Recent Studies in Earthquakes

Slow and Fast Slip Events:How Could We Study Them??

Kevin M. Brown

kmbrown@ucsd.edu: Scripps Institution of Oceanography, University of California, San Diego,

La Jolla, CA 92093-0244, United States

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Experimental Investigation of a Transition Between Stick-slip and Creep as Function of Temperature, Slip-Rate, and Normal stress

A possible origin of slow slip events

Erica Mitchell, Kevin Brown, Yuri Fialko Scripps Institution of Oceanography, UCSD

AGU - T16

Transitions to Unstable Behavior in 3D Phase Space

Temp

EffectiveNormal stress

Strain rate

Slow

Fast a

s

a

Complex Transition Zones

a - aseismic

s - seismic

T=25°CNormal Stress=1MPaForcing Velocity=10-2mm/s

NovaculiteT=200°CNormal Stress=4MPaForcing Velocity=10-4mm/s

T=200°CNormal Stress=4MPaForcing Velocity=10-5mm/s

T=500°CNormal Stress=8MPaForcing Velocity=10-3, 10-2, 3x10-2mm/s

Slip Time=4.8sStress Drop=.01MPaSlip Velocity=2.5x10-4mm/s

Slip Time=90sStress Drop=0.043MPaSlip Velocity=5.78x10-5mm/s

Slip Time= < 0.1sStress Drop= 0.664MPaSlip Velocity= >>1mm/s

1) 2)

Increasing Instability

4)3)

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Onset of Melting

Change in physics with slip velocity

Fabric/localization

Thermal

Residual

Peak

Fric

tion

Coe

ffic

ient

Do Field Studies

And Get Lucky!

Interface locking (assuming 85mm/yr convergence white contours) from on-land campaign GPS [Norabuena et al.,2004] is shown relative to earthquake b-values, and interface seismicity (yellow circles) used to determine b. TheFisher seamount subducts nearly normal to the trench, and corresponds with undulations in locking and high b-values [from Ghosh et al., (2006)].

What is partial plate coupling??

Geodetically coupled region does not appear to be related to smectite to illite transition (too cold)

We already know the Nicoya area of Costa Rica is dangerous (Mw 7.8, potentially tsunamogenic) and interesting

Newman

OBS, flowmeter box

ACDP Current meter

Remote chambers

Early micro-observatories

Marine Hydrologic Instrumentation on a Cold Seep

Nicoya

aaa

30

5

147

25

10

16

1127

29

28

2612

13

30

5

147

25

10

16

1127

29

28

2612

13

Costa Rica

Nicoya

Peninsula

9° 30’N

10° 00’N

10° 30’N

11° 00’N

87° 00’W 86° 30’W 86° 00’W 85° 30’W 85° 00’W

9° 15’N

9° 45’N

10° 15’N

10° 45’N

86° 45’W 86° 15’W 85° 45’W 85° 15’W

9° 30’N

10° 00’N

10° 30’N

11° 00’N

9° 15’N

9° 45’N

10° 15’N

10° 45’N

87° 00’W 86° 30’W 86° 00’W 85° 30’W 85° 00’W86° 45’W 86° 15’W 85° 45’W 85° 15’W

OBS/CAT meter position

Correlations Between Flow

and Noise

• Correlation between noise and flow anomalies is clear.

• Sites show flow anomalies of differing signs.

Brown, Deshone Dorman, Schwartz, Tryon

“Seismic Noise” Manifestations

ComparisoComparison of wind n of wind and noiseand noise

A) Regional RMS noiseaverage of 8 instruments

B) Significant wave heightoff Costa Rica from meteorology using Tolman'sWaveWatchIII program

C) RMS records, and regional average noise. Note that site 5 stands out

Tremor or Many Micro Earthquakes?

See Dorman Poster

Spectrogram of event 3 at site 3Record length is 12 hours

See Le Roy Dormans poster

Spectrogram of event 3 at site 5Record length is 12 hours

See Le Roy Dormans poster- From duel frequency studies-noise is similar too but not exactly like Julian 1994 tremor

Hydrologic Manifestations

The finite element model: Abacusa fully coupled model (LaBonte,Brown, Fialko)

Numerical calculation of:• displacement• pore pressure• fluid flow•(tilt and volumetric strain •also possible)

Costa Ricasubduction zone geometry

oceanicplate

continentalplate

Flow patterns differ between Near/Far Field Earthquake - Depth > Rupture length

Near surface flow/pore pressure signalsare a combination of poro-elastic effects and surfacebuckling/flexure effects

The finite element model: Abacusa fully coupled model (LaBonte, Brown, Fialko)

Updip Propagating Slow Slip

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flow

rat

e

time

=1 =0.05=0.5

OUT FLOWAT SURFACE

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Downdip propagating slow slip

flow

rat

e

time

=1=0.05 =0.5

Down dip propagation, = 0.4Duration of slow slip event = 22daysSite 2 is 6.5km distant from trench

model

Maximum extent= 0.5km-15km Max. prop. velocity= 0.7km/day Min. dislocation distance= ~30cm

Days since event initiation

Flo

w r

ate

(mm

/day

)

Updip propagation

Downdip propagation

flow

rat

e

time

flow

rat

e

time

=0.5

=0.2

=1

One Strategy for a Subduction Zone Earthquake Observatory

1) Buoys- Marine Geodetics2) Boreholes3) Cables

Stress episodicallyreleased near the toe

Continuous marine geodetic measurements can determine where the up dip limit lies and how it stresses are released episodically

Locked to the trench

Locked to the trench

Note on land geodetic measurementsare not sensitive to offshore locking patterns only the down dip limit!

88

6

4

2

0

SeisCORK Concept- for time lapse VSPs and offset VSPs- for monitoring micro-earthquake activity

DYNASEISE: Geodynamic and SeisCORK Observatories in the Central America Subduction System, Costa Rica

Kevin Brown, Ralph Stephen, Andrew Newman, Tom Pettigrew, Bob Petit, Kirk McIntosh, Sue

Bilek, Nathan Bangs, LeRoy Dorman, Glen Spinelli, Susan Schwartz, Pete Lafemina

GEOCE Buoys

CRSZ-1

Hilton Fischer, Brown

He Ra numbers

Gradie

nt

Wedge mantle boundary

GEOCE BUOY

An ORION-Type ObservatorysMultiple Components:

1) Horizontal marine GPS (cm)

2) Verticalseabed height changes (<1cm) oceanographic variability (internal density and

dynamic height changes

4) Absolute sea height Buoy GPS

5) Communications (acoustic andradio/satellite)

GEOCE system

1) XYZ seabed displacement

2) Corrected for oceanic noise

3) Initial system engineering test is ongoing off SIO

4) Highest data retrieval rates will be obtained by line of sight wireless modem.

5) Really useful for slow slip and aseismic creep events

Conclusion

• A very wide variety of methods and techniques are required to make progress!