Department of Conservation - Cal Geological Survey Programs

DOC/California Geological Survey
Earthquake Programs


The State of California has promulgated codes and regulations to build safer structures since the 1933 Long Beach earthquake. The Department of Conservation's California Geological Survey is involved in a variety of earthquake safety-related projects. These involve research, mapping and monitoring. CGS employs geologists, seismologists and other Earth scientists, and is one of the oldest geological surveys in the United States.


Seismic Hazard Mapping Program

This program is a direct result of post-Loma Prieta legislation (the Seismic Hazard Mapping Act of 1990). CGS has produced maps covering all or parts of more than 150 communities, mainly in Ventura, Orange and Los Angeles counties. More recently, mapping work has been done in San Francisco, Alameda and Santa Clara counties. Each map encompasses 60 square miles.

These maps show areas prone to the secondary earthquake hazards of liquefaction and landslides throughout California’s principal urban and major growth areas. Local governments use the maps to regulate development. Cities and counties can withhold permits until geologic or soils investigations are conducted for specific sites and mitigation measures are incorporated into the development plans. Property owners can use the maps to check whether they're in a seismic hazard zone. If they are, that fact must be disclosed to a potential buyer when the property is sold.

Liquefaction -- a phenomenon in which sandy soil saturated with groundwater temporarily behaves like quicksand -- was a major cause of damage in the Loma Prieta quake, particularly in the Marina district. Loma Prieta also caused thousands of landslides. Several residences in the Santa Cruz Mountains were badly damaged by slides, and two lanes were blocked on Highway 17 by a large slide.

CGS also publishes Seismic Hazard Mapping bulletins which announce new map releases, answer frequently asked questions about the maps, and offer tips about investigating and mitigating Seismic Hazard Zones.

Earthquake Planning Scenarios

CGS, working with the Governor's Office of Emergency Services, has completed eight scenarios for active faults in urban areas. These scenarios are for lifelines; they don't predict how much damage will be done to buildings. They help local planners anticipate problems and solutions in the event of a major earthquake. For example, the scenarios advise planners on which roads are likely to be open for emergency vehicles, where to have staging areas, where to house displaced people and whether they can expect aid from neighboring communities.

The scenarios are for the San Andreas fault (one for Northern California, one for Southern California), the San Diego-Tijuana area, the Newport-Inglewood Fault, the San Jacinto Fault, the Hayward Fault, the Rodgers Creek Fault and Humboldt and Del Norte Counties.

Strong Motion Instrumentation Program (SMIP) and the California Integrated Seismic Network

Established in 1971 after the San Fernando earthquake, SMIP installs monitoring devices called "accelerographs" throughout California to measure the vertical and horizontal response of buildings and soils to strong earthquake shaking. The analysis of this data is used to recommend changes to seismic design codes and practices for buildings, bridges and other structures and to aid emergency response personnel in the event of a disaster.

There are more than 1,000 "accelerographs" installed throughout the state. Since Loma Prieta, monitors have been placed on 60 bridges around the state. The San Mateo Bridge has 120 sensors and the Golden Gate Bridge 95. There are 12 seismic sensors installed in the State Capitol.

Through the SMIP program, CGS entered into TriNet, a partnership with the California Institute of Technology and the U.S. Geological Survey. TriNet is a seismic network for earthquake research, monitoring and early warning in Southern California. TriNet was completed in 2001. It includes nearly 600 monitoring stations and has the capability to provide information faster than ever before on where the most damaging shaking has occurred in an earthquake. At the time of the Loma Prieta quake, seismologists had to visit each monitoring device to collect the information. The new generation of "accelerographs" contains a modem that dials a computer at CGS/SMIP within seconds of an event in the magnitude 3.5 range or greater. It also pages CGS and Caltrans officials.

Accurate information is an important commodity in the wake of an earthquake because it saves lives, time, effort and money. The goal of TriNet is to provide preliminary "ShakeMap" information within 10 minutes of a quake and a detailed "ShakeMap" showing shaking intensity within half an hour. These maps will help authorities concentrate their recovery efforts more effectively. The maps show the severity of shaking in terms of peak ground acceleration, peak velocity and instrumental intensity -- the parameters most familiar to major groups of users, especially engineers and emergency managers.

The TriNet program has been expanded into Northern California and is now called the California Integrated Seismic Network (CISN). In the northern part of the state, the Department of Conservation and USGS are partnering with UC Berkeley. Funding from the Governor’s Office of Emergency Services will maintain the existing stations in Southern California and add approximately 25 stations per year in Northern California through 2006.

Alquist-Priolo Maps

The Alquist-Priolo Earthquake Fault Zoning Act was passed after the 1971 San Fernando earthquake, and it is an ongoing program. It is designed to prevent damage from surface fault rupture. Faults that meet the zoning criteria of "sufficiently active" (faults that have broken the surface in the past 11,000 years) and "well defined" are included in Alquist-Priolo Earthquake Fault Zones. Development permits in those zones must be withheld until a geologic investigation determines the potential danger of ground rupture. Structures generally must be set back 50 feet from an active fault. To date there are 544 Alquist-Priolo Earthquake Fault Zone maps that affect 36 counties and 100 cities.

Earthquake Probability Studies

CGS has developed an assessment of the seismic hazard in California in cooperation with the USGS. This analysis represents an extensive effort to obtain consensus within the scientific community regarding fault and earthquake parameters that contribute to the seismic hazard. The loss estimate accounts for various sizes of earthquakes on faults in California to estimate the ground shaking that would occur across the state. This seismic hazard model is used for building code development, mitigation and response planning, and earthquake loss estimation.

Earthquake Damage/Loss Estimates

Recent developments have allowed CGS to better model and estimate future earthquake losses, and that information is useful for public planning. A Department of Conservation study estimates that the average annual loss from earthquake damage to buildings only over the next several decades will be $3.9 billion in today's dollars; that's $130 per resident per year. When associated losses to contents, inventory and income are added, the expected annual loss in California increases to about $4.7 billion. That total does not reflect the cost associated with injuries to occupants or other economic losses.

The estimate is based on the CGS/USGS earthquake hazard model and a regional inventory of buildings throughout the state. It incorporates new engineering-based relationships on how buildings will respond to different levels of ground shaking.

The point of the damage/loss estimates is to promote cost-effective mitigation strategies prior to the next major earthquake in California. Historically, there have been calls for mitigation right after disasters, but the interest level fades over time -- until the next major earthquake.

The calculated losses include soil and liquefaction maps, the CGS/USGS public seismic hazard model, and the HAZUS loss estimation model developed by the Federal Emergency Management Agency and the National Institute of Building Sciences. HAZUS evaluates the damage potential of several standard building types using the experience of structure damage from previous earthquakes and structural engineers' judgments regarding the performance of those types of structures during earthquake shaking. The model produces the levels of damage that are expected to occur, on the average, to a group of similar structures.

Not all types of structures are considered in the loss estimation model. For example, damage to airports and most lifelines is not included. Nor are the effects of catastrophic losses caused by damage to facilities such as dams, nuclear power plants, liquefied natural gas facilities or military installations. Fires that often follow earthquakes, social losses and other indirect losses are not included.

Seismic Shaking Hazard Maps of California

This project, done in cooperation with USGS, puts a wide variety of information in one place. Seven maps in one, it is essentially a tutorial on how to understand seismic hazards and is of special interest to engineers. It includes a map of earthquakes, damage areas of historic significant earthquakes, and a characterization of nearly 200 faults. One outstanding feature of the map is that it shows soil types as well as seismic hazards.

Near-Source Maps

The strongest shaking and highest ground acceleration during an earthquake generally occurs closest to the fault involved. These maps show zones near faults in the seismic areas most prone to strong shaking. The maps also define distance contours within two, five, 10 and 15 kilometers of faults. The 1997 Uniform Building Code incorporates a "near-source" factor; construction within the 15-kilometer area near a fault must meet certain design criteria. The standard generally becomes more conservative closer to the fault. CGS has produced more than 200 of these maps in an atlas covering the active zones of California.