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

L’Aquila, Italy 2009 M 6.3 Normal faulting in area often shown as subduction

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

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

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.

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

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

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

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.

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

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

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)

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

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

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

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

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

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.

INTRAPLATE : (< 1 mm/yr) Seismicity varies in space & time 1900-2002 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

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 - 3700 yr ~ 35 mm/yr GPS SLIP RATE 35 mm/yr Z.-K. Shen Geodetic, geologic, & plate motion rates agree

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 7.7 Slip 4 m Sieh et al., 1989 Expect earthquakes about every 4 m / 35 mm/yr or ~ 144 years M >7 mean 132 yr

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

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

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

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