Anomalous radon emanation at local and regional distances preceding earthquakes in the New Madrid seismic zone and adjacent areas of the central mid-continent of North America, 1981–84

Anomalous soil-radon activity, including several spike-like surges over periods of 5, 2, and 2 1/2 months, and a year-long declining trend, preceded the most significant earthquakes of the central mid-continental region of North America during 1981 and early 1984. The 5-month period of erratic weekly radon activity, February–June 1981, was followed by a tremor of magnitudeM=4.0, 40 km from the monitoring site in the New Madrid seismic zone. An unusual earthquake swarm in central Arkansas, 160 km from the New Madrid seismic zone and 230 km from the monitoring site in the seismic zone, began in January 1982, shortly after a year-long declining trend in anomalous radon emanation. Earthquakes of magnitudes 4.5, 4.1, and 4.0 occurred at the swarm's outset in early 1982, fitting a pattern anticipated for the New Madrid seismic zone on the basis of the radon activity. Two periods of coincident peak radon emanation have since been observed in the Arkansas and New Madrid seismic regions, as have synchronous seismic pulses for the two separate areas.Two more recent periods of highly erratic soil-radon emanation, March–May 1983 and November–January 1984, were followed by a 4.3 earthquake in southwestern Illinois on 15 may 1983, and 3.5–3.6 tremors and swarm activity in the New Madrid seismic zone in late January and mid-February 1984. Prior to the 4.3 event, radon peaked at three widely separated monitoring sites 1–3 weeks before the tremor at distances of 120, 225, and 320 km from the epicentral region, the station at 225 km, in the New Madrid seismic zone, recording the longest period of anomalous radon activity. As for the recent 3.5–3.6 tremors of 1984, seismic activity of this magnitude had been anticipated for January or February on the basis of the amplitude of the November radon anomaly.These observations provide further evidence of (a) the existence of soil-radon anomalies precursory to the large earthquakes in this intraplate region, (b) the utility of such anomalies in anticipating events of small to moderate magnitudes for the region, and (c) the occurrence of regional-scale strain events prior to some of the larger mid-continental earthquakes.A very recent radon anomaly, the strongest yet to be detected in the seven years of monitoring in the mid-continental region, occurred in the New Madrid seismic zone from mid-February through mid-June 1984. A 4.0 earthquake occurred one month after a peak in the radon activity. The amplitude and duration of the anomaly suggest that a significant change in the state of stress or strain may have occurred in the mid-continental region during 1984.