This study summarizes our investigation of the 17 May 2012 M W-RMT 4.8 earthquake nearTimpson, Texas, the largest earthquake recorded historically in eastern Texas. To identify preshocks and aftershocks of the 17 May event we examined the arrivals recorded at Nacogdoches (NATX) 30 km from the 17 May epicenter, at nearby USArray Transportable Array stations, and at eight temporary stations deployed between 26 May 2012 and mid-2013. At NATX we identified seven preshocks, the earliest occurring in April 2008. Reliably located aftershocks recorded by the temporary stations lie along a 6 km long NW-SE linear trend corresponding to a previously mapped basement fault that extends across the highest-intensity (MMI VII) region of the 17 May main shock. Earthquakes in this sequence are relatively shallow-with focal depths ranging from 1.6 to 4.6 km. Evidence supporting these depths include: hypocentral locations of exceptionally well-recorded aftershocks, S-P intervals at the nearest stations, and comparisons of synthetics and observed seismograms.
IntroductionOn 17 May at 0812 UTC a M W-RMT 4.8 earthquake occurred near Timpson, Texas. The quake awoke numerous residents of Nacogdoches, Texas, 50 km to the southwest of Timpson and caused significant damage to chimneys, fireplaces, and brick veneer siding 5 km southwest of Timpson. The 17 May earthquake is the largest earthquake in the historical record in East Texas (Figure 1). This paper discusses this earthquake and the sequence of preshocks and aftershocks, including an M W-RMT 4.0 foreshock on 10 May 2012 at 1515 UTC and aftershocks occurring on 25 January 2013 at 701 UTC (m bLg 4.1) and 2 September 2013 at 1652 (m bLg 4.1) and 1851 (M W-RMT 4.3).Regional tectonics in eastern Texas is dominated predominately by salt bodies; however, the Mt. Enterprise fault zone, a system of approximately east-west trending Cretaceous-Paleogene faults, is situated north and west of the epicentral area (Figure 1) [see Ewing, 1990]. In the epicentral area of the 2012 earthquake Jackson [1982] and Geomap Company [2012] also indicate a northwest-southeast trending fault that is roughly parallel to and slightly east of the Rusk-Shelby county line.Seismicity was rare in this region prior to the events analyzed in this study. The nearest events discussed by Frohlich and Davis [2002] were a M4.0 8 January 1891 Rusk, Texas, event, 80 km to the west of the epicentral region and the M3.0 9 June 1981 Center, Texas, earthquake, 25 km to the southeast. However, some investigations have suggested the 1891 Rusk event might be spurious-a thunderstorm or a tornado-and the 1981 Center earthquake was only locatable because it was recorded by a temporary local network deployed between June 1981 and August 1982[Pennington and Carlson, 1984. This network also recorded a microearthquake occurring on 11 December 1981 and located 25 km west of the 2012 epicenter.FROHLICH ET AL.
