Aftershock Investigation of the October 24, 1997, Earthquake Near Atmore, Alabama

Starting 22 hours after the mainshock, we deployed an array of portable seismographs to record aftershocks of the October 24, 1997, Atmore earthquake (mb 4.9) in southern Alabama where there are no local and few regional permanent seismograph stations. Initially we installed two RefTek* seismographs operating in continuous record mode, four Kinemetrics K2 accelerographs operating in triggered mode, and two MEQ-800 continuous analog seismographs running at 72 dB to monitor the activity. No aftershocks occurred for 62 hours after the mainshock, so the first six recording sites were chosen using intensity information from interviews of residents in an area near the NEIC preliminary location of the mainshock. That array configuration later proved to only cover azimuths northwest clockwise through southeast from the aftershocks. After we had recorded and located the first aftershocks, we installed four more RefTek seismographs sent by IRIS to cover the southeast through northwest quadrants. The digital array recorded 25 aftershocks at two or more stations, 8 at three or more, and 5 at four or more. The largest aftershock, a M3.7 event on October 26,1997, had a peak acceleration of 0.03 g. We operated an eleven-station array for 42 days following the mainshock, and a four station array continued to monitor the epicentral area for an additional month.

Hypocenter Determinations

Events were located using HYPOELLIPSE (Lahr,1997; we are grateful for the assistance and advice he gave us) and a simplified version of the P-wave velocity structure. S-wave velocities were calculated from the P-wave values by estimates of Vp/Vs that ranged from 4.0 at the surface with a P-wave velocity of 1.676 km/s to 1.78 at a depth of 1.439 km with P-wave velocity of 3.627 km/s.

Directivity/Radiation-pattern Amplification

The body-wave accelerations produced by the M3.7 and M2.9 aftershocks were an order-of-magnitude larger at station STA than at stations PIN, WAD, HUX, and WAD. We initially believed that this discrepancy may have been caused by either instrument malfunction or strong site response differences. Subsequent records of smaller aftershocks and a huddle test of the instruments in question (K2 accelerometers) indicated that the instrument responses were comparable and reliable.

Note that the coda amplitudes, unlike the corresponding body-wave amplitudes are very similar from station to station. This suggests that the large amplitudes at station STA are caused by directivity and/or radiation pattern.

Additional evidence for directivity being the cause of the large amplitude of station STA is the narrow pulse-width of the P-wave at that station relative to those of the P-waves of the other stations (Note that the pulses of the M2.9 aftershock are more complex than those of the M3.7 aftershock).