San Francisco’s ‘Big One’ In 1906 Was Third In A Series Of Quakes
April Flowers for redOrbit.com – Your Universe Online
Geologic evidence that supports historical narratives for two earthquakes in the 68 years prior to the 1906 quake that devastated San Francisco has been discovered by a research team led by the University of Oregon.
The evidence, discovered by doctoral student Ashley R. Streig, places the two earthquakes—in 1838 and 1890—on the San Andreas Fault. Previously, many researchers had theorized this placement based on written accounts about damage to Spanish-built missions in the Monterey and San Francisco Bay areas. As with the 1906 quake, these two were surface-rupturing events.
Streig is continuing the work by digging deeper into the region’s geological record—layers of sediment along the fault. Her goal is to determine if the ensuing seismically quiet years make up a normal pattern of quake frequency along the fault or not. The results of the current study were published in a recent issue of Bulletin of the Seismological Society of America.
The team, which included Ray Weldon, professor of the UO’s Department of Geological Sciences and Streig’s doctoral advisor, and Timothy E. Dawson of the Menlo Park office of the California Geological Survey, were the first to fully map the active fault trace in the Santa Cruz Mountains using a combination of on-the-ground observations and airborne Light Detection and Ranging (LIDAR), a remote sensing technology. This mountain range runs approximately 39 miles from south of San Francisco to near San Juan Batista, with Hazel Dell being located east of Santa Cruz and north of Watsonville.
“We found the first geologic evidence of surface rupture by what looks like the 1838 and 1890 earthquakes, as well as 1906,” said Streig. She was first introduced to major earthquakes at age 11 during the 1989 Loma Prieta Earthquake on a deep sub-fault of the San Andreas Fault zone when her family was forced to camp outside their home and baseball’s World Series was disrupted.
The 1906 quake ruptured 296 miles of the San Andreas Fault. In contrast, the 1838 and 1890 quakes ruptured shorter portions, possibly limited to the Santa Cruz Mountains.
“This is the first time we have had good, clear geologic evidence of these historic 19th century earthquakes,” she said. “It’s important because it tells us that we had three surface ruptures, really closely spaced in time that all had fairly large displacements of at least half a meter and probably larger.”
Ax-cut wood chips, charcoal fragments and tree stumps from early logging efforts were discovered by the team in unexpectedly deep layers of sediment, approximately five feet below ground. They also documented evidence of three earthquakes since the logging had occurred. The team dug 16 trenches along the fault at the Hazel Dell site in the mountain range in 2008, 2010 and 2011, to uncover the logging evidence.
Streig and her team used high-resolution carbon dating of the tree-rings from the wood chips and charcoal to confirm these are post European deposits. The geologic evidence observed at the site coincides with written accounts describing local earthquake damage, including damage to Spanish missions in 1838, and in a USGS publication of earthquakes in 1890 catalogued by an astronomer from Lick Observatory.
Additional anecdotal reports from individuals in the Hazel Dell region in 1906 made to geologists note cracks from the 1906 earthquake had occurred just where they had 16 years earlier, in 1890. Streig and her colleagues say the 1890 earthquake was probably centered in the Hazel Dell region. They observed another sediment displacement at the Hazel Dell site that matched the timeline of the 1906 quake.
The team also concluded that another historically reported quake, in 1865, was not surface rupturing, but it was probably deep. Like the 1989 event, the 1865 quake occurred on a sub zone of the San Andreas Fault. Streig said that conventional thinking suggests that the San Andreas Fault always ruptures in a long-reaching fashion similar to the 1906 earthquake. Her findings, however, point to more regionally confined ruptures as well.
“This all tells us that there are more frequent surface-rupturing earthquakes on this section of the fault than have been previously identified, certainly in the historic period,” Streig said. “This becomes important to earthquake models because it is saying something about the connectivity of all these fault sections — and how they might link up.”
Streig thinks the frequency of quakes in the Santa Cruz Mountains must have been terrifying for settlers during the 68-year period.
“This study is the first to show three historic ruptures on the San Andreas Fault outside the special case of Parkfield,” Weldon said, referring to a region in mountains to the south of the Santa Cruz range where six magnitude 6-plus earthquakes occurred between 1857 and 1966. “The earthquakes of 1838 and 1890 were known to be somewhere nearby from shaking, but now we know the San Andreas Fault ruptured three times on the same piece of the fault in less than 100 years.”
Having multiple paleoseismic sites close together on a major fault, according to Weldon, has led geologists to realize that interpretations based on single-site evidence probably aren’t reliable. “We need to spend more time reproducing or confirming results rather than rushing to the next fault if we are going to get it right,” he said. “Ashley’s combination of historical research, C-14 dating, tree rings, pollen and stratigraphic correlation between sites has allowed us to credibly argue for precision that allows identification of the 1838 and 1890 earthquakes.”
“Researchers at the University of Oregon are using tools and technologies to further our understanding of the dynamic forces that continue to shape our planet and impact its people,” said Kimberly Andrews Espy, vice president for research and innovation and dean of the UO Graduate School. “This research furthers our understanding of the connectivity of the various sections of California’s San Andreas Fault and has the potential to save lives by leading to more accurate earthquake modeling.”