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Comprehensive Seismic Hazard Review

San Francisco

Johanna Vaughan

- Senior Seminar 2015 -

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Content

Geography & Geology……………………………………….…..……………(3)

Tectonic Setting & Fault Kinematics…………………………………...…….(4)

Earthquake History & Potential Rupture of Active Faults………………...….(5)

Appendix A – Figures & Tables……………..………………………………..(8)

Appendix B – References……………………………………………………(18)

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The San Francisco region of California encompasses a diverse tectonic setting, entailing

historical evidence for catastrophic earthquake events, making the region a primary focus for

seismic hazard analysis. Understanding the potential for seismic hazard in this region strengthens

efforts towards mitigating seismic risk, as both qualities are linearly related through the

magnitude of vulnerability concerning life, property, economy, and natural resources. The highly

developed region bordering the eastern portion of San Francisco Bay alone has a population of

1.46 million (Catchings, 2006) distributed amongst several prominent cities such as Berkeley,

Fremont, and Oakland. The close proximity of several tectonic structures to one another facilities

the complex likelihood of earthquake hazard occurrence. Active fault zones are of high concern

as they pose significant seismic hazard potential for the San Francisco region. The scope of

seismic hazard is analyzed through the exploration of qualitative aspects such as related geologic

setting, tectonic behavior, historical data and modeling analysis. Understanding seismic hazard

potential for the San Francisco region serves to build a better foundation for earthquake risk

assessment and response in the case of a catastrophic earthquake event.

Geography & Geology

The coastal range of California is one of eleven geologic provinces (Figure 1.), and is

unique in geographic location as the strike-slip boundary separating the Pacific and North

American plates is roughly aligned along strike with the San Andreas fault system. Large scale

plate motions exhibit dextral strike-slip faulting along the coastal range forming pull apart basin

complexes such as the San Francisco Bay Basin (Prims, 1995). The tension resulting from

extensional forces promotes subsidence in the region, making geography susceptible to

deposition of sediments and sand from hydrological systems (Catchings, 2006). Depositional

pathways include streamflow from the Sacramento-San Joaquin River Delta (Figure 2.). The San

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Francisco region is divided into three distinct depositional timeframes (Figure 3.) where the most

recent Quaternary deposits consist of Alluvium and Bay Area Mud. This geologic combination

increases seismic hazard for the region bordering the San Francisco Bay, as these deposits

respond with little resistance to seismic wave propagation (Borcherdt & Gibbs, 1976). The

loosely consolidated nature of these deposits when compared with neighboring geologic units

relates to spectral amplification of ground motion (Table 1.) in response to seismic action. Both

vertical and horizontal ground motion amplification is higher for Alluvium and particularly Bay

Area Mud. Average intensity increments (Table 2.) calculated from spectral amplification are

also higher for these units (Borcherdt & Gibbs, 1976).

Tectonic Setting & Fault Kinematics

Several active fault zones (Table 3.) including the San Andreas, San Gregorio, Hayward,

Rogers Creek, and Calaveras have high potential for seismic hazard in the San Francisco region

(Figure 4.). These fault zones facilitate a complex array of movement in association with large

scale tectonic movement of the Pacific and North American plates. Complexity is derived from

vast geographic segmentation of these active fault zones in conjunction with variability amongst

slip rates (Table 3.). The San Andreas exhibits the highest magnitude of slip (Niemi & Hall,

1999) as well as segmentation, and the reoccurrence interval for this zone is more precisely

known when compared with the Hayward fault zone. The spatial distribution of fault zones

(Figure 4.) shows particular complexity where the San Andreas and San Gregorio zones form a

fault junction (Figure 5.) west of San Francisco. A complex association also exists where the

Hayward and Rogers Creek fault zones form a dilatational step over (Figure 7.) east of the San

Andreas. Fault junction kinematics aid in the formation of basin complexes where the shear

exerted between faults causes crustal block migration away from the San Andreas (Parsons et al.,

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2005). Counterclockwise rotation of the San Gregorio over a three million year time range

causes eastward stepping right-lateral fault formation. This behavior correlates with the existence

of multiple fault zone and depositional basins in San Francisco (Parsons et al., 2005). The eastern

step over zone (Figure 6.) between the Hayward and Rogers Creek fault zones is of seismic

hazard concern due to close fault convergence (4 km near surface). Slip along fault zones is

relieved through basin formation and the creation of normal faulting perpendicular to strike

(Parsons et al., 2003). Seismic hazard potential is relevant as the critical distance for an

earthquake to jump a dilatational step over is 4-5 km (Lettis et al., 2002). Through aeromagnetic

surveying the total magnetic field distribution (Figure 8.) indicates low density sediments

existing between converging fault zones where the most recent subsidence has occurred. The

unique tectonic settings and large slip rates of the San Andreas and Hayward fault zones

distinguish these systems regionally for elevated seismic hazard potential.

Earthquake History & Potential Rupture of Active Faults

Historical earthquake evidence (Figure 9.) supports the concern for seismic hazard potential

particularly along the active San Andreas and Hayward fault zones as several notable

earthquakes have occurred such as the 1968 Mw 6.8 (Hayward fault zone) and 1989 Mw 6.9

(San Andreas fault zone) events. Event reoccurrence modeling is dependent upon historical data

analysis, and suggests notable concern for future hazard potential along the Hayward fault zone

(Figure 12.) as it displays particular readiness for a Mw>6.7 event. The Hayward fault zone has a

reoccurrence interval of 140 years (over a 2000 year time range), yet 147 years have passed since

the last major event (Field et al., 2015). The remnants of the 1968 Mw 6.8 event are preserved as

organic rich deposits (Lienkaemper & Williams, 1999) visible through cross-sectional analysis

(Figure 10.). These deposits orthogonally truncate previously horizontally deposited layers and

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indicate a large scope of destruction following the earthquake event. Modeling analysis of the

Hayward fault zone shows dense strain accumulation (Figure 11.) as several velocity vectors of

moderate slip rate (approximately 7 mm/yr) are shown to cluster the surface fault trace. Fault

kinematics reveal continuous movement in small increments, where the lack of significant

movement is correlated to a high probability for major seismic events (D'Alessio et al., 2005).

The most concerning aspect of seismic hazard in association with the Hayward fault zone is the

continuous increase in dextral slip rate along the fault zone over the past decade (Field et al.,

2015). As a result the Hayward fault zone reveals concern for the reoccurrence of a large scale

earthquake (Mw = 6.8+/-0.2) similar in hazard potential to the 1968 event.

Characterization of the San Francisco region under the premises of seismic hazard

accounts for an active and complex tectonic setting. There exists a lack of precision, and a need

for refinement in understanding kinematic motion related to large scale interactions between the

Pacific and North American plate boundaries. Techtronic structures influencing seismic hazard

include the evolution of pull apart basin complexes, facilitating the deposition of sediment

around the Coastal Bay region. Quaternary deposits of Bay Mud and Alluvium have vast

geographic distribution and increased seismic hazard potential as these deposits are most

susceptible to elevated spectral amplification of ground motion. Understanding basin

morphology of the region and fault kinematics in relation to velocity structure are the most

important elements in modeling strong ground motions of potential earthquakes in the San

Francisco Bay region (Kim et al., 2012). The coastal region east of San Francisco Bay exhibits

high seismic risk probability as the concern for seismic hazard and vulnerability are

distinguishably higher due to location and population density. In particular the region

surrounding the Hayward fault zone is overdue for a large scale earthquake event, as a lack of

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movement and strain buildup could produce a magnitude event similar to the 1968 earthquake.

Modeling analysis reveals sparse earthquake occurrence along the Hayward fault zone coupled

with dense slip velocity distribution, and the increase in slip magnitude over the past decade.

However, modeling analysis lacks the certainty of kinematic behavior needed to precisely assess

seismic hazard probability. Focusing seismic hazard assessment towards the Hayward fault

system would increase the accuracy of mitigating seismic risk for the densely populated and

urbanized area of San Francisco.

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Appendix A - Figures & Tables

Figure 1. (Sloan, 2006) Comprehensive geomorphic map of California showing the location of San Francisco in Red. San Francisco is part of the Costal Range Providence extending 400 miles from the Klamath Mountains (North) to the Transverse Ranges (South).

Figure 2. (USGS, 2012) Google map overlay displaying San Francisco region morphology. The Coastal Range consists of Major Mountain (red) & Basin (blue) systems depicting dynamic geographic expression. The Sacramento-San Joaquin River Delta system feeds into the San Fransisco Bay Basin, bordered by the Santa Cruz Mountains to the west and Diablo Range to the east. The low lying topography of the Central Valley truncates abruptly with northwest striking mountain systems.

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Figure 3. (Stoffer, 2002) Regional geologic map displaying three distinct depositional timeframes and related geologic units spatially distributed throughout greater Californian coast. Quaternary deposits shown in yellow, have origin related to California’s interglacial period (125 kyr). These deposits consist of weak Bay Area Mud and Alluvium.

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Table 1. (Borcherdt & Gibbs, 1976) Average spectral amplification for ground motion shown in relation to San Francisco region geology (Figure 3.). Both Vertical and Horizontal components are displayed, with Bay Mud and Alluvium having the highest amplifications for ground motion.

Table 2.(Borcherdt & Gibbs, 1976) Geology of San Francisco region correlated to average intensity increments calculated through the use of an intensity increment formula; Intensity Increment = 0.27+2.70*log(Average Horizontal Spectral Amplifications)*(2). Both Bay Mud and Alluvium exhibit high intensity increments.

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Table 3. Custom made table showing quantitative and qualitative aspects of major fault zones in San the Francisco region. The San Andreas Fault zone exhibits the greatest slip rate range as well as segmentation. Notice that the recurrence interval is more precisely known relative to other fault zones. All fault zones display a wide range of geomorphic expression. Uniquely the Calaveras fault zone is associated with large-scale landslides, stemming primarily from geographic location and close proximity to the Diablo Mountain Range.

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Figure 4. Custom created Google Earth image depicting 100 km x 100 km area encompassing the San Francisco region. Colour coding corresponds to Table 1. The location of major fault zones are shown with relatively similar northwest strike, and close proximity to one another. Four major fault zones exist within the 50 km x 50 km area surrounding San Francisco are indicated by the white rectangle. These major fault systems exhibit dextral strike-slip motion, and in relation to large scale tectonics, pose potential seismic hazard.

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Figure 5.Custom Google Earth image depicting the location of a major fault junction between the San Andreas and San Gregorio fault systems. The area of crustal and extensional basin formation is indicated by the white circle. Here kinematics involve eastward stepping right-lateral fault formation.

Figure 6. (Parsons et al., 2005) Coincident seismic reflection/refraction analysis shows potential fault kinematics along the San Andreas Fault zone in relation to extensional basin formation and fault zone migration over a 3 Mya time range from left to right. The red indicates the location of the pacific plate and yellow depicts the zone migration existing within the fault junction between the San Andreas and San Gregorio fault systems. The trace of the San Andreas Fault zone is indicated by the red dashed line.

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Figure 7. Custom created Google Earth image depicting Rogers Creek and Hayward fault zone kinematics. The location of a critical dilatational step over zone between the faults is encompassed by the white circle. The fault zones converge within 4 km near surface, and the formation of hazardous normal faulting striking perpendicular to fault surface traces..

Figure 8. (Parsons et al., 2003) Total field magnetic data resulting from aeromagnetic surveying with magnetic field intensity in nT and colour resolution. Red to maroon values indicate magnetic highs whereas blue to green colour values indicate magnetic lows. The Rodgers Creek and Hayward fault zones are shown with solid black lines and can be seen to border the San Francisco Bay to the east.

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Figure 9. (Tuttle & Sykes, 1992) Proposed surface rupture areas in association with historical earthquake events along major fault zones near San Francisco. Notice that historical ruptures along the Hayward and San Andreas Fault systems exhibit large magnitudes.

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Figure 10. (Lienkaemper & Williams, 1999) Here the location of black organic rich deposits are clearly depicted in relation to the 1968 M6.8 earthquake along the Hayward fault zone.

Figure 11. (D'Alessio et al., 2005) Spatial distribution of slip vectors along Hayward and San Andreas Fault systems showing the rate of strain accumulation. Velocity vectors are shown in yellow directionally indicate dextral strike slip faulting kinematics on a large scale. The Bay Area Velocity Unification program in conjunction with Berkley University continue to monitor and analyze active fault kinematics in the San Francisco region.

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Figure 12. (Field, 2014) Current earthquake likelihood relative to long-term likelihood for fault systems within the San Francisco region. Low earthquake potential is depicted in blue whereas high earthquake likelihood is shown in red.

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Appendix B - References

Borcherdt, R. D., & Gibbs, J. F. (1976). Effects of local geological conditions in the San Francisco Bay region on ground motions and the intensities of the 1906 earthquake. Bulletin of the Seismological Society of America, 66(2), 467-500. Catchings, R. D., Borchers, J. W., Goldman, M. R., Gandhok, G., Ponce, D. A., & Steedman, C. E.

(2006). Subsurface structure of the east bay plain ground-water basin; san francisco bay to the hayward

fault, alameda county, california. Open-File Report - U.S.Geological Survey, , 68. Retrieved from

http://ezproxy.lib.vt.edu:8080/login?url=http://search.proquest.com/docview/51537326?accountid=14826

D'Alessio, M. A., Johanson, I. A., Bürgmann, R., Schmidt, D. A., & Murray, M. H. (2005). Slicing up the

San Francisco Bay Area: Block kinematics and fault slip rates from GPS‐derived surface

velocities. Journal of Geophysical Research: Solid Earth (1978–2012), 110(B6).

Field, E.H., and 2014 Working Group on California Earthquake Probabilities, 2015, UCERF3: A new

earthquake forecast for California’s complex fault system: U.S. Geological Survey 2015–3009, 6 p.,

http://dx.doi.org/10.3133/fs20153009.

Google Earth Map overlay from Dartmouth Flood Observatory, University of Colorado, USGS, 2012

Kelson, K. I., G. D. Simpson, W. R. Lettis, and C. C. Harden, Holocene slip rate and recurrence of the northern Calaveras fault at Leyden Creek, eastern San Francisco Bay region, J. Geophys. Res., 101, 5961-5975, 1996.

Kim, A., Dreger, D. S., & Larsen, S. (2010). Moderate earthquake ground-motion validation in the san francisco bay area. Bulletin of the Seismological Society of America, 100(2), 819-825. doi:http://dx.doi.org/10.1785/0120090076

Lettis, W., J. Bachhuber, R. Witter, C. Brankman, C. E. Randolph, A. Barka, W. D. Page, and A. Kaya

(2002). Influence of releasing stepovers on surface fault rupture and fault segmentation: examples from

the 17 August 1999 Izmit earthquake on the North Anatolian fault, Turkey, Bull. Seism. Soc. Am. 92, 19–

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Lienkaemper, J. J., & Williams, P. L. (1999). Evidence for surface rupture in 1868 on the Hayward fault in

north Oakland and major rupturing in prehistoric earthquakes. Geophysical research letters, 26(13), 1949-

1952.

Lienkaemper, J. J., and G. Borchardt, Holocene slip rate of the Hayward fault at Union City, California, J.

Geophys. Res., 101, 6099-6108, 1996

Niemi, T. M., and N. T. Hall, Late Holocene slip rate and recurrence of great earthquakes on the San Andreas fault in northern California, Geology, 20, 195-198, 1992. Parsons, T., Sliter, R., Geist, E. L., Jachens, R. C., Jaffe, B. E., Foxgrover, A., ... & McCarthy, J. (2003).

Structure and mechanics of the Hayward–Rodgers creek fault step-over, San Francisco bay, California.

Bulletin of the Seismological Society of America, 93(5), 2187-2200.

Parsons, T., Bruns, T. R., & Sliter, R. (2005). Structure and mechanics of the san andreas-san gregorio

fault junction, san francisco, california. Geochemistry, Geophysics, Geosystems - G3, 6(1), 7.

doi:http://dx.doi.org/10.1029/2004GC000838

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Prims, J., & Furlong, K. P. (1995). Subsidence of San Francisco Bay: Blame it on Salinia. Geology, 23(6),

559-562.Sloan, D. (2006). Geology of the San Francisco Bay region. Berkeley, University of California

Press.

Stoffer, P.W., and Gordon, L.C., eds., 2001, Geology and Natural History of the San Francisco Bay Area:

A Field-Trip Guidebook, U. S. Geological Survey Bulletin 2188, p. 61-86

http://www.nps.gov/goga/learn/education/loader.cfm?csModule=security/getfile&PageID=146483

Tuttle, M. P., & Sykes, L. R. (1992). Re-evaluation of several large historic earthquakes in the vicinity of

the Loma Prieta and peninsular segments of the San Andreas fault, California. Bulletin of the

Seismological Society of America,82(4), 1802-1820.

Weber, G. E., and J. M. Nolan, Determination of late Pleistocene-Holocene slip rates along the San Gregorio fault zone, San Mateo County, California, U.S. Geol. Sum. Open Fzle Rep., 95-210, 805-807, 1995

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