Hayward Fault Zone
The Hayward Fault Zone is a right-lateral strike-slip geologic fault zone capable of generating destructive earthquakes. The fault was first named in the Lawson Report of the 1906 San Francisco Earthquake in recognition of its involvement in the earthquake of 1868. This fault is about long, situated mainly along the western base of the hills on the east side of San Francisco Bay. It runs through densely populated areas, including Richmond, El Cerrito, Berkeley, Oakland, San Leandro, Castro Valley, Hayward, Union City, Fremont, and San Jose.
The Hayward Fault is parallel to the San Andreas Fault, which lies offshore and through the San Francisco Peninsula. To the east of the Hayward Fault lies the Calaveras Fault. In 2007, the Hayward Fault was discovered to have merged with the Calaveras Fault east of San Jose at a depth of, with the potential of creating earthquakes much larger than previously anticipated. Some geologists have suggested that the Southern Calaveras should be renamed as the Southern Hayward.
North of San Pablo Bay is the Rodgers Creek Fault, which was shown in 2016 to be linked with the Hayward Fault under San Pablo Bay to form a combined Hayward-Rodgers Creek Fault that is long, stretching from north of Healdsburg through Santa Rosa down to Alum Rock in San Jose. Another fault further north, the Maacama Fault, is also considered to be part of the "Hayward Fault subsystem".
While the San Andreas Fault is the principal transform boundary between the Pacific plate and the North American plate, the Hayward-Rodgers Creek Fault takes up its share of the overall displacement of the two plates.
Tectonic setting
The Pacific plate is a major section of the Earth's crust, gradually expanding by the eruption of magma along the East Pacific Rise to the southeast. It is also being subducted far to the northwest into the Aleutian Trench. In California, the plate is sliding northwestward along a transform boundary, the San Andreas Fault, toward the subduction zone. At the same time, the North American plate is moving southwestward relative to the Earth's core, but southeastward relative to the Pacific plate, due to the latter's much faster northwestward motion. The westward component of the North American plate's motion results in some compressive force along the San Andreas and its associated faults, thus helping lift the Pacific Coast Ranges and other parallel inland ranges to the west of the Central Valley, in this region most notably the Diablo Range. The Hayward Fault shares the same relative motions of the San Andreas. As with portions of other faults, a large extent of the Hayward Fault trace is formed from a narrow complex zone of deformation which can span hundreds of feet in width.The transform boundary defined by the San Andreas Fault is not perfectly straight, and the stresses between the Pacific and North American plates are diffused over a wide region of the West, extending as far as the Walker Lane east of the Sierra Nevada. The Hayward Fault is one of the secondary faults in this diffuse zone, along with the Calaveras Fault to the east and the San Gregorio Fault, west of the San Andreas.
The complete fault zone, including the Rodgers Creek fault, is divided by seismologists into three segments – Rodgers Creek, Northern Hayward, and Southern Hayward. It is expected that these segments may fail singly or in adjacent pairs, creating earthquakes of varying magnitude. The Association of Bay Area Governments in concert with other government agencies has sponsored the analysis of local conditions and the preparation of maps indicative of the destructive potential of these earthquakes. The various ABAG maps shown below represent some of the more likely possible combinations.
While there are indications that a substantial earthquake on a nearby parallel fault can release stress and so also decrease the near-term probability of an earthquake, the opposite appears to be true concerning sequential segments. A release on a major segment can substantially increase the likelihood of an earthquake on an adjacent fault segment, increasing the likelihood of two major regional earthquakes within a period of a few months.
Rodgers Creek Fault Zone
The connection between the Rodgers Creek Fault Zone and the Hayward Fault Zone was unclear until 2015 when a survey of the floor of San Pablo Bay found that the ends of the two faults were smoothly linked between Point Pinole and Lower Tubbs Island. An alternate prior hypothesis suggested that the Hayward Fault and Rodgers Creek Fault were probably connected by a series of en echelon fault strands beneath San Pablo Bay. The new finding means that the Rodgers-Hayward system together could produce a quake with a magnitude as high as 7.2. It is also considered possible that a major seismic event on either fault may involve movement on the other, either concurrently or within an interval of up to several months. The Association of Bay Area Governments has prepared ground shaking maps that include a possible concurrent scenario.In October 2016, scientists found definitive evidence that the Rodgers Creek Fault and the Hayward Fault are linked together under San Pablo Bay. A simultaneous rupture of the connected Hayward-Rodgers Creek Fault – about 118 mi long from just north of Healdsburg down to Alum Rock in San Jose – could result in a major earthquake of magnitude 7.4 that "would cause extensive damage and loss of life with global economic impact". It has been suggested that the name "Rodgers Creek Fault" be retired and that the entire 118-mile fault be known as the "Hayward Fault".
Calaveras Fault
The Calaveras Fault is continuous from the Sunol area south to Hollister. It was long believed that there was no connection between the Hayward Fault and the Calaveras, but geological studies suggest that the two may be connected. If true, this link would have significant implications for the potential maximum strength of earthquakes on the Hayward, since this strength is determined by the maximum length of the fault rupture and this rupture could extend beyond the juncture point and so include some portion of the Calaveras.Earthquakes
The largest quake on the Hayward Fault in recorded history occurred in 1868, with an estimated magnitude of 7.0. It occurred on the southern segment of the fault, receiving its name from the nascent town of Hayward where it was determined the quake's epicenter was located. However, the 1868 quake caused much damage throughout the then sparsely settled Bay Area, including the city of San Francisco. In fact, the 1868 event became known as the "Great San Francisco earthquake" until the larger tremor in 1906.Many seismologists believe that the 1906 San Francisco earthquake, which occurred on the San Andreas fault, reduced the stress on many faults in the Bay Area including the Hayward fault, creating an "earthquake shadow": a quiescent period following a major earthquake. 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.
The 1868 earthquake occurred well before the East Bay region was extensively urbanized. The following year, in 1869, the William Meek Estate became one of the first developments in the area, built on 3,000 acres in what became known as the Cherryland district of Eden Township. Recent renovations of the Meek Mansion have revealed that with the 1868 earthquake still fresh in minds of residents of the time, some unusual diagonal bracing was built into the original construction. Although its magnitude was less than the 1906 San Francisco earthquake, the intensity of shaking experienced in the Hayward area may have been greater than in 1906 due to the proximity of the Hayward Fault.
Earlier earthquakes have been detected by trench exposure and associated radiocarbon dating. Combined with the historic record, the last five major events were in 1315, 1470, 1630, 1725, and 1868, which have intervals of about 140 years. The longest time was the 160-year period between 1470 and 1630. As of, it has been years since the 1868 event.
Probability of future activity
scientists state that a major earthquake occurring on the zone is "increasingly likely". When the next major earthquake occurs on the fault, damage will be catastrophic. More than 1.5 trillion U.S. dollars in property exists in the affected area, and more than 165 billion US dollars in damage would likely result if the 1868 earthquake were to reoccur. Since the fault runs through heavily populated areas, more than 5 million would be affected directly. Water could be cut off to 2.4 million people living in California's San Francisco Bay Area.For the thirty years following 2014, the probability of there being one or more magnitude 6.7+ earthquakes on the Hayward Fault during that time frame was estimated at 14.3 percent. This is compared to 6.4 percent for the San Andreas Fault, which can have larger earthquakes but is farther away from a significant portion of the urbanized parts of the Bay Area. Earlier assessments suggest that the Hayward, Rodgers Creek, and Calaveras faults may be more likely to fail in the next few decades than previously thought.
The 140th anniversary of the 1868 event was in 2008, and the average time between the last five major events is also averaged at 140 years. Recent estimates of the damage potential of a major Hayward Fault earthquake by a professional risk management firm indicate the potential for huge economic losses, of which only a small percentage is insured against earth movement..
Image:BayareaUSGS.jpg|thumb|left|225px|USGS satellite photo of the San Francisco Bay Area. Light gray areas are heavily urbanized regions.
Depending upon seasonal weather conditions at the time of a major event a seismic event could be followed by urban wildfires compounded by damage to water systems or massive landslides in saturated soils. In addition to direct damage the effects on commerce due to damaged infrastructure would also be substantial. Experience with large area urban destruction such as caused by earthquake, hurricane, and firestorms has shown that complete rebuilding can take up to a decade, owing to various factors.
The progressively more severe reports and estimates of event probability and consequences have awakened a broad interest in training people for emergency response. It is becoming widely understood that professional fire fighting, police, and medical services will be overwhelmed by a major event and that neighbors will have to assist each other as best they can. Several jurisdictions in the affected area have implemented volunteer Community emergency response team programs to augment the professional response services.
In 2012, USGS scientists said the fault was due for another magnitude 6.8 to 7.0 earthquake, with the California Geological Survey concurring, stating they believe there is a 31 percent chance of a magnitude-6.7 earthquake or greater along the Rodgers Creek-Hayward Fault in the next 30 years.
In March 2015, the United States Geological Survey released "UCERF3: A New Earthquake Forecast for California's Complex Fault System". The UCERF3 represents the best available science to date, and it now considers "multifault ruptures" and "fault readiness", in addition to historical seismicity, in the calculus of earthquake forecasting. The upshot, for those who live in the San Francisco Bay Area, is that experts say there is a 72% chance of experiencing a magnitude 6.7 or greater earthquake before 2045. Moreover, they had a 51% chance of a M≥7, a 20% chance of a M≥7.5 and a 4% chance of a M≥8 when all the mapped faults in the region are taken in to account.