The 6th Joint Meeting of the UJNR Panel on Earthquake Research, 2006/11/9-10

Deep slow earthquake families in the Nankai Trough subduction zone:

non-volcanic tremor, slow slip event and very low frequency earthquake

Kazushige Obara,Yoshihiro Ito, Shutaro Sekine, Hitoshi Hirose and Katsuhiko Shiomi （NIED)

Shallow VLFE

Philippine Sea plate

Nankai Trough

Long-termSSE

Deep LF TremorDeep VLFE

Short-termSSE

Transition zone

Stable sliding

Accretionary prism

Decollement Seismogenic zone

Seismic slip in accretionary prism

Aseismic stick-slip on the plate boundary

Shallow Very Low Frequency Eq (S-VLFE)

Short-term Slow Slip Event (S-SSE)

Deep Low Frequency Tremor

Deep Very Low Frequency Eq (D-VLFE) Seismic slip of small patch on S-SSE fault plane

Slow Earthquake Families in Nankai Zone

Micro crack failurearound S-SSE

Source time duration (or predominant time period) of slow earthquakes in Nankai subduction zone

Shallow Ito, P014VLFE

1000s 100s 10s 1s 0.1s1000d 100d 10d 1d 0.1d

Deep

Long-termSSE

Short-termSSE

VLFE LF Tremor

(LF Earthquake)included(0.5~5years) (2~5days)

Tremor

Slow Slip

High-quality dense seismic network

with Tiltmeter

NIED Hi-net(800stations)

Epicentral distribution of tremor

Deep tremorNIED Hi-net stations

Iso-depth contour of intra-slab seismicity

Nankai

Tonankai

Tokai

anticipated seismogenic zone

Tremor activity in Shikoku

East

Center

West

Three major segments

2001 2002 2003 2004 2005 2006

3mon.

3mon.

6mon.

(SW)

(NE)

Tremor and Tilt change in the western part of Shikoku

(Time window:14days)

5days 5days 5days 6days

0 50 100 150 200[km]

Tilt change and Tremor migration in W. Shikoku HIYH

(SW) (NE)

(SW)

(NE)

(SW)

(NE)

9

Short-term SSE & Tremor, 2002 AugustAugust 6-8 August 9-12

Cross sectional view of the tremor

Based on JMA catalog

TremorRegular Earthquake

Cross sectional view of the tremor

Based on JMA catalog

TremorRegular Earthquake

plate boundary from reflection survey (Baba, 2002)Short-term SSE

Oceanic Moho from receiver function analysis(Shiomi et al.,2004)

Tremor energy release in the active stageTremor energy release in the active stagewith shortwith short--term slow slip in west of Shikokuterm slow slip in west of Shikoku

Aug.-Sep. 2003

The periodicity of 12 hours == effect of earth tideThe periodicity of 12 hours == effect of earth tide

Tremor triggered by M7.6 Indonesia earthquakeTremor triggered by M7.6 Indonesia earthquake

P T-phase

Surface wave

TremorsTremors

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 slip１．Locked for 6 months at the transition zone２．Strain accumulation due to the slab descending３．Weakening of frictional strength by fluid from dehydration４．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 tremor１．Micro crack failure triggered by sudden increasing of fluid pressure by occurrence of slow slip２．Minor tremor triggered by small fluctuation of fluid pressure by teleseismic wave

Migrating tremor and slow-slip eventin Tokai and Kii area, January 2006

Total Mｗ ~ 6.2

Vel.=10km/dayJanuary, 2006

Kii TokaiIseBay

(SW)

(NE)

Tremor activity in Kii and Tokai area

(NE)

Tokai

Kii

Ise Bay

6mon.

3mon.

6mon.

?(SW)

Migration of tremor in Kii area

(NE)

Tokai

Kii

Ise Bay

(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.

ShiomiP23

Nankai

Tonankai

Tokai

Anticipated Seismogenic zone

Deep tremorNIEDHi-net stations

Iso-depth contour of intraslab seismicity

Deep Slow Earthquakes in Nankai Zone

6mon.

6mon.

6mon.

3mon. 3mon.

Recurrence interval

?

Short-term slow slip eventDuration: 2~5 days

Long-term slow slip eventDuration:0.5~5 years

Deep Very Low Frequency Earthquake

Seismic records of tremor

Superposing long period signals

Radial-comp.

Clear long period signals

Apparent Vel.~6km/s

Radial-comp.

Very Low Frequency (VLF) earthquake

Spectra of VLF earthquake

noise

TremorVLF

0.01 0.1 1 10Frequency(Hz)

Vel

ocity

spe

ctru

m(m

)

Predominant freq. = 20 sec.

CMT solution of VLF earthquakes in 2006

Focal mechanism = Thrust typeFocal depth = 30~40km

Slab surfaceVLF earthquake 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 earthquake１．strain accumulation at small patch due to slow slip on surrounded area２．seismic slow slip with high fluid pressure ３． No VLF without SSE period

●Three types of slow earthquakes in transition zone

○Short-term slow slip ＝ Stick slip on the plate boundaryDeep VLF earthquake ＝ seismic slow slip at small patchDeep LF tremor ＝ chain-like crack failure

○reflecting inhomogeneous structure in transition zone

Conclusion

考察

深部超低周波地震とスロースリップとの関係

・どちらもプレート境界面で発生。

・スロースリップが深部超低周波地震を誘発する。

・すべり量が大きい場合に深部超低周波地震が発生。

深部超低周波地震と深部低周波微動との関係・超低周波地震が微動をトリガーする（ことがある）。

小パッチのすべりによって、その周囲の微小クラック破壊を誘発？

プレート境界での「すべり」現象と活動サイクル

海洋プレートの沈み込み

アスペリティ

長期的スロースリップ

流体

脱水反応

深部低周波微動短期的スロースリップ

定常すべり

小繰り返し地震

巨大地震（アスペリティ）

時間

変位

地震

時間変位

時間

変位

定常すべり

時間

変位

100年

10年半年間隔

数年

数日継続

NE

SW

紀伊半島

愛知県

SW

NE

紀伊半島・愛知県における微動の時空間分布

0

50

100

150

200

2003/1/1 0:00 2004/1/1 0:00 2004/12/31 0:00 2005/12/31 0:00

同時期に活動

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 4/12003年 2004年 2005年 2006年

200

100

0

[km]NE

SW

NE

SW

四国東部・中部における微動活動時空間分布

「Hi-net傾斜計で確認された短期的スロースリップ」を伴う微動活動

東部と中部は独立に３ヶ月周期で活動（連動する場合もある）

東部

中部

傾斜変化は東部のみ

四国西部における微動活動時空間分布

「Hi-net傾斜計で確認された短期的スロースリップ」を伴う微動活動

長期的スロースリップ

長期的スロースリップ期間を除き、半年周期

Relationship between migration of tremor and tilt

[Winter]Tilt vector :S-down E-downTremor : NE SW

[Summer]Tilt vector :SE-down S-downTremor : SW NE

東海・紀伊半島における微動活動時系列

JMA歪計で確認された短期的スロースリップ（小林・他,2006）

AB

C

「Hi-net傾斜計で確認された短期的スロースリップ」を伴う微動活動の開始位置

微動と超低周波地震一致一致

不一致

Improvement of Detection Capability

１９９８２００１Magnitude

Cum

ulat

ive

Num

ber

Before Hi-netAfter

Start point of tremor activity in W. ShikokuRupture initiation of short-term SSE

Long-term slow slip

Pattern A

Pattern BPattern B’

Start point of tremor activity with short-term slow slip detected by NIED tiltmeter

0 50 100 150 200 km

Migration of tremor in W. ShikokuPattern A［NE SW］

0 50 100 150 200 km

Migration of tremor in W. ShikokuPattern B［SW NE］

0 50 100 150 200 km

Migration of tremor in W. ShikokuPattern B’［Bungo Channel NE］

●Segmentation of tremor and slow slip activity

・Segment size ：20～100km・各セグメント内で活動（破壊）が移動（速度：約10km/day）・活動開始場所は毎回異なるが、パターン化される。

（四国西部では３箇所）

・隣接セグメントが連動することがある。

（２００６年１月の三重・愛知における活動）

OneOne--hour record chart at the station IKTHhour record chart at the station IKTHin Shikoku Island, Southwest Japanin Shikoku Island, Southwest Japan

(2(2001/8/17 4 a.m.)001/8/17 4 a.m.)

Time (s)

Tim

e (m

in)

Clustering of tremor activity

Shikoku

Kii Tokaieastwestcenter

IKTH HIYHHIYH

KWBH YNDH

1 hour continuous seismograms observed at 1 hour continuous seismograms observed at 3 stations in the western part of Shikoku3 stations in the western part of Shikoku

4 am, 17 August 20014 am, 17 August 2001

Time [sec] Time [sec]

Time [sec]

Tim

e [m

in]

Tim

e [m

in]

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.ShikokuRupture process of short-term SSE

Long-term slow slip

Start point of tremor activity with short-term slow slip detected by NIED tiltmeter