New study eyes Southern California tsunami risks

Modeling shows possible outcome of offshore earthquakes

This is a map of regional peak tsunami amplitude in meters resulting from an earthquake on the Pitas Point and Lower Red Mountain fault system. The thin solid black line indicates the coastline and the thick black line indicates the Pitas Point fault trace. The fault trace is where the fault surface intersects the seafloor; it is seen as a straight line in the east-west direction. Note that significant regional tsunami inundation occurs. Credit Kenny Ryan, UC Riverside.
New computer modeling details potential coastal inundation from earthquakes off the shoreline of Southern California, Image courtesy
Kenny Ryan, UC Riverside.

Staff Report

FRISCO — New computer modeling by seismologists at the University of California, Riverside shows that parts of Southern California could be more at risk from tsunamis than previously thought.

After taking a close look at the consequences of earthquakes along the  Pitas Point and Red Mountain faults — located off the shore of Ventura — the researchers said those faults could send a massive tsunami surging inland several kilometers, with greater flooding than currently shown by the state of California’s reference inundation line.

The new modeling helped show in detail how deformation of the seafloor and the shape of the coastline would affect the direction of a tsunami, potentially putting the relatively flat coastal plan around Ventura and Oxnard at risk. The study results appear in Geophysical Research Letters.To see a video, click here.

“The hazard from earthquake-generated tsunamis in the Ventura/Oxnard area has received relatively little attention,” said Kenny J. Ryan, a graduate student in the Department of Earth Sciences at UC Riverside  “Unfortunately, the Ventura/Oxnard area has relatively flat topography along the coast, so a tsunami can inundate that area quite effectively.”

“Our study is different in that we use a dynamic earthquake model to calculate seafloor displacement from the earthquake,” said coauthor David D. Oglesby, a professor of geophysics in whose lab Ryan works. “Dynamic models such as these calculate movement in time by looking at the forces on and around the fault in time. They are physics-based, and fault slip distribution and ground motion are calculated results of the models.”

The modeling results showed that a tsunami generated by a magnitude 7.7 earthquake along the Pitas Point and Red Mountain could reach the Santa Barbara coastline within five minutes. Part of the tsunami would be refracted, rotating counterclockwise in the direction of Ventura and Oxnard. Other parts of the tsunami waves would reflected off the regional coastline. These refracted and reflected waves focus toward Ventura and Oxnard in 15-20 minutes and begin to inundate that area in less than 30 minutes.

“The models result in large tsunami amplitudes northward and eastward of the fault due to the shape of the coastline and seafloor,” Ryan explained. “The probability of such an event in a given time frame is low compared to smaller earthquake events. Nonetheless, it is crucial to investigate the possible effects from such rare but plausible earthquake and tsunami scenarios so that a full hazard assessment can be made. Results from such modeling efforts can help reveal potential regions of high tsunami hazard.”

Previous research has shown that the faults in the Ventura basin in Southern California are capable of generating earthquakes of magnitude 7 or greater as well as significant local tsunamis generated by faulting and landslides.

“Our study describes one potential earthquake and tsunami scenario along the Pitas Point and Red Mountain faults, and is designed to illustrate the usefulness of rupture modeling in determining tsunami inundation,” Ryan cautioned. “It is not intended to give an overall distribution of all possible earthquakes and tsunami hazards in this region. Our models simply give an indication of what may be possible in this region.”

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