San Andreas Fault Outlined in Clouds

May 23, 2007


Provided by: Ravi Kanda, Caltech
Summary authors & editors: Ravi Kanda

Early morning clouds clearly delineate the San Andreas Fault (SAF) in southern California. This view towards the east is a striking illustration of the orographic effect. The SAF is a linear trough, bordered by escarpments on either side. Low level clouds rolling down over the southwestern escarpment (San Gabriel Mountains above Los Angeles Basin - hidden below the carpet of clouds along the bottom-right of the photo) evaporate within the trough of the fault, then re-condense as they get pushed up and over the northeastern escarpment (dark linear feature below the cloud line near the image's center). They finally disperse in the dry air of the Mojave Desert.

The San Andreas Fault demarcates the plate boundary between the North American and Pacific plates and extends over 1,000 miles (1,600 km) from near the Gulf of California, in Mexico, to just off the coast of Mendocino, California. The photo was taken from just south of Quail lake (linear "sag pond" at the bottom left corner of photo) just east of the intersection of the northwest-southeast trending San Andreas Fault (SAF) and the northeast-southwest trending Garlock Fault (GF) -- right in the middle of the big "S"-bend in the SAF. See []. Both faults are young and have been active since the Miocene (~25 Ma). The relatively "flat" but high Mojave Desert - with an elevation range of ~ 2,000-6,000 ft (610-1,830 m) - appears between these two faults, extending from near the bottom-left corner of this photo to the horizon.

The inset roughly corresponds to my perspective from the flight while taking the above photo. It was extracted from a southern California DEM. See []. The San Gabriel Mountains above Los Angeles basin, which form the southwestern escarpment of the SAF appear as a linear strand going from the center of this map to the right-center. These mountains are transpressive features produced by the bend in the SAF strike-slip system mentioned above. An approximate "North" arrow is shown in the inset to orient the viewer.

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