Empirical Observations Regarding Reverse Earthquakes, Blind Thrust Faults, and Quaternary Deformation: Are Blind Thrust Faults Truly Blind?

Transcription

1 Bulletin f the Seismlgical Sciety f America, Vl. 87, N. 5, pp , Octber 1997 Empirical Observatins Regarding Reverse Earthquakes, Blind Thrust Faults, and Quaternary Defrmatin: Are Blind Thrust Faults Truly Blind? by William R. Lettis, Dnald L. Wells, and Jhn N. Baldwin Abstract Active thrust faults pse a significant seismic hazard wrldwide. Many f these faults include "blind" thrusts, where the prpagating fault tip des nt reach the Earth's surface, and "buried" faults, where the gemrphic expressin f the fault is bscured by subsequent sedimentatin and/r ersin. This raises the issue f whether cnventinal gelgic, gemrphic, and paleseismic methds can be used t identify and characterize thrust faults fr the assessment f seismic hazards r whether these faults smetimes are truly "blind." We cmpiled a data base f 148 wrldwide mderate- t large-magnitude thrust/reverse earthquakes t evaluate whether r nt the event ccurred n a fault that culd have been identified prir t the earthquake n the basis f recgnizable Quaternary surface defrmatin (i.e., a pre-existing fault r fld). Analysis f the data shws that interplate reverse earthquakes almst always are assciated with pre-existing Quaternary defrmatin that was r culd have been recgnized prir t the earthquake. In particular, mst interplate reverse earthquakes are assciated with an active reverse fault at the surface and/r an active anticline. In cntrast, intraplate reverse earthquakes seldm ccur n faults assciated with pre-existing recgnizable surface defrmatin. We cnclude that thrust faults can be detected in interplate regins with careful Quaternary gelgic and gemrphic mapping; furthermre, the absence f Quaternary surface defrmatin can be used t infer the absence f an underlying active blind thrust fault in interplate tectnic settings. Hwever, the data shw that Quaternary gelgic mapping techniques alne likely are insufficient t characterize blind thrusts in intraplate regins. In these areas, inclusin f a flating r randm earthquake may be necessary t assess earthquake hazards. Intrductin Numerus mderate- t large-reverse earthquakes have ccurred thrughut the wrld n faults that d nt intersect the Earth' s surface (Tables 1 and 2). In Califrnia, mderatet large-reverse "blind" earthquakes at Calinga in 1983 (Mw 6.4), Whittier Narrws in 1987 (Mw 6.), and Nrthridge in 1994 (M W 6.7) underscre the need t identify and characterize buried r blind thrust (r reverse) faults as seismic surces fr seismic hazard evaluatins. Seismlgic and gelgic studies f these Califrnia earthquakes shw that they ccurred n mderate- t lw-angle thrust faults that d nt intersect the Earth's surface but underlie areas f active anticlinal defrmatin (Namsn and Davis, 1988; Haukssn et al., 1988; Davis and Namsn, 1994). Gedetic leveling data fr the 1983 Calinga earthquake (Stein, 1985), the Whittier Narrws earthquake (Lin and Stein, 1989), and the Nrthridge earthquake (Hudnut etal., 1996) demnstrate that reverse slip n the causative blind thrust fault prduced c-seismic uplift f an verlying anticline. In additin, structural mdels based n gemetrically cnstrained retrde- frmable crss sectins (Suppe, 1983; Namsn and Davis, 1988; Davis et al., 1989; Davis and Namsn, 1994) shw a genetic relatinship between thrust faults and the grwth f verlying anticlines. Thus, althugh many thrust and reverse faults are blind in that they d nt intersect the Earth's surface, c-seismic displacement n these faults cmmnly prduces anticlinal surface defrmatin that is recgnizable (Bullard and Lettis, 1993). The results frm these studies raise an imprtant questin: T what extent are seismgenic thrust (r reverse) faults assciated with recgnizable surface defrmatin, r, cnversely, t what extent are active thrust faults nt assciated with recgnizable surface defrmatin? T answer this questin, we cmpiled a data set f 148 wrldwide mderate- t large-magnitude (=>5.4) thrust earthquakes. The data cmpilatin includes an assessment f the seismlgic, gelgic, and tectnic setting f each earthquake and an assessment f whether the event was r was nt assciated with pre-existing recgnizable Quaternary surface defr- 1171

2 1172 W.R. Lettis, D. L. Wells, and J. N. Baldwin marin. Fr each earthquake, we asked the questin: Did this earthquake ccur n a fault that a gelgist culd have identified prir t the event n the basis f recgnizable surface defrmatin? We define recgnizable surface defrmatin as Quaternary surface faulting r flding that is detectable by a field gelgist applying cnventinal investigative techniques. Fr this study, we infrmally define detectable defrmatin as defrmatin ccurring at sufficient rates and dimensins as t be detected and mapped within the reslving pwer and detectin threshld f cnventinal investigative techniques used by Quaternary gelgists (e.g., gelgic mapping, trenching, drilling, gemrphic analyses; see Bullard and Lettis, 1993). In paleseismlgy, blind thrust faults differ frm buried thrust faults. Blind thrust faults are thse in which the prpagating tip f the fault des nt reach the Earth's surface because (1) the fault gemetry precludes it, (2) the fault tip has nt yet reached the surface, r (3) the rate f sedimentatin is higher than the rate f prpagatin f the fault tip. Buried thrust faults are thse where the fault trace at the Earth's surface has been bscured by subsequent depsitin and/r ersin. In this study, we are cncerned with bth blind and buried thrust faults as ptential surces f earthquakes. Fr the purpse f this discussin nly and because ur data base cannt accurately differentiate between these types f faults, we use the term "blind" thrust fault t encmpass bth types f thrust faults. The questin addressed in this study is whether reverse earthquakes n thrust faults, in general, prduce recgnizable surface defrmatin r whether sme r all f these faults are truly "blind" t the eye f the paleseismlgist. The answer t this questin is essential fr the assessment f blind thrust faults fr inclusin as seismic surces in deterministic and prbabilistic seismic hazard studies. Gelgists and seismlgists must knw whether blind thrust faults can be identified using cnventinal gelgic, gemrphic, and seismlgic investigatins; whether additinal investigative techniques must be develped, r whether seismic hazard studies in cntractinal tectnic dmains must incrprate a randm r flating earthquake t accunt fr earthquakes n blind thrust faults. An additinal gal is t cmpare reverse earthquakes ccurring in interplate and intraplate tectnic settings and t assess whether bth f these tectnic envirnments may be treated similarly in the identificatin and characterizatin f blind thrust faults. We hypthesize that repeated c-seismic displacements n blind thrust faults prduce detectable defrmatin at the Earth's surface that can be used t help characterize the lcatin, size, and recency f earthquakes alng these faults, particularly in interplate tectnic settings. The earthquake data base prepared fr this study enables us t evaluate and dcument the relatinship between reverse earthquakes and surficial defrmatin and t evaluate the utility f cnventinal Quaternary gelgic investigative techniques fr identifying and characterizing blind thrust faults. Selectin Criteria fr the Earthquake Data Base Wrldwide reverse earthquakes were cmpiled and analyzed t assess their assciatin with c-seismic defrmatin and pre-existing Quaternary defrmatin. The preliminary data base cnsists f 148 shallw depth, mderate- t large-magnitude (M w 5.4 t 8.7) reverse earthquakes that ccurred in cntinental crust. Earthquakes that ccurred at the plate interface bundary f subductin znes, r as intraslab events, were nt included in this study. These earthquakes prduce a unique pattern and style f surface uplift and subsidence ver brad regins that is beynd the scpe f this article t characterize. Data fr each earthquake were cmpiled primarily frm published literature and gelgic maps. Fr the perid 199 t 1995, we reviewed and cmpiled data frm the Preliminary Determinatin f Epicenters data base published by the U.S. Gelgical Survey. Fr the perid befre 199, we included in the data base nly earthquakes fr which gelgic field studies and/r seismlgic studies were published. In additin, the data base used by Wells and Cppersmith (1994) was reviewed and updated t include recent earthquakes and recent literature. The cmpleteness and quality f data varied greatly depending n the effects f the earthquake (e.g., prperty damage, lss f life, surface defrmatin), recency f the earthquake (lder histrical earthquakes generally have less cmplete and/r lwer quality infrmatin), and gegraphic lcatin (e.g., events in Nrth America and Japan generally have been mre thrughly studied than events in suthern Asia, Suth America, and the frmer Sviet Unin). The data base includes earthquakes that have published reverse (thrust) fcal mechanisms (defined by a rake f 45 t 135 ) r lder earthquakes withut published fcal mechanisms, where the maximum dip-slip cmpnent was reprted t be larger than the maximum lateral-slip cmpnent. In additin, several reprtedly strike-slip earthquakes that had a large cmpnent f reverse slip at the surface were included in the data base. We cnsider these blique-slip earthquakes t be relevant t ur study because cmpressinal cmpnents rarely are reprted in the literature, except where detailed studies (i.e., trenching) have been perfrmed. Cntinental earthquakes f M w _-< 5.9 were nt systematically evaluated because few f these earthquakes are assciated with surface defrmatin and, therefre, typically are prly dcumented in the literature. Wells and Cppersmith (1993) evaluated the ccurrence f surface faulting fr a data base f 276 cntinental earthquakes. This study cncluded that the prbability f surface rupture decreases frm 42% at Mw 5.9 t 12% at Mw 5.. Because this relatinship des nt accunt fr flding, uplift, r subsidence at the surface, the prbability f surface rupture r ther surface defrmatin ccurring is expected t be higher at any magnitude level. The shape f the regressin line in Figure 1 shws that the prbability f surface rupture increases rap-

3 Empirical Observatins Regarding Reverse Earthquakes, Blind Thrust Faults, and Quaternary Defrmatin W 18 I rr I'- _.7- ill O.6- n-" m.5- LI. ~_.4" < m.3- O tr _.2- I I I I I I I 1) and thse with recgnized c-seismic surface rupture (Table 2). Tables 1 and 2 include a summary f the earthquake surce parameters, related Quaternary defrmatin, tectnic setting (intraplate r interplate), and references (Appendix A). Earthquake surce parameters include the lcatin, earthquake name, date, magnitude, and the nature f c-seismic defrmatin (e.g., surface rupture length, surface displacement, fld grwth, and surface uplift). Related Quaternary defrmatin include descriptins f assciated faults, flds, and reginal fld and thrust belts. In additin, the tables include a rating system fr the relative cnfidence in the data used t evaluate the c-seismic defrmatin and related Quaternary defrmatin, and a classificatin system fr the assciatin f the earthquake with Quaternary defrmatin..1- i i i i E u i MAGNITUDE (Mw) Figure 1. Empirical relatinship shwing the prbability f surface rupture as a functin f magnitude. Line shws linear regressin fr lgit transfrmatin f data (magnitude and ccurrence f surface rupture). Data base includes shallw depth cntinental earthquakes, Mw 4. t 8.5, ccurring frm 1847 t 1992 (frm Wells and Cppersmith, 1993). idly in the range f Mw 5.7 t 6.4. The cutff f Mw 5.9 was selected at the level at which mre than 5% f the earthquakes are expected t cause surface defrmatin; this cutff magnitude als was based n the availability f published data fr histrical earthquakes. Of the 148 earthquakes in the preliminary data base, 28 were f Mw <= Twenty f these earthquakes were deleted frm the data base because there was little r n data. The remaining eight smaller earthquakes (M w 5.4 t 5.8) were retained in the data base primarily because f their thrugh assessment by the scientific cmmunity, and ur ability t cnfidently assess the presence r absence f assciated surface defrmatin. In additin, 14 earthquakes in the preliminary data base were identified as dependent events (i.e., aftershcks, r earthquakes that ccurred at the same lcatin as a previus earthquake). The dependent earthquakes were retained in the data base but were nt cunted as individual earthquakes in the analysis f the assciatin f earthquakes and Quaternary defrmatin. Thus, the final data base cnsists f 114 earthquakes that ccurred between 1812 and 1995, including 16 wrldwide reverse earthquakes f Mw >= 5.9 frm thrughut the wrld and eight well-studied reverse earthquakes f Mw 5.4 t 5.8 that ccurred in Nrth America and Australia. Figure 2 shws the lcatins f the earthquakes included in the final data base. The earthquakes were separated int tw categries: thse withut recgnized c-seismic surface rupture (Table 9. Tempral Cmpleteness and Gegraphic Distributin f the Earthquake Data Base We evaluated the tempral cmpleteness and gegraphic distributin f the data base t assess whether the final data base prvides a representative sampling f the wrldwide ccurrence f cntinental reverse earthquakes. First, we reviewed the annual frequency f the earthquakes in the preliminary data base. Fr the perid frm 1985 t 1996, the preliminary data base includes tw t eight earthquakes per year, with an average f abut fur earthquakes per year. Frm 1896 t 1985, the preliminary data base includes an apprximate average f ne earthquake per year. Tw earthquakes that ccurred in 1812 and 1847 als are included in the data base. Review f the Harvard Centrid Mment Tensr (Harvard CMT) catalg f 1977 t 1995 indicates that an apprximate average f six t eight earthquakes per year meet the selectin criteria fr ur data base. Based n the wrldwide data frm the Harvard CMT catalg, the preliminary data base cmpiled fr this study includes mre than 5% f the ttal number f earthquakes that meet the selectin criteria fr the perid 1985 t 1995, and nearly 2% f the estimated ttal number f earthquakes that wuld meet the selectin criteria fr the perid 1896 t Thus, the preliminary data base cmpiled fr this study is large enugh t prvide a representative sample f the histrical recrd f reverse earthquakes. Furthermre, we nte that n published infrmatin was identified fr the earthquakes nt included in the preliminary data base; thus there wuld have been little benefit frm adding additinal earthquakes t the data base. Secnd, we reviewed the gegraphic distributin f earthquakes in the final data base f 114 earthquakes. The distributin f earthquakes varies with respect t gegraphic lcatin, rates f defrmatin, and availability f published literature and data. We cmpared the wrldwide distributin f earthquakes in the final data base (listed by cuntry in Table 3) t the distributin f earthquakes frm the Harvard CMT catalg and t the lcatin f majr active nshre reverse fault and fld systems (Yeats et al., 1997). With

4 1174 W.R. Lettis, D. L. Wells, and J. N. Baldwin Figure 2. Lcatin f earthquakes listed in Tables 1 and 2. Sme dts represent the lcatin f mre than ne earthquake. respect t gegraphical distributin, the largest number f earthquakes ccurred in Iran, China, the United States, and the frmer Sviet Unin. The final data base appears t be well represented fr parts f Suth America (Argentina, Clumbia, and Ecuadr), mst f Nrth America (Califrnia and Canada), Africa (Algeria), Australia, New Zealand, India, the frmer Sviet Unin and eastern Eurpe, Iran, and the rest f Asia (China, Japan, Nepal, Pakistan, and Taiwan). The data base is under-represented (in terms f numbers f earthquakes in the Harvard CMT catalg) fr Central America, parts f Nrth America (Alaska), parts f Suth America (Chile and Peru), and west and sutheast Asia (Afghanistan, Indnesia, Philippines, and Papua New Guinea). Little infrmatin has been published n the effects f earthquakes, particularly in the M w 6 t 7 range, fr many f the underrepresented regins. Based n this cmparisn, we cnclude that the final data base includes representative earthquakes frm mst cntinental regins where active reverse faulting and flding are ccurring and, therefre, prvides a gd representatin f the wrldwide ccurrence f cntinental reverse earthquakes. Tectnic Setting Earthquakes were included frm bth interplate and intraplate tectnic envirnments. Interplate reverse earth- quakes generally ccur alng transcurrent and transfrm plate margins (areas having significant lateral and cnvergent slip), and alng cnvergent plate margins dminated by crustal shrtening (e.g., verriding plate in subductin znes and cntinent--cntinent cnvergent margins). Examples f plate margin systems include the San Andreas fault system (transfrm) and the Himalayan decllement (cnvergent). The data set als includes intraplate earthquakes that have ccurred in stable cntinental regins (SCRs; e.g., eastern Nrth America, Australia, Peninsular India) as defined by Jhnstn (1992) and by the Electric Pwer Research Institute (EPRI, 1994). These regins are characterized by a general lack f Quaternary structures and by lw rates f earthquake recurrence (EPRI, 1994). Of the 114 earthquakes in the final data base, 95 are interplate events (62 cnvergent and 33 transpressive), and 19 are intraplate events (Tables 1 and 2). Earthquake Classificatin Earthquakes in the data base were classified, first, by level f cnfidence in the data (i.e., Cnfidence Rating) and, secnd, by assciatin r lack f assciatin with recgnizable surface faulting and/r flding (Defrmatin Classificatin). Fr each earthquake in the data base, a numerical rating and class was assigned fr cnfidence level and fr style f assciated defrmatin, respectively (Tables 1 and

5 Empirical Observatins Regarding Reverse Earthquakes, Blind Thrust Faults, and Quaternary Defrmatin ). The earthquake rating and defrmatin classificatin systems are described belw. Figures 3 and 4 present flwcharts that illustrate the decisin prcess fr assigning the numerical defrmatin classificatin fr each earthquake. The flwchart is a decisin-tree analysis cmpsed f five branches, each f which is discussed belw. Cnfidence Rating Given the disparate amunt, detail, and quality f available wrldwide gelgic and seismlgic infrmatin, it is imprtant t categrize earthquakes with respect t ur level f cnfidence in frmulating a judgment as t the presence r absence f assciated pre-existing recgnizable Quaternary surface defrmatin. The earthquakes in the data base were assigned t ne f three cnfidence categries based n (1) the amunt, detail, and quality f data available fr the earthquake; and (2) ur cnfidence in assessing the presence r absence f assciated pre-existing Quaternary surface defrmatin. These categries are (1) high cnfidence, (2) lw cnfidence, and (3) indeterminate (n cnfidence), each f which is described belw. Branch 1 f the flwchart subdivides the data set based n ur cnfidence in the data. High-cnfidence earthquakes are listed in Figure 3 and lwcnfidence earthquakes are listed in Figure 4. Indeterminate earthquakes are excluded frm further analysis. The numerical cnfidence rating fr each earthquake als is listed in Tables 1 and 2. A high-cnfidence rating represents an earthquake that has been extensively studied, and we are cnfident in ur assessment f whether the earthquake is r is nt assciated with pre-existing Quaternary defrmatin. The lcatin, magnitude, fcal mechanism, surce structure, and characteristics f c-seismic defrmatin are well knwn, and detailed gelgic maps are available t assess the presence r absence f assciated pre-existing Quaternary defrmatin. A ttal f 51 earthquakes were assigned a high-cnfidence rating. A lw-cnfidence rating represents an earthquake that has nt been studied extensively because its gegraphic lcatin (accessibility), date f ccurrence, and/r ccurrence in a regin where the gelgy f the earthquake epicentral regin is nt well knwn frm published maps. We fund that sufficient data cmmnly are available t dcument an earthquake's seismlgic character (lcatin, magnitude, fcal mechanism, c-seismic defrmatin) but that available gelgic maps and/r gelgic studies are insufficient t assess with high cnfidence the presence r absence f preexisting Quaternary defrmatin. A ttal f 48 earthquakes were assigned a lw-cnfidence rating. An indeterminate r n-cnfidence rating was assigned t thse earthquakes where gelgic r seismlgic data are insufficient t render a judgment as t the earthquake's assciatin with c-seismic r Quaternary defrmatin. Fifteen (15) earthquakes were assigned the indeterminate r ncnfidence rating. Defrmatin Rating Branches 2 thrugh 5 f the flwchart subdivide the data base accrding t the presence r absence f c-seismic defrmatin and assciatin with pre-existing Quaternary defrmatin, as fllws: Branch 2 assesses the presence r absence f surface rupture. Branch 3 assesses the relatins, if any, between an earthquake and a reginal fld belt. Branches 4 and 5 evaluate the relatinship f the earthquake t pre-existing Quaternary flds and faults, respectively. T make the assessment fr branches 4 and 5, we asked the fllwing questin fr each earthquake: Did this earthquake ccur n r near a fault r fld that culd have been identified prir t the event based n recgnizable pre-existing surface defrmatin (i.e., surface fault rupture r fld defrmatin)? Based n the answer t this questin, we assigned each earthquake t ne f seven classificatins: Class Assciatin with Quaternary Surface Defrmatin Earthquake ccurred n a knwn Quaternary thrust r reverse fault. Earthquake ccurred beneath a knwn Quaternary fld. Earthquake is assciated with bth a Quaternary thrust fault and fld (bth class 1 and 2). Earthquake prbably is assciated with a Quaternary thrust fault and/r fld, r with a reginal Quaternary fld and thrust belt. Earthquake prbably is nt assciated with a knwn Quaternary thrust fault r fld, r a reginal Quaternary fld and thrust belt. Earthquake is nt assciated with a knwn Quaternary thrust fault r fld. Assciatin unknwn. Insufficient data fr judgment (n cnfidence). Nte that the classificatins depend, in part, n the detail in the available data. Fr instance, classificatins 1, 2, 3, and 6 are based n data that prvide cnclusive evidence f the assciatin r lack f assciatin f the earthquake with surface defrmatin. Earthquakes given a high-cnfidence rating typically were assigned t ne f these defrmatin classificatins. Classes 4 and 5 reflect greater uncertainty in the earthquake's assessment. Earthquakes assigned t these classificatins typically included cmplex events fr which the relatinship between the earthquake and Quaternary defrmatin is nt clear. Earthquakes given a lw-cnfidence rating typically were assigned t ne f these defrmatin classificatins. Class 7 included nly thse earthquakes fr which the available infrmatin was insufficient t make any judgment with respect t the assciatin with pre-existing defrmatin; therefre, all n-cnfidence (indeterminate) earthquakes were assigned a defrmatin classificatin f 7. The defrmatin classificatins assigned t each earthquake (Text cntinues n page 1137)

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21 Empirical Observatins Regarding Reverse Earthquakes, Blind Thrust Faults, and Quaternary Defrmatin 1191 Table 3 Percentage f Earthquakes fr Each Cuntry Number f Percent f Cuntry Earthquakes Data Base Iran % China United States Frmer Sviet Unin Japan India, Pakistan, Nepal 8 7. Australia 8 7. Canada Taiwan New Zealand Algeria Argentina Afghanistan Peru Clumbia Others Ttal are listed in Tables 1 and 2; each defrmatin classificatin is described belw. Classes 1, 2, and 3 apply t earthquakes assciated with a specific Quaternary structure. Class 1 applies t an earthquake that is described in the literature as having ccurred n r near a mapped Quaternary fault. Fr example, the 1929 M s 7.8 White Creek earthquake (event 78) is assciated with the active White Creek fault in New Zealand (Table 2). Class 2 applies t earthquakes that ccurred beneath r near a knwn Quaternary fld. Fr example, the 1983 Mw 6.4 Calinga earthquake (event 35) ccurred beneath the actively grwing Calinga anticline (Table 1). Class 3 includes thse earthquakes assciated with bth an active fault and a fld. Fr example, the 1952 Mw 7.4 Kern Cunty earthquake (event 88) ruptured the active White Wlf fault in Califrnia, and the rupture extended t the base f the active Wheeler Ridge anticline (Table 2). Class 4 applies t earthquakes that prbably are assciated with pre-existing Quaternary surface defrmatin, but a direct assciatin cannt cnfidently be made based n available infrmatin. This classificatin is assigned when either (1) the epicentral lcatin f the earthquake is nt well cnstrained, and assciatin t a fault r a fld cannt be made cnfidently, r (2) available gelgic maps are at scales f 1:5, r smaller, s that assciatin t a fault r fld cannt be dcumented ther than t state that the earthquake ccurred in the vicinity f a knwn thrust fault r fld, r within a Quaternary fld and thrust belt. Fr example, the 1992 M w 6.2 earthquake in Pakistan (event 6) ccurred within a Quaternary reginal fld belt, the Karakram Himalayan Crystalline thrust zne. The causative fault fr the earthquake is unknwn, but because there are Quaternary faults and flds in the prximity f the epicenter, we infer that the earthquake is related t these mapped struc- tures. In additin, available fcal mechanism data were reviewed t evaluate the cnsistency f the fcal plane rientatin with the trend f lcal and reginal faults and flds. Earthquakes nt assciated with a knwn Quaternary structure were assigned t class 5 r 6. Class 5 applies t earthquakes that prbably are nt assciated with Quaternary surface defrmatin. These earthquakes typically have prly cnstrained gelgic and/r seismlgic data, but the data are sufficient t suggest the absence f assciated defrmatin. Fr example, the 1947 Ms 7.8 earthquake in Daft, China (event 86), is nt assciated with a knwn Quaternary fld and thrust belt, and the clsest fault r f1d is 2 krn frm the epicenter. Class 6 reflects a higher level f cnfidence than class 5. Class 6 applies t earthquakes fr which sufficient gelgic and seismlgie data are available t cnfidently preclude any assciatin with pre-existing Quaternary defrmatin. Fr example, the 197 M w 6. earthquake near Lake Mackay, Australia (event 19) ccurred in a regin where undefrmed Quaternary depsits verlie Meszic sedimentary rcks. Class 7 applies t thse earthquakes fr which infrmatin is insufficient t assess whether the earthquake was assciated with knwn Quaternary defrmatin. Fr example, the 1975 Mw 6.6 earthquake in Lice, Turkey (event 98), ruptured an existing Micene thrust fault, and its fcal mechanism is subparallel t anticlinal fld axes in Micene bedrck. Available gelgic maps d nt shw Quaternary depsits in the regin frm which t assess the presence r absence f Quaternary defrmatin, and n pst-earthquake gelgic studies have been perfrmed t evaluate Quaternary defrmatin in the earthquake epicentral regin. We exclude these indeterminate earthquakes frm further evaluatin because they cannt be used t assess the presence r absence f assciated Quaternary defrmatin. Evaluatin f the Earthquake Data Base Fr the 114 earthquakes in the final data base, 51 are well dcumented (high cnfidence), 48 are mderately well dcumented (lw cnfidence), and 15 are prly dcumented (n cnfidence) with respect t the earthquake surce parameters and/r gelgical character f the earthquake (Table 4). We analyzed the 51 high-cnfidence earthquakes as a subset f the cmbined 99 high- and lw-cnfidence earthquakes t evaluate ptential bias in the results related t increased uncertainty in classificatin f the earthquakes. In additin, we assign ur greatest certainty t judgments based nly n the high-cnfidence earthquakes. The cnfidence rating and defrmatin classificatin fr interplate and intraplate earthquakes are summarized in Figures 5 and 6. Analysis f the earthquakes by classes clearly shws differences between interplate and intraplate earthquakes with respect t assciatin with recgnizable Quaternary defrmatin. Reverse earthquakes ccurring in intraplate tectnic settings generally are nt assciated with pre-existing

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23 Empirical Observatins Regarding Reverse Earthquakes, Blind Thrust Faults, and Quaternary Defrmatin ] ] 93 Lw Surface Event Assciated with Event Assciated Cnfidence 1 Rupture Reginal Fld Belt with Knwn Quaternary Fld Event Assciated with Knwn Quaternary Fault Earthquake Defrmatin Classificatin / 11 / ~rbable, %\ ,74,75,76,8,82, / 86,89,97,99,1, 15,11,112,114, 115) / 73,74,75,76,8,82,~ 89, 97,99,1,15, 11,112 bable 8,1 (13) % ~ 73,74,75,76,82,89, 97,99,15,112 / / ~e~rbable ' 1 %\ 8 74,82,89,112 prbable 73, 75, 76, 97, 15 %\ 99 prbable % \ ,114 (2) bable 6,7,9,1,12,14,15,16,18, 2,23,24,25,26,27,28,29, 3,32,33,37,41,47,8,52, 53,55,56,57,59,62,63,67, 73,74, 75, 76, 8,82,86,89, 97,99,1,15,11,112,11z (48) 6,7,9,1,12,14,15, 16,18,2,23,24,25, 26,27,28,29,3,32, 33,37,41,47,5,52, 53,58,56,57,59,62, 63,67 (33) \ 86,114 prbable %\ Represents an earthquake nt extensively studied, but has sufficient data t dcument the earthquakes seismgenic behavir Bld number indicates an intraplate earthquake / ~ prbable 114 prbable / %,, %\ 86 6, 15, 16, 18,2,23,24, f 26, 27, 28, 29, 3, 32, /.~/ 41, 47, 5, 59, 63, 67/nrnbable 15,16,18,47,5,63 (47prbable 16, 18, 5, 63 \- \ (19) %\ 15, 47 ~37, 9, 1, 12, 14, 25, 33, / F \6,2,23,24,26,27,28, / % 6 % 29, 3, 32, 41, 59, 67 ('%~prbable 2,3, 24,32, 26,41,28,59, 29,67 "> N 23, 27 / <~ ; r bable / * N 7, 9, 1, 12, 14, 25, 83, 37, 52, 53, 55, 56, 57, 62 ~r~/bable %\ <.~;rjbab~e 9, 1,53,14,25,33,52,56, 57, 62 %\ 7, 12, 37, Figure 4. Flw chart fr lw-cnfidence earthquakes.

24 1194 W.R. Lettis, D. L. Wells, and J. N. Baldwin Cnfidence Rating Table 4 Number f Earthquakes per Cnfidence Level Number f Earthquakes With Surface Rupture Withut Surface Rupture Ttal Interplate Intraplate Interplate Intraplate High Lw N cnfidence Ttal I,<,, z O INTERPLATE REVERSE EARTHQUAKES Earthquakes with Surface Rupture (37) High ] Lw 1 N Cnfidence t Cnfidence I Cnfidence ~ d 4~5'6'7' 1 '~3'4'5'6'7 Earthquakes withut Surface Rupture (58) High I LOW 1 NO Cnfidence I Cnfidence I Cnfidence I'~3'4'5'6'TI~3'4 EARTHQUAKE DEFORMATION CLASSIFICATION INTRAPLATE REVERSE EARTHQUAKES Earthquakes with Surface Rupture Earthquakes withut Surface Rupture I (7) (12) High Lw N High Lw N Cnfidence Cnfidence Cnfidence' Cnfidence Cnfidence Cnfidence (6) (1) (7) (4) (1) EARTHQUAKE DEFORMATION CLASSIFICATION EXPLANATION OF CLASSIFICATIONS Class 1 - Knwn Quaternary thrust fault Class 2 - Knwn Quaternary fld Class 3 - Bth a Quaternary fault and fld Class 4 - Prbable Quaternary thrust fault and/r fld Class 5 - Prbably nt assciated with a knwn Quaternary thrust fault r fld Class 6 - Nt assciated with a knwn Quaternary thrust fault r fld Class 7 - Insufficient data fr judgement Figure 5. Numerical distributin f high- and lwcnfidence interplate and intraplate earthquakes fr classes 1 thrugh 7. 5'~ 25 _ Quaternary defrmatin (class 6). Reverse earthquakes ccurring in interplate tectnic settings almst always are assciated with Quaternary surface defrmatin that culd have been recgnized prir t the earthquake (class 1, 2, 3, r 4). Of the 81 high- and lw-cnfidence interplate earthquakes, 4 (49%) are assciated with a knwn active fault (class 1), active fld (class 2), r bth (class 3), and 37 (46%) prbably are assciated with a knwn Quaternary fault r fld, r ccur in a regin f knwn Quaternary flding and thrust faulting (class 4). T illustrate the detail and data necessary t classify knwn versus prbable assciatin between blind thrust earthquakes and Quaternary defrmatin, we cnsider the recent Nrthridge, Califrnia, earthquake. In 1994, the M w 6.7 Nrthridge earthquake (event 64) ccurred in an area nt assciated with any bvius Quaternary anticline r tpgraphic high (Haukssn et al., 1995). Hwever, detailed retrdefrmable crss sectins cnstructed by Huftile and Yeats (1996) shw that Pleistcene strata within the hanging wall f the Nrthridge thrust fault have been uplifted as a result f repeated displacements n the buried thrust fault. In additin, wrk in prgress by Hitchcck and Kelsn (1997) shws that a late Quaternary anticline is present in the nrthern San Fernand Valley in the hanging wall f the Nrthridge thrust fault. With careful examinatin f these surficial features, Huftile and Yeats (1996) and Hitchcck and Kelsn (1997) suggest that the Nrthridge thrust fault culd have been identified by a gelgist befre the earthquake. Hwever, given the difficulty in recgnizing this defrmatin, we assign the Nrthridge earthquake t class 4, prbably assciated with a Quaternary fault r fld. Thus, 95% (77 f 81) f the high- and lw-cnfidence reverse earthquakes in interplate settings are, r prbably are, assciated with recgnizable Quaternary defrmatin at the Earth's surface. If ne cnsiders nly the high-cnfidence earthquakes, 82% (31 f 38) are assciated with a knwn active fault (classes 1 and 3) expsed at the Earth's surface, and 89% (34 f 38) are assciated with an active fault and/r fld (classes 1, 2, and 3). Althugh fur lwcnfidence earthquakes appear nt t be assciated with Quaternary defrmatin (events 23, 55, 86, and 99), these earthquakes ccur near lder faults and/r flds f unknwn activity. Further studies may help t characterize the activity f these structures and the causative structure f the earthquakes. These data supprt the cnclusin that reverse earthquakes in interplate tectnic settings almst always are assciated with areas f knwn Quaternary flding and/r thrust faulting. The data further suggest that, wrldwide, mst reverse earthquakes ccurred n active faults that were r culd have been recgnized prir t the earthquake. Of the 38 high- and lw-cnfidence earthquakes, 31 (82%) are assciated with a knwn fault (class 1) r a knwn fault and fld (class 3). Thus, mst ptential surces f reverse earthquakes in the wrld culd have been identified and characterized by cnventinal paleseismic investigatins (mapping, trenching,

25 Empirical Observatins Regarding Reverse Earthquakes, Blind Thrust Faults, and Quaternary Defrmatin 1195 High Cnfidence (38) INTERPLATE REVERSE EARTHQUAKES (81) Lw Cnfidence (43) lo c Lg With Surface Rupture (21) Withut Surface Rupture (17) SUBTOTAL with Surface Rupture (14) Withut Surface Rupture (29) 9 SUBTOTAL TOTAL 1 77 ~ I, LL 5 'i,,, 1, O O LU I 25 14,,,,, 13.5 " t EARTHQUAKE DEFORMATION CLASSIFICATION INTRAPLATE REVERSE EARTHQUAKES (18) ] High Cnfidence Lw Cnfidence (13) i (5) With Surface Withut Surface With Surface Withut Surface Rupture (6) Rupture (7) SUBTOTAL Rupture (1) Rupture (4) SUBTOTAL TOTAL m 87 O 75! ~ 75 -ip- 67 n- < LU ~) 5 5 UJ m t t EARTHQUAKE DEFORMATION CLASSIFICATION EXPLANATION OF CLASSIFICATIONS Class 1 - Knwn Quaternary thrust fault Class 2 - Knwn Quaternary fld Class 3 - Bth a Quaternary fault and fld Class 4 - Prbable Quaternary thrust fault and/r fld Class 5 - Prbably nt assciated with a knwn Quaternary thrust fault r fld Class 6 - Nt assciated with a knwn Quaternary thrust fault r fld Figure 6. Percentage distributin f high- and lw-cnfidence interplate and intraplate earthquakes fr classes 1 thrugh 6. etc.) f faults expsed at the surface. Califrnia, hwever, appears t be an exceptin t this generalizatin. Only tw f six (33%) f the high cnfidence reverse earthquakes in Califrnia ccurred n a specific active fault at the Earth's surface (1952 Kern Cunty and 1971 San Fernand earthquakes, events 88 and 96). The ther fur high-cnfidence earthquakes are assciated with a knwn active fld but nt an active fault at the Earth's surface (1983 Calinga, 1985 Avenal-Nrth Kettleman Hills, 1987 Whittier Narrws, and 1994 Nrthridge earthquakes, events 36, 4, 45, and 64, respectively). Althugh the Califrnia data set is small, it may demnstrate that there is significant variability in the expressin f reverse faulting in different tectnic settings and/r gegraphic regins f the wrld. Fr example, 33 f 43 (77%) lw-cnfidence earthquakes ccurred in areas f knwn Quaternary flding and thrust faulting, but we can nt cn- fidently assign these earthquakes t either a fld r a fault. In many f these cases, the earthquake may have ccurred n a fault that des nt reach the Earth's surface. Further wrk wuld be required t assess whether these earthquakes are assciated with a fault, a fld, r bth. These results shw that the majrity f thrust faults in interplate regins are nt "blind" but exhibit detectable surface defrmatin. Surface defrmatin typically is expressed as a pre-existing fault trace r fld shwing gemrphic and/ r gelgic evidence f Quaternary defrmatin, r regins f Quaternary mnclinal warping r tilting. The defrmatin generally can be recgnized using cnventinal gelgic and gemrphic mapping techniques. Specialized analytical studies such as quantitative mrphmetdc analyses (e.g., terrace prfiles, residual uplift maps, drainage pattern analyses) may be needed t identify and characterize regins f subtle flding r warping. Fr instance, event 29 (Table

26 1196 W.R. Lettis, D. L. Wells, and J. N. Baldwin 1), the 1977 Mw 6.9 Khurgu earthquake f Iran, was nt assciated with c-seismic surface rupture. Based n its prximity t faults and the active Zagrs fld belt, hwever, detailed studies such as quantitative mrphmetfic analyses likely wuld identify subtle defrmatin f verlying Quaternary depsits and gemrphic surfaces (e.g., class 4). Many active blind thrust faults have been identified because f their assciatin with recent mderate- t largemagnitude reverse earthquakes. Because we are cnveniently armed with 2/2 hindsight, it is easy t cnclude that mst f the causative structures culd have been detected befre the earthquake ccurred. We avid this ptential bias in ur interpretatin by using the perspective f a Quaternary gelgist evaluating the regin befre the earthquake. Specifically, we fcus n infrmatin that was published and gelgic cnditins that culd have been identified even if n earthquake had ccurred in the regin. Because the verwhelming majrity f interplate earthquakes are assciated with Quaternary surficial defrmatin, the cnclusin that blind thrust faults are nt truly blind appears t be valid and well dcumented. Cnversely, reverse earthquakes ccurring in intraplate tectnic settings generally are nt assciated with Quaternary surface defrmatin that culd have been recgnized prir t the earthquake (Figs. 5 and 6). Eighteen high- and lw-cnfidence intraplate reverse earthquakes are included in the data base (Table 4). Only tw earthquakes, the 7 Febmary 1812 New Madrid, Missuri, earthquake (event 1) and the 9 September 1993 Killari, India, earthquake (event 114) appear t be assciated with pre-existing Quaternary defrmatin. Paleseismic studies (Russ, 1979, 1982; Kelsn and Simpsn, 1996) shw that part f the Reelft scarp and assciated flding and secndary faulting prbably predate the 1811 t 1812 earthquake sequence. A recent study f the near-surface stratigraphy and structure alng the 1993 Killari earthquake ruptures revealed an inverted fault scarp, where the hanging wall was preferentially erded belw the elevatin f the ftwall alng an existing reverse fault (Rajendran et al., 1996). The ther 16 intraplate earthquakes (89%, Fig. 5) ccurred in regins where there are few t n Quaternary depsits r where there is n recgnizable Quaternary defrmatin. Of these 16 earthquakes, eight ccurred n an existing fault identified by gelgic mapping r gephysical surveying; hwever, these faults did nt shw evidence f pre-existing Quaternary defrmatin. Fr example, the M w 5.8 Marryat Creek earthquake (event 17) in Australia ruptured an existing fault in lwer Prterzic crystalline rcks. There was n evidence f Quaternary displacement alng this fault. The ther eight intraplate earthquakes ccurred in regins characterized by pre-quaternary bedrck and extremely lw depsitinal rates, neither f which are cnducive t detecting active faulting. We als cmpared ur classificatin results with earthquake magnitude t evaluate any magnitude dependence in the assciatin with Quaternary structures. As shwn in Figure 1, the ccurrence f surface rupture is magnitude depen- dent such that the prbability f c-seismic surface rupture (fr all types f faulting) increases frm apprximately 4% at M w 5.9 t apprximately 9% at M w 7.2. Based n this bservatin, we expect that a similar relatinship exists between magnitude and c-seismic fld grwth. We als evaluated the ccurrence f faults and flds with respect t the magnitude f earthquakes in the final data base. We nte that there is n implicit relatinship between the size f reverse earthquakes and the assciated structures because the bserved earthquake may nt be the largest r "characteristic" earthquake assciated with the bserved thrust fault r fld. Specifically, the characteristic (largest) earthquakes t ccur n a structure wuld have the largest seismic mment release and wuld result in the largest displacement n the assciated fault and/rfld. Fr example, the 28 June 1991 M w 5.6 Sierra Madre earthquake is assciated with the Sierra Madre fault. The mrphlgy and extent f the surface trace f this fault indicates that the bserved surface defrmatin results frm much larger earthquakes, n the rder f Mw 6.5 t 7.. Thus, the 1991 Sierra Madre earthquake des nt represent the upper end f the distributin f magnitudes f earthquakes expected t ccur n the Sierra Madre fault and des nt cntribute significantly t the develpment f the bserved surface fault. The earthquake magnitudes and classificatins fr the data base cmpiled fr this study are cnsistent with the abve bservatin. With respect t a definitive assciatin nly with Quaternary faults (class 1), r with Quaternary faults and flds (classes 1 and 3), earthquake magnitudes range frm M w 5.6 t 8.2, with a slight increase in number f earthquakes fr larger magnitudes (->_7.). This apparent increase in number f earthquakes with increasing magnitude is at least partly a functin f the increasing likelihd that studies were cmpleted fr the larger-magnitude earthquakes (such that they are included in the data base). With respect t a definitive assciatin nly with Quaternary flds (class 2), fr a data set f three earthquakes, magnitudes range frm M w 6. t 6.4. In cntrast, fr assciatin with Quaternary faults and flds (class 3), earthquake magnitudes range frm M w 6. t 7.8 and cluster tward the upper end f this magnitude range. Thus, althugh there is n clear dependence between magnitude and Quaternary faults, the ccurrence f Quaternary flds withut surface faults appears t crrespnd t mderate-magnitude earthquakes (Mw 6. t 6.4). The data set fr this assciatin (class 2 nly) is very small (three earthquakes); additinal data wuld be necessary t define the maximum magnitude f earthquakes likely t ccur in areas where Quaternary flds ccur in the absence f Quaternary faults. An upper bund t the maximum magnitude f earthquakes ccurring n such flds may be inferred frm Figure 1. This figure shws that the prbability f surface rupture is very high (>9%) fr earthquakes greater than M w 7.2. Thus, it appears that the presence f nly Quaternary flds in a regin indicates that the maximum magnitude f ptential earthquakes likely is limited t the range f Mw 6.5 t 7.2.

27 Empirical Observatins Regarding Reverse Earthquakes, Blind Thrust Faults, and Quaternary Defrmatin 1197 Cnclusins The earthquake data base described in this study shws that blind r buried thrust faults in interplate regins generally prduce surface defrmatin that can be detected by gelgic and gemrphic mapping. The data base shws that 82% are assciated with a knwn active fault expsed at the Earth's surface, and 89% are assciated with either a Quaternary thrust fault r fld that culd have been recgnized prir t the event (classes 1, 2, and 3). The rest f the highcnfidence reverse earthquakes (11%) prbably are assciated with an active fld r fault at the Earth's surface (class 4). Thus, all the high-cnfidence earthquakes appear t be assciated with recgnizable Quaternary surface defrmatin. Lw-cnfidence earthquakes als shw a strng assciatin (91%) with surface defrmatin (classes 1, 2, 3, and 4). Furthermre, based n the data cmpiled fr this study and n analysis f the prbability f surface rapture fr earthquakes wrldwide, it appears that earthquakes larger than apprximately Mw 6.5 t 7.2 are unlikely t ccur where Mv flds ccur in the absence f Quaternary surface faults. The earthquake data base, therefre, supprts the cnclusin that blind thrust faults in interplate regins are nt truly "blind." Seismgenic thrust faults typically reach the Earth's surface. When they d nt reach the surface, they defrm the surface in a manner that can be identified and characterized by a gelgist cnducting detailed gemrphic and gelgic mapping and/r structural analysis. In additin, the available gelgic r gedetic data shw that nearly all f the interplate earthquakes in the data base prduced cseismic surface defrmatin (surface rupture, flding, r tilting), suggesting that repeated events wuld prduce gelgic and gemrphic evidence f surface defrmatin. Cnventinal paleseismic investigatin techniques, which include detailed mapping, scarp prfiling, and varius subsurface investigative techniques (e.g., trenching, drilling, gephysical explratin) are summarized by McCalpin (1996). Mst f these techniques are riented tward identifying and characterizing active faults at the Earth's surface. Paleseismic techniques t assess active flds and the assciatin between flds and underlying faults are summarized by Bullard and Lettis (1993). These techniques rely n detailed Quaternary gelgic and gemrphic mapping and mrphmetric analyses t identify and characterize regins f active flding. Mrphmetric analyses that have been successful in cnstraining the lcatin, pattern, gemetry, and amunt f fld defrmatin include (1) cnstructin and cmparisn f range-crest prfiles and piedmnt/muntainfrnt junctin prfiles; (2) cnstructin f lngitudinal stream channel and terrace prfiles; (3) cnstructin f drainage net maps and derivative prducts such as subenvelpe, envelpe, and residual maps; (4) assessment f range frnt and stream channel sinuusity; and (5) cnstructin f a reginal slpe map (Bullard and Lettis, 1993). With respect t gegraphic trends in the data base, ne might infer that thrust faulting in suthern Califrnia alng the transpressive San Andreas fault system is unusual. Althugh this interpretatin is permissive frm the data base, we believe it is incrrect. The suthern Califrnia regin cntains numerus seismgenic thrust faults that d nt reach the Earth's surface, and these thrust faults typically underlie well-frmed anticlines that lack c-existing surface faulting. In additin, structural mdels (e.g., Suppe, 1983; Namsn and Davis, 199) shw an excellent kinematic relatinship between fault slip and grwth f the verlying anticline. Hwever, suthern Califrnia has been well studied relative t ther parts f the wrld. The earthquake data base includes 37 high- and lw-cnfidence earthquakes that "prbably" are assciated with a Quaternary fault and/r fld (class 4). We believe that detailed gelgic studies wuld shw that many f these earthquakes ccurred n buried thrust faults beneath Quaternary anticlines, particularly in the Zagrs fld belt f Iran. The results frm this study are significant fr the assessment f seismic hazards. Thrust faults are knwn t pse a significant seismic hazard in many regins f the wrld. The data indicate that blind faults in interplate regins can be identified by thrugh and detailed gelgic investigatins. We als believe that the reverse hlds true: that if detailed gemrphic and gelgic mapping and structural analyses dcument the absence f active flding and/r thrust faulting at the surface, then an active blind r buried thrust fault prbably is nt present in that regin. The data base als indicates that cnventinal appraches t identifying Quaternary defrmatin are unlikely t identify mst surces f reverse/thrust earthquakes in intraplate tectnic settings. In thse regins, reverse earthquakes typically have ccurred in areas that lack recgnizable Quaternary defrmatin, r in areas where n Quaternary depsits are present (e.g., n truly blind thrust r reverse faults). The exceptin might be thse areas where Quaternary depsits are present, such as alng the New Madrid seismic zne in the vicinity f the 1811 t 1812 New Madrid earthquakes. Alternative techniques, including seismlgic, gephysical, and gedetic methds, may help identify and characterize blind thrust faults in these regins. Because these techniques als may nt identify all ptential surces f reverse earthquakes, additinal appraches (such as including a randm r flating earthquake within a gelgically/seismically defined surce zne) als may be necessary t assess earthquake hazards in intraplate regins. Acknwledgments This research was supprted by the U.S. Gelgical Survey's Natinal Earthquake Hazards Reductin Prgram under Award # G The views and cnclusins in this reprt are thse f the authrs and shuld nt be interpreted as necessarily representing the fficial plicies, either expressed r implied, f the U.S. Gvernment. We als thank Pacific Gas and Electric Cmpany (San Francisc) fr financial supprt fr cmpiling the initial data base f this study. Additinal supprt fr this study was

28 1198 W.R. Lettis, D. L. Wells, and J. N. Baldwin prvided by the Prfessinal Develpment Prgrams f William Lettis & Assciates, Inc., and Gematrix Cnsultants, Inc. References Bullard, T. and W. Lettis (1993). Quaternary fld defrmatin assciated with blind thrust faulting, Ls Angeles Basin, Califrnia, J. Gephys. Res. 98, Davis, T. L. and J. S. Namsn (1994). A balanced crss sectin f the 1994 Nrthridge earthquake, suthern Califrnia, Nature 372, Davis, T. L., J. S. Namsn, and R. F. Yerkes (1989). A crss sectin f the Ls Angeles area: seismically active fld and thrust belt, the 1987 Whittier Narrws earthquake, and earthquake hazard, J. Gephys. Res. 94, Electric Pwer Research Institute (1994). The earthquakes f stable cntinental regins, Prject RP , , and Reprt prepared fr Seismicity Owners Grup and Electric Pwer Research Institute by Gematrix Cnsultants, San Francisc, Califrnia, and Center f Earthquake Research and Infrmatin, Memphis, Tennessee. Haukssn, E., L. M. Jnes, T. L. Davis, L. K. Huttn, A. G. Brady, P. A. Reasenberg, A. J. Michael, R. F. Yerkes, P. Williams, G. Reagr, C. W. Stver, A. L. Bent, A. K. Shakal, E. Etheridge, R. L. Prcella, C. G. Bufe, M. J. S. Jhnstn, and E. Cranswick (1988). The Whittier Narrws earthquake in the Ls Angeles metrplitan area, Califrnia, Science 239, Haukssn, E., L. M. Jnes, and L. K. Huttn (1995). The 1994 Nrthridge earthquake sequence in Califrnia, seismlgical and tectnic aspects, J. Gephys. Res. 1, t2,335-12,355. Hitchcek, C. S., and K. I. Kelsn (1997). Late Quaternary tectnic defrmatin in the nrthern San Fernand Valley, Califrnia, in Prc. f the Nrthridge Earthquake Research Cnference, Califrnia Universities fr Research in Earthquake Engineering (CUREe), August 2-22, Hudnut, K. W., Z. Shen, M. Murray, S. McClusky, R. King, T. Herring, B. Hager, Y. Feng, P. Fang, A. DnneUan, and Y. Bck (1996). Cseismic displacements f the 1994 Nrthridge, Califrnia earthquake, Bull. Seism. Sc. Am. 86, Huftile, G. J. and R. S. Yeats (1996). Defrmatin rates acrss the Placerita (Nrthridge Mw = 6.7 aftershck zne) and Hpper Canyn segments f the western Transverse Ranges defrmatin belt (supplement), Bull. Seism. Sc. Am. 86, $3-S18. Jhnstn, A. C. (1992). The stable cntinental regin earthquake data base, EPRI Rept. RP , Electr. Pwer Res. Inst., Pal Alt, Califrnia. Kelsn, K. and G. Simpsn (1996). Multiple late Hlcene earthquakes alng the Reelft fault, central New Madrid seismic zne, J. Gephys. Res. 11, Lin, J. and R. S. Stein (1989). C-seismic flding, earthquake recurrence, and the 1987 surce mechanism at Whittier Narrws, Ls Angeles basin, Califrnia, J. Gephys. Res. 94~ McCalpin, J. P. (1996). Paleseismlgy, Academic Press, San Dieg, Califrnia, 583 pp. Namsn, I. S. and T. L. Davis (t988). Structural transect f the western Transverse Ranges, Califrnia: implicatins fr lithsperic kinematics and seismic risk evaluatin, Gelgy 16, Namsn, J. S. and T. L. Davis (199). Late Cenzic fld and thrust belt f the suthern Cast Ranges and Santa Maria Basin, Califrnia, Am. Assc. Petrl. Gel. Bull 74, Rajendran, C. P., K. Rajendran, and B. Jhn (1996). The 1993 Killari (Latur), central India, earthquake: an example f fault reactivatin in the Precambrian crust, Gelgy, 24, Russ, D. P. (1979). Late Hlcene faulting and earthquake recurrence in the Reelft Lake area, nrthwestern Tennessee, Gel. Sc. Am. BulL 9, Russ, D. P. (1982). Style and significance f surface defrmatin in the vicinity f New Madrid, Missuri, in Investigatins f the New Madrid, Missuri, Earthquake Regin, F. A. McKewn and L. C. Pakiser (Editrs), U.S. Gel. Surv. Prfess. Pap L Stein, R. S. (1985). Evidence f surface flding and subsurface fault slip frm gedetic evaluatin changes assciated with the 1983 Caliaga, Califrnia earthquake, in Mechanics f the May 2, 1983, Calinga, Califrnia Earthquake, M. J. Rymer and W. L. Ellswrth (Editrs), U.S. Gel. Surv. Open-File Rept , Suppe, J. (1983). Gemetry and kinematics f fault-bend flding, Am. J. Sci. 283~ Wells, D. L. and K. J. Cppersmith (1993). Likelihd f surface rupture as a functin f magnitude, Seism. Res. Let. 64, n. 1, p. 54. Wells, D. L. and K. J. Cppersmith (1994). New empirical relatinships amng magnitude, rupture length, rupture width, rupture area and surface displacement, Bull. Seism. Sc. Am. 84, Yeats, R. S., K. Sieh, and C. R. Allen (1997). The Gelgy f Earthquakes, Oxfrd University Press, New Yrk, 32 pp. Appendix A (References) The list f references fr the earthquakes listed in Tables 1 and 2 can be viewed and dwnladed electrnically frm the Bulletin f the Seismlgical Sciety f America Wrld Wide Web Site at The references als are available by request frm the authrs. William Lettis & Assciates, Inc Btelh Drive, Suite 262 Walnut Creek, Califrnia Vice mail: (51) ; fax: (51) ; lettis@lettis:cm and baldwin@lettis.cm (W.R.L., J.N.B.) Gematrix Cnsultants, Inc. 1 Pine Street, 1th Flr San Francisc, Califrnia Vice mail: (415) ; fax: (415) ; gematrix.cm (D.L.W.) Manuscript received 2 December 1996.