abstract
A nine-station telemetered array was installed around the Boulder Basin portion of Lake Mead, Nevada-Arizona. During years of monitoring, approximately 1,360 events were detected in the magnitude range −1.0 ≦ ML ≦ 2.9, and half this number were locatable. Many of the events, which ranged in depth from 0 to 13 km, can be associated with mapped faults. In particular, epicenter lineations clearly indicate activity on portions of steeply dipping faults on the east side of Boulder Basin, which generally confirms Carder's earlier work (Carder, 1945, 1948, 1970). The strike of these faults is approximately north. Focal mechanisms are in agreement with this strike and show right-lateral motion on near-vertical faults. The tension axis for this solution is oriented northwest-southeast in agreement with the stress pattern for other parts of the Basin and Range Province.
Although lake load increased 20 per cent during the monitoring period, neither number of events nor energy release shows a correlation with this change. Carder (1970) found similar results for periods after 1949. A higher b value (1.45) was obtained than in the past, and lower monthly energy release was observed than in the late 1930's and 1940's when seismic activity at Lake Mead was first recorded. A 2-yr seismicity map of southern Nevada, including the monitoring period, shows that Lake Mead activity is now no greater than that of the surrounding area.
The pre-Lake Mead seismic history is not well known because the founding of Boulder City occurred just a few years before Lake Mead was impounded. The existence of earthquakes in the region before the reservoir was filled has been questioned in the liteature. However, a search of the Las Vegas Review Journal for felt reports pre- and postimpoundment and compilation of a catalog of the largest events from the literature confirm that there was a significant increase in the number of felt events at Las Vegas after the reservoir was filled.
The results indicate that the filling of Lake Mead has triggered release of tectonic stresses having the same orientation as the regional stress field. These stresses are probably being released in a way that is more dependent on tectonic stress buildup than on small changes in pore pressure (∼ 1 bar) due to fluctuating lake level. Given that the shear strength of rock decreases with increasing pore pressure, one might explain the decreasing energy release since the 1940's and the high b value as due to a decrease in elastic-energy density of the rock. A test of this hypothesis based on the order of magnitude of energy released seems to support it.
