SEISMICITY IN THE WHITE RIVER FAULT ZONE, MISSISSIPPI AND ARKANSAS

The White River Fault Zone (WRFZ) extends 280 km from near Newport, Arkansas on the northwest end to near Grenada, Mississippi on the southeast end. Fisk (1944) used aerial photography to first delineate the WRFZ. In Arkansas this structural zone coincides with the orientation of the White River and the southern terminus of Crowley's Ridge. In Panola County, Mississippi, the Mississippi River bluff line for 20 km has the same N 40 W orientation as the overall WRFZ. This zone is generally represented as about 15 km wide. In the past 25 years some 15 earthquakes have been recorded within or closely adjacent to the WRFZ, most in the range of magnitude 1 to 3. The December 1931 magnitude 5 earthquake near Charleston, Mississippi was the largest earthquake in the state and located within the WRFZ. Investigations of two recent seismic events in Panola County, Mississippi illustrate the variability of surface effects to be expected with future earthquakes in this area. The Courtland (1999) and Curtis Station (2002) earthquakes were both shallow (5 km depth), had similar magnitudes (2.8 and 2.7, respectively), and were only 30 km apart. The intensities and felt areas, however, were markedly different. The Courtland event was felt over an area of about 260 km2 and was assigned a maximum intensity value of IV, whereas the Curtis Station event was felt over only 10 km2 and had a maximum intensity value of II. The Curtis Station earthquake epicenter was in the Mississippi River flood plain while the Courtland earthquake epicenter was in the Tertiary hills to the east. These significant differences in surface effects of two similar earthquakes are likely related to differences in subsurface sediment and soil responses to seismically-induced vibrations. Thick sequences of unconsolidated alluvium, typical of Mississippi River flood plain deposits, appear to have attenuated the particular style of seismic energy more efficiently than the Tertiary sediments.