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TABLE 1Criteria for Identifying Stable Continental Regions (SCRs)Time Interval * SCR Identification Criteria †Neogene Period(0–23 Ma)1. No rifting or major extensionor transtension after Paleogene2. No deformation of orogenicforeland after Early Cretaceous3. No orogenic activity afterEarly CretaceousPaleogene Period Allowed Not allowed Not allowed(23–65.5 Ma)Late Cretaceous Epoch Allowed Not alowed Not allowed(65.5–99.6 Ma)Early Cretaceous Epoch Allowed Allowed Allowed(99.6–145.5 Ma)* In parentheses, age range of the interval of geologic time, from Gradstein et al. (2004). Ma, millions of years ago.† After Kanter (1994). Allowed: deformation of this age and kind does not disqualify an area from being an SCR. Kanter listeda fourth criterion of no major anorogenic intrusions younger than Early Cretaceous. The criterion is not necessary forevaluation of the Southeast Asian SCRs.Coppersmith et al. (1987) and Coppersmith (1994) suggestedthat the CEUS and other areas worldwide that are tectonicallyanalogous to it may have similar values of Mmax. Coppersmithet al. (1987) suggested combining the historical earthquakesof geologically similar regions into a dataset large enough topotentially provide robust lower bounds on Mmax, or perhapsto be candidates for Mmax itself. Accordingly, Kanter (1994)expressed “tectonically analogous” in terms of four criteria thatbroadly characterize the tectonics of the CEUS and centraland eastern Canada (Table 1). Johnston et al. (1994) used theterm stable continental region (SCR) for an area that meets allfour criteria. Other continental areas are not considered to betectonic analogs of the CEUS and are classified as active continentalcrust (ACR).Participants in the CEUS Mmax workshop were acutelyaware that the geologic variables that control the value ofCEUS Mmax are poorly known (see discussions throughoutWheeler 2009b). Furthermore, the distinction between SCRsand ACRs is not clear in all continental areas. For example, tectonismyoung enough to classify an area as active crust accordingto the criteria of Table 1 may be sparse or unrecognized.Alternatively, the tectonism might not be clearly rifting, orogenicactivity, or deformation of an orogenic foreland. In caseswhere the distinction between stable and active crust is enigmatic,focusing attention on the brittle upper crust can helpto make the distinction. In other cases argument by geologicor tectonic analogy can clarify the distinction. Later sectionsdescribe illustrative cases in and around eastern Mongolia andin Indochina, respectively.With these uncertainties in mind, Kanter (1994) utilizedher criteria to define eight SCRs. Each continent contains atleast one SCR. The CEUS forms the southern half of the NorthAmerica SCR. Johnston et al. (1994) compiled geological andseismological information on SCR earthquakes worldwide.As already mentioned, the recent Mmax workshop produceda recommendation that future estimates of CEUS Mmax forseismic-hazard analyses should utilize the global SCR catalogof Johnston et al. (1994) (Wheeler 2009b, 141–143). Theglobal catalog shows that, after North America, the muchsmaller China SCR in Southeast Asia has the most historicalearthquakes of M 6.0 or larger (Figure 1). Consequently the1994 China SCR and its three parts as shown in Figure 1 areimportant tectonic analogs in estimating CEUS Mmax.PurposeSince the definition and delineation of SCRs in 1994, manypapers on the geophysics and tectonics of Southeast Asia haveappeared in English-language Western journals, for exampleYin (2010) and papers cited there. My purpose is to reassessthe 1994 China SCR and its earthquakes in light of the newinformation presented in these papers, in order to improve estimatesof CEUS Mmax. The 1994 China SCR of Kanter (1994)includes two thin bands of active continental crust that are centeredon large, active, strike-slip fault systems. The thin bandsdivide the 1994 China SCR into three parts that are labeledMO, CH, and IO in Figure 1. The rest of this paper utilizesthe new information and the criteria in Table 1 to update theMongolia, 2011 China, and Indochina SCRs of Figure 1. Theupdate will result in reclassifying most of the 1994 China SCRof Figure 1 as active crust. Nearly all of the Mongolia SCR willretain its classification as SCR crust, as will the southwesternpart of the 2011 China SCR.SOUTHEAST ASIAN SCRsContinental Extension and Alkaline Igneous RocksInformation published since the early 1990s (for example, Yin2010) shows that much of Southeast Asia is undergoing horizontalextension. Reassessing the SCR with the new informationrequires determining which parts of the 1994 ChinaSCR have undergone Neogene extension (Table 1). Geodeticand geophysical data and mapped extensional faults and continentalrifts provide well-known indicators of continentalextension.It may be less well known that dated alkaline igneous rocks,when combined with geologic field relations showing relativeages of faulting, eruption, and intrusion, can determine boththe occurrence of continental rifting and its age. Worldwide,972 Seismological Research Letters Volume 82, Number 6 November/December 2011