Page 444 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

408 part III The Earth–Atmosphere Interface
▲Figure 13.19 Seismic activity in or near Canada, 1627 to 2012. Damaging earthquakes (M > 5) are strongly associated
with the Western Cordillera and the valleys of the Ottawa and St. Lawrence Rivers. [Used by permission of the Minister of Public Animation Works and Government Services Canada; Natural Resources Canada, Geological Survey of Canada.] Elastic Rebound
rate, can also induce seismicity by causing subsidence of the ground and enhancing slippage along faults.
Both fluid injection and fluid extraction are common activities associated with oil and natural gas drilling and with geothermal energy production. Hydraulic fracturing, or fracking, associated with shale gas extraction injects large quantities of fluid to break up subsurface rock, mak- ing this process a probable cause for induced seismicity (please review the Chapter 1 Geosystems Now, and see The Human Denominator, Figure HD 13, at the end of this chapter). In 2013, scientists linked increased earth- quake activity in Colorado, New Mexico, and Oklahoma to fluid injection associated with fracking. Enhanced Geo- thermal Systems, discussed in Focus Study 12.1, also use fracking, which has resulted in seismic activity at several locations, including the Geysers in California. A 2012 Na- tional Research Council report found only a minimal risk of seismicity associated with energy technologies.
Earthquake Forecasting
The maps in Figure 13.20 plot earthquake hazards in Can- ada and the United States. These occurrences give some indication of relative risk by region. The Geological Sur- vey of Canada is responsible for monitoring earthquake ac- tivity and hazards in Canada (www.earthquakescanada.nrcan .gc.ca/index-eng.php). In the United States, the National Earthquake Hazards Reduction Program is a multi-agency
program that includes the Advanced National Seismic System, which provides key data for the hazard map (earthquake.usgs.gov/monitoring/anss/).
A major challenge for scientists is to predict earth- quake occurrences. One approach to earthquake fore- casting is the science of paleoseismology, which stud- ies the history of plate boundaries and the frequency of past earthquakes. Paleoseismologists construct maps that estimate expected earthquake activity based on past performance. An area that is quiet and overdue for an earthquake is a seismic gap; such an area possesses ac- cumulated strain. The area along the Aleutian Trench subduction zone had three such gaps until the great 1964 Alaskan earthquake filled one of them. A seismic gap in the Cascadia subduction zone contributes significantly to forecasts of a major earthquake in the southwest British Columbia-Pacific Northwest region.
A second approach to forecasting is to observe and measure phenomena that might precede an earthquake. Dilatancy refers to the slight increase in rock volume produced by small cracks that form under stress and ac- cumulated strain. One indication of dilatancy is a tilting and swelling in the affected region in response to strain, as measured by instruments called tiltmeters. Another indicator of dilatancy is an increase in the amount of radon (a naturally occurring, slightly radioactive gas) dis- solved in groundwater. At present, earthquake hazard