Remote Earthquake Triggering: (Fault) Failure is Not Always an

Option

Heather Savage and Emily Brodsky

UC Santa Cruz

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

QuickTime™ and a decompressor

are needed to see this picture.

Remote Triggering

From Denali Earthquake

Gomberg et al., 2004

Triggered Seismicity from the Denali Earthquake

2002 Denali, Alaska Mw 7.9 Earthquake

Husker and Brodsky, 2004

Courtesy of Anthony 2004

•5 MPa normal stress•Tectonics stress: background shear loading rate of 5 µm/s•Oscillating stress: Vlp = V0 + Asin(t)•Fault zone materials: granite blocks, glass beads

Laboratory Setup

0.5

MP

a

Time

Shear Stress

Stick-slip Failure

Transient

Trigger

t2t1

Savage and Marone, 2008

She

ar S

tres

s (M

Pa)

Loa

d P

oint

Vel

ocit

y (µ

m/s

)

Amplitude Dependence

10

20

30

40

50 TriggeredNon-triggered

0 10 20 30 40 50 60

6 mm layer

Amplitude (µm/s)

D

10

20

30

40

50

granite -smooth

granite -rough

A

10

20

30

40

502 mm layerB

10

20

30

40

503 mm layerC

Increasing gouge thickness

No gouge

Inte

r-ev

ent T

ime

(s)

Amplitude (µm/s)

1.4

1.6

1.8

2

2.2

2.4

p936

B

RoughGranite Surface

1.4

1.6

1.8

2

2.2

2.4

p900

C

3 mm thick layer

1.4

1.6

1.8

2

2.2

2.4

665 670 675 680 685 690 695 700

p961

Time (s)

D

6 mm thick layer

1

1.5

2

2.5

3

3.5

p1173

A

SmoothGranite Surface

Sh

ear

Str

ess

(M

Pa)

Time (s)Savage and Marone, 2008

Triggering Intensity = Normalized Seismicity Rate

Change

t2t1

Seismicity Rate: =1/t

Normalized Seismicity Rate Change: n (2- 1)/ 1 Felzer and Brodsky, 2005

Changes in Lab Seismicity

Rate

0.1

1

10 100

6 mm gouge layer

Velocity Amplitude (µm/s)

Triggering Threshold

0.1

1

10 100

Granite Surfaces2 mm gouge layer3 mm gouge layer6 mm gouge layer

Velocity Amplitude (µm/s)

Changes in Lab Seismicity

Rate

Remote Triggering on the Nicoya Peninsula, Costa Rica

Seismic data courtesy of S. Schwartz

Solomon Islands - 04/01/07

Peru - 08/15/07

So. Sumatra - 09/12/07

Indonesia - 09/12/07

Fiji - 12/09/07

Sichuan - 05/12/08

New Zealand - 07/15/09

Remote Earthquakes

New Zealand 2009, recorded in Costa Rica

Seismic data courtesy of S. Schwartz

0.1

1

10

104 105 106

Velocity Amplitude (nm/s)

Costa Rica

Indonesia 09/12/2007

Peru 08/15/2007

Solomon Islands 04/01/07

So. Sumatra 09/12/07

Fiji 12/09/07

New Zealand

0715/09

Sichuan 04/12/08

Remote Triggering in the Western US, Transportable Array

Remote Earthquakes

New Zealand - 07/15/09

Samoa - 09/29/09

Sumatra - 09/30/09

Samoa 2009, recorded in Eastern Wyoming

Costa Rica and E. Wyoming

0.1

1

10

104 105 106

Velocity Amplitude (nm/s)

Sumatra 09/30/09

New Zealand 07/15/09

0.1

1

10 100

Granite Surfaces2 mm gouge layer3 mm gouge layer6 mm gouge layer

Velocity Amplitude (µm/s)

Conclusions

• Triggered seismicity is a function of strain amplitude (bigger earthquakes trigger more earthquakes) but is also dependent on interseismic history

• Large earthquakes that closely follow previous large events are very inefficient at triggering additional seismicity

• Experiments suggest that fault zone properties determine the relationship between triggering intensity and strain amplitude– Preliminary seismic observations suggest subduction zones spend more time close to critical failure than old crustal

faults