Tilts Associated with Small and Medium Size Earthquakes

Abstract

Tilt observations from nearby local earthquakes in the magnitude range from 3 to 5 and larger teleseisms show an amplitude reduction with epicentral distance R of the form A (in seconds of arc)=10M-4.1×R-1. The result is compared to static elastic dislocation models. Velocities of the tilt propagation are in the range 2.1 to 2.8km/sec for near earthquakes (focal point to station), and up to 3.3km/sec for teleseisms. In view of the discrepancies between strain and tilt observations and static elastic dislocation theory, the tilt reduction with distance, and the propagation velocities, the strain and tilt steps are viewed as a surfacetype propagation phenomenon.The earthquake-associated tilts and first motions imply a south-southeast to north-north-west compression along the northern edge of the Tanana Basin. Earthquake locations and depths indicate active faults south of Fairbanks along the northern edge of a gravity low, along the Chatanika River, and on the northern extension of the Minto fault. The first two of these faults have not been known before.