Earthquakes No One Remembers

Today we will be discussing earthquakes that no one remembers. No, definitely no one; the most recent we will be discussing occurred in 1857. Earthquakes, by their nature, aren’t things we can predict, but of course that doesn’t stop scientists from trying to model them and see if there are any patterns. There is some debate on which type of model is best, or if perhaps some models suit some regions better than others.
Comparing model results and observed earthquakes is not an easy task. This is mostly because empirical observations only go back so far in history, and the large timescales between earthquakes, especially larger ones that potentially come hundreds or even thousands of years apart. There are two ways of approaching this; one is to look at the offset of geological features but has the flaw of assuming that they come from successive earthquakes. While this covers a lot of ground, it leaves open possible errors in terms of when the earthquake(s) occurred and the potential contribution from smaller earthquakes. The second method uses paleoseismology, or the study of prehistoric earthquakes using geological methods. Paleoseismology finds and date the evidence of past earthquakes, though errors may stem from radiocarbon dating and the long distances involved. A combination of these two methods creates a much more complete picture of past earthquakes.
This brings us to today’s article, “Paleoearthquakes at Frazier Mountain, California delimit extent and frequency of past San Andreas Fault ruptures along 1857 trace” written by Katherine Scharer, Ray Weldon II, Ashley Streig, and Thomas Fumal. Published in the American Geophysical Union’s Geophysical Research Letters in 2014, this takes a paleoseismological approach to the southern San Andreas Fault (SSAF). 
The historical record in this area is complex to begin with. The extent of land affected by earthquakes overlaps for about 50 kilometers (31 miles) between the magnitude 7.7 Wrightwood earthquake in 1812, and the magnitude 7.7 to 7.9 Fort Tejon earthquake in 1857. This showed a stretch of about 60 kilometers (37 miles) where the surface was moved 9 meters (30 feet), twice the average. Using paleoseismology, it was later found that at least part of this was likely due to the result of multiple earthquakes.
This map shows the epicenter of the M7.7-7.9 Fort Tejon Earthquake with a star and labels a few locations in California. The bright green line traces the San Andreas Fault. Image Credit: United States Geological Survey.
This map shows the epicenter of the M7.7-7.9 Fort Tejon Earthquake with a star and labels a few locations in California. The bright green line traces the San Andreas Fault. Image Credit: United States Geological Survey.
Today’s article focused on an approximately 200 kilometer (124 mile) stretch where there is much uncertainty, near Frazier Mountain. The team dug a series of trenches 3 to 9 meters (10 to 30 feet) deep to reveal layers of rock that recorded earthquakes and could be dated. Radiocarbon dating was used; this uses an isotope (variant) of carbon that slowly decays at a steady rate in order to see how old something is. 37 samples  from charcoal and plants being radiocarbon dated yielded a timeline for the geological record. There is about fifty years give or take on the dates returned, as is typical for this method.
In addition to the Frazier Mountain site, there was also data from the locations of Bidart Fan, Little Rock, Pallett Creek, and Wrightwood. The oldest time that all locations had in common was about 1350 AD. Some areas, like Frazier Mountain, had evidence of six or seven earthquakes (one deposit could have been caused by a non-earthquake source and thus is ambiguous), while others had evidence of only three earthquakes. Comparing the records and times suggests only one other earthquake on the same scale of the 1857 quake, sometime around 1550. The evidence suggests that the length of ruptures varies, suggesting one type of modeling over another, though more tests would be needed to verify the more appropriate test.
One remaining question is whether or not the 1857 earthquake was unusual. In itself it seems to be not outside the norm, but it was followed by silence. On the other hand, the suggested 1550 earthquake was followed by several smaller events. Which is more typical of this region is a question we cannot yet answer.

 

Bibliography
Scharer, K., R. Weldon II, A. Streig, and T. Fumal (2014), Paleoearthquakes at Frazier Mountain, California delimit extent and frequency of past San Andreas Fault ruptures along 1857 trace, Geophys. Res. Lett., 41, 45274534, doi:10.1002/2014GL060318.
Today’s main article.
United States Geological Survey. The Great M7.9 1857 Fort Tejon Earthquake. USGS, n.p., n.d. Web.
Some information on the 1857 earthquake, and the source for the image in today’s article.
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