Michael C. Stickney

Seismicity Remotely Triggered by the Magnitude 7.3 Landers, California, Earthquake

The magnitude 7.3 Landers earthquake of 28 June 1992 triggered a remarkably sudden and widespread increase in earthquake activity across much of the western United States. The triggered earthquakes, which occurred at distances up to 1250 kilometers (17 source dimensions) from the Landers mainshock, were confined to areas of persistent seismicity and strike-slip to normal faulting. Many of the triggered areas also are sites of geothermal and recent volcanic activity. Static stress changes calculated for elastic models of the earthquake appear to be too small to have caused the triggering. The most promising explanations involve nonlinear interactions between large dynamic strains accompanying seismic waves from the mainshock and crustal fluids (perhaps including crustal magma).

Seismotectonics of the 20 August 1999 Red Rock Valley, Montana, Earthquake

On 20 August 1999, a magnitude 5.3 earthquake occurred in southwestern Montana, ending a 25-year hiatus for magnitude 5+ seismicity in Montana. This earthquake occurred in the central part of the Red Rock Valley, a northwest-trending graben bounded by late Pleistocene and Holocene faults. A focal depth of 12.4 km and a normal-faulting focal mechanism suggest that this earthquake resulted from continued graben development, although not along graben-bounding faults mapped at the surface. The 1999 Red Rock Valley earthquake occurred near the northern end and within the footwall block of the northeast-dipping Red Rock fault. We deployed a temporary network close to the mainshock epicenter and located 65 aftershocks over 3 days, including a magnitude 4.0 aftershock on 26 August that allowed determination of P -wave travel-time delays for regional seismograph stations. Using these station delays to improve relative hypocenter locations, we recomputed hypocenter locations for >1000 Red Rock Valley area earthquakes that occurred since 1989. Relocated hypocenters avoid the Holocene portion of the Red Rock fault but do surround it. The mainshock epicenter location is close to that of a magnitude 5.0 earthquake in 1965.

Revised Velocity Structure of Western Montana

Using a one-dimensional (1D) layered earth approach, we determined a new crust and upper-mantle velocity model for western Montana to improve earthquake hypocenter locations. P -wave arrival times recorded on 280 stations from 1432 well-recorded earthquakes provided input for a sparse damped least-squares sensitivity matrix. We solved for 1D velocity structure, refractor depths, station corrections, and hypocentral positions by minimizing travel-time residuals. The new model has three layers with P -wave velocities of 5.70, 6.12, and 6.53 km/sec, with corresponding interface depths of 7.0, 19.8, and 39.7 km below the surface. The upper-mantle velocity is 8.00 km/sec. We determined station corrections and show that the Yellowstone caldera is the only regional geologic feature not adequately modeled by our new 1D velocity model. Use of the new model and station corrections reduced hypocenter location uncertainties and travel-time residuals, implying improved hypocenter locations for western Montana earthquakes. Online material : Table of seismographic stations.

Kinematic evidence for the effect of changing plate boundary conditions on the tectonics of the northern U.S. Rockies

We derive surface velocities from GPS sites in the interior Northwest U.S. relative to a fixed North American reference frame to investigate surface tectonic kinematics from the Snake River Plain (SRP) to the Canadian border. The Centennial Tectonic Belt (CTB) on the northern margin of the SRP exhibits west-directed extensional velocity gradients and strain distributions similar to the main Basin and Range Province (BRP) suggesting that the CTB is part of the BRP. North of the CTB, however, the vergence of velocities relative to North America switches from westward to eastward along with a concomitant rotation of the principal stress axes based on available seismic focal mechanisms, revealing paired extension in the northern Rockies and shortening across the Rocky Mountain Front. This change in orientation of surface velocities suggests that the change in the boundary conditions on the western margin of North America influences the direction of gravitational collapse of Laramide thickened crust. Throughout the study region, fault slip rate estimates calculated from the new geodetic velocity field are consistently larger than previously reported fault slip rates determined from limited geomorphic and paleoseismic studies.