Thursday, December 29, 2011
Induced seismicity under Lake Mead
Seismic activity under Lake Mead appears to have increased this past spring and summer, which I speculated was due to the rapid refilling of the lake from melting snowpack upstream and releases from Lake Powell. [Right, Satellite images of Lake Mead taken between 1990 (upper left), 1995 (upper right) and 2009 (lower left) show the dropping lake level. The red color in the lower right image (difference between 1990 and 2009) shows where the water level has dropped. These false-color images use TM bands 7,4,2. Photo credit, NASA/USGS]
Science reporter Tom Beal wrote an article about this last week in the Arizona (Tucson) Daily Star.
Now, Phil Pearthree, Chief of the AZGS Environmental Geology Section (which also runs the Arizona Broadband Seismic Network) did a little research and found the Lake Mead area experienced a burst of activity in the late 1930's that coincided with initial lake filling, and elevated rates of seismicity compared with rates for the area prior to dam construction continued until the mid-1960's. The largest event was M 5, with 8 events of M 4.5. These events correlated fairly well, but not perfectly, with annual maxima in lake levels. No moderate events have occurred since 1964, when Lake Powell was completed and annual lake level variations were damped down. Detailed studies of short intervals since then did not show a correlation between seismicity and lake level, but looking at the whole period between the mid-1930's and the present, there probably is a positive correlation between lake levels and seismicity.
Lake levels have risen fairly dramatically this year from the all-time lows of late last year. The current reservoir storage is far below levels of the early 2000's, and essentially is back to a level that was more typical of the past decade. However, the rate of rise during the past year has been much greater than any other interval in this record, and this may be recreating the changing lake levels of the pre-Lake Powell reservoir period when there were more lake fluctuations and more seismicity.
Lake Mead is included in a number of papers and books as a type case for induced seismicity.
David W. Simpson, ‘Seismicity Changes associated with reservoir loading’, Engineering Geology 10 (1976) 123-150, p.123
Bulletin of the Seismological Society of America. Vol. 66, No. 5, pp. 1657 1681. October 1976
SEISMIC STUDY OF EARTHQUAKES IN THE LAKE MEAD, NEVADA-ARIZONA REGION
by A. M. Roger and W. H. K. Lee
Advanced dam engineering for design, construction, and rehabilitation, by Robert B. Jansen, p744
Earthquakes related to reservoir filling, by Joint Panel on Problems Concerning Seismology and Rock Mechanics, National Academy of Sciences (U.S.), National Academy of Engineering, National Research Council (U.S.). Division of Earth Sciences, 1972
at 7:36 PM