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Southern California Geology

 

Earthquake Hazards in Southern California

Southern Californians are well acquainted with the hazards created by earthquakes because the region is traversed by many fault zones capable of producing seismic events. Earthquake-generated hazards include:

 

  • Hazards due to fault rupture at the Earth's surface
  • Hazards due to ground shaking (ground acceleration)
  • Hazards due to shaking-induced ground failure (liquefaction and landslides)

A useful summary of the kinds of earthquake hazards facing southern Californians can be found in the various chapters of Ziony (1985).

 

Ziony, J.I., ed., 1985, Evaluating earthquake hazards in the Los Angeles region-an earth-science perspective: U.S. Geological Survey Professional Paper 1360. 505 p.

Glossaries of Earthquake Terminology

 


Earthquake-hazard Studies by the U.S. Geological Survey (USGS)

The USGS Earthquake Hazards Program conducts research and mapping studies in southern California that answer questions about:

 

  • How earthquakes occur (earthquake mechanisms and sources)
  • How frequently they have occurred in the past (paleoseismology and earthquake recurrence)
  • What is the long-term slip-rate for each fault zone or segment (as a measure of long-term strain accumulation)?
  • What are the earthquake-induced hazards, where are they located, and what is their probability of occurrence?

The hazards program also conducts post-earthquake investigations that map ground-rupture patterns, evaluate ground-response patterns, and determine regional stress distribution throughout southern California prior to and after a major earthquake event. Extensive post-earthquake investigations have been conducted for the 1968 Borrego Mountain earthquake, the 1971 San Fernando earthquake, the 1979 Imperial Valley earthquake, the 1992 Landers earthquake, the 1994 Northridge earthquake, and the 1999 Hector Mine earthquake. Some online USGS earthquake resources include:

 

Frequently asked questions about earthquakes

Basic Educational Information

Seismolinks, a nifty online listing of internet resources dealing with earthquake-hazards, seismology, earthquake geology, and plate tectonics

An index map showing real-time earthquakes in California. Another resource that shows where earthquakes are occuring is the southern California shakemap website

A useful discussion of how information telemetered from field instruments is used to determine earthquake parameters and locations on the index map

A discussion of how earthquake epicentral locations are determined

A chronologic listing of earthquakes in California and Nevada , together with a listing of their locations and magnitudes

Geologic information about the Hector Mine earthquake

Investigations of the 1994 Northridge earthquake

The Earthquake Hazards Program has a National Seismic Hazard Mapping Project that has produced seismic-hazard maps for the United States. These include online maps showing seismic hazards for California . Navigate to the Frequently Asked Questions link of the National Seismic Hazards Project for clear answers to questions about seismic hazards, especially as represented by probabilistic ground-shaking estimates. In particular, read a short description of how the maps are used and a general-information pamphlet about how seismic-risk maps save lives and property ).

The USGS also maintains a National Earthquake Information Center that distributes information about earthquakes to the lay public and to various Federal, State, and County agencies.


Earthquake-hazard studies by the California Geological Survey

The California Geological Survey Seismic Hazards Evaluation and Zoning Program carries out a systematic program of earthquake-hazards studies in southern California. The program is described in Chapters 4, 5, and 6 of CDMG Special Publication 117 (CSG was formerly California Division of Mines and Geology --CDMG) that identifies State guidelines for evaluation and mitigation of seismic hazards; a facts sheet provides a brief overview of this seismic-hazards mapping program. Useful CGS resources describing earthquake hazards include:

 

a state-wide probability map (including southern California) showing the likelihood that ground-shaking of a particular strength will be exceeded for a specified time period;

an Atlas of Probabilistic Seismic Hazard Maps produced for 1:250,000-scale quadrangles of southern California and the rest of the state (for example, the Santa Ana 1:250,000-scale quadrangle );

a probabilistic seismic hazard assessment for the State of California

An index of CDMG seismic-hazards evaluations in southern California shows the extent of CDMG earthquake-hazard evaluations, including released and ongoing investigations. The index points to 1:24,000-scale quadrangle maps where susceptibility to liquefaction and to earthquake-induced landsliding is displayed online using small-scale map graphics. Some evaluations include down-loadable .pdf files of evaluation reports that contain chapters describing liquefaction and landslide hazards (for example, see seismic-hazard evaluations for the Pasadena 7.5-minute quadrangle and the San Fernando 7.5-minute quadrangle ).

CGS's Program of fault-zone mapping under the auspices of the California Alquist-Priolo Earthquake Fault Zoning Act is a significant source of information about earthquake hazards in southern California. An online index of available fault maps identifies available Alquist-Priolo fault-zone maps, each showing fault lines plotted on 1:24,000-scale 7.5-minute quadrangle maps.

CGS maintains a Strong Motion Data Center that allows web-site visitors to examine strong-motion wave data recorded at various stations for specific southern California earthquakes.


Earthquake-hazard Studies by the Southern California Earthquake Center (SCEC)

The Southern California Earthquake Center (SCEC) is a consortium of University, Federal, and State geoscience groups that conducts research on the character, origin, mechanisms, and hazards of earthquakes in southern California as well as the history of southern California earthquakes in the recent geologic past. The SCEC data center is a valuable portal to many different online information products, including:

 

a general-interest publication on earthquakes in southern California

an explanation of the relationship between earthquakes and faults

a series of online resources that describe historic southern California earthquakes , including:

 

 


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