THE SAN ANDREAS FAULT SYSTEM
The historical record of major earthquakes affecting California, western Nevada, and northernmost Baja California (Table 6.1) includes basic seismologic data on 206 of the largest earthquakes occurring between 1769 and 1989. This catalog lists all known events of M>=6 and includes new and updated information on their locations.
The record of seismicity within the San Andreas fault system and surrounding regions is both geographically and temporally uneven and incomplete before the introduction of practical seismographic instrumentation around the turn of the 20th century. In general, the density and distribution of people who left written accounts of their experiences determines the reliability of the catalog during the preinstrumental period. From the establishment of the Franciscan missions beginning in 1769 until their secularization in the 1830's, detailed accounts of events that damaged the missions are available, and these accounts form the primary source material for earthquakes occurring during this period. Life in California was a constant struggle for survival at that time; posting to a mission evidently was considered a hardship assignment, and so essentially nothing was recorded about events that were only felt, even when they were destructive at nearby missions. After secularization and before the gold rush, the quality of the record degrades with the cessation of the annual reports of the missions. Other sources of records also are notably weak during the Mexican period, from the early 1830's until 1846.
The discovery of gold in 1848 transformed the written record of earthquakes with the advent of newspapers throughout the gold fields in the Sierran foothills and in the San Francisco Bay region. Printed accounts of earthquakes have been extensively used, notably by Toppozada and others (1988), to quantify the seismicity of California from 1850 onward. They estimated that their historical catalog is probably complete for the San Francisco Bay region and central Sierra Nevada from 1850 on for earthquakes of M~~6. The same level of completeness is not achieved, however, for the San Andreas fault system in southern California until the 1890's. Statewide, the catalog of earthquakes is substantially complete for earthquakes of M~~7 after about 1850 (see Agnew, 1985). The quality of the catalog for central Nevada, where much significant 20th century seismicity has occurred, is less complete. Questions remain today about purported events as late as 1903 in this region (Slemmons and others, 1959).
Reports of the local effects of earthquakes continue to play a major role in determining the locations and sizes of earthquakes well into the 20th century. The earliest seismographs capable of systematically detecting California and Nevada earthquakes were installed throughout the world by John Milne beginning in 1896. Seismograms from these instruments and their successors provide useful instrumental magnitudes from 1898 onward. However, not until the development of the Wood-Anderson seismograph and its deployment throughout California beginning in 1926 do instrumental measurements fully supplant noninstrumental magnitudes and epicentral locations.
The objective in assembling a single catalog from these many sources, spanning many different types and qualities of information, has been to achieve uniform spatial coverage without sacrificing any events of historical significance. M=6 was chosen as the threshold magnitude because probably all events of this magnitude are known from the instrumental period beginning in 1898, and the preinstrumental record is reasonably complete at this level in some areas for an additional half-century. All earthquakes with at least one reported magnitude of at least 6.0 have been included in the catalog. Because magnitude is an estimated quantity and has some inherent uncertainty, events with reported magnitudes within a few tenths of a unit of 6.0 are also included. In addition to those earthquakes with cataloged magnitudes, original documents for others with reported high intensities or of particular historical significance have been reexamined in an attempt to refine their locations and magnitudes.
A word of introduction should be added about earthquake locations and magnitude scales and their use in this chapter. Earthquakes are complex physical processes generated by sudden slip on faults, and as such they can only be grossly characterized by simple concepts. Two seismologic conventions are in common use for assigning a single geographic coordinate to an earthquake: One measures the center of energy release, frequently as estimated from the intensity distribution for preinstrumental events; the other measures the location of the initial point of rupture, or hypocenter, as determined from seismic travel-time measurements. Either point on the Earth's surface above the hypocenter or the center of the intensity distribution is sometimes referred to as the epicenter, and each type of location appears in Table 6.1, with preference given to instrumental epicenters. Fortunately, the geographic differences between these distinct physical measures become significant only for the largest events, M~~7, when viewed at the scale of the entire San Andreas fault system.
Magnitude, as commonly used to compare the sizes of different earthquakes, also represents an extreme Simplification of the earthquake process and by itself cannot fully characterize the size of any event. Traditionally seismologists have developed a suite of magnitude scales, each with its own purpose and range of validity to measure an earthquake. Because no single magnitude scale can be systematically applied to the entire historical record, a summary magnitude, M, is introduced here to facilitate comparisons between events. As described below in the subsection entitled "Quantification of Earthquakes and Magnitude Scales," M is taken as the surface-wave magnitude (Ms), when available, and as a modified intensity magnitude (MI) during the preinstrumental era. Generally speaking, M provides a better relative measure of the static, geologic increment of fault slip in the earthquake than it does of the severity of shaking.
The earthquake history of California, western Nevada, and northern Baja California presented here has apparent limitations and can doubtlessly be improved through further research. Nevertheless, it provides a firm observational basis for assessing the tectonic implications of the 2-century-long seismic history, as well as of the prospects for future earthquake activity.