Migrating earthquakes and faults switching on and off: a new view of intracontinental earthquakes Seth Stein Northwestern University Mian Liu University of Missouri Eric Calais Purdue University “How wonderful that we have met with a paradox. Now we have some hope of making progress.” Niels Bohr

Plate Boundary Earthquakes Major fault loaded rapidly at constant rate Earthquakes spatially focused & temporally quasi-periodic Past is good predictor Intraplate Earthquakes Tectonic loading collectively accommodated by a complex system of interacting faults Loading rate on a given fault is slow & may not be constant Earthquakes can cluster on a fault for a while then shift Past can be poor predictor Plate A Plate B Earthquakes at different time Stein, Liu & Wang 2009

You must unlearn what you have learned. Alan Kafka New insight into intraplate earthquakes & hazards

PACIFIC NORTH AMERICA Insights from New Madrid seismic zone M 7 earthquakes in Small quakes continue (M>6 about every 175 years) with little damage Big ones might happen again sometime Don’t know why, when, how dangerous

New Madrid 1991: because paleoseismology shows large events in 900 & 1450 AD, like those of GPS studies started, expecting to find strain accumulating consistent with large events ~500 years apart Contradictory results emerged at first….

One group found high deformation rates similar to California, implying large upcoming earthquakes Science, 1992

Science, April 1999 More detailed study found little or no motion: Seismicity migrates Recent cluster transient, possibly ending Hazard overestimated

As data improve, maximum possible motion keeps decreasing E. Calais < 0.2 mm/yr No sign of large earthquake coming Long time needed to store up slip for future large earthquake For steady motion, M 7 at least 10,000 years away M 8 100,000

Large earthquake cluster in past 2000 years isn’t representative of long term NMSZ behavior Lack of significant fault topography, jagged fault, seismic reflection, and other geological data also imply that recent pulse of activity is only a few thousand years old Recent cluster may be ending ?? 9k7k6k4k12k3k1kToday Portageville CycleReelfoot CycleNew Madrid Cycle Slip Cluster Slip Cluster Slip Cluster Quiescent Holocene Punctuated Slip New Madrid earthquake history inferred from Mississippi river channels Holbrook et al., 2006

Tuttle (2009) Meers fault, Oklahoma Active 1000 years ago, dead now Obermeier, (1998) Wabash: M~7 6 Kybp Faults in broad region active in past show little present seismicity Seismicity migrates among faults due to fault interactions (stress transfer)

“Large continental interior earthquakes reactivate ancient faults … geological studies indicate that earthquakes on these faults tend to be temporally clustered and that recurrence intervals are on the order of tens of thousands of years or more.” (Crone et al., 2003) Now recognize similar behavior in other continental interiors

during the period prior to the period instrumental events Earthquakes in North China Large events often pop up where there was little seismicity! Ordos Plateau Shanxi Graben Bohai Bay Beijing 1303 Hongtong M 8.0 Liu, Stein & Wang 2011 Weihi rift

during the period prior to the period instrumental events Earthquakes in North China Large events often pop up where there was little seismicity! Ordos Plateau Shanxi Graben Bohai Bay Beijing 1556 Huaxian M 8.3 Weihi rift Liu, Stein & Wang 2011

during the period prior to the period instrumental events Earthquakes in North China Large events often pop up where there was little seismicity! Ordos Plateau Shanxi Graben Bohai Bay Beijing 1668 Tancheng M 8.5 Weihi rift Liu, Stein & Wang 2011

during the period prior to the period instrumental events Earthquakes in North China Large events often pop up where there was little seismicity! Ordos Plateau Shanxi Graben Bohai Bay Beijing 1679 Sanhe M 8.0 Weihi rift Liu, Stein & Wang 2011

during the period prior to the period instrumental events Earthquakes in North China Large events often pop up where there was little seismicity! Ordos Plateau Shanxi Graben Bohai Bay Beijing 1966 Xingtai M Tangshan M Haicheng M 7.3 Weihi rift Liu, Stein & Wang 2011

No large (M>7) events ruptured the same fault segment twice in past 2000 years In past 200 years, quakes migrated from Shanxi Graben to N. China Plain Historical Instrumental Shanxi Graben Weihi rift

2011 M 9.1 Tohoku, 1995 Kobe M 7.3 & others in areas mapped as low hazard Geller 2011 Problems with hazard mapping increasingly recognized Bad luck or bad map?

2001 hazard map M7 earthquake shaking much greater than predicted for next 500 years

NY Times 3/21/11 Increasing recognition that map failures are common

Deadliest earthquakes since 2000 Have much greater shaking than predicted by GSHAP hazard map Peresan et al. (2011) After Kossobokov & Nekrasova (2011) Intensity difference Intensity difference among the observed values and those predicted by GSHAP

Accuracy of hazard map prediction depends on accuracy of answers assumed to hierarchy of four basic questions Where will large earthquakes occur? When will they occur? How large will they be? How strong will their shaking be? Uncertainty & map failure result because these are often poorly known, especially within continents

USGS 2008 Wenchuan earthquake (Mw 7.9) was not expected: map showed low hazard

Hazard map - assumed steady state - relied on lack of recent seismicity Didn’t use GPS data showing 1-2 mm/yr Earthquakes prior to the 2008 Wenchuan event Aftershocks of the Wenchuan event delineating the rupture zone M. Liu

Hazard maps are like ‘Whack-a-mole’ - you wait for the mole to come up where it went down, but it’s likely to pop up somewhere else.

Variability in recurrence is significant and much larger than often assumed Significant errors can result from short record, events missed from record, size underestimated Commonly used log-normal with  ~ 0.2 T av (Nishenko & Buland, 1987) can underestimate

Time dependent predicts lower until ~2/3 mean recurrence Results depend on both model choice & assumed mean recurrence Assumed probability of large earthquake & thus hazard depend on recurrence model & position in earthquake cycle Hebden & Stein, 2008 Not clear which model works best where

2% in 50 yr (1/2500 yr) 154% %106 Effect larger in Memphis Large uncertainty in maps

Newman et al., 2001 PREDICTED HAZARD ALSO DEPENDS GREATLY ON - Assumed maximum magnitude of largest events -Assumed ground motion model -Neither are well known since large earthquakes rare 180% 275%

What to do Realistically assess uncertainties and present them candidly to allow users to decide how much credence to place Develop methods to objectively test hazard maps and thus guide future improvements

Comparing map predictions shows the large uncertainties resulting from different assumptions Shows contributions to logic tree before subjective weighting

Testing analogy: evidence-based medicine objectively evaluates widely used treatments Although more than 650,000 arthroscopic knee surgeries at a cost of roughly $5,000 each were being performed each year, a controlled experiment showed that "the outcomes were no better than a placebo procedure."

Some hazard map testing challenges 1)Short time record: can in some cases be worked around. For example, North China record probably has almost or all M7s in 2000 years. Paleoseismology can go back even further, with higher probability of missing some. 2)Subjective nature of hazard mapping, resulting from need to chose faults, maximum magnitude, recurrence model, and ground motion model. This precludes the traditional method of developing a model from the first part of a time series and testing how well it does in the later part. That works if the model is "automatically" generated by some rules (e.g. least squares, etc). In the earthquake case, this can't be done easily because we know what happens in the later part of the series.

3) Biases due to new maps made after a large earthquake that earlier maps missed. Frankel et al, 2010 Before 2010 Haiti M7After 2010 Haiti M7 4X

A posteriori changes to a model are "Texas sharpshooting:” shoot at the barn and then draw circles around the bullet holes.

4) Overparameterized model (overfit data): Given a trend with scatter, fitting a higher order polynomial can give a better fit to the past data but a worse fit to future data Analogously, a seismic hazard map fit to details of past earthquakes could be a worse predictor of future ones than a less detailed map How much detail is useful? Linear fit Quadratic fit

Approaching intracontinental seismic zones as a complex system is necessary to improve understanding of midcontinental tectonics, the resulting earthquakes, and the hazards they pose. Summary Unlike plate boundary faults that give quasi-periodic earthquakes, interacting fault networks in midcontinents predict complex variability of earthquakes. Conventional seismic hazard assessment, which assumes steady behavior over years, can overestimate risks in regions of recent large earthquakes and underestimate them elsewhere.