APPLICATIONS OF GEOMORPHIC FEATURES PRODUCED BY EXTREME STORM EVENTS: UNRAVELING THE SEISMIC AND CREEP COMPONENTS OF FAULT SLIP

The southernmost San Andreas Fault (SAF) in California last experienced an earthquake resulting in surface rupture in 1726 CE. Palaeoseismic and neotectonic studies have revealed that this section has a long-term slip rate of approximately 20 mm/yr. Creep meter, InSAR and paleoseismic studies in the area suggest this section is creeping at about 3 mm/yr. Differentiating creep and coseismic slip rates is critical to understanding palaeoseismic records and developing estimates of future slip per event. In addition, landforms such as rills and channels offset by creep can be used to assess mechanisms of formation of rills as their ages can be estimated by the amount of creep offset.

The southern SAF traverses the Coachella Valley of California. This area of California is arid and receives less than 70 mm/yr of rainfall. Major precipitation events are rare and generally relate to hurricanes or subtropical depressions that are tied to El Nino periods. Significant 1-day rainfall events of >180 mm have been recorded. These events are capable of reactivating the landscape, developing rills, and creating new channels. Two such storms, 1939 San Pedro Tropical Depression and 1858 Hurricane, have been recorded in the area. The 1939 storm, known as the “Lash of St Francis”, was downgraded from a category 1 storm to a depression just prior to landfall in San Pedro, CA and resulted in 180 mm of rain in 3 hours in the Coachella Valley. The 1858 Hurricane was recorded as a category 1 at landfall. No rainfall records from the Coachella Valley are available from this time period.

A Phantom 4 Pro was used to acquire high-resolution, low altitude aerial imagery of sites along the southern SAF. The <25 m altitude resulted in sub-cm resolution imagery and was used to generate orthophoto mosaics and point clouds, which were converted into digital surface models, hillshades and slope maps. Analysis resulted in the identification of two clusters of rill offsets: 24±3 cm and 50±4 cm. Using 3 mm/yr as the estimated creep rate results in 80±1 yr and 167±1 yr since creation of the rill; 80 years ago is 1939 CE and 167 years ago is 1852 CE. The two identified clusters of rill offsets almost certainly correlate to the 1939 San Pedro storm and the 1858 Hurricane.