Map Sheet 49 shows epicenters of potentially damaging earthquakes, magnitude 5 or greater, which have occurred in California since 1800. In addition, an inset map shows areas that have been damaged by earthquakes
MMI VII or greater, and the minimum number of times such damage has occurred.
The image to the left is a thumbnail view of the map. The actual map measures 27" x 37".
The 200 year earthquake history is divided into three nearly equal time periods, distinguished by red, blue and green on the epicenter map and bar graph, to show the changes in earthquake occurrence and identification with time.
The map shows that since 1800, earthquakes capable of damaging unreinforced buildings have occurred at least six times in each of these regions: Los Angeles to San Fernando, San Francisco Bay to Santa Cruz, and Eureka to Cape Mendocino. The epicenters of 800 magnitude 5 or greater temblors are shown, including some centered offshore of California, as well as bordering regions of Nevada, Oregon and Mexico. A list of
earthquakes of magnitude 5.5 and greater, sorted by latitude for easy locating, is listed on the map
California Geological Survey seismologists compared old newspaper and other written accounts of earthquake shaking with accounts of the shaking from earthquakes recorded by modern instruments to determine the most likely epicenters and magnitudes of pre-1932 earthquakes. Most of the information about earthquakes that occurred before the 1849 gold rush comes from reports from the 21 missions near the coast from San Diego to Sonoma. Information about earthquakes from 1850 to 1931 is based largely on newspaper reports. Post-1932 data is from the seismological laboratories at Caltech and UC Berkeley, which began to routinely determine the epicenters and magnitudes of earthquakes in Southern and Northern California, respectively, in 1932 and 1942, supplemented by data from the United States Geological Survey starting in the 1970s. As California's population increased and spread across the state, earthquakes began to be reported in new areas — a fact that is reflected in the color-coded map.
The map indicates that California, on the average, has experienced one magnitude 6 or greater earthquake each year since 1850. It also shows that some areas have been relatively quiet seismically in the last 90 years but were very active in the previous 100 years. The relatively quiet areas include the San Andreas system of faults, which includes the San Jacinto and the Hayward-Rodgers Creek faults. Strong earthquakes also have occurred away from major known surface faults, such as near Vacaville in the Sacramento Valley 1892.
Epicenter map legend: colored circles define the three time periods covered on this map; circle size denotes magnitude of earthquake.
Time distribution of earthquake occurrences.
Inset from Damage Map, showing San Franscisco Bay area; image is approximately twice the size of the actual map.
Damage Map, from MS 49. Number next to color refers to minimum number of damage occurrences.