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Topographic/bathymetric map of onshore/offshore Southern California, with height and depth in meters. The Red Mountain and Pitas Point faults are considered in this study. Triangles indicate direction of dip; faults without triangles are considered strike-slip. Letters show approximate (central) city locations: SB = Santa Barbara; V = Ventura; O = Oxnard. Inset shows the map boundary in black.
Image credit: Kenny Ryan, UC Riverside
A new study led by University of California Riverside seismologists shows a computer modeled tsunami from a simulated earthquake causing severe damage to the Ventura and Oxnard areas, reports UCR Today.
The significance of this data proves that Ventura/Oxnard is not only threatened by outside earthquakes, but locals ones off the coastline. The simulated earthquake takes place in the Ventura basin and first develops south but then turns unexpectedly towards Ventura.
Findings state a 7.7 magnitude earthquake on the Pitas Point and Red Mountain faults, located off Ventura’s coast, would result in many parts of the coastline being affected by a tsunami wave for over a mile inland, in some places even farther than indicated by the state of California’s reference.
“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. “For our study, the shape of the coastline and seafloor produce the most interesting effects on the tsunami, causing a southward moving tsunami to refract – and therefore rotate – and focus on the Ventura/Oxnard area.
Unfortunately, the Ventura/Oxnard area has relatively flat topography along the coast, so a tsunami can inundate that area quite effectively.”
A magnitude 7.7 earthquake generated by the researchers’ models along the Pitas Point and Red Mountain faults results in the following scenario, reports UCR Today:
The earthquake occurs much more rapidly than the tsunami. First, the fault slips (within the first 20 seconds of the model) and seismic waves propagate outward in all directions. The seafloor is permanently deformed from the earthquake. This happens in less than a minute.
The tsunami is generated by the permanent vertical displacement of the seafloor, and begins to propagate outward through the ocean.
Part of the tsunami propagates north and arrives at the northward coastline, where Santa Barbara is located, in approximately five minutes. Also, part of the tsunami propagates south toward the deeper water in the Santa Barbara Channel. Because of deeper water here and the local bathymetry, this southward propagating tsunami begins to refract after five to ten minutes, rotating counterclockwise in the direction of Ventura and Oxnard.
Meanwhile, some of the tsunami waves are being 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 entire regional coastline sees a tsunami wave train that inundates many parts of the coastline in the region. The tsunami inundation in Ventura/Oxnard is significant in the model owing to a combination of factors:
large slip and seafloor displacement from the modeled earthquake scenario, refraction, focusing, and Ventura/Oxnard’s flat topography that facilitates water flowing inland.
“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.”
The faults in the Ventura basin have the capabilities of generating earthquakes of magnitude 7 or greater, as well as significant tsunamis.
These local tsunamis can impact the coastline within minutes, for Ventura it’s between 15-30 and sections of Santa Barbara within 5 minutes.
Courtesy of edhat.com
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