S23C-01 INVITED
Constraints on the Rupture of the October 21, 1868, Hayward Earthquake Determined From the Distribution of Modified Mercalli Intensity
The October 21, 1868, Hayward earthquake was the most damaging earthquake to occur in California in the half- century following the 1848 annexation. The earthquake shattered the city centers of Oakland and San Francisco, and cracked brick buildings as far away as Santa Rosa and Gilroy. We have re-evaluated MMI intensites at the 124 sites with damage or felt reports compiled by Toppozada et al. (1981), and added 26 sites where we obtained reports from newspapers and historical narratives. We used the 1878 Thompson and West Atlas of Alameda County to locate most of the specific buildings that were reported as damaged. The resulting ShakeMap interpolates the distribution of intensity along the Hayward fault, and the extent of shaking throughout the greater Bay Area and the San Joaquin Delta. Surprisingly, the highest intensities (MMI 8-9 to 9) are clustered near the middle of the fault rupture, in Hayward, San Leandro, and San Lorenzo. The intensities are lower (MMI 7- 8) at the ends of the fault rupture, in Berkeley and Warm Springs. The lack of strong shaking at either end of the fault rupture makes it hard to discern the rupture direction: the intensities observed at regional distances suggest that the rupture was stronger to the northwest towards Petaluma (MMI 7) and Martinez (MMI 7) than to the southeast towards Calaveras Valley (MMI 6-7) and Gilroy (MMI 6-7). The relatively low intensities in Oakland and Berkeley (MMI 7-8) suggest that the shallow locked zone near Piedmont, which Simpson et al. (2001) infer from the distribution of fault creep, did not rupture in the earthquake. This result appears to contradict Yu and Segall's (1996) conclusion that the fault slipped > 1 m in Berkeley. Given the large proportion of aseismic slip on Hayward fault, both observed geologically at the surface and inferred geodetically at depth, it is natural to propose that the rupture process of the 1868 earthquake comprised a series of disjoint asperity ruptures with variable rupture directions, and a substantial amount of dynamically forced slip.
S23C-02 INVITED
Probabilistic seismic hazard in the San Francisco Bay area based on a simplified viscoelastic-cycle model of fault interactions
We construct a viscoelastic-cycle model of plate boundary deformation that includes the effect of time-dependent interseismic strain accumulation, coseismic strain release, and viscoelastic relaxation of the substrate beneath the seismogenic crust. For a given fault system, time-averaged stress changes at any point (not on a fault) are constrained to zero, i.e., kinematic consistency is enforced for the fault system. The dates of last rupture, mean recurrence times, and the slip distributions of the (assumed) repeating ruptures are key inputs into the viscoelastic cycle model. This simple formulation allows construction of stress evolution at all points in the plate boundary zone for purposes of probabilistic seismic hazard analysis (PSHA). Stress evolution is combined with a Coulomb failure stress threshold at representative points on the fault segments to estimate the times of their respective future ruptures. In our PSHA we consider uncertainties in a four-dimensional parameter space: the rupture peridocities, slip distributions, time of last earthquake (for pre-historic ruptures) and Coulomb failure stress thresholds. We apply this methodology to the San Francisco Bay region using a recently-determined fault chronology of area faults. Assuming single-segment rupture scenarios, we find that future rupture probabilities of area faults in the coming decades are the highest for the southern Hayward, Rodgers Creek, and northern Calaveras faults. This conclusion is qualitatively similar to that of Working Group on California Earthquake Probabilities [2003], but the probabilities derived here are significantly higher. Given that fault rupture probabilities are highly model dependent, no single model should be used to assess to time-dependent rupture probabilities. We suggest that several models, including the present one, be used in a comprehensive PSHA methodology, as was done in Working Group on California Earthquake Probabilities [2003].
S23C-03
3D Image Tour of the Hayward Fault in the East Bay, San Francisco Bay Region, California
A 3D image tour of the Hayward Fault begins at its northern land-based terminus at Point Pinole from where it continues northward under the waters of San Pablo Bay. From Point Pinole, the Hayward Fault extends southward for about 90 kilometers through the urbanized landscape of the East Bay region, passing through the cities of Richmond, Berkeley, Oakland, San Leandro, Hayward, Fremont, and other communities. At its southern end, the fault forms a series of oblique reverse faults, but at depth it connects with the Calaveras Fault as a through-going structure along the western foothills of the Diablo Range east of the greater San Jose area. This presentation focuses on access to the Hayward Fault in public places where features impacted by active fault creep can be viewed. Features include offset curbs, fractures in sidewalks, parking areas, buildings, and damage to other infrastructure in the active fault zone. Additional images highlight landscape features and historic landmarks along the fault, including those that were impacted by the 1868 Hayward earthquake, and those that were or were engineered both with and without consideration of the location of the fault. Earthquake data and geologic interpretations of the subsurface along the fault zone are also presented. This presentation, and an associated website, is for educational audiences with the intent of promoting public awareness and earthquake preparedness. This work is part of the ongoing outreach and public education efforts by the U.S. Geological Survey in cooperation with the 1868 Hayward Earthquake Alliance in anticipation of the 140th anniversary of the great earthquake. The use of 3D imagery enhances the educational value of the presentation and provides a unique perspective on the subject matter. Red-and-cyan 3D viewing glasses will be available at the presentation. http://3dparks.wr.usgs.gov
S23C-04 INVITED
Asperities on the Hayward fault resolved by PS-InSAR, GPS and boundary element modeling
The Hayward fault shows mixed behaviors -- the majority of the fault surface creeps interseismically, but it is capable of supporting large earthquakes. One explanation for this is that discrete areas of the fault are locked, and accumulating strain, even as surrounding areas creep around them. These locked areas are likely to rupture as asperities in future 1868-like earthquakes. We use PS-InSAR and GPS measurements of surface deformation velocities to identify the creep-rate distribution of the fault, and therefore the areas that are not creeping. We solve first for a kinematic model of the distribution of slip rate on the fault surface using a 3D mesh of triangular dislocation elements that is defined by precisely relocated microseismicity. Then, guided by that solution, we calculate a range of scenarios of boundary element models to resolve the dimensions of the fault area that is fully locked. Slip on deep dislocations underlying the major faults in the San Francisco Bay Area drives the system. Shallow fault elements are either chosen to be locked (zero displacement) or creeping frictionlessly (zero shear traction). The distribution of locked areas which best explains the surface deformation velocities is our preferred model. We find that the pattern of surface velocities is consistent with a 40-km-long locked asperity at the base of the upper crust, extending from Oakland to Union City. The top of this locked zone is shallowest (~4~km) on the 20 km stretch between Oakland and Hayward. In all, around 20% of the fault surface is locked and accumulating a moment deficit equivalent to a M6.5-6.6 earthquake per century. An open question for seismic hazard estimates concerns the seismic potential of the creeping areas - are they also susceptible to rupture, or would any accumulated moment deficit be released instead as aseismic afterslip?
S23C-05
The Hayward Fault Exposed! 20,000 Visitors Made it a Success
Last year, as part of the commemoration of the anniversary of the 1906 earthquake, an exhibit was built that gave the public a chance to better understand earthquakes and the faults that create them, and how to be prepared for a major earthquake. Open for six months, the exhibit in Fremont Central Park attracted more than 20,000 visitors from throughout the San Francisco Bay area and beyond. The main draw was the opportunity to descend into a 12-foot-deep excavation that provided up-close views of the Hayward fault itself. Visitors came to see the fault but stayed to hear its story and view displays about being prepared for the coming quake and the science behind it. The Hayward fault is an excellent subject to spark public interest. The large 1868 earthquake, which was known as "the great San Francisco earthquake" until 1906, caused the Hayward fault to slip up to 6 feet in areas that are now densely urbanized with homes and town centers. Further, the fault has been researched extensively, revealing that we are currently in the time window during which the next big earthquake, perhaps a repeat of the 1868 earthquake, is likely to occur along the Hayward fault. And to top it off, the fault experiences tectonic creep that provides fairly dramatic evidence of fault movement by cracking and offsetting curbs, parking lots, and streets near the exhibit site. Visitor feedback was overwhelmingly positive. Local groups came en masse and were spurred into developing plans for responding to a large earthquake in their community. School children came on field trips, saw what a fault looks like and how fault movement affects what they think of as static features of their world. Many visitors mentioned that such an exhibit should be a permanent Bay Area attraction. Two years in planning, the event required large amounts of volunteer time, sponsorship funds, agreement from the local government, and dedication from its developers. A permanent exhibit would undoubtedly be successful. It is the funding and support of the local government that are the biggest challenges. Now that the idea of an Earthquake and Fault Exhibit has proven successful, the common pre-exhibit question of: "Who would want to see a big hole in the ground?" is easy to answer.
S23C-06 INVITED
Before the 1868 Earthquake: An 1800-year geologic history of surface ruptures along the southern Hayward fault
We summarize* geologic evidence of the occurrence and timing of paleoearthquakes on the southern Hayward fault as interpreted from trenches excavated within a sag pond at the Tyson's Lagoon site in Fremont, northern California. We use the information to estimate the mean value (μ) and aperiodicity (σ/μ) of the fault's recurrence interval (RI), two fundamental parameters for estimation of regional seismic hazard. In addition to evidence for the 1868 Hayward earthquake, we present evidence for at least ten paleoruptures since about AD 170. We document these events with evidence for ground rupture, such as the presence of blocky colluvium at the base of the main trace fault scarp, and with corroborating evidence such as simultaneous liquefaction or increase in deformation immediately below event horizons. The mean RI is 170 ± 82 yr (1σ, standard deviation of the sample), aperiodicity is 0.48, and individual intervals may be expected to range from 30 yr to 370 yr (at a 95.4% confidence level). The mean RI is consistent with the recurrence model of Working Group on California Earthquake Probabilities (2003) (mean, 161 years; range: 99 yr (2.5%); 283 yr (97.5%)). We note that the mean RI for the five most recent events may have been only 138 ± 58 yr (1σ). Hypothesis tests for the shorter RI do not demonstrate that any recent acceleration has occurred compared to the earlier period or the entire 1800-yr record, principally because of inherent uncertainties in the event ages. *Based on work published in the December 2007 issue of Bulletin of the Seismological Society of America
S23C-07
Developing a Hayward Fault Greenbelt in Fremont, California
The Math Science Nucleus, an educational non-profit, in cooperation with the City of Fremont and U.S. Geological Survey has concluded that outdoor and indoor exhibits highlighting the Hayward Fault is a spectacular and educational way of illustrating the power of earthquakes. Several projects are emerging that use the Hayward fault to illustrate to the public and school groups that faults mold the landscape upon which they live. One area that is already developed, Tule Ponds at Tyson Lagoon, is owned by Alameda County Flood Control and Conservation District and managed by the Math Science Nucleus. This 17 acre site illustrates two traces of the Hayward fault (active and inactive), whose sediments record over 4000 years of activity. Another project is selecting an area in Fremont that a permanent trench or outside earthquake exhibit can be created that people can see seismic stratigraphic features of the Hayward Fault. This would be part of a 3 mile Earthquake Greenbelt area from Tyson Lagoon to the proposed Irvington BART Station. Informational kiosks or markers and a "yellow brick road" of earthquake facts could allow visitors to take an exciting and educational tour of the Hayward Fault's surface features in Fremont. Visitors would visually see the effects of fault movement and the tours would include preparedness information. As these plans emerge, an indoor permanent exhibits is being developed at the Children's Natural History Museum in Fremont. This exhibit will be a model of the Earthquake Greenbelt. It will also allow people to see a scale model of how the Hayward Fault unearthed the Pleistocene fossil bed (Irvingtonian) as well as created traps for underground aquifers as well as surface sag ponds. http://msnucleus.org
S23C-08
New Airborne LiDAR Survey of the Hayward Fault, Northern California
We present a digital elevation model (DEM) constructed from newly acquired high-resolution LIght Detection and Ranging (LIDAR) data along the Hayward Fault in Northern California. The data were acquired by the National Center for Airborne Laser Mapping (NCALM) in the spring of 2007 in conjunction with a larger regional airborne LIDAR survey of the major crustal faults in northern California coordinated by UNAVCO and funded by the National Science Foundation as part of GeoEarthScope. A consortium composed of the U. S. Geological Survey, Pacific Gas & Electric Company, the San Francisco Public Utilities Commission, and the City of Berkeley separately funded the LIDAR acquisition along the Hayward Fault. Airborne LIDAR data were collected within a 106-km long by 1-km wide swath encompassing the Hayward Fault that extended from San Pablo Bay on the north to the southern end of its restraining stepover with the Calaveras Fault on the south. The Hayward Fault is among the most urbanized faults in the nation. With its most recent major rupture in 1868, it is well within the time window for its next large earthquake, making it an excellent candidate for a "before the earthquake" DEM image. After the next large Hayward Fault event, this DEM can be compared to a post-earthquake LIDAR DEM to provide a means for a detailed analysis of fault slip. In order to minimize location errors, temporary GPS ground control stations were deployed by Ohio State University, UNAVCO, and student volunteers from local universities to augment the available continuous GPS arrays operated in the study area by the Bay Area Regional Deformation (BARD) Network and the Plate Boundary Observatory (PBO). The vegetation cover varies along the fault zone: most of the vegetation is non-native species. Photographs from the 1860s show very little tall vegetation along the fault zone. A number of interesting geomorphic features are associated with the Hayward Fault, even in urbanized areas. Sag ponds and push up ridges can easily be followed along the fault zone, as well as more subtle features. Landslides along the western flanks of the East Bay Hills were also imaged. We expect that these new LIDAR images will allow us to detect subtle geomorphic features associated with active faulting that may reveal previously undetected active strands or better delineate active strands in areas of pervasive landsliding (as well as better mapping of the landslides themselves). We also anticipate that they will aid in land use planning and identification of new paleoseismic sites. The LIDAR data are freely available at www.earthscope.org. http://www.earthscope.org/