October 21, 2008 marks the 140th anniversary of the M7 1868 Hayward earthquake. This large earthq... more October 21, 2008 marks the 140th anniversary of the M7 1868 Hayward earthquake. This large earthquake, which occurred slightly before 8 AM, caused extensive damage to San Francisco Bay Area and remains the nation's 12th most lethal earthquake. Property loss was extensive and about 30 people were killed. This earthquake culminated a decade-long series of earthquakes in the Bay Area which started with an M~6 earthquake in the southern Peninsula in 1856, followed by a series of four M5.8 to M6.1 sized earthquakes along the northern Calaveras fault, and ended with a M~6.5 earthquake in the Santa Cruz Mountains in 1865. Despite this flurry of quakes, the shaking from the 1868 earthquake was the strongest that the new towns and growing cities of the Bay Area had ever experienced. The effect on the brick buildings of the time was devastating: walls collapsed in San Francisco, Oakland, and San Jose, and buildings cracked as far away as Napa, Santa Rosa, and Hollister. The area that was strongly shaken (at Modified Mercalli Intensity VII or higher) encompassed about 2,300 km2. Aftershocks continued into November 1868. Surface cracking of the ground along the southern end of the Hayward Fault was traced from Warm Springs in Fremont northward 32 km to San Leandro. As Lawson (1908) reports, "the evidence to the northward of San Leandro is not very satisfactory. The country was then unsettled, and the information consisted of reports of cow- boys riding on the range". Analysis of historical triangulation data suggest that the fault moved as far north as Berkeley, and from these data the average slip along the fault is inferred to be about 1.9 ± 0.4 meters. The paleoseismic record from the southern end of the Hayward Fault provides evidence for 10 earthquakes before 1868. The average interval between these earthquakes is 170 ± 80 years, but the last five earthquakes have had an average interval of only 140 ± 50 years. The 1868 Hayward earthquake and more recent analogs such as the 1995 Kobe earthquake are stark reminders of the awesome energy waiting to be released from below the east side of the San Francisco Bay along the Hayward Fault. The population at risk from a Hayward Fault earthquake is now 100 times larger than in 1868. The infrastructure in the San Francisco Bay Area has been tested only by the relatively remote 1989 M6.9 Loma Prieta earthquake. To help focus public attention on these hazards, the 1868 Hayward Earthquake Alliance has been formed, consisting of public and private sector agencies and corporations (see their website www.1868alliance.org). The Alliance is planning a series of activities leading up to the 140th anniversary on October 21, 2008. These include public forums, conferences, commemoration events, publications, websites, videos, and public service announcements.
October 21, 2008 marks the 140th anniversary of the M7 1868 Hayward earthquake. This large earthq... more October 21, 2008 marks the 140th anniversary of the M7 1868 Hayward earthquake. This large earthquake, which occurred slightly before 8 AM, caused extensive damage to San Francisco Bay Area and remains the nation's 12th most lethal earthquake. Property loss was extensive and about 30 people were killed. This earthquake culminated a decade-long series of earthquakes in the Bay Area which started with an M~6 earthquake in the southern Peninsula in 1856, followed by a series of four M5.8 to M6.1 sized earthquakes along the northern Calaveras fault, and ended with a M~6.5 earthquake in the Santa Cruz Mountains in 1865. Despite this flurry of quakes, the shaking from the 1868 earthquake was the strongest that the new towns and growing cities of the Bay Area had ever experienced. The effect on the brick buildings of the time was devastating: walls collapsed in San Francisco, Oakland, and San Jose, and buildings cracked as far away as Napa, Santa Rosa, and Hollister. The area that was strongly shaken (at Modified Mercalli Intensity VII or higher) encompassed about 2,300 km2. Aftershocks continued into November 1868. Surface cracking of the ground along the southern end of the Hayward Fault was traced from Warm Springs in Fremont northward 32 km to San Leandro. As Lawson (1908) reports, "the evidence to the northward of San Leandro is not very satisfactory. The country was then unsettled, and the information consisted of reports of cow- boys riding on the range". Analysis of historical triangulation data suggest that the fault moved as far north as Berkeley, and from these data the average slip along the fault is inferred to be about 1.9 ± 0.4 meters. The paleoseismic record from the southern end of the Hayward Fault provides evidence for 10 earthquakes before 1868. The average interval between these earthquakes is 170 ± 80 years, but the last five earthquakes have had an average interval of only 140 ± 50 years. The 1868 Hayward earthquake and more recent analogs such as the 1995 Kobe earthquake are stark reminders of the awesome energy waiting to be released from below the east side of the San Francisco Bay along the Hayward Fault. The population at risk from a Hayward Fault earthquake is now 100 times larger than in 1868. The infrastructure in the San Francisco Bay Area has been tested only by the relatively remote 1989 M6.9 Loma Prieta earthquake. To help focus public attention on these hazards, the 1868 Hayward Earthquake Alliance has been formed, consisting of public and private sector agencies and corporations (see their website www.1868alliance.org). The Alliance is planning a series of activities leading up to the 140th anniversary on October 21, 2008. These include public forums, conferences, commemoration events, publications, websites, videos, and public service announcements.
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