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Map Showing Seacliff Response to Climatic and Seismic Events, Seabright Beach, Santa Cruz County, California

September 1, 2002


The coastal cliffs along much of the central California coast are actively retreating. Large storms and periodic earthquakes are responsible for most of the documented sea cliff slope failures. Long-term average erosion rates calculated for this section of coast do not provide the spatial or temporal data resolution necessary to identify the processes responsible for retreat of the sea cliffs where episodic retreat threatens homes and community infrastructure. Research suggests that more erosion occurs along the California coast over a short time scale, during periods of severe storms or seismic activity, than occurs during decades of normal weather or seismic quiescence.

This is the third map in a series of maps prepared to document the processes of short-term sea cliff retreat through the identification of slope failure styles, spatial variability of failures, and temporal variation in retreat amounts in an area that has been identified as an erosion hotspot. This map presents sea cliff failure and retreat data from the Seabright Beach section, California, which is located on the east side of Santa Cruz along the northern Monterey Bay coast. The data presented in this map series provide high-resolution spatial and temporal information on the location, amount, and processes of sea cliff retreat in Santa Cruz, California. These data show the response of the sea cliffs to both large magnitude earthquakes and severe climatic events such as El Ni?os; this information may prove useful in predicting the future response of the cliffs to events of similar magnitude. The map data can also be incorporated into Global Information System (GIS) for use by researchers and community planners. During this study we developed a method for investigating short-term processes of sea cliff evolution using rectified photographic stereo models. This method allows us to document the linear extent of cliff failures, the spatial and temporal relationship between failures, and the type or style of slope failure.

Seabright Beach extends 0.9 km from San Lorenzo Point on the west to the Santa Cruz Yacht Harbor on the east. The cliffs at Seabright Beach are completely protected from wave attack by a wide beach. The protective beach is a relatively recent feature that formed after the emplacement of the Santa Cruz Yacht Harbor jetty in 1963-1964. Prior to the completion of the jetty, the cliffs at Seabright Beach were subject to daily wave attack. The data in this study are post-jetty construction; therefore, the sea cliff failures and cliff retreat are the result of nonmarine processes (rainfall, groundwater and seismic shaking). The 8 to 15 m high cliffs at Seabright Beach are composed of the Miocene to Pliocene Purisima Formation, which is overlain by unconsolidated Pleistocene terrace deposits. The relative thickness of these units varies along the length of the cliff. At the west end of Seabright Beach, including San Lorenzo Point, nearly the entire cliff section is composed of Purisima Formation and is capped by less than 2 m of terrace deposits. In this exposure, the Purisima Formation is a moderately weathered, moderately indurated massive sandstone. The height of the cliffs and the thickness of the Purisima Formation decrease to the east. In the cliffs immediately adjacent to the harbor, the entire exposure is composed of terrace deposits. Toe-slope debris and wind-blown sand form a nearly continuous fan along the cliff base that obscure the lower portion of the cliff.

This study documents the impacts of earthquakes and large storms to the sea cliffs in the Seabright Beach section. The first event is the 1989 Loma Prieta earthquake, a M7.1 earthquake that caused widespread damage to the area stretching from Santa Cruz to the San Francisco Bay. The epicenter of the earthquake was located in the Santa Cruz Mountains, approximately 9 km inland from the coast. Extensive block and debris falls, induced by the seismic shaking, occ

Publication Year 2002
Title Map Showing Seacliff Response to Climatic and Seismic Events, Seabright Beach, Santa Cruz County, California
DOI 10.3133/mf2400
Authors Cheryl J. Hapke, Bruce M. Richmond, Mimi M. D'Iorio
Publication Type Report
Publication Subtype USGS Numbered Series
Series Title Miscellaneous Field Studies Map
Series Number 2400
Index ID mf2400
Record Source USGS Publications Warehouse
USGS Organization Western Coastal and Marine Geology