Phillips, Precise microearthquake locations and fluid flow in the geothermal reservoir at
Soultz-sous-Forêts, France, Bull. Seism. Soc. Am., 90(2000), 1, 212–228.
9) F. Waldhauser and W. L. Ellsworth, A double-difference earthquake location algorithm: Method andapplication to the North Hayward fault, California, Bull. Seism. Soc. Am., 90(2000), 6, 1353–1368.
10) C. A. Rowe, R.C. Aster, B. Borchers and C.J. Young, An automatic, adaptive algorithm forrefining phase picks in large seismic data sets, Bull. Seism. Soc. Am., 92(2002), 1660-1674.
11) T. Matsuzawa, N. Uchida, T. Igarashi, T. Okada and A. Hasegawa, Repeating earthquakes andquasi-static slip on the plate boundary east off northern Honshu, Japan, Earth Planets Space,56(2004), 803–811.
12) N. Uchida, A. Hasegawa, T. Matsuzawa and T. Igarashi, Pre- and post-seismic slow slip on theplate boundary off Sanriku, NE Japan associated with three interplate earthquakes as estimatedfrom small repeating earthquake data, Tectonophysics, 385(2004), 1–15.
13) H. Moriya, H. Niitsuma and R. Baria, Multiplet-clustering analysis reveals structural details within
the seismic cloud at the Soultz geothermal field, Bull. Seism. Soc. Am., 93(2003), 1606–1620.
14) N. Hirata and M. Matsu’ra, Maximum-likelihood estimation of hypocenter with origin timeeliminated using nonlinear investigation technique, Phys. Earth Planet. Inter., 47(1987), 50-61.
15) P. Reasenberg and D. Oppenheimer, U.S. Geol. Surv. Open File Rep., 109(1989), 85-0739．
16) T. Sagiya, A decade of GEONET: 1994 – 2003 - The continuous GPS observation in Japan and its
impact on earthquake studies -, Earth Planets Space, 56(2004), xxix-xli.