Jan 7, 2016 M=4.8 Oklahoma mainshock and its M=4.4 foreshock felt throughout north-central Oklahoma and southern Kansas

Jan 8, 2016: Quake Insights

The Jan 7, 2016 M=4.8 event followed 30 seconds after a M=4.4 at a distance of 4 km (3 mi) to the southwest. This means that the M=4.8 could have been triggered by the seismic waves of the foreshock, or it could indicate a 4-km-long rupture on a fault striking northeast.A quake of this size typically ruptures a fault area of about 3-4 km by 3-4 km, consistent with this inference. If so, the focal mechanism would indicate left-lateral slip. 

Foreshocks are rare among natural earthquakes; depending on how they are counted, they only occur several percent of the time. There were an additional 20 recorded aftershocks over an area of 7 km (4 mi). The depth of the quake could be as little as 2 km (1 mi) and as deep as 12 km (8 mi); there are inconsistencies in the current depth estimates. The other recent Oklahoma quakes were also strike-slip.

Near the site of today’s temblor, there are surprisingly few deep injection wells, at least as of March 2015.  This might indicate that the well closest to these quakes was very recently drilled, or that these quakes are much farther from the fluid source than is typical.

Ross Stein & Volkan Sevilgen, Temblor on Jan 8, 2016

Data: USGS, Oklahoma Geological Survey, Oklahoma Corporation Commission