L.A. County Tops List of California Regions Most Prone to Earthquake Damage

L.A. County Tops List of California Regions Most Prone to Earthquake Damage

By | 2018-01-12T17:35:00+00:00 June 12th, 2017|Earthquake Protection, Earthquakes, Seismic Retrofit|

Anyone who lives in California knows that our state’s calamitous reputation for earthquakes is fueled by the intricate web of volatile fault lines that crisscross our landscape.

The closer you are to the convergence of two tectonic plates, the more likely you are to be impacted by seismic activity. Here in Southern California, the junction of two major faults – the San Andreas and San Jacinto – creates a grim seismic scenario where both might at any time rupture simultaneously, causing an epic magnitude-7.5 earthquake.

Buildings in the Greater Los Angeles area have up to a 5 percent chance of being damaged by an earthquake this year, according to the United States Geological Service.

In fact, Los Angeles County ranks as No. 1 for estimated annualized earthquake loss at 30.6 percent, the USGS reported. A U.C. Berkeley study performed recently by the California Geological Survey supported that finding, ranking potential damage in Southern California higher than that of any other part of the state, including San Francisco.

Why is L.A. Most Vulnerable?
Projected earthquake damage is calculated based on the strength or intensity of the ground shaking and the ability of a structure to withstand it.

The U.S. Geological Survey last year completed an analysis of earthquake risks in California by community. San Bernardino was put at the top of the list, due to its proximity to both the San Andreas and San Jacinto faults. Eureka came in second because it is located near faults that are believed to be capable of beastly quakes with magnitudes of 8 or 9.

More recently, the USGS shifted its focus to Southern California’s Grapevine area north of Los Angeles, where major quakes have occurred on average every 100 years.

The last major tremblor to strike the Grapevine area was more than 160 years ago, jolting the earth along a shocking 185 miles of the San Andreas fault.

A repeat of that 1857 quake could cause considerable damage and shake the ground with such a force that Central Los Angeles could experience a couple of minutes of jolting – considerably more than the 15 seconds of shaking caused by the 1994 Northridge quake.

In other words, the epic tremblor sleeping restlessly under our feet would be something few Southern Californians have ever experienced. Some seismologists say it would create rifts along the fault measuring 9 feet on average.

Likewise, the California Geological Survey determined that due to the anticipated ground motions, soils and development in Los Angeles County, our region faces the potential of significant annualized earthquake loss.

A repeat of San Francisco’s 1906 earthquake would cause up to $54 billion in damage, the California Geological Survey said. Likewise, in Southern California, a magnitude 7.1 quake along the Puente Hills fault would bring an estimated $69 billion in damages. Other calculations for Southern California include $49 billion from a 6.9 magnitude quake on the Newport-Inglewood fault; $30 billion from a 7.1 magnitude event along the Palos Verdes fault; $29 billion from a 6.8 event on the Whittier fault; and $24 billion for a 6.7 event on the Verdugo fault.

One of the major reasons the USGS put L.A. County at the top of annualized earthquake loss has to do with the population density, the types of buildings located here and the likelihood that they may be damaged when a major quake strikes.

Is Your Building at Risk?

Seismologists and structural engineers have identified certain buildings that are most likely to sustain damage in a major earthquake. These include:

  • Soft-Story built before 1978
    • These structures, with parking on the ground floor and units built above, are prone to collapse during major earthquakes.
  • Unreinforced Masonry built before 1975
    • The facades of these buildings can collapse in a quake.
  • Non-Ductile Concrete built before 1977
    • Limited lateral resisting capacity makes these structures brittle.
  • Steel Moment Frame built before 1996
    • These buildings can sustain brittle fracturing of the steel frames at welded joints between beams and columns.

A detailed USGS scientific assessment of potential damage from a magnitude-7.8 San Andreas earthquake in Southern California estimated that 300,000 structures would be damaged.

That’s one in every 16 buildings in the region.

The land on either side of the San Andreas fault is moving in opposite directions at a rate of roughly an inch a year – about the same rate that your fingernails grow. The shifting of the earth creates underground pressure that builds and occasionally lets loose with seismic activity.

Most earthquakes are small, virtually imperceptible, but scientists universally agree that we are long overdue for “The Big One.” It’s just a matter of time.

Want to know more? Optimum Seismic is teaming up with the Apartment Association of Greater Los Angeles on a series of educational seminars to help inform members and address any questions about seismic retrofit laws, policies, procedures and programs. You are welcome to attend our next seminar at:

AAGLA Headquarters
4 p.m. to 6 p.m. Tuesday, July 18
621 S. Westmoreland Ave.
Los Angeles, CA 90005

Please call us at 323-OPTIMUM to RSVP or for a list of other upcoming dates and locations.

About the Author:

Ali Sahabi
Ali Sahabi has been a licensed General Engineering Contractor (GEC) since 1993, and is a principal at Optimum Seismic, Inc. He has completed hundreds of seismic retrofitting and adaptive reuse projects for multifamily residential, commercial, and industrial buildings throughout California.

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