Geologists Release First New Seismic Hazard Model in Five Years, Risk to Pasadena Reaffirmed

Published : Friday, December 13, 2019 | 6:38 AM

View of John Muir School, showing damage from the March 10 1933 Long Beach earthquake. Image courtesy USGS

The United States Geological Survey on Wednesday issued its first new National Seismic Hazard Model report in five years which in part reaffirmed Pasadena’s high risk but by contrast raised potential ground motion estimates for some Southern California cities by up to 50 percent stronger.

The report predicts where earthquakes are likely to occur and at what frequency and strength.

Scientists said the Los Angeles Basin is so well-examined because of extensive oil-prospecting studies that earthquake predictions for Pasadena remain largely unchanged in the new study.

But recalculated ground motion studies based on more new data elsewhere indicate that cities including Long Beach and Ventura could experience much more violent temblors than previously thought.

The study’s new maps show areas where the basin regions in California are miles deep. The basins over the years have collected sediment that flows down from higher elevations, and these pockets of sediment don’t move the same way solid rock does.

“We worry about taller buildings in the basins,” said Mark Petersen, a research geophysicist with the USGS. “The fact that they’re so deep is what we worry about. The update helps us to understand better, in more detail for different types of levels.”

Pasadena is in a unique situation, where it sits between the mountains and the L.A. basin, where sediment collects. Some coastal cities are on much shakier ground.

Experts agree that Californians are much better prepared than when the magnitude 6.7 1994 Northridge earthquake struck, killing 60 and injuring 9,000.

But the new study suggests more should be done, especially in retrofitting vulnerable buildings.

Pasadena’s new building retrofitting codes go into effect January 1, 2020.

The codes affect some 472 local “soft-story” residential buildings housing approximately 4,500 Pasadenans.

This type of structure, exemplified by apartment and office buildings with ground floor tuck-in parking underneath the main structure, is characterized by weak load resistance and vulnerability to collapsing during earthquakes.

Soft story failure is believed to have been responsible for nearly half of all homes that became uninhabitable after the 1989 Lorna Prieta and 1994 Northridge earthquakes, accounting for significant human loss and property damage.

A City ordinance passed in February requires identified buildings in Pasadena to be earthquake retrofitted for safety.