Abstract. Since July 1994 an unusually persistent swarm of earthquakes (M< 4.0) has been in progress at the Hengill triple junction, SW Iceland. Activity is clustered around the center of the Hr6mundartindur volcanic system. Geodetic measurements indicate a few centimeters uplift and expansion of the area, consistent with a pressure source at 6.5 + 3 km depth beneath the center of the volcanic system. The system is within the stress field of the south Iceland transform zone, and the majority of the recorded earthquakes represent strike-slip faulting on subvertical planes. We show that the secondary effects of a pressure source, modeled as a point source in an elastic half-space, include horizontal shear that perturbs the regional stress. Near the surface, shear stress is enhanced in quadrants around the direction of maximum regional horizontal stress and diminished in quadrants around the direction of minimum regional stress. The recorded earthquakes show spatial correlation with areas of enhanced shear. The maximum amount of shear near the surface caused by the expanding pressure source exceeds 1 •tstrain, sufficient to trigger earthquakes if the crust in the area was previously close to failure.

S U M M A R YIt is well known that similar earthquakes, i.e. earthquakes having almost identical waveforms, allow extremely accurate relative timing of the seismic arrivals. This has traditionally been used for achieving accurate relative locations of clusters of similar earthquakes. The arrival time differences between similar events depend not only on their relative location but also on the absolute location of the group. Moving a pair of events 200m while retaining their relative locations can cause a 1 ms change in the time difference between the first arrivals of the events at a station 6km distant. A change in time difference of l m s can easily be estimated by cross-correlating the waveforms of the two earthquakes. We use the accurate relative timings to improve absolute locations of groups of similar events, as well as to obtain extremely accurate relative locations. The absolute locations from relative timings are expected to have errors that are independent of the errors associated with locations based on absolute arrival time observations. We analyse data from five earthquake sequences, comprising a total of 96 earthquakes, recorded by a regional network in southern Iceland. One of the clusters is located within the on-land spreading ridge in south-western Iceland, and the other four are within the South Iceland seismic zone, a transform zone between overlapping branches of the spreading ridge. The events vary in magnitude between M , -0.3 and 2.8. After determining the absolute and relative locations of each swarm, we estimate the orientation of a best-fitting plane through the hypocenters. The mean distance of events from a best-fitting plane varies between 4 and 15m for the five swarms. This is comparable to the formal error estimates for the relative locations. Together with (nonunique) fault-plane solutions, the relative locations constrain the fault planes and the type of faulting. Faulting within the nascent transform zone in southern Iceland is predominantly strike slip on near-vertical N-S striking planes, in agreement with the orientation of mapped earthquake fractures in the area. The earthquakes within the spreading zone clearly define a fault plane striking parallel to the ridge and dipping 63". Each group of similar events probably represents repeated slip on the same fault.

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