Source of volcanoes may be much closer than thought: Geophysicists challenge traditional theory underlying origin of mid-plate volcanoes

Traditional thought holds that hot updrafts from the Earth's core cause volcanoes, but researchers say eruptions may stem from the asthenosphere, a layer closer to the surface.
Credit: Virginia Tech

A long-held assumption about the Earth is discussed in today's edition of Science, as Don L. Anderson, an emeritus professor with the Seismological Laboratory of the California Institute of Technology, and Scott King, a professor of geophysics in the College of Science at Virginia Tech, look at how a layer beneath the Earth's crust may be responsible for volcanic eruptions.

The discovery challenges conventional thought that volcanoes are caused when plates that make up the planet's crust shift and release heat.

Instead of coming from deep within the interior of the planet, the responsibility is closer to the surface, about 80 kilometers to 200 kilometers deep -- a layer above the Earth's mantle, known as the as the asthenosphere.

"For nearly 40 years there has been a debate over a theory that volcanic island chains, such as Hawaii, have been formed by the interaction between plates at the surface and plumes of hot material that rise from the core-mantle boundary nearly 1,800 miles below the Earth's surface," King said. "Our paper shows that a hot layer beneath the plates may explain the origin of mid-plate volcanoes without resorting to deep conduits from halfway to the center of the Earth."

Traditionally, the asthenosphere has been viewed as a passive structure that separates the moving tectonic plates from the mantle.

As tectonic plates move several inches every year, the boundaries between the plates spawn most of the planet's volcanoes and earthquakes.

"As the Earth cools, the tectonic plates sink and displace warmer material deep within the interior of the Earth," explained King. "This material rises as two broad, passive updrafts that seismologists have long recognized in their imaging of the interior of the Earth."
The work of Anderson and King, however, shows that the hot, weak region beneath the plates acts as a lubricating layer, preventing the plates from dragging the material below along with them as they move.

The researchers show this lubricating layer is also the hottest part of the mantle, so there is no need for heat to be carried up to explain mid-plate volcanoes.

"We're taking the position that plate tectonics and mid-plate volcanoes are the natural results of processes in the plates and the layer beneath them," King said.

Source: Virginia Tech

The Foreshock series controls earthquake rupture

GPS measurements of the displacement vectors. Credit: GFZ

A long lasting foreshock series controlled the rupture process of this year's great earthquake near Iquique in northern Chile. The earthquake was heralded by a three quarter year long foreshock series of ever increasing magnitudes culminating in a Mw 6.7 event two weeks before the mainshock. The mainshock (magnitude 8.1) finally broke on April 1st a central piece out of the most important seismic gap along the South American subduction zone. An international research team under leadership of the GFZ German Research Centre for Geosciences now revealed that the Iquique earthquake occurred in a region where the two colliding tectonic plates where only partly locked.

The Pacific Nazca plate and the South American plate are colliding along South America's western coast. While the Pacific sea floor submerges in an oceanic trench under the South American coast the plates get stressed until occasionally relieved by earthquakes. In about 150 years time the entire plate margin from Patagonia in the south to Panama in the north breaks once completely through in great earthquakes. This cycle is almost complete with the exception of a last segment -- the seismic gap near Iquique in northern Chile. The last great earthquake in this gap occurred back in 1877. On initiative of the GFZ this gap was monitored in an international cooperation (GFZ, Institut de Physique du Globe Paris, Centro Sismologico National -- Universidad de Chile, Universidad de Catolica del Norte, Antofagasta, Chile) by the Integrated Plate Boundary Observatory Chile (IPOC), with among other instruments seismographs and cont. GPS. This long and continuous monitoring effort makes the Iquique earthquake the best recorded subduction megathrust earthquake globally. The fact that data of IPOC is distributed to the scientific community in near real time, allowed this timely analysis.

Ruptures in Detail
The mainshock of magnitude 8.1 broke the 150 km long central piece of the seismic gap, leaving, however, two large segments north and south intact. GFZ scientist Bernd Schurr headed the newly published study that appeared in the lastest issue of Nature Advance Online Publication: "The foreshocks skirted around the central rupture patch of the mainshock, forming several clusters that propagated from south to north." The long-term earthquake catalogue derived from IPOC data revealed that stresses were increasing along the plate boundary in the years before the earthquake. Hence, the plate boundary started to gradually unlock through the foreshock series under increasing stresses, until it finally broke in the Iquique earthquake. Schurr further states: "If we use the from GPS data derived locking map to calculate the convergence deficit assuming the ~6.7 cm/yr convergence rate and subtract the earthquakes known since 1877, this still adds up to a possible M 8.9 earthquake." This applies if the entire seismic gap would break at once. However, the region of the Iquique earthquake might now form a barrier that makes it more likely that the unbroken regions north and south break in separate, smaller earthquakes.

International Field Campaign
Despite the fact that the IPOC instruments delivered continuous data before, during and after the earthquake, the GFZ HART (Hazard And Risk Team) group went into the field to meet with international colleagues to conduct additional investigations. More than a dozen researchers continue to measure on site deformation and record aftershocks in the aftermath of this great rupture. Because the seismic gap is still not closed, IPOC gets further developed. So far 20 multi-parameter stations have been deployed. These consist of seismic broadband and strong-motion sensors, continuous GPS receivers, magneto-telluric and climate sensors, as well as creepmeters, which transmit data in near real-time to Potsdam. The European Southern astronomical Observatory has also been integrated into the observation network.

Source: GFZ GeoForschungsZentrum Potsdam, Helmholtz Centre

Oklahoma earthquakes induced by wastewater injection by disposal wells, study finds

House damage in central Oklahoma from the magnitude 5.6 earthquake on Nov. 6, 2011. Credit: Brian Sherrod, USGS

The dramatic increase in earthquakes in central Oklahoma since 2009 is likely attributable to subsurface wastewater injection at just a handful of disposal wells, finds a new study to be published in the journal Science on July 3, 2014.

The research team was led by Katie Keranen, professor of geophysics at Cornell University, who says Oklahoma earthquakes constitute nearly half of all central and eastern U.S. seismicity from 2008 to 2013, many occurring in areas of high-rate water disposal.

"Induced seismicity is one of the primary challenges for expanded shale gas and unconventional hydrocarbon development. Our results provide insight into the process by which the earthquakes are induced and suggest that adherence to standard best practices may substantially reduce the risk of inducing seismicity," said Keranen. "The best practices include avoiding wastewater disposal near major faults and the use of appropriate monitoring and mitigation strategies."

The study also concluded:

• Four of the highest-volume disposal wells in Oklahoma (~0.05% of wells) are capable of triggering ~20% of recent central U.S. earthquakes in a swarm covering nearly 2,000 square kilometers, as shown by analysis of modeled pore pressure increase at relocated earthquake hypocenters.
• Earthquakes are induced at distances over 30 km from the disposal wells. These distances are far beyond existing criteria of 5 km from the well for diagnosis of induced earthquakes.
• The area of increased pressure related to these wells continually expands, increasing the probability of encountering a larger fault and thus increasing the risk of triggering a higher-magnitude earthquake.

"Earthquake and subsurface pressure monitoring should be routinely conducted in regions of wastewater disposal and all data from those should be publicly accessible. This should also include detailed monitoring and reporting of pumping volumes and pressures," said Keranen. 'In many states the data are more difficult to obtain than for Oklahoma; databases should be standardized nationally. Independent quality assurance checks would increase confidence. "

Source: Cornell University

The Disaster planning: Risk assessment vital to development of mitigation plans

Wildfires and flooding affect many more people in the USA than earthquakes and landslide and yet the dread, the perceived risk, of the latter two is much greater than for those hazards that are more frequent and cause greater loss of life. Research published in the International Journal of Risk Assessment and Management, suggests that a new paradigm for risk assessment is needed so that mitigation plans in the face of natural disasters can be framed appropriately by policy makers and those in the emergency services.

Maura Knutson (nee Hurley) and Ross Corotis of the University of Colorado, Boulder, explain that earlier efforts for incorporating a sociological perspective and human risk perception into hazard-mitigation plans, commonly used equivalent dollar losses from natural hazard events as the statistic by which to make decisions. Unfortunately, this fails to take into consideration how people view natural hazards, the team reports. Moreover, this can lead to a lack of public support and compliance with emergency plans when disaster strikes and lead to worse outcomes in all senses.

The researchers have therefore developed a framework that combines the usual factors for risk assessment, injuries, deaths and economic and collateral loss with the human perception of the risks associated with natural disasters. The framework includes risk perception by graphing natural hazards against "dread" and "familiarity." These two variables are well known to social psychologists as explaining the greatest variability in an individual's perception of risk, whether considering earthquakes, landslides, wildfires, storms, tornadoes, hurricanes, flooding, avalanche, even volcanic activity. "Understanding how the public perceives the risk for various natural hazards can assist decision makers in developing and communicating policy decisions," the team says.

The higher the perceived risk of a natural disaster, the more people want to see that risk reduced and that means seeing their tax dollars spent on mitigation and preparation. For example, far more money is spent on reducing earthquake risk than on reducing the risk from wildfires, perhaps because the perceived risk is much greater, even though both will cause significant losses of life and property. The team's new framework for risk assessment will act as an aid in decision making for these types of situations as well as perhaps even offering a way to give members of the public a clearer understanding of actual risk rather than perceived risk.

Source: Inderscience Publishers

Earthquakes: The next 'Big One' for the San Francisco Bay Area may be a cluster of major quakes

Acluster 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."

Source: Seismological Society of America

The NASA model provides a 3-D look at L.A.-area quake

JPL scientists modeled the March 28, 2014 magnitude 5.1 quake near Los Angeles based on USGS seismic data. This model image shows how the quake may appear to airborne radar, such as NASA's UAVSAR, which will survey the area soon. Blue shades indicate the greatest surface displacement. Credit: NASA/JPL-Caltech/USGS/Google Earth

On March 28, residents of Greater Los Angeles experienced the largest earthquake to strike the region since 2008. The magnitude 5.1 quake was centered near La Habra in northwestern Orange County about 21 miles (33 kilometers) east-southeast of Los Angeles, and was widely felt throughout Southern California. There have been hundreds of aftershocks, including one of magnitude 4.1.

Scientists at NASA's Jet Propulsion Laboratory, Pasadena, Calif., have developed a model of the earthquake, based on the distribution of aftershocks and other seismic information from the U.S. Geological Survey.

A new image based on the model shows what the earthquake may look like through the eyes of an interferometric synthetic aperture radar, such as NASA's Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR). JPL scientists plan to acquire UAVSAR data from the region of the March 28 quake, possibly as soon as this week, and process the data to validate and improve the results of their model.

The model image is online at: http://www.jpl.nasa.gov/spaceimages/details.php?id=pia18041
The earthquake is believed to be associated with the Puente Hills Thrust fault, a blind thrust fault (meaning it does not break the earth surface) that zigzags from Orange County northwest through downtown Los Angeles. The same fault was responsible for the magnitude 5.9 Whittier Narrows earthquake on Oct. 1, 1987, which caused eight fatalities, injured several hundred and left about $360 million in property damage.

The NASA model is based on a fault estimated to be 5.6 miles (9 kilometers) long, 3.1 miles (5 kilometers) deep and 1.9 miles (3 kilometers) wide. The modeled fault segment dips upward through the ground at a 60-degree angle. The model estimated that in this earthquake, one side of the fault moved at a slanted angle horizontally and vertically 3.9 inches (10 centimeters) relative to the other side. The model also estimated the maximum displacement of Earth's surface from the quake at approximately 0.4 inch (1 centimeter), which is at the threshold of what is detectable with UAVSAR. The region of greatest ground displacement is indicated by the darker blue area located in the right center of the image.

In Nov. 2008, NASA JPL scientists began conducting a series of UAVSAR flights over regions of Northern and Southern California that are actively deforming and are marked by frequent earthquakes. About every six months, the scientists precisely repeat the same flight paths to produce images of ground deformation called interferograms. From these data, 3-D maps are being created for regions of interest, including the San Andreas and other California faults, extending from the Gulf of California in Mexico to Santa Rosa in the northern San Francisco Bay.

UAVSAR, which flies on a NASA C-20A aircraft from NASA's Armstrong Flight Research Center in California, measures ground deformation over large areas to a precision of 0.04 to 0.2 inches (0.1 to 0.5 centimeters).

By comparing the repeat-pass radar observations, scientists hope to measure any crustal deformations that may occur between observations, allowing them to "see" the amount of strain building up on fault lines, and giving them a clearer picture of which faults are active and at what rates they're moving, both before earthquakes and after them. The UAVSAR fault mapping project is designed to substantially improve knowledge of regional earthquake hazards in California. The 3-D UAVSAR data will allow scientists to bring entire faults into focus, allowing them to understand faults not just at their surfaces, but also at depth. When integrated into computer models, the data should give scientists a much clearer picture of California's complex fault systems.

The scientists are estimating the total displacement occurring in each region. As additional observations are collected, they expect to be able to determine how strain is partitioned between individual faults.
The UAVSAR flights serve as a baseline for pre-earthquake activity. As earthquakes occur during the course of this project, the team is measuring the deformation at the time of the earthquakes to determine the distribution of slip on the faults, and then monitoring longer-term motions after the earthquakes to learn more about fault zone properties.

Airborne UAVSAR mapping can allow a rapid response after an earthquake to determine what fault was the source and which parts of the fault slipped during the earthquake. Information about the earthquake source can be used to estimate what areas were most affected by an earthquake's shaking to guide rescue efforts and damage assessment.

The model was developed as part of NASA's QuakeSim project. The JPL-developed QuakeSim is a comprehensive, state-of-the-art software tool for simulating and understanding earthquake fault processes and improving earthquake forecasting. Initiated in 2002, QuakeSim uses NASA remote sensing and other earthquake-related data to simulate and model the behavior of faults in 3-D both individually and as part of complex, interacting systems. This provides long-term histories of fault behavior that can be used for statistical evaluation. QuakeSim also is used to identify regions of increased earthquake probabilities, called hotspots.

NASA's Earthquake Data Enhanced Cyber-Infrastructure for Disaster Evaluation and Response (E-DECIDER) project, which provides tools for earthquake disaster management and response using remote sensing data and NASA earthquake modeling software, published the model results, along with automatically generated deformation models and aftershock forecasts on a La Habra earthquake event page: http://e-decider.org/content/la-habra-earthquake-march-2014

For more information about UAVSAR, visit: http://uavsar.jpl.nasa.gov/

Source: NASA/Jet Propulsion Laboratory

The Magnetic anomaly deep within Earth's crust reveals Africa in North America

The repeated cycles of plate tectonics that have led to collision and assembly of large supercontinents and their breakup and formation of new ocean basins have produced continents that are collages of bits and pieces of other continents. Figuring out the origin and make-up of continental crust formed and modified by these tectonic events is vital to understanding Earth's geology and is important for many applied fields, such as oil, gas, and gold exploration.

In many cases, the rocks involved in these collision and pull-apart episodes are still buried deep beneath Earth's surface, so geologists must use geophysical measurements to study these features.
This new study by Elias Parker Jr. of the University of Georgia examines a prominent swath of lower-than-normal magnetism -- known as the Brunswick Magnetic Anomaly -- that stretches from Alabama through Georgia and off shore to the North Carolina coast.

The cause of this magnetic anomaly has been under some debate. Many geologists attribute the Brunswick Magnetic Anomaly to a belt of 200 million year old volcanic rocks that intruded around the time the Atlantic Ocean. In this case, the location of this magnetic anomaly would then mark the initial location where North America split from the rest of Pangea as that ancient supercontinent broke apart. Parker proposes a different source for this anomalous magnetic zone.

Drawing upon other studies that have demonstrated deeply buried metamorphic rocks can also have a coherent magnetic signal, Parker has analyzed the detailed characteristics of the magnetic anomalies from data collected across zones in Georgia and concludes that the Brunswick Magnetic Anomaly has a similar, deeply buried source. The anomalous magnetic signal is consistent with an older tectonic event -- the Alleghanian orogeny that formed the Alleghany-Appalachian Mountains when the supercontinent of Pangea was assembled.

Parker's main conclusion is that the rocks responsible for the Brunswick Magnetic Anomaly mark a major fault-zone that formed as portions of Africa and North America were sheared together roughly 300 million years ago -- and that more extensive evidence for this collision are preserved along this zone. One interesting implication is that perhaps a larger portion of what is now Africa was left behind in the American southeast when Pangea later broke up.

Source: Geological Society of America

The Magnitude 8.2 earthquake off Chile: Thrust faulting at shallow depths near the Chilean coast

The April 1, 2014 M8.2 earthquake in northern Chile occurred as the result of thrust faulting at shallow depths near the Chilean coast. The location . Credit: Image courtesy of U.S. Geological Survey

Amagnitude 8.2 earthquake struck off Chile on April 1, 2014 at 23:46:46 UTC, according to the U.S. Geological Survey.

The following is information from the USGS event page on this earthquake.

Tectonic Summary
The April 1, 2014 M8.2 earthquake in northern Chile occurred as the result of thrust faulting at shallow depths near the Chilean coast. The location and mechanism of the earthquake are consistent with slip on the primary plate boundary interface, or megathrust, between the Nazca and South America plates. At the latitude of the earthquake, the Nazca plate subducts eastward beneath the South America plate at a rate of 65 mm/yr. Subduction along the Peru-Chile Trench to the west of Chile has led to uplift of the Andes mountain range and has produced some of the largest earthquakes in the world, including the 2010 M 8.8 Maule earthquake in central Chile, and the largest earthquake on record, the 1960 M 9.5 earthquake in southern Chile.

The April 1 earthquake occurred in a region of historic seismic quiescence -- termed the northern Chile or Iquique seismic gap. Historical records indicate a M 8.8 earthquake occurred within the Iquique gap in 1877, which was preceded immediately to the north by an M 8.8 earthquake in 1868.

A recent increase in seismicity rates has occurred in the vicinity of the April 1 earthquake. An M6.7 earthquake with similar faulting mechanism occurred on March 16, 2014 and was followed by 60+ earthquake of M4+, and 26 earthquakes of M5+. The March 16 earthquake was also followed by three M6.2 events on March 17, March 22, and March 23. The spatial distribution of seismicity following the March 16 event migrated spatially to the north through time, starting near 20oS and moving to ~19.5oS. The initial location of the April 1 earthquake places the event near the northern end of this seismic sequence. Other recent large plate boundary ruptures bound the possible rupture area of the April 1 event, including the 2001 M 8.4 Peru earthquake adjacent to the south coast of Peru to the north, and the 2007 M 7.7 Tocopilla, Chile and 1995 M 8.1 Antofagasta, Chile earthquakes to the south. Other nearby events along the plate boundary interface include an M 7.4 in 1967 as well as an M 7.7 in 2005 in the deeper portion of the subduction zone beneath inland Chile.

Seismotectonics of South America (Nazca Plate Region)
The South American arc extends over 7,000 km, from the Chilean margin triple junction offshore of southern Chile to its intersection with the Panama fracture zone, offshore of the southern coast of Panama in Central America. It marks the plate boundary between the subducting Nazca plate and the South America plate, where the oceanic crust and lithosphere of the Nazca plate begin their descent into the mantle beneath South America. The convergence associated with this subduction process is responsible for the uplift of the Andes Mountains, and for the active volcanic chain present along much of this deformation front. Relative to a fixed South America plate, the Nazca plate moves slightly north of eastwards at a rate varying from approximately 80 mm/yr in the south to approximately 65 mm/yr in the north. Although the rate of subduction varies little along the entire arc, there are complex changes in the geologic processes along the subduction zone that dramatically influence volcanic activity, crustal deformation, earthquake generation and occurrence all along the western edge of South America.
Most of the large earthquakes in South America are constrained to shallow depths of 0 to 70 km resulting from both crustal and interplate deformation. Crustal earthquakes result from deformation and mountain building in the overriding South America plate and generate earthquakes as deep as approximately 50 km. Interplate earthquakes occur due to slip along the dipping interface between the Nazca and the South American plates. Interplate earthquakes in this region are frequent and often large, and occur between the depths of approximately 10 and 60 km. Since 1900, numerous magnitude 8 or larger earthquakes have occurred on this subduction zone interface that were followed by devastating tsunamis, including the 1960 M9.5 earthquake in southern Chile, the largest instrumentally recorded earthquake in the world. Other notable shallow tsunami-generating earthquakes include the 1906 M8.5 earthquake near Esmeraldas, Ecuador, the 1922 M8.5 earthquake near Coquimbo, Chile, the 2001 M8.4 Arequipa, Peru earthquake, the 2007 M8.0 earthquake near Pisco, Peru, and the 2010 M8.8 Maule, Chile earthquake located just north of the 1960 event.

Large intermediate-depth earthquakes (those occurring between depths of approximately 70 and 300 km) are relatively limited in size and spatial extent in South America, and occur within the Nazca plate as a result of internal deformation within the subducting plate. These earthquakes generally cluster beneath northern Chile and southwestern Bolivia, and to a lesser extent beneath northern Peru and southern Ecuador, with depths between 110 and 130 km. Most of these earthquakes occur adjacent to the bend in the coastline between Peru and Chile. The most recent large intermediate-depth earthquake in this region was the 2005 M7.8 Tarapaca, Chile earthquake.

Earthquakes can also be generated to depths greater than 600 km as a result of continued internal deformation of the subducting Nazca plate. Deep-focus earthquakes in South America are not observed from a depth range of approximately 300 to 500 km. Instead, deep earthquakes in this region occur at depths of 500 to 650 km and are concentrated into two zones: one that runs beneath the Peru-Brazil border and another that extends from central Bolivia to central Argentina. These earthquakes generally do not exhibit large magnitudes. An exception to this was the 1994 Bolivian earthquake in northwestern Bolivia. This M8.2 earthquake occurred at a depth of 631 km, which was until recently the largest deep-focus earthquake instrumentally recorded (superseded in May 2013 by a M8.3 earthquake 610 km beneath the Sea of Okhotsk, Russia), and was felt widely throughout South and North America.
Subduction of the Nazca plate is geometrically complex and impacts the geology and seismicity of the western edge of South America. The intermediate-depth regions of the subducting Nazca plate can be segmented into five sections based on their angle of subduction beneath the South America plate. Three segments are characterized by steeply dipping subduction; the other two by near-horizontal subduction.

The Nazca plate beneath northern Ecuador, southern Peru to northern Chile, and southern Chile descend into the mantle at angles of 25° to 30°. In contrast, the slab beneath southern Ecuador to central Peru, and under central Chile, is subducting at a shallow angle of approximately 10° or less. In these regions of "flat-slab" subduction, the Nazca plate moves horizontally for several hundred kilometers before continuing its descent into the mantle, and is shadowed by an extended zone of crustal seismicity in the overlying South America plate. Although the South America plate exhibits a chain of active volcanism resulting from the subduction and partial melting of the Nazca oceanic lithosphere along most of the arc, these regions of inferred shallow subduction correlate with an absence of volcanic activity.

Source: U.S. Geological Survey

After & Before major earthquake, silence: Dynamic stressing of a global system of faults results in rare seismic silence

In the global aftershock zone that followed the major April 2012 Indian Ocean earthquake, seismologists noticed an unusual pattern -- a dynamic "stress shadow," or period of seismic silence when some faults near failure were temporarily rendered incapable of a large rupture.

The magnitude (M) 8.6 earthquake, a strike-slip event at intraoceanic tectonic plates, caused global seismic rates of M≥4.5 to spike for several days, even at distances tens of thousands of kilometers from the mainshock site. But beginning two weeks after the mainshock, the rate of M≥6.5 seismic activity subsequently dropped to zero for the next 95 days.

Why did this rare period of quiet occur?
In a paper published today in the Bulletin of the Seismological Society of America (BSSA), Fred Pollitz of the U.S. Geological Survey and co-authors suggests that the Indian Ocean earthquake caused short-term dynamic stressing of a global system of faults. Across the planet, there are faults that are "close to failure" and ready to rupture. It may be, suggests Pollitz and his colleagues, that a large quake encourages short-term triggering of these close-to-failure faults but also relieves some of the stress that has built up along these faults. Large magnitude events would not occur until tectonic movement loads stress back on to the faults at the ready-to-fail levels they reached before the main shock.

Using a statistical model of global seismicity, Pollitz and his colleagues show that a transient seismic perturbation of the size of the April 2012 global aftershock would inhibit rupture in 88 percent of their possible M≥6.5 earthquake fault sources over the next 95 days, regardless of how close they were to failure beforehand.

This surprising finding, say the authors, challenges the previously held notion that dynamic stresses can only increase earthquake rates rather than inhibit them. But there are still mysteries about this process; for example, the global rate of M≥4.5 and M≥5.5 shocks did not decrease along with the larger shocks.


Source: Seismological Society of America

The Urbanization exposes French cities to greater seismic risk

French researchers have looked into data mining to develop a method for extracting information on the vulnerability of cities in regions of moderate risk, creating a proxy for assessing the probable resilience of buildings and infrastructure despite incomplete seismic inventories of buildings. The research exposes significant vulnerability in regions that have experienced an 'explosion of urbanization.'

"Considering that the seismic hazard is stable in time, we observe that the seismic risk comes from the rapid development of urbanization, which places at the same site goods and people exposed to hazard" said Philippe Gueguen, co-author and senior researcher at Université Joseph Fourier in Grenoble, France. The paper appears today in the journal Seismological Research Letters (SRL).

Local authorities rely on seismic vulnerability assessments to estimate the probable damage on an overall scale (such as a country, region or town) and identify the most vulnerable building categories that need reinforcement. These assessments are costly and require detailed understanding of how buildings will respond to ground motion.

Old structures, designed before current seismic building codes, abound in France, and there is insufficient information about how they will respond during an earthquake, say authors. The last major earthquake in France, which is considered to have moderate seismic hazard, was the 1909 magnitude 6 Lambesc earthquake, which killed 42 people and caused millions of euros of losses in the southeastern region.

The authors relied on the French national census for basic descriptions of buildings in Grenoble, a city of moderate seismic hazard, to create a vulnerability proxy, which they validated in Nice and later tested for the historic Lambesc earthquake.

The research exposed the effects of the urbanization and urban concentrations in areas prone to seismic hazard.

"In seismicity regions similar to France, seismic events are rare and are of low probability. With urbanization, the consequences of characteristic events, such as Lambesc, can be significant in terms of structural damage and fatalities," said Gueguen. "These consequences are all the more significant because of the moderate seismicity that reduces the perception of risk by local authorities."

If the 1909 Lambesc earthquake were to happen now, write the authors, the region would suffer serious consequences, including damage to more than 15,000 buildings. They equate the likely devastation to that observed after recent earthquakes of similar sizes in L'Aquila, Italy and Christchurch, New Zealand.

Source: Seismological Society of America

What it takes to heal a disaster-ravaged forest: Case study in China

Jindong Zhang, a post-doctoral research associate in CSIS, spent several months over a period of four years in Wolong dodging landslides, mudslides and rubble strewn roads to survey forest recovery at a finer scale than can be observed from satellites and getting a better handle on the nuances of tree species, height and soil conditions. The data was then combined with that from satellite imagery.

Recovering from natural disasters usually means rebuilding infrastructure and reassembling human lives. Yet ecologically sensitive areas need to heal, too, and scientists are pioneering new methods to assess nature's recovery and guide human intervention.

The epicenter of China's devastating Wenchuan earthquake in 2008 was in the Wolong Nature Reserve, a globally important valuable biodiversity hotspot and home to the beloved and endangered giant pandas. Not only did the quake devastate villages and roads, but the earth split open and swallowed sections of the forests and bamboo groves that shelter and feed pandas and other endangered wildlife. Persistent landslides and erosion exacerbated the devastation.

Typically such natural damage is assessed with remote sensing, which can be limited in fine details. Scientists at Michigan State University (MSU) and in China embarked on a dangerous boots-on-the-ground effort to understand how well the trees, bamboo and critical ground cover were recovering. Their work, which is relevant to disaster areas worldwide, is reported in this week's Forest Ecology and Management.

"Across the world, people are investing billions of dollars to protect valuable natural areas, as well as making enormous investments in restoring such areas after natural disasters," said Jianguo "Jack" Liu, director of MSU's Center for Systems Integration and Sustainability, and a co-author. "It's important we develop ways to understand the fine points of how well recovery efforts are working, so we can direct resources in the right places effectively."

Jindong Zhang, a post-doctoral research associate in CSIS, spent several months over a period of four years in Wolong dodging landslides, mudslides and rubble strewn roads to survey forest recovery at a finer scale than can be observed from satellites and getting a better handle on the nuances of tree species, height and soil conditions. The data was then combined with that from satellite imagery.
What was found was that much of the natural areas were on the road to recovery, and that China's $17 million effort at replanting native trees and bamboo were helping in areas handicapped by poor soil and growing conditions.

"Our evaluation of the Wolong restoration project will have a guiding role in the restoration scheme areas across the entire area affected by the earthquake, Zhang said. "Our study indicated that forest restoration after natural disasters should not only consider the forest itself, but also take into account the animals inhabiting the ecosystem and human livelihoods."

They also noted that such efforts could benefit from more targeting of areas most favored by pandas. The replanting efforts were done by local residents.

"We witnessed pretty intense periods when it seemed like everyone in the target areas were out planting," said co-author Vanessa Hull, a CSIS doctoral candidate who studies panda habitat in Wolong. "My field assistants also joined in on the village-wide efforts. It was pretty neat to see."
But a potential downside to such efforts was that most of the available labor was near villages, and pandas shy from human contact. That meant that some of the best assisted-forest recovery was in areas not favored by pandas. Hull noted, however, that there could be an upside to that. Healthier forests could mean local residents have less need to venture into more far-flung panda-friendly forests.

"We wanted to know if the benefit of this effort was matching up to the investment -- which was
significant," Hull said. "It's an important question, and the world needs good ways to evaluate it as natural disasters are growing in frequency and intensity."


Source: Michigan State University
Summary: Recovering from natural disasters usually means rebuilding infrastructure and reassembling human lives. Yet ecologically sensitive areas need to heal, too, and scientists are pioneering new methods to assess nature's recovery and guide human intervention. A new study focused on the epicenter of China's devastating Wenchuan earthquake in 2008, a globally important valuable biodiversity hotspot and home to the beloved and endangered giant pandas. Not only did the quake devastate villages and roads, but the earth split open and swallowed sections of the forests and bamboo groves that shelter and feed pandas and other endangered wildlife. The study indicated that forest restoration after natural disasters should not only consider the forest itself, but also take into account the animals inhabiting the ecosystem and human livelihoods.

The San Francisco's big 1906 earthquake was third of a series on San Andreas Fault

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. Credit: Image courtesy of 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 kilometers (296 miles) 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."

The U.S. Geological Survey funded the research through grants 08-HQ-GR-0071, 08-HQ-GR-0072, G10AP00064, G10AP0065 and G11AP20123. A Geological Society of America Student Research Grant to Streig funded the age-dating of the team's evidence at the Lawrence Livermore National Laboratory's Center for Accelerator Mass Spectrometry.

Source: University of Oregon
Summary: Geologists have uncovered geologic evidence that supports historical narratives for two earthquakes in the 68 years prior to San Francisco's devastating 1906 disaster.

Earthquake behind the Shroud of Turin image? Radiation from earthquake could have led to 'wrong' 1988 dating

Semitransparent front copy of the Turin Shroud.
Credit: The Society for Imaging Science and Technology

Neutron radiation caused by 33 A.D. earthquake could have led to "wrong" 1988 radiocarbon dating of Shroud, suggest researchers

An earthquake in Old Jerusalem might be behind the famous image of the Shroud of Turin, says a group of researchers led by Alberto Carpinteri of the Politecnico di Torino in Italy in an article published in Springer's journal Meccanica. They believe that neutron radiation caused by an earthquake could have induced the image of a crucified man -- which many people believe to be that of Jesus -- onto the length of linen cloth, and caused carbon-14 dating done on it in 1988 to be wrong.

The Shroud has attracted widespread interest ever since Secondo Pia took the first photograph of it in 1898: about whether it is Jesus' purported burial cloth, how old it might be, and how the image was created. According to radiocarbon dating done in 1988, the cloth was only 728 years old at the time. Other researchers have since suggested that the shroud is much older and that the dating process was incorrect because of neutron radiation -- a process which is the result of nuclear fusion or nuclear fission during which free neutrons are released from atoms -- and its interaction with the nuclei of other atoms to form new carbon isotopes.

However, no plausible physical reason has yet been proposed to explain the origin of this neutron radiation. Now Carpinteri's team, through mechanical and chemical experimentation, hypothesizes that high-frequency pressure waves generated in Earth's crust during earthquakes are the source of such neutron emissions. This is based on their research into piezonuclear fission reactions, which are triggered when very brittle rock specimens are crushed under a press machine. In the process, neutrons are produced without gamma emissions. Analogously, the researchers theorize further that neutron flux increments, in correspondence to seismic activity, should be a result of the same reactions.

The researchers therefore believe that neutron emission from a historical earthquake in 33 A.D. in Old Jerusalem, which measured 8.2 on the Richter Scale, could have been strong enough to cause neutron imaging through its interaction with nitrogen nuclei. On the one hand, this could have created the distinctive image on the Shroud through radiation imagery, while on the other, it could have increased the level of carbon-14 isotopes found on the linen fibres that could have confused the 1988 radiocarbon dating tests.

"We believe it is possible that neutron emissions by earthquakes could have induced the image formation on the Shroud's linen fibres, through thermal neutron capture on nitrogen nuclei, and could also have caused a wrong radiocarbon dating," hypothesizes Carpinteri.

Source: Springer Science+Business Media
Summary: Neutron radiation caused by 33 A.D. earthquake could have led to "wrong" 1988 radiocarbon dating of Shroud, suggest researchers. An earthquake in Old Jerusalem might be behind the famous image of the Shroud of Turin, says a group of researchers. They believe that neutron radiation caused by an earthquake could have induced the image of a crucified man - which many people believe to be that of Jesus - onto the length of linen cloth, and caused carbon-14 dating done on it in 1988 to be wrong.