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MONTEREY PENINSULA WATER MANAGEMENT DISTRICT

187 Eldorado • Suite E • P.O. Box 85 • Monterey, CA 93940 • (408) 6494866

'l'ipril 13, 1984

Earl Hart California Division of Mines and Geology Room 1004, Ferry Building San Francisco, California 94111

Dear Mr. Hart:

Enclosed is a memorandum by Dr. Robert Curry on his observations of Quaternary and recent fault activity in the Monterey Bay Region. The Monterey Peninsula Water Management District strongly feels that these observations warrant further investigation. Indeed, the tectonics of the entire Santa Lucia Mountains are poorly understood and are worthy of re-evaluation. Keeping our "wish list" within practical bounds, however, we would like to encourage the Division of Mines and Geology to give #1 priority to the Tularcit6s Fault, as the geomorphic evidence of active faulting as sited by Dr. Curry lies within one mile of the proposed site for a new dam on the Carmel River.

If we can be of any assistance, please give us a call.

Sincerely,

?~~~"""---Frances Krebs Hydrologist

FK/df

Enclosure:·, Curry's Memorandum

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RECEIVED I\ Ii; I ·.·~ '! ·:: f.:' ... '.·

MEMORANDUM M.P.\V.M.D.

ObEervations oo Quaternary and Recent fault activity, Central Coastal California

Robert R. Curry, College Eight, Univ. California Santa Cruz, 950~~;;;,

Karen McNally - UCSC, Earth Science Monterey Peninsula Water Management i)jgtrlct ,,,.,-­Calif. Div. Mines and Geology Gary Greene, Ken laJale, Malcolm Clark - USGS Joe Clark, University of Pennsylvania

& other interested pu1:l.es

Field ObEervati.on 1982-1984

Field work that I have been conducting, both alone and with cJa :s of students, has ronv:lnced me that recent fault activity is &gnifi.cantly underestimated throughout much of central coastal California. In the interest of stt.mulatl,ng further fl.eld work and :Iii.ult evaluation, I wish 'ID informally Etla.re eome fl.eld Olea vati.ons and tentative :incomplete hypotheses. Inferred rates presented here are, of neceesl.ty, irnprec:lse, and are presented aily for romparlson with rublisbed estimates of others.

1. REGIONAL PICTURE '. 1

In the Monterey Bay and Big Sur areas, }:llSt work has sugge;ted that the Sa.linian Block :Is bounded by two major light-lateral fuult systems. There :Is general agreement that the San Andreas :is the eastern bound:ing fault, but leS3 agreement on the western edge. Since the kx:ati.ons of the grouµ; of fault zones that have been called tn1> &!r-Nacimjento and tile San Gregar.lo-HCJe€li are not clee.r.ly defined and since the offshore fault Eegments and oo-Slore 93gments are of necei:sl.ty difficult 'ID link J?ecetEe of our :inability 'ID map accurately in the nearmore zone, much room for interpretation remains.

'lbe hypothesis that seems best 'ID :fit the :fl.eld data that I have observed t:hrqug~ut coast.al Califc;iruia s.iggest that a system of fault zones bounds the west side .of the Sal:lnian Block,· and that plilnary amoog the.<E is a rontl.nuots fault zone abollt 1 km wide that seem~ best identified by ttie name ''San Gregorio. - Sur - HOEgd.". Very tentative and incomplete evidence that I shall cite herein suggests that this fault zone has accommodated light-lateral offset of oo the order of ooe-tblrd 'ID· one-half that of the San Andreas Fault wer the }:llSt i:everal hundred tbol.Bllld years. In mcst: cases where this fault "romes amore", as for exampl.e between Pescadero and Ano Nuevo and between the Little Sur ·River and tts exit paint south of the 'IDwn of Big Sur, the geom<r}irlc evidence of :lil.ulting is every 1:xi.t as dramatic and clear'J.y related to recent offset as is seen along the San Andreas.

1tcr exampl.e: Greene, H.G., W.H.K. Lee, D.S. McCulloch, and E.E. Brabb, 1973, Faults and. ,Earthquakes in the Monterey Bay Region, California. U.S. Geal. Survey-HOD Bas1.c Data Contr.lb. 58; and Map MF-618. -Far historic thinking, see ala:>: Jennings, C; W. et al, 1975, Fault Map of Califor­nia, Calif. Div. Mines and Geology, Geol. Data S~ Map No. 1.

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• What :Is puti.cularly of interest is the ample geomarphic evidence of the "non­

r:lgi.dness" of the Salinian Block. A great number of fault zones croe; diagooally in a northwesterly direction between the San Andreas and the San Gregarl.o-Sur-HO€gri. Amcng these, the Butano, Zayante, King Ci.ty-Rinconada, Tul1lrci:tos, Cypress Paint, and Palo Colorado-Nacimiento are amoog the mcst prominent. Many of these mow cl.ear signs of acti.v:lty within the last 10,000 years. Most are mapped in S.tes of ruch great tectonic and erosional activity that :It is the exception, rather than the rule, that permits relic land Sll'faces, &li.ls, or deposl.tional units acroe; the faults to have rurvi.ved for mare than a few lrundred to at mcst a few thoU3a.Jld yeara. Thus, evaluation of fil.ult acti.v:lty :is ~riously hindered in the rugged upland areas of the Santa Lucia and Santa Cruz mountains. For this reaoon, I believe :It may imprudent to conclude that the abrence of direct simple evidence of Quaternary offset in these Salinian Block faults in any way ruggests inactivity. The few places where young deposl.tional features extend acroes fil.ul1s, ruch as the alluv:la.l. :funs of the King City-Rinconada fault zone and the landslides and alluvial fill in the Tularc.itos fault zone, as well as the maj:Jl' deformation of late Quaternary river and marine ter­races as along the Carmel Valley, in the Bixby - Pt Sur area and throughout the Big Sur Coast routh of the Big Sur River, all give evidence of very considerable tectonic activity throughout the late Quaternary and puti.cularly in the last 100,000 yeara.

The question of the character of the tectonic activity west of the San Andreas must await further work by EEismciogists and geologists. Clearly, the eaismicity of the Salinian Block during historic ti.mes is apparently less than that of the San Andreas fuult zone. It is pcEEible that tectonic activity in the Salinian Block is largely aseismic, perhaµ; beca~ that block is west of the San Andreas fault zone where moot of the plate-margin compressional EEism·ogenic tectonic activity takes place now. Alternatively, it is pcEEible that the two maj'.:r plate-margin fil.ult zones of the San Andreas and the San Gregorio-Sur-Hosgri alternate as sites of primary seismogenic offoot. On ti.me intervals of perhaµ; hundreds to thoU3a.Jlds of years, mej::r portions of oonti.nuo\E plate margin motion may be accommodated on first one and then tile other fil.ult zone. Historically, between the Santa Ynez Range and San Fran­cisco B::ty, EEismogeni.c offoot has apparently been concentrated on the San Andreas. However, ~ µ.irely on geomarphic and pedol.ogic evidences, compi.rable orders of magni.tud.; of rates of offset appear to have characterized the western margin of the Salinian Block within a period of at maximum a few thousands of years. Histor.l.c seismic activity of tile San Gregarlo-Sur-Hosgri has been reviewed by Coppersmith and Grlggi;2 and by W.J. Gawthrop, pp. 45-56 in the same volume (op cit, 1978). ·

In either ~. it is apparent from the geometry of fil.ult planes and character of deformation wi1hin tile Salinian Block that this block :Is being generally uplifted and Sieared as a component of the primary plate boundary. Where evidence for Recent offoot :is clear, as for example on the Tularcitos and King City - Rinconada fil.ult syst.ems, both strike-slip (right-lateral) and reverred fil.ulti.ng is characterlstl.c. Locally; as far example in the upper Tularcitos Valley near Rana Creek, small ten­sional gravtty block faults occur in bloclls actiacent to predominantly strike-slip fault planes.

2co~, J.K. and G.B. Griggs, 1978, MorJ:iiology, recent activtty, and i:elsmi­ci.ty of ·the San Gregorio fault zone: pp. 33-43 in San Gregorio-HOE{¢ Fault zone, Calif., E.A. Silver and W.R. Normark (eds), Calif. Dim Mines and Geal.. Sp. Rept. 137.

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2. SITE-8PECIFIC OBSERVATIONS

Three sltes are being looked at in s:>me detail. There are Tularcttos Creek Val­ley near Tularc.ttos Ranch (Tularcitos Fault), lower Butano Creek Valley near Pes­cadero (San Gregarlo-Sur-Hoegri Fault), and ttie Pt. Sur region (Pa.lo Colorado and San Gregario-Sur-Hoegri faults). Other ol:JEErvations at the mouth of the Ca.I'mel River (Cypress Point-San FranclS}uito Fault) and along ttie eastern rose of the Sierra de Salinas (King Cl.ty-Rinconada Fault) are pertinent but will not he adcires3ed here.

~:!: Tularcitos Fault system

Two striking geomari:mc features occur in the Tularcitos Valley near Tularcitos Ranch (ooe Plate 1). There are ttie large cloeed depression immediately alongEide Collllty Rd G16 about 1 km SE of the j.mction of Rana Creek and Tularcitos Creek, and the very large ( 6 km x 1.3 km ) landslide feature in the mme area oo the north slde of Tularc.itos Ridge. Other equally striking but mare obscure related features include mg-ponds, disrupted 1ributary drainage, and abrupt changes in longitudinal profile in an alluvial valley.

The closed depression Siown oo Plate 1 is almost one-half kilometer in diameter and about 4 m deep. What :Is most lll!USUal is that it lies immediately actiacent to the course of Tularcitos Creek, and the bed of that stream is about 2 m higher than the bottom of ttie depressi.on. The stream flows in an alluvial fill into which it 1s 1.ocally incised up to 3 to 4 m, but at the }Xlint of th1s cloeed depression, the inci­si.on is less than 1 m. Effectively, only the inved roadway on the narrow alluvial terrace remmnt between the depression and the water courae prevents diversion of the stream. The origin of the depression is unclear. However, the roils on the floor of the l:nsin are :fine-grained well-developed colluvial depositional roils derived from the actiacent hilliides, and are not of fluvial origin. Clearly, the topograi::tJic exµ ssion of the prerent depression must post-date the last overtopping of the Slal­low wid ·marginally adequate Tularcitos Creek channel. The fill ooquence into which the creek preoontly is incised is moot probably of historic or very late prehistoric orlgln. Oak 1rees rn the terrace rurface, and within the incised channel appear to be oo the order of 100 years old. Historic fl.oocling of the lower Carmel River canyon, to which the Tularci.tos is tributary, was last egnificantly overbank with modification of the uppermost valley-floor alluvium in 1911, and perhaps again in 1914. The uppermost alluvial airface that comprises the low barrier between the preoont '!'.±..-c!tos Creek and the actiacent depression is free of egnlficant &Ill de1iel.opment and without any dlscernible weathering profile. Its character is entirely compi.ra.ble to that of the "1911 surface" further downvalley, although in the arid Tularcitos Valley ate, simple abrence of rurface alteration could persist on surfaces of ooveral lrundred years age. However, using the kinds of compmi.ble pedogenic development med to establish the chronoeequence in the faulted alluvium of the Ventura--Ojai area as outlined by Keller, Johns:>n and otheref, it is very improb­able that the upper Tularcitos Valley fill ooquence has remained free of surface water flows for mare than 300 years. Any ruch flow would be expected, under today's

3 . Keller, E.A., D.L. Johns:>n, M.N. Clark, and T.K. Rockwell, 1980, Tectonic geomor­

phology and earthquake hazard, North Flank, Central Ventura Basin, Calif.; Final Tech. Rept. for U.S. Geol. Survey, 167 p. -PublEhed as the Guidebook to the 1981 Pacific Cell Friends of the Pleistocene Meeting, July, 159 p.; and in rummary as pp. 42-£0 in Asscx:. Am. Geographers, Field Trip Guide, 1981; R. F. Logan (ed).

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conditions, to :flll. the cl.ored depression and wash freely available alluvial materi­als :Into tt. If the 1911 flood did not mcxlify ttrts ate, it is probable that the road berm :Itself of cnly about 0.1 m height may have protected it. Thus, the depres­s!.on as a geomcri:tJic feature below the aqja.cent Iiver bed, can be only a few hundred years old at the maximum. Longer term SJbS.dence may have created the small "emtay­ment" in the Tertiary bedrock and may have resulted in infilling by stream'-bame alluvllim piiDr 1D the cU?Tent continued anomalous topograJXiic development. So topo­graJXiically anomalom is tllis ate that even the USGS 7.5' Rana Creek Quad. m:ISES this and other cl.<Jald depressions, albeit plausible that at a 40-foot contour-interval, chance would preclude their notation.

The landslide :Is not mapped as SJch on the reconruils3ance-level regional map done by Tom Dibblee :In 1972 (as enown as the bas:l-map :In Plate 1). This appears oo inspectl.cin 1x> be a maasl.ve deep-reated rotational-translational block slide primarily in bas:lment quartz dlorite rocks. Bared upon the undeformed nature of post-slide fault traces through the slide ma.ss, ttrts appears 1x> have been a SJdden &ogle-event slide, pcl93!.bly asrocia ted with rec tonic ace el era ti.on on the Tularcltos or nearby faults. In cnly a few small fPQts is the toe of the slide mass deformed by further mllffi :lll:Uure. Cl<Jald depressions within the blocky slide mass are not yet fully filled with alluvium. The very prominent heads:arp :is not much erosionally modified, despite a height of 200 m. Alluvial :flll. at the bas:l of the head-ocarp, mapped by Dibblee as "Older Alluvium", :Is relatively little dissected and appears to be actively forming by coalescing debris funs off the headscarp 1x>day. A prominent trace of the Tulareltos fl.ult crossing through mcst of the slide ma.ss near its toe has an effective valley-side-upward Eellffi of motion, creating mere local unfilled cloeed depressions :In the fl.ult ma.ss.

The age of the landslide :Is uncertain. It has evidently deflected the mrun course of Tularctt.os Creek northward around its toe and may be respcmsi.bl.e for effec­tively daming the valley itself, creating ttie alluvial fill basin.· All of the maj:r tributary streams entering tile valley from the Sierra de Salinas to the northeast have rather abrupt changes in longitudinal profiles with a distinct flattening of gradient where tlleir lower ool.II'EeS are drowned in alluvllim. The quantities of allu­vial :fill that could have been trapped by the slide would argue for rome antiquity of thousands of yearn er more, although the lack of filling of cl.oeed depres:;l.ons oo the slide SJr:fu.ce and lack of si.gnl.fi.cant weathering profiles argue for lee5er antiquity. At any rate, it :Is certainly a late Quaternary feature, and ma;t probably late­Pleist:o<'.enP. to Holocene. ·

Other evidences of recent tectonic activity include sag ponds or sag-pond like features :In both the tributary valley alluvial :fills and in Tertiary Monterey Eilale on the northeast &de of the valley, immediately east of the landslide depoS.t. A ser.les of springs and EeeJB indicate relative upward movement of the main valley at the mouth of Sycamcre Gulch and especially the canyon immediately west of it. The

' fault line thus defined P' s .)Et ffiuthwest of the Tularcltos Ranch buildings where a large pond may, in put, be a natural oonsequence of Recent fl.ult activity, with the valley-si.de upward motion effectively darning the small tributary valley. Although th:ls "mtural" dam appears 1x> be mruntained and augmented by human interven­tion, the geometry of the situation is very compll'll.ble 1x> that of the beforementloned clcEed depression further upvalley. Where this pres.imed fl.ult trace within the val-ley all\1Vh1m prqjects a:>utheastward into Tertiary bedrock on the northeast sl.de of the valley east of Rana Creek, a eeiies of cloeed depressions, eag ponds, and clear l:lnear fuult B!a.rJB of wh9diary normal :faults trending north and northeast are encountered. I have not traced this very fur, but roil p.ts in the depres:;l.ons along

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thes;i faults reveal active oolluvia.l infilling and ruggESt continuing deposl.tional procee:;es far at mcst a few hundred years. Another potentially important observation is the existence of two active nick-paints in the main Tularcitos Creek valley. If faulting :5 active and jg serving to raise the alluvial floor of the valley relative to its lower non-alluvial OOUIB3, then episoclk faulting could be expected to give rise to locali2'.ed downcuttlng. A constructed concrete drop-structure 1n the stream 1n the lower part of the alluvial valley, near BM 870, sei;arates areas of markedly ctifferent local incision, and has temporar:ily reduced incision above :It (east) by 1-2 m. The main stream in the fault-controlled alluvial valley sequence above the pri-mary 1rlbutary Rana Creek, has undergone a recent acceleration of downcuttlng associ­ated with a marked increase in sediment 1ransport accompuiying 'the 1982 and 1983 run­off seasons. The main stream has downcut about 0.3 to 0.6 m and headcut about 0.4 km within its already incised channel above its junction w:lth Rana Creek. Elevation of the bed of this uppermcst portion of Tularcitos Creek is controlled locally by a cul-vert in the main stem above the Rana Creek 1ributary junction at about the posl.tion of the 960-ft contour. Accelerated downcuttlng and headcutting are evident by the expoeure of oo.k roots oompletely bridging the channel from wall to wall that had become estalillshed far a period of at least 30 yeaxs below the bed of this ei;ilemeral channel. Thes3 changes in longitudinal profile of the stream in its the main fault­aligned canyon ruggest a non-equilibrium state far that longitudinal profile. This could be aSRY"Jated w:lth changed land \Ee, fire ruppree>!.on, climatic change and other factors as well as w:lth tectonic movement.

In conclt&on, the Tularcitos Fault system in the upper Tularci.tos Valley appears to me to demrnstrate definite tectonic activity in Holocene time. Folding in the Tertiary Santa Margarita and underlying units as mapped by Dibblee ruggests compr• mfonal forces in a northeastern clirection. Evidences of faulting that I have observed eimilarly rupports NE-SW compree:;ion and local NW-SE exteneion and Iight­lateral motion. Active and inactive landslides of several types in the Tertiary rocks ;;.s· well as one inactive large 1ranslational-rotational slide in granitic l::ese­ment attest to slope instability that may in part be triggered by !Eismic accelera­tion.

2.2. pt. ::;..,· Area faulting

Unfilled s:i.g ponds and a fault-line trough on a narrow precariol.5 bench hanging 100 m er mere above the ocean where the Big Sur fault zone last :is seen on land j.lSt south of thP 1nwn of Big Sur as seen from Highway 1, has always ruggested cbnsider­ahl.e a.ctl.vity. It :Is bard to conceive of any way that ruch a bench could remain as a topographic feature uncovered by colluv:lum far mere than a few thot5and years at mast. The approximate 6 km tight-lateral offset of the Big Sur River valley, where it CIUUXJS the fault identifies the eenee of motion, and the absence of alder ter­races and prea!nce of those farmed after the Marble Cone fire along the Iiver where it fallows the fault zone ruggests late Pleistocene (lem than 200,000 yr) maximum age far that 6 km reach of the Iiver. Had that offset valley e:idsted when the lowest subaerial marine terrace was being eroded, rome remnant of an alluvial teITace graded to the marine terrace tnre level mould exist (as we eee far example :in the lower Carmel River Valley).

The massl.ve actl.ve landslides along the im:Uacted fault zone between the mouth of the Little Sur and the Big Sur livers makes interpretation for that 10 km zone difficult. EroS.on in active gullies along the hillS.de east of Highway 1 immedi-ately east of Pt. Sur expooos a very complex sequence of debris slide and colluvial flil sequences capping what may be the back edge of a marine terrace remnant about

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60-80 m above prerent sea level. Classes working in these gullies, puti.cularly after the 1982 winter storms, have Jieced together a very Eitetchy hypotheS.s of alternating cutting and :f!.lling, with intervening periods of pedogeneSs. These same ero&onal bldland gullies that exIJ(S3 sections along much of the blck-terrace areas of the Big Sur coast today, were :In the i;est filled and reexcavated multiple timES Despite very small (< 1 sq.km.) to nonexistent present wurce areas far these gul­lies, the progree;;lve changes :In lli.hologies and character.istlcs of the fill requences wggest considerable changes :In their headward areas through time along the fault zone. Since this whole sequence rests upon what appears to be the ''first" 90,000 to 125,000 yr marine terrace, these must pest-date that last miQT high sea­level stand time.

A Magnitude 5.0 event Jan. 22 of this year at 9:40 PM has been tentatively located by the USGS at 36 degrees 22.4' N and 121 degrees 51.8' W, which would be 3-4 km NE of the fault, close to the Palo Colorado fault at Bixby Mountain. An a.ft­ermock at 10:59 PM local time that day of M 3.8 was located by the USGS farther west at 36 deg. 20.1' N and 121 deg. 55.6' W, which puts tt jiEt a few hundred meters offshore directly oo the prqjection of the Sur Fault j.Jst north of the mouth of the Little Sur River. Unfortunately, the acc1m1.cy of location of epi.centers :In this region :is suboptimal, requiring triangulation from paints in the Salinas Valley and Monterey through very inhom~eneo\E ba.9'!ment. A 5 km locational error :Is probably not unreas:mable. There :is a genuine need far a good offshore recoverable sea-bottom and local a:i-Ehore recording reism:lc network to develop a better understanding of the Sa.lin:lan ffiock and the San Gregorio-Sur-Ha;gti system.

Careful :field review of the coast along Highway 1 in the vicinity of the Palo Colorado fault withln the 1st mcnth following the event failed to reveal any evidence of SJrfil.ce rupture rn that fault er thooo immediately acijacent to tt. January and February were very· dry, with cruy a few mm of local :ralllfall •. Exporure of the faults in the coastal bluffs and rocky headlands :is very good. Well maintained and very lightly \Eed roadbeds a:i Highway 1, reµi.ved :In Eeveral areas in the late fall of 1983, with wbEequent very light travel beca\Ee of clOSJre of the highway further routh, created an ideal tatala rasa far analysis of any surface disturbance at the coast. The carefully-engineered rlgid-arch high-span concrete tridges immediately actlacent to :fault traces, with surveyed bench marks, provide ideal geodetic control and .an ideal 'artifact' upon which to measure and evaluate offset. No evidence of surfaoo d:ist:urbance was found far the M 5.0 event on the Palo Colorado fault. Unlike earlier :Interpreters, I am not convinced that the Palo Colorado fault is µI.rt cif and 1n line with the main offshore expre$1.ons of the San Gregorio-Sur-ff~ system.

At Rocky Creek, a m!\}'.lr fault trace is reen about 100 m E of the bridge. The trace :Is evident as an unvegetated gaEh nearly vertically cutting the left bank of Rocky Creek. Another rxffihle trace oo the other Eide of the !ridge 1s about 200 m offshore, based- upon apµi.rent offset of the headland ,and a fl.uvial fill requence now offset from its apµi.rent Rocky Creek source eeen in the terrace remnant oo the next headland N of the !:ridge. That :fault trace can be Eeen offshore during ti.mes of calm surf as a line of kelp. The Rocky Creek Bridge, built in 1932 as Br 44-36, has a USGS vertical control BM in the NW abutment W 1317, installed :In 1ITT7. This &te was last reviewed 2-17-84.

At Bixby Creek bridge (Br 44-19 of 1932) new µi.ving near the abutments was und1st:urbed. Bixby Creek 1t.self appears offset about 2 km along what may be a trace of the main San Gregorio-Sur-Ha;gti system locaJly called the Serra Hill Fault. It :Is expoE;Ed as a zone of ffiear j.Jst east of Higllway 1 along the old Coast Roa.cl abut 0.5 km east of the N abutment of the bridge •. A USGS B~ (G261) established :In 1935 1s

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in the northwest corner of that abutment. The well expoead mear zone can be reen in the old Coast Rd. cut and has much evident sllckensides oo both Sdes Of the creek canyon. However, no evidence of rurfuce disturbance of any kind could be found. Tlrls ate was last :lnspect.ed 2-17-84.

At little Sur ru.ver l:ridge (Br 44-20), built in 1953, and located at a j.mction of reveral mapped :fault traces, rurfuce disturbance was clearly evident. The bridge is a rigid flat structure curved along a radius that is concave reaward, with tts axis trending roughly north-routh. The tiver flows approximately westward and the highway turns :far :Inland to croes :It at a nrurow area about 1 km inland from the gen­eral coastline jJst N of Pt. Sur. The l:ridge is built as a slab, wpported by vl'rti-cal members in the center of the tiver channel, but resting oo a steel Slip-plate facing the top of the abutments. The abutments are maESl.ve deep reinforced JXJured concrete S:ructures, recessed where the !:ridge slab lies upon them. Thus the bridge span is not connect.ed to tts abutments.

At the l:ridge, if ooe 11S>1.1me:> that the central i:pan remained S:.a.tiona.ry, the north abutment moved relatively ooutheast and the oouth abutment moved relatively northwest. Independent of the central ::pan and relative ooly to the abutments, the south abutment moved with a western component, relative to the north abutment, and initially moved with a north component Slffi.clent to buckle the roadway at both abut­ments and Slatter the foundation of the north abutment by dziving the epan into it. One would need to get the detailed engineering construction dra wll!gs from Caltrans to reconstruct the full Iicture. There are many control !ins and 1ll.rgets oo the bridge structure that will perlltit careful examination of offset. Ba.Eed sl.mply upon field measurements made 2-17-84, 26 days after the event the followll!g incomplete interpre­tation can be made.

Plate 2 is a sketch of tile lrtdge at the little Sur tiver. Net rlght lateral offset of at least 16 mm 3s demcnstrated by the 'misilignments' of the abutments and central epan. Jn met, the abutments have alS'.> apparently pulled aµi.rt teru:ionally, and rotated oomewhat. Net extension between the east side of tile two abutments, or compree;;lon of the central i:pan of the bridge, for a total of 95 mm can be demoo­strat.ed by by meas.uing the old and new pos:tions of the edge of tile central i:pan on the slip plates of the abutmen1B. Apparently, initial first-motion compi •onal farces of the oouth abutment moving northwestward "threw" the central epan into the north abutment, perhaps cloang mid-epan exparu:ion j:lints. At any rate, the roadway south of the s:>uth abutment was was upbuckled compi BS'onally at the oouth abutment, and the epan :!!:self struck the north abutmait with wch farce that whole mllESi.ve abutment fractured diagonally to come 1D rest with tile fracture open about 4-5 mm.

The creek bed carried an active stream ~g coane si.nd. Any Sirface rupture healed before I saw tt. The ptincipal mult traces at the bridge sl.te appear to be directly in tile creek bed, but tile main Sur :tault zone can be Ellen clearly trend:lng SE from the tridge sl.te. Cattle grazing in s:>ft cl.ay-Iich BJll rendered tracing rupture difficult in tile immedlate vicinity of the l:ridge. However, the old Coast Road Cl'C s all tile mult traces 3-5 km SE. Here further field work is war­ranted. A preliminary traven:e from PJ.co Blanco mountain to Highway 1 along public rights-of-way revealed mveral non-definitive evidences of rurfuce dlsttn'ba.nce, includmg freshly dislodged bloclls of S'.>d and small liquifaction flows. ·

2.3. Butano Creek disturbances

Much further north along the eme fuult system, the San Gregarlo-Sur-HOEgd. oomes a&lare near Ano Nuevo. Gazos, little Butano, Butano, and ~ero creeks are

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all imracted by movement of the fuult. Pes::adero Ma.rm has begun 1I> accelerate its rate of :Infilling by redimmt, borne primarily by Butano Creek. This acceleration begins primarily :In late historic times wtth a rate of sediment transport :lnc:reaSng fCll" each SJilEequent etorm flow through 1983. This :Is ni.cely documented by analysis of equential maps and aerial p:iotos of the ma.rm area showing prograd:lng ~g mnd del'tas. In a march fCll" the source of the rediment and the ca\J9eS of its increased rates of transport or decreare :In rate of flushing through the lagoon, I began 1I> wCll"k on the anomalotB course of Butano Creek.

Many plalBl.ble reconstructions are poeEi.ble, but all demand tight-lateral offset of on the crder of 6 km minimum :In the last 100,000 to 300,000 years. This :Is the mme crder of offset as pootulated far this fuult in the Big Sur area, although here at Pescadero there :Is a much stronger poeEl.hllity of working out multiple li.mlling dates for determination of offset rates. Others working rere have made obearvatlons of tight-lateral :fault offset and come 1I> similar conclusions I.Bing different lines . 4 of evidence.

Tbe course of the ~nt Butano Creek northwestward along the fuult zone 1I> its present j.mctlon wttb Pes::adero Creek in the lagoon :Is quite anomalous. large water-gap; ex:lst :In the :fault s.:arp-bounded upland west of the fuult line. The moot prominent of these :Is AITOyo Frtjales (Bean Hollow!). 'Ib:ls appears to be the west­ward offset extenS.on of either Little Butano or, m<re probably, Gazos Creek. Where they meet the fuult, all the watercmnres south of Pes::adero are everely d:lsrupted with local impounded all!Mal fi.ll sequences that are now being incised 1I> release sediment 1I> the lagoon. Evm the moot minor gullies are offset, consistent with recent fuult motion. The course of Little Butano :Is remarkable in that it has incised everal meters :Into tts fault-line alluv:lal fi.ll along the extreme western edge of the fuult zone valley. Apparently, this location was at one .time the lowest paint :In the Cl'OS3 S?ction of that valley, but 1I>day that lowest· paint :Is B:rlftlng eastward 1I> the east S.de of the north-south-trending valley, leaving the stream course ''!rung'' like an :lnigatlon ditch along the edge of the valley. Two hYPotheses come tn mind. First, there :Is a remote poeEl.bili.ty that some s:Jrt of open fissure system or even human intervention somehow localized flow of groundwater and later sur:fii.ce (lajnage along the last trace of active fuult movement on the west Eide of the valley. Mare probably, there :Is a compressional tbr!Et-fuult component of motion on the San Gregar.lo-Sur-Hoegrl system wttb both tight lateral and west-side upward motion. Such a high-angle reverre component would help to explain the water gaps and stream capture along the :fault line through the propoeitlon that individual offset even'IB were Of SJffi.cient magnitude to accomplish the oft-propoe:!cl but probably geomor;irlcally rare :Instantaneous defeat and deflection of water courses such as Lit­tle Butano Creek. Continued slow deflection with a west-Eide upward component could

4Clark, J.C., and E.E. Brabb, 1978, stratigraphic contrasts acroe; the San Gre­gor.lo Fault, Santa Cruz Mountains, West Central California. pp 25-34 in CaJjf. Div. Mines and Geol.. Sp. Rept 137. --Weber, G.E., and K.R. IBJaie, 1977, late Pleistocene· and Holocene Tec1I>nics of the San Gregor.lo Fault Zone between Moss Beach and Paint Ano Nuevo, San Mateo Co., Calif. Geol. Soc. Amer.lea Abs. with Programs, v. 9, no. 4, p. 524. -Le.Joie, K.R. and others, 1979, Quaternary Tectonics of Coastal Santa Cruz and San Mateo Counties, California, as :Indicated by deformed marine teITa.Ces and alluv1al depcats (with other pipers); pp 61-119 in Weber; G.E. , et al, (eds), Field Trip Gui.de, Coastal Tectt>nics and CoastalGeoioglc Hazards :In Santa Cruz and San Mateo Counties, California; Coniillerlan Section, Geol. Soc~ America. · · , ·

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be tilting the valley eastward. Thrust components of this primarily right-lateral fault have been propored for the Sur Thrust and Sur Hill Thrust in the tittle Sur river area5 and in the Hosgrl offshore area6• Similar obeervatl.ons of uplift and tilting of the western (Pigeon Point) Block locally have been made by laJale, et al (op cit, 1979, p. 69) and Weber has identifi.ed a ttU"\Et component in the gen~ right-lateral motion of th1s fault zone.

Right-lateral offset rate :Is estimated to be 6 km :in the last few hundred tho1Eand years !:med upon assumed matching of Am:iyo Frljoles with the Gazes Creek watershed, and little Butano watershed with tl:te present cot!I're of Butano Creek west of the :fault zone (s:!e Plate 3). Dating of that offset :Is OOsed upon marine terrace inc:lsl.on and upon interpretation of the alluv1al fill sequences in the :fault valley as of late-glacial to post-glacial age. The!E requences are well expored by red1s:3ec­tion under present hlg~sea-level condiiions. The redimection is very recent tnsed upon steep channel walls in uncons::il.idated alluvium, high historic rediment input to the lagoon from th1s s:>uroe, and complete abrence of terrace or cut and fill evi­dences except tt:tore associated with historic dlsturl>ances in the watershed within the last 100 years. The youthful character of the fill itself is demcnstrated by the facts that the :lliult valley itself :Is cut into a younger marine terrace of at maximum 200-300,000 years age, that the fill requence is straightforward and ample without evidences of still-stands er S'.l!l formation in this area of very rapld mil forma-tion, and by obeervation that there are no apparent river terrace remnants along the walls of :fault valley. I interpret this to mean that the stream COt!I're deflection ma:t likely occurred at a ti.me .er ti.mes of lower sea levels when headwa.rd cutting along tlle weak :fault zone was enhanced and any earlier :fill could be removed by eI'Cr

slon. The present fill must postdate the last rise in sea level of 18,000 to 12,000 years ago.

A }iermissihle interpretation would be that only 3 km of total offset has occurred Eince the uplift of a marine terrace that now is deformed downward to the the northwest and :Is about 100 m above rea level opposite tlle "outlet" of tlle upper Butano Valley at the :fault line. In this interpretation, Ll.ttle Butano Creek would have formed AITOyo Frljoles and been only very recently captured by Butano Creek in perhaJE a Sngle tectonic event during the Holocene after the fault valley fill was depo&ted. If we assign an age of 200-300,000 years to the 100 m teITace, which is locally the "second" terrace and may be equivalent to either the Davenport (Cement)

or Western Drlve teml.ces of Bra.dley7, an overall offset rate of on the amer of ,1 m per 100 years would be indicated. A problem with this interpretation is that the present lower cot!I're of Butano Creek west of the fault is through a rather under:lit valley that eeems mere readily ass!gned to ttie watershed of little Butano Creek than to the Butano Creek watershed, ~ ti.me> its s..ze. If Ll.ttle Butano is assigned to Lower Butano valley, and Gazes Creek to AITOyo Frljoles, then we must propore a rate of :fault motion of 2 m per century. laJale, Weber, and others (op cit, 1979) esti­mate' 1.6 m per century of cumulative offset on this fault zone at Ano Nuevo using offset of temi.ce moreline angles.

5see the compJa:tion by Greene, et al, op cit, 1973. 6as reported in the .press at tlle ti.me of ttrls writing as attributed to an unctted

study by James K. Crouch.

. 7Bradley, W.C., and G.B. Griggs, 1976, Form, genesis, and deformation of central California wave-cut platforms. Geol. Soc. America Bull. v. B7, p. 433-449.

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Whatever the :Interpretation, many questions are raised by the rela:tionships :In this Pescadero area. The ti.ming af the capture of the headwaters af the San Larenm watermed by Pescadero Creek 1X> nearly double tts wateraied area ls unknown, as :Is the role of the Butano :fault and the San Gregarlo :In promoting the beadward cutting of PeEK:adero Creek 1X> behead the San I.orenm. The apparent under.lltnEEB of the present mouth of Pea:adero and Butano Creeks, which exit 1X> the sea in a single chan­nel CE bedrock at sea level in the sJrf :woe ls very enigmatic. The buried low-ma level channel north of the prerent outlet seems as ·m:lsfit as the mouth of the Carmel to handle the discharge of the prerent waterEhed areas of Butano and PeEcadero Creeks. Jn the case of the Carmel, active past-glacial offset CE the Cypres:; fu!nt­San Franclsiuito :fault his !inched off the 18,000 year-old low-rea-level channel. Could local :faulting CE the San Gregor:lo-Sur-H~ system be doing the si.me thing at Pes::a.dero?

3. DISCUSSION AND CONCLUSIONS

The San Gregarlo-Sur-Ha;grl system :Is characterized by rates of lateral dis­placement that are con.slstent over large geographic distances and are compi.tlble w:!th geodetic and theoretical analyses af rates of motion of the Pacifl.c Plate relative to the North American Plate. Average rates of offset on the central part!on of the San Andreas as det.erm:lned by geodetic mea&Jrements far the period 1907 to l!m. have been found 1X> be 3.2 plu;-m:lnts 0.5 cm/yr.8 Relative motion of the Pacific and North Amer­

ican plates 1::ased upon sea floor spread:lng :Is 5.5 to 5.8 cm/yr.9 The net difference of about 2.5 cm/yr ls very clore 1X> the estimated current offset rates oo ttle San Gregarlo-Sur-Ha;grl system of 2-3 cm/yr 1::ased upon rather crude geomcr}irl.c evidences

from the PeEcadero and Big Sur areas. A recent preprint by Minster and Jordan10

applies 1pherical vector analysis 1X> re&llution of geologic and geodetic . data far Cl"IElta1 motion :In western United States. Their independent analysis predicts strike-slip motion west of ttle San Andreas of no leS3 6 nor mere ttlan 25 mm/yr w:!th a 4-13 mm/yr comp1. sdmml component, which ttley apply in a rlg!d-plate model to ttle San GregarJo-H~ system. They utilize virtually ttle si.me geometr.lc concept of a 420-km long San Gregarlo-HOEgrl ~m as I do with ttie si.ngle exception that ttiey appear to accept the idea that the Sur faults are put of the Nac.imiento rather than San Gregarlo-HOEgli continuum (see Plate 4 attached, from their report).

Evidence within the Sallnlan twck wggests that it does not act as a ligid plate and that :faulting and folding there :Is actl.ve and :in perfect agreement with regional comp1 dm1al 9refB vectors as deduced from fault plane solutions of

8savage, J.C., and R. O. Burford, 1~3. Geodetic determination of relative plate motion in centra.l California. JGR, v. 78, no. 5, p. 832-838.

9 Atwater, Tanya, l~O. Implications of plate tectonics far the Cenozoic tectonics of western North America. Geal. Soc. Amer.lea. Bull., v. 81, p. 3513-3536. ---, and P. Molnar, 1~3. Relative motion of the Pa.cifi.c and North American Plates deduced from sea-floor spread:lng :In the Atlantic, Indian, and South Pa.cifi.c Oceans. pp 136-148 in Proceed. Conf. on Teet. Problems of the San Andreas Fault Sys­tem, Stanfcrd Univ. Geol. SM. Puhl XIII., Kovack, R.C. and A. Nur (eds).

10Minster,. J.B. and T.H. Jordan, 1004, Vector constraints on Quaternary deforma­tion of the Western United States east and west of the San Andreas fault; :In Tecton­ics and Sedimentation along the California Margin, J.K. Crouch and sJf. Bachman (eds), S.E.P.M., Pac. Section, in pres;. .

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McEvfily as presented by Zoback and Zoback11 and by the vector eolut:.i.oos of Minster and Jordan (1984, op c:l.t). 'lbe Sa.linian l:iock :Is ampl:y a wedge of crustal rocm that :Is caught at the :lnt.erfil.ce between the Pacl.fi.c and North American plat.es and :Is being meared and compi: d :In a stnlightfCJl'ward geometrically conast.ent pi.ttem. Faults wtthln 1he eotl.re l:iock that are geometrically conSst:ent wtth regional pi.t­tems mould all be conS.dered tD be actt.ve :In the Quateniary unleES demaistra.t.ed tD the contrary. RaJid uplift and actt.ve eroEional puce make conveotl.onal :Iii.ult ana.eysls diffl.cult :In the momltaino'IS pu1s of the lilock. However, where Pleistocene and Holocene depoS.ts exist wtthln the mOIDltainO'IS areas in esooMatl.on wtth :Iii.ult&, there :Is ample evklence of :Iii.ult actt.v:lty contt.nul.ng into the Holocene.

11 Zoba.ck, M.L., and M. ZOback, 1980, State of stres3 in the ooterm:lno'IS United States. JGR, v. 85, p. 6113-6156. .

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