Stress Drops and Ground Motions from Induced Earthquakes in Oklahoma and Kansas: Are They Different From Tectonic Earthquakes?

Session: Mechanisms of Induced Seismicity: Pressure Diffusion, Elastic Stressing and Aseismic Slip
Type: Oral
Date: 4/29/2020
Time: 11:00 AM
Room: 215 + 220
Description: 

We developed a ground motion model (GMM) for shallow induced earthquakes due to fluid injection (Mw 2.5 and greater) in Oklahoma and southern Kansas. Inputs into the model were based on an inversion of a subset of strong motion/broadband data to estimate stress drop, Q(f) and kappa. The shallow nature of induced earthquakes in a low Q crustal environment and the resulting potentially lower stress drops have been suggested as a cause for lower ground motions as compared to tectonic earthquakes. Our GMM was developed based on the point-source stochastic simulation method rather than an empirical approach due to the general lack of data at higher magnitudes (Mw > 5.0). An inversion of both recorded tectonic and induced earthquake data in the NGA-East dataset supplemented by data from 40 Oklahoma and southern Kansas induced events was performed. The inversion for stress drops of the 50 induced earthquakes (Mw 2.5 to 5.8) and 43 mostly shallow tectonic earthquakes in the central and eastern U.S. (CEUS) show that stress drops of the induced earthquakes are signifcantly lower than the CEUS tectonic earthquakes. Ground motions assuming both single-corner and double-corner point-source models were numerically simulated based on the inversion results. The simulation results using both models are very similar. The GMM was developed for horizontal PGA, PGV and 5%-damped PSA for 0.01 to 10.0 sec for Mw 2.5 to 7.5 at hypocentral distances from 2 to 500 km. Our simulation-based model was compared with the database of recordings and with two recently developed GMMs for induced earthquakes in Oklahoma. Our GMM gives lower ground motions at short distances (< 50 km) and short- to moderate-periods and somewhat higher motions at long-periods (> 1.0 sec). We are currently evaluating the basis for the increase in ground motion scaling that is observed in the data at distances less than 20 km as observed by others (e.g., Atkinson, 2015).

Presenting Author: Ivan Wong

Authors