Kelin WangPacific Geoscience Centre, Geological Survey of Canada

Observing an Earthquake Cycle

Within a Decade

(Drawn by Roy Hyndman)

1. Stress and strain evolve in earthquake cycles. Presently observed interseismic deformation is a snapshot of a changing field.

2. Earthquake cycle is a common process. There are fundamental similarities between earthquake cycles of different subduction zones.

3. Study of multiple subduction zones that are presently at different phases of earthquake cycles will help us understand the full cycle.

4. This will require us to distinguish between common/fundamental processes and site-specific processes.

Important points

Land observations

• Downdip limit of the seismogenic zone• Frictional behavior of deeper part of the fault• Mantle rheology

• Updip limit of the seismogenic zone• Frictional behavior of shallow part of the fault

Offshore observations

Sumatra: A few years after a great earthquake

Courtesy Kelly Grijalva and Roland Burgmann

Alaska: ~ 40 years after a great earthquake

M = 9.2, 1964

Freymueller et al. (2008)

GPS data:

Green: Klotz et al. (2001)Red: Wang et al. (2007)

Chile: ~ 40 years after a great earthquake

M = 9.5, 1960

Cascadia: ~ 300 years after a great earthquake

Wells and Simpson (2001)

Inter-seismic 2 (Cascadia)

Inter-seismic 1(Alaska, Chile)

Co-seismic

Coast line

Coast line

Post-seismic (Sumatra)

Rupture

Stress relaxation

Stress relaxation

Afterslip

Locking

Afterslip and transient slow slip: short-lived, fault frictionStress relaxation: long-lived, mantle rheology

ETS

Viscoelastic stress relaxation model for Chile, viscosity 2.5 1019 Pa s

(c)

1995 Antofagasta earthquake, N. Chile (Mw = 8.0)

1993-95 Displacements (dominated by co-seismic)

1996-97 Velocities (2 years after earthquake)

Data from Klotz et al. (1999) and Khazaradze and Klotz (2003)

Inter-seismic 2 (Cascadia)

Inter-seismic 1(Alaska, Chile)

Co-seismic

Coast line

Coast line

Post-seismic (Sumatra)

?

?

?

?

?

Coast line

Coast line

?

?coseismicslip (2 m contours)

Hsu et al. (2006)

GPS stations

Coseismic (contours) and 1-yr postseismic (color) slip of 2005 Nias-Simeulue earthquake

Coast line

Coast line

?

GPSA off Peru (Gagnon et al., 2005)

Updip segment is not slipping. Fully relaxed?

Coast line

Coast lineVery-low-frequency earthquakes possibly

in Nankai accretionary prism

(Ito and Obara, 2006)

?

Coast line

Coast line

?

?

Fluid pressure during a VLF episode

VLF events

Near-trench boreholes off Mutoto

Davis et al. (2006)

b 0.04

b -0.01

Average stress

~ 15 MPa

Stress drop~ 4 MPa

b > 0Stress

increaseA few MPa

Evidence for a velocity-strengthening shallow segment:• Lack of evidence for massive trench-breaking rupture• Slip patterns from inversion of seismic/tsunami/geodetic data• Inferences based on continental earthquakes• Real-time monitoring at Hokkaido (2003) and Sumatra (2005)

• Soft frontal prism sediment• Presence of stable-sliding minerals • Dilatancy of granular gouge material upon fast shearing• Inability to localize deformation during fast shearing

Mechanism of the velocity-strengthening behavior:

Studying the shallow segment is as important as studying the seismogenic zone

Importance I:

Tsunamigenic seafloor

deformation

Importance I:

Tsunamigenic seafloor

deformation

Earthquakes of same moment magnitude

Less strengthening of the shallow segment leads to trench-breaking rupture.

Trench-breaking rupture causes less seafloor uplift.

Importance II: Deformation of the frontal prism(Dynamic Coulomb wedge)

Inter-seismic: lower basal stress

Co-seismic: Basal fault strengthens; greater compression and pore fluid pressure within the prism

Cumulative effects of numerous great earthquakes control wedge taper

(Park et al., 2002)

Importance III: Coseismic activation of megasplay

Seismogenic Zone

Co-seismic

Post-seismic

Stress increase; resisting slip

Rupture;Stress drop

Stress decrease

Locked;Stress increase

Immediately following an earthquake

? Rate depends on friction properties

Rate depends on mantle rheology

Longer time after the earthquake

Slip quickly slows down

Stress increases butshortening slowly slows down

Fully locked?

Issues to be resolved by observations

• Does deformation at different stages of the earthquake cycle leave different signatures in rock samples?

• How far does coseismic rupture propagate updip? How common or rare is trench-breaking rupture?

• How do the frontal prism and splay faults respond to megathrust motion during, after, and between earthquakes?

• How does pore fluid pressure within the frontal prism and along the megathrust evolve in earthquake cycles?

• How does the coseismically strengthened shallow segment of megathrust relax after the earthquake? What is the time scale of the relaxation?

• How does the oceanic mantle respond to earthquake cycles? What viscosity model and value? Is it similar to the mantle wedge?

• … …

1. Stress and strain evolve in earthquake cycles. Presently observed interseismic deformation is a snapshot of a changing field.

2. Earthquake cycle is a common process. There are fundamental similarities between earthquake cycles of different subduction zones.

3. Study of multiple subduction zones that are presently at different phases of earthquake cycles will help us understand the full cycle.

4. This will require us to distinguish between common/fundamental processes and site-specific processes.

Important points

1. Study multiple subduction zones that are presently at different phases of earthquake cycles

2. Monitor strain, pore fluid pressure, etc., correlate with land-based networks

3. Transects of shallow boreholes

4. Monitor locked and creeping segments

Suggestions for SEIZE

Earthquake followed by

locking

Different along-strike rupture lengths and slip magnitudes(surface velocities 35 years after an earthquake;

mantle viscosity 2.5 x 1019 Pa s)

Coast line

Coast lineVery-low-frequency earthquakes in

Nankai accretionary prism

?