Geologic Setting

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Westline Drive

The Pacific Ocean defines the western boundary of the area. The maximum site elevation is approximately 450 feet along the eastern boundary that coincides with Longview Drive. In the western portion of the area, the topography slopes moderately to very steep from the backyards of residences along Westline Drive, down toward the ocean. Downhill of the escarpment, the topography slopes moderately to very-steeply downward the northwest to the level open space at Mussel Rock. The maximum relief between Westline Drive and open space is approximately 200 feet.

Earthquake history

The shaking intensity for Daly City during the 1906 earthquake was measured at IX-violent to X –very violent (Refer to map # 1). By visiting one of the geologic sites at Mussel Rock, Longview Playground via Longview Drive, one can observe a fault that was formed during the 1906 San Francisco Earthquake. The fault zone consists of hundreds of offsets along the San Andreas Fault. The most recent of these offsets was caused by the 1906 Earthquake. The trace of the 1906 fault crosses Longview Drive 100 feet south of the Longview Playground and then continues northwest down the landslide scarp just to the north of the steep gully below the playground. The trace then crosses the north end of the landfill and goes into the ocean about 500 feet north of the landfill. There is little evidence of the trace now, since it is largely covered by houses, roads, landfill and beach sand (Geologic Trips pg. 117).

A M5.3 earthquake occurred near Daly City on March 22, 1957, the largest event on the northern San Francisco peninsula since 1906. It was located within a kilometer of the "locked" section of the San Andreas Fault and roughly 20 km southeast of the presumed epicenter area of the 1906 earthquake, offshore from the Golden Gate. Available local and regional first motion data were originally interpreted to suggest primarily dip-slip movement on a near vertical and a near horizontal plane indicating that, despite its location, this event was not a typical right--lateral strike slip San Andreas earthquake.

During the 1989 Loma Prieta Earthquake the shaking intensity for Daly City was measured at VI- moderate (Refer to map #2). The residents of #35 Westline Drive reported that the earthquake resulted in extensive re-cracking of Westline Drive and residential damage.

Land movements and landslides

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