California's Hayward Fault Revealed: Most Dangerous Urban Fault In America?

The Hayward Fault has ruptured about every 140 years for its previous five large earthquakes. October 21, 2008, marks the 140th Anniversary of the 1868 approximate Magnitude 7 earthquake. Two and half million people now live along the Hayward Fault and seven million people in the region would feel a repeat event of the same magnitude.

The US Geological Survey presented new studies at the American Geophysical Union's annual conference in San Francisco on December 11.

Scientists explained the significance of new high resolution elevation models, imagery of active deformation and creep, new results of age-dating studies, refinements and innovations to an 1868 earthquake ShakeMap, 3-D geometric and mechanical models, and ground motion simulations showing rupture along the fault.

The October 21, 1868 Hayward Earthquake -- 140 Years Later

This large earthquake caused extensive damage to the San Francisco Bay Area and remains the nations 12th most lethal earthquake, resulting in about 30 fatalities.  The shaking from this earthquake was the strongest that the the new towns and growing cities of the Bay Area had ever experienced, producing devastating effects on brick buildings and walls and cracking buildings as far away as Napa, Santa Rosa, and Hollister. 

Analysis of triangulation data suggest that the fault moved as far north as Berkeley with an average inferred slip rate of 1.9 m.  The average interval between the 10 earthquakes before 1868 is 170 years, with the last five earthquakes having an average interval of only 140 years.  The population at risk from a large Hayward fault earthquake is now 100 times greater than in 1868 and the infrastructure in the San Francisco Bay Area has been tested only by the relatively remote 1989 magnitude 6.9 Loma Prieta earthquake. 

Liquefaction Scenarios in the Northern Santa Clara Valley

The spatial distribution of the probability of liquefaction in the northern Santa Clara Valley was predicted for a repeat of an earthquake like the 1868 Hayward fault for surficial geologic units and scenario maps were produced depicting the estimated probability of liquefaction.  For a magnitude 7 earthquake probabilities range from 0.1 to 0.2 along Coyote and Guadalupe Creeks, but are less than 0.05 throughout the Valley.  Liquefaction probabilities are substantially higher for a magnitude 7.8, 1906-type earthquake, along the San Andreas fault, exceeding 0.3 along Coyote and Guadalupe Creeks for a 1.5 m water table depth.  Predicted probabilities are highest in areas where liquefaction and lateral spreading were reported following the 1868 and 1906 earthquakes. 

Three-Dimensional Geologic Model of the Hayward-Calaveras Fault Junction

A 3-D geologic model was presented depicting the Hayward-Calaveras fault junction as a complex network of faults at the surface and as a single through-going fault below about 5 km. Deep seismicity suggests a simple connection between the southern Hayward and central Calaveras Faults, whereas geologic mapping at the Earth's surface shows a complex zone of deformation between the fault traces.  The new 3-D geologic model can be used as a basis for earthquake process modeling and local and regional ground motion simulations, which will aid in further characterizing the potential seismic hazard along the Hayward-Calaveras fault system.  The increased length of a combined southern Hayward and central Calaveras fault rupture could generate an earthquake greater than magnitude 7 based on magnitude-area relationships, posing a significant seismic hazard to the San Francisco Bay region.

Segmentation of the Calaveras-Hayward Fault System 

For the purpose of estimating seismic hazard, the Calaveras and Hayward Faults have been considered as separate structures and analyzed and segmented based largely on their surface-trace geometry and the extent of the 1868 Hayward Fault earthquake.  Recent relocations of earthquakes and 3-D geologic mapping have shown, however, that at depths associated with large earthquakes the fault geology and geometry is quite different than that at the surface.  Using default geometry and inferred from these studies the Hayward and Calaveras Faults are treated as a single system and divided into segments that differ from the previously accepted segments.  This segmentation is similar to that based on the extent of the 1868 fault rupture, but is now related to an underlying geologic cause. 

The direct connection of the Hayward and central Calaveras faults at depth suggests that earthquakes larger than those previously modeled should be considered.  The additional segmentation of the central Calaveras fault proposed here may explain the observation that this segment seems to generate characteristic moderate earthquakes rather than the larger earthquakes that could be generated by rupture of the longer central Calaveras segment.

Geophysical Investigations along the Hayward Fault and Their Implications on Earthquake Hazards

Geophysical studies indicate that the Hayward fault follows a pre-existing basement structure and that local geologic features play an important role in earthquake seismicity.  The recent creeping trace of the Hayward fault extends from San Pablo Bay to Fremont, and together with its northern extension, the Rodgers Creek Fault, is regarded as one of the most hazardous faults in northern California.  The Hayward fault is predominantly a right-lateral strike-slip fault and is characterized by distinct linear gravity and magnetic anomalies that correlate with changes in geology, structural trends, creep rates, and clusters of seismicity. 

These correlations indicate the existence of fault-zone discontinuities that probably reflect changes in mechanical properties, and may play a role in defining fault segment locations where recurring seismic ruptures may tend to nucleate or terminate.  Combined modeling and seismicity data indicate that the dip of the fault surface varies and ultimately connects with the central Calaveras fault.  An earthquake cluster and a bend in the fault associated with a gravity and magnetic high along a distinct mafic geologic feature suggests that this feature influences fault geometry and behavior, and may serve as a nucleation point for large earthquakes on the fault.

A Virtual Tour of the 1868 Hayward Earthquake In Google Earth

The 1868 Hayward earthquake has been overshadowed by the subsequent 1906 San Francisco earthquake that destroyed much of San Francisco.  Nonetheless, a modern recurrence of the 1868 earthquake would cause widespread damage to the densely populated Bay Area, particularly in the East Bay communities that have grown up virtually on top of the Hayward fault.  Scientific concern has been heightened by paleoseismic studies suggesting that the recurrence interval for the past five earthquakes on the southern Hayward fault is 140-170 years. 

An educational Web site is being constructed using Google Earth to illustrate the cause and effect of the 1868 earthquake drawing upon scientific and historic information to visually illustrate complex scientific concepts in a way that is understandable to a non-scientific audience.  The website will provide information about regional plate tectonics and faulting in western North America, to more specific information about the 1868 Hayward earthquake including models of ground shaking and historic photographs. Earthquake engineering concerns will be stressed, including population density, vulnerable infrastructure, and lifelines.  

Constraints on the Rupture of the 1868 Hayward Earthquake 

The 1868 Hayward earthquake was the most damaging earthquake to occur in California in the half-century following the 1848 annexation, shattering the city centers of Oakland and San Francisco, and cracking brick buildings as far away as Santa Rosa and Gilroy.  A reevaluation of modified Mercalli intensity at sites with damage or felt reports, together with additional reports from newspapers and historical narratives, was used to construct a ShakeMap along the Hayward fault and throughout the greater Bay Area and the San Joaquin Delta.  Surprisingly, the highest intensities are clustered near the middle of the fault rupture in Hayward, San Leandro, and San Lorenzo. 

The intensities are lower at the ends of the fault rupture, in Berkeley and Warm Springs.  Intensities observed a regional distances suggest that the rupture was stronger to the northwest towards Petaluma and Martinez then to the southeast towards Calaveras Valley and Gilroy.  The relatively low intensities in Oakland and Berkeley suggest that the shallow locked zone near Piedmont did not rupture in the earthquake.  Given the large proportion of aseismic slip on the 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.

New Airborne LiDAR Survey of the Hayward Fault, Northern California

A digital elevation model (DEM) is presented constructed from newly acquired high-resolution Light Detection and Ranging (LiDAR) data along the Hayward Fault in northern California.  Airborne LiDAR data were collected within a 106 km long by 1 km wide swath encompassing the Hayward fault from San Pablo Bay 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 and, with its most recent rupture in 1868, 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 detailed analysis of fault slip.  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, and landslides along the western flanks of the East Bay Hills are apparent in the imagery.  It is expected that these new LiDAR images will allow detection of subtle geomorphic features associated with active faulting that may reveal previously undetected active strands or better delineate active strands in areas prone to  landslides, as well as allowing better mapping of the landslides themselves.