EARTHQUAKES AND THEIR HAZARDS IN WESTERN COLORADO: IMPLICATIONS TO SEISMIC DESIGN AND SAFETY. Ivan G. Wong

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2 EARTHQUAKES AND THEIR HAZARDS IN WESTERN COLORADO: IMPLICATIONS TO SEISMIC DESIGN AND SAFETY by 1 Ivan G. Wng In many parts f the western United States, the lack f significant histrical seismicity has prvided a misleading picture f the actual level f seismic hazards that may exist in a regin. Pale seismic studies in the past tw decades, hwever, have prvided a tl fr deciphering the prehistric earthquake recrd and a mre reliable basis fr assessing the surces, sizes, and frequencies f future damaging events. Western Clrad is generally thught t pssess a lw level f seismic hazard because the histrical earthquake recrd indicates nly lw t mderate seismicity, and knwn late-quaternary faults appear t be relatively few in number with lw activity rates (Kirkham and Rgers, 1981). Hwever, relatively few paleseismic studies have been perfrmed t date. Western Clrad is lcated in a transitin zne between the seismically active Intermuntain seismic belt t the west and the significantly less active cratnic Midcntinent east f the Frnt Range. Based n the brief ISO-year histrical recrd, the largest earthquake knwn t have ccurred in western Clrad, as well as the state as a whle, is the enigmatic 1882 mment magnitude (Mw) 6.6 earthquake (McGuire et al., 1982; Kirkham and Rgers, 1985; 1986; Spence et a/., 1996) (Figure I). This event may have ccurred in the nrthern Frnt Range, althugh its lcatin has been the subject f cnsiderable debate (Spence et al., 1996). The secnd mst significant event knwn t have ccurred is the 1960 Richter magnitude (ML) 5Y2 earthquake near Mntrse (Figure 1) which als ccurred prir t adequate seismgraphic cverage. Significant advancements have been made in ur understanding f earthquakes, their rupture prcess, and assciated tectnic stresses in western Clrad in the past 15 years because f imprved, albeit still sparse, reginal seismgraphic cverage since 1983, and tw aftershck mnitring studies using prtable instruments. Cnsistent with the histrical recrd, pst-1983 earthquake activity is distributed thrughut mst f western Clrad (Figure 1). The apparent mderate t high level f seismicity in the Frnt Range is prbably in part an artifact f the fcused seismic mnitring since 1983 (Figure Vice-President and Manager, Seismic Hazards Branch, Wdward-Clyde Federal Services, th Street, Suite 200, Oakland, CA 94607; phne (510) ; igwngxo@wcc.cm. X:\CR600IE&SHCOL.DOC

3 2). Events rarely appear t be assciated with specific gelgic structures. pssibly due t the lack f dense seismgraphic cverage utside f the Frnt Range. Seismicity in western Clrad is typically characterized by diffuse, small earthquakes (up t abut ML 4 t 5) and swann activity (sequences f events with n mainshck) (Figures 1 and 2). Swanns ften ccur in regins f relatively high heat flw which is generally the case fr the Suthern Rcky Muntains. In the Frnt Range, sme seismicity appears t spatially crrelate with a few Tertiary and Quaternary faults althugh further studies will be needed t establish an assciatin (Btt et af., 1996) (Figure 2). Based n the analyses f several small earthquakes (Wng and Humphrey, 1986; Ely et af., 1986), mst ntably, swanns near Carbndale in 1984 (Gter et af., 1988) and Crested Butte in 1986 (Wng, 1991; Btt and Wng, 1995), and activity in the Frnt Range (Btt et al., 1996), seismicity appears t be the result f the reactivatin f thrust/reverse faults f Laramide age. Such reactivatin appears t be extensinal in nature because the few earthquake fcal mechanisms which are available exhibit nrmal faulting in respnse t a nrtheast-suthwest riented minimum principal stress (Wng, 1986). Earthquakes in western Clrad, as elsewhere in the western U.S., are generally cnfined t the upper 15 t 20 km f the crust, althugh sme events ccur slightly deeper in the Frnt Range. Estimates f earthquake recurrence based n the recrd f histrical and cntemprary seismicity suggest that events f ML 5~, similar t the 1960 earthquake, ccur nly nce abut every 150 t 200 years, and fr ike events, every 1000 years r s. In cntrast, recent gelgic investigatins suggest that sme faults in western Clrad may be capable f generating earthquakes up t Mw 7 (Unruh et al., 1993a, 1993b, 1994, 1996; Nller et af., 1992); althugh these events are prbably rare. Such faults prbably have recurrence intervals f many tens f thusands t pssibly mre than 100,000 years. Results frm deterministic seismic hazard analyses indicate relatively strng ptential earthquake grund shaking at sites near active r ptentially active faults. These grund mtins may be t cnservative fr seismic design purpses, because f the lng recurrence intervals between estimated maximum earthquakes alng these faults. Fr sites distant frm seismgenic faults, the maximum backgrund (randm) earthquake is the dminant cntributr t seismic hazard. The 1882 earthquake is significant t bth deterministic and prbabilistic hazard assessments in the regin because the event has nt been assciated with a specific fault. Thus, it must be cnsidered t be representative f the minimum magnitude backgrund earthquake that culd ccur in western Clrad. Recent site-specific prbabilistic seismic hazard analyses, which incrprate earthquake recurrence, indicate lw t mderately high grund shaking hazard (less than 0.1 t 0.5 g) in western Clrad fr return perids f 500 t 10,000 years. These grund mtins are X :ICR600\E&SHCOL.DOC

4 higher than previusly published values due t ur imprved knwledge f earthquake surces and the inclusin f uncertainties in grund mtin estimatin. Fr example, the recent U.S. Gelgical Survey natinal grund shaking hazard maps indicate 500-year return perid peak grund acceleratins f nly 0.03 t 0.10 g and 2500-year values f 0.08 t O.30g. Thus it wuld seem prudent, given the uncertainties regarding the pssible ccurence f mderate as well as large infrequent earthquakes in western Clrad, t perfnn site-specific seismic hazard evaluatins, particularly fr imprtant and critical facilities. REFERENCES Btt, J.D.J., and Wng, I.G., 1995, The 1986 Crested Butte earthquake swarm and its implicatins fr seismgenesis in Clrad, Bulletin f the Seismlgical Sciety f America, v. 85, p Btt, J.D.1., Wng, LG., Ake, J., Unruh, J., Nichll, J., and Butler, D., 1996, Cntemprary seismicity f the Central Frnt Range, Clrad (abs.), Abstracts with Prgrams, Gelgical Sciety f America, v. 28, p. A-283. Ely, R.W., Wng, LG., and Chang, P.S., 1986, Netectnics f the Uncmpahgre uplift, eastern Utah and western Clrad, in Cntributins t Clrad Sesimicity and Tectnics - A 1986 Update, W. Rgers and R. Kirkham (eds.), Clrad Gelgical Survey Special Publicatin 28, p Gter, S.K., Presgrave, B.W., Nenrissey, R.F., and Langer, C.L., 1988, The Carbndale, Clrad earthquake swarm f April-May 1984, U.S. Gelgical Survey Open File Reprt , 8 p. Kirkham, R.M. and Rgers, W.P., 1981, Earthquake ptential in Clrad, Clrad Gelgical Survey Bulletin 43, 171 p. Kirkham, RM. and Rgers, W.P., 1985, Clrad earthquake data and interpretatins 1867 t 1985, Clrad Gelgical Survey Bulletin 46, 106 p. Kirkham, RM. and Rgers W.P., 1986, An interpretatin f the Nvember 7, 1882 Clrad earthquake, Clrad Gelgical Survey Open-File Reprt 86-8, 36 p. McGuire, RK., Krusi, A., and Oaks, S.D., 1982, The Clrad earthquake f Nvember 7, 1882: size, epicentral lcatin, intensitities, and pssible causative fault, Muntain Gelgist v. 19, p Nller, 1.S., Unruh, 1.R, Lettis, W.R., and Wng, LG., 1992, Seismtectnic evaluatin, Smith Frk Prject, Crawfrd Dam, Clrad River Strage Prject, Crystal Dam, Mrrw Pint Dam, Blue Mesa Dam, Bstwick Prject, Silver Jack Dam, suthwestern Clrad, unpublished reprt prepared fr U.S. Bureau f Reclamatin. Spence, W., Langer, c.j., and Chy, G.L., 1996, Rare, large earthquakes at the Laramide Defnnatin Frnt--Clrad (1882) and Wyming (1984), Bulletin f the Seismlgical Sciety f America, v. 86, p X:'CR600' &SHCOL.DOC

5 Unruh. 1.R.. Nller. 1.S.. Lettis. W.R.. \\Tng. I.G.. Sawyer. T.L.. and Btt a. Quaternary faults f the central Rcky Muntains, Clrad: A new reginal seismtectnic evaluatin (abs.), Abstracts with Prgrams, Gelgical Sciety f America, v. 25, p Unruh, J.R., Wng, I.G., Btt, 1.D.1., Silva, W.1., and Lettis, W.R., 1993b, Seismtectnic evaluatin, Rifle Gap Dam, Silt Prject, Ruedi Dam, Frying Pan Arkansas Prject, nrthwestern Clrad, unpublished reprt prepared fr U.S. Bureau f Reclamatin. Unruh, 1.R., Wng, I.G., Hitchcck, C.S., Btt, 1.D.J., Silva, W.J., and Lettis, W.R., 1994, Seismtectnic evaluatin, Puebl Dam, Frying Pan-Arkansas Prject, suth-central Clrad, unpublished reprt prepared fr U.S. Bureau f Reclamatin. Unruh, 1.R., Wng, I.G., Knudsen, K.L., Btt, J.D.J., Becker, A., Silva, W.l., and Lettis, W.R., 1996, Seismtectnic evaluatin, Rattlesnake and Flatirn Dams, Clrad-Big Thmpsn Prject, nrth-central Clrad, unpublished reprt prepared fr U.S. Bureau f Reclamatin. Wng, I.G., 1986, Tectnic stresses in Clrad and their implicatins t seismicity, in Cntributins t Clrad Seismicity and Tectnics - A 1986 Update, W. Rgers and R. Kirkham (eds.), Clrad Gelgical Survey Special Publicatin 28, p Wng, I.G., 1991, The 1986 Crested Butte, Clrad earthquake swarm, Muntain Gelgist, v. 28, p Wng, I.G. and Humphrey, J. R., 1986, The 14 August 1983 Cimarrn, Clrad, earthquake and the Cimarrn fault, Muntain Gelgist v. 23, p X:\CR600\E&SHCOL.DOC

6 Wy 41 --I---- ~----~ L UT ~ '\<,,~ 0 ~~ (j),~ (j). (j)~" Frt Cllins 0" cd ~- 0" N' ". "'(i) 0' ~...1 (j). - J Q (j) A ~; l L ~ 0 Bulder Q ~ Q :~,. ~ ~~+_!il 1984 (j) ~ ~ 0 CARBONDALE ~~ cd _ Glertwd SWARM (i) 0 (l) Springs (j) ~ -. (j) ft',\, Q Leadville ~ (j) - Q - \ GRInd Junctin c# 0 C).Aspen ~- (j) (j) ~ G5 ~ (j) (j) 1986 ~ fl. (j) ~ _r.'l.. ~ CRESTED BUTTE' Cjl Crested G,)'"" "-.. SWARM Butte (j) (i) 0 (j)mntrse ell cd / (i) (i)~ (i) -Gunnisn ~ (i) ~~..,.. 0 ~ ~ ~~ 0 ~ J ' "(i)~ Q cpj' \ \1~Y"- (j) ~ 0 Clrad Springs (j) NB ROCKY MOUNTAIN ARSENAL EARTHQUAKES MAGNITUDE (j) 3.0 (j) 4.0 (j) (j) Ci) KS Q Q (j) (j) 50 I,, I I I kilmeters NM OK Figure 1. Histrical seismicity (1870 t 1992) and ptentially active faults in Clrad. Nte Frnt Range seismicity is shwn n Figure 2.

7 40.0 LONGMONT + Ci) " + \ ī I 1 Q I & CASTLE Q R0CKQ + Great ~ Plains ~ ~+ I N r Magnitude ML 1 Q 2 Q 3 4Ci) Fault I ~./ Figure 2, Micrseismicity, , and Cenzic faults in the Frnt Range 45