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4.13: Faults, Earthquake Faults, and Earthquakes in Southern California

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    • Contributed by Miracosta Oceanography 101
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    Faults, Earthquake Faults, and Earthquakes in SoCal (Southern California)

    Southern California is a very geologically active region. The maps below are very useful for understanding the nature of earthquake hazards in the region. Figure 4.43 shows the location of major historic earthquakes including regions where the major fault displayed surface ruptures and the number of years between major ground-rupturing events where they've been studied in important locations.

    Figure 4.44 is a map showing SoCal's regional seismic activity as illustrated with the location of earthquake data recorded between the years of 1970 to 2010. It is interesting to study this map to see which faults, or fault systems were most active within this time window. Faults that do not show a lot of seismic activity on this map may indicate three possible scenarios: 1) the fault is no longer active, 2) the fault already experience an earthquake, and has released most of its stored up energy before 1970, or 3) the fault is locked up and is potentially going to possibly create a major earthquake in the future. It is interesting to study the landscape geography (both topography and bathymetry) relative to the location of the faults on this map. In most cases, the faults are associated with a mountain front (both on land and offshore).

    Figure 4.45 shows a map of some of the major earthquake faults in Southern California, displaying characteristics of the faults below the surface. Faults shown as narrow lines are have a vertical orientation, whereas the wider lines show that the faults penetrate into the crust at a low angle (thrust faults). Many of the fault show a component of both horizontal or vertical segments. Almost all the faults are interconnected with other faults in the region. These maps show that the potential for major earthquake may occur both on land or offshore. The ones located offshore could possibly generate massive tsunamis along the SoCal coastline.

    Map showing the location of major earthquake faults, many of which showed ground rupture. Map of earthquake faults and earthquake epicenters (1970 to 2010) in Southern California. Map of low-angle (thrust) and high angle faults in Southern California.
    Figure 4.43. Map showing the location of California's major historic earthquakes including those that displayed significant ground rupture (before 2016). Figure 4.44. Map showing the location of major faults and the epicenters of earthquakes (1970 to 2010) in Southern California. The base map of this image displays the rugged nature of the landscape associated with this fault zones as well as the bathymetry of the borderlands associated with islands offshore. Figure 4.45. Map showing the detail of many of the faults in Southern California. The width of the color-shaded areas of the different faults shows the general angle that these earthquake faults descend into the crust. Vertical fault are narrow lines, whereas low-angle thrust faults are wider. Some faults show both low-angle and vertical components in different segments of the faults.