Wednesday, August 24, 2011
Human Activity Is Officially Acknowledged to Cause Earthquakes
The United States Geological Survey is America's official expert on earthquakes. It's the Federal agency charged with monitoring, reporting on, researching and stressing preparedness for earthquakes.
So I was surprised to read the following statement by the USGS:
Earthquakes induced by human activity have been documented in a few locations in the United States, Japan, and Canada. The cause was injection of fluids into deep wells for waste disposal and secondary recovery of oil, and the use of reservoirs for water supplies. Most of these earthquakes were minor. The largest and most widely known resulted from fluid injection at the Rocky Mountain Arsenal near Denver, Colorado. In 1967, an earthquake of magnitude 5.5 followed a series of smaller earthquakes. Injection had been discontinued at the site in the previous year once the link between the fluid injection and the earlier series of earthquakes was established. (Nicholson, Craig and Wesson, R.L., 1990, Earthquake Hazard Associated with Deep Well Injection--A Report to the U.S. Environmental Protection Agency: U.S. Geological Survey Bulletin 1951, 74 p.)
Injection Wells Can Induce Earthquakes
The New York Times noted in February:
Researchers with the Arkansas Geological Survey say that while there is no discernible link between earthquakes and gas production, there is “strong temporal and spatial” evidence for a relationship between these quakes and the injection wells.
For decades, scientists have been researching induced seismicity, or how human activity can cause earthquakes. Such a link gained attention in the early 1960s, when hundreds of quakes were recorded in Colorado a few years after the Army began injecting fluid into a disposal well near the Rocky Mountain Arsenal.
Lawrence Berkeley Laboratory points out:
Lawrence Berkeley Lab provides details:
Induced seismicity [i.e. earthquakes] in oil and gas production has been observed ever since the 1930s, i.e., ever since large scale extraction of fluids occurred. The most famous early instance was in Wilmington, California, where the oil production triggered a series of damaging earthquakes. In this instance the cause of the seismicity was traced to subsidence due to rapid extraction of oil without replacement of fluids.
In the last decade a number of examples on earthquake activity related to oil and gas production as well as injection of liquids under high pressure have been observed, although not with as serious consequences as for Wilmington. Almost all induced seismicity associated with petroleum extraction can be traced to either fluid injection or extraction. In some recent cases injection of produced water (excess water extracted during oil and gas extraction) has produce significant seismic activity. Examples are in Colorado and Texas where gas and oil production yield large volumes of water that must be put back underground. In some cases the water cannot be put back exactly where it was produced and over pressurization of the water causes induced seismicity.
Fluid pressures play a key role in causing seismicity. Explained in simple terms, fluids can play a major role in controlling the pressures that are acting on the faults. The fluid pressure in the pores and fractures of the rocks is called the pore pressure.
Injecting fluids into the subsurface is one way of increasing the pore pressure and thus allowing the faults and fractures to “fail” more easily, thus inducing an earthquake.
That is why in many cases induced seismicity is 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. Figure 2 is an example of induced seismicity being caused by water injection. Figure 2 is a cross section of the earth showing the location of the earthquakes (green dots), the locations of injection wells (thick blue lines) and production wells (thin lines, these wells extract fluid). Note the large number of events associated with the injection wells.
Figure 2. Example of injection related seismicity; note the close correlation between water injection wells and the location of the seismicity.
"Fracking" Can Cause Earthquakes
Lawrence Berkeley Lab also points out that hydrofracturing (or "fracking" for short) can cause earthquakes:
Another type of induced seismicity is that which is associated with “hydrofracturing”. Hydrofracturing is done by injecting fluid into the subsurface to create distinct fractures in order to link existing fractures together in order to create permeability in the subsurface. This is done to extract in situ fluids (such as oil and gas). Hydrofracturing is distinct from many types of shear induced seismicity because hydrofracturing is by definition only created when the forces applied create a type of fracture called a tensile fracture, creating a “driven” fracture. Shear failure has been observed associated with hydrofracturing operations, as the fluid leaks off into existing fractures, but due to the very high frequency nature of tensile failure ( seismic source at the crack tip only) only the associated shear failure is observed by microseismic monitoring . However, hydofracturing is such a small perturbation it is rarely, if ever, a hazard when it is used to enhance permeability in oil and gas or other types of fluid extraction activities. To our knowledge hydrofracturing to intentionally create permeability rarely creates unwanted induced seismicity large enough to be detected on the surface even with very sensitive sensors, let alone be a hazard or an annoyance. In fact the very small seismic shear events created from the shear failure associated with the hydrofracture process are used to map the location of the induced permeability and as management toll to optimize fluid production. If not for the very small shear events it would be much more difficult to understand the effect of hydrofracturing because the seismic energy created from the “main fract” is to low to be detected, even from he most sensitive instruments at the surface of the earth Figure 3 is an example of how seismicity is used to map these hydrofractures. Last but not least another reason that the seismic risk is so low associated with hydrofracture operations in that they are of relatively low volume and short durations ( hours or days at the very most) compared to month and years for other type of fluid injections described above.
Figure 3. Cross section through a stimulation well showing six different stages of hydrofracture stimulation and the associated seismicity (magnitude -1.0 to -2.5) during the entire hydrofracture (less than 24 hours) Warpinski et al 2005.
AP reported in February:
Scott Ausbrooks, geohazards supervisor for the Arkansas Geological Survey, said the quakes are part of what is now called the Guy earthquake swarm – a series of mild earthquakes that have been occurring [in Arkansas] periodically since 2009. A similar swarm occurred in the early 1980s when a series of quakes hit Enola, Ark.
Ausbrooks said geologists are still trying to discover the exact cause of the recent seismic activity but have identified two possibilities.
"It could just be a naturally occurring swarm like the Enola swarm, or it could be related to ongoing natural gas exploration in the area," he said.
A major source of natural gas in Arkansas is the Fayetteville Shale, an organically-rich rock formation in north-central Arkansas. Drillers free up the gas by using hydraulic fracturing or "fracking" – injecting pressurized water to create fractures deep in the ground.
Ausbrooks said geologists don't believe the production wells are the problem, but rather the injection wells that are used to dispose of "frack" water when it can no longer be re-used. The wastewater is pressurized and injected into the ground.
Ausbrooks said the earthquakes are occurring in the vicinity of several injection wells.
[Police Chief Dave Martini] the earthquakes started increasing in frequency over the past week and that the disposal well has seen an increase in use recently.
Websites Ask Whether Fracking Caused the Virgina Quake
I have no idea whether or not this is true, and have been too busy to look at the supposed evidence of drilling near the epicenter of the earthquake.
But given that some human activity is officially acknowledged to be able to induce earthquakes, it's worth asking these types of questions.