Re-thinking Southern California earthquake scenarios in Coachella Valley, San Andreas Fault

The Coachella Valley segment of the southernmost section of the San Andreas Fault in California has a high likelihood for a large rupture in the near future, since it has a recurrence interval of about 180 years but has not ruptured in over 300 years. -  UMass Amherst and Google Earth
The Coachella Valley segment of the southernmost section of the San Andreas Fault in California has a high likelihood for a large rupture in the near future, since it has a recurrence interval of about 180 years but has not ruptured in over 300 years. – UMass Amherst and Google Earth

New three-dimensional (3D) numerical modeling that captures far more geometric complexity of an active fault segment in southern California than any other, suggests that the overall earthquake hazard for towns on the west side of the Coachella Valley such as Palm Springs and Palm Desert may be slightly lower than previously believed.

New simulations of deformation on three alternative fault configurations for the Coachella Valley segment of the San Andreas Fault conducted by geoscientists Michele Cooke and Laura Fattaruso of the University of Massachusetts Amherst, with Rebecca Dorsey of the University of Oregon, appear in the December issue of Geosphere.

The Coachella Valley segment is the southernmost section of the San Andreas Fault in California. It has a high likelihood for a large rupture in the near future, since it has a recurrence interval of about 180 years but has not ruptured in over 300 years, the authors point out.

The researchers acknowledge that their new modeling offers “a pretty controversial interpretation” of the data. Many geoscientists do not accept a dipping active fault geometry to the San Andreas Fault in the Coachella Valley, they say. Some argue that the data do not confirm the dipping structure. “Our contribution to this debate is that we add an uplift pattern to the data that support a dipping active fault and it rejects the other models,” say Cooke and colleagues.

Their new model yields an estimated 10 percent increase in shaking overall for the Coachella segment. But for the towns to the west of the fault where most people live, it yields decreased shaking due to the dipping geometry. It yields a doubling of shaking in mostly unpopulated areas east of the fault. “This isn’t a direct outcome of our work but an implication,” they add.

Cooke says, “Others have used a dipping San Andreas in their models but they didn’t include the degree of complexity that we did. By including the secondary faults within the Mecca Hills we more accurately capture the uplift pattern of the region.”

Fattaruso adds, “Others were comparing to different data sets, such as geodesy, and since we were comparing to uplift it is important that we have this complexity.” In this case, geodesy is the science of measuring and representing the Earth and its crustal motion, taking into account the competition of geological processes in 3D over time.

Most other models of deformation, stress, rupture and ground shaking have assumed that the southern San Andreas Fault is vertical, say Cooke and colleagues. However, seismic, imaging, aerial magnetometric surveys and GPS-based strain observations suggest that the fault dips 60 to 70 degrees toward the northeast, a hypothesis they set out to investigate.

Specifically, they explored three alternative geometric models of the fault’s Coachella Valley segment with added complexity such as including smaller faults in the nearby Indio and Mecca Hills. “We use localized uplift patterns in the Mecca Hills to assess the most plausible geometry for the San Andreas Fault in the Coachella Valley and better understand the interplay of fault geometry and deformation,” they write.

Cooke and colleagues say the fault structures in their favored model agree with distributions of local seismicity, and are consistent with geodetic observations of recent strain. “Crustal deformation models that neglect the northeast dip of the San Andreas Fault in the Coachella Valley will not replicate the ground shaking in the region and therefore inaccurately estimate seismic hazard,” they note.

This work was supported by the National Science Foundation.
More: http://geosphere.gsapubs.org/content/10/6/1235.abstract

Re-thinking Southern California earthquake scenarios in Coachella Valley, San Andreas Fault

The Coachella Valley segment of the southernmost section of the San Andreas Fault in California has a high likelihood for a large rupture in the near future, since it has a recurrence interval of about 180 years but has not ruptured in over 300 years. -  UMass Amherst and Google Earth
The Coachella Valley segment of the southernmost section of the San Andreas Fault in California has a high likelihood for a large rupture in the near future, since it has a recurrence interval of about 180 years but has not ruptured in over 300 years. – UMass Amherst and Google Earth

New three-dimensional (3D) numerical modeling that captures far more geometric complexity of an active fault segment in southern California than any other, suggests that the overall earthquake hazard for towns on the west side of the Coachella Valley such as Palm Springs and Palm Desert may be slightly lower than previously believed.

New simulations of deformation on three alternative fault configurations for the Coachella Valley segment of the San Andreas Fault conducted by geoscientists Michele Cooke and Laura Fattaruso of the University of Massachusetts Amherst, with Rebecca Dorsey of the University of Oregon, appear in the December issue of Geosphere.

The Coachella Valley segment is the southernmost section of the San Andreas Fault in California. It has a high likelihood for a large rupture in the near future, since it has a recurrence interval of about 180 years but has not ruptured in over 300 years, the authors point out.

The researchers acknowledge that their new modeling offers “a pretty controversial interpretation” of the data. Many geoscientists do not accept a dipping active fault geometry to the San Andreas Fault in the Coachella Valley, they say. Some argue that the data do not confirm the dipping structure. “Our contribution to this debate is that we add an uplift pattern to the data that support a dipping active fault and it rejects the other models,” say Cooke and colleagues.

Their new model yields an estimated 10 percent increase in shaking overall for the Coachella segment. But for the towns to the west of the fault where most people live, it yields decreased shaking due to the dipping geometry. It yields a doubling of shaking in mostly unpopulated areas east of the fault. “This isn’t a direct outcome of our work but an implication,” they add.

Cooke says, “Others have used a dipping San Andreas in their models but they didn’t include the degree of complexity that we did. By including the secondary faults within the Mecca Hills we more accurately capture the uplift pattern of the region.”

Fattaruso adds, “Others were comparing to different data sets, such as geodesy, and since we were comparing to uplift it is important that we have this complexity.” In this case, geodesy is the science of measuring and representing the Earth and its crustal motion, taking into account the competition of geological processes in 3D over time.

Most other models of deformation, stress, rupture and ground shaking have assumed that the southern San Andreas Fault is vertical, say Cooke and colleagues. However, seismic, imaging, aerial magnetometric surveys and GPS-based strain observations suggest that the fault dips 60 to 70 degrees toward the northeast, a hypothesis they set out to investigate.

Specifically, they explored three alternative geometric models of the fault’s Coachella Valley segment with added complexity such as including smaller faults in the nearby Indio and Mecca Hills. “We use localized uplift patterns in the Mecca Hills to assess the most plausible geometry for the San Andreas Fault in the Coachella Valley and better understand the interplay of fault geometry and deformation,” they write.

Cooke and colleagues say the fault structures in their favored model agree with distributions of local seismicity, and are consistent with geodetic observations of recent strain. “Crustal deformation models that neglect the northeast dip of the San Andreas Fault in the Coachella Valley will not replicate the ground shaking in the region and therefore inaccurately estimate seismic hazard,” they note.

This work was supported by the National Science Foundation.
More: http://geosphere.gsapubs.org/content/10/6/1235.abstract

Severe drought is causing the western US to rise

The severe drought gripping the western United States in recent years is changing the landscape well beyond localized effects of water restrictions and browning lawns. Scientists at Scripps Institution of Oceanography at UC San Diego have now discovered that the growing, broad-scale loss of water is causing the entire western U.S. to rise up like an uncoiled spring.

Investigating ground positioning data from GPS stations throughout the west, Scripps researchers Adrian Borsa, Duncan Agnew, and Dan Cayan found that the water shortage is causing an “uplift” effect up to 15 millimeters (more than half an inch) in California’s mountains and on average four millimeters (0.15 of an inch) across the west. From the GPS data, they estimate the water deficit at nearly 240 gigatons (62 trillion gallons of water), equivalent to a six-inch layer of water spread out over the entire western U.S.

Results of the study, which was supported by the U.S. Geological Survey (USGS), appear in the August 21 online edition of the journal Science.

While poring through various sets of data of ground positions from highly precise GPS stations within the National Science Foundation’s Plate Boundary Observatory and other networks, Borsa, a Scripps assistant research geophysicist, kept noticing the same pattern over the 2003-2014 period: All of the stations moved upwards in the most recent years, coinciding with the timing of the current drought.

Agnew, a Scripps Oceanography geophysics professor who specializes in studying earthquakes and their impact on shaping the earth’s crust, says the GPS data can only be explained by rapid uplift of the tectonic plate upon which the western U.S. rests (Agnew cautions that the uplift has virtually no effect on the San Andreas fault and therefore does not increase the risk of earthquakes).

For Cayan, a research meteorologist with Scripps and USGS, the results paint a new picture of the dire hydrological state of the west.

“These results quantify the amount of water mass lost in the past few years,” said Cayan. “It also represents a powerful new way to track water resources over a very large landscape. We can home in on the Sierra Nevada mountains and critical California snowpack. These results demonstrate that this technique can be used to study changes in fresh water stocks in other regions around the world, if they have a network of GPS sensors.”

Severe drought is causing the western US to rise

The severe drought gripping the western United States in recent years is changing the landscape well beyond localized effects of water restrictions and browning lawns. Scientists at Scripps Institution of Oceanography at UC San Diego have now discovered that the growing, broad-scale loss of water is causing the entire western U.S. to rise up like an uncoiled spring.

Investigating ground positioning data from GPS stations throughout the west, Scripps researchers Adrian Borsa, Duncan Agnew, and Dan Cayan found that the water shortage is causing an “uplift” effect up to 15 millimeters (more than half an inch) in California’s mountains and on average four millimeters (0.15 of an inch) across the west. From the GPS data, they estimate the water deficit at nearly 240 gigatons (62 trillion gallons of water), equivalent to a six-inch layer of water spread out over the entire western U.S.

Results of the study, which was supported by the U.S. Geological Survey (USGS), appear in the August 21 online edition of the journal Science.

While poring through various sets of data of ground positions from highly precise GPS stations within the National Science Foundation’s Plate Boundary Observatory and other networks, Borsa, a Scripps assistant research geophysicist, kept noticing the same pattern over the 2003-2014 period: All of the stations moved upwards in the most recent years, coinciding with the timing of the current drought.

Agnew, a Scripps Oceanography geophysics professor who specializes in studying earthquakes and their impact on shaping the earth’s crust, says the GPS data can only be explained by rapid uplift of the tectonic plate upon which the western U.S. rests (Agnew cautions that the uplift has virtually no effect on the San Andreas fault and therefore does not increase the risk of earthquakes).

For Cayan, a research meteorologist with Scripps and USGS, the results paint a new picture of the dire hydrological state of the west.

“These results quantify the amount of water mass lost in the past few years,” said Cayan. “It also represents a powerful new way to track water resources over a very large landscape. We can home in on the Sierra Nevada mountains and critical California snowpack. These results demonstrate that this technique can be used to study changes in fresh water stocks in other regions around the world, if they have a network of GPS sensors.”

New Oso report, rockfall in Yosemite, and earthquake models

From AGU’s blogs: Oso disaster had its roots in earlier landslides

A research team tasked with being some of the first scientists and engineers to evaluate extreme events has issued its findings on disastrous Oso, Washington, landslide. The report studies the conditions and causes related to the March 22 mudslide that killed 43 people and destroyed the Steelhead Haven neighborhood in Oso, Washington. The team from the Geotechnical Extreme Events Reconnaissance (GEER) Association, funded by the National Science Foundation, determined that intense rainfall in the three weeks before the slide likely was a major issue, but factors such as altered groundwater migration, weakened soil consistency because of previous landslides and changes in hillside stresses played key roles.

From this week’s Eos: Reducing Rockfall Risk in Yosemite National Park

The glacially sculpted granite walls of Yosemite Valley attract 4 million visitors a year, but rockfalls from these cliffs pose substantial hazards. Responding to new studies, the National Park Service recently took actions to reduce the human risk posed by rockfalls in Yosemite National Park.

From AGU’s journals: A new earthquake model may explain discrepancies in San Andreas fault slip

Investigating the earthquake hazards of the San Andreas Fault System requires an accurate understanding of accumulating stresses and the history of past earthquakes. Faults tend to go through an “earthquake cycle”-locking and accumulating stress, rupturing in an earthquake, and locking again in a well-accepted process known as “elastic rebound.” One of the key factors in preparing for California’s next “Big One” is estimating the fault slip rate, the speed at which one side of the San Andreas Fault is moving past the other.

Broadly speaking, there are two ways geoscientists study fault slip. Geologists formulate estimates by studying geologic features at key locations to study slip rates through time. Geodesists, scientists who measure the size and shape of the planet, use technologies like GPS and satellite radar interferometry to estimate the slip rate, estimates which often differ from the geologists’ estimations.

In a recent study by Tong et al., the authors develop a new three-dimensional viscoelastic earthquake cycle model that represents 41 major fault segments of the San Andreas Fault System. While previous research has suggested that there are discrepancies between the fault slip rates along the San Andreas as measured by geologic and geodetic means, the authors find that there are no significant differences between the two measures if the thickness of the tectonic plate and viscoelasticity are taken into account. The authors find that the geodetic slip rate depends on the plate thickness over the San Andreas, a variable lacking in previous research.

The team notes that of the 41 studied faults within the San Andreas Fault system, a small number do in fact have disagreements between the geologic and geodetic slip rates. These differences could be attributed to inadequate data coverage or to incomplete knowledge of the fault structures or the chronological sequence of past events.

New Oso report, rockfall in Yosemite, and earthquake models

From AGU’s blogs: Oso disaster had its roots in earlier landslides

A research team tasked with being some of the first scientists and engineers to evaluate extreme events has issued its findings on disastrous Oso, Washington, landslide. The report studies the conditions and causes related to the March 22 mudslide that killed 43 people and destroyed the Steelhead Haven neighborhood in Oso, Washington. The team from the Geotechnical Extreme Events Reconnaissance (GEER) Association, funded by the National Science Foundation, determined that intense rainfall in the three weeks before the slide likely was a major issue, but factors such as altered groundwater migration, weakened soil consistency because of previous landslides and changes in hillside stresses played key roles.

From this week’s Eos: Reducing Rockfall Risk in Yosemite National Park

The glacially sculpted granite walls of Yosemite Valley attract 4 million visitors a year, but rockfalls from these cliffs pose substantial hazards. Responding to new studies, the National Park Service recently took actions to reduce the human risk posed by rockfalls in Yosemite National Park.

From AGU’s journals: A new earthquake model may explain discrepancies in San Andreas fault slip

Investigating the earthquake hazards of the San Andreas Fault System requires an accurate understanding of accumulating stresses and the history of past earthquakes. Faults tend to go through an “earthquake cycle”-locking and accumulating stress, rupturing in an earthquake, and locking again in a well-accepted process known as “elastic rebound.” One of the key factors in preparing for California’s next “Big One” is estimating the fault slip rate, the speed at which one side of the San Andreas Fault is moving past the other.

Broadly speaking, there are two ways geoscientists study fault slip. Geologists formulate estimates by studying geologic features at key locations to study slip rates through time. Geodesists, scientists who measure the size and shape of the planet, use technologies like GPS and satellite radar interferometry to estimate the slip rate, estimates which often differ from the geologists’ estimations.

In a recent study by Tong et al., the authors develop a new three-dimensional viscoelastic earthquake cycle model that represents 41 major fault segments of the San Andreas Fault System. While previous research has suggested that there are discrepancies between the fault slip rates along the San Andreas as measured by geologic and geodetic means, the authors find that there are no significant differences between the two measures if the thickness of the tectonic plate and viscoelasticity are taken into account. The authors find that the geodetic slip rate depends on the plate thickness over the San Andreas, a variable lacking in previous research.

The team notes that of the 41 studied faults within the San Andreas Fault system, a small number do in fact have disagreements between the geologic and geodetic slip rates. These differences could be attributed to inadequate data coverage or to incomplete knowledge of the fault structures or the chronological sequence of past events.

The next ‘Big One’ for the Bay Area may be a cluster of major quakes

A cluster of closely timed earthquakes over 100 years in the 17th and 18th centuries released as much accumulated stress on San Francisco Bay Area’s major faults as the Great 1906 San Francisco earthquake, suggesting two possible scenarios for the next “Big One” for the region, according to new research published by the Bulletin of the Seismological Society of America (BSSA).

“The plates are moving,” said David Schwartz, a geologist with the U.S. Geological Survey and co-author of the study. “The stress is re-accumulating, and all of these faults have to catch up. How are they going to catch up?”

The San Francisco Bay Region (SFBR) is considered within the boundary between the Pacific and North American plates. Energy released during its earthquake cycle occurs along the region’s principal faults: the San Andreas, San Gregorio, Calaveras, Hayward-Rodgers Creek, Greenville, and Concord-Green Valley faults.

“The 1906 quake happened when there were fewer people, and the area was much less developed,” said Schwartz. “The earthquake had the beneficial effect of releasing the plate boundary stress and relaxing the crust, ushering in a period of low level earthquake activity.”

The earthquake cycle reflects the accumulation of stress, its release as slip on a fault or a set of faults, and its re-accumulation and re-release. The San Francisco Bay Area has not experienced a full earthquake cycle since its been occupied by people who have reported earthquake activity, either through written records or instrumentation. Founded in 1776, the Mission Dolores and the Presidio in San Francisco kept records of felt earthquakes and earthquake damage, marking the starting point for the historic earthquake record for the region.

“We are looking back at the past to get a more reasonable view of what’s going to happen decades down the road,” said Schwartz. “The only way to get a long history is to do these paleoseismic studies, which can help construct the rupture histories of the faults and the region. We are trying to see what went on and understand the uncertainties for the Bay Area.”

Schwartz and colleagues excavated trenches across faults, observing past surface ruptures from the most recent earthquakes on the major faults in the area. Radiocarbon dating of detrital charcoal and the presence of non-native pollen established the dates of paleoearthquakes, expanding the span of information of large events back to 1600.

The trenching studies suggest that between 1690 and the founding of the Mission Dolores and Presidio in 1776, a cluster of earthquakes ranging from magnitude 6.6 to 7.8 occurred on the Hayward fault (north and south segments), San Andreas fault (North Coast and San Juan Bautista segments), northern Calaveras fault, Rodgers Creek fault, and San Gregorio fault. There are no paleoearthquake data for the Greenville fault or northern extension of the Concord-Green Valley fault during this time interval.

“What the cluster of earthquakes did in our calculations was to release an amount of energy somewhat comparable to the amount released in the crust by the 1906 quake,” said Schwartz.

As stress on the region accumulates, the authors see at least two modes of energy release – one is a great earthquake and other is a cluster of large earthquakes. The probability for how the system will rupture is spread out over all faults in the region, making a cluster of large earthquakes more likely than a single great earthquake.

“Everybody is still thinking about a repeat of the 1906 quake,” said Schwartz. “It’s one thing to have a 1906-like earthquake where seismic activity is shut off, and we slide through the next 110 years in relative quiet. But what happens if every five years we get a magnitude 6.8 or 7.2? That’s not outside the realm of possibility.”

The next ‘Big One’ for the Bay Area may be a cluster of major quakes

A cluster of closely timed earthquakes over 100 years in the 17th and 18th centuries released as much accumulated stress on San Francisco Bay Area’s major faults as the Great 1906 San Francisco earthquake, suggesting two possible scenarios for the next “Big One” for the region, according to new research published by the Bulletin of the Seismological Society of America (BSSA).

“The plates are moving,” said David Schwartz, a geologist with the U.S. Geological Survey and co-author of the study. “The stress is re-accumulating, and all of these faults have to catch up. How are they going to catch up?”

The San Francisco Bay Region (SFBR) is considered within the boundary between the Pacific and North American plates. Energy released during its earthquake cycle occurs along the region’s principal faults: the San Andreas, San Gregorio, Calaveras, Hayward-Rodgers Creek, Greenville, and Concord-Green Valley faults.

“The 1906 quake happened when there were fewer people, and the area was much less developed,” said Schwartz. “The earthquake had the beneficial effect of releasing the plate boundary stress and relaxing the crust, ushering in a period of low level earthquake activity.”

The earthquake cycle reflects the accumulation of stress, its release as slip on a fault or a set of faults, and its re-accumulation and re-release. The San Francisco Bay Area has not experienced a full earthquake cycle since its been occupied by people who have reported earthquake activity, either through written records or instrumentation. Founded in 1776, the Mission Dolores and the Presidio in San Francisco kept records of felt earthquakes and earthquake damage, marking the starting point for the historic earthquake record for the region.

“We are looking back at the past to get a more reasonable view of what’s going to happen decades down the road,” said Schwartz. “The only way to get a long history is to do these paleoseismic studies, which can help construct the rupture histories of the faults and the region. We are trying to see what went on and understand the uncertainties for the Bay Area.”

Schwartz and colleagues excavated trenches across faults, observing past surface ruptures from the most recent earthquakes on the major faults in the area. Radiocarbon dating of detrital charcoal and the presence of non-native pollen established the dates of paleoearthquakes, expanding the span of information of large events back to 1600.

The trenching studies suggest that between 1690 and the founding of the Mission Dolores and Presidio in 1776, a cluster of earthquakes ranging from magnitude 6.6 to 7.8 occurred on the Hayward fault (north and south segments), San Andreas fault (North Coast and San Juan Bautista segments), northern Calaveras fault, Rodgers Creek fault, and San Gregorio fault. There are no paleoearthquake data for the Greenville fault or northern extension of the Concord-Green Valley fault during this time interval.

“What the cluster of earthquakes did in our calculations was to release an amount of energy somewhat comparable to the amount released in the crust by the 1906 quake,” said Schwartz.

As stress on the region accumulates, the authors see at least two modes of energy release – one is a great earthquake and other is a cluster of large earthquakes. The probability for how the system will rupture is spread out over all faults in the region, making a cluster of large earthquakes more likely than a single great earthquake.

“Everybody is still thinking about a repeat of the 1906 quake,” said Schwartz. “It’s one thing to have a 1906-like earthquake where seismic activity is shut off, and we slide through the next 110 years in relative quiet. But what happens if every five years we get a magnitude 6.8 or 7.2? That’s not outside the realm of possibility.”

San Francisco’s big 1906 quake was third of a series on San Andreas Fault

University of Oregon doctoral student Ashley Streig shows a tree stump on which tree-ring dating indicates the tree was cut prior to the earthquake of 1838 on the San Andreas Fault in the Santa Cruz Mountains. -  University of Oregon
University of Oregon doctoral student Ashley Streig shows a tree stump on which tree-ring dating indicates the tree was cut prior to the earthquake of 1838 on the San Andreas Fault in the Santa Cruz Mountains. – University of Oregon

Research led by a University of Oregon doctoral student in California’s Santa Cruz Mountains has uncovered geologic evidence that supports historical narratives for two earthquakes in the 68 years prior to San Francisco’s devastating 1906 disaster.

The evidence places the two earthquakes, in 1838 and 1890, on the San Andreas Fault, as theorized by many researchers based on written accounts about damage to Spanish-built missions in the Monterey and San Francisco bay areas. These two quakes, as in 1906, were surface-rupturing events, the researchers concluded.

Continuing work, says San Francisco Bay-area native Ashley R. Streig, will dig deeper into the region’s geological record — layers of sediment along the fault — to determine if the ensuing seismically quiet years make up a normal pattern — or not — of quake frequency along the fault.

Streig is lead author of the study, published in this month’s issue of the Bulletin of the Seismological Society of America. She collaborated on the project with her doctoral adviser Ray Weldon, professor of the UO’s Department of Geological Sciences, and Timothy E. Dawson of the Menlo Park office of the California Geological Survey.

The study was 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. The Santa Cruz Mountains run for about 39 miles from south of San Francisco to near San Juan Batista. Hazel Dell is 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, whose introduction to major earthquakes came at age 11 during the 1989 Loma Prieta Earthquake on a deep sub-fault of the San Andreas Fault zone. That quake, which disrupted baseball’s World Series, forced her family to camp outside their home.

Unlike the 1906 quake that ruptured 470 km (296 mi) of the fault, the 1838 and 1890 quakes ruptured shorter portions of the fault, 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.”

The team identified ax-cut wood chips, tree stumps and charcoal fragments from early logging efforts in unexpectedly deep layers of sediment, 1.5 meters (five feet) below the ground, and document evidence of three earthquakes since logging occurred at the site. The logging story emerged from 16 trenches dug in 2008, 2010 and 2011 along the fault at the Hazel Dell site in the mountain range.

High-resolution radiocarbon dating of tree-rings from the wood chips and charcoal confirm these are post European deposits, and the geologic earthquake evidence 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.

Additionally, in 1906 individuals living near the Hazel Dell site reported to geologists that cracks from the 1906 earthquake had occurred just where they had 16 years earlier, in 1890, which, Streig and colleagues say, was probably centered in the Hazel Dell region. Another displacement of sediment at the Hazel Dell site matched the timeline of the 1906 quake.

The project also allowed the team to conclude that another historically reported quake, in 1865, was not surface rupturing, but it was probably deep and, like the 1989 event, occurred on a sub zone of the San Andreas Fault. Conventional thinking, Streig said, has suggested that the San Andreas Fault always ruptures in a long-reaching fashion similar to the 1906 earthquake. This study, however, points 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.”

The frequency of the quakes in the Santa Cruz Mountains, she added, must have been a terrifying experience 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.”

More broadly, Weldon said, having multiple paleoseismic sites close together on a major fault, geologists now realize that interpretations gleaned from 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.”

San Francisco’s big 1906 quake was third of a series on San Andreas Fault

University of Oregon doctoral student Ashley Streig shows a tree stump on which tree-ring dating indicates the tree was cut prior to the earthquake of 1838 on the San Andreas Fault in the Santa Cruz Mountains. -  University of Oregon
University of Oregon doctoral student Ashley Streig shows a tree stump on which tree-ring dating indicates the tree was cut prior to the earthquake of 1838 on the San Andreas Fault in the Santa Cruz Mountains. – University of Oregon

Research led by a University of Oregon doctoral student in California’s Santa Cruz Mountains has uncovered geologic evidence that supports historical narratives for two earthquakes in the 68 years prior to San Francisco’s devastating 1906 disaster.

The evidence places the two earthquakes, in 1838 and 1890, on the San Andreas Fault, as theorized by many researchers based on written accounts about damage to Spanish-built missions in the Monterey and San Francisco bay areas. These two quakes, as in 1906, were surface-rupturing events, the researchers concluded.

Continuing work, says San Francisco Bay-area native Ashley R. Streig, will dig deeper into the region’s geological record — layers of sediment along the fault — to determine if the ensuing seismically quiet years make up a normal pattern — or not — of quake frequency along the fault.

Streig is lead author of the study, published in this month’s issue of the Bulletin of the Seismological Society of America. She collaborated on the project with her doctoral adviser Ray Weldon, professor of the UO’s Department of Geological Sciences, and Timothy E. Dawson of the Menlo Park office of the California Geological Survey.

The study was 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. The Santa Cruz Mountains run for about 39 miles from south of San Francisco to near San Juan Batista. Hazel Dell is 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, whose introduction to major earthquakes came at age 11 during the 1989 Loma Prieta Earthquake on a deep sub-fault of the San Andreas Fault zone. That quake, which disrupted baseball’s World Series, forced her family to camp outside their home.

Unlike the 1906 quake that ruptured 470 km (296 mi) of the fault, the 1838 and 1890 quakes ruptured shorter portions of the fault, 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.”

The team identified ax-cut wood chips, tree stumps and charcoal fragments from early logging efforts in unexpectedly deep layers of sediment, 1.5 meters (five feet) below the ground, and document evidence of three earthquakes since logging occurred at the site. The logging story emerged from 16 trenches dug in 2008, 2010 and 2011 along the fault at the Hazel Dell site in the mountain range.

High-resolution radiocarbon dating of tree-rings from the wood chips and charcoal confirm these are post European deposits, and the geologic earthquake evidence 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.

Additionally, in 1906 individuals living near the Hazel Dell site reported to geologists that cracks from the 1906 earthquake had occurred just where they had 16 years earlier, in 1890, which, Streig and colleagues say, was probably centered in the Hazel Dell region. Another displacement of sediment at the Hazel Dell site matched the timeline of the 1906 quake.

The project also allowed the team to conclude that another historically reported quake, in 1865, was not surface rupturing, but it was probably deep and, like the 1989 event, occurred on a sub zone of the San Andreas Fault. Conventional thinking, Streig said, has suggested that the San Andreas Fault always ruptures in a long-reaching fashion similar to the 1906 earthquake. This study, however, points 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.”

The frequency of the quakes in the Santa Cruz Mountains, she added, must have been a terrifying experience 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.”

More broadly, Weldon said, having multiple paleoseismic sites close together on a major fault, geologists now realize that interpretations gleaned from 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.”