GEOLOGIC SETTING AND ACTIVITY OF FAULTS IN THE SAN FERNANDO AREA, CALIFORNIA By CARL M. WENTWORTH and R. F. YERKES U.S. GEOLOGICAL SURVEY With a section on SEISMOLOGICAL ENVIRONMENT By CLARENCE R. ALLEN SEISMOLOGICAL LABORATORY, CALIFORNIA INSTITUTE OF TECHNOLOGY The faulting associated with the San Fernando sediments has both limbs overturned and overridden earthquake of February 9, 1971, occurred in the by thrusts (Bailey and Jahns, 1954, p. 92-96). Transverse Ranges structural province, a region Very young deformation in the Transverse Ranges noted for its strong and relatively young tectonic is not restricted to the Ventura Basin, however, and deformation. This is, however, the first example of late Quaternary faulting is known to occur from historic surface faulting within the interior of that near Point Conception eastward to the San Andreas province. fault. The Transverse Ranges structural province The February 9, 1971, faulting broke the ground trends eastward across southern California (across surface near the north margin of the San Fernando the center of fig. 1) and constitutes a region of late Valley opposite the "great bend" in the San An­ Cenozoic north-south shortening that lies athwart dreas fault south of the San Joaquin Valley (fig. 1). the northwesterly regional trend of the dominant The geometry of the crustal blocks in this region is San Andreas fault system. In the past 30 million such that the Transverse Ranges structural block years the San Andreas fault, master fault of the sys­ southwest of the San Andreas fault is constrained tem, has undergone about 200 km of right-lateral and compressed against the bend as the block moves strike slip in southern California (Crowell, 1962). northwestward along the fault. During the past 5 to During historic time, surface faulting in southern 10 million years' this compressive deformation has California has largely been confined to this and re­ thrust the mountain blocks up over adjacent valleys lated right-lateral strike slip faults in the San An­ to the south. The movement has occurred along dreas system. Notable exceptions are the historic north-dipping reverse or thrust-fault systems such ruptures of the east- and northeast-trending White as the Sierra Madre and Santa Susana, which trend Wolf and possibly Big Pine faults, and now the west and northwest along the southern margins of 1971 San Fernando faulting, all of which have dis­ the mountain blocks. The most impressive product played at least some component of left-lateral strike of this uplift is the bold southern front of the San slip (fig:. 1). Gabriel Mountains, which stands some 1,500 m The Transverse Ranges province consists of nu­ above the San Fernando and Los Angeles areas to merous east-trending mountain ranges and valleys the south. Additional horizontal shortening and typified by late Cenozoic structural deformation and uplift of one of the blocks occurred along the rup­ strike slip, reverse, and thrust faults of ~imilar tures that accompanied the San Fernando earth­ trend (Bailey and Jahns, 1954; Jennings and quake. Strand, 1969). The most spectacular development of Although segments of the surface traces of many the structure of this province is in the Ventura of the Transverse Ranges faults have been mapped Basin, where an immense Cenozoic syncline that in­ and interpreted in the context of regional structural cludes a thickness of 1,500 m of marine Pleistocene history, the abundant but relatively unobtrusive evi- 6 THE SAN FERNANDO, CALIFORNIA, EARTHQUAKE OF FEBRUARY 9, 1971 7 50 100 M ILES UNDERWATER CONTOURS IN fEET \ ' \ ' ~ N \ A v E \ 114" .... R T ) / 1 34" I \ EXPLANATION Historically and recently active faults Possible extensions of historically and recently active faults, and related inactive faults Inferred extensions of active or related inactive faults beneath surficial deposits or the sea floor ~ Fauft seg ments with surface or postulated submarin e slip during historical earth ­ quakes Or with continuous or intermittent aseismic fault slippage 120" ll8" MAJOR HISTORICALLY AND RECENTLY ACTIVE FAULTS OF THE SOUTHERN CALIFORNIA REGION FIGURE 1.-Major historically and recently active faults of the southern California region. dence of their recent geologic activity has not been this sense the earthquake itself was no great sur­ widely recognized or appreciated. Study of the 1971 prise. An earthquake of about this same magnitude San Fernando faulting should reinforce our confi­ occurred in 1968 in the Borrego Mountain area 220 dence in using this geologic data to evaluate the fu­ km southeast of Los Angeles, but damage was small ture behavior of faults. because-unlike the 1971 event-it occurred in a re­ mote location. SEISMOLOGICAL ENVIRONMENT Between 1934 and .1971, which is the interval In the years prior to 1971, the area of the San during which epicentral locations of southern Cali­ Fernando earthquake had been one of low to moder­ fornia earthquakes have been listed by the Califor­ ate seismic activity-not unlike that of many other nia Institute of Technology, only about 10 earth­ parts of California. Indeed, the 1934-63 strain­ quakes of magnitude 3.0 and greater occurred release maps (Allen and others, 1965) indicated that within the area that corresponds to the epicentral the northern San Fernando Valley was seismically region of the San Fernando earthquake. None of less active than most other parts of the greater Los these was large, although a few were felt locally, Angeles area. Certainly there was nothing in the such as the magnitude 4.0 shock of August 30, 1964, very recent seismic history to suggest that this that was centered under southern San Fernando. area, more than any other area, was particularly Previous to 1934, two shocks are of special interest: likely to experience a magnitude 6.6 earthquake. It the magnitude 5.2 earthquake of August 30, 1930, must be kept in mind, however, that an earthquake was probably much closer to San Fernando than the of at least this magnitude occurs somewhere in the original assignment of the epicenter location in southern California region on the average of about Santa Monica Bay suggests, and it is particularly once every 4 years (Allen and others, 1965), and in noteworthy because it caused some minor damage to 8 THE SAN FERNANDO, CALIFORNIA, EARTHQUAKE OF FEBRUARY 9, 1971 E X PLAN AT ION UN CO NFORMITY r~···-:::· ·j- - ~ Contact :: .Cl~ .. :·: Qa ~ ..A.. ..A.. ...? ·· .. ~ ·.' .·: .... -··?- ·· . High -angle fault Thrust fault ""t:: Alluvial deposits Q.J [ tT:W }J Faults """' Oc, depos i ts in ac.:tive stream c h a nne ls : uncun ~ t:: Mint Canyon Formation O<l> >- Q.J Da shed where approximatel y l ocated or inferred ; ~u"'"' solida.ted g ravel , sand , s!lr , and t:lay a:: u 0 F : e se nt only nurth of San G a briel fault; conti · dotted where concealed. queri ed where doubtful .~o Oa, uncunsolidau~ d gravel , sand, silt , and c l ay: <{ aJo z 11 e ntal, soft, poorly to well-cunsulldat ed '"c3 commonly uverl<::lin h y suil i ~ o:::r: a:: '- sediments; upper part conglomerate and voarse ­ ------ ,_"0 w Q.J g ra ined sandston e , mudston e , and t uff: Iuwe r Tectonic ruptures associated with earthquake t':l Q.Jt:: c. 1-- c. part poorly sorted co nglom erate with interbedded of February 9, 1971 [/). c_Ol <{ ::::;, ::l coar~e-grained sandstone, siltstone, and mud­ > s G (J r5J z Stream terrace deposits sron e, coar se b asal vo nglumer ate co nsists of Landslide angular fragments of underlying basem e nt "l'j Sligh1ly con soli d a t e d gravel , sand , si!r and csay Only selected large lalldslldes shown, arrows t':l ;·oc ks >­ UNCONFORMITY UNCONFORMITY cr: indicate general direction of move ment ~ o<{ z f­ > er: z .... w t;1 <l> C0EJIHlli4 f- ~ Saugus Formation Topanga Formation Geology comp 1led by R . F . Yerke s an d 0 0 Q.J Q.J C. ~. \ . Wentworth, 197 1 - t:: OT s. llOrth and cast part of map : s lightly to t:: Sands : oJJ e a nd cobble -bould er conglom e r ate , poorly "<:;Q.J <l> n t:: u muduratu ly consolidated t:Darse -g r a incd sand· u sorted . m as siv e . commonly well indurated 0: 0 0 • > r stone, conglumera te. and mudston < ~ UNCONFORMITY H Q.J[i) i "l'j ~ -~ Os . southwe st part of m ap : s lightly .:on solid a ted Q.J Q.J~ 0 t.:ung l- omerare and cu arse -~ r ai ned sand:sr urJ e ;;:; sa.. T sr, Sunshine R anch Memb er, of Pliocene age, in ~ "0 z 0:: west part of map; siltstone with interbedded i .._:;; andstone and conglomerate. locally interfingers Volcanic rocks :; wi th uppe r part of P ico Formation Basaltic flows . tuff breccia s . and flow brec c.:ias , t':l h a rd and dense to• soft. v esi c ular. and amyg­ > daloidal ~ '"c3 Q.J 0 ~ t:: Pico Formation Q.J z .0 Q.J "J t:: o<{ cj u .\1arine , soft siltstone. mudstone, and fine-gr a ined >­ ::s Q.J >­ 0 cr: (J)OC sandston e with interbedd e d coarse-grained sa nd­ o<{ s...Og Undifferentiated lower Tertiary and Upper > - Q.JQ.JQ.J ::Jo<{ ~ 0:: stone and resist a nt pebble conglome r ate; upper f­ c.<.>~ Cretaceous rocks o­ er: c. 0: 0: wr­ t':l part interfingers with Saugu ~ Formation and w :::.Q;Il.. Chi e fly sandstone, s iltstone, and p e bble co n­ u cr: .lower part with T owsley Formation g lomerate commonl y referred to Martinez and -<t:w 0 f- '--o "l'j (.)"' Chico Form ations;commonly well i ndurated and r-f­ "<:; 0: w "l'j t:: Q.J resistant.