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Originally posted by pasiphae
reply to post by openeyeswideshut
i agree. if you work in that industry they are going to make sure you have no doubt that drilling, etc. is safe. they don't want the people doing all the work ever thinking what they are doing could cause major problems.
They were not reporting less than M4.0 earthquakes before 1970, so there is no good way to know how many small earthquakes there were. After they started injecting and greater withdrawal, the companies were asked to start monitoring more closely, the USGS increased their coverage, and LBL/Berkeley put in a network. So it is a VERY closely monitored area post-1970, and the monitoring quality progressively increased with time. I'd guess that the current threshold of detection stabilized in 1990 or so, so you can compare data easily thereafter. In any case, you are correct that there is no doubt that seismicity has increased with production and injection. I guess the real question is whether this actually increases the seismic risk for anyone in the region. Many people think not, which is why development continues.
The following is a pretty good summary of what we know:
It is 12 MB in size.
As you can see, there are lots of people who have followed the situation and much research has been done. This is not a subject that has been ignored.
At Yellowstone, my worry would simply be that geothermal development would ruin the thermal features. I am not so worried that it would induce powerful earthquakes or start a volcanic eruption. Fortunately, we won't need to find out, because geothermal development is extremely, extremely unlikely.
My understanding is that the long-term risk from big strike-slip faults is not perceptibly different at The Geysers than it is elsewhere along those strike-slip faults. My impression from the reading I've done over the years is that any increased seismic risk posed by energy extraction at The Geysers is minimal.
There are no "risk-free" energy sources. They all have their warts. Geothermal energy is one of the least polluting and most green. Even solar and wind have their hidden costs.
The USGS summary of seismicity at the Geysers on the EHP FAQ site is:
Given the increase in rate of 4+ seismicity, shown by the studies, it would seem to me that previous, bigger, tectonic quakes along the area faults have also occurred at shallow enough depths to make me raise again the question of potential triggered tectonic rupture from injection induced seismicity. I can infer from these documents that the depths of the induced quakes could be close enough in depth to aggravate existing fault points near similar depths that have shown tectonic, shallow rupture before.
Do you know to what depths compressional or shear wave energy can reach from a 4.5 Mw at say 5 km depth, downwards? Aren't there limits to that? Isn't some or most of the energy reflected back off of deeper crust layers? And that essentially, at some depth point the tighter packed layers, under much greater pressure, have enough rigidity to repel the energy generated from above? Seems like I have witnessed this a bit with very shallow sub mag 2 quakes, where the sub 2 Hz frequency response of instruments is noticeably less. I suspect this kind of condition for the shallow quakes that happened in Clintonville, WI, for example.
But still, if it could be shown that enough of this energy propagation was reaching known focal depth points of larger tectonic quake hypocenters, then I don't see why at all that triggering would be out of the question. To me it would be entirely possible. Especially if the orientation of the injection induced focal mechanisms were such that they propagated particularly harmful energy into a tectonic focal point at a similar depth. Didn't Denali into Yellowstone illustrate pretty clearly what can happen when the worst kind of energy is directed through focal mechanism orientation?
It is possible that a magnitude 5 could occur, but larger earthquakes are thought to be unlikely. In order for a larger earthquake to occur, it is necessary that a large fault exist. For example, the 1906 magnitude (M) 7.8 San Francisco earthquake ruptured nearly 300 miles of the San Andreas Fault. At The Geysers no such continuous fault is known to exist.