THE HENGILL GEOTHERMAL AREA, ICELAND - VARIATION OF TEMPERATURE-GRADIENTS DEDUCED FROM THE MAXIMUM DEPTH OF SEISMOGENESIS

Given a uniform lithology and strain rate and a full seismic data set, the maximum depth of earthquakes may be viewed to a first order as an isotherm. These conditions are approached at the Hengill geothermal a rea, S. Iceland, a dominantly basaltic area. The likely strain rate ca lculated from thermal and tectonic considerations is 10(-15) s(-1), an d temperature measurements from four drill sites within the area indic ate average, near-surface geothermal gradients of up to 150 degrees C km(-1) throughout the upper 2 km. The temperature at which seismic fai lure ceases for the strain rates likely at the Hengill geothermal area is determined by analogy with oceanic crust, and is about 650 +/- 50 degrees C, The topographies of the top and bottom of the seismogenic l ayer were mapped using 617 earthquakes located highly accurately by pe rforming a simultaneous inversion for three-dimensional structure and hypocentral parameters. The thickness of the seismogenic layer is roug hly constant and about 3 km. A shallow, aseismic, low-velocity volume within the spreading plate boundary that crosses the area occurs above the top of the seismogenic layer and is interpreted as an isolated bo dy of partial melt. The base of the seismogenic layer has a maximum de pth of about 6.5 km beneath the spreading axis and deepens to about 7 km beneath a transform zone in the south of the area. Beneath the high -temperature part of the geothermal area, the maximum depth of earthqu akes may be as shallow as 4 km. The geothermal gradient below drilling depths in various parts of the area ranges from 84 +/- 9 degrees C km (-1) within the low-temperature geothermal area of the transform zone to 138 +/- 15 degrees C km(-1) below the centre of the high-temperatur e geothermal area. Shallow maximum depths of earthquakes and therefore high average geothermal gradients tend to correlate with the intensit y of the geothermal area and not with the location of the currently ac tive spreading axis.