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A small, but noticeable earthquake of Magnitude 3.6 struck San Francisco, CA minutes ago, at 10:18 PM eastern US time (7:18 PM local time) at a very shallow depth of only 8km.  The quake is small compared to others that take place in California, but it struck very near the densely populated city, and is taking place while other severe earthquake and volcano activity is also happening in Hawaii.   

Geologists and others are wondering if the shaking and eruptions in Hawaii are having an effect on the unstable tectonics of the US west coast; in this case, the Hayward Fault in California?

Hayward Fault

The Hayward fault is a historically active, right–lateral, strike–slip that extends from San Pablo Bay to the Evergreen area east of San Jose, and closely follows the western front of the East Bay Hills. The Hayward fault runs through some of the most densely populated areas in California, making it a significant seismic hazard to both people and infrastructure. It is a part of the San Andreas Fault System, a group of both major and minor faults in western California that, at depth, compose the boundary between the North American and Pacific plates. The San Andreas Fault System accommodates the majority of the movement between the Pacific and North American plates, releasing accumulated stress and strain through earthquakes and aseismic creep.

The most recent major earthquake on the Hayward fault occurred on October 21st of 1868 and is estimated to have been between magnitude 6.8 and 7 and is thought to have caused 40 km of surface rupture on the southern part of the fault. Despite the relatively sparse population, it killed 30 people and caused significant damage throughout the Bay Area due to poor construction methods. The damage was concentrated in the East Bay, particularly near Hayward. Prior to the famous 1906 earthquake, the 1868 earthquake was known as the ‘Great San Francisco Earthquake’. This area has urbanized significantly in the ~150 years since that earthquake, increasing the risk associated with a potential repeat event. Geologic studies at Tule Pond (Fremont) on the southern segment of the Hayward Fault have shown that the average interval between the past 5 earthquakes has been 140 ± 50 years. The average interval of the past 11 earthquakes on this segment of the fault is 170 ± 80 years.

The Hayward Fault is an example of a creeping fault in the Bay area; some stress is released gradually by steady, small movements at the surface that are not usually associated with any earthquake activity. Much of the Hayward Fault near the ground surface experiences between 2.7 and 7.6 mm of creep each year (McFarland et. al., 2016). The highest creep rates on the Hayward fault are found on its southern portion near Fremont, CA. Creep at the ground surface on the Hayward fault is expressed by deformed or offset roads, curbs and buildings.

The 2014 Uniform California Earthquake Rupture Forecast 3 (UCERF 3) estimates that there is a 33% chance of a M 6.7 or greater EQ in the next 30 years on the Hayward–Rodgers Creek fault system.


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