BIG EMPHASIS on MIGHT
I'll repeat before we begin: BIG EMPHASIS on MIGHT.
I've been documenting the continuing research into the increase in seismicity occuring east of the Rockies in the United States. Almost all of it is concentrated in a handful of states, like Oklahoma, where four wastewater fluid injection wells have been potentially linked to 20% of all the cumulative seismicity that's occuring in the region.
However, while the methods by which humans can induce seismicity are fairly well known and have been for decades, telling the difference between induced seismicity and natural seismicity is not as straightforward.
A short note in the October 2014 Bulletin of the Seismological Society of America suggests there might be a way.
Again, big emphasis on might.
Nature covered the story in August:
Expanded oil and gas operations in the central and eastern United States have triggered earthquakes as large as magnitude 5.7, as drillers inject wastewater back into the ground. But seismologists now report a bit of good news: such 'induced' quakes appear to shake the ground less than a naturally occurring earthquake of the same magnitude would.
That is good news because less shaking means less damage. “Maybe induced earthquakes aren’t quite as fearsome as they may seem,” says Susan Hough, a seismologist at the US Geological Survey (USGS) in Pasadena, California, whose work is published in the Bulletin of the Seismological Society of America. “The hazard from induced quakes is going to be down a notch relative to tectonic quakes.”
But the observation holds only for areas more than 10 kilometres from the earthquake’s epicentre. Anyone close to the drilling would still feel as much shaking as a natural quake would bring. “This might lead to a recommendation that deep injection wells should be kept 10 kilometres away from population centres,” says Hough.
Dr. Susan Hough used the USGS's "Did you Feel It" data collection system to complete an analysis on 11 earthquake events suspected of having been triggered/induced by wastewater injection. If you haven't read up on DYFI, I suggest that you do. It really is an excellent system.
The above is an example of a DYFI map and in fact it's from an earthquake highly suspected of having been
triggered by an earlier earthquake that was highly suspected of having been
induced by wastewater injection.
The USGS explains DYFI here.
Calculating Community Internet Intensities
The Community Internet Intensity Map (CIIM) summarizes the questionnaire responses provided by you and other Internet users. An intensity value has been assigned to each community from which we have received a filled-out CIIM questionnaire; each intensity value reflects the effects of earthquake shaking on the people and structures in the community. For convenience, we define "communities" to be ZIP code regions. We consider all the filled-out questionnaires from a given ZIP code and assign a single intensity to the ZIP code. The form of the questionnaire and the method for assignment of intensities are based on an algorithm developed by Dengler and Dewey (1998) for determining a "Community Decimal Intensity". The decimal intensity values computed by the algorithm of Dengler and Dewey have been rounded off to integers for the Community Internet Intensity Map and represented by Roman Numerals.
A Community Internet Intensity Map is made and updated every five minutes following a significant earthquake and then less frequently as additional data are received. ZIP code areas for which data have been received are color-coded according to the intensity scale below the map; ZIP codes in gray are those for which we have not yet received data. At first only a few ZIP codes will have intensities assigned, but over time others will be assigned as data come in. Individual ZIP code zones may change color as a new consensus is reached (that is, data from more respondents may change the average intensity value for a ZIP code). Check back often and remember to reload your browser to see the update! Note the date and time on the lower left corner of the map to keep track of the updates.
Since it was introduced it has provided a huge and rich database that is slowly being used to research all sorts of things like seismic hazard mapping and other research, all designed with our safety in mind. If you feel an earthquake, report what you felt to the USGS. Even if you don't feel it too. All of those reports are used.
Dr. Hough determined through her analysis that the 11 suspected quakes all had low-levels of shaking. Now, some of these quakes were rather small, but the largest in the sample was M5.7 (its DYFI is above). The possible reason why are that induced earthquakes may have low stress drops.
Remember what we've discussed about strain and stress before:
Even though the entire process is fairly complicated, it can be explained pretty easily. The injection of fluid into a well, especially under pressure, raises the pressures beneath the ground. It isn't that the fluids are lubricating the faults. That's not what is happening. Instead the fluid injection and its pressures change the stress conditions that are acting on any possible subsurface faults. Because all of the continents have an overall stress and strain field due to their motions relative to each other, a fault that is oriented in a favorable fashion may find its clock advanced significantly forward if the local stress and strain field is changed.
Alexandra Witze, in the above linked Nature article, explains "stress drop" a different way that is also useful:
The relatively low levels of shaking suggest that induced earthquakes have a low 'stress drop', a measure of how an earthquake behaves. Two earthquakes of the same magnitude can have different stress drops. Imagine two trucks rumbling across the same distance: one moves in short, fast jerks; the other moves slowly and smoothly. The second truck would have the lower stress drop
Now as I was writing this, an M3.7 shook
Oklahoma near Stillwater and Perry. Despite the earthquake's moderate size, depth, and location in a part of the nation where its seismic waves would attenuate and be felt over a large distance, there appears to be a low level of shaking.
This study also suggests that aformentioned M5.7 (the largest quake in the November 2011 Central Oklahoma earthquake sequence) may not have been triggered by an earlier, likely injection-induced quake that day but itself was also injection-induced. Unlike the August 2011 Central Virginia quake which was of a similar magnitude, it did not cause damage at large distances despite it being felt across a wide swath of the Central United States. If you remember, the Central Virginia earthquake was felt in a region from Atlanta well into Ontario and Quebec, caused damage as far away as Brooklyn, and did some not-insignificant damage across Washington D.C and its surrounding suburbs. The Prauge quake was damaging but its damage zone was small, although I am unsure if a detailed damage survey was ever done. This is an area of debate and active study however, so stay tuned.
Determining the hazard from earthquakes due to wastewater injection and other human processes is important (and the USGS would like to quantify this in its hazard mapping as it currently does not) so this study is just one small piece in building the guidebook to understanding human and earth systems linkages.
But again, big emphasis on my part on the word might.
The study is "Shaking from Injection-Induced Earthquakes in the Central and Eastern United States" by Susan E. Hough.