Abstract

The Arkansas swarm is the largest earthquake swarm ever recorded in the Central or Eastern United States. Three-component digital and single-component analog recordings for 88 earthquakes occurring during a 12-day period provide a quality data set of manageable proportions for study of the swarm's seismic properties. These 88 earthquakes occurred within a small volume of about 45 km3 at depths from 4 to 7 km. Composite focal mechanisms indicate predominantly strike-slip motion with the P axis oriented NE-SW. Seismic wave velocity studies indicate that significant temporal and spatial variations of seismic wave velocities are present. The Vp/Vs ratio ranges from as low as 1.59 to as high as 1.78 with the majority of values falling between 1.67 to 1.70. An anomalous region immediately north of the center of swarm activity is found where both the P- and S-wave velocities and Vp/Vs ratio increase dramatically starting about 1 day before the 5 July Md = 3.8 event. Two strong secondary arrivals between the direct P and S arrivals can be clearly identified, especially at stations NNW of the activity. These secondary arrivals must be converted P waves from S waves incident on two deep discontinuities.

The above observations are compatible with a small, highly heterogeneous swarm source volume within which the ambient stress regime can vary rapidly in both time and space. Some identified clustering and Vp/Vs changes are clearly precursory to the major events of the sequence but no simple, repeating pattern is evident in this restricted data sample.

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