Potential for large earthquake off coast of Sumatra remains large


The subduction zone that brought us the 2004 Sumatra-Andaman earthquake and tsunami is ripe for yet another large event, despite a sequence of quakes that occurred in the Mentawai Islands area in 2007, according to a group of earthquake researchers led by scientists from the Tectonics Observatory at the California Institute of Technology (Caltech).

"From what we saw," says geologist Jean-Philippe Avouac, director of the Tectonics Observatory and one of the paper's lead authors, "we can say with some confidence that we're probably not done with large earthquakes in Sumatra."

Their findings were published in a letter in the December 4 issue of the journal Nature.

The devastating magnitude 9.2 earthquake that occurred off the western coast of Sumatra on December 26, 2004—the earthquake that spawned a lethal tsunami throughout the Indian Ocean—took place in a subduction zone, an area where one tectonic plate dips under another, forming a quake-prone region.

It is that subduction zone that drew the interest of the Caltech-led team. Seismic activity has continued in the region since the 2004 event, they knew. But have the most recent earthquakes been able to relieve the previous centuries of built-up seismic stress?

Yes . . . and no. Take, for instance, an area just south of the 2004 quake, where a magnitude 8.6 earthquake hit in 2005. (That same area had also been the site of a major earthquake in 1861.) The 2005 quake, says Avouac, did a good job of "unzipping" the stuck area in that patch of the zone, effectively relieving the stresses that had built up since 1861. This means that it should be a few centuries before another large quake in that area would be likely.

The same cannot be said, however, of the area even further south along that same subduction zone, near the Mentawai Islands, a chain of about 70 islands off the western coasts of Sumatra and Indonesia. This area, too, has been hit by giant earthquakes in the past (an 8.8 in 1797 and a 9.0 in 1833). More recently, on September 12, 2007, it experienced two earthquakes just 12 hours apart: first a magnitude 8.4 quake and then a magnitude 7.9.

These earthquakes did not come as a surprise to the Caltech researchers. Caltech geologist and paper coauthor Kerry Sieh, who is now at the Nanyang Technological University in Singapore, had long been using coral growth rings to quantify the pattern of slow uplift and subsidence in the Mentawai Islands area; that pattern, he and his colleagues knew, is the result of stress build-up on the plate interface, which should eventually be released by future large earthquakes.

But was all that accumulated stress released in 2007? In the work described in the Nature letter, the researchers analyzed seismological records, remote sensing (inSAR) data, field measurements, and, most importantly, data gathered by an array of continuously recording GPS stations called SuGAr (for Sumatra Geodetic Array) to find out.

Their answer? The quakes hadn't even come close to doing their stress-reduction job. "In fact," says Ali Ozgun Konca, a Caltech scientist and the paper's first author, who did this work as a graduate student, "we saw release of only a quarter of the moment needed to make up for the accumulated deficit over the past two centuries." (Moment is a measure of earthquake size that takes into account how much the fault slips and over how much area.)

"The 2007 quakes occurred in the right place at the right time," adds Avouac. "They were not a surprise. What was a surprise was that those earthquakes were way smaller than we expected."

"The quake north of this region, in 2005, ruptured completely," says Konca. "But the 2007 sequence of quakes was more complicated. The slippage of the plates was patchy, and it didn't release all the strain that had accumulated."

"It was what we call a partial rupture," adds Avouac. "There's still enough strain to create another major earthquake in that region. We may have to wait a long time, but there's no reason to think it's over."

Source: California Institute of Technology



www.physorg.com/news147533054.html

Great Indian Ocean earthquake of 2004 set off tremors in San Andreas fault
Space & Earth science / Earth Sciences
In the last few years there has been a growing number of documented cases in which large earthquakes set off unfelt tremors in earthquake faults hundreds, sometimes even thousands, of miles away.

New research shows that the great Indian Ocean earthquake that struck off the Indonesian island of Sumatra on the day after Christmas in 2004 set off such tremors nearly 9,000 miles away in the San Andreas fault at Parkfield, Calif.

"We found that an earthquake that happened halfway around the world could trigger a seismic signal in the San Andreas fault. It is a low-stress event and a new kind of seismic phenomenon," said Abhijit Ghosh, a University of Washington doctoral student in Earth and space sciences.

"Previous research has shown that this phenomenon, called non-volcanic tremor, was produced in the San Andreas fault in 2002 by the Denali earthquake in Alaska, but seeing this new evidence of tremor triggered by an event as distant as the Sumatra earthquake is really exciting," he said.

Ghosh is to present the findings next week (Dec. 17) in a poster at the American Geophysical Union annual meeting in San Francisco.

The Indian Ocean earthquake on Dec. 26, 2004, was measured at magnitude 9.2 and generated tsunami waves that killed a quarter-million people. It was not known, however, that an earthquake of even that magnitude could set off non-volcanic tremor so far away.

The San Andreas fault in the Parkfield region is one of the most studied seismic areas in the world. It experiences an earthquake of magnitude 6.0 on an average of every 22 years, so a variety of instruments have been deployed to record the seismic activity.

In this case, the scientists examined data from instruments placed in holes bored in the ground as part of the High-Resolution Seismic Network operated by the University of California, Berkeley, as well as information gathered by the Northern California Seismic Network operated by the U.S. Geological Survey.

Signals corresponding with non-volcanic tremor at precisely the time that seismic waves from the Indian Ocean earthquake were passing the Parkfield area were recorded on a number of instruments as far as 125 miles apart.

"It's fairly obvious. There's no question of this tremor being triggered by the seismic waves from Sumatra," Ghosh said.

Scientists have pondered whether non-volcanic tremor is related to actual slippage within an earthquake fault or is caused by the flow of fluids below the Earth's surface. Recent research supports the idea that tremor is caused by fault slippage.

"If the fault is slipping from tremor in one place, it means stress is building up elsewhere on the fault, and that could bring the other area a little closer to a big earthquake," Ghosh said.

Monitoring tremor could help to estimate how much stress has built up within a particular fault.

"If the fault is closer to failure, then even a small amount of added stress likely can produce tremor," he said. "If the fault is already at low stress, then even high-energy waves probably won't produce tremor."

The work adds to the understanding of non-volcanic tremor and what role it might play in releasing or shifting stress within an earthquake-producing fault.

"Our single-biggest finding is that very small stress can trigger tremor," Ghosh said. "Finding tremor can help to track evolution of stress in the fault over space and time, and therefore could have significant implications in seismic hazard analysis."

Source: University of Washington


www.physorg.com/news148136737.html

’Earthquake-safe’ city sits on fault line

KONYA - Konya, which is known as the province where the risk of an earthquake is the lowest in Turkey, has fractures on the ground resulting from a fault line. The fractures are reported to be unique to Turkey and exceptional in the world.

Associate Professor Yaşar Eren from Selçuk Unviersity said they conducted research in the central Anatolian province of Konya’s Selçuk city.

The fractures, which are filled with alluvium, indicate that Konya's fault line is potentially active and could produce an earthquake measuring six or 6.5 on the Richter Scale, Eren said.

The fractures occurred approximately 10,000 to 20,000 years ago, Eren said, adding that they are exceptional with their average depth of two meters, which can reach 25 meters, and their average length of 200 meters.

"These are like earthquake records for Konya. They indicate that in the past an earthquake occurred in Konya," he said.

The fractures are a significant source of research and data for scientists and they should be protected, Eren said.

Associate Professor Tahir Nalbantçılar, on the other hand, said they offered to help the Selçuk municipality found a geology museum in the area. Mayor Adem Esen is positive about the project, Nalbantçılar said. "We offered to exhibit samples of rocks, fossils, and minerals that are significant for natural and geological sciences in the museum," he said.

Country earthquake prone
Turkey is a country where the majority of its soil is geographically on earthquake zones. The geography Turkey has experienced devastating earthquakes in its history. Most recently, Turkey in 1999 had the Marmara Earthquake that killed around 17,000 people, according to official data. The earthquake struck late on the night of Aug.17, 1999, and hit the industrial city of Kocaeli and nearby cities in which the population density is high, increasing the number of causalities. More than 40,000 people were injured in the earthquake.



www.hurriyet.com.tr/english/...57905.asp

Study finds troubling pattern of Southern California quakes
The southern stretch of the San Andreas fault has had a major temblor about every 137 years, according to new research. The latest looks to be overdue.
By Jia-Rui Chong
January 24, 2009
Large earthquakes have rumbled along a southern section of the San Andreas fault more frequently than previously believed, suggesting that Southern California could be overdue for a strong temblor on the notorious fault line, a new study has found.

The Carrizo Plain section of the San Andreas has not seen a massive quake since the much-researched Fort Tejon temblor of 1857, which at an estimated magnitude of 7.9 is considered the most powerful earthquake to hit Southern California in modern times.

But the new research by UC Irvine scientists, to be published next week, found that major quakes occurred there roughly every 137 years over the last 700 years. Until now, scientists believed big quakes occurred along the fault roughly every 200 years.

The findings are significant because seismologists have long believed this portion of the fault is capable of sparking the so-called Big One that officials have for decades warned will eventually occur in Southern California.

"It's been long enough since 1857 that we should be concerned about another great earthquake that ruptures through this part of the fault," said Ken Hudnut, a geophysicist at the U.S. Geological Survey in Pasadena who was not involved in the study.


Many scientists thought the Carrizo area produced relatively infrequent but large-scale earthquakes such as the Fort Tejon temblor. The new work suggests the area produces more quakes but also ones of a smaller magnitude than Fort Tejon, said Ray Weldon, a University of Oregon geologist who was not involved in the research but reviewed the paper for the Journal of Geophysical Research.

Such temblors, experts warned, would likely be at least as big as the 1994 Northridge quake, which had a magnitude of 6.7.

"Even moderate earthquakes on the San Andreas can cause considerable damage, so the overall hazard and risk has gone up," Weldon said.

The section of the San Andreas fault threading through the dry Carrizo Plain is one of the most famous and photographed parts of the fault because creek beds and other features on one side of the fault have clearly shifted away from matching features on the other side. About 100 miles northwest of Los Angeles, the Carrizo area was one of the main sections that ruptured in the 1857 quake. That rupture, roaring southwest into the Los Angeles Basin, rocked parts of the region so hard that men were thrown to the ground.

Lisa Grant Ludwig, a principal investigator on the study, first visited the Carrizo Plain about 20 years ago, digging trenches in an area west of the Panorama Hills known as the Bidart Fan.

By looking at the pattern of soils and using radiocarbon dating on charcoal deposits, she found evidence of five large earthquakes dating back to the early 1200s. She found a gap of some 400 years between the 1857 earthquake and the one before, but only about 100 years separating the three preceding quakes.

Back then, the earthquake age estimates were very rough and the samples had to be fairly large, about the size of a jelly bean. Ludwig saved field notes and hundreds of soil samples in glass vials in her garage for more than 15 years, hoping that radiocarbon dating techniques would improve.

Once the technology improved, Ludwig and her colleagues could date samples with much higher precision and analyze charcoal flakes as small as the tip of a pencil.

They went back to her archive, and the redating effort, led by scholar Sinan Akciz, found that the four big earthquakes before the 1857 temblor probably occurred around 1310, 1393, 1585 and 1640.

"We were better able to constrain the dates and show that actually these five earthquakes were pretty evenly spaced," Ludwig said.

Because they are looking at only a handful of earthquakes, scientists can't be sure that the pattern will hold, Ludwig said.

"But we know it increases the probability of an earthquake," she said. "There's not any way I can look at the data and be comforted by it."

Ludwig's team has dug some new trenches in the area to supplement the redating project, hoping to find new soil samples that show the increased frequency of large earthquakes.

Results won't be finalized for a few months, Ludwig said, but preliminary analysis suggests that the time interval between earthquakes may be even shorter, something on the order of 100 years.



www.latimes.com/news/local...34479.story

Locations of strain, slip identified in major earthquake fault

Deep-sea drilling into one of the most active earthquake zones on the planet is providing the first direct look at the geophysical fault properties underlying some of the world's largest earthquakes and tsunamis.

The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is the first geologic study of the underwater subduction zone faults that give rise to the massive earthquakes known to seismologists as mega-thrust earthquakes.

"The fundamental goal is to sample and monitor this major earthquake-generating zone in order to understand the basic mechanics of faulting, the basic physics and friction," says Harold Tobin, University of Wisconsin-Madison geologist and co-chief scientist of the project.

Tobin will present results from the first stage of the project Sunday, Feb. 15, at the 2009 American Association for the Advancement of Science meeting in Chicago.

Subduction zone faults extend miles below the seafloor and the active earthquake-producing regions — the seismogenic zones — are buried deep in the Earth's crust. The NanTroSEIZE project, an international collaboration overseen by the Integrated Ocean Drilling Program, is using cutting-edge deep-water drilling technology to reach these fault zones for the first time.

"If we want to understand the physics of how the faults really work, we have to go to those faults in the ocean," Tobin explains. "Scientific drilling is the main way we know anything at all about the geology of the two-thirds of the Earth that is submerged."

The decade-long project, to be completed in four stages, will use boreholes, rock samples, and long-term in situ monitoring of a fault in the Nankai Trough, an earthquake zone off the coast of Japan with a history of powerful temblors, to understand the basic fault properties that lead to earthquakes and tsunamis. The project is currently is its second year.

Subduction zone faults angle upward as one of the giant tectonic plates comprising Earth's surface slides below another. Tremendous friction between the plates builds until the system faults and the accumulated energy drives the upper plate forward, creating powerful seismic waves that make the crust shake and can produce a tsunami. But although both shallow and deep parts of the fault slip, only the deep regions produce earthquakes.

During the first stage of the project, the team found evidence of extensive rock deformation and a highly concentrated slip zone even in shallow regions that do not generate earthquakes. One rock core from a shallow part of the fault contains a narrow band of finely ground "rock flour" revealing a fault zone between the upper and lower plates that is only about two millimeters thick — roughly the thickness of a quarter.

Above deeper portions of the fault, the team discovered layers of displaced rock and evidence of prolonged seismic activity that suggest a region known as the megasplay fault is likely responsible for the largest tsunami-generating plate slips.

"A fundamental goal was to understand how the faults at depth connect up toward the Earth's surface, and we feel that we've discovered the fault zone that's the main culprit," Tobin says.

The next stage of drilling will commence this May, with plans to drill additional boreholes into the plate above deep regions of the fault zone. In addition to collecting cores for comparison to those from shallower parts of the fault, the scientists will install sensors in these holes to set up a deep-sea observatory monitoring physical stresses, movement, temperature and pressure.

Additional information about the first NanTroSEIZE expeditions is available at www.jamstec.go.jp/chikyu/en...index.html .

Source: University of Wisconsin-Madison


www.physorg.com/news153919676.html

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New monitoring stations detect 'silent earthquakes' in Costa Rica

After installing an extensive network of monitoring stations in Costa Rica, researchers have detected slow slip events (also known as "silent earthquakes") along a major fault zone beneath the Nicoya Peninsula. These findings are helping scientists understand the full spectrum of motions occurring on the fault and may yield new insights into the events that lead to major earthquakes.

A slow slip event involves the same fault motion as an earthquake, but it happens so slowly that the ground does not shake. It can be detected only with networks of modern instruments that use the Global Positioning System (GPS) to measure precisely the movements of the Earth's crust over time.

Susan Schwartz, a professor of Earth and planetary sciences at the University of California, Santa Cruz, leads a team that has installed a permanent network of 13 GPS monitoring stations and 13 seismic stations on Costa Rica's Nicoya Peninsula.

"At least two slow slip events have occurred beneath the Nicoya Peninsula since 2003," Schwartz said. "When we recorded the first one in 2003, we had only 3 GPS stations. By 2007, we had 12 GPS stations and over 10 seismic stations, so the event that year was very nicely recorded."

The National Science Foundation (NSF) has funded the work by Schwartz and others to install monitoring equipment in Costa Rica. Schwartz, who directs UCSC's Keck Seismological Laboratory, has been working in the region since 1991. At the annual meeting of the American Association for the Advancement of Science (AAAS) in Chicago, she will describe results from the past decade of fault-zone monitoring in Central America.

"The newest discovery is the occurrence of these slow slip events. But there has been a decade of focused effort in this area that has significantly advanced our knowledge of the Central America seismogenic system," Schwartz said. "Initially, we focused on areas of the fault that are locked up, which slip in an earthquake. The slow slip is occurring in regions that are not strongly locked, and a big question is whether that is loading the locked area, making it more likely to break, or relieving stress on the fault."

Schwartz said she does not think slow slip events significantly increase the likelihood of a major earthquake on a locked portion of the fault. She noted, however, that scientists are still at an early stage in terms of understanding the implications of different kinds of fault motion and translating that information into earthquake hazard assessments.

Flanked by active tectonic margins on both the Pacific and Caribbean coasts, Costa Rica is one of the most earthquake-prone and volcanically active countries in the world. Just off the west coast is the Middle America Trench, where a section of the seafloor called the Cocos Plate dives beneath Central America, generating powerful earthquakes and feeding a string of active volcanoes. This type of boundary between two converging plates of the Earth's crust is called a subduction zone--and such zones are notorious for generating the most powerful and destructive earthquakes.

The slow slip phenomenon was first observed at subduction zones where hundreds of GPS and seismic instruments are deployed: the Cascadia fault zone (off the coast of Washington and British Columbia) and Japan's Nankai Trough. At these and most other subduction zones, the part of the plate boundary where earthquakes originate, called the seismogenic zone, lies beneath the ocean. But in Costa Rica, the seismogenic zone runs right beneath the Nicoya Peninsula.

"It's a perfect opportunity to study the seismogenic zone using a network of land-based instruments," Schwartz said.

The 2007 slow slip event in Costa Rica involved movement along the fault equivalent to a magnitude 6.9 earthquake. But it took place over a period of 30 days rather than the 10 seconds typical for an earthquake of that size, and such slow motion does not radiate the seismic energy associated with normal earthquakes. The instruments did pick up seismic tremor, however, which Schwartz likened to a lot of very small earthquakes. Tremor activity is also associated with slow slip events in Japan and Cascadia, but there are some differences in Costa Rica, Schwartz said.

"Costa Rica has a different type of subduction zone from the well-studied ones in Japan and Cascadia," she said. "One thing that makes it interesting is that the temperature is much cooler at the depth range where slip occurs, and that is helping us work out the role of fluids in generating slow slip."

Ultimately, the goal of this research is not only a better understanding of subduction zones, but also better assessments of earthquake hazards. Schwartz said her Costa Rican colleagues have been working to educate the population of Nicoya about earthquakes and related hazards. With a growing population along the coast, the region faces a potential tsunami threat as well as the possibility of a major earthquake, she said.

Source: University of California - Santa Cruz






www.physorg.com/news153929276.html

Magnitude 4.3 Earthquake Shakes Northern California, Reveals New Fault
April 2, 2009


A magnitude 4.3 earthquake struck near Santa Clara Valley Calif., this week near Mt. Hamilton, revealing a fault scientists did not know existed, according to the U.S. Geological Survey in Menlo Park, Calif.

According to its summary of the event, the quake occurred on a north-south oriented fault about 3 km to the east of the of the Calaveras fault as defined by the past 40 years of earthquake epicenters. "The fault has no name and is not mapped at the surface of the earth."

USGS said the fault is similar in orientation and tectonics to the fault that was ruptured by the Mt. Lewis sequence in 1986 (M5.7). However, the Mt. Lewis sequence lasted for 491 days and contained 1930 aftershocks. In contrast, as of Mar 30, there were only 4 located aftershocks with the largest having a magnitude of 1.2. "Although this quake is on a similar structure, it is not exhibiting the similar behavior," USGS said. The group said it was unlikely that this recent earthquake will trigger a significant earthquake on this section of the Calaveras fault.

Source: USGS


www.insurancejournal.com/news/...56.htm

Solid Earth Tide Triggers Quakes
Michael Reilly, Discovery News

April 6, 2009 -- This high tide is bound to wash away more than just your sand castle. A new study has found that bulges in Earth's crust -- solid Earth tides -- trigger about 1 percent of earthquakes.

As Earth and the moon grind through their gravitational ballet, our planet gets tugged hard near the equator. The force is so strong that as the moon passes overhead each day, it pulls Earth's surface up 30 centimeters (11.8 inches).

Read all about quakes in Discovery Earth's Seismic Week Wide Angle.

Scientists have known about this effect for over a century and have speculated that it might cause earthquakes. Writing in the journal Earth and Planetary Science Letters, Laurent Metivier of Paris Diderot University in France and a team of researchers now claim they've found a distinct connection between solid Earth tides and earthquakes.

The team analyzed the largest set of earthquake data ever assembled, a global record of 442,412 quakes since 1973. In amongst Earth's tiny shivers and mega-tremors they discovered a daily cycle: earthquake probability was enhanced as the moon passed overhead, pulling against the bedrock and, for a few hours at a time, easing the stress that normally keeps faults locked.

The effect is most pronounced in smaller and shallower earthquakes, and harder to detect in tremors above magnitude 4.0.

"Theoretically it will impact big earthquakes too," Metivier said. "But the main problem is that there aren't enough big earthquakes to make a correlation."

John Vidale of the University of Washington said bigger quakes may be less sensitive to tidal forces because they occur on huge faults that can extend deep into the crust. Below about 20 kilometers' (12.4 miles') depth, rocks are under such pressure that tidal forces barely affect them.

Overall, the triggering effect is much weaker than expected, though. Tectonic plates build stress slowly over centuries, but the stress tides exert on faults each day is far greater. If earthquakes only happened the moment a fault reached a critical "breaking point" level of stress, they would always occur right as the moon exerts its maximum tidal force on the fault -- earthquake high tide.

"It's clear that tidal stress is much faster, so you'd expect every earthquake to be triggered that way," Vidale said. "But this is not what we see. There's a response time to loading -- it takes days of pushing before the fault gives way. This give us a number that shows just how hard it is to start an earthquake."



dsc.discovery.com/news/2009...quake.html

Silent Quakes Build Stress Along Mega Fault Line
Michael Reilly, Discovery News

Feb. 2, 2009 -- A bizarre form of earthquake, which happens over the course of two to three weeks but makes barely a rumble, are lending important clues to the Cascadia subduction zone in the Pacific northwest, one of the most dangerous fault zones on Earth.

For the last decade, slow-slip earthquakes have been measured in fault zones all over the world, baffling scientists. Though the 'quakes' release as much energy as a normal earthquake between magnitude 6.0 and 6.5, they produce almost no shaking.

But researchers have measured separate, small tremors at around the same time as the silent quakes. And in a new study in the journal Science, a team of seismologists show the two events are really one in the same.

Using a combination of seismic sensors and Global Positioning System (GPS) measurements of ground movements in northwestern Washington and British Columbia, the team has pinpointed the tremors as coming from between 30 and 45 kilometers deep in the crust. There, the fault is heavily lubricated with water, and the Juan de Fuca tectonic plate slides peacefully beneath the North American plate.

Near the surface, things are less placid. Every 500 years or so the Cascadia megathrust fault unleashes a hellish earthquake in excess of magnitude 9.0. Geologic records tell of tsunamis similar in size to the 2004 Indian Ocean wave that killed a quarter million people.

By connecting the slow-slip events and tremors directly to the deeper parts of the fault, seismologists can begin to unravel mysteries that could affect millions of people.

"This has two advantages," Kenneth Creager of the University of Washington, a co-author of the study said. "We can locate the tremor more quickly, and we can infer the amount of slip taking place. That allows us to estimate how much stress there's going to be on the plate."

The probability of a devastating quake also increases slightly with each slow-slip event, which scientists call an episodic tremor and slip (ETS) event. On average, one occurs every 15 months.

"Every day there's a probability that a magnitude 9 earthquake will occur," Creager said. "The probability goes up during one of these events."

But this should not be cause for alarm, Herb Dragert of the Geological Survey of Canada said.

"The amount of stress transferred is miniscule," he said. "The only time this becomes important is as you start to approach critical stress along the fault."

Since no one knows what critical stress for Cascadia is, a devastating earthquake can't be predicted.

Geologic records indicate the last major quake along the fault was in 1700. Mega-quakes have recurred in as little as 200 years, or as long as 700 years, though, so it could still be centuries before the next one hits.

"We should not start worrying or anguishing about these ETS events, but it's something we should take into consideration," Dragert said. "If in a couple of hundred years I'm still alive, though, I'm going to start holding my breath every time we get one of them."




dsc.discovery.com/news/2009...uakes.html

Satellites show how Earth moved during Italy quake

(PhysOrg.com) -- Studying satellite radar data from ESA's Envisat and the Italian Space Agency's COSMO-SkyMed, scientists have begun analysing the movement of Earth during and after the 6.3 earthquake that shook the medieval town of L'Aquila in central Italy on 6 April 2009.

Scientists from Italy's Istituto per il Rilevamento Elettromagnetico dell' Ambiente (IREA-CNR) and the Istituto Nazionale di Geofisica e Vulcanologia (INGV) are studying Synthetic Aperture Radar (SAR) data from these satellites to map surface deformations after the earthquake and the numerous aftershocks that have followed.

The scientists are using a technique known as SAR Interferometry (InSAR), a sophisticated version of 'spot the difference'. InSAR involves combining two or more radar images of the same ground location in such a way that very precise measurements - down to a scale of a few millimetres - can be made of any ground motion taking place between image acquisitions.

The InSAR technique merges data acquired before and after the earthquake to generate 'interferogram' images that appear as rainbow-coloured interference patterns. A complete set of coloured bands, called 'fringes', represents ground movement relative to the spacecraft of half a wavelength, which is 2.8 cm in the case of Envisat's ASAR.

The first Envisat data, acquired after the earthquake on 12 April, were made immediately available to the scientists.

"We produced an interferogram just a few hours after the Envisat acquisition by combining these data with data acquired before the earthquake on 1 February. We were pleased that we were able to immediately see the pattern of the earthquake," said Riccardo Lanari of IREA-CNR in Naples, Italy.

The Envisat interferogram, as explained by Stefano Salvi from INGV's Earthquake Remote Sensing Group, shows nine fringes surrounding a maximum displacement area located midway between L'Aquila and Fossa, where the ground moved as much as 25 cm (along a line between the satellite's orbital position and the earthquake area).

"By using available 3D ground displacements from five GPS location sites around the affected area, we were able to confirm the preliminary results obtained with Envisat data," Salvi said.

The COSMO-SkyMed constellation, which is currently made up of three satellites, allows for frequent data. This means new interferograms can be calculated every few days.

The COSMO-SkyMed data together with the Envisat data and possibly SAR data from other satellites will ensure a dense sampling of the ground deformation around the L'Aquila area in the next months, which could make this earthquake one of the most covered by SAR Interferometry measurements.

To ensure all scientists are able to contribute to the analysis of the earthquake, ESA is making its Earth observation dataset collected over the L'Aquila area freely accessible with an innovative fast data download mechanism. The dataset will be continuously updated with the newest Envisat acquisitions.

Source: European Space Agency


www.physorg.com/news159015099.html

Studies offers new picture of Lake Tahoe's earthquake potential


For more than a decade, scientists at Scripps Institution of Oceanography at UC San Diego have been unraveling the history of fault ruptures below the cobalt blue waters of Lake Tahoe one earthquake at a time. Two new studies by the Scripps research team offer a more comprehensive analysis of earthquake activity in the Lake Tahoe region, which suggest a magnitude-7 earthquake occurs every 2,000 to 3,000 years in the basin, and that the largest fault in the basin, West Tahoe, appears to have last ruptured between 4,100 and 4,500 years ago.

These studies, led by a team of Scripps researchers including Graham Kent, Neal Driscoll, Jeff Babcock and Alistair Harding, collected new data on earthquake history along three active faults in the region. These new data suggest that the most recent ruptures along the West Tahoe and Incline Village faults each produced nearly 4-meter-high offsets. The most recent event along the Incline Village Fault occurred about 575 years ago.

"These studies taken together show that the West Tahoe Fault is capable of a magnitude-7 earthquake - similar to large earthquakes that have occurred on the nearby Genoa Fault - but with the added danger of nearly 500 m of overlying water, which is capable of spawning a large tsunami wave," said Kent, a research geophysicist at Scripps.

Jeff Dingler, lead author on a paper in the April online issue of Geological Society of America Bulletin (GSA Bulletin) and former Scripps Oceanography graduate student, used a high-resolution seismic imaging technique, known as CHIRP, to supply a comprehensive view of faulting beneath the lake. Scripps' Neal Driscoll developed the new digital CHIRP profiler for this study, which provided an unprecedented picture of deformation within the sedimentary layers that blanket the floor of Lake Tahoe, laying the groundwork for more detailed fault studies that continue today.

In a complementary paper, published in the April issue of the Bulletin of the Seismological Society of America (BSSA), Scripps graduate student Danny Brothers investigated the rupture history of the West Tahoe Fault in greater detail. Using comprehensive CHIRP and coring surveys of Fallen Leaf Lake, where the West Tahoe Fault crosses the southern end of the lake, the study confirmed the suspected fault length of over 50 km (31 miles). When combined with the rupture offset size observed across the fault from CHIRP imagery, the analysis suggests an upper limit of a magnitude-7.3 earthquake for the basin's most dangerous fault.

This new analysis, coupled with a slip-rate approaching 0.8 mm/year and the rupture timeline taking place between 4,100 and 4,500 years ago, places the West Tahoe Fault near the end of its characteristic earthquake cycle. Researchers caution that some degree of variability is to be expected. Such an earthquake could produce tsunami waves some three to 10 meters (10 to 33 feet) high, colleagues at the University of Nevada, Reno, have shown.


Lake Tahoe, which straddles the California and Nevada border in the Sierra Nevada region, is one of the world's deepest freshwater lakes. At more than 501 meters (1,645 feet) deep, the lake covers 191 square miles in a basin prone to earthquakes and catastrophic landslides. The West Tahoe Fault runs along the west shore of the lake and comes onshore at Baldwin Beach, then passes through the southern third of Fallen Leaf Lake, where it descends into Christmas Valley near Echo Summit.

Source: University of California - San Diego



www.physorg.com/news160229642.html

Typhoons trigger earthquakes on Taiwan: scientists

Surprised scientists say that typhoons which hit Taiwan unleash long, slow earthquakes, a phenomenon that may save the island from devastating temblors.

Seismologists installed movement sensors in boreholes at depths of 200-270 metres (650-870 feet) in eastern Taiwan, monitoring a spot where two mighty plates, the Philippine Sea Plate and the Eurasian plate, bump and jostle in an oblique, dipping fault.

Over five years, researchers saw a remarkable link between tropical storms and "slow" earthquakes, a seismic beast first identified three decades ago.

Slow quakes entail a slippage in the fault that unfolds progressively over hours or days, rather than a sudden, violent release of the kind that destroys buildings and lives.

The sensors noted 20 such slow earthquakes, 11 of which coincided with typhoons, during the study period.

The 11 quakes were all stronger and characterised by more complex seismic waveforms than other "slow" events.

"These data are unequivocal in identifying typhoons as triggers of these slow quakes. The probability that they coincide by chance is vanishingly small," said co-author Alan Linde of the Carnegie Institution for Science in the United States.

A typhoon causes a fall in atmospheric pressure -- and the researchers suggest that this in turn reduces pressure on the land over the fault.

As a result, one side of the fault lifts slightly, causing the pressure that has been building up inside to be released.

"This fault [in Taiwan] experiences more or less constant strain and stress buildup," Linde said in a press release.

"If it's close to failure, the small perturbation due to the low pressure of the typhoon can push it over the failure limit.

"If there is no typhoon, stress will continue to accumulate until it fails without the need for a trigger."

The typhoon does not work as a seismic trigger on faults that lie on the seabed because water moves into the area, dampening out any difference in pressure, they theorise.

Often considered a curse, typhoons -- for Taiwan -- could in fact could be a blessing.

A storm could act as a pressure valve, preventing strain from building up to the point where the fault ruptures devastatingly.

The Nankai Trough, in southwestern Japan, also lies on the convergence of the Philippine Sea and Eurasian plates.

The plates are converging at about four centimetres (2.5 inches) per year, which is about half that of the activity in Taiwan.

In theory, Taiwan should be more vulnerable than the Nankai Trough because of the greater slippage, but the record shows that it has had no great earthquakes and relatively few large quakes, said Linde.

By comparision, the Nankai Trough is capable of unleashing a true monster, a magnitude-8 earthquake, every 100 to 150 years.

The paper, published in the British journal Nature, is led by Chiching Liu of the Institute for Earth Sciences at Academic Sinica, Taipei.




www.physorg.com/news163859251.html

Deep tremors may foretell quake

by Staff Writers
Los Angeles (UPI) Jul 10, 2009
Tremors deep within the San Andreas Fault suggest California should not become complacent about future earthquakes, a leading seismologist said.
"The San Andreas fault is changing down deep and it's changing down deep in places where large earthquakes have happened in the past," said Robert Nadeau, a research seismologist at the University of California, Berkeley.

Seismic activity in the central part of the fault has increased in the years since the magnitude 6.5 San Simeon quake in 2003 and the magnitude 6.0 Parkfield quake in 2004, Nadeau and his team said in a study published Friday in the journal Science.

Unusually strong tremors preceded the Parkfield quake three weeks before it struck, leading scientists to believe such tremors could provide an early warning single to a big quake, said Greg Beroza, a seismologist at Stanford University.

Earthquakes usually generate clear seismic waves with sharp onsets, while tremors vibrate quietly and can continue for days. Tremors usually occur in a deeper, softer part of the Earth's crust, rather than in the upper part typically thought to generate earthquakes, the Los Angeles Times reported Friday.


www.terradaily.com/reports/..._999.html

Lightning-Fast Earthquake Could Hit Southern California
By Steven Mikulan in City News

Just when Southern Californians have begun to pat themselves on the backs for having the
​world's stiffest earthquake building codes, there now comes news of "supershears." These are not competitors to Supercuts, but a newly discussed type of earthquake known for the incredibly high velocities by which it travels -- and compacts the earth.

New Scientist writers Richard Fisher describes "one that slipped at such blistering speeds that the rip in the Earth overtook its own seismic waves. This created the earthquake equivalent of a sonic boom, capable of striking anything in its path like a hammer blow."

According to Fisher's article, which appeared online Wednesday, evidence shows that this little-understood phenomenon occurs more often than previously suspected and that a "superhighway" of supershear faults grids the planet where 60 million people live. Although a brief moment during a 1979 Imperial Valley earthquake seemed to fit the description of a supershear, such quakes had long been relegated to the realm of theory -- until Turkey's 1999 Izmit quake. That 7.6 earthquake was measured as spreading five kilometers per second. (Five kilometers is about 3.1 miles. The speed of sound is 343 meters per second.)

Fisher says that some geologists now believe the 1906 San Francisco earthquake may have been a supershear. More ominously, he notes that not only is California's San Andreas Fault part of the supershear superhighway, but says that even the state's well-fortified buildings may be no match for the "mach fronts" generated by supershears, and quotes a Caltech scientist as noting that less-rigorously built structures five kilometers outside of L.A. would be especially at risk.



blogs.laweekly.com/ladaily/...ould-hit/

Groundwater Radon Points to Dangerous Faults
Michael Reilly, Discovery News

Aug. 25, 2009 -- Spikes in the tiny amounts of radioactivity in groundwater may help scientists find dangerous hidden faults, or even indicate an impending earthquake, according to a new study.

Uranium is common in many rocks around the world. As it decays it leaves behind radon, among other by-products, which can escape into ground water or the atmosphere as a gas.

Now a team of researchers led by Alberto Gonzalez-Diez of the University of Cantabria in Spain are using radon to determine what faults may be ready to rupture, and to discover previously unknown danger zones. They've published their findings in the journal, Geomorphology.

Scientists have been tantalized for decades by hints of a connection between radon levels and quakes. Faults and small cracks in bedrock make way for water and gas to escape, they theorize. In areas where rocks routinely rupture during earthquakes, these fissures -- and the radon that dissolves out of the rocks -- should be more prevalent than elsewhere.

To test this relationship the team measured 47 natural springs in the north of Spain. Springs associated with known faults indeed had significantly larger concentrations of dissolved radon, they found. But they also found some springs had high radon but no known faults nearby.

Gonzalez-Diez believes the the high-radon springs are probably associated with previously unknown faults.

Susan Hough of the United States Geological Survey said the method could be useful in the eastern and central United States, regions riddled with ancient faults -- many of them unmapped.

"The saying goes 'You've got faults without earthquakes and earthquakes without faults,'" she said. "The problem is there are lots of faults, but it's really hard to say which one will produce an earthquake."

Looking for radon spikes might help seismologists identify hidden, active faults that could be potentially dangerous.

But using this relationship to predict earthquakes has proven unreliable.

Some researchers have claimed to be able to predict earthquakes using radon. Last March, Italian scientist Giampaolo Guiliani caused a sensation when he cited measurements of radon gas to claim that an earthquake was imminent near the town of Sulmona.

A few weeks later, a magnitude 6.3 quake struck a few miles away, devastating the nearby city of L'Aquila and killing hundreds of people.

However, no one has ever been able to make consistent predictions. Gonzalez-Diez said that may one day be possible; for now his team is focusing his work on identifying previously unknown faults.

"We know there's a correlation between radon and earthquakes," he said, "but we're very far from being able to predict when an earthquake will occur."



dsc.discovery.com/news/2009...aults.html

Fault Monitoring Breakthrough May Help Predict Quakes
Michael Reilly, Discovery News

Sept. 30, 2009 -- A breakthrough method of measuring the strength of earthquake-prone faults is shedding new light on the vast interconnectivity of quakes around the world, according to a study published in Nature.

This new development could in time help scientists predict the likelihood of earthquakes, which have most recently rocked the South Pacific.

Taka'aki Taira of the University of California, Berkeley and a team of researchers looked at small, reliable tremors along the Parkfield section of the San Andreas Fault recorded between 1987 and 2008. These "repeating earthquakes" usually occur in exactly the same spot, at regular intervals, and always have the same magnitude.

But they faltered on three occasions. In 2004, the San Andreas roared to life along the Parkfield section, rupturing with a magnitude 6.0 earthquake.

The rocks were shattered, and the repeating quakes went haywire. The team deduced that fluids were lubricating newly opened cracks and causing small tremors.

The next two anomalies occurred after major earthquakes: first following the 1992 magnitude 7.0 tremor near Landers, Calif., and then again after the devastating magnitude 9.1 Sumatra-Andaman quake that killed almost a quarter of a million people half a world away.

Each time, the repeating quakes increased in frequency but dipped in strength, indicating that rocks were slipping faster and more loosely.

In short, earthquakes from nearby as well as thousands of miles away were weakening the fault ever so slightly.

"A change in fault strength changes the likelihood of an earthquake occurring," Taira said.

Since 2004, the team noted that there has been an increase in the number of magnitude 8.0 and greater earthquakes around the world. Though it's still too early to draw any firm conclusions, they believe the Sumatra disaster may have weakened faults and triggered temblors in many far-flung regions.

It's even possible that the quake jostled faults near the Samoan Islands enough to cause yesterday's shaker.

And while Taira said it's a long way from predicting future quakes, the team's measurements allow them to make guesses as to how much a fault has weakened. That in turn could improve estimates about when a fault is at increased risk of going critical.

"In terms of directly measuring faults strength, we still don't know how strong a fault is," Stephen Mill of the University of Bonn said. "We just know relative to last Tuesday, it's a little bit weaker now, or it's a little bit stronger now."

Still, Miller agreed that measuring such relative changes is important to gauging the likelihood of a quake. He added that being able to detect how and when pressurized water and liquid carbon dioxide (CO2) move through faults is a crucial step in understanding, and one day predicting, earthquakes.



dsc.discovery.com/news/2009...oring.html