nonvolcanic deep tremor associated with …cais.gsi.go.jp/ujnr/ujnr4p/obara-nied.pdf · nonvolcanic...

NonvolcanicNonvolcanic deep tremor associated with deep tremor associated with subductionsubduction in Southwest Japanin Southwest Japan

Kazushige Kazushige ObaraObara (NIED)(NIED)

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)

IKTHIKTH HIYHHIYH

KWBHKWBH YNDHYNDH

OneOne--hour record chart at four stationshour record chart at four stationsin Shikoku Island (2001/8/17 4 a.m.)in Shikoku Island (2001/8/17 4 a.m.)

A scene of drillingA scene of drilling

Discovery of tremor

sparsely distributed seismic network

It is impossible to discriminate betweenthe tremor and cultural noises.

after Hi-net densely distributed high-sensitivity seismic network

The amplitude pattern is the same at some neighbor stations.

natural phenomena

before Hi-net

U

DN

SE

W

U

DN

S

EW

One-hour seismogram at IKTH

Including an isolated pulseIncluding an isolated pulse Random wave trainRandom wave train

enable to locate as enable to locate as a lowa low--frequency eventfrequency event

impossible to pick phasesand locate the source

U

DN

SE

W

U

DN

S

EW





Very difficult to detect and monitor the tremor activityby normal hypocentral determination method

U

DN

SE

W

U

DN

S

EW





Very difficult to detect and monitor the tremor activityby normal hypocentral determination method

Normal event

Low-frequency tremor

The tremor is characterized by low-frequency componentscompared with the same size microearthquakes

Envelope seismogram at 8 stations in ShikokuEnvelope seismogram at 8 stations in Shikoku

(2001/8/17 4am)(2001/8/17 4am)

Time (min)

Three days envelope seismograms in ShikokuThree days envelope seismograms in Shikoku

TBEH

KWBH

Time(min)

Comparison of 2 envelopes

Measurment of time difference

TBEH

KWBH

TBEH

KWBH

Time difference = 3sTime difference = 3s

HypocentralHypocentral determination with envelope correlationdetermination with envelope correlation

TBEH

KWBH

Time(min)



TBEH

KWBH

TBEH

KWBH



Time difference measurmentfor every station pairs

Spatial Smoothing

TBEH

KWBH

Time(min)



TBEH

KWBH

TBEH

KWBH




Spatial Smoothinghypocentral determinationwith S wave velocity

５

１０１５２０２５

Arrival time difference (s)

TBEH

KWBH

Time(min)



TBEH

KWBH

TBEH

KWBH




Spatial Smoothinghypocentral determinationwith S wave velocity

５

１０１５２０２５

Arrival time difference (s)

Continuous Continuous hypocentralhypocentral determinationdetermination

Depth-distribution of normal earthquakes in the subducting

Philippine sea plate (Nakamura, et.al., 1997)

Distribution of tremors (2001/1/1Distribution of tremors (2001/1/1--2001/12/31)2001/12/31)

Distribution of tremors (2002/1/1Distribution of tremors (2002/1/1--2002/10/25)2002/10/25)

Based on JMA catalog

Cross sectional view of the low-frequency events

SpaceSpace--time plot of tremor (2001/1/1time plot of tremor (2001/1/1--2001/12/31)2001/12/31)

not analyzed

Time sequence of tremor (2000/9/1Time sequence of tremor (2000/9/1--2002/6/30)2002/6/30)

The frequency in units of 1-hour periods in which the tremor was recognized in each region is plotted.

Time sequence of tremor (2002/1/1Time sequence of tremor (2002/1/1--2002/10/27)2002/10/27)

Envelope seismograms in ShikokuEnvelope seismograms in Shikoku(2000/12/31 23:30(2000/12/31 23:30--2001/1/1 00:00)2001/1/1 00:00)

Time(min)


Time(min)

Normal eventM3.6

depth=10km


Time(min)

Normal eventM3.6

depth=10kmTriggered tremorsTriggered tremors

Distribution of tremors in 2000/12/31 23pmDistribution of tremors in 2000/12/31 23pm

◆Normal events(M3.6)（based on the normal hypocentral

determination method）

No.36 is the location of the normal event(M3.6) determined by the envelope correlation method

Distribution of tremors in 2000/12/31 23pmDistribution of tremors in 2000/12/31 23pm

◆Normal events(M3.6)（based on the normal hypocentral

determination method）

No.36 is the location of the normal event(M3.6) determined by the envelope correlation method

誘発された微動誘発された微動Triggered tremorsTriggered tremors

Location of tremorLocation of tremor

••　　Length of the source area is about 600km.Length of the source area is about 600km.••　　NonNon--volcanic areavolcanic area••　　Along the strike of the Along the strike of the subductingsubducting Philippine sea platePhilippine sea plate

(corresponding to the seismic activity in the slab (corresponding to the seismic activity in the slab with the depth range from 35 to 45km)with the depth range from 35 to 45km)

••　　Depth of the tremor = 30km(Depth of the tremor = 30km(++10km)10km)　　MohoMoho discontinuity?discontinuity?

Lower crust?Lower crust?　　Upper boundary of the Philippine sea Upper boundary of the Philippine sea plabplab??

••　　There is no tremor around There is no tremor around KiiKii Channel and Channel and E.ShikokuE.Shikoku

Time sequence of tremorTime sequence of tremor

••　　There are active periods and quiet periods.There are active periods and quiet periods.––　　Active periodActive period：： ~ a few weeks~ a few weeks––　　Quiet period Quiet period ：： ~ a few months~ a few months

••　　In an active period, active and quiet stages exist.In an active period, active and quiet stages exist.••　　Tremors are sometimes triggered by nearbyTremors are sometimes triggered by nearby

major earthquakesmajor earthquakes••　　The source area migrates with the velocity of The source area migrates with the velocity of

9 ~ 13km/day.9 ~ 13km/day.••　　A tremor activity is related to shallow swarm.A tremor activity is related to shallow swarm.

Cause of tremorCause of tremor

••　　Long duration and mobility Long duration and mobility ••　　Consistency with slab geometryConsistency with slab geometry

FluidFluid generated by slab dehydrationgenerated by slab dehydrationA certain temperature/pressure condition A certain temperature/pressure condition Inhomogeneous structureInhomogeneous structure

・　・　Possible mechanismsPossible mechanisms1. Chain1. Chain--like fracture with supercritical fluidlike fracture with supercritical fluid2. 2. FuildFuild flowflow3. 3. ……………………………………………………..

Moho

Philippine sea sla

b

TremorLow-frequencymicroearthquakes

stable s

liding

Interseismic locked zone

microearthquakes

in the slab

slab dehydration

Further StudyFurther Study

・ Depth and focal mechanism of tremor•• Underground structure in southwest Japan Underground structure in southwest Japan subductionsubduction

zone (zone (MohoMoho, plate boundary,,,), plate boundary,,,)•• Similarities and differences with other deep lowSimilarities and differences with other deep low--

frequency events (near volcanoes, active faults)frequency events (near volcanoes, active faults)

•• Other Other subductionsubduction zone? zone?

nonvolcanic deep tremor associated with …cais.gsi.go.jp/ujnr/ujnr4p/obara-nied.pdf · nonvolcanic...

Documents