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 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
Source time duration (or predominant time period) of slow earthquakes in Nankai subduction zone Shallow VLFE Ito, P014 1000d 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)
NIED Hi-net (800stations) High-quality dense seismic network Tremor with Tiltmeter Slow Slip
Epicentral distribution of tremor Deep tremor NIED Hi-net stations Iso-depth contour of intra-slab seismicity Tokai Tonankai Nankai anticipated seismogenic zone
Tremor activity in Shikoku (NE) Three major segments East 3mon. Center West 3mon. 6mon. (SW) 2001 2002 2003 2004 2005 2006
Tremor and Tilt change in the western part of Shikoku 5days 5days 5days 6days (Time window :14days)
Tilt change and Tremor migration in W. Shikoku (NE) (SW) (NE) HIYH 0 50 100 150 200[km] (SW) (NE) (SW)
August 6-8 August 9-12 9
Cross sectional view of the tremor Tremor Regular Earthquake Based on JMA catalog
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
Tremor energy release in the active stage with short-term term slow slip in west of Shikoku Aug.-Sep. 2003 The periodicity of 12 hours == effect of earth tide
Tremor triggered by M7.6 Indonesia earthquake P T-phase Tremors Surface wave
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
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
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
Tremor activity in Kii and Tokai area (NE) Tokai 6mon. Ise Bay Kii 6mon. 3mon. (SW)?
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
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
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
Deep Very Low Frequency Earthquake
Seismic records of tremor
Superposing long period signals Radial-comp.
Clear long period signals Very Low Frequency (VLF) earthquake Radial-comp. Apparent Vel.~6km/s
Spectra of VLF earthquake Velocity spectrum(m) VLF noise Tremor Predominant freq. = 20 sec. 0.01 0.1 1 10 Frequency(Hz)
CMT solution of VLF earthquakes in 2006 Focal mechanism = Thrust type Focal depth = 30~40km VLF earthquake Slab surface Tremor
Deep VLF earthquakes with tremor and slip of episode 2006 in Tokai and Kii area VLF earthquake Tremor Tilt
Deep VLF earthquakes with tremor and slip of episode 2006 in W. Shikoku VLF earthquake Tremor Tilt
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
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
100 10
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 4/1 2003 2004 2005 2006 SW 100 0
Hi-net
Hi-net
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
JMA,2006 C B A Hi-net
Improvement of Detection Capability Before Hi-net After Cumulative Number Magnitude
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
Migration of tremor in W. Shikoku Pattern A NE SW 0 50 100 150 200 km
Migration of tremor in W. Shikoku Pattern B SW NE 0 50 100 150 200 km
Migration of tremor in W. Shikoku Pattern B Bungo Channel NE 0 50 100 150 200 km
Segmentation of tremor and slow slip activity Segment size 20 100km 10km/day
One-hour record chart at the station IKTH in Shikoku Island, Southwest Japan (2001/8/17 4 a.m.) Time (min) Time (s)
Clustering of tremor activity Shikoku west center east Kii Tokai
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
3 days envelope traces in Shikoku
Outline - Tremor & Short-term slow slip - 2006 episode in Tokai - Very low frequency earthquake -Summary
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