All shook up
by Christina Rowland
Jul 05, 2012 | 2243 views | 0 0 comments | 8 8 recommendations | email to a friend | print
Map courtesy of the USGS
This part of Texas is classified as at lower seismic hazard, as shown by the darker blue color. The hazard level increases as the color shifts to teal and yellow.
Map courtesy of the USGS This part of Texas is classified as at lower seismic hazard, as shown by the darker blue color. The hazard level increases as the color shifts to teal and yellow.
Earthquakes in South Texas are rare, but when there is one, it tends to shake people up, and they go looking for an explanation based on either scientific fact or pure speculation.

One of the most popular explanations and blame is on hydraulic fracturing used in oil and gas construction.

The National Research Council at the request of the U.S. Congress and U.S. Department of Energy released a preliminary report called Induced Seismicity Potential in Energy Technologies earlier this month that investigates whether hydraulic fracturing can indeed cause induced seismic events.

“The process of hydraulic fracturing a well as presently implemented for shale gas recovery does not pose a high risk for inducing felt seismic events,” the report states as one of its major findings.

So, while the report says it is not impossible totally, it is highly unlikely that if it did happen it would even be felt.

“About 35,000 hydraulically fractured shale gas wells exist in the United States; only one case of felt seismicity (M-2.8) in the United States has been described in which hydraulic fracturing for shale gas development is suspected, but not confirmed...”

A second finding of the report was “injection for disposal of wastewater derived from energy technologies into the subsurface does pose some risk for induced seismicity, but very few events have been documented over the past several decades relative to the large number of disposal wells in operation.”

A chart included with the report had wastewater disposal wells in Arkansas, Colorado and Ohio linked to induced seismic activity.

“Although only a few induced seismic events have been linked to these disposal wells, the occurrence of these events has generated considerable public concern. Examination of these cases has suggested causal links between the injection zones and previously unrecognized faults in the subsurface.”

This is not the case in all situations, but the report states that it is one of the possibilities.

Another possibility the report talks about for induced activity is the imbalance of fluids.

“...the net fluid balance (total balance of fluid introduced and removed) appears to have the most direct consequence on changing pore pressure in the subsurface over time. Energy technology projects are designed to maintain a balance between the amount of fluid being injected and the amount of fluid being withdrawn, such as geothermal, and most oil and gas development may produce fewer induced seismic events than technologies that do not maintain fluid balance.”

So, while it is possible for seismic activity to be induced by some oil and gas production activities, it is highly unlikely, and if it was to happen it is even less likely it would actually be felt.

While seismic events happen around the county daily, the report states “seismic events M (magnitude) greater than 2.0 have the possibility of being felt, particularly if they occur at shallow depths, but smaller seismic events (M<2.0) generally are not felt.”

The most recent earthquake in South Texas happened near Tilden on Sunday, June 24, at 3:55 a.m.

The U.S. Geological Survey reported the earthquake to be a 3.4 on the Richter Magnitude Scale.

The epicenter of the quake was located eight miles east-southeast of Tilden. And the quake happened at a depth of 3 miles, according to U.S. Geological Survey measurements.

Jayne Varga, McMullen County administrative assistant, said she didn’t feel the earthquake.

She said most people she’s asked also felt nothing.

Earthquakes have been recorded in Texas since 1882, according to USGS, long before oil and gas production came to be.

There are hundreds of fault lines around the state, and a group of faults that run along the coast make up the Gulf Margin Faults. The Gulf Margin Faults run along the coast from northern Mexico into Texas and follow the coastline all the way to Florida.

“...the belt of gulf-margin normal faults from Florida through Texas has strikingly low historical seismicity; the stress field and seismogenic potential of the underlying crust are unknown; and, therefore, the ability of the fault belt to generate significant seismic ruptures that could cause damaging ground motion is unclear,” according to R.L. Wheeler, who compiled the Quaternary fault and fold database of the United States for the Gulf-margin normal faults (1999).

So, while some blame oil and gas for increased seismic activity in South Texas, others blame the fault lines that run along the entire coast. There is not enough information for either answer to be entirely the reason in all cases, but it is important to know that both possibilities exist.

Christina Rowland is the regional editor at the Bee-Picayune and can be reached at 358-2550, ext. 119, or at
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