ABSOLUTE AND RELATIVE LOCATIONS OF SIMILAR EVENTS WITH APPLICATION TOMICROEARTHQUAKES IN SOUTHERN ICELAND

It is well known that similar earthquakes, i.e. earthquakes having alm ost identical waveforms, allow extremely accurate relative timing of t he seismic arrivals. This has traditionally been used for achieving ac curate relative locations of clusters of similar earthquakes. The arri val time differences between similar events depend not only on their r elative location but also on the absolute location of the group. Movin g a pair of events 200 m 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 6 km distant. A change in time difference o f 1 ms can easily be estimated by cross-correlating the waveforms of t he two earthquakes. We use the accurate relative timings to improve ab solute locations of groups of similar events, as well as to obtain ext remely accurate relative locations. The absolute locations from relati ve timings are expected to have errors that are independent of the err ors associated with locations based on absolute arrival time observati ons. We analyse data from five earthquake sequences, comprising a tota l of 96 earthquakes, recorded by a regional network in southern Icelan d. One of the clusters is located within the on-land spreading ridge i n south-western Iceland, and the other four are within the South Icela nd seismic zone, a transform zone between overlapping branches of the spreading ridge. The events vary in magnitude between M(L) -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 hypo centers. The mean distance of events from a best-fitting plane varies between 4 and 15 m for the five swarms. This is comparable to the form al error estimates for the relative locations. Together with (non-uniq ue) 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-vertica l N-S striking planes, in agreement with the orientation of mapped ear thquake fractures in the area. The earthquakes within the spreading zo ne clearly define a fault plane striking parallel to the ridge and dip ping 63 degrees. Each group of similar events probably represents repe ated slip on the same fault.