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Introduction to the Hayward Fault
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Although regional inhabitants had known about or experienced many earthquakes, the Hayward Fault “introduced itself” to the modern world in the form of the 1868 Hayward earthquake. The 1868 earthquake was originally called the Great San Francisco earthquake prior to the 1906 earthquake on the San Andreas Fault. The name, Hayward Fault, wasn’t applied until A.C. Lawson used the term in his 1908 report to the new California Earthquake Commission after the 1906 San Francisco earthquake.
The Hayward Fault is a major earthquake fault that runs for nearly 50 miles (70 km) through the East Bay of the San Francisco Bay region (Fig. 2-1). The fault and its associated greater system of Bay Area faults, have been known to generate great earthquakes in the past, and will continue doing so in the future. For its length, the Hayward Fault has probably been studied more than any fault in the world. And for good reason—the Hayward Fault is a known killer. The fault runs through, or near to, some of the most densely urbanized areas in North America. These also support a large portion of California's economy—the region encompassing the fault is host to a maze of transportation, energy, water, telecommunications, waste disposal, and emergency infrastructure that supports millions of people.
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Location map of the Hayward Fault
Click here for a larger view. Fault systems in the San Francisco Bay region are shown on a satellite composite image modified from Google Earth. Faults that have had major earthquakes in historic times (since 1776) are shown in red. Fault that display movement or earthquake history in roughly the last 10,000 years (Holocene Epoch) are shown in orange. Older faults that display evidence of having activity in the last 2 million years (Quaternary Period) are shown in yellow. Fault data from Graymer and others, 2006. (Fault data from Graymer and others, 2006)
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| Earthquakes and ShakeMap of the 1868 Earthquake |
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Click on images for a larger view. |
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The Hayward Fault has a long and complex geologic history, and the character and history of the fault is continuously debated. Like almost all major earthquake faults in the San Francisco Bay region, the Hayward Fault is a right-lateral strike-slip fault, with the western side of the fault moving northward relative to the eastern side. The modern "Hayward Fault" is perhaps tens of millions of years old, but also is imprinted on older fault systems that formed millions of years earlier. Rocks of many ages and compositions are found along the fault—some of them are "far traveled" by the cumulative movements associated with the plate-tectonic evolution of western North America. The East Bay fault system (which includes the Hayward Fault) has accumulated as much at 107 miles (175 km) of right-lateral offset in the last 12 million years. Of this total, the Hayward Fault is responsible for about 60 miles (100 km) of the offset, with 60 kilometers having occurred in the past 6 million years (Graymer and others, 2002).
The southern end of the Hayward Fault is complex. At depth in the region south of Fremont, the Hayward Fault merges with the Central Calaveras Fault. In this region, the fault plane of the Hayward Fault dips gently at the surface, but grows increasingly steeper with depth (based on seismic and other geophysical data—see the geologic block diagram and map_hf4.html at Mission Peak near Fremont). Farther north, the plane of the Hayward Fault is steeper, approaching nearly vertical in profile (see the cross sections near San Leandro, Berkeley, and Richmond). At Point Pinole Regional Shoreline, the Hayward Fault runs offshore beneath San Pablo Bay. Beneath San Pablo Bay, the Hayward Fault probably merges with other fault systems in the North Bay, possibly the Rodgers Creek Fault.
A Creepy Fault
In earthquake terminology, creep is the slow, more or less continuous movement occurring on faults due to ongoing tectonic deformation that doesn’t happen during major earthquakes. The Hayward Fault is actively moving, year-by-year. This is unlike other earthquake faults in the region that are locked between major earthquakes (like the San Andreas Fault in the Santa Cruz Mountains, San Francisco Peninsula, and north of the Golden Gate), or faults that are inactive or typically ancient faults that are no longer active. Detailed USGS investigations into movements along the Hayward Fault show the fault creep averages about a half-inch (7 mm) per year. However, this creep occurs at different rates and in different locations and can also vary significantly from one year to the next. Just as creep is an observable surficial phenomenon, fault motion is also taking place at different levels in the earth extending downward to the base of the brittle crust—roughly 8 miles (12 km) in the San Francisco Bay region.
Just as surface creep varies, motion rates at depth vary as well. Some areas-at-depth along the fault zone may be temporarily locked whereas other locations are gradually moving. When stresses build up in the locked portions the fault will eventually rupture, or break, releasing energy in the form of an earthquake. Small, almost imperceptible earthquakes happen daily along the Hayward and other Bay Area faults. Larger (and fortunately, less frequent) earthquakes typically occur deeper in the crust or when larger locked sections of the fault yield to rupture. For more information about earthquakes and current earthquake activity in the region, check out the USGS Maps of Recent Earthquake Activity in California-Nevada (http://quake.usgs.gov/recenteqs/latestfault.htm).
Geologic cross sections across the Hayward Fault (with Google Earth satellite image for reference).
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| Mission Peak (near Fremont) |
San Leandro/south Oakland |
Berkeley |
Richmond/Point Pinole |
| Click on images for a larger view. |
Observing the Hayward Fault
The selected fieldtrip destinations described in this report include access to the fault in public places in regional parks, college campuses, public institutions, or in urban areas where features can be accessed from street, sidewalk, or in parking areas. Some of the selected destinations described in this report are not on the Hayward Fault. However, they are included because they provide information about the regional geology and geologic history, landscape features, or important infrastructure or historical sites that share a heritage with the earthquakes and creep history associated with the Hayward Fault.
Fault and landscape features |
Places where best to see them |
Offset curbs, sidewalks, and walls |
Contra Costa College, Berkeley Memorial Stadium, Monclair Village, downtown Hayward, and Fremont Central Park area |
En-echelon cracks |
Contra Costa College, downtown Hayward, Arroyo Agua Caliente Park (South Fremont) |
Fault scarps |
Point Pinole, Hayward |
Sag Ponds |
Lake Temescal (Berkeley), Tule Lake and Lake Elisabeth (Fremont) |
Linear valleys, shutter ridges |
Lake Temescal, Hayward, views from Oakland Hills |
Landslides |
Point Pinole, Mission San Jose (Mission Peak Landslide) |
Cultural or historic features |
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Historic buildings from 1868 era |
Downtown San Leandro, Mission San Jose |
Museums and exhibits |
Mission San Jose, Tule Pond, Hayward Area Historical Society Downtown Museum, Oakland Zoo |
Field trips to examine the Hayward Fault show the relationships of the fault to the landscape that it has created. Large geomorphic features associated with fault systems, such as rift valleys and linear mountain fronts, may be easy to observe from a distance. However, many of the features that can be observed up close along the fault trace are subtle and may not be recognizable without guidance. Features such as cracks in pavement or sidewalks occur practically everywhere, and most form from ground settling after construction. In some cases, offsets in sidewalks and curbs are generated by rapid tree growth or the natural down slope movement of soil and rock rather than by the slow movements associated with creep along a fault. Also, the idea that a fault can be drawn as a simple narrow line on a map is often different from what can be observed on the ground. Fault traces can be complex—on a map scale they may appear as a line, but up close they may appear complex because of the variable physical properties of surficial materials (such as asphalt, concrete, buildings, soil, bedrock, etc.). Note that the term creep is also used to describe the movement of soil and landslide deposits. Faults and landslides produce many similar landscape features and may occur together. However, fault movement is caused by stresses from tectonic forces in the crust, whereas landslides are surficial processes that move materials downslope under the force of gravity.
The trace of a fault trace can change over time—measured in periods of thousands to millions of years. The trace of a fault evolves, splits, may move to a more "convenient" (easier to rupture) location, or may even stop due to local bedrock conditions. Meanwhile, erosion is competing with the tectonic forces that are moving one-side-of-the-fault versus the other, or uplifting in one area while sinking (subsiding) in another. Over long periods of time, the impact of movement along the East Bay fault system has also resulted in the uplift of mountains (the Berkeley-Oakland-or-East Bay Hills. Meanwhile, both surficial erosion and sediment deposition, and sea level changes (associated with the formation and melting of continental ice sheets), have impacted the surface of the land between what is now the hills (to the east) and the bay (to the west). For most of its length the Hayward Fault crosses (and offsets) old alluvial fans descending from the hills onto a gentle sloping surface (locally called the Piedmont) before merging with the bays. Year-by-year, and millennium-by-millennium, streams have carved channels down the slope, and across the trace of the fault. Likewise, the fault has continued to move, either by slow-moving surface creep or the sudden jump during a major earthquake every century-or-two (1-2 meters offsets are typical during great earthquakes). These competing forces, tectonics versus erosion, have gone on in the past, and will continue into the future. These processes have created many of the landscape features along the trace of the fault.
The critical factor is when, where, what, and how a major earthquake on the Hayward Fault will impact the San Francisco Bay Area. Worse case scenarios are dire, and negative impacts will affect the entire nation's economy and social fabric. The most likely scenario is a repeat of an earthquake like the 1868 earthquake that ruptured a significant portion of the Hayward Fault (from Fremont to Berkeley). Geologist suggest that the Hayward Fault and the Rodgers Creek Fault in Sonoma County (north of San Pablo Bay) are probably interrelated, although their trends are offset by several miles in map view. When asked, many geologists who have studied the Bay Area fault systems agree that one of the worst-case scenarios for an earthquake in the Bay Area would be if the Hayward and Rodgers Creek faults were to rupture at the same time.
The recent Alum Rock earthquake of October 30, 2007 (8:05 PM) occurred near the junction of the Hayward and Calaveras faults. The earthquake struck about seven miles east of the city of Milpitas. The moderate earthquake had a magnitude of 5.4 and ruptured at a depth of five miles. This moderate earthquake resulted in minor damage throughout the San Jose and southern East Bay region and briefly received much media attention. However, this earthquake was miniscule compared to the potential for a great earthquake on the Hayward Fault or other major faults in the San Francisco Bay Area.
The most recent great earthquake on the Hayward Fault occurred in 1868, and the estimated frequency of the past five major earthquakes is, on average, about every 140 years (USGS Earthquake Hazard Program, 2007). Simple math illustrates that we are due, almost overdue, for another major earthquake on the Hayward Fault. While the date, duration, magnitude, and regional impact of the earthquake are still speculative, it occurrence is exceedingly probable (USGS Earthquake Hazard Program, 2003). We can't prevent it from happening so earthquake preparedness is essential! See the USGS Earthquake Preparedness website: <http://quake.usgs.gov/prepare/prepare.html>.) |
