Relative relocation of seismic events following the June 15, 1991 eruption of Mount Pinatubo

The June 15, 1991 climactic eruption of Mount Pinatubo (Philippines) was followed by intense seismicity which remained at a high level for several months with thousands of recorded events. In contrast to the pre-eruptive seismicity which was focused in two groups below the summit area, post-eruptive events were widely distributed below and around the volcano. We located about 10,000 events among the triggered recordings available from July 1 to mid December 1991 and applied precise relative relocation methods to these events. The classification of these events indicates the presence of several large families of similar volcano-tectonic events. The precise relocation using cross-correlation delays delineates several planar features which could be interpreted as fault planes. These are mostly subvertical, with often a roughly east-west strike. The small number of stations available (6) limits the quality of the relocations. In an effort to test the robustness of our results we carried out several tests. Two of the major sources of error are poor delays caused by mis-alignments at one (or several) station(s) and large uncertainties for poorly constrained events. The one-by-one elimination of a station in the relocation process provides a first way to test the influence of each station on the shapes defined by relocated events. A Monte Carlo simulation assuming Gaussian errors on takeoff and azimuth angles and on delays provides error ellipses for each relocation. Despite relatively large uncertainties (commonly about 100-200 m in horizontal and 200-500 m in vertical), the values for the better constrained events are smaller than the extent of the relocated clusters. These tests indicate that reliable features can be detected with the small Pinatubo network, and we expect that similar success can be attained for other cases with sparse network geometries.