Microearthquakes, swarms, and the geothermal areas of Iceland

Abstract

More than 2100 microearthquakes were recorded and crudely located by using data from portable seismographs operated in Iceland during the summer of 1968. Another 600 events were located more precisely in three areas by using data from tripartite arrays. The earth­ quakes recorded are largely confined to 13 regions that are generally less than 100 in area. Most of the well-located events are at depths of 2 to 6 km but some less well located events may be as deep as 13 km. The microearthquakes are largely confined to the upper few kilometers of the oceanic layer, or layer 3 (Vp ::::: 6.5 km/sec in Iceland). Geothermal areas in Iceland that are structurally related to a large number of faults and fissures generally have high microearthquake activity. Geothermal areas that have few fissures and appear to be structurally related to acidic intrusions contain little or no microearthquake activity. The distribution of zones of micro earthquake activity generally supports the hypothesis of a transform fault in southern Iceland. It appears that the stress along this fault is being relieved in geothermal areas by numerous micro earthquake swarms occurring more or less con­ tinuously. Outside the geothermal areas, mainshock-aftershock sequences seem to be the dominant mode of stress release. The swarms can be attributed to weakening of the crust by fluids or fluid pressure. During the summers of 1967 and 1968, surveys of microearthquake activity were conducted for the first time and largely on a reconnaisance basis throughout a substantial part of Iceland. Results from the field program of 1967 are presented by Ward et al. [1969]. This paper presents information gathered in 1968 and an analysis of microearthquake phenomena during both years. Microearthquakes discussed here are earth­ quakes as small as magnitude -1. Up to 100,000 microearthquakes might occur in an area where only one event of magnitude greater than or equal to 4 is located during the same