Significant strides in science have been made to better understand potential ground shaking from induced earthquakes, which are earthquakes triggered by man-made practices.

Earthquake activity has sharply increased since 2009 in the central and eastern United States. The increase has been linked to industrial operations that dispose of wastewater by injecting it into deep wells.

The U. S. Geological Survey (USGS) released a report today that outlines a preliminary set of models to forecast how hazardous ground shaking could be in the areas where sharp increases in seismicity have been recorded. The models ultimately aim to calculate how often earthquakes are expected to occur in the next year and how hard the ground will likely shake as a result. This report looked at the central and eastern United States; future research will incorporate data from the western states as well.

This report also identifies issues that must be resolved to develop a final hazard model, which is scheduled for release at the end of the year after the preliminary models are further examined. These preliminary models should be considered experimental in nature and should not be used for decision-making.

USGS scientists identified 17 areas within eight states with increased rates of induced seismicity. Since 2000, several of these areas have experienced high levels of seismicity, with substantial increases since 2009 that continue today. This is the first comprehensive assessment of the hazard levels associated with induced earthquakes in these areas. A detailed list of these areas is provided in the accompanying map, including the states of Alabama, Arkansas, Colorado, Kansas, New Mexico, Ohio, Oklahoma, and Texas.

Scientists developed the models by analyzing earthquakes in these zones and considering their rates, locations, maximum magnitude, and ground motions.

“This new report describes for the first time how injection-induced earthquakes can be incorporated into U.S. seismic hazard maps,” said Mark Petersen, Chief of the USGS National Seismic Hazard Modeling Project. “These earthquakes are occurring at a higher rate than ever before and pose a much greater risk to people living nearby. The USGS is developing methods that overcome the challenges in assessing seismic hazards in these regions in order to support decisions that help keep communities safe from ground shaking.”

In 2014, the USGS released updated National Seismic Hazard Maps, which describe hazard levels for natural earthquakes. Those maps are used in building codes, insurance rates, emergency preparedness plans, and other applications. The maps forecast the likelihood of earthquake shaking within a 50-year period, which is the average lifetime of a building. However, these new induced seismicity products display intensity of potential ground shaking from induced earthquakes in a one-year period. This shorter timeframe is appropriate because the induced activity can vary rapidly with time and is subject to commercial and policy decisions that could change at any point.

These new methods and products result in part from a workshop hosted by the USGS and the Oklahoma Geological Survey. The workshop, described in the new report, brought together a broad group of experts from government, industry and academic communities to discuss the hazards from induced earthquakes.

Wastewater that is salty or polluted by chemicals needs to be disposed of in a manner that prevents contaminating freshwater sources. Large volumes of wastewater can result from a variety of processes, such as a byproduct from energy production. Wastewater injection increases the underground pore pressure, which may lubricate nearby faults thereby making earthquakes more likely to occur. Although the disposal process has the potential to trigger earthquakes, most wastewater disposal wells do not produce felt earthquakes.

Many questions have been raised about whether hydraulic fracturing—commonly referred to as “fracking”—is responsible for the recent increase of earthquakes. USGS’s studies suggest that the actual hydraulic fracturing process is only occasionally the direct cause of felt earthquakes.

Reference:
Read the newly published USGS report, “Incorporating Induced Seismicity in the 2014 United States National Seismic Hazard Model—Results of 2014 Workshop and Sensitivity Studies.”. DOI: 10.3133/ofr20151070

Note: The above story is based on materials provided by USGS.

More than 143 million Americans living in the 48 contiguous states are exposed to potentially damaging ground shaking from earthquakes, with as many as 28 million people likely to experience strong shaking during their lifetime, according to research discussed at the annual meeting of Seismological Society of America. The report puts the average long-term value of building losses from earthquakes at $4.5 billion per year, with roughly 80 percent of losses attributed to California, Oregon and Washington.

“This analysis of data from the new National Seismic Hazard Maps reveals that significantly more Americans are exposed to earthquake shaking, reflecting both the movement of the population to higher risk areas on the west coast and a change in hazard assessments,” said co-author Bill Leith, senior science advisor at USGS. By comparison, FEMA estimated in 1994 that 75 million Americans in 39 states were at risk from earthquakes.

Kishor Jaiswal, a research contractor with the U.S. Geological Survey (USGS), presented the research conducted with colleagues from USGS, FEMA and California Geological Survey. They analyzed the 2014 National Seismic Hazard Maps and the latest data on infrastructure and population from LandScan, a product of Oak Ridge National Laboratory.

The report focuses on the 48 contiguous states, where more than 143 million people are exposed to ground motions from earthquakes, but Leith noted that nearly half the U.S. population, or nearly 150 million Americans, are at risk of shaking from earthquakes when Alaska, Puerto Rico and Hawaii are also considered.

In the highest hazard zones, where 28 million Americans will experience strong shaking during their lifetime, key infrastructure could also experience a shaking intensity sufficient to cause moderate to extensive damage. The analysis identified more than 6,000 fire stations, more than 800 hospitals and nearly 20,000 public and private schools that may be exposed to strong ground motion from earthquakes.

Using the 2010 Census data and the 2012 replacement cost values for buildings, and using FEMA’s Hazus program, researchers calculated systematically the losses that could happen on any given year, ranging from no losses to a very high value of loss. However, the long-term average loss to the buildings in the contiguous U.S. is $4.5 billion per year, with most financial losses occurring in California, Oregon and Washington states.

“Earthquakes remain an important threat to our economy,” said Jaiswal. “While the west coast may carry the larger burden of potential losses and the greatest threat from the strongest shaking, this report shows that the threat from earthquakes is widespread.”

Note: The above story is based on materials provided by Seismological Society of America.

It may seem unlikely that a large earthquake would take place hundreds of kilometers away from a tectonic plate boundary, in areas with low levels of strain on the crust from tectonic motion. But major earthquakes such as the Mw 7.9 2008 Chengdu quake in China and New Zealand’s 2011 Mw 6.3 quake have shown that large earthquakes do occur and can cause significant infrastructure damage and loss of life. So what should seismologists look for if they want to identify where an earthquake might happen despite the absence of historical seismic activity?

Roger Bilham of the University of Colorado shows that some of these regions had underlying features that could have been used to identify that the region was not as “aseismic” as previously thought. Some of these warning signs include debris deposits from past tsunamis or landslides, ancient mid-continent rifts that mark the scars of earlier tectonic boundaries, or old fault scarps worn down by hundreds or thousands of years of erosion.Earth’s populated area where there is no written history makes for an enormous “search grid” for earthquakes. For example, the Caribbean coast of northern Colombia resembles a classic subduction zone with the potential for tsunamigenic M>8 earthquakes at millennial time scales, but the absence of a large earthquake since 1492 is cause for complacency among local populations.

These areas are not only restricted to the Americas. Bilham notes that in many parts of Asia, where huge populations now reside and critical facilities exist or are planned, a similar historical silence exists. Parts of the Himalaya and central and western India that have not had any major earthquake in more than 500 years could experience shaking at levels and durations that are unprecedented in their written histories.

Note: The above story is based on materials provided by Seismological Society of America.

It’s the first thing that geologist Todd Halihan asks on a sunny spring afternoon at Oklahoma State University in Stillwater: “Did you feel the earthquake? My mother-in-law just called to complain that the house was shaking.”

Halihan’s mother-in-law has been calling a lot lately. Fifteen quakes of magnitude 4 or greater struck in 2014 — packing more than a century’s worth of normal seismic activity for the state into a single year. Oklahoma had twice as many earthquakes last year as California — a seismic hotspot — and researchers are racing to understand why before the next major one strikes.

Whatever they learn will apply to seismic hazards worldwide. Oklahoma’s quakes have been linked to underground wells where oil and gas operations dispose of waste water, but mining, geothermal energy and other underground explorations have triggered earthquakes from South Africa to Switzerland. This week, at a meeting of the Seismological Society of America in Pasadena, California, scientists will discuss how the risk from human-induced quakes differs from that of natural quakes — and how society can prepare for it.

In Oklahoma, the earthquakes have unleashed a frenzy of finger-pointing, with angry residents suing oil and gas companies over damage to their homes. The industry and politicians are locked in fierce debates about whether the quakes are induced, but the unprecedented shaking across central and northern parts of the state matches almost exactly with the activity of water-disposal wells. “There are some who will argue that it is purely natural,” says Halihan. “But by now it’s pretty clear it’s not.”

Companies drill into the ground to extract oil and gas mixed with salt water, essentially the brine from a long-fossilized sea. They separate out the fuels and then inject the salt water into deep disposal wells (there are more than 4,600 in Oklahoma). State regulations require that the salt water be disposed of in rock layers below those that hold drinking water.

Stress fracture

Much of the liquid ends up in a rock formation called the Arbuckle, which underlies much of Oklahoma and is known for its ability to absorb huge volumes of water. But in many places the Arbuckle rests on brittle, ancient basement rocks, which can fracture along major faults under stress. “The deeper you inject, the more likely it is that the injected brine is going to make its way into a seismogenic fault zone, prone to producing earthquakes,” says Arthur McGarr, who leads research on induced quakes at the US Geological Survey (USGS) in Menlo Park, California.

Oil and gas companies operate disposal wells across the central United States, and although Oklahoma stands out for the sheer volume of waste water, other states may be getting triggered earthquakes. A report in Nature Communications this week, for example, links brine injection to a series of quakes that began in November 2013 near Azle, Texas.

The basic physics of the process has been understood since the 1970s, when scientists from the USGS pumped water down a well in Rangely, Colorado, and recorded how earthquake activity rose and petered out as they varied the amount of fluid. The question now is which faults are likely to rupture in Oklahoma, and how large an earthquake they might produce.

Whether a fault breaks in an earthquake depends on how it sits in relation to the stresses that compress Earth’s crust. The movement of tectonic plates is squeezing Oklahoma from east to west, so most of the earthquakes are happening along faults oriented northwest to southeast, or northeast to southwest. Other faults are less likely to rupture, says McGarr.

The biggest earthquake ever recorded in Oklahoma was a magnitude-5.6 event near the town of Prague in November 2011, and many seismologists think that it was induced by nearby disposal wells. Theoretical work suggests that the potential size of a quake grows with the volume of fluid injected into the ground. The biggest disposal wells in Oklahoma inject more than 60 million litres of waste water each month.

Austin Holland, the state seismologist at the Oklahoma Geological Survey in Norman, estimates that the chance of another earthquake of magnitude 5 or greater striking the state in the next year is about 30%. “That is not the kind of lottery we want to win,” he says.

Oklahoma has designated buffer zones, requiring extra scrutiny for disposal wells within 10 kilometres of sites of earthquake swarms or quakes of magnitude 4 or greater. As of 18 April, operators must also prove they are not injecting into or near basement rocks, or must cut their disposal volumes by half.

Yet oil and gas companies hold great political power in Oklahoma, and regulators continue to emphasize what they call uncertainty in linking injection wells to quakes. “We felt a big quake one Friday night and I knew we had permitted a brand-new Arbuckle disposal well not three miles from my house,” said Tim Baker, director of the oil and gas division of the Oklahoma Corporation Commission, which regulates drilling, at a town-hall meeting in suburban Oklahoma City this month. “I drove to that well to inspect it on Saturday morning, and it wasn’t even turned on. That’s how complex this issue is.”

The related — and controversial — technique of hydraulic fracturing, in which water is injected into rock to open cracks so oil and gas can flow more easily, has also been linked to earthquakes, but to a much lesser extent. The fracking involves injecting less water for shorter periods of time, and has not been tied to any earthquakes greater than magnitude 4 (ref. 5).

Seismic survey

One group of geologists wants to explore exactly how disposal wells might cause earthquakes. The team hopes to find a remote corner of Oklahoma and inject fluids deep underground while monitoring seismicity, in a modern analogue to the 1970s experiments in Colorado. “It’s a very ambitious goal, but we want to do a controlled field-scale experiment,” says Ze’ev Reches, a geophysicist at the University of Oklahoma in Norman and a co-leader of the project. But with Oklahomans already on edge, it is not clear whether the team could pull off such an experiment. So far, it remains hypothetical.

For now, seismologists are just trying to keep up with the quakes. The state geological survey recently gave up naming earthquake swarms, because the quakes simply never stopped, says Amberlee Darold, an agency seismologist. (The survey used to name swarms after nearby towns; it now identifies huge swathes of continuous activity by county.)

In the 15-storey brick Earth sciences building on the University of Oklahoma campus in Norman, statues celebrate the state’s ‘wild­catters’ who made it big in oil and gas, and a well-manicured garden nearby is dedicated to their achievements. Holland and Darold labour in the building’s dark basement, compiling a database of Oklahoma’s faults and trying to make sure that every earthquake is documented.

Many scientists are worried that the state’s buildings are not constructed to standards that consider seismic risk, and are concerned about how old brick-and-mortar structures would hold up in a large earthquake. The USGS issues national seismic-hazard maps every few years, but has never included the risk from induced quakes. This year, for the first time, the agency is developing induced-seismicity hazard maps for Oklahoma and surrounding states. The first of these is likely to be out by the end of 2015, says McGarr.

In Cushing, almost 60 kilometres north of Prague, crude-oil pipelines from across the continent meet. Fences topped with razor wire are meant to protect huge oil-storage tanks from a terrorist attack, but will not help if a major earthquake strikes, says Halihan.

In the meantime, he sits and waits to hear about the next quake. If he does not want to rely on his mother-in-law, Halihan can track the tremors by watching the movement of a small brass marker pinned to his office wall. It used to shake about once a week. Now it does so almost every day.

Note : The above story is based on materials provided by Nature.

A seismology team led by Southern Methodist University (SMU), Dallas, finds that high volumes of wastewater injection combined with saltwater (brine) extraction from natural gas wells is the most likely cause of earthquakes occurring near Azle, Texas, from late 2013 through spring 2014.
In an area where the seismology team identified two intersecting faults, they developed a sophisticated 3D model to assess the changing fluid pressure within a rock formation in the affected area. They used the model to estimate stress changes induced in the area by two wastewater injection wells and the more than 70 production wells that remove both natural gas and significant volumes of salty water known as brine.

Conclusions from the modeling study integrate a broad-range of estimates for uncertain subsurface conditions. Ultimately, better information on fluid volumes, flow parameters, and subsurface pressures in the region will provide more accurate estimates of the fluid pressure along this fault.

“The model shows that a pressure differential develops along one of the faults as a combined result of high fluid injection rates to the west and high water removal rates to the east,” said Matthew Hornbach, SMU associate professor of geophysics. “When we ran the model over a 10-year period through a wide range of parameters, it predicted pressure changes significant enough to trigger earthquakes on faults that are already stressed.”

Model-predicted stress changes on the fault were typically tens to thousands of times larger than stress changes associated with water level fluctuations caused by the recent Texas drought.

“What we refer to as induced seismicity — earthquakes caused by something other than strictly natural forces — is often associated with subsurface pressure changes,” said Heather DeShon, SMU associate professor of geophysics. “We can rule out stress changes induced by local water table changes. While some uncertainties remain, it is unlikely that natural increases to tectonic stresses led to these events.”

DeShon explained that some ancient faults in the region are more susceptible to movement — “near critically stressed” — due to their orientation and direction. “In other words, surprisingly small changes in stress can reactivate certain faults in the region and cause earthquakes,” DeShon said.

The study, “Causal Factors for Seismicity near Azle, Texas,” has been published online in the journal Nature Communications.

The study was produced by a team of scientists from SMU’s Department of Earth Sciences in Dedman College of Humanities and Sciences, the U.S. Geological Survey, the University of Texas Institute for Geophysics and the University of Texas Department of Petroleum and Geosystems Engineering. SMU scientists Hornbach and DeShon are the lead authors.

SMU seismologists have been studying earthquakes in North Texas since 2008, when the first series of felt tremors hit near DFW International Airport between Oct. 30, 2008, and May 16, 2009. Next came a series of quakes in Cleburne between June 2009 and June 2010, and this third series in the Azle-Reno area northwest of Fort Worth occurred between November 2013 and January 2014. The SMU team also is studying an ongoing series of earthquakes in the Irving-Dallas area that began in April 2014.

In both the DFW sequence and the Cleburne sequence, the operation of injection wells used in the disposal of natural gas production fluids was listed as a possible cause of the seismicity. The introduction of fluid pressure modeling of both industry activity and water table fluctuations in the Azle study represents the first of its kind, and has allowed the SMU team to move beyond assessment of possible causes to the most likely cause identified in this report.

Prior to the DFW Airport earthquakes in 2008, an earthquake large enough to be felt had not been reported in the North Texas area since 1950. The North Texas earthquakes of the last seven years have all occurred in areas developed for natural gas extraction from a geologic formation known as the Barnett Shale. The Texas Railroad Commission reports that production in the Barnett Shale grew exponentially from 216 million cubic feet a day in 2000, to 4.4 billion cubic feet a day in 2008, to a peak of 5.74 billion cubic feet of gas a day in 2012.

While the SMU Azle study adds to the growing body of evidence connecting some injection wells and, to a lesser extent, some oil and gas production to induced earthquakes, SMU’s team notes that there are many thousands of injection and/or production wells that are not associated with earthquakes.

The area of study addressed in the report is in the Newark East Gas Field (NEGF), north and east of Azle. In this field, hydraulic fracturing is applied to loosen and extract gas trapped in the Barnett Shale, a sedimentary rock formation formed approximately 350 million years ago. The report explains that along with natural gas, production wells in the Azle area of the NEGF can also bring to the surface significant volumes of water from the highly permeable Ellenburger Formation — both naturally occurring brine as well as fluids that were introduced during the fracking process.

Subsurface fluid pressures are known to play a key role in causing seismicity. A primer produced by the U.S. Department of Energy explains the interplay of fluids and faults:

“The fluid pressure in the pores and fractures of the rocks is called the ‘pore pressure.’ The pore pressure acts against the weight of the rock and the forces holding the rock together (stresses due to tectonic forces). If the pore pressures are low (especially compared to the forces holding the rock together), then only the imbalance of natural in situ earth stresses will cause an occasional earthquake. If, however, pore pressures increase, then it would take less of an imbalance of in situ stresses to cause an earthquake, thus accelerating earthquake activity. This type of failure…is called shear failure. Injecting fluids into the subsurface is one way of increasing the pore pressure and causing faults and fractures to “fail” more easily, thus inducing an earthquake. Thus, induced seismicity can be caused by injecting fluid into the subsurface or by extracting fluids at a rate that causes subsidence and/or slippage along planes of weakness in the earth.”

All seismic waveform data used in the compilation of the report are publically available at the IRIS Data Management Center. Wastewater injection, brine production and surface injection pressure data are publicly available at the Texas Railroad Commission (TRC). Craig Pearson at the TRC, Bob Patterson from the Upper Trinity Groundwater Conservation District; scientists at XTO Energy, ExxonMobil, MorningStar Partners and EnerVest provided valuable discussions and, in some instances, data used in the completion of the report.

“This report points to the need for even more study in connection with earthquakes in North Texas,” said Brian Stump, SMU’s Albritton Chair in Earth Sciences. “Industry is an important source for key data, and the scope of the research needed to understand these earthquakes requires government support at multiple levels.”

Reference:
Matthew J. Hornbach, Heather R. DeShon, William L. Ellsworth, Brian W. Stump, Chris Hayward, Cliff Frohlich, Harrison R. Oldham, Jon E. Olson, M. Beatrice Magnani, Casey Brokaw, James H. Luetgert. Causal factors for seismicity near Azle, Texas. Nature Communications, 2015; 6: 6728 DOI: 10.1038/ncomms7728

Note: The above story is based on materials provided by Southern Methodist University.