>>Dr. Robert Puls: What is the definition of an earthquake? It is when two bodies of
rocks slide past each other or rupture. Energy is radiated out form the rupture in all directions.
These are called seismic waves. Seismic waves travel and are detected at seismic stations
and help locate and determine the size of the earthquake or it's magnitude.
We have had increased seismic activity in Oklahoma the last three years. On November
5th, 2011 the largest earthquake ever recorded struck near Prague, Oklahoma. It was a magnitude
5.7 on the Richter scale. The quake was reportedly felt far away as Illinois, Kansas, Tennessee
and Texas. I felt it in Ada, Oklahoma. A number of earthquakes have been felt since, mostly
in the 3 to 4 magnitude range on the Richter scale. This is a logarithmic scale, so a magnitude
5 earthquake is a hundred times greater than a 3 magnitude earthquake. Recent repeated
quakes have been occurring in south central Oklahoma and in the Edmond area. In 2013 a
study was published in the journal Science, which linked earthquakes in Midwestern United
States to underground injection of wastewater. It was postulated that a net fluid volume
increase after years of injection lowered effective stress on reservoir bounding faults.
Decades long lags between the commencement of fluid injection and the onset of induced
earthquakes are thought possible. A more recent study published in the journal Geophysical
Research Letters connects earthquakes in the Youngstown Ohio area in 2010 and 2011 to wastewater
injection to the Northstar 1 injection well.
Earthquake triggering can be caused by both natural and anthropogenic factors. Natural
causes are: dynamically by the passage of seismic waves, statically by local stress
changes from previous earthquakes, also from natural fluid movement. The latter may be
the cause of many after shocks. Hydrologic loads also can cause earthquakes. Anthropogenic
causes are: mining and oil production from the subsurface, from fluid injection and form
geothermal production and thermal contraction. Seismic events occur with varying degrees
of intensity. Many more are smaller rather than larger. The energy released reached the
Earth's surface and cause noticeable shaking. Earthquakes of magnitude less than 3 are generally
not felt at the surface. Damage to structures, if any, depends on the amount of energy reaching
the surface, the geomechanical characteristics of the soil, and the condition of the structures.
The epicenter is the location of the surface above the slip event. The hypocenter is the
event's actual location in the subsurface. Induced seismicity is a term that refers to
seismic events linked with human activity. It's well understood to be either or both,
an increase in sheer stress with accompanying mass changes and thermal changes, or increase
in pore pressure. This can be from fluid injected under pressure but also can be from fluid
injected under little or no pressure, which can generate a hydraulic head, which can increase
pore pressure.
Induced Seismicity from fluid injection has become a greater concern over the past few
years with a significant number of new possible cases induced seismicity can occur when faults
are already near failure and pore pressure is increased. Fluids injected can originate
from multiple types of activities including produced waters from hydraulic fracturing
and enhanced oil recovery from carbon dioxide injections.
The National Academy of Sciences recently released a report on induced seismicity. The
major findings of the report were that hydraulic fracturing does not pose a high risk. Wastewater
disposal does pose some risk, but the frequency of known events is low. Carbon capture and
storage may have potential for inducing seismic events but much is unknown about it. The report
further indicated that methodologies can be developed for quantitative, probabilistic
hazard assessments of induced seismicity risk. And that there is a need for federal agencies
to coordinate on induced seismicity response actions.
With respect to underground injection control wells and induced seismicity there are more
than 150,000 class two injection wells in the U. S. Oklahoma has more than ten thousand.
Strong linkages have been posed for five events in the U. S. in the past few years. Some of
these are still undergoing review. It is thought by some that extended injection can lead to
seismic events. Very low seismic is common and does not pose any problem. A small subset
of injection wells are sited at locations with unexpected geological features. On occasion
these can cause seismic activities that can be felt at the surface. In the U. S., 150,000
to 200,000 injection wells operate each day. Rarely do these wells cause noticeable seismic
activity. This next table show induced seismicity from injection in terms of both best documented
cases to date and also some general observations. For instance in 1976 in Rangely, Colorado
this case was published in the journal Science. In Paradox Valley, Colorado this was also
published in the bulletin of Seismology Society of America. Another case was in Germany, and
also another one in Basel, Switzerland. And then one at the Rocky Mountain Arsenal in
Colorado, in 1981. Again, published in the Journal of Geophysical Research.
The general observations observed for these different cases are that earthquakes generally
occur first near the well and migrate away from the well with time. Earthquakes have
a clear temporal correlation to injection. Time and spatial distribution of earthquakes
can generally be related to diffusion of pore pressure. Earthquakes can occur over long
distances, some over twenty kilometers. Modifying injection parameters can alter earthquake
production.
The Underground Injection Control programs conducted by the states and by EPA are examining
this issue of earthquakes and underground injection. They are going to be formulating
new guidance on this topic in the very near future.