Stephen P. Horton

The 6 June 2003 Bardwell, Kentucky, Earthquake Sequence: Evidence for a Locally Perturbed Stress Field in the Mississippi Embayment

This article describes an unusually well-behaved, unusually well-documented central and eastern United States (ceus) earthquake sequence. Detailed analysis of regional and local waveform data from the 6 June 2003 Bardwell, Kentucky, earthquake indicates that the mainshock has the seismic moment of M 0 1.3 (±0.5) × 10 15 N m ( M w 4.0) and occurred at a depth of about 2 (±1) km on a near-vertical fault plane. A temporary seismic network recorded 85 aftershocks that delineate an east-trending fault approximately 1 km in length. The hypocenters illuminate a vertical plane between 2.0 and 2.7 km depth. The centroid of the aftershock distribution is at 36.875° N, 89.010° W and a depth of 2.4 km. The aftershock cluster is interpreted as a circular fault area with a radius of 0.44 (±0.03) km. This source radius yields a static stress drop, Δ σ = 67 (±14) bars for the mainshock. The focal mechanism for the mainshock has strike = 90°, dip = 89°, and rake = −165° with a subhorizontal P axis trending 135°. A formal stress inversion based on the focal mechanisms of the mainshock and ten aftershocks indicates the maximum compressive stress trends 104° with a plunge of 5°. The local stress field near Bardwell is therefore rotated about 40° clockwise relative to 65° for eastern North America as a whole. The Bardwell earthquakes have the opposite sense of slip to earthquakes with east-trending nodal planes that occur near New Madrid, Missouri. This requires a significant local rotation of the stress field over a distance of 60 km.

Three‐Dimensional Seismic‐Velocity Model for the Unconsolidated Mississippi Embayment Sediments from H/V Ambient Noise Measurements

Abstract The horizontal‐to‐vertical (H/V) technique by Nakamura (1989) was applied to data from 30 new field stations and 64 other broadband temporary and permanent seismic stations within the Mississippi Embayment of the central United States to develop a 3D model of unconsolidated sediment shear‐wave velocity structure. Using the Dart (1992) map of sediment thickness as a basis, two self‐consistent models of average shear‐wave velocity versus sediment thickness were developed by utilizing the theoretical linear relationship between the frequency of the H/V peak and shear‐wave velocity. One model was based on the observation that the H/V peak period T p (s) versus sediment thickness h (m) was seen to be approximately linear with the relationship T p =0.003266 h +1.084. The second model was developed by considering peak frequency f p versus sediment thickness parameterized to follow ln f p =8.325×10 −7 h 2 −0.00232 h −0.01796. Overall, the models show low‐average shear‐wave velocity near the edge of the Mississippi Embayment with velocities increasing with increasing sediment thickness, consistent with increased sediment compaction. These models will be useful in studies of site resonance and amplification for earthquake‐shaking hazards and for wave propagation computations for the region.

Characteristics of Induced/Triggered Earthquakes during the Startup Phase of the Guy–Greenbrier Earthquake Sequence in North–Central Arkansas

ABSTRACT Between August 2010 and June 2011, an intense sequence of induced earthquakes occurred along the Guy–Greenbrier fault in central Arkansas due to fluid injection at nearby waste disposal wells. A previous study by Horton (2012), limited to ∼1000 earthquakes having M D >∼2.0, illuminated the ∼13  km fault. We present an updated catalog of 17,395 earthquakes that is complete between 0≤ M L ≤4.4 between August 2010 and 20 October 2010, with earthquakes located using an updated 1D velocity model for the region. The inclusion of the small‐magnitude events reveals that seismicity starts below the SRE injection well a month earlier than estimated using only M D >2 events. During this period of time, the seismicity migrated from north to south, enhancing the resolution of three joined sections that form the northern ∼7.3  km portion of the fault, which plunges southward. The seismogenic zone covers the lower portion of the Paleozoic sedimentary layers and extends into the crystalline Precambrian basement (∼3  km z M L >3 events constrained within the basement. A b ‐value of 1.01 was obtained for the updated catalog during this period, with the b ‐value varying between 1.37 and 0.72 for different clusters of events. The seismicity pattern at depth is coincident with structural geologic features observed within the Fayetteville Shale (at ∼1500  m depth).