Norimitsu Nakata, Ph.D. - US grants

Induced earthquakes, or seismicity caused by human activity such as wastewater injection or secondary oil recovery, have been increasing in the midwestern United States since 2009. The state of Oklahoma has become known as "the new earthquake capital", with earthquake rates of M>3 exceeding those of California in the last few years. The 3 September 2016 Mw5.8 Pawnee, Oklahoma earthquake occurred within the region of induced seismicity, near wastewater disposal wells, and was the largest earthquake ever recorded within the state. The earthquake occurred five years after the second largest earthquake, the similarly sized Mw5.7 November 2011 Prague earthquake. Near Pawnee, local residents reported strong ground-shaking, damage to houses, cracks on the ground, and concerns about safety. Students from Cornell University and Colombia University, in collaboration with local universities including the University of Oklahoma, Oklahoma State University, and University of Tulsa, actively participated in the rapid deployment of 25 seismic stations around the epicentral region. Students of the institutions involved have been actively involved in learning about sensors, network designs, and earthquake physics.

The main fault the ruptured during the Pawnee event was an unmapped fault intersecting mapped Labette Fault, which has hosted a significant number of aftershocks include the largest aftershock (M3.9). The aftershock sequence and related seismic activity in the region that has been recorded since the rapid (within 2 days) deployment of seismic instrumentation will help study aftershocks and the likely triggered seismicity on the regional fault systems. In the coming weeks, we will deploy an additional 20 short-period stations that will record data for 12 months. This combined network will provide information on fault structure, aftershock decay, duration, and interaction with injection rate variations. The data will help to understand the characteristics of faults that host large damaging earthquakes, and will help to assess earthquake hazards in Oklahoma and the midcontinent. The broader impact of this rapid deployment will help understand the regional tectonic framework in northern Oklahoma and may elucidate the relationship between this recent seismic activity with the fluid injection activities in the region.