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Abstract. Using detailed microearthquake data, we present a stress tensor inversion scheme with new methods for selecting the fault planes and allowing for errors in the focal mechanisms. The nonuniqueness of earthquake focal mechanisms is accounted for in our inversion scheme through the introduction into the inversion of a range of well-fitting focal mechanisms for each event. The range of focal mechanisms significantly improves the quality of the estimated stress tensor. Relative localization of clusters of microearthquakes is used to obtain information about which nodal plane could be correct fault plane. The clusters frequently fall on a common fault plane, and if there are acceptable focal mechanisms where one nodal plane has orientation similar to the common plane, we assume this is the correct fault plane for the event. If there is no predefined fault plane, we utilize a simple Mohr-Coloumb failure criterion to obtain a physical choice of fault plane between the two nodal planes in the focal mechanism. The nodal plane with highest relative instability is chosen as the fault plane. Differences between the instability and the standard slip angle criterion are investigated. The new inversion scheme has been applied to microearthquake data from the •lfus area in the vicinity of the southwest Iceland triple junction. We estimate an oblique strike-slip state of stress, maximum horizontal stress at N30øE, and minimum horizontal stress at N60øW, with significant normal faulting influence. The instability fault selection criterion predicts very well the orientation of faults mapped by relative localization.

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.

New deformation data from the Askja volcano, Iceland, show that the volcano's caldera has been deflating continuously for over 20 years, and confirm that the rate of subsidence is slowing down. The decay in subsidence rate can be fitted with a function of the form e −t/τ , where τ is 39 years. Reanalysis of GPS data from 1993-1998 show that these data can be fitted with a model calling for two Mogi point sources, one shallow, and another one much deeper (16.2 km depth). Pressure decrease occurs in both sources. The deeper source is responsible for observed horizontal contraction towards Askja at distances that cannot be explained by the shallower source. Plate spreading of 19 mm/year distributed evenly over about 100-km-wide zone is also favoured by the data.

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