What can geodesy tell us about rifting & subduction?

Matt PritchardCornell

Summary:

Geodesy is a tool that reveals real-time processes. We need the rest of the community to decipher them.

We need new observations from multiple rifts & subduction zones

Multi-sensor approach:LAND: All subduction zones with cGPSSEA: Advance seafloor geodesySPACE: Include InSAR

What can InSAR tell us about rifting & subduction?

Matt PritchardCornell

Rifting:Comparison of 3 rifting events indicates

varying importance of magma & earthquakes

Subduction zones:1) More detailed earthquake slip maps

2) Overlapping slow & fast slip

2) Can we predict locations of large earthquakes?

3) Role of upper-plate faulting?

Geodetic2005-2009 Dabbahu, Afar

Map from Juliet Biggs

2007 Gelai, Tanzania

Recent rifting events

2009 Harrat LunayyirSaudi Arabia

6 May – 28 Oct 2005; from Tim Wright, U. Leeds

July 17th

July 23rdJuly 21st

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Biggs et al, GJI, 2009

Time

Temporal evolution of Tanzania eventR

iftin

g E

vent

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Data Model

Modeling: Dyke + fault + Magma chamber

Biggs et al, GJI, 2009

Summary of 3 recent rifting events studied with seismology &

geodesy

From: Baer & Hamiel, 2009

Central Andes earthquakes1992-2007

4 earthquakes > Mw 8.0 (3 shallow, 1 event > 600 km deep)

1995 Mw 8.1 Antofagasta, Chile2001 Mw 8.5 Arequipa, Peru2007 Mw 8.1 Pisco, Peru

8 additional earthquakes > Mw 7.0 (5 shallow, 3 events > 200 km deep)

1996 Mw 7.7 Nazca, Peru1998 Mw 7.1 Antofagasta, Chile2001 Mw 7.6 Arequipa, Peru2005 Mw 7.8 Tarapaca, Chile (110 km deep!)2007 Mw 7.7 Tocopilla, Chile

(Pritchard et al., 2002; Salichon et al., 2003; Pritchard et al., 2006; Peyrat et al., 2006; Delouis & Legrand, 2007; Pritchard et al., 2007; Pritchard & Fielding, 2008; Loveless et al., 2010)

Example: 1998 Mw 7.1 earthquake

Cross-sectionMap view

Map fault slip before, during, & after earthquakes

Joint seismic & geodetic estimates of earthquake and after-slip 1995-2007 in northern Chile

Loveless & Pritchard, 2010

Role of along-strike variabilityConflicting(?) SEIZE summary slides

2D profiles are not universal!

Evidence for after-slip within the “normal” earthquake zone2003 Tokachi-Oki (below)1995 Chile (Pritchard & Simons, 2006), etc.

This profile reveals new features recently discovered phenomena

Compare actual & predicted interferograms from different slip models

Slip models from seismic and sparse geodetic data do not match InSAR

Why slip distributions matter

Coulomb stress change on upper plate faults from1995 Mw 8.1 earthquake

7 co-seismic slip models predict different direction of slip:

Normal fault motion encouraged by some models

Reverse motion discouraged or neutral in our joint model

From Loveless & Pritchard (2008)

Model 1: Delouis et al., 1997; Model 2: Klotz et al., 1999; Model 3: Xia et al., 2003; Model 4: Pritchard et al., 2002; Model 5: Ihmle and Ruegg, 1997; Model 6: Pritchard et al., 2006; Model 7: Pritchard et al., 2006

Splay fault motion during 1964 Alaska earthquake

Profile in next slide

•Co-seismic splay fault motion(?) during 1946 Nankaido, Japan earthquake (Kato, 1983)

•No evidence for upper-plate motion in northern Chile (Loveless & Pritchard, 2008) or Cascadia (Finnegan et al., 2008)

•Patton Bay and Hanning Bay faults moved up to 8 m

From: Plafker, 1972

2007 Sumatra earthquake sequence

Mw 8.4 Sept. 12, 200712 hours later Mw. 7.9

Konca et al., 2008

Interferograms from ALOS satelliteImages from: Geological Survey Institute, Japan

Conclusions

Some lessons learnedDifferent amounts of aseismic deformation during rifting eventsAlong-strike variations in subduction zones can be as important as down-dip changesNo single parameter seems to control location of “asperities”

The way forward:Sea (Seafloor Geodesy & OBS): Focused sites

Options for vertical & horizontal continuous or campaign measurements See Chadwell SEIZE talk

Land (GPS & seismology): All subduction zones(?) for real-time tsunami warning

MARGINS could lead international effortWhy MARGINS? Addresses scientific & societal objectives

Space (InSAR): Everywhere subaerialFree data available in the future (MARGINS should support this)Purchase archived data over all MARGINSWhy MARGINS? Global scale beyond capability of individual PI

Geodesy addresses many questions:Spatial & temporal distribution of fault slip (including updip limit)Post-seismic deformation & rheology of fault/rift zonesDetailed picture of seismogenesisRole of magma & other fluids

Gravity anomalies & slip in southern Peru

Nazca

Rid

ge

Frac

ture

Zone

From: Loveless et al., 2010

3) Empirical features (origin unknown)• Shape of the coastline• Existence of peninsulas• Gravity lows associated with large earthquakes -- proxy for lower & upper plate structure (Song & Simons, 2003; Wells et al., 2003)

Could we have large areas of slip (asperities) in South American

earthquakes?1) Lower plate structure

• Temperature (irrespective of age, Newman et al., 2002)• Seamounts (e.g., Small & Scholz, 1997; Bilek et al., 2003) • Fracture zones (e.g., Robinson et al., 2006)• Plate dip (Mitsui & Hirahara, 2006)

2) Upper plate structure• Isostatic anomaly proxy for lithologic changes (Sobiesak, 2004)• Upper crustal faults oblique to coastline (e.g., Collot et al., 2004)

4) Interface properties:• Sediment: Composition, amount or thickness (e.g., Ruff, 1989)• Magnitude or importance of subduction erosion• Normal stresses on plate interface (e.g., Scholz & Campos, 1995)

No: No single property can explain co-seismic “asperities” for all earthquakes

Comparing various physical properties of subduction zones with co-seismic slip location

Gradients in the gravity field (Llenos & McGuire, 2007) are perhaps useful

From: Loveless et al., 2010

Some ideas for MARGINS successor program

Assess along-strike variations by deploying geodetic arrays in all subduction zonesUnderstand the factors that control slow slip Added benefit for real-time tsunami warningWould also achieve Wang’s goal of measuring different stages of the seismic

cycle via observations in different subduction zones

Seafloor geodesy: (From SEIZE summary & Chadwell talk)What types (vertical pressure gauges; horizontal acoustic, etc.?)?Where to deploy? (up-dip limit; densifying in seismic gaps; post-seismic, etc.?)How many instruments could we afford, given other objectives?

Incorporate InSAR into MARGINS to achieve SEIZE & RCL goalsAchieves SEIZE goal of comparing different subduction zonesPurchase archived data complementary to existing available datasetsSupport future acuisitions via Sentinel & DESDynI

Monitor strain, pore-pressure, tremor & seismic activity in the submarine wedge (SEIZE summary)

Can off-shore observations give us predictive capability for megathrust slip properties?

Detailed study of off-shore faults & topography, seismicity, tomography, etc. is useful for comparing with fault slip on the megathrust