MARCH 17, 2014 "ENCINO" EARTHQUAKE
Postulated Significance for Malibu

For the reader's clarity, a geologic fault is incorporeal. It is not an object, but a location - actually a surface - between two earth masses that have moved differentially. The "trace" of a fault is the line defined by its intersection with the earth's surface. A "fault zone," on the other hand, is corporeal - a volume within which more or less parallel faults occur. Thus, the RHSCI zone extends within or at the base of the Santa Monica Mountains block.

Along the RHSCI zone, the Santa Monica Mountains block currently is moving west and being forced upward presumably consistent with the inferred and continuing clockwise movement of the Transverse Ranges microplate. As a result, faults along the Malibu coast have modes that generate initial focal mechanism solutions indicating thrusting along north-dipping fault surfaces of either dip-slip or left-lateral oblique slip, all of which means that the mountain block is moving upward and generally somewhat southwestward over the adjacent block to the south. Vedder, et al. (1986) indicate offsets of RHSCI zone faults in Santa Monica Bay during Holocene time, i.e., about the past 11,000 years, and certainly other such offsets have been recorded in during offshore petroleum exploration. Probably, however, none is known to have occurred in relation to any recorded seismic event in the RHSCI zone offshore. Should sudden movement offset the sea bottom in the RHSCI zone, a tsunami would be generated. Indeed, such movements may have occurred but were so minor that tsunamis they generated had magnitudes too small to have been noticed.

The Encino event produced a focal mechanism presumably similar to many related to earthquakes with epicenters offshore in the RHSCI zone, three of which are shown by Yerkes and Lee (1987, Pl. 4.1). Of these, two were reverse oblique-slip and one, the February 21, 1973 Point Mugu event, was reverse dip-slip. All the earthquakes reported in Santa Monica Bay have magnitudes mostly in the range of 2.0 to 2.5, except that of the Point Mugu event which was 5.9. Comes now the Encino event with a significantly smaller magnitude of 4.4, but still in a class with that of the Point Mugu, well above all others recorded in the RHSCI zone offshore from Malibu.

The Encino event was well removed from the surface expression of the RHSCI zone - in fact 4.5 miles northerly the closest point along it, deemed here for present purposes to be on the Hollywood fault trace close to the intersection of Sunset Boulevard and Stone Canyon Road near UCLA, as mapped by Dibblee (1991), and some 7.5 miles north-northeast from the nearest point in the offshore RHSCI zone at the mouth of Potrero Canyon. The significance of the Encino event is that it suggests a stress regime involving reverse faulting is to be expected throughout the Santa Monica Mountain block, just as is more widely known to the west in Ventura and Santa Barbara according to Yerkes and Lee (op. cit.). As to the offshore section of the RHSCI zone, the Encino event is statistically meaningless and is not, so far as the available data indicate, a precursor to similar activity there. To date, as a practical matter discussed elsewhere in this web site, Malibu's best defense against the risk of tsunamis remains: [i] a warning system, coupled with [ii] predetermined paths of emergency pedestrian evacuation from near-shore areas to higher ground to be accomplished in a period of a few minutes.

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