Preliminary Conclusion

Earthquake families were recognised by cross correlation of different earthquakes recorded on the same channel, using programs written by M. Iguchi, and after stacking to improve waveform clarity, the onset times of the stack were used to locate the family focus. These times, when used with the new 4 km/s model, give an epicentre 120 m from the lava lake, and depth 340 m below it, with an RMS time residual of 0.02 s. Using the old 2.1 km/s model, and the onset times of each of the 8 separate explosions in the best family, the epicentres were a mean distance of 2170 m +/−2470 m from the lava lake, and depth 6.2 +/− 4.4 km below it. The mean RMS residual was 0.33 +/−0.14 second.

Thus the new higher velocity model results in foci for explosion earthquakes which are shallower and nearer the lava lake than for the old velocity model. However, it is the stacking which produces the improvement in error statistics, not the change in velocity.