Geologists Identify Areas of Ventura at Risk Following Major Quakes

Most of downtown Ventura could be at risk of liquefaction during a major earthquake, while landslides could be a danger in the hills above the city, according to a seismic hazards map issued by state geologists this week.

The liquefaction zone stretches from the ocean to the area north of Poli Street. Liquefaction occurs when sandy soil becomes saturated by ground water and then is shaken.

The state Department of Conservation map also identifies a landslide hazard zone above much of the inhabited section of Poli Street.

It is the 20th of 21 state maps to be released for the county. The 21st map, encompassing an area north of Santa Paula, is out in draft form and will become final in a few months. The maps, along with others for areas throughout Southern and Northern California, can be seen at https://gmw.consrv.ca.gov/shmp.

The maps identify areas at risk from secondary effects of earthquakes and are aimed at alerting city planners, developers and home buyers about the potential hazards of landslides and liquefaction.

Paul Calderwood, a senior planner for the city of Ventura, said that while the report contained no surprises, “it heightens people’s awareness a little bit.” The city already requires developers to conduct a soil and geological study before building and to address any relevant concerns.

The downtown Ventura report became final July 2.

The state mandates that builders test ground stability if a potential hazard is identified. In addition, home sellers must disclose the hazard to potential buyers.

Although about a dozen faults stretch across Ventura County, they were not included in the seismic hazard maps, said Allan Barrows, a senior engineering geologist with the department. One of them, the Ventura fault, runs roughly along Poli Street in the newest area mapped.

The report identifies dangers in the event of a 6.8 or greater quake. There is a 10% risk of such a major quake occurring in the next 50 years, Barrows said.

The most effective result of the maps, he said, would be improving future construction.

Barrows said that many local governments already required the quake-preparedness measures the department was requiring through the maps. The effort marked the first time a regulatory map had been done, he said, and “nudges” local governments to conform to state standards.