Map showing the location of NEFZ (black) at the top of the Ellenburger formation, inferred faults (dashed) at the top of the Ellenburger formation, injection wells (red squares), two production wells (API 36734045 and 36734139) with significant brine production near the faults (pink arrows) and earthquake epicentres (coloured circles) recorded by the temporary seismic network (triangles) (a). The red star in the inset of a shows the map location. The black scale bar in a is 2 km. Grey (white) triangles indicate the locations of active (inactive) seismic stations. Line X–X' in (a) shows the location of the cross-section shown in (b). We interpret two faults based on earthquake location and consistent with industry interpretations: a primary normal fault and a shallower antithetic normal fault. Graphic: Hornbach, et al., 2015

By Alicia Chang, Tim Talley, Donna Bryson, and Jon Fahey
23 April 2015

LOS ANGELES (AP) – With the evidence coming in from one study after another, scientists are now more certain than ever that oil and gas drilling is causing hundreds upon hundreds of earthquakes across the U.S.

So far, the quakes have been mostly small and have done little damage beyond cracking plaster, toppling bricks and rattling nerves. But seismologists warn that the shaking can dramatically increase the chances of bigger, more dangerous quakes.

Up to now, the oil and gas industry has generally argued that any such link requires further study. But the rapidly mounting evidence could bring heavier regulation down on drillers and make it more difficult for them to get projects approved.

The potential for man-made quakes "is an important and legitimate concern that must be taken very seriously by regulators and industry," said Jason Bordoff, founding director of the Center on Global Energy Policy at Columbia University.

He said companies and states can reduce the risk by taking such steps as monitoring operations more closely, imposing tighter standards and recycling wastewater from drilling instead of injecting it underground.

A series of government and academic studies over the past few years — including at least two reports released this week alone — has added to the body of evidence implicating the U.S. drilling boom that has created a bounty of jobs and tax revenue over the past decade or so.

On Thursday, the U.S. Geological Survey released the first comprehensive maps pinpointing more than a dozen areas in the central and eastern U.S. that have been jolted by quakes that the researchers said were triggered by drilling. The report said man-made quakes tied to industry operations have been on the rise.

Scientists have mainly attributed the spike to the injection of wastewater deep underground, a practice they say can activate dormant faults. Only a few cases of shaking have been blamed on fracking, in which large volumes of water, sand and chemicals are pumped into rock formations to crack them open and free oil or gas.

"The picture is very clear" that wastewater injection can cause faults to move, said USGS geophysicist William Ellsworth.

Until recently, Oklahoma — one of the biggest energy-producing states — had been cautious about linking the spate of quakes to drilling. But the Oklahoma Geological Survey acknowledged earlier this week that it is "very likely" that recent seismic activity was caused by the injection of wastewater into disposal wells.

Earthquake activity in Oklahoma in 2013 was 70 times greater than it was before 2008, state geologists reported. Oklahoma historically recorded an average of 1.5 quakes of magnitude 3 or greater each year. It is now seeing an average of 2.5 such quakes each day, according to geologists.

Angela Spotts, who lives outside Stillwater, Oklahoma, in an area with a number of wastewater disposal wells, said the shaking has damaged her brick home. She pointed to the cracked interior and exterior walls, and windows and kitchen cabinets that are separating from the structure.

"There's been no doubt in my mind what's causing them," Spotts said. "Sadly, it's really taken a long time for people to come around. Our lives are being placed at risk. Our homes are being broken."

Yet another study, this one published Tuesday in the journal Nature Communications, connected a swarm of small quakes west of Fort Worth, Texas, to nearby natural gas wells and wastewater disposal. [more]

Scientists convinced of tie between earthquakes and drilling

Map view of modelled excess pressures at a depth of ~2,500 m for May 2009 (a), January 2010 (b), January 2011 (c) and December 2013 (d,e). The model uses average monthly reported water injection rates and the Dupuit–Theim equation to estimate bottom-hole pressure values. Pressure above hydrostatic averages 0.58 MPa for injector well #1 and 0.28 MPa for injector well #2 during injection. Ellenburger permeability is assumed constant at 5 × 10−14 m2; boundary conditions are open along the side and closed at the top and bottom. We apply an average rate of brine production based directly on reported TRC G-10 water production values for the 70 largest water producing production wells in the region. The images show the system before injection (a) through the onset of seismicity (e). Black lines, the NEFZ location at the top of the Ellenburger formation; red squares, injector locations; pink arrows, approximate location of two large brine production wells that are located both near the faults and near reported earthquakes swarms within the Ellenburger (grey circles with white outlines). Note that the most significant amount of brine removal occurs along the fault trend (a). Graphic: Hornbach, et al., 2015

ABSTACT: In November 2013, a series of earthquakes began along a mapped ancient fault system near Azle, Texas. Here we assess whether it is plausible that human activity caused these earthquakes. Analysis of both lake and groundwater variations near Azle shows that no significant stress changes were associated with the shallow water table before or during the earthquake sequence. In contrast, pore-pressure models demonstrate that a combination of brine production and wastewater injection near the fault generated subsurface pressures sufficient to induce earthquakes on near-critically stressed faults. On the basis of modelling results and the absence of historical earthquakes near Azle, brine production combined with wastewater disposal represent the most likely cause of recent seismicity near Azle. For assessing the earthquake cause, our research underscores the necessity of monitoring subsurface wastewater formation pressures and monitoring earthquakes having magnitudes of ~M2 and greater. Currently, monitoring at these levels is not standard across Texas or the United States.

Causal factors for seismicity near Azle, Texas

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