Scientists confirm Sierra Nevada 200-year megadroughts

University of Nevada, Reno, researchers were joined by a Scripps Institution of Oceanography research team, spending many days on Fallen Leaf Lake to gather sonar and side-scan radar data to study earthquake faults and paleoshorelines. The low-tech boat was adorned with high-tech hardware, such as gyroscopes used on rockets, to gather high-resolution images of the lake bottom. Using standing trees they found submerged under 130 feet of water, the team confirmed and reported in their paper, a culmination of a comprehensive high-tech assessment of Fallen Leaf Lake -- a small moraine-bound lake at the south end of the Lake Tahoe Basin -- that stands of pre-Medieval trees in the lake suggest the region experienced severe drought at least every 650 to 1,150 years during the mid- and late-Holocene period. -  Photo by Mike Wolterbeek, University of Nevada, Reno
University of Nevada, Reno, researchers were joined by a Scripps Institution of Oceanography research team, spending many days on Fallen Leaf Lake to gather sonar and side-scan radar data to study earthquake faults and paleoshorelines. The low-tech boat was adorned with high-tech hardware, such as gyroscopes used on rockets, to gather high-resolution images of the lake bottom. Using standing trees they found submerged under 130 feet of water, the team confirmed and reported in their paper, a culmination of a comprehensive high-tech assessment of Fallen Leaf Lake — a small moraine-bound lake at the south end of the Lake Tahoe Basin — that stands of pre-Medieval trees in the lake suggest the region experienced severe drought at least every 650 to 1,150 years during the mid- and late-Holocene period. – Photo by Mike Wolterbeek, University of Nevada, Reno

The erratic year-to-year swings in precipitation totals in the Reno-Tahoe area conjures up the word “drought” every couple of years, and this year is no exception. The Nevada State Climate Office at the University of Nevada, Reno, in conjunction with the Nevada Drought Response Committee, just announced a Stage 1 drought (moderate) for six counties and a Stage 2 drought (severe) for 11 counties.

Reno, Lake Tahoe and the Sierra Nevada are no strangers to drought, the most famous being the Medieval megadrought lasting from 800 to 1250 A.D. when annual precipitation was less than 60 percent of normal. The Reno-Tahoe region is now about 65 percent of annual normal precipitation for the year, which doesn’t seem like much, but imagine if this were the “norm” each and every year for the next 200 years.

Research by scientists at the University of Nevada, Reno and their partners at Scripps Institution of Oceanography in San Diego indicates that there are other instances of such long-lasting, severe droughts in the western United States throughout history. Their recent paper, a culmination of a comprehensive high-tech assessment of Fallen Leaf Lake – a small moraine-bound lake at the south end of the Lake Tahoe Basin – reports that stands of pre-Medieval trees in the lake suggest the region experienced severe drought at least every 650 to 1,150 years during the mid- and late-Holocene period.

“Using an arsenal of cutting edge sonar tools, remotely operated vehicles (ROVs), and a manned submersible, we’ve obtained potentially the most accurate record thus far on the instances of 200-year-long droughts in the Sierra,” Graham Kent, director of the Nevada Seismological Laboratory said. “The record from Fallen Leaf Lake confirms what was expected and is likely the most accurate record, in terms of precipitation, than obtained previously from a variety of methods throughout the Sierra.”

Kent is part of the University of Nevada, Reno and Scripps research team that traced the megadroughts and dry spells of the region using tree-ring analysis, shoreline records and sediment deposition in Fallen Leaf Lake. Using side-scan and multibeam sonar technology developed to map underwater earthquake fault lines such as the West Tahoe fault beneath Fallen Leaf Lake, the team also imaged standing trees up to 130 feet beneath the lake surface as well as submerged ancient shoreline structure and development. The trees matured while the lake level was 130 to 200 feet below its modern elevation and were not deposited by a landslide as was suspected.

The team, led by John Kleppe, University of Nevada, Reno engineering professor emeritus, published a paper on this research and is presenting its findings in seminars and workshops.

“The lake is like a ‘canary in a coal mine’ for the Sierra, telling the story of precipitation very clearly,” Kent said. “Fallen Leaf Lake elevations change rapidly due to its unique ratio between catchment basin and lake surface of about 8 to 1. With analysis of the standing trees submerged in the lake, sediment cores and our sonar scanning of ancient shorelines, we can more accurately and easily trace the precipitation history of the region.”

Water balance calculations and analysis of tree-ring samples undertaken by Kleppe, Kent and Scripps scientists Danny Brothers and Neal Driscoll, along with Professor Franco Biondi of the University’s College of Science, suggest annual precipitation was less than 60 percent of normal from the late 10th century to the early 13th century. Their research was documented in a scientific paper, Duration and severity of Medieval drought in the Lake Tahoe Basin, published in the Quaternary Science Reviews in November 2011.

Tree-ring records and submerged paleoshoreline geomorphology suggest a Medieval low-lake level of Fallen Leaf Lake lasted more than 220 years. More than 80 trees were found lying on the lake floor at various elevations above the paleoshoreline.

“Although the ancient cycle of megadroughts seems to occur every 650 to 1150 years and the last one was 750 years ago, it is uncertain when the next megadrought will occur. With climate change upon us, it will be interesting to see how carbon dioxide loading in the atmosphere will affect this cycle,” Kent said.

Professor Paula Noble, in the University’s College of Science’s Department of Geological Sciences and Engineering, is expanding this research to include the fine-scale study of climate change through out the Holocene (about 12,000 years) using recently collected 40-foot-long sediment cores in Fallen Leaf Lake.

New observations on the San Andreas Fault in Santa Cruz Mountains, Seattle Fault Zone

San Andreas Fault in Santa Cruz Mountains – large quakes more frequent than previously thought.

Recent paleoseismic work has documented four surface-rupturing earthquakes that occurred across the Santa Cruz Mountains section of the San Andreas Fault (SAF) in the past 500 years. The research, conducted by the U.S. Geological Survey, with assistance from the California Geological Survey, suggests an average recurrence rate of 125 years, indicating the seismic hazard for the area may be significantly higher than currently recognized. The observations help fill a gap in data on the seismic activity of the SAF in northern California, particularly south of San Francisco.

Geologists Thomas Fumal and Tim Dawson conducted paleoseismic studies at Mill Canyon, near Watsonville, California. They documented evidence for four earthquakes, the most recent being the 1906 M 7.8 San Francisco event. They conclude that each of the three earthquakes prior to the 1906 quake was a large magnitude event that likely ruptured most, or all, of the Santa Cruz Mountains segment, producing similar physical deformation as the 1906 quake.

In addition to filling in a data gap about the SAF in this region, this research adds to the understanding of how the SAF behaves, in particular whether individual segments of the fault system can produce destructive earthquakes and how often. This study joins to a growing body of work that suggests the SAF produces a wider array of magnitudes than previously appreciated in the current seismic hazard models.

“Timing of Large Earthquakes during the past 500 years along the Santa Cruz Mountains Segment of the San Andreas Fault at Mill Canyon, near Watsonville, California,” published by BSSA, Vol. 102:3.

Author: Thomas Fumal, U.S. Geological Survey.

Media contact: timothy.dawson@conservation.ca.gov

Seattle Fault Zone – 900-930 AD earthquake larger than previously thought

A fresh look at sedimentary evidence suggests the 900-930 AD rupture of the Seattle fault possibly produced a larger earthquake than previously recognized. The Seattle fault zone, a series of active-east-west trending thrust faults, poses seismic threat to the Puget Sound region.

The 900-930 AD rupture is the only known large earthquake along the Seattle Fault, making geological records of prehistoric events the only clues to the earthquake potential of the fault.

While a graduate student at the University of Washington, Maria Arcos looked at tsunami and debris flow deposits – both evidence of a paleo-quake – in the coastal marsh at Gorst, Washington. She also identified evidence of at least three meters of uplift that preceded a tsunami, which was followed by a sandy debris flow from Gorst Creek, and suggests that the 900-930 AD quake covered a greater geographic area than previous fault interpretations.

The revised height and width of deformation caused by the quake may influence current interpretations of the Seattle fault’s structure. This study found a minimum of three meters of uplift at Gorst, which is double the amount of previous fault models for the same location. A broader zone of deformation, says Arcos, may indicate either a wider zone of slip along the dip of the fault, a shallower dip or splay faults farther to the south.

**
“The A.D. 900 – 930 Seattle Fault Zone Earthquake with a Wider Coseismic Rupture Patch and Postseismic Submergence: Inferences from New Sedimentary Evidence,” published in BSSA Vol 102:3; DOI number 10.1785/0120110123.

Author: Maria Elizabeth Martin Arcos is currently employed by AMEC and can be reached at beth.arcos@amec.com.

Scientists confirm Sierra Nevada 200-year megadroughts

University of Nevada, Reno, researchers were joined by a Scripps Institution of Oceanography research team, spending many days on Fallen Leaf Lake to gather sonar and side-scan radar data to study earthquake faults and paleoshorelines. The low-tech boat was adorned with high-tech hardware, such as gyroscopes used on rockets, to gather high-resolution images of the lake bottom. Using standing trees they found submerged under 130 feet of water, the team confirmed and reported in their paper, a culmination of a comprehensive high-tech assessment of Fallen Leaf Lake -- a small moraine-bound lake at the south end of the Lake Tahoe Basin -- that stands of pre-Medieval trees in the lake suggest the region experienced severe drought at least every 650 to 1,150 years during the mid- and late-Holocene period. -  Photo by Mike Wolterbeek, University of Nevada, Reno
University of Nevada, Reno, researchers were joined by a Scripps Institution of Oceanography research team, spending many days on Fallen Leaf Lake to gather sonar and side-scan radar data to study earthquake faults and paleoshorelines. The low-tech boat was adorned with high-tech hardware, such as gyroscopes used on rockets, to gather high-resolution images of the lake bottom. Using standing trees they found submerged under 130 feet of water, the team confirmed and reported in their paper, a culmination of a comprehensive high-tech assessment of Fallen Leaf Lake — a small moraine-bound lake at the south end of the Lake Tahoe Basin — that stands of pre-Medieval trees in the lake suggest the region experienced severe drought at least every 650 to 1,150 years during the mid- and late-Holocene period. – Photo by Mike Wolterbeek, University of Nevada, Reno

The erratic year-to-year swings in precipitation totals in the Reno-Tahoe area conjures up the word “drought” every couple of years, and this year is no exception. The Nevada State Climate Office at the University of Nevada, Reno, in conjunction with the Nevada Drought Response Committee, just announced a Stage 1 drought (moderate) for six counties and a Stage 2 drought (severe) for 11 counties.

Reno, Lake Tahoe and the Sierra Nevada are no strangers to drought, the most famous being the Medieval megadrought lasting from 800 to 1250 A.D. when annual precipitation was less than 60 percent of normal. The Reno-Tahoe region is now about 65 percent of annual normal precipitation for the year, which doesn’t seem like much, but imagine if this were the “norm” each and every year for the next 200 years.

Research by scientists at the University of Nevada, Reno and their partners at Scripps Institution of Oceanography in San Diego indicates that there are other instances of such long-lasting, severe droughts in the western United States throughout history. Their recent paper, a culmination of a comprehensive high-tech assessment of Fallen Leaf Lake – a small moraine-bound lake at the south end of the Lake Tahoe Basin – reports that stands of pre-Medieval trees in the lake suggest the region experienced severe drought at least every 650 to 1,150 years during the mid- and late-Holocene period.

“Using an arsenal of cutting edge sonar tools, remotely operated vehicles (ROVs), and a manned submersible, we’ve obtained potentially the most accurate record thus far on the instances of 200-year-long droughts in the Sierra,” Graham Kent, director of the Nevada Seismological Laboratory said. “The record from Fallen Leaf Lake confirms what was expected and is likely the most accurate record, in terms of precipitation, than obtained previously from a variety of methods throughout the Sierra.”

Kent is part of the University of Nevada, Reno and Scripps research team that traced the megadroughts and dry spells of the region using tree-ring analysis, shoreline records and sediment deposition in Fallen Leaf Lake. Using side-scan and multibeam sonar technology developed to map underwater earthquake fault lines such as the West Tahoe fault beneath Fallen Leaf Lake, the team also imaged standing trees up to 130 feet beneath the lake surface as well as submerged ancient shoreline structure and development. The trees matured while the lake level was 130 to 200 feet below its modern elevation and were not deposited by a landslide as was suspected.

The team, led by John Kleppe, University of Nevada, Reno engineering professor emeritus, published a paper on this research and is presenting its findings in seminars and workshops.

“The lake is like a ‘canary in a coal mine’ for the Sierra, telling the story of precipitation very clearly,” Kent said. “Fallen Leaf Lake elevations change rapidly due to its unique ratio between catchment basin and lake surface of about 8 to 1. With analysis of the standing trees submerged in the lake, sediment cores and our sonar scanning of ancient shorelines, we can more accurately and easily trace the precipitation history of the region.”

Water balance calculations and analysis of tree-ring samples undertaken by Kleppe, Kent and Scripps scientists Danny Brothers and Neal Driscoll, along with Professor Franco Biondi of the University’s College of Science, suggest annual precipitation was less than 60 percent of normal from the late 10th century to the early 13th century. Their research was documented in a scientific paper, Duration and severity of Medieval drought in the Lake Tahoe Basin, published in the Quaternary Science Reviews in November 2011.

Tree-ring records and submerged paleoshoreline geomorphology suggest a Medieval low-lake level of Fallen Leaf Lake lasted more than 220 years. More than 80 trees were found lying on the lake floor at various elevations above the paleoshoreline.

“Although the ancient cycle of megadroughts seems to occur every 650 to 1150 years and the last one was 750 years ago, it is uncertain when the next megadrought will occur. With climate change upon us, it will be interesting to see how carbon dioxide loading in the atmosphere will affect this cycle,” Kent said.

Professor Paula Noble, in the University’s College of Science’s Department of Geological Sciences and Engineering, is expanding this research to include the fine-scale study of climate change through out the Holocene (about 12,000 years) using recently collected 40-foot-long sediment cores in Fallen Leaf Lake.

New observations on the San Andreas Fault in Santa Cruz Mountains, Seattle Fault Zone

San Andreas Fault in Santa Cruz Mountains – large quakes more frequent than previously thought.

Recent paleoseismic work has documented four surface-rupturing earthquakes that occurred across the Santa Cruz Mountains section of the San Andreas Fault (SAF) in the past 500 years. The research, conducted by the U.S. Geological Survey, with assistance from the California Geological Survey, suggests an average recurrence rate of 125 years, indicating the seismic hazard for the area may be significantly higher than currently recognized. The observations help fill a gap in data on the seismic activity of the SAF in northern California, particularly south of San Francisco.

Geologists Thomas Fumal and Tim Dawson conducted paleoseismic studies at Mill Canyon, near Watsonville, California. They documented evidence for four earthquakes, the most recent being the 1906 M 7.8 San Francisco event. They conclude that each of the three earthquakes prior to the 1906 quake was a large magnitude event that likely ruptured most, or all, of the Santa Cruz Mountains segment, producing similar physical deformation as the 1906 quake.

In addition to filling in a data gap about the SAF in this region, this research adds to the understanding of how the SAF behaves, in particular whether individual segments of the fault system can produce destructive earthquakes and how often. This study joins to a growing body of work that suggests the SAF produces a wider array of magnitudes than previously appreciated in the current seismic hazard models.

“Timing of Large Earthquakes during the past 500 years along the Santa Cruz Mountains Segment of the San Andreas Fault at Mill Canyon, near Watsonville, California,” published by BSSA, Vol. 102:3.

Author: Thomas Fumal, U.S. Geological Survey.

Media contact: timothy.dawson@conservation.ca.gov

Seattle Fault Zone – 900-930 AD earthquake larger than previously thought

A fresh look at sedimentary evidence suggests the 900-930 AD rupture of the Seattle fault possibly produced a larger earthquake than previously recognized. The Seattle fault zone, a series of active-east-west trending thrust faults, poses seismic threat to the Puget Sound region.

The 900-930 AD rupture is the only known large earthquake along the Seattle Fault, making geological records of prehistoric events the only clues to the earthquake potential of the fault.

While a graduate student at the University of Washington, Maria Arcos looked at tsunami and debris flow deposits – both evidence of a paleo-quake – in the coastal marsh at Gorst, Washington. She also identified evidence of at least three meters of uplift that preceded a tsunami, which was followed by a sandy debris flow from Gorst Creek, and suggests that the 900-930 AD quake covered a greater geographic area than previous fault interpretations.

The revised height and width of deformation caused by the quake may influence current interpretations of the Seattle fault’s structure. This study found a minimum of three meters of uplift at Gorst, which is double the amount of previous fault models for the same location. A broader zone of deformation, says Arcos, may indicate either a wider zone of slip along the dip of the fault, a shallower dip or splay faults farther to the south.

**
“The A.D. 900 – 930 Seattle Fault Zone Earthquake with a Wider Coseismic Rupture Patch and Postseismic Submergence: Inferences from New Sedimentary Evidence,” published in BSSA Vol 102:3; DOI number 10.1785/0120110123.

Author: Maria Elizabeth Martin Arcos is currently employed by AMEC and can be reached at beth.arcos@amec.com.

Super-eruptions may have surprisingly short fuses

This three-dimensional perspective view of Long Valley, Calif., was created from data taken by the Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar on board the space shuttle Endeavour. -  NASA/JPL
This three-dimensional perspective view of Long Valley, Calif., was created from data taken by the Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar on board the space shuttle Endeavour. – NASA/JPL

Enormous volcanic eruptions with potential to end civilizations may have surprisingly short fuses, researchers have discovered.

These eruptions are known as super-eruptions because they are more than 100 times the size of ordinary volcanic eruptions like Mount St. Helens. They spew out tremendous flows of super-heated gas, ash and rock capable of blanketing entire continents and inject enough particulate into the stratosphere to throw the global climate into decade-long volcanic winters. In fact, there is evidence that one super-eruption, which took place in Indonesia 74,000 years ago, may have come remarkably close to wiping out the entire human species.

Geologists generally believe that a super-eruption is produced by a giant pool of magma that forms a couple of miles below the surface and then simmers for 100,000 to 200,000 years before erupting. But a new study suggests that once they form, these giant magma bodies may only exist for a few thousand years, perhaps only a few hundred years, before erupting.

“Our study suggests that when these exceptionally large magma pools form they are ephemeral and cannot exist very long without erupting,” said Guilherme Gualda, the assistant professor of earth and environmental sciences at Vanderbilt University who directed the study, which appears in the May 30 issue of the journal PLoS ONE.

The study was performed on the remnants of the Bishop Tuff, the Long Valley super-eruption that occurred in east-central California 760,000 years ago. Using the latest methods for dating the process of magma formation, Gualda and his colleagues found several independent lines of evidence that indicate the magma pool formed within a few thousand years, perhaps within a few hundred years, before it erupted, covering half of the North American continent with smoldering ash.

These giant magma pools tend to be shaped like pancakes and are 10 to 25 miles in diameter and one half to three miles deep. In the beginning, the molten rock in these pools is largely free from crystals and bubbles. After they form, however, crystals and bubbles form gradually and progressively change the magma’s physical and chemical properties, a process that halts when an eruption takes place. As far as geologists can tell, no such giant crystal-poor magma body currently exists that is capable of producing a super-eruption. The research team believes this may be because these magma bodies exist for a relatively short time rather than persisting for hundreds of thousands of years as previously thought.

According to Gualda, the estimates for the 100,000 year-plus lifetimes of these giant magma bodies appears to be an artifact of the method that geologists have used to make them. The measurements have been made using zircon crystals. Zircons are commonplace in volcanic rocks and they contain small amounts of radioactive uranium and thorium, which decay into lead at a set rate, allowing scientists to accurately determine when the crystals formed. They are extremely useful for many purposes because they can survive most geologic processes. However, the fact that zircons can withstand the heat and the forces found in a magma chamber means that they are not good at recording the lifetimes of crystal-poor magma bodies.

Gualda and his colleagues took a different approach in his studies of the Bishop Tuff. They determined crystallization rates of quartz – the most abundant mineral in the deposits – to gather information about the lifespan of these giant magma bodies. They developed four independent lines of evidence that agreed that the formation process took less than 10,000 years and most likely between 500 to 3,000 years before the eruption. They suggest that the zircon crystal measurements record the extensive changes that take place in the crust required before the giant magma bodies can begin forming as opposed to the formation itself.

“The fact that the process of magma body formation occurs in historical time, instead of geological time, completely changes the nature of the problem,” said Gualda. Instead of concluding that there is virtually no risk of another super-eruption for the foreseeable future because there are no suitable magma bodies, geologists need to regularly monitor areas where super-eruptions are likely, such as Yellowstone, to provide advanced warning if such a magma body begins to form.

According to a 2005 report by the Geological Society of London, “Even science fiction cannot produce a credible mechanism for averting a super-eruption. We can, however, work to better understand the mechanisms involved in super-eruptions, with the goal of being able to predict them ahead of time and provide a warning for society. Preparedness is the key to mitigation of the disastrous effects of a super-eruption.”

Landslides linked to plate tectonics create the steepest mountain terrain

The Landsat satellite image at left shows a huge lake on the Tsangpo River behind a dam created by a landslide (in red, lower right of the lake) in early 2000. The image at right shows the river following a catastrophic breach of the dam in June 2000. -  U.S. Geological Survey/NASA
The Landsat satellite image at left shows a huge lake on the Tsangpo River behind a dam created by a landslide (in red, lower right of the lake) in early 2000. The image at right shows the river following a catastrophic breach of the dam in June 2000. – U.S. Geological Survey/NASA

Some of the steepest mountain slopes in the world got that way because of the interplay between terrain uplift associated with plate tectonics and powerful streams cutting into hillsides, leading to erosion in the form of large landslides, new research shows.

The work, presented online May 27 in Nature Geoscience, shows that once the angle of a slope exceeds 30 degrees – whether from uplift, a rushing stream carving away the bottom of the slope or a combination of the two – landslide erosion increases significantly until the hillside stabilizes.

“I think the formation of these landscapes could apply to any steep mountain terrain in the world,” said lead author Isaac Larsen, a University of Washington doctoral student in Earth and space sciences.

The study, co-authored by David Montgomery, a UW professor of Earth and space sciences and Larsen’s doctoral adviser, focuses on landslide erosion along rivers in the eastern Himalaya region of southern Asia.

The scientists studied images of more than 15,000 landslides before 1974 and more than 550 more between 1974 and 2007. The data came from satellite imagery, including high-resolution spy satellite photography that was declassified in the 1990s.

They found that small increases in slope angle above about 30 degrees translated into large increases in landslide erosion as the stress of gravity exceeded the strength of the bedrock.

“Interestingly, 35 degrees is about the same angle that will form if sand or other coarse granular material is poured into a pile,” Larsen said. “Sand is non-cohesive, whereas intact bedrock can have high cohesion and should support steeper slopes.

“The implication is that bedrock in tectonically active mountains is so extensively fractured that in some ways it behaves like a sand pile. Removal of sand at the base of the pile will cause miniature landslides, just as erosion of material at the base of hill slopes in real mountain ranges will lead to landslides.”

The researchers looked closely at an area of the 150-mile Tsangpo Gorge in southeast Tibet, possibly the deepest gorge in the world, downstream from the Yarlung Tsangpo River where the Po Tsangpo River plunges more than 6,500 feet, about 1.25 miles. It then becomes the Brahmaputra River before flowing through the Ganges River delta and into the Bay of Bengal.

The scientists found that within the steep gorge, the rapidly flowing water can scour soil from the bases, or toes, of slopes, leaving exposed bedrock and an increased slope angle that triggers landslides to stabilize the slopes.

From 1974 through 2007, erosion rates reached more than a half-inch per year along some 6-mile stretches of the river within the gorge, and throughout that active landslide region erosion ranged from 0.15 to 0.8 inch per year. Areas with less tectonic and landslide activity experienced erosion rates of less than 0.15 inch a year.

Images showed that a huge landslide in early 2000 created a gigantic dam on a stretch of the Po Tsangpo. The dam failed catastrophically in June of that year, and the ensuing flood caused a number of fatalities and much property damage downstream.

That event illustrates the processes at work in steep mountain terrain, but the processes happen on a faster timescale in the Tsangpo Gorge than in other steep mountain regions of the world and so are more easily verified.

“We’ve been able to document the role that landslides play in the Tsangpo Gorge,” Larsen said. “It explains how steep mountain topography evolves over time.”