Stress Drop and Depth Controls on Ground Motion From Induced Earthquakes

Abstract:

Induced earthquakes in the central United States pose a risk to local populations, but there is not yet agreement on how to portray their hazard. A large source of uncertainty in the hazard arises from ground motion prediction, which depends on the magnitude and distance of the causative earthquake. However, ground motion models for induced earthquakes may be very different than models previously developed for either the eastern or western United States. A key question is whether ground motions from induced earthquakes are similar to those from natural earthquakes, yet there is little history of natural events in the same region with which to compare the induced ground motions. To address these problems, we explore how earthquake source properties, such as stress drop or depth, affect the recorded ground motion of induced earthquakes. Typically, due to stress drop increasing with depth, ground motion prediction equations model shallower events to have smaller ground motions, when considering the same absolute hypocentral distance to the station. Induced earthquakes tend to occur at shallower depths, with respect to natural eastern US earthquakes, and may also exhibit lower stress drops, which begs the question of how these two parameters interact to control ground motion. Can the ground motions of induced earthquakes simply be understood by scaling our known source-ground motion relations to account for the shallow depth or potentially smaller stress drops of these induced earthquakes, or is there an inherently different mechanism in play for these induced earthquakes? We study peak ground-motion velocity (PGV) and acceleration (PGA) from induced earthquakes in Oklahoma and Kansas, recorded by USGS networks at source-station distances of less than 20 km, in order to model the source effects. We compare these records to those in both the NGA-West2 database (primarily from California) as well as NGA-East, which covers the central and eastern United States and Canada. Preliminary analysis indicates that the induced ground motions appear similar to those from the NGA-West2 database. However, upon consideration of their shallower depths, ground motion behavior from induced events seems to fall in between the West data and that of NGA-East, so we explore the control of stress drop and depth on ground motion in more detail.