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Abrupt Along-Strike Change in Tectonic Style: San Andreas Fault Zone, San Francisco Peninsula

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Abrupt Along-Strike Change in Tectonic Style: San Andreas Fault Zone, San Francisco Peninsula University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln USGS Staff -- Published Research US Geological Survey 1999 Abrupt along-strike change in tectonic style: San Andreas fault zone, San Francisco Peninsula Mary Lou Zoback U.S. Geological Survey, [email protected] Robert C. Jachens U.S. Geological Survey Jean A. Olson Timble Navigation Company Follow this and additional works at: https://digitalcommons.unl.edu/usgsstaffpub Part of the Earth Sciences Commons Zoback, Mary Lou; Jachens, Robert C.; and Olson, Jean A., "Abrupt along-strike change in tectonic style: San Andreas fault zone, San Francisco Peninsula" (1999). USGS Staff -- Published Research. 467. https://digitalcommons.unl.edu/usgsstaffpub/467 This Article is brought to you for free and open access by the US Geological Survey at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in USGS Staff -- Published Research by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. B5, PAGES 10,719-10,742, MAY 10, 1999 Abrupt along-strike change in tectonic style: San Andreas fault zone, San Francisco Peninsula Mary Lou Zoback, Robert C. Jachens,and Jean A. Olson• u.s. GeologicalSurvey, Menlo Park, California Abstract. Seismicityand high-resolutionaeromagnetic data are usedto define an abrupt changefrom compressionalto extensionaltectonism within a 10- to 15-km-widezone along the San Andreas fault on the San FranciscoPeninsula and offshore from the Golden Gate. This 100-km-longsection of the San Andreas fault includesthe hypocenter of the Mw - 7.8 1906 San Franciscoearthquake as well as the highestlevel of persistent microseismicityalong that -470-km-long rupture. We define two distinctzones of deformationalong this stretchof the fault usingwell-constrained relocations of all post- 1969 earthquakesbased a joint one-dimensionalvelocity/hypocenter inversion and a redeterminationof focal mechanisms.The southernzone is characterizedby thrust- and reverse-faultingfocal mechanismswith NE trending P axesthat indicate "fault-normal" compressionin 7- to 10-km-widezones of deformation on both sidesof the San Andreas fault. A 1- to 2-km-wide vertical zone beneath the surface trace of the San Andreas is characterizedby its almostcomplete lack of seismicity.The compressionaldeformation is consistentwith the young,high topographyof the Santa Cruz Mountains/CoastRanges as the San Andreasfault makesa broad restrainingleft bend (-10 ø) throughthe southernmostpeninsula. A zone of seismicquiescence -15 km long separatesthis compressionalzone to the southfrom a zone of combinednormal-faulting and strike-slip- faultingfocal mechanisms(including a M L = 5.3 earthquakein 1957) on the northernmostpeninsula and offshoreon the Golden Gate platform. Both linear pseudo- gravitygradients, calculated from the aeromagneticdata, and seismicreflection data indicatethat the San Andreas fault makes an abrupt -3-km right step lessthan 5 km offshorein this northernzone. A similarright-stepping (dilatational) geometry is also observedfor the subparallelSan Gregorio fault offshore.Persistent seismicity and extensionaltectonism occur within the San Andreas right stepoverregion and at least 15 km along-strikeboth to the SE and NW. The 1906 San Franciscoearthquake may have nucleatedwithin the San Andreas right stepover,which may help explain the bilateral nature of rupture of this event. Our analysissuggests two seismichazards for the San FranciscoPeninsula in addition to the hazard associatedwith a M - 7 to 8 strike-slip earthquakealong the San Andreas fault: the potential for a M • 6 normal-faulting earthquakejust 5-8 km west of San Franciscoand a M -- 6 + thrust faulting event in the southernpeninsula. 1. Introduction This fault is part of the San Gregorio-Hosgri fault system,a mostlylate Tertiary, plate boundaryfault zone with a cumula- Approximately100 km southwestof the San FranciscoPen- tive offset of 100-150 km, that extends from Point Sal in insula the very linear central California segmentof the San southernCalifornia and mergeswith the San Andreas fault on Andreas fault bifurcates,with the westerlystrand heading to- the Golden Gate platform [Grahamand Dickinson,1978; Clark ward the peninsulaas the SanAndreas fault proper (Figure 1). et al., 1984]. Geologic data, including trenching of the San The Hayward-Calaverasfault systemsin the East Bay form an Grcgorio fault, indicate multiple Holocene surfacerupturing eastern, more northerly striking strand. Together these two eventsand a sliprate of -5 mm yr-1 [Simpsonet al., 1997; strandsaccount for roughly-32 mmyr -• right-lateralshear, Working Group on Northern California EarthquakePotential, or approximately85% of the plate motion east of the Sierra 1996]. Nevada [WorkingGroup on Northern California Earthquake Four significanthistoric earthquakeshave occurredon the Potential,1996; Williamset al., 1994]. A third strike-slipfault San FranciscoPeninsula (Figure 1): (1)aM w • 6.8-7.5 event zone subparallelto the San Andreas fault, the San Gregorio in 1838 on the central peninsula,which on the basisof first- fault, crossesthe westernmostpart of the peninsulaand the hand accountsof the surface rupture appears to be a right- shelfoffshore from the Golden Gate (Golden Gate Platform). lateral strike-slipevent on the San Andreasfault [Louderback, 1947; Ellsworth, 1990; Tuttle and Sykes,1992; Toppozadaand 1Nowat TrimbleNavigation Company, Sunnyvale, California. Borchardt,1998; Bakun, 1999]; (2) the M w = 7.8 1906 San This paper is not subjectto U.S. copyright.Published in 1999 by the Francisco earthquake with an epicenter probably located American GeophysicalUnion. somewherenear where the San Andreas fault goes offshore, Paper number 1998JB900059. southof the Golden Gate [Bolt, 1969;Boore, 1981; Waldet al., 10,719 10,720 ZOBACK ET AL.: SAN ANDREAS FAULT ZONE, SAN FRANCISCO PENINSULA 122'45' 122'30' 122ø15 ' 122' 38' 38' 37'45' 37o45- .:- 37'30' -•?111i,37'30' -fif•ß- *.::. Menlo ::•... ß :,:3 '•' :., •;'•-'.•:::.::.. Jose 37'15' 37' ';;'6"•'•.• • 122'16' 122' Figure 1. Shadedtopography map showingQuaternary faults in the greater San FranciscoBay area [Bor- tugnoet al., 1992]. The inset showsmajor strike-slipfault systemsin central California. Large circlesindicate approximateepicenters of large historic earthquakes.The location of the 1838 epicenter is unknown;it is plotted on the central peninsulanear the center of the assumedrupture. 1993; Thatcheret al., 1997]; (3) the M,• - 6.9 1989 Loma can plate boundary(including the East Bay fault systems)for Prieta earthquakeon the southernmostpeninsula; and (4) a the past 160 years [Bakun, 1999]. M • 6.5 event in 1865 probablylocated near the city of San Whereasgeologic, historic earthquake, and geodeticdata all Jose,which may have been a thrust or oblique thrust event indicate that the dominant style of deformation in the San similar to the 1989 Loma Prieta earthquakeand possiblynot FranciscoPeninsula region is right-lateral strike-slipfaulting, on the San Andreas fault [Tuttle and Sykes,1992; Jaum• and changesin geometry of the San Andreas fault systemin the Sykes,1996]. While there is considerablecontroversy about the vicinity of the peninsula coincide with marked along-strike magnitudeof the 1838 event (estimatesrange from 6.8 to 7.5 variations in its geomorphicand topographicexpression. In [Bakun,1999; Toppozada and Borchardt,1998]), at a minimum crossingthe southernpeninsula, the San Andreas fault under- the 1838 and 1906 earthquakessum to about two thirds of the goesa broad bend to the left (a 12- to 15-km westwardbend expectedmoment releasefor the entire Pacific-North Ameri- over ---90-kmlength of fault, resultingin a ---10ø-12ø compres- ZOBACK ET AL.: SAN ANDREAS FAULT ZONE, SAN FRANCISCO PENINSULA 10,721 sional"restraining" bend), analogousto the "Big Bend" in the the north; for more information on this event the reader is Transverse Ranges of southern California but on a much referred to Reasenberg[1997], Simpson [1994], Spudich [1996], smallerscale. As in the Big Bend regionin southernCalifornia, and Wellsand Vidale [1999]. there is a zone of uplift (25-30 km wide) straddlingthe San Andreasfault that is particularlypronounced on the SW side, 2. Offshore Fault Locations Inferred From forming the Santa Cruz Mountains. The 1989 Loma Prieta event occurredwithin this bend region and exhibited nearly Aeromagnetic Data equal componentsof right lateral strike-slipand reversefault Only limited informationhas been availableon the morphol- motion (rake = 140ø) [Oppenheimer,1990]. Both geodeticdata ogy of the San Andreas fault offshore on the Golden Gate and surface deformation features suggestthat this event did platform. The lack of a clear fault trace on the seafloor in not occur on the sameportion of San Andreas fault that rup- high-resolutionseismic profiling [Cooper,1973] as well as the tured in 1906 [Thatcheret al., 1997; Prenticeand Schwartz, lack of alignedoffset featureson detailed (1-m contour) ba- 1991; Segall and Lisowski, 1990]. However, this event drew thymetrymaps (T. E. Chase,U.S. GeologicalSurvey (USGS), attentionto the seismicpotential of the convergentcomponent written communication,1994) indicatesthat sedimentarypro- of plate motion in the Bay area. cesseson the shelf outpacetectonic processes in determining On the central and northern peninsulathe active strand of geomorphicexpression of the fault zone. In fact, the coastline the San Andreas fault is straight, with an
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