Hayward Fault Zone
The Hayward Fault Zone is located in northern California in the San Francisco Bay Area. It is parallel to and east of its more famous (and much longer) sister fault, the San Andreas Fault. To the east of the Hayward Fault lies the Calaveras Fault and beyond that the Clayton-Marsh Creek-Greenvile Fault and their northern and southern extensions via other faults. These four fault structures are the major faults in California at the latitude of San Francisco.
For recent activity in the region shown above see the USGS map for this location. The "live" maps will also show all of the names of faults shown on the map as you rollover with the cursor.
The nearest aligned fault to the north, the Rodgers Creek Fault Zone, is considered by many experts to be an extension of the Hayward Fault Zone in that a massive rupture may result in slippage on both these fault segments as well as the currently undisclosed connection between them - under San Pablo Bay. The Association of Bay Area Governments (ABAG) has prepared ground shaking maps that include this possible scenario (these are shown below to the right)
As the Pacific Plate moves slowly northward under the forces of plate tectonics, these faults are both locked and stressed and periodically one will rupture over some portion of its length, releasing energy in the form of earthquakes. Most of the earthquakes are small and fairly frequent (in a geological time scale). The last truly major earthquake in the region was the 1906 San Francisco Earthquake, its 16 foot (5meter) fault movement causing substantial destruction of masonry buildings prior to the subsequent and far more destructive fire in the city. Much of the substantial destruction due to ground motion in the larger region was little noted in the reporting. It is apparent that the 1906 earthquake reduced the stress on the Hayward fault, creating an "earthquake shadow". Since the 1906 San Andreas event there have been no moderately strong earthquakes on the Hayward fault as were seen before that earthquake. It also appears likely that this quiet period in the earthquake shadow is ending, as projected by the rate of plate motion and the stress state of other faults in the region. Of all the region's large faults, the Haward + Rodgers Creek fault system is considered most likely to create a major destructive earhquake. Lesser destructive earthquakes have been occurring in the region at random 15 to 30 year intervals - typically causing spillage of merchandise and occasionally, structural failures in lower stories and chimneys. This type of event is considered normal in California's "Earthquake Country" by geologists and long term natives, although disquieting to recent immigrants to the region. Most aware residents just live with the conditions and are glad to not experience tornados and hurricanes. There is an element of denial among most residents as they are largely under-prepared for a major event. A major event on either the Haward or San Andreas could produce a minute or more or intense shaking, as was felt in the great Kobe earthquake. The ground conditions in that region of Japan are quite similar to those in the East Bay and that earthquake destroyed what where considered to be modern and well engineered structures. This was seen especially in the failure of elevated urban road structures due to soil failure.
The magnitude of an earthquake, as measured by the Richter magnitude scale, is proportional to the length of the rupture, while the ground motion in a large region surrounding the fault is highly dependant upon the local soil conditions and (as recently recognized in the Loma Prieta earthquake) reflected energy from deep discontinuities in the earth's structure.
The most recent major earthquake on the Hayward Fault occurred in 1868, well before the East Bay region was extensively urbanized. Some believe that this earthquake may have in fact been of greater intensity than that of the San Andreas 1906 event, particularly insofar as its perceived ground motion is concerned.
For additional ground hazard maps similar to those shown at the right see the ABAG map index for this location. Some maps on this site are for more specific regions within the larger area.
The Hayward fault is considered to be particularly dangerous due to the poor soil conditions in the alluvial plain that drops from the East Bay Hills to the eastern shoreline of San Francisco Bay. At the lower elevations near the bay the soil is mostly water saturated mud and sand, which tends to amplify the effects of an earthquake and so producing significantly greater ground motion. Additionally, the soil itself can fail, turning into a liquid mud from the agitation, a mud unable to support buildings erected upon once-firm soil. This region is also covered with dense low-rise urban development, most of which was built soon after the 1906 San Francisco earthquake, but before good earthquake resistant construction practices had been developed. All though many structures have undergone seismic retrofitting there are a large number of dangerous un-reenforced masonry (mostly brick) structures and chimneys, which can be extremely hazardous to occupants in a large earthquake.
In its northern extent the Hayward Fault lies directly beneath the portion of highway 13 that is south of highway 24, extending north directly under Lake Temescal and further under the centerline of the football field of Memorial Stadium at the University of California, Berkeley. There are no plans at this time to replace the stadium, even though its "O" shape may possibly be split into two "C"s. A suitable site for a replacement is available immediately to the east in Wildcat Canyon, although this would require an east-west alignment rather than the traditional north-south orientation. While there is only a small probablity of an earthquake on this fault while the stadium is occupied the results could be deadly.
Some of the cities in the eastern bayshore and south bay region near this fault include Richmond, Berkeley, Emeryville, Oakland, San Leandro, San Lorenzo, Castro Valley, Hayward, Fremont, Milpitas, Niles, and portions of San Jose.
Similar dangerous soil conditions and insufficiently resistant buildings are also on the southern, western and northern boundaries of San Francisco and San Pablo bays and would also be severely affected by a major earthquake on the Hayward fault. As that portion includes the so-called Silicon Valley, the potential economic disruption due to destruction or works in progress and the dismantling of microelectronics fabrication plants could have an economic effect extending worldwide. The current estimates of the probabliity of a major earthquake range up to 70 percent within the thirty year period 2000-2029. A recent quiet period following many years of minor activity is considered to be particularly ominous by many, although geologists have not yet been able to predict earthquakes with any useful accuracy. They do warn that all residents of the region should be prepared for a large event and its subsequent effects (e. g., lack of water, firefighting, first aid, etc.) and that much life-safety protective work remains to be done.
It is the likelihood of a severe earthquake on the Hayward Fault (rather than the San Andreas Fault) that has spurred a substantial effort to retrofit and sometimes replace large structures at risk, particulary the San Francisco-Oakland Bay Bridge, the San Francisco and Oakland city halls, the Bay Area Rapid Transit under-bay tube slip joint, and numerous elevated rail, road, and pedestrian structures and overpasses. Much work remains to be done in the region and progress may be hampered by budget constraints imposed by trickle down federal-state-regional deficits, design and construction delays due to state and local political bickering over design, and unexpectedly high steel and concrete costs due to the extensive construction work being done in China.
External link
- http://eastbayexpress.com/issues/2005-02-16/news/feature_print.html It's Not Our Fault — An exceptional essay on the origins, frequency, and effects of large earthquakes in the S. F. Bay area.