1. DISCREPANCIES BETWEEN DATA INDICATING TIME- AND SPACE-VARIABLE DEFORMATION
Seth Stein
Northwestern University
Timothy Dixon
University of Miami
Eric Calais
Purdue University
Mian Liu
University of Missouri
Anke Friedrich
University of Munich
“When you come to a fork in the road, take it” Yogi Berra

2. L’Aquila, Italy M 6.3
Normal faulting in area often shown as subduction

3. Wenchuan, China 2008 M 7.9 Was this really a surprise?
No M > 7 events known in area ~ 1500 years

4. Techniques sample plate motion and crustal deformation in different ways
Often they agree, but sometimes they don’t
Discrepancies can be due to errors, but often both data are correct & reflect real changes in space and time
As data improve, more of the apparent changes are real
Some of these are happening today

5. Motions between plates sometimes change on scales of tens of millions of years
The geometry and rate of motion within plate boundary zones can change dramatically on million-year timescales although motion between the major bounding plates remains constant.
The locus of slow deformation within plates migrates on time scales as short as thousands of years, as fault systems turn on and off.

6. SPACE GEODESY & GEOLOGIC PLATE MOTION MODELS GENERALLY AGREE
Plate motions over a few years observed by space geodesy very similar to predictions of NUVEL-1 or similar geologic models describing average motions over past 3 Ma
Hence plate motions are generally steady, presumably because viscous asthenosphere damps episodic motions at plate boundaries
However, in places geological models and space geodesy disagree, showing changing motion
SPACE GEODESY & GEOLOGIC PLATE MOTION MODELS GENERALLY AGREE
Robbins et al., 1993

7. NUBIA-SOUTH AMERICA
GPS rate slower than NUVEL-1 (0-3 Ma spreading rate) is part of long-term slowing shown by marine magnetic data
Sella et al., 2002

8. NAZCA - SOUTH AMERICA: CONVERGENCE SLOWS AS ANDES RISE
Marine magnetic, NUVEL-1, and GPS data show slowing of Nazca - South America convergence
Slowing associated with rise of Andes, and accelerated shortening in thrust belt, implying complicated feedbacks
Hindle et al., 2002

9. Motions between plates sometimes change on scales of tens of millions of years
The geometry and rate of motion within plate boundary zones can change dramatically on million-year timescales although motion between the major bounding plates remains constant.
The locus of slow deformation within plates migrates on time scales as short as thousands of years, as fault systems turn on and off.

10. ADRIA MICROPLATE: NOW MOVES NORTHEASTWARD WRT EURASIA
Focal mechanisms and GPS find Adria bounded by convergent boundaries in the Dinarides and the Venetian Alps, extensional boundary in the Apennines and moving northeastward away from western Italy (~ Eurasia).
Nubia
GPS wrt Eurasia
Devoti et al., 2008
Stein & Sella, 2005

11. ADRIA MICROPLATE: MIO-PLIOCENE TIME: SOUTHWEST CONVERGENCE
Adria subducted southwestward beneath Italy
Apennines part of thrust belt extending south to Sicily
Arc evolved in association with opening of Tyrrhenian sea since about 15 Ma, interpreted as back arc spreading associated with rollback of the Adria slab.
As subduction migrated eastward, western Italy microplate rotated counterclockwise with respect to Eurasia
Eurasia
Western Italy
Adria
Nubia
After Rosenbaum & Lister, 2004

12. GPS & EARTHQUAKES vs. GEOLOGY SHOW PLEISTOCENE MOTION CHANGE
Subduction and back arc spreading ceased within past 2 Ma, making Italy west of the Apennines almost part of Eurasia.
Adria - Eurasia motion then caused a shift from convergence to extension in the Apennines.
Eurasia
W. Italy
Adria
Eurasia
Adria
Stein & Sella (2004); modified from Malinverno and Ryan (1986)

13. PACIFIC - NORTH AMERICA PLATE BOUNDARY ZONE relative to North America
Deformation within boundary zone changes although major plate motion doesn’t
GPS site velocities
relative to North America
Intermountain seismic belt
Colorado Plateau
Bennett et al., 1999

14. Deformation at various scales I: recent (GPS data)
A. Friedrich

15. Deformation at various scales II: 100 years (GPS + Historic seismicity)
A. Friedrich

16. Deformation at various scales III: 10 ka (+ Holocene surface ruptures)
Recent (GPS); 100 years (Historic seismicity)
A. Friedrich

17. Deformation at various scales IV: 1 Ma (Quaternary fault traces)
Recent (GPS); 100 years (Historic seismicity); 10 ka (Holocene surface ruptures)

18. Motions between plates sometimes change on scales of tens of millions of years
The geometry and rate of motion within plate boundary zones can change dramatically on million-year timescales although motion between the major bounding plates remains constant.
The locus of slow deformation within plates migrates on time scales as short as thousands of years, as fault systems turn on and off.

19. INTRAPLATE : (< 1 mm/yr) Seismicity varies in space & time
NORTH AMERICA
PLATE BOUNDARY: SAN ANDREAS
30-45 mm/yr
Steady motion gives quasi-periodic earthquakes
PACIFIC
INTRAPLATE : (< 1 mm/yr) Seismicity varies in space & time

20. GEOLOGIC SLIP RATE - 3700 yr ~ 35 mm/yr
San Andreas: GPS site motions show deformation accumulating that will be released in future earthquakes
Like a deformed fence
GEOLOGIC SLIP RATE yr ~ 35 mm/yr
GPS SLIP RATE 35 mm/yr
Z.-K. Shen
Geodetic, geologic, & plate motion rates agree

21. Expect earthquakes about every 4 m / 35 mm/yr or ~ 144 years
GPS site motions consistent with paleoseismic earthquake recurrence, showing steady motion
1906 San Francisco M Slip 4 m
Sieh et al., 1989
Expect earthquakes about every 4 m / 35 mm/yr or ~ 144 years
M >7 mean 132 yr

22. NEW MADRID M7 earthquakes in 1811-12
Paleoseismology - primarily paleoliquefaction - shows events ~ 1450 and 900 AD
Calais & Stein, 2009
GPS data do not require motion, and restrict any motion to being very slow.
Long time would be needed to store up the slip for a future large earthquake
For steady motion, M 7 is at least 10,000 years away: M 8 100,000

23. Recent large earthquake cluster likely ended
Large earthquake cluster in past 2000 years isn’t representative of long term NMSZ behavior
Recent large earthquake cluster likely ended
Tuttle (2009)
Seismicity migrates among faults due to fault interactions (stress transfer)
Many faults active in past show little present seismicity

24. MIGRATING SEISMICITY China
H. Wang
In North China, not one M>7 earthquake repeated in the same place since 1300 AD.
Since 1700 seismicity has been migrating from Shanxi Grabens to North China plain
Li, Liu & Stein, 2009

25. Discrepancies between data sometimes show that this is the case
We know that plate boundaries, plate motions, and intraplate deformation change over time, but are sometimes surprised to find it happening today
Discrepancies between data sometimes show that this is the case
If so, changes from the recent past shouldn’t surprise us
Meers fault, Oklahoma
Active 1000 years ago, dead now