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San Diego less vulnerable to, but not free of, fracking

Two clarifications regarding this news item: First, hydraulic fracturing generally does not cause earthquakes that can be felt at the ground surface. Microearthquakes are generated at depths of several thousand feet below ground level where the actual fracturing process is taking place within the target shale. The larger earthquakes that Oklahoma (OK) has been experiencing are virtually all the result of the injection of wastewater (produced water) down EPA Class 2 injection wells. This injected water increases the fluid pressure along faults present in the crystalline basement rock (in the case of OK), decreases effective stress, and induces earthquakes. This U.S. Geological Survey (USGS) webpage nicely sums up these concepts: https://earthquake.usgs.gov/research/induced/myth… Second, OK has not supplanted California as the big earthquake state. I had the good fortune to attend a lecture recently by Dr. Susan Hough, a USGS seismologist who works in Pasadena. She was in Kansas City (where I live) to address seismic activity in the Midwest, including OK. She explicitly stated that this idea that OK has a more dangerous seismic risk than California is a myth. The bottom line is that although there has been increased earthquake activity in OK due to wastewater injection, California lies on the boundary between the North American and Pacific tectonic plates. This transcurrent plate boundary (the San Andreas and associated faults) has the potential to generate catastrophic earthquakes, such as those California has experienced repeatedly since settlement by Europeans during the mid-18th century. California will always be the big earthquake state (in terms of the release of seismic energy) compared to OK.
— December 6, 2017 6:19 p.m.

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