Cracks

A somewhat surprising aspect of the October 17, 1989 Loma Prieta earthquake was the lack of recognizable surface faulting or rupture along the trace of the San Andreas Fault. Aftershock patterns (see the first page) demonstrated that actual fault rupture teminated approximately 5 km below the ground surface. Although no throughgoing fault rupture occurred at the surface, a 5 km wide zone of complex ground cracking formed along the summit and seaward flank of the Santa Cruz Mountains in the Summit/Skyland Ridge area between Highway 17 and the forest of Nisene Marks. These ground cracks varied widely in dimension and lateral extent, often occurring in complex interconnected patterns both along the upper slopes of ridges as well as along ridgetops themselves.

Following the earthquake geologists from UCSC, several local consulting firms and government agencies (USGS, CDMG, and S.C. County) mapped the ground crack patterns and engaged in a number of research projects and geotechnical studies designed to evaluate the origin and significance of these features as well as any hazards associated with them. These investigations revealed that the ground cracking was associated with several different earthquake-related phenomena, including incipient landsliding, ridge crest spreading, and flexural slip along bedding planes within the steeply dipping sedimentary rocks underlying the Summit/Skyland Ridge area.

Tranbarger house crack, Summit Rd., Santa Cruz Mtns (photos by Jeff Marshall): The Tranbarger's pink house and the spectacular crack running across their yard were made famous by the international press who flocked to this site several kilometers from the earthquake's epicenter. The crack was not an actual surface rupture of the fault, as many initially assumed (for one thing, it has a left lateral offset). This crack, and many others like it, was due to tensional forces resulting from uplift during the quake (see below). Another contributing factor was "ridge top shattering." Ridge top shattering is due to the focussing of seismic energy towards the ridgetops of the Santa Cruz Mountains. When seismic energy reaches the surface in such cases, it essentially shatters the surface outward. The photograph on the right gives you an idea of the scale of these cracks.

Head scarp of landslide, Readwood Estates, Santa Cruz Mountains (photo by Dan Orange): This large landslide is over 300 m wide and contains three houses. This feature is the result of a slope failure caused by the shaking, and possibly extensional faulting as well.

Road crack, Summit Road, Santa Cruz Mountains (photo by Dan Orange): Cracks and landslides caused havoc for commuters during the quake. In fact, the only unaffected route from Santa Cruz to the South Bay was Mt. Charlie Rd., the 19th century stagecoach road.

This diagram shows the uplift that occurred during Loma Prieta EQ. The broad uplift caused tensional stresses at the surface which are expressed as large cracks as seen in the photographs above. The cracks are not surface ruptures of the San Andreas Fault. In fact, they occur a few kilometers west of the fault.