abstract
From October 1974 through June 1978, P-wave arrival times were measured from large quarry-blast signals as a means of investigating possible temporal variations of seismic velocity across the Wasatch Front, Utah. A new technique was devised that eliminated the need for field-measured origin times and permitted accurate measurements of the velocity with an average standard deviation of ±0.10 sec for 24 stations. The velocity data revealed remarkable stability for the observation period. However, during the study no earthquakes of magnitude greater than ML 3.6 occurred close to any monitored ray path. Velocity anomalies at 14 stations were identified for the study, but none could be correlated with earthquake activity. A 1978 earthquake swarm near Magna, Utah (maximum magnitude ML 3.3), revealed no significant anomaly. Also, aftershocks as large as magnitude ML 3.6, of the 1975, ML 6.0 Pocatello Valley earthquake showed no velocity perturbations. These results suggest that velocity variations before small earthquakes along the Wasatch Front provide inadequate precursory information. However, because no large shocks greater then ML 3.6 occurred near monitored ray paths during the study, we cannot assess the effectiveness of the method for larger more damaging earthquakes.
