1. What can geodesy tell us about rifting & subduction?
Matt Pritchard
Cornell
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 cGPS
SEA: Advance seafloor geodesy
SPACE: Include InSAR

2. What can InSAR tell us about rifting & subduction?
Matt Pritchard
Cornell
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?

3. Recent rifting events 2009 Harrat Lunayyir Saudi Arabia Geodetic
Dabbahu, Afar
2007 Gelai, Tanzania
Map from Juliet Biggs

4. 6 May – 28 Oct 2005; from Tim Wright, U. Leeds4

5. Temporal evolution of Tanzania event
July 23rd
July 21st
July 17th
Rifting Event
STOP
Time
Biggs et al, GJI, 2009

6. Modeling: Dyke + fault + Magma chamber
Data
Biggs et al, GJI, 2009

7. Summary of 3 recent rifting events studied with seismology & geodesy
From: Baer & Hamiel, 2009

8. Central Andes earthquakes 1992-2007
4 earthquakes > Mw 8.0
(3 shallow, 1 event > 600 km deep)
1995 Mw 8.1 Antofagasta, Chile
2001 Mw 8.5 Arequipa, Peru
2007 Mw 8.1 Pisco, Peru
8 additional earthquakes > Mw 7.0
(5 shallow, 3 events > 200 km deep)
1996 Mw 7.7 Nazca, Peru
1998 Mw 7.1 Antofagasta, Chile
2001 Mw 7.6 Arequipa, Peru
2005 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)

9. Example: 1998 Mw 7.1 earthquake
Cross-section
Map view

10. Map fault slip before, during, & after earthquakes
Joint seismic & geodetic estimates of earthquake and after-slip in northern Chile
Loveless & Pritchard, 2010

11. Role of along-strike variability Conflicting(?) SEIZE summary slides
Evidence for after-slip within the “normal” earthquake zone
2003 Tokachi-Oki (below)
1995 Chile (Pritchard & Simons, 2006), etc.
This profile reveals new features recently discovered phenomena
2D profiles are not universal!

12. Compare actual & predicted interferograms from different slip models
Slip models from seismic and sparse geodetic data do not match InSAR

13. Why slip distributions matter
Coulomb stress change on upper plate faults from
1995 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

14. Splay fault motion during 1964 Alaska earthquake
Profile in next slide

15. Patton Bay and Hanning Bay faults moved up to 8 m
From: Plafker, 1972
•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)

16. 2007 Sumatra earthquake sequence
Interferograms from ALOS satellite
Images from: Geological Survey Institute, Japan
Mw 8.4 Sept. 12, 2007
12 hours later Mw. 7.9
Konca et al., 2008

17. Conclusions Some lessons learned
Different amounts of aseismic deformation during rifting events
Along-strike variations in subduction zones can be as important as down-dip changes
No single parameter seems to control location of “asperities”
Geodesy addresses many questions:
Spatial & temporal distribution of fault slip (including updip limit)
Post-seismic deformation & rheology of fault/rift zones
Detailed picture of seismogenesis
Role of magma & other fluids
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 effort
Why MARGINS? Addresses scientific & societal objectives
Space (InSAR): Everywhere subaerial
Free data available in the future (MARGINS should support this)
Purchase archived data over all MARGINS
Why MARGINS? Global scale beyond capability of individual PI

18. Gravity anomalies & slip in southern Peru
From:
Loveless et al., 2010
Nazca Ridge
Fracture Zone

19. 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)
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)
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)

20. 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

21. Some ideas for MARGINS successor program
Incorporate InSAR into MARGINS to achieve SEIZE & RCL goals
Achieves SEIZE goal of comparing different subduction zones
Purchase archived data complementary to existing available datasets
Support future acuisitions via Sentinel & DESDynI
Assess along-strike variations by deploying geodetic arrays in all subduction zones
Understand the factors that control slow slip
Added benefit for real-time tsunami warning
Would also achieve Wang’s goal of measuring different stages of the seismic cycle via observations in different subduction zones
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
Monitor strain, pore-pressure, tremor & seismic activity in the submarine wedge (SEIZE summary)
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?