Extended Ground Motion Duration from Unique Earthquakes in Oklahoma

Abstract:

Oklahoma has seen a reduction of induced earthquakes associated with wastewater disposal in the last several years. However, the number of earthquakes associated hydraulic fracturing activities in the state increased concurrent with the decrease wastewater-disposal linked events. During that period, we have observed an increase in pairs of earthquakes that occur closely both in space and time. Some of the events occur with such a short difference in origin time that the ground motion elongates the event duration at a single site. Ground motion duration, in association with other motion parameters such as amplitude, frequency content, and energy, is viewed as a significant factor in seismic risk assessment. The significant duration of an earthquake may be influenced by the complexity of the event source or the site of the recording station. We classify local earthquakes in Oklahoma as single, twin/multiples, or complex-source events based on the P and S phase pair. By defining the ground motion duration on the basis of the Arias intensity and determining the duration using Husid plots, we describe event twins or multiples as two or more single events with identifiable body wave phases whose significant duration overlap occur in close proximity in both time and space, and complex events as earthquakes with multiple phases due to their complex source-time function or due to crustal reverberations. We identify at least 21 twin pairs and over 85 complex-source earthquakes in the last year since June 2019. A comparative analysis of the significant duration shows an increasing duration with epicentral distance for the three sets of earthquakes but at a higher rate for the twins, complex-source events and then singles in that order.

Occurrence of the earthquakes with the characteristics we describe also coincide with an inflation in the intensity of reported felt ground motions in the SCOOP and STACK area, relative to the magnitude of the source event. We thus examine whether the relationship between the ground motion duration influences the felt intensity of a particular event, as reported in the “Did You Feel It” (DYFI) program. We plan to present updated research that has implications for better understanding felt ground motions of earthquakes induced by hydraulic fracturing.