スライド 1

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1 The 6th Joint Meeting of the UJNR Panel on Earthquake Research, 2006/11/9-10 Kazushige Obara,Yoshihiro Ito, Shutaro Sekine, Hitoshi Hirose and Katsuhiko Shiomi NIED) Deep slow earthquake families in the Nankai Trough subduction zone: non-volcanic tremor, slow slip event and very low frequency earthquake

Slow Earthquake Families in Nankai Zone Shallow Very Low Frequency Eq (S-VLFE) Seismic slip in accretionary prism Short-term Slow Slip Event (S-SSE) Aseismic stick-slip on the plate boundary Deep

2 Slow Earthquake Families in Nankai Zone Shallow Very Low Frequency Eq (S-VLFE) Seismic slip in accretionary prism Short-term Slow Slip Event (S-SSE) Aseismic stick-slip on the plate boundary Deep Very Low Frequency Eq (D-VLFE) Deep Low Frequency Tremor Seismic slip of small patch on S-SSE fault plane Micro crack failure around S-SSE Nankai Trough Shallow VLFE Accretionary prism Decollement Seismogenic zone Transition zone Philippine Sea plate Long-term SSE Deep LF Tremor Deep VLFE Short-term SSE Stable sliding

3 Source time duration (or predominant time period) of slow earthquakes in Nankai subduction zone Shallow VLFE Ito, P d 100d 10d 1d 0.1d 1000s 100s 10s 1s 0.1s Long-term SSE Deep (0.5~5years) Short-term SSE (2~5days) VLFE LF Tremor included (LF Earthquake)

4 NIED Hi-net (800stations) High-quality dense seismic network Tremor with Tiltmeter Slow Slip

5 Epicentral distribution of tremor Deep tremor NIED Hi-net stations Iso-depth contour of intra-slab seismicity Tokai Tonankai Nankai anticipated seismogenic zone

6 Tremor activity in Shikoku (NE) Three major segments East 3mon. Center West 3mon. 6mon. (SW)

7 Tremor and Tilt change in the western part of Shikoku 5days 5days 5days 6days (Time window :14days)

8 Tilt change and Tremor migration in W. Shikoku (NE) (SW) (NE) HIYH [km] (SW) (NE) (SW)

9 August 6-8 August

10 Cross sectional view of the tremor Tremor Regular Earthquake Based on JMA catalog

11 Cross sectional view of the tremor Short-term SSE plate boundary from reflection survey (Baba, 2002) Oceanic Moho from receiver function analysis(shiomi et al.,2004) Tremor Regular Earthquake Based on JMA catalog

12 Tremor energy release in the active stage with short-term term slow slip in west of Shikoku Aug.-Sep The periodicity of 12 hours == effect of earth tide

13 Tremor triggered by M7.6 Indonesia earthquake P T-phase Tremors Surface wave

14 Observational result of short-term slow slip Occur and migrate with active tremor recur with a period of 6 months (W. Shikoku, Kii, Tokai) slip duration of 2~5 days Reverse faulting at the depth of the plate boundary Stick-slip on plate boundary Scenario of short-term slow slip Locked for 6 months at the transition zone Strain accumulation due to the slab descending Weakening of frictional strength by fluid from dehydration Slip event occurs periodically as a stable process Control factor plate motion velocity, fluid generation rate, segment size, overriding structure, etc

15 Observational result of tremor Activated by slow slip affected by earth tide migrate with slow slip Isolated tremor with large amplitude occurs at plate boundary (Shelly et al.,2006),with thrust type focal mechanism (Ide et al.,2006) Minor tremor frequently occurs Triggered by teleseismic wave Miyazawa et al.,2006 Chain-like crack failure in inhomogeneous layer around plate boundary generated by underplating Scenario of tremor Micro crack failure triggered by sudden increasing of fluid pressure by occurrence of slow slip Minor tremor triggered by small fluctuation of fluid pressure by teleseismic wave

16 Migrating tremor and slow-slip event in Tokai and Kii area, January 2006 (NE) Kii Ise Bay Tokai (SW) January, 2006 Vel.=10km/day Total M ~ 6.2

17 Tremor activity in Kii and Tokai area (NE) Tokai 6mon. Ise Bay Kii 6mon. 3mon. (SW)?

18 Migration of tremor in Kii area (NE) Tokai Ise Bay Kii (SW) Start point of tremor activity with short-term slow slip detected by NIED tiltmeter

19 Interpretation of the episode 2006 in Tokai Small ridge structure of Philippine Sea plate at Ise Bay The ridge acts as a barrier. Usually, tremor and SSE occur on both sides of ridge independently. In the Kii segment, tremor usually starts from the Ise Bay. Episode 2006 started from the south end of the Kii segment, migrated to NE, and passed over the small ridge. Shiomi P23

20 Deep Slow Earthquakes in Nankai Zone Deep tremor NIEDHi-net stations Iso-depth contour of intraslab seismicity Recurrence interval 6mon. 3mon. 6mon. 3mon.? 6mon. Tonankai Tokai Nankai Anticipated Seismogenic zone Short-term slow slip event Duration: 2~5 days Long-term slow slip event Duration:0.5~5 years

21 Deep Very Low Frequency Earthquake

22 Seismic records of tremor

23 Superposing long period signals Radial-comp.

24 Clear long period signals Very Low Frequency (VLF) earthquake Radial-comp. Apparent Vel.~6km/s

25 Spectra of VLF earthquake Velocity spectrum(m) VLF noise Tremor Predominant freq. = 20 sec Frequency(Hz)

26 CMT solution of VLF earthquakes in 2006 Focal mechanism = Thrust type Focal depth = 30~40km VLF earthquake Slab surface Tremor

27 Deep VLF earthquakes with tremor and slip of episode 2006 in Tokai and Kii area VLF earthquake Tremor Tilt

28 Deep VLF earthquakes with tremor and slip of episode 2006 in W. Shikoku VLF earthquake Tremor Tilt

29 Observational result of deep VLF earthquake Occur and migrate with active tremor and short-term SSE Predominant in 20 seconds and radial component Apparent velocity ~ 6km/s Thrust type focal mechanism near plate boundary Seismic slow slip of small patch surrounded by short-term slow slip fault plane Scenario of deep VLF earthquake strain accumulation at small patch due to slow slip on surrounded area seismic slow slip with high fluid pressure No VLF without SSE period

30 Conclusion Three types of slow earthquakes in transition zone Short-term slow slip Stick slip on the plate boundary Deep VLF earthquake seismic slow slip at small patch Deep LF tremor chain-like crack failure reflecting inhomogeneous structure in transition zone

33 100 10

34 NE SW SW NE NE [km] NE 200 SW 1/1 4/1 7/1 10/1 1/1 4/1 7/1 10/1 1/1 4/1 7/1 10/1 1/1 4/ SW 100 0

35 Hi-net

36 Hi-net

37 Relationship between migration of tremor and tilt [Winter] Tilt vector :S-down E-down Tremor : NE SW [Summer] Tilt vector :SE-down S-down Tremor : SW NE

38 JMA,2006 C B A Hi-net

40 Improvement of Detection Capability Before Hi-net After Cumulative Number Magnitude

41 Start point of tremor activity in W. Shikoku Rupture initiation of short-term SSE Pattern A Pattern B Pattern B Long-term slow slip Start point of tremor activity with short-term slow slip detected by NIED tiltmeter

42 Migration of tremor in W. Shikoku Pattern A NE SW km

43 Migration of tremor in W. Shikoku Pattern B SW NE km

44 Migration of tremor in W. Shikoku Pattern B Bungo Channel NE km

45 Segmentation of tremor and slow slip activity Segment size km 10km/day

46 One-hour record chart at the station IKTH in Shikoku Island, Southwest Japan (2001/8/17 4 a.m.) Time (min) Time (s)

47 Clustering of tremor activity Shikoku west center east Kii Tokai

48 Time [min] Time [min] Time [sec] Time [sec] 1 hour continuous seismograms observed at 3 stations in the western part of Shikoku 4 am, 17 August 2001 IKTH HIYH KWBH Time [sec] YNDH

49 3 days envelope traces in Shikoku

50 Outline - Tremor & Short-term slow slip episode in Tokai - Very low frequency earthquake -Summary

51 Migration of tremor in W.Shikoku Rupture process of short-term SSE Long-term slow slip Start point of tremor activity with short-term slow slip detected by NIED tiltmeter

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untitled World topography Most earthquakes occur in subduction zones Trench-Arc-Backarc system Landward side Seaward side Back- Fore- (Widiyantoro et al., 1999 EPSL) Seismicity cross section across Japan trench