rassessment of e stable continental regions of utheast so...

Seismological Research Letters Volume 82, Number 6 November/December 2011 971doi: 10.1785/gssrl.82.6.971

E A S T E R N S E C T I O N

R E S E A R C H L E T T E R S

reassessment of stable Continental regions

of southeast asiaRussell L. Wheeler

Russell L. WheelerU. S. Geological Survey

ABSTRACT

Probabilistic seismic-hazard assessments of the central and eastern United States (CEUS) require estimates of the size of the largest possible earthquake (Mmax). In most of the CEUS, sparse historical seismicity does not provide a record of moder-ate and large earthquakes that is sufficient to constrain Mmax. One remedy for the insufficient catalog is to combine the catalog of moderate to large CEUS earthquakes with catalogs from other regions worldwide that are tectonically analogous to the CEUS (stable continental regions, or SCRs). After the North America SCR, the largest contribution of earthquakes to this global SCR catalog comes from a Southeast Asian SCR that extends from Indochina to southeasternmost Russia. Integration and interpretation of recently published geologi-cal and geophysical results show that most of these Southeast Asian earthquakes occurred in areas exposing abundant alka-line igneous rocks and extensional faults, both of Neogene age (last 23 million years). The implied Neogene extension pre-cludes classification of the areas as SCR crust. The extension also reduces the number of moderate and large Southeast Asian historical earthquakes that are available to constrain CEUS Mmax by 86 percent, from 43 to six.

INTRODUCTION

Most probabilistic seismic-hazard assessments of the cen-tral and eastern United States (CEUS: east of the Rocky Mountains) and elsewhere worldwide require an estimate of Mmax, the moment magnitude of the largest earthquake that is thought to be possible within a specified area (Wheeler 2009a,b). Wheeler (2009a) cited example assessments, includ-ing Risk Engineering Inc. et al. (1986), Johnston et al. (1994), and Petersen et al. (2008). The value of Mmax is important in probabilistic computations for building codes and for design of critical structures such as nuclear power plants (Mueller 2010).

Accurate estimates of Mmax are more important for nuclear reactors than building codes because reactor designs require consideration of smaller annual probabilities of unexpectedly strong ground motions (Petersen et al. 2008; Office of Nuclear Regulatory Research 2007).

The historical record of the CEUS contains earthquakes of moment magnitude M 7.0 or larger only at the seismic zones of New Madrid, Missouri; Charleston, South Carolina; and per-haps Charlevoix, Quebec (Ebel 1996, 2011; Johnston 1996c; Hough et al. 2000; Bakun and Hopper 2004). Elsewhere in the CEUS, sparse seismicity suggests that large earthquakes may have recurrence intervals longer than the historical record, which is generally two to four centuries long. Wherever suf-ficient paleoseismic work has been done in the CEUS outside the New Madrid, Charleston, and Charlevoix zones, findings document occurrences of prehistoric earthquakes larger than any in the historical record that occurred at intervals longer than the historical record (Madole 1988; Crone and Luza 1990; Crone, Machette, and Bowman 1997; Crone, Machette, Bradley et al. 1997; Obermeier 1998; McNulty and Obermeier 1999; Tuttle et al. 2006; Cox et al. 2010). If recurrence inter-vals are that long, then earthquakes larger than any observed historically are possible. If such an earthquake is not in the his-torical record, then Mmax may not have been observed and it must be estimated by other means. Indirect methods based on physics, statistics, or the geologic properties of small areas have generally given Mmax estimates that lack strong supporting evidence (Chinnery 1979; Coppersmith et al. 1987; Wheeler 2009a). Another approach was needed and the next section summarizes it.

Stable Continental RegionsA recent workshop on CEUS Mmax concluded that identifi-cation and study of global tectonic analogs of the CEUS and their seismicity is the preferred approach to the problems aris-ing from short historical records (Wheeler 2009b, 141–143).

972 Seismological Research Letters Volume 82, Number 6 November/December 2011

Coppersmith et al. (1987) and Coppersmith (1994) suggested that the CEUS and other areas worldwide that are tectonically analogous to it may have similar values of Mmax. Coppersmith et al. (1987) suggested combining the historical earthquakes of geologically similar regions into a dataset large enough to potentially provide robust lower bounds on Mmax, or perhaps to be candidates for Mmax itself. Accordingly, Kanter (1994) expressed “tectonically analogous” in terms of four criteria that broadly characterize the tectonics of the CEUS and central and eastern Canada (Table 1). Johnston et al. (1994) used the term stable continental region (SCR) for an area that meets all four criteria. Other continental areas are not considered to be tectonic analogs of the CEUS and are classified as active conti-nental crust (ACR).

Participants in the CEUS Mmax workshop were acutely aware that the geologic variables that control the value of CEUS Mmax are poorly known (see discussions throughout Wheeler 2009b). Furthermore, the distinction between SCRs and ACRs is not clear in all continental areas. For example, tec-tonism young enough to classify an area as active crust accord-ing to the criteria of Table 1 may be sparse or unrecognized. Alternatively, the tectonism might not be clearly rifting, oro-genic activity, or deformation of an orogenic foreland. In cases where the distinction between stable and active crust is enig-matic, focusing attention on the brittle upper crust can help to make the distinction. In other cases argument by geologic or tectonic analogy can clarify the distinction. Later sections describe illustrative cases in and around eastern Mongolia and in Indochina, respectively.

With these uncertainties in mind, Kanter (1994) utilized her criteria to define eight SCRs. Each continent contains at least one SCR. The CEUS forms the southern half of the North America SCR. Johnston et al. (1994) compiled geological and seismological information on SCR earthquakes worldwide. As already mentioned, the recent Mmax workshop produced a recommendation that future estimates of CEUS Mmax for seismic-hazard analyses should utilize the global SCR cata-log of Johnston et al. (1994) (Wheeler 2009b, 141–143). The global catalog shows that, after North America, the much

smaller China SCR in Southeast Asia has the most historical earthquakes of M 6.0 or larger (Figure 1). Consequently the 1994 China SCR and its three parts as shown in Figure 1 are important tectonic analogs in estimating CEUS Mmax.

PurposeSince the definition and delineation of SCRs in 1994, many papers on the geophysics and tectonics of Southeast Asia have appeared in English-language Western journals, for example Yin (2010) and papers cited there. My purpose is to reassess the 1994 China SCR and its earthquakes in light of the new information presented in these papers, in order to improve esti-mates of CEUS Mmax. The 1994 China SCR of Kanter (1994) includes two thin bands of active continental crust that are cen-tered on large, active, strike-slip fault systems. The thin bands divide the 1994 China SCR into three parts that are labeled MO, CH, and IO in Figure 1. The rest of this paper utilizes the new information and the criteria in Table 1 to update the Mongolia, 2011 China, and Indochina SCRs of Figure 1. The update will result in reclassifying most of the 1994 China SCR of Figure 1 as active crust. Nearly all of the Mongolia SCR will retain its classification as SCR crust, as will the southwestern part of the 2011 China SCR.

SOUTHEAST ASIAN SCRs

Continental Extension and Alkaline Igneous RocksInformation published since the early 1990s (for example, Yin 2010) shows that much of Southeast Asia is undergoing hori-zontal extension. Reassessing the SCR with the new infor-mation requires determining which parts of the 1994 China SCR have undergone Neogene extension (Table 1). Geodetic and geophysical data and mapped extensional faults and con-tinental rifts provide well-known indicators of continental extension.

It may be less well known that dated alkaline igneous rocks, when combined with geologic field relations showing relative ages of faulting, eruption, and intrusion, can determine both the occurrence of continental rifting and its age. Worldwide,

TABLE 1Criteria for Identifying Stable Continental Regions (SCRs)

Time Interval* SCR Identification Criteria†

Neogene Period(0–23 Ma)

1. No rifting or major extension or transtension after Paleogene

2. No deformation of orogenic foreland after Early Cretaceous

3. No orogenic activity after Early Cretaceous

Paleogene Period(23–65.5 Ma)

Allowed Not allowed Not allowed

Late Cretaceous Epoch(65.5–99.6 Ma)

Allowed Not alowed Not allowed

Early Cretaceous Epoch(99.6–145.5 Ma)

Allowed Allowed Allowed

* 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 listed

a fourth criterion of no major anorogenic intrusions younger than Early Cretaceous. The criterion is not necessary for evaluation of the Southeast Asian SCRs.

Seismological Research Letters Volume 82, Number 6 November/December 2011 973

igneous rocks of alkaline compositions are spatially associated with continental rifts (Bailey 1974; Neumann and Ramberg 1978; Keller and Hoover 1988; McKenzie and Bickle 1988; Wilson 1989). The spatial association is generally accepted as implying that extension produces alkaline melts (Wilson 1989). Furthermore, common crustal rocks have melting tempera-tures well below those of basalts. This implies that melting in the mantle generates alkaline basalts. Petrological modeling calculations of McKenzie and Bickle (1988) and of Barry et al. (2003) imply that the melting of peridotite, a common mantle rock, yields alkaline basaltic melts at depths exceeding 70 km. In laboratory experiments, the initial melting of peridotite under mantle pressures and temperatures produces small amounts of alkaline basaltic melts (Jaques and Green 1980; Olafsson and Eggler 1983; Takahashi and Kushiro 1983). Both the melting experiments and the petrological calculations show that addi-tional melting shifts the composition of basaltic melts away from alkaline toward less-alkaline basaltic compositions. Most rift-related alkaline igneous rocks are alkaline basalts. In addition, some rifts also contain alkaline volcanic and intrusive rocks of granitic compositions. Alkaline igneous rocks are known in sev-eral parts of a rift that contains the New Madrid seismic zone,

the most active seismic zone in the CEUS (Figure 1) (see sum-mary of these alkaline rocks in Wheeler 1997). Importantly for the present study, basaltic rocks dominate in Southeast Asia (Whitford-Stark 1987; Yin 2010), for example in the Baikal rift system of the study area (Wilson 1989; Figure 2 this paper). Thus, volcanic rocks of alkaline basaltic composition imply a small amount of extension within the upper mantle.

Exposed or shallow normal or transtensional faults dem-onstrate extension of the upper crust and its seismogenic zone. The larger the extensional fault slips, the more likely it is that brittle extension penetrates into or spans the seismogenic zone. Where both rifts and alkaline basaltic volcanic rocks are pres-ent and are of similar ages, they indicate that extension affects the upper crust, upper mantle, and therefore perhaps the mid-dle and lower crust as well. Rifts without known alkaline vol-canic rocks demonstrate brittle extension of at least the upper-most crust, perhaps including the seismogenic zone. However, alkaline volcanic rocks without recognized, coeval extensional faults are more problematic. Absent known extensional faults, alkaline rocks might indicate that incipient extension in the upper mantle has not extended far enough upward to affect the seismogenic zone.

0 2000Kilometers

100°W60°W

30°N30°N

70°N60°W100°W70°N

*

0 2000Kilometers100°E

140°E100°E

10°N

50°N

10°N

140°E

50°N

MO

CH

IO

Earthquake epicenter

Mainland and coastline of continent

Boundary of stable continental region

Large earthquakes of New Madrid seismic zone*

(A)

(B)

Wheeler, Figure 1

▲ Figure 1. Comparison of sizes and seismicities of North America (A) and China (B) stable continental regions (SCRs). Coastlines and SCR boundaries after Kanter (1994) and Broadbent and Allan Cartography (1994). Epicenters are of earthquakes of magnitude 6.0 or larger on any magnitude scale (Wheeler, in preparation). Both maps use Lambert azimuthal equal area projections and the same scale to aid visual comparison (Broadbent and Allan Cartography 1994). The North America SCR covers 24,007,000 km2 and generated 35 reported earthquakes of magnitude 6.0 or larger over the four-century historical record, whereas the smaller China SCR covers 7,118,000 km2 and has generated 27 such earthquakes over its 15-century historical record (Johnston et al. 1994). Chinese earthquakes appear more numerous in the figure because more of the North American epicenters overlap one another at the scale of the figure. For ease of discussion, I will refer to the single large SCR of part B as the 1994 China SCR, and to its three components as the Mongolia SCR (MO), the 2011 China SCR (CH), and the Indochina SCR (IO).


0 1,000Kilometers

140°

140°

130°E

130°120°

120°

110°

110°

100°

100°

90°

90°

80°

80°

20°

50°

30°

40°

10°

20°

50°

30°

40°

10°

Right lateral strike-slip fault

Left lateral strike-slip fault

Reverse fault (teeth onupthrown wall)

Normal fault (hachure ondowndropped wall)

Epicenter in stablecontinental crust

Epicenter in activecontinental crust

Area of Neogene volcanic rocks

Large crustal block (see text)

Borders of China and Mongolia

Border of stable continental region (SCR) (Kanter, 1994)

EXPLANATION

LPSFB

XX

FS

SBF

LMSFQLFZ

TLF

TKFS

HES

SF

KES

BRSSFZ

ASRR

ASRR

Mongolia SCR

2011China SCR

Indo-China SCR

IndianOcean

SouthChinaSea

YellowSea

EastChinaSea

Taiwan

Hainan Island

Outlines of islands other than Hainan and Taiwan are omitted for clarity.

NORTHCHINABLOCK

SOUTHCHINABLOCK

ECCM

ECCM

Mongolia ChinaRussia

ChinaVietnamLaos

ChinaBurma

North

KoreaChin

a

Siberia

W

Wheeler, Figure 2 ▲ Figure 2. Selected tectonic elements of Southeast Asia. Stable continental regions (SCRs) after Broadbent and Allan Cartography (1994) and Kanter (1994) (See Figure 1 for new SCR names introduced here.) North China block and the adjoining part of the Korean peninsula after Zhang et al. (1984), Zhao et al. (1998), and Kwon et al. (2009). South China block after Ren et al. (2002), Liu et al. (2007), J. Adams (see Wheeler 2009b, 83), Yin (2010), and analyses in this paper. Locations of areas of Neogene volcanic rocks are from the continent-scale map in Figure 1 of Yin (2010). For legibility here I generalized the locations. Each solid square represents the center of a group of approximately five of the locations shown by Yin (2010). Epicenters are of reported earthquakes of magnitude 6.0 or larger on any magnitude scale (Wheeler, in preparation). W, epicenter of Wenchuan earthquake (12 May 2008; M 7.9; 31.00°N, 103.32°E; http://earthquake.usgs.gov/). Faults shown have Neogene movement. Their locations and age assignments are after Peizhen et al. (1991), Ren et al. (2002), Jia et al. (2006), Zhu et al. (2010), and Yin (2010). ECCM, eastern China continental margin. Fault names: ASRR, Ailao Shan–Red River shear zone; BRS, Baikal rift system; HES, Hangay extensional system; KES, Khubsugul extensional system; LMSF, Longmen Shan thrust fault; LPSFB, Lanping-Simao fold belt; QLFZ, Qinling fault zone; SBF, Sichuan Basin fault; SF, Sangaing fault; SFZ, Stanovoy fault zone; TKFS, Tunka fault system; TLF, Tanlu fault; XXFS, Xiangshuihe-Xiaojiang fault system. Lambert azimuthal equal area projection centered at 25°N, 100°E.


Mongolia SCRThe presence or absence of Neogene alkaline igneous rocks and rifting provides a guide to whether the Mongolia SCR should be classified as an SCR or an ACR. The Mongolia SCR includes the eastern half of Mongolia, most of northeastern China, and adjacent areas of Russia (Figure 2). The Mongolia SCR is a com-paratively stable region. ACR crust surrounds this SCR; later sections will summarize the active nature of the North China block on the south, a region of active faults and volcanism on the west and north, and the eastern China continental margin on the east. The Tanlu fault separates the Mongolia and 2011 China SCRs. The Mongolia SCR is moving eastward with respect to a fixed Eurasia (Liu et al. 2007; Wang et al. 2011). The relative motion takes place on a belt of extensional and left-lateral transtensional faulting north of the SCR, between it and the Siberian part of the Eurasian SCR. The belt of faulting comprises the Hangay and Khubsugul extensional systems, the Tunka fault system, the Baikal rift system, and the Stanovoy fault zone (Figure 2). Judging from the motions that Liu et al. used to compute Quaternary rates of fault slips, the Mongolia SCR appears to be moving eastward with respect to Siberia at much less than 1 mm/yr, and possibly as little as 0.1 mm/yr.

Tomography shows that S-wave velocities at 50 km depth are similar across the Mongolia SCR, and the same is true for 100 km depth (Feng and An 2010). From this it appears that crustal and lithospheric thicknesses vary little across the SCR. Seismicity is sparse and geodetically measured velocity and strain are small (Broadbent and Allan Cartography 1994; Liu et al. 2007; Feng and An 2010) (see also the earthquake cata-logs at http://earthquake.usgs.gov/earthquakes; last accessed July 21, 2011).

The compilation map of Ren et al. (2002) shows normal faults that bound Paleogene and older basins throughout the SCR, but no Neogene faults or basins. Active systems of north-erly striking normal faults and easterly striking left-lateral strike-slip faults in western Mongolia do not appear to extend into the SCR, except in its northwestern corner at the north-east-striking normal faults of the Hangay extensional system (Figure 2) (McCalpin and Khromovskikh 1995; Walker et al. 2007; Yin 2010). Walker (2009) did not find active faults in eastern Mongolia and adjacent China, which include most of the Mongolia SCR.

Figure 2 shows that Neogene volcanic rocks are less numer-ous per unit area within the Mongolia SCR than in more tec-tonically active regions, such as the Indochina SCR and China east of the South China block and the Tanlu fault (Ren et al. 2002; Liu et al. 2007; Yin 2010). The Neogene volcanic rocks in the Mongolia SCR are largely alkaline although older volca-nic rocks range more widely in compositions (Whitford-Stark 1987; Basu et al. 1991). Barry et al. (2003) cited computations by McKenzie and Bickle (1988), which imply that generation of significant amounts of alkaline melts would require much more Neogene horizontal extension than appears to have occurred in most of the Mongolia SCR. Consequently, since the definition of the Mongolia SCR in 1994, new information does not demonstrate extension younger than Paleogene except

in the northwestern corner of the Mongolia SCR. The rest of the Mongolia SCR meets the criteria of Table 1 and retains its classification as an SCR.

2011 China SCR

North China BlockThe North China craton is the Chinese part of the Sino-Korean craton, with the remainder being the northern part of the Korean peninsula (for example, Zhang et al. 1984, Zhao et al. 2009, Yang et al. 2010). I follow Kwon et al. (2009) in calling both cratons “blocks” because, as explained later, they underwent Mesozoic and Cenozoic metamorphism, extension, intrusion, and volcanism so that they are no longer cratonic crust (Figure 2; note that the craton boundary is northwest of the boundary between North and South Korea). The Tanlu fault splits the North China block into two parts. The larger part of the block lies entirely west of the 1994 and 2011 China SCRs, whereas the smaller part is within both versions of the SCR. The smaller part is of more interest here, but most of the information on the North China block comes from the active crust of the larger part. Therefore, I will discuss the North China block as a whole. The block is a triangular region in northern China (Figure 2) that is made of early Precambrian crust (Zhang et al. 1984; Zhao et al. 2001; Kwon et al. 2009). The North China block is moving eastward with respect to a fixed Siberia and the Mongolia SCR (Yin 2010). Geologic data including slip rates of individual faults indicate eastward move-ment with respect to Siberia of 1–2 mm/yr in the eastern half of the North China block and 2–4 mm/yr in the western half (Liu et al. 2007). Geodetic data indicate rates consistent with those of Liu et al. (2007) (Wang et al. 2011).

The eastern part of the block is seismically active, whereas the western part is less so (Liu et al. 2007). For example, the 2008 version of the “Centennial” earthquake catalog of Engdahl and Villasenor (2002) lists 17 earthquakes of mag-nitude 6.0 or larger in the eastern part of the block but only one in the western part. Results of P- and S-wave tomography show a low-velocity zone that extends to 300–400 km depth beneath the eastern part of the block (Zhao et al. 2009). S-wave tomography, deep seismic-reflection profiles, and receiver-func-tion imaging show that the crust thins eastward from approxi-mately 45 km in the western part of the block to about 30 km in the eastern part (Li et al. 2006; Zheng et al. 2006; Chen et al. 2009; Feng and An 2010). Zheng et al. (2006) concluded that most of the thinning took place in the lower crust and in a transitional zone between the crust and mantle. The thin-ning resulted from extension that began with widespread Early Cretaceous eruption and intrusion of alkaline basaltic and gra-nitic rocks (Ren et al. 2002; Wu et al. 2005; Zhu et al. 2010). The entire North China block underwent extension by normal and transtensional faulting of early Neogene age, whereas the eastern part of the block and the northern part of the Korean peninsula also underwent late Neogene alkaline igneous activ-ity (Liu et al. 2001; Ren et al. 2002; Zheng et al. 2006; Yu et al. 2008; Yang et al. 2010; Yin 2010). Zhao et al. (2009) inter-


preted the low-velocity zone in the mantle beneath the eastern part of the block in terms of warm mantle material that might have caused the rifting and alkaline igneous activity. Extension is demonstrated in the upper crust by the mapped extensional faults, in the lower crust by the geophysical evidence, and deeper than 70 km (McKenzie and Bickle 1988) in the mantle by the compositions of the igneous rocks. The Neogene exten-sion demonstrates that the North China block and the rest of the Sino-Korean block are no longer a craton (Yang et al. 2008, 2010). The extension also requires reclassifying this part of the 1994 China SCR as active crust (Table 1).

Eastern China Continental MarginThe eastern China continental margin comprises all of the Yellow Sea, the East China and South China seas within 200–600 km of the mainland Chinese coast, and the land east of the North China and South China blocks (Figure 2; Zhou et al. 1995; Yin 2010). Bathymetric, geologic, and geo-physical data show that the continental crust of the margin extends seaward approximately to the edge of the continental shelf (GEBCO World Map Editorial Board 2006; Wang et al. 2006; Yin 2010). Fault-slip data show that the margin is mov-ing south-southwestward past the North and South China blocks (Figure 2; Yin 2010); geodetic data give the rate as 0.7 mm/yr in the north and 1.8 mm/yr in the south (Wang et al. 2011). The continental margin is more seismically active than the South and North China blocks, especially west of Taiwan (Liu et al. 2007; Wang et al. 2011).

The compilations of Yin (2010), Ren et al. (2002), and Sengor and Natal’in (2001) show the continental margin as having undergone Paleogene and older extension. S-wave tomography indicates thick sediments and thin crust and lith-osphere under the margin (Feng and An 2010). China east of the North and South China blocks contains numerous basins bounded by normal faults; most of the basins are of Paleogene age (Ren et al. 2002). In the same part of China and in off-shore basins on the continental shelf, abundant basaltic volca-nic rocks of late Neogene ages are exposed from Hainan Island northeastward to the southern Tanlu fault (Figure 2; Ren et al. 2002; Yin 2010). Many of the basaltic rocks are alkaline (Ho et al. 2003). In the South China Sea, early Neogene thermal sub-sidence of several kilometers was followed by middle Neogene normal faulting (Zhou et al. 1995). The alkaline basalts and normal faulting indicate Neogene extension of the continental margin from Hainan Island to the southern Tanlu fault.

Yin (2010) restricted the definition of the margin north-east of North Korea to offshore continental crust that has been thinned by back-arc extension. However, three onshore areas between the coast and the Tanlu fault must also be considered because they are in the 1994 China SCR, or adjacent to it and along its tectonic grain (Figure 2):

1. The northern Korean peninsula and the northern Yellow Sea are part of the 1994 China SCR. The northern pen-insula underwent Neogene extension as described earlier. Thus, ACR of the Sino-Korean block and the eastern China continental margin surround the northern Yellow

Sea on three sides (Figure 2). The Neogene extension implies that the northern Korean peninsula, and probably the adjacent part of the Yellow Sea, should be reclassified as active crust (Table 1).

2. Northeast of the North China block and east of the Tanlu fault, northeastern China and adjacent Russia are not in the 1994 China SCR. This area contains numerous Neogene basalts, many of them alkaline (Liu et al. 2001; Ren et al. 2002). The alkaline basalts indicate Neogene extension, and Sengor and Natal’in (2001) show latest Cretaceous–early Neogene grabens in the same area as the basalts. The evidence for Neogene extension implies that the area was correctly classified as active crust in 1994.

3. The southern part of the Korean peninsula is part of the 1994 China SCR. Yin (2010) reported that the peninsula appears to have had little Paleogene or Neogene extension. The southern part of the peninsula is outside the Sino-Korean block. Summaries of Korean geology and tecton-ics do not show any evidence of foreland deformation or orogenic activity of Cretaceous or younger age (Table 1) (Exxon Production Research Company 1985; Chang 1997; Kwon et al. 2009). However, the southeastern part of the peninsula exposes a few extensional or transten-sional faults that bound Neogene basins, as well as sparse Neogene alkaline volcanic rocks (Exxon Production Research Company 1985; Ren et al. 2002; Yang et al. 2010; Yin 2010). Two small islands approximately 170 km east of the mainland and 120 km south of it also expose sparse Neogene alkaline volcanic rocks (Reedman and Kim 1997). Kanter (1994) classified the southern island as being within the same northeast-trending belt of Mesozoic continental crust as the southern part of the Korean pen-insula and most of the Yellow Sea (Broadbent and Allan Cartography 1994). The Neogene alkaline rocks exposed on the southern island, together with the Neogene alka-line rocks exposed along trend to the southwest in the eastern China continental margin (Figure 2), suggest that similar Neogene alkaline rocks may be hidden beneath the Yellow Sea. Thus, the evidence for Neogene extension is sparse and whether the southern part of the Korean pen-insula and the Yellow Sea are SCR crust or ACR crust is arguable. Overall, I judge that neither area meets the requirements for classification as an SCR (Table 1).

Regional geologic relations are consistent with the classification of the southern part of the Korean peninsula as probable ACR crust. As pointed out earlier in this section, all other areas east of the Tanlu fault, including the northern part of the Korean peninsula, have undergone Neogene extension. As a result, 500–600 km of the active crust of the North China block sep-arates the southern Korean peninsula from the nearest SCR, which is the Mongolia SCR. This isolation allows the possibil-ity that the southern Korean peninsula might be a small frag-ment of comparatively intact ACR crust that is being carried east-southeastward by Neogene extension of the surrounding more active continental crust. The possibility is consistent with


the small horizontal relative motions and dilational strains between the southern part of the peninsula and adjacent ACRs to the west and southwest (Liu et al. 2007; Wang et al. 2011).

The 1994 and 2011 versions of the China SCR extend as far offshore as 700 km southeast of Hainan Island (Figures 1, 2; Broadbent and Allan Cartography 1994). Bathymetric data show that only the near-shore half of this part of the SCR is on the continental shelf, at water depths of 0–200 m (GEBCO World Map Editorial Board 2006). The offshore half is beyond the shelf edge, at depths of 200–5,000 m. As shown on the map of Broadbent and Allan Cartography (1994), the off-shore half was interpreted as SCR crust that had been highly extended during the Paleogene Period. However, interpreta-tions of more recent marine seismic-reflection profiles between Hainan Island and southern Taiwan show thinned continental crust in the landward part of the surveyed area and unusually thick oceanic crust in the seaward part (Wang et al. 2006). The profile interpretations imply that the crust that was considered to be highly extended SCR crust should now be reclassified as thick oceanic crust.

In summary, information published since 1994 shows that almost the entire eastern China continental margin underwent Neogene extension. The only potential exception is the south-ern part of the Korean peninsula.

South China BlockThe South China block is part of the 1994 China SCR. The block consists of two smaller blocks that joined during late Precambrian time to form crust that is now tectonically stable (Qiu et al. 2000; Li et al. 2002; Zheng et al. 2006; Liu et al. 2007). S-wave tomography shows that the South China block has thinner sedimentary cover and higher upper mantle veloci-ties than the active North China block (Feng and An 2010). Feng and An (2010) inferred that the different upper-mantle velocities indicate thicker lithosphere in the South China block than in the North China block. Seismicity and geodetic strain rates are approximately as low as those of the Mongolia SCR. These comparisons allowed Liu et al. (2007) to suggest that the South China block is being extruded southeastward, away from the India-Eurasia collision zone, as a single intact entity. Shen et al. (2005) and Liu et al. (2007) used the geodetic data to com-pute the rate of southeastward motion as 7–8 mm/yr and 4–6 mm/yr, respectively, with respect to a stable Siberia, and Wang et al. (2011) used the data to calculate a left-lateral slip rate of 0.9 mm/yr between the South China and North China blocks.

Active continental crust bounds the South China block on all sides. On the north, the Qinling fold belt of Paleogene or Neogene age separates the South China and North China blocks (Figure 2; Terman 1974; Ren et al. 2002). The fold belt itself is active continental crust because it is the product of orogenic activity younger than Early Cretaceous (Table 1). The southernmost component of the fold belt is the left-lateral Qinling fault system of Neogene age (Yin 2010). Thus, the Qinling fault system marks the northern boundary of the South China block. On the east, the South China block adjoins the active continental crust of the eastern China continental mar-

gin. On the south, the Neogene Ailao Shan–Red River shear zone separates the South China block from the Indochina SCR. The Ailao Shan–Red River shear zone is tens of kilome-ters wide, many hundreds of kilometers long, and constitutes active continental crust, as described in a later section on the Indochina SCR. The northern edge of the shear zone marks the south boundary of the South China block. On the west, the left-lateral Xiangshuihe-Xiaojiang fault system forms part of the block’s boundary. The fault system formed during the middle Neogene Period and remains active today. The rest of the western boundary of the South China block is along the east-dipping Sichuan Basin fault and the west-dipping thrust faults that crop out at the eastern front of the Longmen Shan range (Figure 2; Yin 2010; Burchfiel et al. 2008).

The Longmen Shan thrust fault comprises several strands that crop out along different sections of the northeast-trend-ing Longmen Shan range front (Hubbard et al. 2010). Most of the exposed strands of the fault crop out within the range and along its southeastern front. The 400-km-wide Sichuan Basin is southeast of the range front and is part of the 1994 China SCR (Kanter 1994). Jia et al. (2006) and Burchfiel et al. (2008) explained that the Sichuan Basin is the foreland basin produced by orogenic movements farther west, including those on the Longmen Shan thrust fault. The orogenic movements continue, as shown by the occurrence of the M 7.9 Wenchuan earthquake in 2008 (Figure 2) and by a similar earthquake that paleoseismic and historical evidence dates at approximately 2 ka (Liu et al. 2010).

From the range front southeastward into the Sichuan Basin is a belt of northeast-trending anticlines and northeast-striking, northwest-dipping reverse faults (Burchfiel et al. 1995). Interpretations of well and seismic-reflection data and of geologic mapping imply that the anticlines and reverse faults are underlain by a nearly horizontal strand of the Longmen Shan fault, which propagated southeastward into the basin (Jia et al. 2006; Hubbard et al. 2010). Southeastward move-ment on the buried fault strand buckled the overlying strata to form the anticlines and reverse faults (Burchfiel et al. 1995, 2008). Elsewhere, similar combinations of anticlines, reverse faults, and one or more underlying thrust faults deformed sedimentary strata of foreland basins and moved the strata outward away from growing mountain ranges, as in the south-ern, central, and northern Appalachian mountains, the south-ern Canadian Rocky Mountains, the Idaho-Montana thrust belt, and the Ouachita mountains of Oklahoma and Arkansas (for example, Rich 1934; Boyer and Elliott 1982; Perry et al. 1984; Rodgers 1970; Arbenz 1988; Hatcher et al. 1990). In the Sichuan Basin, deformation of the basin deposits continued as late as Paleogene and Neogene time (Burchfiel et al. 1995; Kirby et al. 2002; Jia et al. 2006). Thus, the northwestern part of the Sichuan Basin is a deformed orogenic foreland that has been active more recently than the Early Cretaceous. This con-clusion results in the reclassification of the northwestern part of the basin as active crust (Table 1).

Geologic maps and cross-sections show that several of the reverse faults dip southeastward, particularly at or near the


southeast edge of the foreland deformation (Burchfiel et al. 1995; Kirby et al. 2002; Jia et al. 2006; Burchfiel et al. 2008; Hubbard et al. 2010). Such backward-dipping reverse faults at or near the fronts of fold-and-thrust belts are recognized elsewhere (for example, Rodgers 1950; Malik et al. 2010). Accordingly, a line drawn along the southeasternmost out-crops of southeast-dipping thrust faults may approximate the eastern limit of deformation in the Sichuan Basin. Figure 1 of Yin (2010) shows such a line as a southeast-dipping reverse fault labeled the Sichuan Basin fault. I use the Sichuan Basin fault to approximate the western boundary of the South China block (Figure 2).

Figure 2 shows an isolated right-lateral strike-slip fault within the northeastern South China block. Yin (2010) shows the fault as having Neogene movement but provides no other information on the fault. The North America SCR contains four similarly isolated active faults: the normal Cheraw fault in eastern Colorado, the strike-slip Meers fault in southern Oklahoma, the reverse Reelfoot fault in the New Madrid seis-mic zone of southeastern Missouri, and the reverse Ungava fault in northern Quebec (Russ 1979; Madole 1988; Crone and Luza 1990; Adams et al. 1991; Crone, Machette, Bradley et al. 1997). The active faults inside the North America SCR imply that the fault inside the South China block need not dis-qualify that part of the block from being SCR crust. Neither Yin (2010) nor Ren et al. (2002) show any Neogene igneous rocks or additional faults in the South China block. To sum-marize, the South China block retains its classification as SCR crust except for the northwestern Sichuan Basin.

Indochina SCRSince middle Paleogene time the Indochina SCR has been extruded southeastward in response to subduction of the Indian plate beneath the Eurasian plate (Figure 2; Leloup et al. 1995; Yin 2010). The SCR is moving rapidly southeastward relative to a fixed Siberia (Liu et al. 2007; Simons et al. 2007). Wang et al. (2011) calculated rates of left-lateral strike slip of 18 and 10 mm/yr along two sections of the Xiangshuihe-Xiaojiang fault system (Figure 2). The northeastern boundary of the extrud-ing mass is the northwest-striking Ailao Shan–Red River shear zone. The western boundary of the extruding mass is a complex fault network that includes the Sangaing fault and other north- and northwest-striking, mostly right-lateral faults (Leloup et al. 1995; Ren et al. 2002; Yin 2010) and diversely oriented, offshore Paleogene and Neogene grabens (Sengor and Natal’in 2001). Since early Neogene time most of the interior of the Indochina SCR was fragmented by right-lateral transtensional faulting and underwent eruption of alkaline basalts (Rangin et al. 1995; Ren et al. 2002; Ho et al. 2003). S-wave tomogra-phy and teleseismic receiver function analyses indicate thinned crust and lithosphere under the Indochina SCR (Feng and An 2010; Bai et al. 2010). Seismicity is negligible in the SCR (Broadbent and Allan Cartography 1994; Tarr et al. 2010).

The Ailao Shan–Red River shear zone is 20–200 km wide and 800–1,200 km long onshore, with another 500–1,400 km of length suggested under the South China Sea (Terman

1974; Exxon Production Research Company 1985; Leloup et al. 1995). The shear zone underwent roughly 500–700 km of left-lateral strike slip during 40–15 Ma (Leloup et al. 1995). The zone reversed its slip sense at approximately 5 Ma and has accumulated 20–50 km of right-lateral slip since that time. The shear zone contains a core of high-temperature metamorphic and igneous rocks that were uplifted from mid-crustal depths. Flanking the core on its northeast and southwest sides are belts of lower-temperature metamorphic rocks and, farther out from the core, sedimentary basins whose strata were folded and cut by reverse faults that dip inward toward the core. Presumably most of the characteristics of the shear zone and related struc-tures formed during the left-lateral majority of the zone’s evolu-tion. For this reason Figure 2 shows the dominant left-lateral slip sense.

One of the groups of structures related to the shear zone is the Lanping-Simao fold belt (Leloup et al. 1995), which over-laps the north tip of the Indochina SCR (Figure 2). The fold belt comprises elongated folds and related reverse faults, which together deformed Late Cretaceous–early Paleogene shales of one of the sedimentary basins that flank the core of the Ailao Shan–Red River shear zone. The folds and reverse faults accom-modated approximately 40 km of horizontal transport to the west-northwest. This structural style is the same as those of the northwestern Sichuan Basin and the North American analogs listed in the earlier description of the Sichuan Basin. The direc-tion of horizontal transport is kinematically consistent with the orientation and movement sense of late Paleogene and early Neogene left-lateral movement on the Ailao Shan–Red River shear zone. Leloup et al. (1995) described field relations that led them to conclude that the fold belt formed at the same time as the shear zone and formed in the same stress field. These characteristics of the Lanping-Simao fold belt and its similari-ties to the Sichuan Basin and the North American analogs of the Sichuan Basin motivate me to classify the Lanping-Simao fold belt as a deformed foreland that is younger than the Early Cretaceous Epoch. Thus, new information that demonstrates Neogene rifting and post-Cretaceous foreland deformation requires reclassification of the Indochina SCR as active crust (Table 1).

DISCUSSION

In this section I draw on the preceding descriptions to summa-rize whether the Mongolia SCR, North China block, north-ern and southern parts of the Korean peninsula, Yellow Sea, eastern China continental margin, South China block, and Indochina SCR satisfy the criteria of Kanter (1994) as summa-rized in Table 1. Figure 3 summarizes the following discussion. The paucity of alkaline basaltic rocks, the absence of reported extensional faulting, and the laboratory experiments in which heating of mantle rock under mantle pressures produced alka-lic melts first, when considered together indicate that the Mongolia SCR may have undergone incipient Neogene exten-sion at upper-mantle melting depths exceeding 70 km. The extension does not appear to have propagated upward to rift


the upper-crustal seismogenic zone. Therefore, nearly all of the Mongolia SCR retains its SCR classification, at least in the upper crust and for the purpose of reassessing the 1994 China SCR to improve estimates of CEUS Mmax.

Similarly, the South China block should retain its classi-fication as SCR crust (Figure 3). Its lithosphere is thick. Low seismicity and strain rates led Liu et al. (2007) to the interpre-tation that the block is being extruded southeastward as an intact block. I did not find any published evidence of Neogene extensional faulting, Neogene alkaline igneous rocks, or fore-land deformation or orogeny younger than Early Cretaceous within the South China block. I suggest that the block be called the South China SCR.

In contrast, the North China block, eastern China con-tinental margin, and Indochina SCR fail to meet the require-ments for classification as SCR crust (Figure 3). All three areas contain widespread Neogene extensional faulting and abundant Neogene alkaline basaltic rocks. Thus, all three areas underwent Neogene rifting. The Korean peninsula and the Yellow Sea contain lesser amounts of Neogene extensional faulting and alkaline rocks. In addition, the northern tip of the Indochina SCR overlaps the Lanping-Simao fold belt, in which folding and reverse faulting deformed foreland-basin strata after Early Cretaceous time. A better classification for all five areas is active continental crust.

In the introduction I explained that moderate to large his-torical earthquakes in the 1994 China SCR provide valuable constraints on the value of CEUS Mmax. The realization that most of the 1994 China SCR is active crust reduces the number of Southeast Asian earthquakes that are available to constrain CEUS Mmax. Figure 2 shows epicenters of 43 earthquakes

of magnitude 6.0 or larger on any magnitude scale (Wheeler, in preparation). Some epicenters coincide so that the figure appears to show only 39 of them. Thirty-seven of the earth-quakes, or 86 percent of the 43, are now recognized as having occurred in ACRs, leaving only six SCR earthquakes. The 37 active-crust earthquakes had estimated moment magnitudes ranging up to approximately 8. Two of the six stable-crust earthquakes occurred in the eastern part of the Mongolia SCR and four were in or on the border of the South China SCR (Figure 3). Table 2 lists the six SCR earthquakes, and double circles identify their epicenters in Figure 2.

Moderate earthquakes can provide lower bounds on estimates of Mmax, but larger earthquakes can provide tighter constraints on Mmax. Specifically, the tightest con-straints on CEUS Mmax are sizes of earthquakes with about M 6.7 and larger (for example, see Figure 3 of Petersen et al. 2008). As explained earlier, many of Earth’s larger historical earthquakes occurred in China. As explained in this paper, recently published information indicates that nearly all of those larger earthquakes occurred in ACR crust instead of SCR crust. These reclassified earthquakes are lost to estima-tion of CEUS Mmax. Fortunately, the development of paleo-seismology in recent decades could counterbalance this loss. Paleoseismic data allow estimation of the sizes, locations, and ages of moderate to large prehistoric earthquakes (Tuttle 2001; McCalpin 2009). Wheeler (2008) estimated that earthquakes of approximately M 6.5 and larger are the most likely to yield paleoseismic estimates of their magnitudes, locations, and ages. Therefore, paleoseismic magnitude estimates of large prehis-toric SCR earthquakes may partly counterbalance the loss of large Chinese earthquakes. Determination of the impact of

0 1,000Kilometers

140°

140°

120°

120°

100°

100°

80°

80°

20°

40°

20°

40°

Epicenter in stablecontinental crust

Epicenter in activecontinental crust

Border of stable continental region (SCR) (this paper)

Border of stable continental region (SCR) (Kanter, 1994)

EXPLANATION Mongolia SCR

South China SCR

Wheeler, Figure 3

▲ Figure 3. Summary of the changes from the 1994 stable continental regions (SCRs) to the SCRs of this paper (see Discussion text).


this counterbalance on CEUS Mmax is beyond the scope of this paper and will be examined in a future report.

CONCLUSIONS

1. New information that was not available in the early 1990s shows that most of the 1994 China SCR is active conti-nental crust. Only the South China and Mongolia parts of the 1994 SCR meet the criteria for retaining their clas-sifications as SCRs.

2. This finding reduces the number of Southeast Asian mod-erate to large earthquakes that are available to constrain CEUS Mmax from 43 to six.

ACKNOWLEDGMENTS

A. C. Johnston and his colleagues produced the global SCR catalog. The catalog and its extensive documentation of the geo-logic and tectonic contexts of each earthquake form the most valuable source of information with which to constrain CEUS Mmax. Many of the ideas in this paper stemmed from discus-sions with J. Adams, J. Ake, A. C. Johnston, C. S. Mueller, R. L. Wesson, and the other participants in the 2008 Mmax work-shop. Suggestions by R. Gold, C. S. Mueller, M. D. Petersen, and two anonymous reviewers improved the manuscript.

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TABLE 2Moderate to Large Earthquakes in the Mongolia and South China SCRs*

SCR †Date

(YearMoDay)Origin Time(HrMnSecs) Source 1‡ Latitude Longitude Source 2§ M Source 3||

SCH 16310814 unknown J94 29.300 111.700 J94 5.8 J94, J96

SCH 19170124 004812.00 ISS 31.000 114.000 ISS 6.5 C08, J96

MO 19220814 114104.70 C08 52.069 130.539 C08 6.6 C08, J96

SCH 19360401 unknown J94 22.500 109.400 J94 6.8 J94, J96

MO 19410505 151827.00 J94 46.500 126.900 ISS 6.0 J94, J96

SCH 20080525 082149.99 PDE 32.560 105.420 PDE 6.1 GCMT

* Stable Continental Regions (see text).† SCR in which earthquake occurred. SCH, South China SCR; MO, Mongolia SCR.‡ Source catalog for date and origin time. C08, 2008 version of “Centennial” catalog of Engdahl and Villasenor (2002); ISS,

International Seismological Service (available at http://www.isc.ac.uk/); J94, Johnston et al. (1994); PDE, Preliminary Determination of Epicenters, U. S. Geological Survey (available at http://earthquake.usgs.gov/earthquakes/eqarchives/epic/).

§ Source catalog for latitude and longitude of epicenter. || Source catalog for moment magnitude M. GCMT, Global Centroid Moment Tensor (available at http://www.globalcmt.org/

CMTsearch.html). First entry is source catalog of original size estimate (MS or maximum intensity). Second entry indicates that size estimate was converted to M with the look-up tables of Johnston (1996a,b).


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