85 84 Utah Geological Survey Western States Seismic Policy Council Pmceedings
CONTRASTS BETWEEN SHORT- AND LONG-TERM RECORDS OF SEISMICITY cally <1-2 mm/year; thus, recunence intervals for surface rupturing fault are as little as several thousand years (e.g., IN THE RIO GRANDE RIFT-IMPORTANT IMPLICATIONS FOR SEISMIC the Wasatch fault zone) or as long as 10,000 years (e.g., a N ----1 typical Basin and Range fault) t I 00,000 years (e.g., HAZARD ASSESSMENTS IN AREAS OF SLOW EXTENSION Denver ome of the least active Quaternary fau lts in the Rio Grande rift). Obviously, the 150- to 300-year-long record :~ Colorado Michael N. Machette of felt seismicity only portrays a small fraction of the '\ I\ U.S. Geological Survey, MS 966 potentially active faults in these regions. I \ GP I I Geologic Hazards Team-Central Region I I I Finally, in even le s tectonically active regions, such CP SRM' ' ' Denver, Colorado 80225 as the pa i.ve margin of the ea tern U.S. and compres --r -~ -- l~ ([email protected]) sional domain of the stable continental interior of the , ~Taos i-- U.S., Canada, and Australia, fault slip rates are probably J- Los', ( 1 measured in hundredths of a mm/year, and intervals be Alamo~'• :Santa Fe I ABSTRACT tween surface rupturing events may be extremely long \ Arizona : , ~J 1 ., Albuquerque (>100,000 years) or immeasurable. However, these re I f,C~ \ I I I The Rio Grand rift ha a relatively hort and unimpres iv record of hi toricaJ eisouc1ty. However there is abunda.nl gions also eem prone to clustered earthquake activit)'. \ • : New Mexico ( ocorro, 1 vidence of prehistoric (Quaternary) ·urface faulting as ociated with large (M>6) earthquakes. Thi paradox ben.veen his that is, one where seismic structure are recurrently active I \ •Ph oenix torical and prehistoric ei micily (paleo. eismi.city) ha imp rtant impljcation for seismic-hazards a11alyses based primar over relatively hort geologic time frames (hundred to I BR ' ~, GP ily on modern seismicity. thousand of year ), then inactive for long interval · of I ,' .\ : RGR : c Tucson , , ' , 1 " 1 .... • ( 8 Paso The paleo eismic record of faul.Li ng in the rift i poorly documented compared to mo l . eismically active regions, time (thousands to tens of thousands of years). The New 1 _.l..---,:, - ,i,,;;::--...... Douglas. 1- - 1t ' : , Texas mainly becau e ther have been few detailed studie and partly becau:e many or th faults have long recurrence intervals Madrid and Charleston seismic zones (mid-continent and -i,c0 .... --.... _ -~ Jua rer, ~ eastern U.S., respectively) and the Cheraw fault (south I ' (e.g. 10,000-l 00 000 years) which make dating them difficult by radiocarbon methods alone. Nevertheless, a new com I ' I pilation f Quaternary fault data for the rift sugge t Holo ene (<10,000 years) movement on at least 20 faults . Several of eastern Colorado) are examples of faults that have evi ? \ •, Valentine ? the e fau lt have evidence for mulliple movements · Chu , at least 22 large earthquakes of probable M >6.25 were associated dence of short-term clustering but no geologic evi_dence 0 100 200 300 400 500 km ? \ : I with surface ruptures during the past 10,000 years, or one about every 450 years. of similar long-term tectonic activity. MEXICO ' '-...._ / Because the level of seismicity of the Rio Grande rift is generally low, the populace (in general) believes that earth In extensional domains such as the Rio Grande rift, a 0 100 200 300 mi quakes do not pose a significant threat to them wherea the pre. ence of abundant young faults tells a different . tory. From situation exists where the record of historical seisrnicity is a geologic viewpoint, it . eem obvious that modern seismic-hazards asses ments for regions like the Rio Grande rift must relatively short and generally unimpressive. Instrumental Figure 1. lndex map of the Rio Grande rift showing cultural and geo use not only catalog of modern seismicity, but also integrate data from a comprehensive inventory of Quaternary faull's, data exist from the mid-1960s and felt reports go back to graphic feeu 11res mentioned ill the text. Phr~iographic provinC~.\' C{l'OIII especially those tructures showing evidence of movement in the pa t LOO 000 year (a time interval which encompa ses a the 1600s in specific locations (e.g., Santa Fe). Although Fe111ie111an, 193 i ) are as follows: BR, Basm and Range; CP, Colorado Plateau; GP, Great Plains; MR, Middle Rocky Mountains (includes complete earthquake cycle for mo t aU active fault ). A myopic view f earthquake hazard po ed by indjvidual fault (i.e. the paleoseismic record reveals evidence of abundant late Wyoming Basi11); RGR, Rio Grande rift subprovi11 ce (shaded pallenr); having recmTenc interval of 10,000 year · or more) can lead t a complacent attitude that trenglhen. a perception of low Pleistocene (i.e., 10,000-130,000 years ago) and Holocene and SR, Sowhern Rocky Moumains. Solid lines show province bound seismic potential for the region. Without proper caution, this attitude can be manife ted in inapproprhtte con u·uction (<10,000 years ago) faulting in the rift, we only have evi aries; dashed line shows limits of Rio Grande rift as used in this paper. styles, building codes, land-u e policies, and the siting (or relocation) of important or crili,cal facilitie . dence of a single historic surface rupture from an earth quake that occurred a little more than I J.0 years ago in Neogene age that extends almost 1,000 kilometers from northern Sonora, Mexico, outh of Douglas, Arizona. Leadville, Colorado, south into western Texas, southeast INTRODUCTION controlled by their proximity to tectonic plate margi ns, Within the U.S. portion of the rift, the largest felt or ern Arizona, and northern Chihuahua and Sonora (north regional geologic setting, and their current state of ' U·ess. recorded earthquake (Ms 6.3) occurred in 1931 near ernmost Mexico) (figure 1). The rift widens to the south; Valentine, Texas, and although this event did not form a it is a singular 30- to 40-kilometer-wide, haJf-graben north Virtually all modern earthquake-hazards assessments Although there is probably a continuum between these regions, let us con ider that there are three general types demonstrable surface rupture, it may have had as much as of Taos, New Mexico. The down-to-the-west Sangre de are based primarily on historical seismicity coupled with 38 centimeters of slip in the subsurface (Doser, 1987). Cristo fault zone (faults 3 and 4 , figure 2) is the predomi geologic evidence of faulting that is associated with f ei mogenic etting in the United States. This simple characterization allows us to frame the present and Neo This earthquake, and similar-size ones in the Basin and nate rift-bounding structure in this area. South from a line strong ground motion (paleoseismology). The predictive Range Province (see Wells and Coppersmith, 1994), sug between roughly Taos and Los Alamos, New Mexico, the nature of these assessments follows the widely accepted gene seismotectonic setting of the Rio Grande rift. In tectonically active region · e. peciaJly tho e border gest that the threshold for surface ruptures is in the lower rift widens to 40-60 kilometers, with alternating-polarity paradigm that "the past is the key to the future." This seis half of the M 6 range (i.e., M 6.25±0.25) for normal faults half grabens in the Espanola, Santo Domingo, and Albu mological basis for earthquake-hazards assessments is in g convergent or transpres ive plate margins, large M >6) earthquake are a ociated with cata. trophic move that have earthquakes nucleating at depths of 15 kilome querque basins. South of Socorro, the rift becomes even natural, quantitative, and justified through traditional ment on Faults having slip rate that are typically tens o( ters to perhaps 20 kilometers. Thus, there is an apparent wider (60-150 kilometers) and is comprised of 2 or more practice. How ver, a paradox exists in many recently mm/year to everal cm/year and recurrence interval o'I" paradox in the Rio Grande rift between the short-term half grabens or graben . The southern pru1 of the Rio developed countries with short hi. torical record , such as several hundreds (e.g., San Andreas fault) to several thou record of seismicity (relatively aseismic, no surface rup Grande rift lies with a outhea tern pru1 (extension) of the the United States where the recorded history of seismicity sand years long (e.g., blind tlu-ust faults within the Los ture) and the long-term record (abundant evidence of late Basin and Range, the rift's western margin is herein con may be only 100-300 years long and the instrumental Angele ba ·in). Strike-slip and thm t fa utt in coastal Quaternary faults and, hence, paleoseismicity). The man sidered to be near the Arizona/New Mexico border; how record may be considerably shorter. The problem lies in California and the Ca cadia ubduction zone fall in th.is ner in which this disparity is dealt with has important ever, some have placed the boundary of the Neogene Rio the potential disparity between short-term records of broad tegory of tectonically active region . Here th implications for seismic-hazards analyses. Grande rift further east near Deming, New Mexico. Nev strong ground motion reflected by historical sei micity brief record of felt . ei micity ( I 00-J 50 years) may ade ertheless, at the International Border with Mexico, the rift is on the order of about 400 kilometers wide. On the basis and long-term records of trong ground motion indicated quately portray many of U1e active faults. Nevertheless QUATERNARY FAULTING IN THE RIO from paleoseismic studies. This paper reflects on these even seismicity fail to image some sei m geni · sr_ruc GRANDE RIFT of the north-south orientation and youthfulness of fault disparities and the reasons they exist. ture , such as blind thrusts (e.g., the 1994 Northndg ing, the Quaternary Rio Grande rift probably extends from The U.S .. and many other countries have geologically earthquake). The Rio Grande rift lies within the eastern part of the the Animas Valley in southwestern New Mexico and San Bernardino Valley in northern Sonora, Mexico ( on the distinct region. that fall into markedly different sei mo In tectonical1y less-active regions such as the exte 1_1• Basin and Range and the southern part of the Rocky west) to the Lobo Valley near Valentine, Texas (on the genie settings. Commonly, these regions are directly asso sional domains of the Basin and Range Province and R1. Mountain provinces as defined by Fenneman (1931); it ciated with tectonic provinces, their seismic activity being Grande rift of the western U.S., fault slip rates are typi- form a nearly continuous, deep, sediment-filled valley of east) (figure 2). 86 Utah Geological Survey Wes tern States Seismic Policy Council Proceedings 87
earthquake event , the Sangre de Cri to and Organ Moun O 100 200 300 km \ I I I I I N tain fault have evict nee for two parate movements in Table 1. Faults with known or suspected Holocene movement in the Rio Grande rift. 1 the Holocene; thus one should con icier that there have 0 • 100 2~0 m, \ I (Abbreviations: SM, scarp morphology; ST, stratigraphic relations; S, soils; CJ4, radiocarbon dating; T, trenching; Mtns., mountains; • \ I been at lea t 22 large (M >6.25) earthquake as. ociated / 'Alamosa ' I I EQ, earthquake) <;_o_LORADE __ __ ~ with surface ruplur~ in the Rio Grande rift iJ1 the pa t r ~ L __ __ IO 000 ye~· . The e data ugge t a HoJo ene corn po ite Evidence for timing recurrence JJltervaJ (CRI) of about 450 years for tbe above Name of fault Number & general location Time of most (see figure 2) recent event & main reference(s) NEW MEXICO lo, 1 menti?ned portion of the rift. Some portion of the ri ft, t' + , e~pec1ally those settled by Span~ h missionarie along rhe Alamos Rio Grande, have been contrnuously occupied since Sangre de Cristo 1. West front of Sangre de Cristo Early Holocene SM, ST, S, Cl 4, & T; McCalpin,1982. anta Fe nearly A.D. 1600 thu tbe CRI i approaching the time (section A) Mtns., NE of Alamosa, Colorado. t ...1 ,, ,14 .,,,. that the regi n has been occupied by AmeJican. of Euro 2-3(?) events ST & C14; Kirkham & Rogers, 1981. ,,r • Albuquerque pean origin i.e. Lhe hi torical record). The new calcul ated Sangre de Cristo 2. West front of Sangre de Cristo ~I ,, r (section B) Mtns., E of San Luis, Colorado. in Holocene o ' ~!J., CRT i con iderably le · than before (750 year ) becan e T have included fault · from a larger area (fexa , Mexico, Sangre de Cristo 3. West front of Sangre de Cristo Mtns., Middle to early SM & ST; Menges, 1990. ~· 9 1 ~1 I ~ I Socorro and outhern Colorado) and because there are everal (section C) near Cuesta, New Mexico. Holocene ' \ ,. < 10 newly documented fault · in the Ji ting. The mo ·t recent movement on the prehistoric faults eern to have been Sangre de Cristo 4. West front of Sangre de Cristo Mtns., Early (?) Holocene SM & ST; Machette & ) ~~C) I alo.ng the OJ·gan Mountains fau lt about I 000 year ago (section D) near Taos, New Mexico. Personius, 1984. 1/1~1 /' } I (G~le l987) wherea~ the P_itaycachi fault i the only hi - Valle de Vidal 5. East side of Valle Vidal, Late to middle SM; Menges & Walker, 1990. 12 \ '- \ 13 I tone .urface. rupture 1~ the nft (see. table I). Ba,ed on pa t northern New Mexico. Holocene (::,tt_1 ,), ,-, I.' . -),'"111~, I expenence, .1.t eem hkely that with forth r paleo i mic J 1 1 tudie the number of faults with known Holocene move Gua,:_e Mountain 6. West side of Espanola basin, Middle Holocene ST, T, & C14; Kelson & others, 1993. I I 16 LC Pi• ,' \ N of Los Alamos, New Mexico. • 1 ~ - . Y; \_ ,Jd\~B Paso - - -\ - - TEXAS ~ menl will probably increase and the CRl will decrea e 0 17 11 1 it L J \ 1 s\\111 .. 1 ~ ~ omewbat a ha in many other part of the Ba in and Rendija Canyon 7. West side of Espanola basin, Early Holocene '? ST, T, & C14; Kelson & others, 199J. - - , 0 Juarez " van Rance. In contra t, the CRI for the rif i roughly equiva N of Los Alamos, New Mexico. lo !~;.;- p ee it: l :J G.?./11; " '\ \ ~ \.' . Horn , ~enl to that of the Wasatch fault zone (5 active egment ) 18 MEXICO 1s"--~, • · t 20 J rn Utah (Machette and others 1991 table 2) a fault zone Coyote Springs 8. North flank of Ladrone Mtns., Early (?) Holocene SM; Machette & McGimsey, 1983. ? ' \ ? ' \ \ • Valenllne whjch i con iderably more active that those typical of the central New Mexico. Ba in and Range Province. La Jencia 9. West side of La Jencia basin, 1-2(?) events in Holocene SM, ST, S, & T; Machette, 1988. 1!ig11re 2. Generalized map of Quaiemwy faults of the Rio Grande rift Tbere are two major urban center iJ1 the relatively central New Mexico. '.11 so111hem Colorado, New Mexico, western Texas, and 1101·them Mex sparsely populated Rio Grande rifr. The northern center ico. Faults of Holocene (and historic) age are numbered anti descri include · Lo · Alamos, Santa Fe and Albuquerque a region Socorro Canyon 10. East side of Socorro Mtns., Early (?) Holocene SM; Machette & McGimsey, 1983. b din table J. where an e timated 50 percent (i.e. 750 000) of New S of Socorro, New Mexico. Mexico's population lives (ca. 1.5 mi11ion in J 990). Thi Caballo 11. Northern segment of Caballo Late(?) Holocene . SM, ST, C14, & T; Machette, 1987c, Quaternary faults ar both wide pread and common in area i close t?. or incl~de everal Quaternary fault (Lh e ' L. ~ '1 ,,,, .,.!, (Williamsburg scarp) fault, S of Truth or Consequences, 11',tuj =- r,< , .•.\J ,:_I'' ···' Foley & others, 1988. the Rio Grande rift (Nakata and others, 1982· Machette Embudo, Par~J1to Gua.x.e Mountain, Rendija Canyon, and New Mexico. and other 1996) but re]ath(ely young <15,000 year old) Hubbell Sprmg ) that have dear evidence for either urfac~ ruptw-es are re tricted primarily to major range Holocene or late. t Plei tocene movement a ociated with Caballo 12. Central segment of Caballo fault, S of Middle Holocene SM, ST, & T; Machette, 1987c, Foley boundmg faults (Mach tte and Hawley 1996) such as large earthquake . New studies of the Parajito faull indi Truth or Consequences, New Mexico. & others, 1988. cate late t Pleist cene (Kelson and others J993) or po si tho e along the Sangre de Cri to, Jemez Socorro, Mag Alamogordo 13. West flank of Sacramento Mtns, Early (?) Holocene SM & ST; Machette, 1987b. bly you_n ger movement James McCalpin, oral com dalena, Caballo, an Andre Organ, and Franklin Moun Alamogordo, New Mexico. tain (. ee figw-e 2). The paleosei ·mi.c record of fau]tino in mumcatt ~. 1997), whereas two of its sub 'idiary fault the rift i. poorly under'tood mainly becau e there h~ve (the Rend1Ja and Gua~e M untain) are known to b Organ Mountains 14. East flank of Organ Mtns., 2 events in Holocene SM, S, C14, & T; Gile, 1987; been few detailed tudies and partly becau e many of the Holocene (~el on and olher , 1993). The Parajito fault southern New Mexico. Machette, 1987b. ha · a long hJ tory of Quaternary movement-Hs carp on fauJts have long recurrence intervals (i.e., J 0,000-100 000 Early(?) Holocene SM & ST; Machette, 1987b '. 1.1-milli?n-year-ol~ Bandelier Tuff i commonly 120 East Franklin Mountains 15. East flank of Franklin Mtns., years) which makes dating times of movement on I.hem north of El Paso, Texas. meter high; more unportantly the :fault bounds tbe west difficult by radiocarbon methods. Thus, it is difficult to a e s prehi Lori level of seismicity in the rift. How side of the Lo ~ lamos National Laboratory, an important Washburn (zone) 16. East flank of Pelloncillo Mtns., Holocene SM; Machette & others, 1986. research and defen e-related facility. 1n addition there are W. of Animas, New Mexico. e~er, ba ed on.preliminary tudies of fault age and di trib everal other critical facilities (Kirkland Air Force Base ~1on I prev1ou ly uggested a compo ite recurrence and Sandia National Laborat ry) the State Capital and Gillespie Mtn. 17. West flank of Animas Mtns., Holocene SM; Machette & others, 1986. u1tervaJ (CRI) of ab ut 750±250 y ars for a major urface Co~hiti Reservoir within thi region all of which could be Animas Valley, SW New Mexico. rupturing earthquake in the New Mexico part of the Rio su bJeCl to damage from . evere gr und ha.king during Pitaycachi 18. East side of San Bernardino Historic (May 3, 1887) 75 km rupture, scarp height 4 m max., ~rande rift (M~ch~tte,, 1987a). We now have a compila large earthquake on nearby active faults. (Sonora EQ) Valley, NE Sonora, Mexico Bull & Peatthree, 1988. tion of paleose1snnc data for the rift in outhern Colorado . At the . out!lern margin of the rift population i (Sangre de Cristo fault), New Mexico we. t Texa and mamJy concentrated in the El Paso/Juarez metropolitan West Lobo Valley 19. West side of Lob Valley, Holocene (?) SM; Machelle unpubl. data, 1982; northern Mexico (Collin. and other , 1996' Machette and area (estimated population of more than 1.8 million), wiU1 W of Valentine, Texas. Collins & Raney, 1991. other , 1996' re pectively). These data indicate Holocene a mailer urban area at Las Cmces (70 ki lometers to the Amargosa 20. Northeast side of Amargosa Mtns., Early (?) Holocene SM, ST; Collins & Raney, 1991. movement on 20'fauJts (table l), including the one histori north-northwe t). E l Paso has nearly 600,000 people liv NW Chihuahua, Mexico. cal rupnll"e in northern Mexico. In addition to these 20 ing in a relatively mall area (El Pa o County), wherea 88 Utah Geological Survey Western States Seismic Policy Council Proceedings 89
Cfodad Juarez just across the Rio Grande has a burgeon der with Colorado (figure 3). The largest instrumentally has not been traditionally associated with the Rio Grande SEISMIC HAZARDS IN THE RIO GRANDE ing population roughly estimated at 1.2 million in 1990. recorded earthquakes in the rift have been a M 5 in 1989 rift because the earthquake occurred in Mexico and the rift RIFT The largest historical earthquake to affect the El Paso near Bernardo (between Socorro and Belen, New Mexico; has been considered to be a largely U.S. feature. How region was a MM VI (cited in Collins and Raney, 1991). Sanford and others (1991) and a significantly larger Ms ever, many of the rift tructures in New Mexico and west The scarcity of historical surface faulting (figure 2) However both El Paso and Juarez would be threatened by 6.3 earthquake in 1931 near Valentine, Texas (Doser, Texa continue south into Chihuahua and Sonora. The and the general low level of seismicity of the Rio Grande movement of the East Franklin Mountai_n fault, which 1987). Although the Valentine earthquake was not quite 1887 Sonoran earthquake occurred along the Pitachychi rift (figure 3) has led the populace in that area to believe extends through the heart of both urban areas. The East large enough to cause surface faulting, the epicentral area fault (Bull and Pearthree, 1988)-a range-bounding, that earthquakes do not pose a significant threat to the Franklin Mountains fault is just the . outhern one-quarter has several Quaternary faults that have been active in the north-south-trending, high-angle normal fault that is a region. In fact, the most recent series of USGS seismic of a 182-kilometer-long fault zone that extends from south Holocene(?) or latest Pleistocene (table 1; Collins and southward continuation of Quaternary faulting in the San hazard maps (Frankel and others, 1996) indicate 1) low of the International Border with Mexico (in the city of Raney, 1991; Collins and others, 1996). Also, it is inter Bernardino Valley of southwestern New Mexico (see levels of ground acceleration (:s;O.l g) with 10 percent Juarez) north through El Pa. o and Fort Bliss and into New esting to note that on the basis of instrumental data Machette and others, 1986). As such, this fault should be probability of excedance in 50 years and 2) low to moder Mexico along the we t side of White Sand Mi ile Range (through 1977), the Great Plains and Colorado Plateaus - considered as a modern rift structure. It has the character ate (:s;0.3 g) levels of ground acceleration with 2 percent and the Tularosa Basin. Thi fault zone is one of the two provinces considered to be tectonically stable-had i tic of a major surface-ruptming rift fau ll ee for exam probability of excedance in 50 years (see figure 4a and 4b, longer active normal faults in the Rio Grande rift, ex equivalent or greater seismicity than the Rio Grande rift ple Pea.rthree and Calvo 1987; Machette 1988) in that it respectively). For example, 0.10 g may be an appropriate ceeded only by the Sangre de Cristo fault zone in notth (Sanford and others, 1991). The most recent moderate ha, an extremely low lip rate and l ng-recmrence inter threshold for damage to older structures (pre-1965 d wel ern New Mexico and southern Colorado. The most recent size earthquake was an M 5.7 shock that struck Alpine, val (Bull and Pearthree, 1988). Although almost forgotten lings) or dwellings not made resistant to earthquakes (see movement on the East Franklin M untai11 fault probabJy Texas (east of the rift) in April 1995. Thus, modern seis because it occurred about 110 years ago, movement on the Frankel and others, 1996). The seismic-hazard maps was in the latest PleisL cene or early(?) Holocene (J. micity within the rift is remarkable only for its subdued Pitachychi fau ll B nned the longest (75 kilometers) his shown in figure 4a illustrates the domination of moderate Keaton and J. Barnes, written communication, 1995), but level and lack of association with known Quaternary toric utface ruptur of a normal fault in North America. seismicity in areas of slow extension, such as the Rio the fault has a history of recurrent movement as docu faults. Tucson, then a dusty frontier town and now a major urban Grande rift, whereas figure 4b starts to show the affect of mented by Quaternary scarps as much as 60 meters high. Even though the rift seems to be a relatively quite area of Arizona, was subject to strong ground motion and faults on the hazard. Thus, the populace's perceptions In earlier studies, Machette ( 1987b) estimated that this seismogenic region, past history shows its potential for po11ions of the intervening countryside experienced both are well based on what has happened in New Mexico entire fault zone is comprised of five discrete parts (fault large and devastating earthquakes. For example, the great liquefaction and artesian water spouts. over the past several centuries. The areas of highest map- segments), each having recurrence intervals of 10,000- J 87 Sonoran earthquake (Mw 7.4) of norLhern Mexico 20,000 year, . If this i true, then a major surface-ruptur ing earthquake may ccur, on average about once every • • 2,000-4,000 years (CRI) somewhere on the 182-kilome • ter-long fault zone. This fault system and many others are J • invi ible (not imaged) on seismici.ty maps of the Rio UT1CO • Grande rift. AZ- /-NM------GOK- • • I @ I SEISMICITY Santa Fe • I 1918 • Albuquerque + I Seismicity in the rift is relatively diffuse with few • • ' meaningful concentrations or associations with active ' N faults (figure 3). About half of the earthquakes shown in ' Magnitude • • 4-5 figure 3 are within the rift, and the other half are in the I ., Colorado Plateaus Province and, to a lesser extent, the I 1906 •• • 5-6 Socon:o ~ Socorro 6-7 li Great Plains Province adjacent to the rift. Much of the felt swarm (but not recorded) seismicity for the period 1849-1961 I • 7-7.5 I I • • (Northrup, 1976) was concentrated along the Rio Grande \t Valley between Albuquerque and Socorro. The historical I documentation of these earthquakes probably reflects the r • concentration of early settlers in towns along the Rio I •'I Grande and local sources of low to m derat ei micity. - - -- ~ El Paso - - T X - - 1 For example, the swarm of earthquakes that occurred at ~I _ __J Juarez~ , Soccoro in 1906 (MM VIII; Sanford and others, 1991) appears likely a result of magma movement at depth. The • MEXICO ' ~ 193 • 1887 , Valen'tinc Socorro area continues to be a focus of small, but numer Sonora " +• ous earthquakes (figure 3). Conversely, the distribution of • • ) 1995 M~2.5 earthquakes in New Mexico for the period of • \ Alpirw• 1962-86 (figure 5 in Sanford and others, 1991), shows 0 100 200 300 km 0 100 200 300 km only a weak association with the rift and some of the 0 100 200 mi 0 1oo 200 300 km transverse structures such as the Jemez lineament, which I •, Figure 3. Map showing earthquakes with magnitudes 4.0 and gret1ter 0 100 200 mi 0 100 200 mi partly control the geometry of the rift (i.e., accommoda in New Mexico and adjacem regions. Earthquakes, ke red tQ magni tion zones). tude by symboLv, are f rom 1he combined cawlog used by Frankel and Within New Mexico between 1962 and 1986, there other ' (1996). The 1887 Sonora, Mexico earthquake (table /, 11 0. 18) were only six earthquakes larger than M 3.5, and the i 1he lar>esr historic su,face-rupwring earthquake to have occurred in Figure 4. Recent seismic-hazard maps of the New Mexico portion of the Rio Grande rift. Maps show ground-motion hazard ( contoured in percent g) th e region. Other large etmhquakes, sucli as /he 1931 Valemine, Te.n1s at a given level of probability. A) 10 percent probability of exceedance in 50 years. B) 2 percent probability of exceedance in 50 years. Maps based largest (1966 Ms 4.6-4.9; Sanford and others, 1991) ea rrhquake and the 1906 Sonora, New Mexico earthquake (swarm), on Frankel and others, 1996. Data downloaded from U.S. Geological Survey web site at http://gldage.cr.usgs.gov/eq!html/custom.shtml, whzch allows occurred in the Colorado Plateaus province near the bor- are discussed in rhe tex1. you to create custom ground-motin-hazard maps using varying parameters. 91 90 Utah Geological Survey Western States Seismic Policy Council Proceedings
ped ground acceleration are centered over areas where M and others, 1992b for a more complete discussion). Con 8,000 800 Average Slip Rates (mm/yr) >4 earthquakes have occurred historically ( compare figure versely, sparse data from older datums (70,000- to ~ a, 3 with figures 4a and 4b). Interestingly, one of the areas 140,000-year-old lake deposits) indicate much slower slip ai 600 of large predicted ground acceleration(:::: 0.2 g, figure 4b) rates (i.e., 0.1-0.2 mm/yr) on the Wasatch and associated E is centered over the 1887 Sonoran Mw 7.4 earthquake faults. Thus, in the span of one complete climatic cycle .!: of 400 partly as a result of continued M 4 earthquakes in the area (e.g., 120,000 years), the Wasatch fault zone may have 7,000 ~ (which might be aftershocks from 1887 earthquake). had a 10-fold variation in slip rate. This example may be ] 200 Interestingly, the Pitachychi fault, on which this earth somewhat extreme owing to possible lake/rebound LL B quake occurred, has an estimated recurrence interval of affects, but an analysis of slip rates at Socorro, New Mex 100,000 years or more (see Bull and Pearthree, 1987) for ico suggests a similar potential change through time. o....,. _ _,___. _ _.___,_~-~~- 6,000 o 2 4 6 8 large surface ruptures. Thus, although the maps indicate The Socorro Canyon fault zone (fault 10 on table 1 ~ Time, in millions of years potential for moderate ground acceleration associated and figure 2) bounds the eastern margin of the Socorro c" 0 with a M <6 earthquake in Sonora, Mexico area, paleo Mountains, a rotated fault block which is composed of ·~ > seismologic studies predict an extremely long return time Precambrian and Paleozoic rocks and intruded by a mid wQ) for another 1887-like M >7 earthquake. Tertiary volcanic caldera. The northern margin of the 5,000 The paleoseismic record clearly demonstrates that caldera is spectacularly exposed in cross section within potentially large and devastating earthquakes may occur the mountain front escarpment. The range was uplifted somewhere in New Mexico as frequently as every 750 during early rift formation (early Miocene time), but by ------San Acacia basalt years (or less) to as frequently as every 450 years for the late Miocene time ( ca. 9-10 million years ago) the range ------(4.5 Ma) rift. On this basis alone, one cannot argue that large earth was covered by playa deposits of the Popotosa Formation. 4,000 Popotosa playa quakes will not occur on low-slip (long-recurrence) faults. Subsequent rejuvenation of the mountain range by uplift Socorro Canyon (ca. 9 Ma) More likely, one might argue that these faults are typically along the Socorro Canyon fault zone has resulted in about fault zone aseismic for long intervals between movements, and that 500 meters of local relief and about 750 meters of vertical 30 40 0 10 20 most of the seismicity that is occurring in the rift occurs as offset of the 9-million-year-old playa deposits (C, figure Distance, in 1000 ft minor adjustments on a myriad of non-surface rupturing 5). Although these ages and offset amounts are relatively Figure 5. Schenwlic cross section near Socorro, New Mexico, showing evidence/or lat~ ~e11ozaic a,!d Q1101emar~ s!ip rt1te.~ 01 1 tf'.e ~oco1Tu qany~~1 faults. In a sense, the elephant is snoring. From a geo crude estimates, they yield a long-term vertical slip rate of fault zone. Three age datums e.,·ist in the area: playa depo its of 1/te Popotosa Fomic~t1011 (ovfrlam by ca. 9"1111ll1.on~yea1-o/d ih)olae ~J S~co_110 logic perspective, it seems that most major surface-ruptur 0.08 mm/yr (750 min 9 million years). Pee,k) er,!'ly Pliocene basalts (4. J . 4.5 million years old) that }lowed on Rio Gra11de seclune111 ( 1erra L.adrones Fo1:ma1wn), a11d 1J1e uppe11110.i'. p1.ed- ing, normal faults tend to be aseismic (the normal mode), Two additional datums are present across the fault 1110111.'.i·lope swface (Uis Canas; ca. 750,000 years old) related to an a11cie111 ba. in floor of 1he Rio GrMde valley. Uw a11101111 1s of offset are estlmate.1· but can move during M >6.25 earthquakes without signifi zone for temporal comparison of slip rate: 4.1- to 4.5-mil bmed 011 geologic mappi11g and slrai.igrC1phic r latiom; in the C1rea. cant precursory activity (foreshocks). Similarly, the lion year-old basalts and a early Quaternary (ca. 750,000 Wasatch fault zone in Utah (a world-class normal fault years) piedmont surface. These datums have about 200 obvious pattern of sei, mici.ty and thus appear .to be inact!ve (1996) is starting to. reveal relations between the fault pat with a Holocene slip rate of 1-2 mm/year) is currently less meters and 25 meters of vertical offset, respectively (see A (aseismic). T hi relation ugge, t the questwn of wluch terns hown in figu re 6 and lhe location and lateral extent seismic that the surrounding basin-and-range blocks, yet it and B, figure 4). Using the three datums, the slip rate on faults po e more of a eismic hazard in terms of earth of young volcanic center , deep ba in , and the Quater has a proven history of recurrent movement on roughly the Socorro Canyon fault zone appears to have slowed quake ccurrence: a late Holocene (e.g. 1 00_0- to 2 000- nary geology of the basin. 400-year intervals during the Holocene (see Machette and from about 0.18-0.20 mm/yr in the latest Miocene, to year-old) fault with a 40 000-year recunence mterv~ or a The preservation of urficial evidence of thi complex others, 1991, 1992a, 1992b). about 0.05 mm/yr in the Pliocene, and 0.02-0.04 mm/yr in 35,000-year-old fault with a 40 000-year recurrenc~ m_ter strnctural pattern j another complication in eismic-haz the past 750,000 years of the Quaternary. As with the val? The answer may seem obvjous from a geologic (1. e. ard analy is for thi particular area and likely for other Variations in Slip Rate Through Time Wasatch fault zone, there appears to be a 5- to 10-fold deterministic) viewpoint but mo. t probabili ti eismic ba in of the rift. For example large parts of the land change ( decrease in this case) in slip rate and, by infer hazard a. es ments, which are driven by pattern and rate scape in the Rio Grande rift are formed by either high When geologic data are used in probabilistic hazard ence, seismic activity along the Socorro Canyon fault of modem sei. m:icity, ee the you nger faul t as tbe poten level surfaces related to tne eaJly Pleistocene filling of the assessments, one of the most important parameters is the zone. The youngest slip rate is extremely slow (0.02-0.04 tial hazard and minimize (or ignore) the older fault. basfos (prior Lo Rio Grande entrenchment) or are. form~d slip rate (and the associated recurrence interval) of a fault. mm/yr) and the recurrence interval is probably on the Rarely i there adequate fault-timing information to make by late t Pleistocene (LO 000-30,000 year old) ediment 111 In regions such as the Rio Grande rift and Basin and order of 50,000-100,000 years, but the fault is characteris a meaningful deterministic assessment. alluvial-fan complexe or in entrenched tream valleys Range Province (as a whole), reliable slip-rate data are tic of many other major faults within the rift. The most (figure 7). If fault in uch basins have average recurrence difficult to obtain. In many cases, one must use long-term recent surface-rupturing event on the Socorro Canyon Preservation of Paleoseismic Evidence intervals of 50,000-l 00,000 year. , then vii ually all faults slip rates because data for recent deformation (i.e., Holo fault zone appears to have occurred in latest Pleistocene would be recorded on the high-level surfaces whereas cene or late Pleistocene) do not exist. time (Machette and McGimsey, 1983), and thus would be Another problem that mu ·t be considered in regional onJy some 0f the fault would be expected to have dis The main problem with long-term slip rates (those considered to be an active fault in the seismic-hazards a:naly es of fault activity is the pre ervation of the geo turbed urfaces of late Pleistocene or Holocene age. Thus, averaged over several to hundreds of earthquake cycles) is sense. logic record. Detailed analysi of subsurface well-log in regions where fauJts are characterized by long recur that there can be major variations in slip rate through time. gee.logic mapping, and geophysical data from the Albu rence intervals (10,000-100,000 years), one mu t look at a These variations are difficult to document and many times Hazard versus Recurrence Time querque ba in by H~wley (1996) reve~ a complex pat geologic reco.r9 that is at least one recun-ence interval long tern are based on first-order observations such as geomorphol of late Cenozoic deformatJon (figure 6), fru· more (or longer) in order to capture spatial and temporal pat in.ed from reconnais~ance ogy or sedimentation rates. One moderately well docu In regions of high seismicity such as the West Coast, complex then previously determ tern of faulting. Addilional re earch using subsurface mapping of the basin. Although m . t of the fau lts 111 the mented example exists for the Wasatch fault zone. On this many Holocene faults have a record of movement that method (high-Te olulion eismic reflection geophy ical fault, most of the slip-rate data are from faulted deposits subsurface tJ.·jke norlh- oulh, there are a large number of su ggests relatively high rates of slip (>5 mm/year), short . urveys, analysi of water-well data etc.) are needed to of Holocene or latest Pleistocene age (post high stand of recurrence intervals (hundreds to a thousand years), and subordinate faults that strike northeast-southwest. Previ detect potentially active faults in area of y0t~ng. lan~ Lake Bonneville; 15,000 years ago). These slip rates are are often associated with ongoing sei micity. hu ·, these ous mapping diet not reveal surficial evidenc~ .for these scape. One hould note that most of the older build mg. 111 typically 1-2 mm/yr, but some data from the interval faul ts po ·e an obviou, hazard and are recognfaed a such. cro s-ba in fault , perhaps because the preva~lmg para between 12,000 and 15,000 years ago suggest much However as we know, much of the Rio Grande ri ft is digm wa one of Quaternary north-south fa~ I~ng relat~d the bu jness di tricts of Albuquerque and El Paso a:i:e higher slip rates, perhaps related to the catastrophic drain characterized by faults with low lip rates and long recur to ea t-we t extension. However recent surficial ge0Jog1c located close to the Rio Grande on young, low, flood-plain ing and crustal rebound of Lake Bonneville (see Machette rence intervals; they are mo ·t often not associated with mapping and the excellent subsurface work by Hawley surfaces. 92 Utah Geological Survey Western States Seismic Policy Council Proceedings 93
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Figure _6. Lme Ce,wz9ic (Plioce11 A GEOLOGIC PERSPECTIVE information regarding the potential effects of a large earth - l 987b, Preliminary assessment of paleoseismicity at White Sands recent investigations, conclusions, and interpretations, in Gori, P. quake that are possible, but perhaps unlikely to occur. Missile Range, southern New Mexico - Evidence for recency of L. , and Hays, W.W., editors, Assessment of regional earthquake From a geologic viewpoint, it seems obvious that Their intention is to base such plans on a worst-case or faulting, fault segmentation, and repeat intervals for major earth hazards and risk along the Wasatch Front, Utah: U.S. Geological modem seismic-hazard assessments for regions like the near-worst-case earthquake scenario. If response plans quakes in the region: U.S. Geological Survey Open-File Report Survey Professional Paper 1500-A, p. Al-A71. Rio Grande rift must use not only catalogs of modem seis are in place and can operate effectively for such an earth 87-444, 46 p. Mccalpin, J.P., 1982, Quaternary geology and neotectonics of the west micity, but also must integrate data from a comprehen quake, response to smaller-magnitude earthquakes can -1987c, Preliminary assessment of Quaternary faulting near Truth or flank of the northern Sangre de Cristo Mountains, south-central sive inventory of Quaternary faults, especially those easily be accomplished. Probabilistic ground-motion haz Consequences, New Mexico: U.S . Geological Survey Open-File Colorado: Colorado School of Mines Quarterly 77, no. 3, 97 p., 1 structures showing evidence of movement in the past ard maps for the engineering design of buildings of stan Report 87-652, 40 p. pl., scale I :250,000. 100,000 years. By doing so, the geologic data will portray dard construction have customarily used a 1/500 annual -1988, Quaternary tectonic history of the La Jencia fault, central New Menges, C.M., 1990, Late Quaternary fault scarps, mountain-front the true potential for surface-rupturing earthquakes on a probability of exceedance (roughly 10 percent in 50 years) Mexico: U.S. Geological Survey Professional Paper 1440, 82 p., landforms, and Pliocene-Quaternary segmentation on the range In time frame equivalent to the average earthquake cycle as a standard (see figure 4a). some circumstances, a 25 figs ., 23 tables, 2 pls., scale 1:48,000. bounding fault zone, Sangre de Cristo Mountains, New Mexico, lower probability (2 percent in 50 years, figure 4b) might Chapter 7, in Krinitzsky, E.L., and Slemmons, D.B., editors, Neo (10,000- to 100,000- year recurrence intervals). Once Machette, M.N., and Hawley, J.W., 1996, Neotectonics of the Rio accomplished, the probability of occurrence of large be used for facilities of special interest. These ground tectonics in earthquake evaluation: Geological Society of America motion hazard maps generally will not reflect the influ Grande rift, New Mexico: Geological Society of America Reviews in Engineering Geology, v. 8. p. 131-156. earthquakes on individual structures may prove to be Abstracts with Program, v. 28, no. 7, p. A-271. extremely low, but the location of these potential, strong ence of the large rare earthquakes in the rift that have Menges, C.M., and Walker, J. , 1990, Geomorphic analyses of scarps ground-motion-generating structures will be known in ~ecurrence times of thousands of years since the probabil Machette, M.N., and McGimsey, R.G., 1983, Quaternary and Pliocene along the eastern border of the Valle Vidal, north-central New Mex faults in the Socorro and western part of the Fort Sumner 1 x 2 relation to urban areas and critical facilities. In addition, ity of that earthquake occurring in any 50-year period is ico, in Bauer, P.W., Lucas, S.G., Mawer, C.W., and McIntosh, quadrangles, central New Mexico: U.S. Geological Survey Mis the hazard posed by numerous low-slip-rate faults within low. On the other hand, design of important or critical W.C., editors, Tectonic development of the southern Sangre de facilities such as nuclear and defense facilities, reservoirs cellaneous Field Studies Map MF-1465-A, 12 p., 1 pl., scale a given radius (e.g., 50-100 kilometers) of a town, city, or Cristo Mountains: New Mexico Geological Society Guidebook, hospitals, and any structures that should remain in servic~ 1:250,000. critical facility results in a composite recurrence interval 41st Field Conference, p. 431-438. following a large earthquake are generally engineered to Machette, M.N., and Personius, S.F., 1984, Map showing Quaternary Nakata, J.K., Wentworth, C.M., and Machette, M.N., 1982, Quaternary (CRI) that can be just a fraction (i.e., 1110th) of each indi stricter standards and may make use of ground-motion vidual fault. A myopic approach of not appreciating the and Pliocene faults in the east part of the Aztec 1 x 2 quadrangle fault map of the Basin and Range and Rio Grande rift provinces, estimates at very low probability levels (i.e., 1/10,000 and west part of the Raton 1 x 2 quadrangle, northern New Mex western United States: U.S. Geological Survey Open-File Report potential for earthquake hazards posed by individual annual or 5 percent in 500 years). In this case, the occur structures (i.e., recurrence intervals of 10,000 years or ico: U.S. Geological Survey Miscellaneous Field Studies Map 82-579, 2 pis., scale 1:2,500,000. rence of rare, but large earthquakes on long-recurrence MF-1465-B, 14 p., 1 pl., scale 1:250,000. more) can lead to a complacent attitude that strengthens a faults is of considerable importance. Thus, as paleoseis Northrup, S.A., 1976, New Mexico's earthquake history, 1849-1975: perception of low seismic potential. Without proper cau Machette, M.N., Personius, S.F., Menges, C.M., and Pearthree, P.A., New Mexico Geological Society Special Publication 6, p. 77-87. mological information for the western United States 1986, Map showing Quaternary and Pliocene faults in the Silver tion, this attitude can be manifested in inappropriate con Pearthree, P.A., and Calvo, S.S., 1987, The Santa Rita fault zone-Evi becomes more widely known to user communities and to City 1 x 2 quadrangle and the Douglas 1 x 2 quadrangle, south struction styles, building codes, land-use policies, and the the public, there likely will be concerns and controversies dence for large-magnitude earthquakes with very long recurrence siting (or relocation) of important or critical facilities. eastern Arizona and southwestern New Mexico: U.S. Geological intervals, Basin and Range Province of southeastern Arizona: Bul over what is considered acceptable level of risk. Clearly, Survey Miscellaneous Field Studies Map MF-1465-C, 20 p., 1 pl. , Various mitigation strategies require different por re olation of such seismic- afety issues will require the letin of the Seismological Society of America, v. 77, p. 90-116. scale 1: 250,000. trayals of earthquake hazard. Emergency planning and cooperation and participation of a wide range of users of Sanford, A.R., Jaksha, L.H., and Cash, DJ., 1991, Seismicity of the disaster response plans, for example, generally require earthquake-hazard information and the research community. Machette, M.N., Personius, S.F. , Kelson, K.I., and Seager, W.R., 1996, Rio Grande rift in New Mexico" (Chapter 12), in Slemmons, D.B, New digital map and computer database of Quaternary faults and Engdahl, E.R., Zoback, M.D., and Blackwell, D.D., editors., Neo folds in New Mexico: Geological Society of America Abstracts tectonics of North America: Geological Society of America REFERENCES with Program, v. 28, no. 7, p. A-377. Decade of North American Geology Map Volume 1, p. 185-228. Machette, M.N., Personius, S.F., Nelson, A.R., Schwartz, D.P., and Schwartz, D.P., and Coppersmith, K.J., 1984, Fault behavior and char Bull, W.A., and Pearthree, P.A., 1988, Frequency and size of Quaternary 60 p., 3 appendices, 1 pl., scale 1:24,000. Lund, W.R., 1991, The Wasatch fault zone, Utah-Segmentation acteristic earthquakes-Examples from the Wasatch and San and history of Holocene earthquakes, in Hancock, P.L, Yeats, R.S., surface ruptures of the Pitaycachi fault, northeastern Sonora, Mexico: Frankel, A., Mueller, C., Barnhard, T., Perkins, D., Leyendecker, E.V., Andreas fault zones: Journal of Geophysical Research, v. 89, no. Bulletin of the Seismological Society of America, v. 78, p. 956-978. and Sanderson, D.J., editors, Characteristics of active faults (Spe B7, p. 5681-5698. Dickman, N., Hanson, S., and Hopper, M., 1996, National Seismic cial Issue): Journal of Structural Geology, v. 13, p. 137-149. Collins, E.W., and Raney, J.A., 1991, Tertiary and Quaternary structure Hazards Maps-Documentation June 1996: U.S. Geological Survey Wells, D.L., and Coppersmith, K.J., 1994, New empirical relationships and paleotectonics of the Hueco basin, trans-Pecos Texas and Chi Open-File Report 96-532, 110 p. (see also http://gldage.cr.usgs.gov/eq/). Machette, M.N., Personius, S.F., Nelson, A.R., 1992a, The Wasatch among magnitude, rupture length, rupture width, rupture area, and fault zone, U.S.A.: Annales Tectonicae, v. 6, Supplement, p. 5-39. huahua, Mexico: [Texas] Bureau of Economic Geology Geological Hawley, J.W., 1996, Hydrogeologic framework of potential recharge surface displacement: Bulletin of the Seismological Society of Circular 91-2, 44 p. areas in the Albuquerque basin, central Valencia County (Chapter 1), -1992b, Paleoseismology of the Wasatch fault zone-A summary of America, v. 84, no. 4, p. 974-1002. Collins, E.W., Raney, J.A. , Machette, M.N. , Haller, K.M., and Dart, R.L., in Whitworth, T.M., and Hawley, J.W, editors, Hydrogeology of 1996, Map and data for Quaternary faults in west Texas and adjacent potential recharge and hydrogeochemical modeling of proposed arti parts of Mexico: U.S. Geological Survey Open-File Report 96-002, ficial-recharge methods in basin- and valley-fill aquifer systems, 74 p., 1 plate, scale 1:500,000. Albuquerque Basin, New Mexico: New Mexico Bureau of Mines and Mineral Resources Open-File Report 402-D, various pagination. Doser, D.l., 1987, The 16 August 1931 Valentine, Texas, earthquake Evidence for normal faulting in west Texas: Bulletin of the Seismo Kelson, K.I, Hemphill-Haley, M.A, Wong, LG., Gardner, J.N., and logical Society of America, v. 77, p. 2005-2017. Reneau, S.L., 1993, Paleoseismologic studies of the Parajito fault system, western margin of the Rio Grande rift near Los Alamos: Fenneman, N.M., 1931, Physiography of western United States: New Geological Society of America Abstracts with Program, v. 25, no. 5, York, McGraw-Hill Book Co., 534 p. p. 61-62. Gile, L.H., 1987, Late Holocene displacement along the Organ Moun Kirkham, R.M., and Rogers, W.P. , 1981 , Earthquake potential in Col tains fault: New Mexico Bureau of Mines and Mineral Resources orado - A preliminary evaluation: Colorado Geological Survey Bul Bulletin 116, 43 p. letin 43, 171 p., 3 pis., scale 1: 500,000. Foley, L.L., Laforge, R.C., and Piety, L.A., 1988, Seismotectonic study Machette, M.N., 1987a, Neotectonics of the Rio Grande rift, New Mex for Elephant Butte and Caballo Dams, Rio Grande project, New ico: Geological Society of America Abstracts with Program, v. 19, Mexico: U.S. Bureau of Reclamation Seismotectonic Report 88-9, no. 7, p. 754.