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EASTERN SECTIONRESEARCH LETTERSReassessment of Stable Continental Regionsof Southeast AsiaRussell L. WheelerRussell L. WheelerU. S. Geological SurveyABSTRACTProbabilistic seismic-hazard assessments of the central andeastern United States (CEUS) require estimates of the size ofthe largest possible earthquake (Mmax). In most of the CEUS,sparse historical seismicity does not provide a record of moderateand large earthquakes that is sufficient to constrain Mmax.One remedy for the insufficient catalog is to combine thecatalog of moderate to large CEUS earthquakes with catalogsfrom other regions worldwide that are tectonically analogousto the CEUS (stable continental regions, or SCRs). After theNorth America SCR, the largest contribution of earthquakesto this global SCR catalog comes from a Southeast Asian SCRthat extends from Indochina to southeasternmost Russia.Integration and interpretation of recently published geologicaland geophysical results show that most of these SoutheastAsian earthquakes occurred in areas exposing abundant alkalineigneous rocks and extensional faults, both of Neogene age(last 23 million years). The implied Neogene extension precludesclassification of the areas as SCR crust. The extensionalso reduces the number of moderate and large Southeast Asianhistorical earthquakes that are available to constrain CEUSMmax by 86 percent, from 43 to six.INTRODUCTIONMost probabilistic seismic-hazard assessments of the centraland eastern United States (CEUS: east of the RockyMountains) and elsewhere worldwide require an estimateof Mmax, the moment magnitude of the largest earthquakethat is thought to be possible within a specified area (Wheeler2009a,b). Wheeler (2009a) cited example assessments, includingRisk Engineering Inc. et al. (1986), Johnston et al. (1994),and Petersen et al. (2008). The value of Mmax is important inprobabilistic computations for building codes and for design ofcritical structures such as nuclear power plants (Mueller 2010).Accurate estimates of Mmax are more important for nuclearreactors than building codes because reactor designs requireconsideration of smaller annual probabilities of unexpectedlystrong ground motions (Petersen et al. 2008; Office of NuclearRegulatory Research 2007).The historical record of the CEUS contains earthquakes ofmoment magnitude M 7.0 or larger only at the seismic zones ofNew Madrid, Missouri; Charleston, South Carolina; and perhapsCharlevoix, Quebec (Ebel 1996, 2011; Johnston 1996c;Hough et al. 2000; Bakun and Hopper 2004). Elsewhere inthe CEUS, sparse seismicity suggests that large earthquakesmay have recurrence intervals longer than the historical record,which is generally two to four centuries long. Wherever sufficientpaleoseismic work has been done in the CEUS outsidethe New Madrid, Charleston, and Charlevoix zones, findingsdocument occurrences of prehistoric earthquakes larger thanany in the historical record that occurred at intervals longerthan the historical record (Madole 1988; Crone and Luza1990; Crone, Machette, and Bowman 1997; Crone, Machette,Bradley et al. 1997; Obermeier 1998; McNulty and Obermeier1999; Tuttle et al. 2006; Cox et al. 2010). If recurrence intervalsare that long, then earthquakes larger than any observedhistorically are possible. If such an earthquake is not in the historicalrecord, then Mmax may not have been observed and itmust be estimated by other means. Indirect methods based onphysics, statistics, or the geologic properties of small areas havegenerally given Mmax estimates that lack strong supportingevidence (Chinnery 1979; Coppersmith et al. 1987; Wheeler2009a). Another approach was needed and the next sectionsummarizes it.Stable Continental RegionsA recent workshop on CEUS Mmax concluded that identificationand study of global tectonic analogs of the CEUS andtheir seismicity is the preferred approach to the problems arisingfrom short historical records (Wheeler 2009b, 141–143).doi: 10.1785/gssrl.82.6.971Seismological Research Letters Volume 82, Number 6 November/December 2011 971