Variations of fault slip per event on the Carrizo Segment, San Andreas fault
Department of Geological Sciences Research
In a study appearing in the current issue of the journal Geology, we report that about 95% of displacement at the Carrizo Plain on the San Andreas fault northwest of Los Angeles occurs in big earthquakes. Excavations across the fault allow to collect for information about earthquakes of the past couple of millennia. We found that most of the motion along this stretch of the San Andreas fault occurs during rare but large earthquakes.
Of the six offsets discovered in the excavations, three and perhaps four were offsets of 7.5 to 8m, similar in size to the offset during the great earthquake of 1857. The third and fourth events, however, displacement was only 1.4 and 5.2m. Offsets of several meters are common when the rupture length is very long and the earthquake is very large. For example, the earthquake of 1857 had a rupture length of about 360 kilometers (225 miles), extending from near Parkfield to Cajon Pass. So, the five events that created offsets measuring between 5.2 and 8m likely represent earthquakes that had very long ruptures and magnitudes ranging from 7.5 to 8m. Taken together, these five major ruptures of this portion of the San Andreas fault account for 95% of all the displacement that occurred there over the past thousand years or so.
The practical significance of the study is that earthquakes along the San Andreas, though infrequent, tend to be very large. Years ago, paleoseismic research showed that along the section of the fault nearest Los Angeles the average period between large earthquakes is just 130 years. Ominously, 147 years have already passed since the latest large rupture, in 1857.
Jing Liu (Division of Geological and Planetary Sciences, Caltech, Pasadena, CA), Yann Klinger (Laboratoire de Tectonique, Institut de Physique du Globe de Paris, Paris, France), Kerry Sieh (Division of Geological and Planetary Sciences, Caltech, Pasadena, CA), and Charles Rubin (Department of Geological Sciences, Central Washington University, WA)
We document the precise sizes, but not the dates, of the six latest offsets across the San Andreas fault at Wallace Creek, California. Three and perhaps four of these, including the latest in 1857, show dextral offset of 7.5Ð8 m. The third and fourth offsets, however, are just 1.4 and 5.2 m. The predominance of similar offsets for the latest six events suggests that the fundamental properties of the fault system that control slip size do not vary greatly from event to event. The large offsets imply that ruptures involving this site are typically more than 200 km long.
A basic tenet in probabilistic earthquake forecasts is the assumption that large earthquakes occur by repeating characteristic slip along a fault [Working Group on California Earthquake Probabilities, 1988; Working Group on California Earthquake Probabilities, 1995]. The presumption that we can model source recurrence as regular progression of repeatable earthquakes begin with H. F. Reid's formulation of elastic rebound theory . The basis for such an understanding is intuitive, where seismicity on a fault is represented by repeating slip patterns that reflect along strike differences in fault friction [e.g. Stuart, 1986; Rundle, 1988, and Ward, 1993]. The underlying assumption is that earthquake recurrence intervals are controlled by relatively uniform stress buildup and complete strain release. If, on the other hand, strain release rates are irregular, models of source recurrence and calculations of earthquake probabilities will need to be modified.
along the Carrizo segment (Figure
1a) is one of the most promising sites to investigate slip
per event. Here, small gullies have incised across the simple
trace of the San Andreas fault (Figure
1b). Channels A and B have downcut about 2 m into Pleistocene
alluvium northeast of the fault. Both channels A and B are associated
with small, right-lateral offset gullies on the southwestern side
of the fault. Each channel was offset 9.5 m during the 1857 earthquake.
However, where the channels intersect the fault is uncertain,
due to subsequent degradation along the base of the fault scarp.
The uncertainties of channel morphology and its precise intersection
along the fault give different offset values. Because these geomorphic
observations lead to uncertainties, we are excavating series of
3-D excavations on a few of the channel pairs, just south of Wallace
Creek, in order to determine the exact geometry of the channels
adjacent to the fault that will give greatly refined slip per
event values. Click here for a detailed aerial
view of Wallace Creek.
Below are a few figures and photographs taken south of Wallace Creek. Click on each image for a high-resolution image
Additional information about the Carrizo Plain, including a virtual field trip, can be found at http://www.scec.org/wallacecreek