Microearthquake survey of Median Valley of the Mid-Atlantic ridge at 36°30′N

Abstract

MICROEARTHQUAKE activity in the median valley of the Mid-Atlantic Ridge near 36°30′N has been monitored using expendable radio-sonobuoy arrays. Similar techniques have been used by Reid et al.¹ in the Gulf of California, Macdonald and Mudie² near the Galapagos spreading centre and recently by Reid and Macdonald³ near fracture zones A and B shown in Fig. 1 (ref. 4). This study provides area coverage not obtained by Reid and Macdonald whose experiment was confined to the fracture zone regions. A new method of sonobuoy position determination was used that provides relative buoy locations (locations with respect to each other) to within ±10 m. The ultimate positioning capability in latitude and longitude depends on the accuracy of the primary navigation system available. Satellite navigation can yield absolute positions to within ±100 m. An estimated actual accuracy of ±200 m is ascribed to the experiments reported here. Accuracy of this order allows correlation of epicentre locations with topographic features of similar scale size. Recently acquired bathymetric data (K. C. Macdonald and colleagues, in preparation) indicate that significant features of the ridge near 36°30′N, such as transform faults and valley walls, can be defined by scale sizes of some hundreds of metres, A total of 112 events definitely attributable to seismic activity were recorded in 72 h of listening, yielding an average rate of occurrence of 1.5 events h⁻¹.