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TOPIC 2: How does the challenge of predicting hazards differ between earthquakes - at plate boundaries - In plate boundary zones - within plates? Earthquake locations map narrow plate boundaries, broad plate boundary zones & regions of intraplate deformation DIFFUSE BOUNDARY ZONES INTRAPLATE NARROW BOUNDARIES Stein & Wysession, 2003 5.1-4 PLATE BOUNDARY PLATE BOUNDARY ZONE INTRAPLATE A. Newman Mountain building by continental collision produced boundary zone extending 1000’s of km northward from the nominal plate boundary at the Himalayan front. COLLISION BETWEEN INDIAN AND EURASIAN PLATES: SPACE GEODETIC MOTIONS. Total plate convergence taken up several ways. About half occurs across locked Himalayan frontal faults such as the Main Central Thrust These faults are part of the interface associated with the underthrusting Indian continental crust, which thickens crust under high Himalayas. Larson et al., 1999 COLLISION BETWEEN INDIAN AND EURASIAN PLATES: SPACE GEODETIC MOTIONS. GPS data also show along-strike motion behind the convergent zone, in the Tibetan Plateau, presumably because the uplifted and thickened crust spreads under its own weight. Extension is part of a large-scale process of crustal "escape" or "extrusion" in which large fragments of continental crust are displaced eastward by the collision along major strike-slip faults. Larson et al., 1999 NORTH AMERICA - PACIFIC PLATE MOTION 48 mm/yr PLATE BOUNDARY: SAN ANDREAS 30-45 mm/yr 1900-2002 NORTH AMERICA PACIFIC PLATE BOUNDARY ZONE: 1-10 mm/yr OFF MAIN BOUNDARY INTRAPLATE : (< 1 mm/yr) CONTINENTAL STRIKE SLIP BOUNDARY ZONE Stein, 1993 PLATE BOUNDARY Most of the plate motion occurs on narrow (< 100 km wide) boundary Earthquakes result primarily from plate motion Earthquake recurrence directly reflects known plate motion Earthquake locations don’t change for long time (Myr) Past plate motion, present plate motion, geological, seismological, and geodetic rates are consistent and so give consistent estimates of earthquake hazard 1906 SAN FRANCISCO EARTHQUAKE Average 4 m of motion West side moved north Motion along hundreds of miles of San Andreas Fault USGS ELASTIC REBOUND MODEL PROPOSED Over many years, rocks on opposite sides of the fault move, but friction on the fault "locks" it and prevents slip Eventually strain stored is more than fault rocks can withstand, and the fault slips in earthquake Before plate tectonics, no idea why motion occurred Even so, F. Omori used Japanese experience to predict that a similar earthquake was at least 100 years away SAN ANDREAS FAULT GPS FAR FIELD SLIP RATE 35 mm/yr GEOLOGIC SLIP RATE 3700 yr ~ 35 mm/yr Locked strain will be released in next earthquake Since last earthquake in 1857 ~ 5 m slip accumulated Z.-K. Shen Agreement between paleoseismology, plate motion, & GPS shows that large earthquakes take up most of the motion mean 132 yr s 105 yr Although time between earthquakes is variable 36 mm/yr * 132 yr ~ 5 m slip ~ magnitude 7.7 earthquake Sieh et al., 1989 VARIATIONS IN RECURRENCE TIME MAY BE DUE TO DIFFERENCES IN EARTHQUAKE SIZE AND STRESS TRANSFER R. Stein et al., 1997 NORTH AMERICA - PACIFIC PLATE MOTION 1900-2002 NORTH AMERICA 48 mm/yr PACIFIC PLATE BOUNDARY ZONE: 1-10 mm/yr OFF MAIN BOUNDARY PLATE BOUNDARY ZONE (1-10 mm/yr) Some of the plate motion occurs in broad (< 1000 km wide) zone away from boundary Earthquakes result from plate motion and local effects (topography) Earthquake recurrence does not reflect known plate motion Earthquake locations change on intermediate time (10 - 100 Kyr) Geological, seismological, and geodetic rates are usually consistent and so give consistent estimates of earthquake hazard PACIFIC NORTH AMERICA PLATE BOUNDARY ZONE Earthquakes away from San Andreas San Andreas Fault system GPS site velocities relative to North America Bennett et al., 1999 Intermountain seismic belt Eastern California shear zone Colorado Plateau Wasatch fault, Salt Lake City, Utah M~7 earthquakes in past 6000 years None in past 500 years GPS shows strain building up for future earthquakes WASATCH FAULT: GPS & EARTHQUAKES AGREE 1 mm/yr -> 1 m/ 1000 yrs -> M7 M 7 expected ~ 1000 yr from seismicity GPS consistent - shows ~1-2 mm/yr extension Stein et al., 2005 Chang et al., 2006 PLATE INTERIOR (2< mm/yr) Plate interior deforms slowly far away from boundary Don’t know what causes earthquakes, probably indirect result from plate motion, mantle flow, and local effects (topography, sediment, glacial) Earthquake recurrence does not reflect known plate motion Earthquake locations change on short time scales (100s - Kyr) Geological, seismological, and geodetic rates can differ and so give different estimates of earthquake hazard CONTINENTAL INTRAPLATE EARTHQUAKES Complex regional system of interacting faults Seismicity migrates between faults due to stress transfer Seismicity varies in space and time Earthquakes can occur on fault for a while, then move Past can be poor predictor A complex system whose behavior depends on the interactions between components that can’t be viewed in isolation McKenna, Stein & Stein, 2007 CONTINENTAL INTRAPLATE EARTHQUAKES ARE OFTEN EPISODIC, CLUSTERED & MIGRATING “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) Meers fault, Oklahoma Active 1000 years ago, dead now Liu, Stein & Wang 2011 during the period prior to the period instrumental events Earthquakes in North China Beijing Bohai Bay Ordos Plateau 1303 Hongtong M 8.0 Weihi rift Large events often pop up where there was little seismicity! Liu, Stein & Wang 2011 during the period prior to the period instrumental events Earthquakes in North China Beijing Bohai Bay Ordos Plateau Weihi rift 1556 Huaxian M 8.3 Large events often pop up where there was little seismicity! Liu, Stein & Wang 2011 during the period prior to the period instrumental events Earthquakes in North China Beijing Bohai Bay Ordos Plateau Weihi rift 1668 Tancheng M 8.5 Large events often pop up where there was little seismicity! Liu, Stein & Wang 2011 during the period prior to the period instrumental events Earthquakes in North China 1679 Sanhe M 8.0 Beijing Bohai Bay Ordos Plateau Weihi rift Large events often pop up where there was little seismicity! Liu, Stein & Wang 2011 during the period prior to the period instrumental events Earthquakes in North China 1975 Haicheng M 7.3 Beijing 1976 TangshanBohai Bay M 7.8 Ordos Plateau 1966 Xingtai M 7.2 Weihi rift Large events often pop up where there was little seismicity! No large (M>7) events ruptured the same fault segment twice in past 2000 years Historical Instrumental Weihi rift In past 200 years, quakes migrated from Shanxi Graben to N. China Plain 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. NEW MADRID SEISMICITY: 1811-12 AFTERSHOCKS? Stein & Newman, 2004 Ongoing seismicity looks like aftershocks of 1811-12, as suggested by the fact that the rate & size are decreasing. Moreover, the largest are at the ends of the presumed 1811-12 ruptures Rate-state friction predicts aftershock duration 1/loading rate Plate boundary faults quickly reloaded by steady plate motion after large earthquake Long aftershock sequences in slowly deforming continental interiors Stein & Liu, 2009 Stein & Liu 2009 Faults in continents reloaded much more slowly, so aftershocks continue much longer Lots of seismicity may be aftershocks Aftershock sequence of the 1976 Tangshan earthquake continues today (M. Liu) Effect of major (5 MPa) weak zones Complex space-time variability due to fault interactions via stress transfer Seismicity extends beyond weak zones Short-term seismicity does not fully reflect long-term Variability results from steady platewide loading without local or time-variable loading We are just beginning to study how mid-continent earthquakes work, but it seems that often Hazard assessment based only on the recent earthquake record overestimates the risks in regions of recent large earthquakes and underestimates them where seismicity has been recently quiescent. PLATE BOUNDARY: 1900-2002 Hazard assessment is reasonably good (“B”) NORTH AMERICA PACIFIC All should be better in China due to longer earthquake record PLATE BOUNDARY ZONE: Hazard assessment is adequate (“C”) INTRAPLATE: Hazard assessment is probably poor (“D”)
