[S02-1] World overview of anthropogenic seismicity I

[S02-1-01] Insights into faults, crustal permeability, state of stress and earthquake physics from induced earthquakes in Oklahoma and southern Kansas

^○William Ellsworth¹, Gregory Beroza¹, Yihe Huang², Cornelius Langenbruch¹, Martin Schoenball¹, Rall Walsh¹, Matthew Weingarten¹, Nana Yoshimitsu¹, Mark D. Zoback¹ (1.Stanford University, California, USA, 2.University of Michigan, Ann Arbor, Michigan, USA)

invited

A large region of north-central Oklahoma and southern Kansas in the central U. S. became seismically active starting in 2009. The earthquakes followed the initiation of high volume wastewater disposal by deep injection from newly developed unconventional oil fields and are understood to be primarily a consequence of disposal. These oil fields produce many times more water than oil, which is uneconomic to dispose of except by injection. To date, more than 700 deep disposal wells have injected one billion m3 of primarily co-produced water in to the highly permeable Arbuckle formation near the base of the sedimentary section. Owing to its high permeability (~1 Darcy), the pressure perturbation from individual wells merges into a low-pressure plume (< 1 MPa) spreading across a region that today covers more than 20,000 km2. This relatively small perturbation has activated hundreds of previously unknown basement faults in more than 1500 M >= 3 earthquakes, including the September 2016 Mw 5.8 Pawnee earthquake. These earthquakes characteristically occur on strike slip or normal faults that are near-optimally oriented for failure in the contemporary stress field. Seismicity on each fault characteristically initiates with small events before the occurrence of the largest in the sequence. Stress drops for the largest events average 10 MPa, releasing tectonic stress in what was previously an almost completely aseismic region. Consequently, parts of the crust where earthquakes occur must be near a state of failure equilibrium between the strength of the faults and the stress carried in the North American plate. In response to the seismicity crisis, regulators have mandated large reductions in fluid injection into the Arbuckle formation beginning in mid-2016. Seismicity rates are now falling in the region in response to this change; however, it may be years before the earthquake hazard returns to historic levels.