Low-Frequency Tremor and Slow Slip Around the Probable Source Region

Proceeding of the UJNR 5th Earthquake Research Panel Meeting, 2004 1

Low-frequency tremor and slow slip around the probable source region of the Tokai Earthquake – A new indicator for the Tokai Earthquake prediction provided by uniﬁed seismic networks in Japan –

Noriko Kamaya (Seismological and Volcanological Department, JMA), Akio Katsumata (Meteorological College, JMA) and Yuzo Ishigaki ( Seismological and Volcanological Department, JMA)

Abstract

Enhancement of seismic networks in the Japan Islands of these years revealed occurrence of deep low-frequency continuous tremors of a belt distribution in the southwest Japan, where the subducting Philippine Sea plate reaches depths of 25–40 km. The source region of the tremor is assumed to correspond to boundary region between oceanic and island crusts close to mantle wedge. The east end of the belt tremor distribution is adjacent to the probable source region of the Tokai Earthquake. A slow-slip event has continued in the west of the source region of the Tokai Earthquake since 2000. Synchronous activation of the tremor and slow-slip speed was observed in 2003. This suggests some relationship between the slow-slip and the tremor activity. We expect that the tremor activity could be a new valuable indicator for the Tokai Earthquake prediction. The long duration of the tremor indicates existence of fluid relating to its generation. The most probable fluid is considered to be water produced by dehydration of chlorite and formation of clinopyroxene in the oceanic crust on the basis of data from high pressure and temperature experiments. The northern border of the belt distribution is possibly rimmed by the edge of the mantle wedge because serpentine formation absorbs fluid water.

1 Introduction 1965. The magnitudes are determined from maxi- mum displacement or velocity amplitudes [Tsuboi Since Oct. 1997, short-period seismic records ob- , 1954; Katsumata, 2004; Funasaki et al., 2004]. It tained at most of stations in the Japan Islands is recognized that the lowermost magnitude has have been transmitted to the Japan Meteorologi- continued to decrease year by year since 1970s. cal Agency (JMA). The JMA has processed those More than 130,000 hypocenters are located in and records to make a comprehensive seismic catalog around the Japanese Islands in 2003. Fig. 3 shows in Japan. The short-period instruments include numbers of hypocenter determinations in a year in those installed by universities, the National Re- the JMA (1926–Sep. 1997) and the uniﬁed (Oct. search Institute for Earth Science and Disaster 1997–) seismic catalogs. A level of one thousand a Prevention (NIED), the JMA, other national in- year had continued until mid of 1970s. Since then, stitutes, and local governments. This integrated introductions of high-sensitive seismometers, au- processing lowered the detectable magnitude of tomated digital processing systems and densely earthquakes. Moreover, deployment of the Hi-net distributed seismic networks have increased the [e.g., Okada et al., 2000], which is a dense net- number of located hypocenters. work of highly sensitive short-period instruments The enhanced detection ability revealed oc- installed by the NIED, improved the detectivity currence of low-frequency microearthquakes at remarkably. Hi-net data has been included to pro- depths of 20–40km in the Japan Islands. The duce the seismic catalog since Oct. 2000. JMA has distinguished those deep low-frequency Fig. 1 shows stations which contribute the cur- earthquakes in the seismic catalog since Oct. rent uniﬁed seismic catalog. Seven cable-type 1999. The deep low-frequency earthquakes are in- ocean-bottom seismometry systems are included dicated by red symbols in Fig. 2. Magnitude of in the network. most of those events are less than magnitude 2.0, Fig. 2 shows magnitude-time distribution since and many of those are less magnitude 1.0. Occur- Proceeding of the UJNR 5th Earthquake Research Panel Meeting, 2004 2

4 5 N

4 0 N

3 5 N

3 0 N

JMA N I ED 2 5 N Un i v e r s i t i e s GS J JAMS TEC 0 5 0 0 1 0 0 0 km O t h e r s 1 2 5 E 1 3 0 E 1 3 5 E 1 4 0 E 1 4 5 E

Figure 1: Stations which contribute the current uniﬁed seismic catalog. Proceeding of the UJNR 5th Earthquake Research Panel Meeting, 2004 3 rence of low-frequency events around the Moho work. We describe the activity of the deep tremor, discontinuity in the areas away from volcanoes and discuss its source characteristics in this arti- became recognized based on the seismic catalog cle. In addition to the deep tremor, low-frequency [Nishide et al., 2000]. Although seismicity of low- earthquakes around the Moho discontinuity were frequency events beneath volcanoes has been well- found not only in volcanic regions but also in known [e.g., Hasegawa et al., 1991; Hasegawa and non-volcanic areas. We use a term of deep low- Yamamoto, 1994; Ukawa and Obara, 1993], seis- frequency earthquake (LFE in this article) to de- mic activities away from volcanic areas had not scribe such activities. been noticed. Nishide et al. (2000) recognized low-frequency seismic activities in the east of Aichi Prefecture 2 Seismicity of Deep Low- (34.8◦ ∼ 35.3◦N, 137◦ ∼ 138◦E. See Fig. 8), Frequency Tremors and which was recognized as a part of belt distribution Earthquakes of deep low-frequency tremor along the strike of the subducting Philippine Sea plate [Obara, 2002; In this section, we describe spacial and temporal Katsumata and Kamaya, 2003] after deployment characteristics of the LFT in the southwest Japan. of the Hi-net. Activities of LFE in volcanic and non-volcanic regions are also explained.

2.1 Source Locations of Deep Low- Frequency Tremors in the South- west Japan

The JMA estimates the source locations of LFT by an ordinary hypocenter determination method based on the onset times of enlarging parts of the tremors. This is different from the method of Obara (2002) who estimated the locations of LFT by applying S-wave velocity model to envelopes of tremor waveforms. Fig. 4 shows examples of the observed tremor waveforms. Broken curves in Figure 2: Magnitude-time distribution since 1965 the figure denote calculated arrival times based in the JMA and the unified seismic catalogs. Red small circles denote the events labeled as the deep low- on locations estimated by the JMA. Onsets of the frequency earthquakes. tremor are close to the calculated S travel times. Corresponding onsets of P-waves are seldom seen The deep low-frequency tremor (LFT in this ar- even on the vertical component. Lack of P-wave ticle) is the most notable phenomena which was arrival data makes accuracy of source depths poor. found by the recently deployed dense seismic net- Although most of LFT do not show onsets of P- wave, a small number of LFT segments do show

s 6 e 1 0 onsets of P-wave. Depth of those events are rather k

a 5 precise. Fig. 5 shows an epicenter map and a cross u 1 0 q

h 4 section of the deep low-frequency events with rel- t 1 0 r

a 1 0 3 atively small estimation errors in the southwest E

f 1 0 2 Japan. In the ﬁgure, orange circles show events o

r 1 0 1 occurring within 10 km (epicentral distance) from e

b 0 the centers of volcanoes which have been active in m 1 0 u

N the Quaternary [Committee for Catalog of Quater- 1 9 3 0 1 9 4 0 1 9 5 0 1 9 6 0 1 9 7 0 1 9 8 0 1 9 9 0 2 0 0 0 nary Volcanoes in Japan Y e a r , 1999], and blue circles show events away from volcanoes. The estimated Figure 3: Number of earthquakes listed in the JMA depths of the most of non-volcanic events were dis- (1926–Sep. 1997) and the uniﬁed (Oct. 1997–) seismic tributed from 25 to 40 km. and the average depth catalogs. is about 30 km. Proceeding of the UJNR 5th Earthquake Research Panel Meeting, 2004 4

T i me N / S 0 8 : 4 0 : 0 0 0 8 : 4 0 : 3 0 0 8 : 4 1 : 0 0 0 8 : 4 1 : 3 0 ( JST ) 0

NU . STN )

m 1 0 k ND . TOE (

e c n

a 2 0

t ND .MSK s

i YASUOK

D LG . HRN

l ND . KSH

a 3 0 r

t ND . ACH ND . TNR n

e TAK I SA c i 4 0 ND . SMY p E ND . KGW ND . HTN

5 0 P S P S P S

Figure 4: Example of LFT observed on June 3, 2001. Waveforms are arranged in epicentral distance order from an estimated source location. Broken lines show calculated P and S travel times for enlarging parts of the tremor. Station code are denoted on the right side. ND, NU, and LG signify the NIED, Nagoya University, and local government, respectively. YASUOK and TAKASA are stations of the JMA.

Fig. 6 show source locations of LFT (blue cir- escent periods have been kept for about half year cles) from September, 1999 to September, 2001 from the 4th quarter of 2003 to the 1st quarter of beneath eastern Shikoku (133.6◦ – 134.5◦E) pro- 2004. Then quiescent periods were prolonged in jected onto the vertical structural cross-section 2004, but they are still short compared with the from Kurashimo et al. (2002), which is based on average in 2002. The period of 2003–2004 cor- seismic velocities. The LFT are located in bound- responds to periods of relatively fast slow-slip in ary region between oceanic and island crusts close Tokai area, which is discussed in a latter section. to the mantle wedge. It is notable that the LFT are not distributed boundary regions be- In the region (4), short quiescent periods have tween oceanic crust and mantle wedge. been kept since middle of 2004. It seems that there has been some activity in the region. 2.2 Temporal Characteristics of the Deep Low-Frequency Tremor Ac- In the regions (8), (9) and (10), shortening of tivities quiescent periods appear relatively regularly. It takes about a few months from starting of short- Fig. 7 (b) shows time-space distribution of LFT. ening to returning to regular quiescent period of Most event show episodic activity. Active period about several tens of days. continues for several hours – several days. Fig. 7 (c) and (d) show accumulated numbers In the regions (11) and (12), state of short of events and quiescent periods of the tremor ac- quiescent periods was kept for a few months in tivities in partitioned areas shown in (b). The 2003. The shortening started from the beginning quiescent period in the ﬁgure is deﬁned as a time of 2003, and prolonging had continued until be- interval without any detected tremor events more ginning of 2004. Slow-slip events were observed in than two days. the period by Hirose and Obara (2003) in the re- It is recognized from Fig. 7 (b) and (c) that the gion. The graph of accumulated number of events tremor activities are not stationary. Accumulated also show a unusual rate in the period. numbers and quiescent periods changes with some trends. The activity of LFT should indicate some state In the region (2) (Fig. 7 (b)), shortening of of plate coupling. But its mechanism is to be in- quiescent periods started late in 2002. Short qui- vestigated. Proceeding of the UJNR 5th Earthquake Research Panel Meeting, 2004 5

(a) 1 9 9 9 / 0 9 / 0 1 - 2 0 0 4 / 0 8 / 2 3 (b) E 1

5 3 6 N 6

1 0

1 1

1 2 0 5 0 1 0 0 km W 1 3 2 E 2 0 0 0 2 0 0 1 2 0 0 2 2 0 0 3 2 0 0 4 (c) (d) 1 0 8 1 0 2 1 1 0 1 1 1 0 0 1 1 2 2 1 0 2 2 1 0 1 2 1 0 0 3 1 1 0 2 1 0 1 3 0 3 ) 1 0 2 3 8 7 y 1 0 r a 1 0 1 e 4 d 4 1 0 0 ( b 2 2 1 2 1 0 m 1 0 1 d u 5 0 5

o 1 0 N

2 2 8 1 i 1 0

r 1 d 6 1 0 6 e e 1 0 0 t 7 P 2

1 0 a 1 t l 7 1 0 7 1 0 0 n u 3 4 3 2 e 1 0 m 1 c u 8 1 0 8

s 0 c 1 0 e c 5 6 1 1 0 2 i A 1 0 1

9 u 9 1 0 0 8 0 9 Q 1 0 2 1 0 1 0 1 1 0 1 0 0 6 7 2 1 0 2 1 1 1 0 1 1 1 1 0 0 3 0 4 1 0 2 1 2 1 0 1 1 2 1 0 0

2 0 0 0 2 0 0 1 2 0 0 2 2 0 0 3 2 0 0 4 2 0 0 0 2 0 0 1 2 0 0 2 2 0 0 3 2 0 0 4

Figure 7: Temporal variation of the deep low-frequency tremor activities. (a) Epicenter map; (b) time-space distribution; (c) accumulated number; (d) quiescent period. Accumulated number of events and quiescent period are shown for regions of which numbers are denoted on the right of (b). Proceeding of the UJNR 5th Earthquake Research Panel Meeting, 2004 6

1 9 9 9 / 0 1 / 0 1 - 2 0 0 4 / 0 8 / 2 3

Honshu

Shikoku Kii Kyushu Chan. B Kii Penin. A TQWIJ 0CPMCK6 3 0 N 0 2 0 0 km 1 3 5 E 0

1 0

2 0 Figure 6: Source locations of LFT (blue circles) from 3 0 September, 1999 to September, 2001 beneath eastern 4 0 Shikoku (133.6◦ – 134.5◦E) projected onto the vertical 5 0 structural cross-section from Kurashimo et al. (2002), ( km) which is based on seismic velocities. The horizontal axis represents the distance from shot J2 of Kurashimo, Figure 5: Distribution of LFT and LFE in the south- et al. (2002) which was located near the south coast west Japan (Jan. 1999 – Aug. 2004). Blue circles, of Shikoku Island. Black dots denote ordinary earth- orange ones and red triangles show source locations of quakes. non-volcanic LFE and LFT, LFE close to volcanoes, and active volcanoes in the Quaternary, respectively. The line segments A and B indicate Tonankai and related to volcanic activity even in the cases away Nankaido profiles for which Hyndman et al. (1995) from volcanoes. If we include regions with igneous estimated geotherms, respectively. rocks in the Tertiaries, all LFE-active regions in the northwest Japan are covered. 2.3 Deep Low-Frequency Earthquakes in the Northeast Japan 2.4 Deep Low-Frequency Earthquakes in the Southwest Japan Deep low-frequency earthquakes have been found in wide area in the Japan Islands [Kamaya and In addition to deep low-frequency tremor activi- Katsumata, 2004]. Fig. 8 shows epicenters of the ties, there are some spot distributions of LFE in LFE. Orange circles show LFE occurring within the southwest Japan (gray-filled circles in Fig. 7 10 km (epicentral distance) from the centers of (a)). Some of those clusters are distributed along volcanoes which have been active in the Quater- the volcanic front of the Philippine Sea plate. But nary [Committee for Catalog of Quaternary Vol- some clusters are distributed in fore-arc side of the canoes in Japan, 1999], and blue circles show LFE volcanic front (clusters in 133◦–136◦E and 35.1◦– away from volcanoes which include LFT. The es- 35.7◦N). timated depths of the most of non-volcanic events Fig. 9 (a) and (d) show waveform and spec- were distributed from 25 to 40 km, and the aver- trum of a horizontal (NS) component from a low- age depth is about 30 km. Active volcanoes and frequency earthquake obtained at a station in the Quaternary volcanoes are shown by red tri- Tannan (DP.TNJ, 35.0313◦N, 135.2137◦E) for an angles. In the figure, contours show the tops of event which occurred in Kyoto Pref. (hypocenter: the Pacific and the Philippine Sea plates which 35.1185◦N, 135.5775◦E, 34.7km) at 01:41:41.1, were estimated as upper limits of seismicity in the July 30 2000 JST. Waveforms of events in this plate. region usually show a clear onsets and long du- LFE in the northeast Japan are distributed only ration. This is similar to waveforms of LFE in in the back-arc side of the volcanic front which is volcanic regions. almost coincides with a 110 km depth contour of The bold spectrum curve corresponds to the 30s the Pacific plate. There is no activities in the waveform segment indicated by the bold line in fore-arc side of the volcanic front. The distribu- the waveform. The thin spectrum curve corre- tion indicates the LFE in the northeast Japan are sponds to the noise level as calculated for the 30s Proceeding of the UJNR 5th Earthquake Research Panel Meeting, 2004 7

1 9 9 9 / 0 1 / 0 1 - 2 0 0 4 / 0 8 / 2 3

4 5 N 2 5 0 2 0105 0 1 0 0 5 0

4 0 N

PEJ

6TG

CP 2 5 0 ,CR 3 0 0 4 0 3 5 N 23 0 6 0 4 0 2 0 3 0 6 0 2 0 J QWI 6T MCK 0CP 0 5 0 0 km 1 3 0 E 1 3 5 E 1 4 0 E 1 4 5 E

Figure 8: Distribution of epicenters of low-frequency events near the Mohorovicic discontinuity under the Japan Islands as determined by the Japan Meteorological Agency (JMA) from September, 1999 to August 23, 2004. Orange circles show events within 10km of active volcanoes and volcanoes which have been active in the Quaternary [Committee for catalog of Quaternary volcanoes in Japan, 1999], and blue circles show events away from those volcanoes. Active volcanoes and the Quaternary volcanoes are shown by red triangles. Also shown are parts of depth contours for the upper surfaces of the Paciﬁc Plate and the Philippine Sea Plate. The contours are labeled with depth in km. Proceeding of the UJNR 5th Earthquake Research Panel Meeting, 2004 8

(a) 2 0 0 0 / 0 7 / 3 0 0 1 : 4 1 H : 3 4 km MI D KYOTO PREF DP . TN J NS DLT= 3 4 . 1 km 2 . 8

0 㨰㩖m / s P S - 3 0 - 2 0 - 1 0 0 1 0 2 0 3 0 4 0 ( s ) (b) 2 0 0 1 / 0 6 / 0 3 0 8 : 4 0 H : 3 2 km NE A I CH I PREF NU . STN NS DLT= 4 . 6 km 5 8 0 . 3

0 nm / s P S - 3 0 - 2 0 - 1 0 0 1 0 2 0 3 0 4 0 ( s ) (c) 1 9 9 9 / 1 0 / 1 0 1 3 : 4 6 H : 3 0 km NORTHERN AK I TA PREF TU . GJM NS DLT= 2 1 . 6 km 1 . 5

0 Ǵm / s P S - 3 0 - 2 0 - 1 0 0 1 0 2 0 3 0 4 0 ( s ) (d) (e) 2 0 0 0 / 0 7 / 3 0 0 1 : 4 1 H : 3 4 km MI D KYOTO PREF 2 0 0 1 / 0 6 / 0 3 0 8 : 4 0 H : 3 2 km NE A I CH I PREF DP . TN J NS DLT= 3 4 . 1 km NU . STN NS DLT= 4 . 6 km e e - 8 d d 1 0 u u t t - 8 i

i 1 0 l l p p m m A A

1 0 - 9 l l a a 1 0 - 9 r r t t c c e e p p - 1 0 S S 1 0

1 0 - 1 1 0 0 1 0 1 1 0 2 1 0 - 1 1 0 0 1 0 1 1 0 2 F r e q u e n c y ( Hz ) F r e q u e n c y ( Hz )

(f) 1 9 9 9 / 1 0 / 1 0 1 3 : 4 6 H : 3 0 km NORTHERN AK I TA PREF TU . GJM NS DLT= 2 1 . 6 km 1 0 - 7 e d u t i l

p - 8

m 1 0 A

l a r t

c - 9 e 1 0 p S

1 0 - 1 1 0 0 1 0 1 1 0 2 F r e q u e n c y ( Hz )

Figure 9: Spectra of deep low-frequency events in the Japan Islands. a) and d) A waveform and its spectrum of horizontal (NS) component from a low-frequency earthquake obtained at a station in Tannan (DP.TNJ, 35.0313N, 135.2137E) for an event which occurred in Kyoto Pref. at 01:41:41.1, July 30 2000. The bold spectrum curve corresponds to the 30s waveform segment indicated by the bold line in the waveform. The thin spectrum curve corresponds to the noise level as calculated for the 30s waveform segment indicated by the thin line in the waveform. H and DLT denote the focal depth and epicentral distance. b), c), e) and f) show waveforms and spectra for a low-frequency tremor in southwestern Japan and a low-frequency earthquake away from volcanoes in northeastern Japan, respectively. The records were obtained at Shin-Toyone (NU.STN, 35.1355N, 137.7437E) and Gojome (TU.GJM, 39.9520N, 140.1160E). Proceeding of the UJNR 5th Earthquake Research Panel Meeting, 2004 9 waveform segment indicated by the thin line in the tremor activity. We consider that the deep tremor waveform. The seismic wave from the event has activity would be a indicator of some status of the power in the frequency rage of 1–10 Hz. (b), (c), coupling between the plates. (e) and (f) in Fig. 9 show waveforms and spectra for a low-frequency tremor in southwestern Japan and a low-frequency earthquake away from volcanoes in northeastern Japan, respectively. The spectra of these events do not show no eﬀective spectral contents in a frequency range lower than 1 Hz. It is interesting that the deep tremor and LFE has common characteristics in spectrum.

3 Tokai Slow-Slip and the Tremor

A great earthquake is expected to occur along the Suruga Trough where the Philippine Sea plate Figure 11: Vertical component of crustal deformation due to the Tokai slow-slip observed by the GEONET, subducts beneath the Eurasian plate. The earth- GPS Earth Observation Network of the Geographical quake is named Tokai Earthquake prior to its oc- Survey Institute. currence. Fig. 10 shows the probable source region of the Tokai Earthquake.

rough

Suruga T

Probable Source Region of the Tokai Earthquake Figure 12: Temporal variation of location of the Hamakita GEONET station. Figure 10: Probable source region of the Tokai Earth- quake and epicenters of deep low-frequency events.

In the west of the probable source area, slow- 4 Source Characteristics of LFT slip events has been observed since 2000. Fig. 4.1 Source Force Direction of LFT 11 shows the vertical displacement measured by GEONET, GPS Earth Observation Network of It is important to know the source mechanism of the Geographical Survey Institute of Japan. Fig. the tremor to understand this phenomena. S-wave 12 shows temporal variation of crustal deforma- is dominant on the tremor waveforms. It is con- tion observed at Hamakita GEONET station. sidered that some kind of shear force causes the Relatively high-speed deformation is seen in tremor. 2003–2004 in Fig. 12. Active state of the deep We estimate source-force direction from particle tremor has been continuing in the northwest re- motions of S-waves with a method by Ukawa and gion of the slow-slip area since 2003 (the region 2 Ohtake (1987). We get S-wave polarization direc- in Fig. 7). The fast crustal deformation in 2003– tions from particle motions of every two seconds 2004 is simultaneous with the abnormal tremor ac- with overlapping of one second as shown in Fig. tivity since 2003. We suspect that there would be 13. Lower panel in Fig. 13 show particle motions some interactions between the slow-slip and deep on radial-transverse (R-T), vertical-radial (Z-R) Proceeding of the UJNR 5th Earthquake Research Panel Meeting, 2004 10 and vertical-transverse (Z-T) planes. Polarization Do u b l e Co u p l e data indicated by circles on the upper-right cor- 3 7 N ners of the chart of particle motions are used to estimate source mechanism. 3 6 N We estimate source force directions for time- intervals with enough number of polarization 3 5 N data. Source force directions were estimated for single-force and double-couples models. Fig. 14 3 4 N shows an example of a solution. Red line seg- 3 3 N ments show observed polarization directions, and 0 2 0 0 km color map show residual distribution. Region of 1 3 2 E 1 3 3 E 1 3 4 E 1 3 5 E 1 3 6 E 1 3 7 E 1 3 8 E 1 3 9 E 1 4 0 E 1 4 1 E small residual is rather concentrated. This indi- ( Low e r h em i s p h e r e ) cates that this kind of analysis can bring out in- Figure 15: Inferred P- or T-axes for a double-couple formations on the tremor source mechanism. model projected on a lower hemisphere of each region. Regions are shown with yellow rectangles. S F T : 1 N_ s t : 1 3 DC T : 1 N_ s t : 1 3 A I C : 2 9 . 1 4 A I C : 2 8 . 1 7 S i n g l e F o r c e

TAKNI SDA.MKB TAKNI SDA.MKB 3 7 N ND . KGW ND . KGW ND . NUK ND . NUK ND . HOU P ND . HOU ND . TOE ND . SMY ND . TOE ND . SMY NU . STN NU . TYD NU . S TN NU . TYD 3 6 N ND . ASH ND . ASH ND . HTN ND . HTN YASUOK YASUOK 3 5 N ND . ACH T ND . ACH

LOW LOW 3 4 N

3 6 4 9 6 3 3 2 4 4 5 6 3 3 N Re s ( d e g ) Re s ( d e g ) 0 2 0 0 km 1 3 2 E 1 3 3 E 1 3 4 E 1 3 5 E 1 3 6 E 1 3 7 E 1 3 8 E 1 3 9 E 1 4 0 E 1 4 1 E ( Low e r h em i s p h e r e ) Figure 14: Source force directions inferred from particle motions for single-force (SF) and double-couples Figure 16: Inferred force direction for a single-force (DC) models projected on lower hemispheres. model projected on a lower hemisphere of each region. Regions are shown with yellow rectangles. Inferred P/T axes or force directions are shown in Figs. 15 (double-couple mode) and 16 (single distribution as those in volcanic LFE. Although force model), respectively. In this analysis, P/T the region is in the fore-arc side of the volcanic axes are not distinguished because initial motion front and in non-volcanic areas, the mechanism polarities are not used. Solutions of single force of LFE in the region would be similar to that in (Fig. 16) show relatively clear foci than those for volcanic regions. double couple model. Source force directions of the tremor are distributed along NW-SE direction 4.2 Source Spectrum of the Tremor which is parallel to the direction of the Philippine Sea plate motion. This indicates that the motion It is well-known that LFT show peak spectrum of the Philippine Sea plate related to the tremor around 2–3 Hz. Fig. 17 shows an example of spec- activities. trum of the tremor. The spectrum is calculated On the other hand, many of source-force direc- directly from ground velocity record obtained with ◦ ◦ tions of events around Mt. Fuji (138.7 E, 35.3 N) a broadband instruments (STS-1) without instru- are nearly vertical. Those of events in the western mental response correction. The black bold and ◦ ◦ Tottori area (around 133.3 E, 35.3 N) show simi- blue thin curves show spectra of tremor-active lar tendency. Source-force direction of the tremor period and period without tremor, respectively. is clearly different from that of LFE in volcanic Spectrum level is clearly different in a frequency areas. range of 1–10 Hz. But no clear difference is seen It is also notable that non-volcanic LFE in in other frequency ranges. In the frequency range southwest Japan around 135◦E, 35◦N show similar of 0.1–1 Hz, high-level ground noise due to micro- Proceeding of the UJNR 5th Earthquake Research Panel Meeting, 2004 11

2 0 0 1 / 0 6 / 0 3 0 8 : 4 0 H : 3 0 km NE A I CH I PREF YASUOK YASUOK D= 2 3 . 9 km A= 3 4 . 3 d e g NS ( P ) ( S )

S 0 . 0 5 . 0 1 0 . 0 1 5 . 0 2 0 . 0 2 5 . 0 3 0 . 0 3 5 . 0 4 0 . 0 4 5 . 0 EW ( P ) ( S )

S 0 . 0 5 . 0 1 0 . 0 1 5 . 0 2 0 . 0 2 5 . 0 3 0 . 0 3 5 . 0 4 0 . 0 4 5 . 0 UD ( P ) ( S )

S 0 . 0 5 . 0 1 0 . 0 1 5 . 0 2 0 . 0 2 5 . 0 3 0 . 0 3 5 . 0 4 0 . 0 4 5 . 0 1 0 0 ( nm) 1 3 5 7 9 1 1 1 3 1 5 1 7 1 9 2 1 2 3 2 5 2 7 2 9 3 1 3 3 3 5 3 7 2 4 6 8 1 0 1 2 1 4 1 6 1 8 2 0 2 2 2 4 2 6 2 8 3 0 3 2 3 4 3 6

1 R1 3 2 . 6 T 2 R1 0 8 . 1 T 3 R1 1 6 . 9 T 4 R1 2 2 . 7 T 5 R1 2 0 . 0 T 6 R - 4 2 . 5 T 7 R1 3 0 . 3 T 8 R - 4 2 . 4 T 9 R0 . 7 T 1 0 R - 2 6 . 5 T R R R R R R R R R R

T T T T T T T T T T

Z Z Z Z Z Z Z Z Z Z

R R R R R R R R R R

Z Z Z Z Z Z Z Z Z Z

T T T T T T T T T T

5 0 ( nm) 2 0 ( nm) 2 0 ( nm) 2 0 ( nm) 2 0 ( nm) 2 0 ( nm) 1 0 ( nm) 1 0 ( nm) 1 0 ( nm) 1 0 ( nm)

Figure 13: Particle motions of the tremor. The upper panel shows waveforms of three components, and the lower panel shows particle motions on radial-transverse (R-T), vertical-radial (Z-R) and vertical-transverse (Z-T) planes of every two second period with overlapping of one second. seisms is recognized. In the range of 0.01–0.1 Hz directly. Water would alter physical properties of in which the ground noise is low enough, power of minerals, and may cause tremor directly by mov- the tremor is not recognized. ing or vibration. Hirose and Obara (2003) found slow-slip events We assume some possible sources of the water simultaneous with deep tremor activities. If which is related to the deep tremor. tremor is directly due to a slow-slip event, spectrum level at low frequency is higher than that 1. Extraction of interstitial water from oceanic at high frequency based on source spectrum mod- crust. els [e.g., Aki, 1967]. But no power is observed 2. Dehydration from minerals in the oceanic at frequency ranges of 0.01-0.1Hz. This indicates crust. that there would be different power source for the tremor from the slow-slip. 3. Dehydration from minerals in the mantle of the Philippine Sea plate. 5 Fluid Generation in the 4. Dehydration from minerals in the mantle wedge. Tremor Source Region Although extraction of interstitial water from The continuous tremors remind us of volcanic the oceanic crust might be a possible source, it tremors, which are caused by some kinds of fluid would be difficult to explain why the extraction [e.g., Chouet, 1996]. Deformation of ductile mate- occurs at the restricted depth range of 25–40km. rial may also show long duration like tremor. Since extraction of interstitial water should occur Since a large quantity of water is conveyed gradually, the limited depth range of water supply by subduction of the Philippine Sea plate, water is considered to be related to limited pressure and would be related to the deep tremor, directly or in- temperature condition. Proceeding of the UJNR 5th Earthquake Research Panel Meeting, 2004 12

2 0 0 4 / 0 6 / 2 7 1 6 : 0 0 - - 1 7 : 0 0 in the region to the right of line ”A”, water is

) released by dehydration of chlorite along this line. s / 1 0 - 9 On the other hand, the temperature proﬁle be- m (

neath Kii Peninsula crosses a thick line ”B” at e

d similar depth. Crossing this line, clinopyroxene u t 1 0 - 1 0

i appears. Because clinopyroxene is not a hydrous l

p mineral, the total water content of the basalt de- m

A creases. - 1 1 l 1 0 a r t c e

p 1 0 - 1 2 S

1 0 - 4 1 0 - 3 1 0 - 2 1 0 - 1 1 0 0 1 0 1 F r e q u e n c y ( Hz )

Figure 17: Spectrum of LFT observed at Asahi station (Fig. 18) of the F-net installed by the NIED. The black bold and blue thin curves show spectra of tremor- active period and period without tremor, respectively. rn Japan

aste

Northe

A s a h i la B ninsu Kii Pe ku A Shiko

3 5 N

Figure 19: Phase relationships for water-saturated mid-ocean ridge basalt [Schmidt and Poli, 1998] and temperature profiles for the upper surfaces of the Pa- 1 3 7 E 1 3 7 3 0 ’ E 1 3 8 E cific Plate under northeastern Japan [Iwamori, 1998] and the Philippine Sea Plate beneath the Kii Peninsula Figure 18: Station of Asahi (F-net of the NIED) and and Shikoku [Hyndman et al., 1995] in southwestern epicenters of LFT in June 2004 (red circles). Japan: amph = amphibole, chl = chlorite, cld = chlo- ritoid, cpx = jadeitic or omphacitic clinopyroxene, epi = epidote, gar = garnet, law = lawsonite, zo = zoisite. The temperature profile for the Philippine Sea Plate Fig. 19 shows the phase relationships for beneath Shikoku crosses a thick line ”A” at a depth water-saturated mid-ocean ridge basalt obtained of about 40km. On the other hand, the temperature by Schmidt and Poli (1998). This phase relation- profile beneath Kii Peninsula crosses a thick line ”B” ships are applicable to minerals in a oceanic crust at similar depth. in a subducting slab. The broken curves in the figure show geotherms at the top of the subduct- Fig. 20 shows Phase diagram for water- ing Philippine Sea plate estimated by Hyndman saturated average mantle peridotite, modified et al. (1995) for Tonankai and Nankaido profiles, from a figure by Schmidt and Poli (1998), and respectively (Fig. 5). Depth contours in Fig. 8 temperature profiles for the top of the Pacific were used to convert distance from the trench axis Plate under northeastern Japan [Iwamori, 1998] into depth at the top of the slab. The Philip- and the Philippine Sea Plate beneath the Kii pine Sea plate subducts with a steeper incline be- Peninsula and Shikoku [Hyndman et al., 1995] in neath Kii Peninsula than beneath Shikoku. The southwestern Japan. Serpentine and some other geotherms cross the phase boundary shown by the hydrous minerals including chlorite and amphi- heavy solid curve. Because chlorite can not exist bole in the mantle peridotite also dehydrate under Proceeding of the UJNR 5th Earthquake Research Panel Meeting, 2004 13 the conditions of 600–700◦C and 30–50 km. This and the mantle wedge due to a different mechan- indicates a possibility that hydrous minerals in ical condition from that between the inland crust the descending mantle wedge along with the slab and the oceanic crust. would also release water. Though, temperature at When the Philippine Sea plate reach at the the top of the mantle wedge beneath Shikoku was depth around 50km, melting of the slab would ◦ estimated at about 450-500 C by Hyndman et al. start. On the other hand, the lowest part of (1995). The temperature is considered too low for the mantle wedge, which consists of hydrous peri- serpentine in the mantle wedge to release much of dotite, descends with slab and release water be- water. cause of dehydration of serpentine and chlorite at around the depth of 60-80km. At deeper point, the hydrous peridotite starts melting partially. Such high temperature fluid or magma would

breakdown of serp cause LFE. Fig. 21 shows schematic view of the process that generates LFT and LFE in the southwest breakdown rn Japan of chl Japan.

aste

breakdown Northe of chl Water is produced by dehydration of breakdown Water is fixed Chlorite and formation of Clinopyroxene. of serp by forming serpentine.

sula enin Kii P Belt of LFT epicenters

Shikoku Oceanic Sediment Inland Crust LFT LFE Basalt Mantle Wedge ter idotite Wa Figure 20: Phase diagram for water-saturated av- e otite erage mantle peridotite, modified from a figure by us Per ogit Ecl Perid Schmidt and Poli (1998), and temperature profiles for Hydro the top of the Pacific Plate under northeastern Japan Dehydration of Serpentine Mantle [Iwamori, 1998] and the Philippine Sea Plate beneath Oceanic Dehydration of the Kii Peninsula and Shikoku [Hyndman et al., 1995] Crust Chlorite in southwestern Japan: ’A’ = phase A, amph = amphi- Partial Melting bole, chl = chlorite, cpx = clinopyroxene, gar = garnet, ol = olivine, opx = orthopyroxene, serp = serpentine, Figure 21: Schematic view of the process that gener- sp = spinel, tc = talc. The breakdown of serpentine ates LFT and LFE distributed within and of a belt and and of chlorite, which mean releasing water, are in- north of the belt in western Japan. Short arrows show dicated along the temperature profiles and the phase water being released from the descending Philippine boundaries. Sea Plate. A considerable amount of water is released by both the dehydration of chlorite and the formation LFT is observed in the limited range of slab of clinopyroxene in slab basalt, and the upwelling wa- top depth, 25–40 km. Since the amount of water ter would cause LFT. When the slab comes in contact from dehydration processes in the oceanic crust with the mantle wedge, the upwelling water is fixed as serpentine in the lowest part of the mantle wedge. would increase as temperature and pressure in- Therefore, LFT cannot occur because of the absence crease, a wider belt area would be expected. Ac- of fluid. The gray stars show the hypocentral area for cording to the structure shown by Kurashimo et LFT and LFE. al. (2002), descending oceanic crust starts contact with the mantle wedge under the northern Belt LFT activities are not recognized in the rim of the belt distribution (Fig. 6). Fluid water east of 138◦E line and in Kyushu Island. In addi- relating to the tremor would not be supplied to tion, a gap is recognized around the Kii channel the inland crust in the north of the rim because (Fig. 5). Since the activity is considered to be temperature in the mantle wedge would be low related to dehydration process of hydrous miner- enough for mantle peridotite to absorb ascending als such as chlorite, the inactive areas would in- water forming hydrous minerals. There would be dicate absence of dehydration process due to the another possibility that tremor would not occur following reasons. The Philippine Sea plate moves in the boundary region between the inland crust northwestward in this area [e.g., Seno et al., 1993], Proceeding of the UJNR 5th Earthquake Research Panel Meeting, 2004 14

and the plate moves almost along the iso-depth where Ui(ω), Uj(ω), G(ω) and c are observed spec- curves of the plate under the channel (Fig. 22). If tra at ith and jth stations, gauss filter [Langston, the dehydration process has almost done around 1979], and a constant called as water level. the Kii Peninsula before reaching this area, water Inverse Fourier-transformed function of H(ω) would not be supplied. On the other hand, in the should show a peak at a time of arrival-time differ- area east of 138◦E, there are some volcanoes, Mt. ence of stations of i and j. Fig. 24 shows examples Fuji, Mt. Hakone, and so on. Volcanic activities of the deconvolved functions. The original wave- would affect thermal condition and dehydration forms are shown in Fig. 23. Some of time series in process of the subducting plate. Fig. 24 show clear peaks. We take times of those peaks to locate the source location.

MO ND .MSK D l t = 2 0 . 6 A zm= 8 5 . 9 ND . HOU D l t = 2 1 . 8 A zm= 2 0 0 . 9 LG . HRN D l t = 2 5 . 8 A zm= 1 3 0 . 1 ND . ASH D l t = 2 8 . 2 A zm= 2 7 8 . 1 ND . KSH D l t = 2 9 . 0 A zm= 2 8 7 . 2 ND . ACH D l t = 3 2 . 8 A zm= 3 . 9 ND . SMY D l t = 3 9 . 6 A zm= 2 4 6 . 3 ND . HKW D l t = 3 9 . 9 A zm= 1 0 3 . 9 ND . KGW D l t = 4 5 . 1 A zm= 1 4 1 . 2

5 . 0 ( s )

Figure 23: Waveform data used for the deconvolution. Figure 22: Horizontal displacement vector map based on GPS measurements conducted by the Geographi- ND . ASH d l t = 2 8 . 2 2 ( km) t p k= 0 . 0 0 ( s ) pw i d= 0 . 2 1 ( s ) t p k ( 2 ) = 6 . 2 6 ( s ) cal Survey Institute [Earthquake Research Committee, 0 . 0 2 - 0 . 0 2 ND . KSH d l t = 2 9 . 0 4 ( km) t p k= 0 . 2 0 ( s ) pw i d= 0 . 1 9 ( s ) t p k ( 2 ) = 1 . 4 7 ( s ) 2001], and depth contours for the Philippine Sea Plate 0 . 0 1 7 0 6 - 0 . 0 1 ND . ACH d l t = 3 2 . 8 2 ( km) t p k= 1 1 . 1 4 ( s ) pw i d= 0 . 1 9 ( s ) t p k ( 2 ) = 0 . 9 1 ( s ) in southwestern Japan. Arrows show horizontal dis- 0 . 0 0 7 1 0 . 0 0 ND . SMY d l t = 3 9 . 6 7 ( km) t p k= 2 . 1 0 ( s ) pw i d= 0 . 2 1 ( s ) t p k ( 2 ) = 2 . 7 2 ( s ) placement vectors from May in 1997 to June 2000. The 0 . 0 1 7 2 - 0 . 0 1 fixed station is Fukui. Circles show epicenters of low- ND . HKW d l t = 3 9 . 9 4 ( km) t p k= 2 . 7 3 ( s ) pw i d= 0 . 2 1 ( s ) t p k ( 2 ) = 5 . 0 1 ( s ) 0 . 0 0 7 3 frequency events from September 1 in 1999 to February 0 . 0 0 ND . KGW d l t = 4 5 . 1 9 ( km) t p k= 1 . 6 1 ( s ) pw i d= 0 . 2 8 ( s ) t p k ( 2 ) = 5 . 1 1 ( s ) 0 . 0 0 7 4 20 in 2003. Open circles = Low-frequency events far 0 . 0 0 ND .MKB d l t = 4 5 . 8 7 ( km) t p k= 3 . 3 1 ( s ) pw i d= 0 . 2 1 ( s ) t p k ( 2 ) = 4 . 6 4 ( s ) 0 . 0 0 from active volcanoes and the Quaternary volcanoes, 7 5 7 0 . 0 0 KUROMA d l t = 4 7 . 0 7 ( km) t p k= 4 . 7 5 ( s ) pw i d= 0 . 2 9 ( s ) t p k ( 2 ) = - 0 . 2 1 ( s ) 0 . 0 0 Closed circles = Low-frequency events near active vol- 7 6 0 . 0 0 ND . HTN d l t = 4 7 . 2 1 ( km) t p k= 4 . 2 1 ( s ) pw i d= 0 . 0 9 ( s ) t p k ( 2 ) = 0 . 9 2 ( s ) 0 . 0 0 canoes and the Quaternary volcanoes. Contours are 7 7 0 . 0 0 ND . NUK d l t = 4 7 . 6 8 ( km) t p k= 3 . 4 6 ( s ) pw i d= 0 . 2 2 ( s ) t p k ( 2 ) = 4 . 1 4 ( s ) labeled with depth in km. 0 . 0 0 7 8 0 . 0 0 KAKEGA d l t = 4 9 . 8 7 ( km) t p k= 4 . 9 5 ( s ) pw i d= 0 . 1 1 ( s ) t p k ( 2 ) = 5 . 5 1 ( s ) 0 . 0 0 7 9 0 . 0 0 2 . 0 ( s ) 6 Locating the Tremor with De- Figure 24: Deconvolved waveforms. The top trace is a convolution Technique result of deconvolution by itself. The primary and sec- ondary peaks are marked by red and green segments. The JMA determines the source location of the tremor with an ordinary hypocenter determina- We determine the source locations as to reduce tion method based on onset times of enlarging square of residuals of the arrival-time differences parts of the tremors. With this method, we may as miss to detect the tremor with stationary ampli- obs cal 2 X{(tj − ti) − (tj − ti) } → min. tude levels. The unavailability of P-wave onsets makes the locating precision poor. We are de- The summation is taken over pairs of arrival-time veloping a tremor-locating method using deconvo- differences ti and tj. lution which often used to get receiver functions Fig. 25 shows an estimated location (an circle) [e.g., Ammon, 1991]. and iso-arrival-time-difference curves. We get a function of H(ω) as 7 Conclusions U (ω)U ∗(ω) H(ω) = j i G(ω), φ(ω) Unified processing of seismic data in Japan low- ∗ ∗ φ(ω) = max{Ui(ω)Ui (ω), cUi(ω)Ui (ω)} ered the detectable magnitudes of earthquakes, Proceeding of the UJNR 5th Earthquake Research Panel Meeting, 2004 15

t s d e p= 3 6 . 5 + / - 0 . 7 Sea plate.

ND . KGN It is assumed that the tremor is related to wa- ND . GER 2 6 4 3 5 4 0 ' N 2 9 ter which is produced by dehydration of chlorite KUROKA 2 6 2 3 and formation of clinopyroxene in the subducting 1 7 8 2 7 ND . ACH oceanic crust. The northern rim of the belt re-

7 8 gion seems to correspond to the edge of the man- 9181 4 3 5 2 0 ' N ND . KG I ND . HTN tle wedge. In the north of the edge, ﬂuid water ND . KSH E . OKY ND . ASH 1 8 7 would be depleted to form hydrous minerals such ND .MSK 3 9 as serpentine in the mantle wedge. This would ND . S I Z ND . HKW KUROMA ND . SMY LG . HRN be the cause of the relatively narrow width of the 3 5 N ND . HOU 2 1 ND . OKB belt distribution. ND . AN J ND . NUK ND . KGW

ND .MKB ND . HAZ KAKEGA Acknowledgments 2 7 6

ND . OHS 3 4 4 0 ' N ND . ABN We used seismic data from the National Re-

9 6 search Institute for Earth Science and Disaster 1 3 7 E 1 3 7 2 0 ' E 1 3 7 4 0 ' E 1 3 8 E 1 3 8 2 0 ' E Prevention, Hokkaido University, Hirosaki Uni- 0 5 0 km versity, Tohoku University, University of Tokyo, Nagoya University, Kyoto University, Kochi Uni- Figure 25: Iso-arrival-time-difference curves. Blue curves, red line segments and a circle show iso-arrival- versity, Kyushu University, Kagoshima University, time-difference curves, the pair of stations, the esti- the National Institute of Advanced Industrial Sci- mated source location, respectively. ence and Technology, Tokyo metropolitan government, Shizuoka prefectural government, Kana- gawa prefectural government, the City of Yoko- and activities of deep low-frequency earthquakes hama, the Japan Marine Science and Technology in non-volcanic regions became recognized. After Center, and the Japan Meteorological Agency. the development of the Hi-net in southwest Japan, deep tremor of a belt distribution along the strike of the subducting Philippine Sea plate was found. References Source locations of the deep tremors was estimated with onsets of P and S waves, and it was [1] Ammon, C. J., The isolation of receiver effects found that source depth ranged from 25 to 40 km from teleseismic P waveforms, Bull. Seism. Soc. and that they were distributed around the bound- Am., 81, 2504–2510, 1991. ary region between island and oceanic crusts near [2] Aki, K., Scaling law of seismic spectrum, J. mantle wedge. Geophys. Res., 72, 1217-1231, 1967. The east end of the tremor belt distribution is adjacent to the Tokai slow-slip area. The Tokai [3] Chouet, B. A, Long-period volcano seismicity: slow-slip has been continuing since 2000. Activity its source and use in eruption forecasting, Na- level of the tremor in the region has been kept high ture, 380, 309–316, 1996. since the beginning of 2003, which coincides with the period of relatively fast slow-slip. The region [4] Committee for Catalog of Quaternary Vol- is located in the west of the probable source area of canoes in Japan, eds., Catalog of Quater- the Tokai Earthquake. We expect that the tremor nary volcanoes in Japan (CD-ROM with map), activity could be a new valuable indicator for the The Volcanological Society of Japan, 1999 (in Tokai Earthquake prediction. Japanese). Source force directions inferred from polariza- [5] Earthquake Research Committee, On the tion of S-waves were found to be along the di- long-term evaluation of earthquakes in the rection of the Philippine Sea plate motion, which Nankai trough, 2001 (In Japanese). were clearly different from those of deep low- frequency earthquakes in volcanic regions. The [6] Funasaki and Earthquake Prediction Informa- lack of frequency components lower than 0.1 Hz tion Division, Revision of the JMA velocity indicates that the tremor is not a direct outcome magnitude, Quart. J. Seism., 67, 11-17, 2004 of the slow slip at the boundary of the Philippine (in Japanese with English abstract). Proceeding of the UJNR 5th Earthquake Research Panel Meeting, 2004 16

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