The hybrid earthquakes (dominant frequencies of 3 to 5 Hz) continued to be tiny, all less than magnitude 1 and the vast majority near magnitude 0.5. They are still occurring every 10 to 20 seconds but can only be recorded by the two nearest stations, MGAT (near Gages) and the new station MHR (at Hermitage). Long-period earthquakes (dominant frequency of 2 Hz) became even more frequent than yesterday; about 34 were recorded in all and these had magnitudes between 1.3 and 2.0. Only a few of these events were large enough to locate well; these were between 1.2 to 1.7 km beneath the crater. One volcano-tectonic (rock-breaking) earthquake was recorded during this period; it occurred at about 3 km beneath St. George's Hill and was of about magnitude 1.5. Broadband tremor (dominant frequencies of 2 to 8 Hz) was essentially non-existent during this period. About 18 seismic signals, of the type that can often be visually correlated with rockfalls, were recorded during this period; five of these were confirmed by visual observation.
Measurement of EDM lines on the eastern triangle to Castle and on the Windy Hill-Farrells-St George's Hill triangle were carried out today. The results of these measurements are still being processed.
Very good views of English's Crater were obtained from the helicopter and from within English's Crater today. An expedition to sample rocks from the southern parts of the dome enabled very close examination of dome growth in this area. Dome growth is now largely concentrated in two main areas of English's Crater. Two elongate ridges, oriented northeast-southwest, which resemble whale backs, have become quite prominent during the past week of activity. The top of the north whale back, which has become increasingly visible from several areas on the western flank of the volcano, is now at least as high as the top of Castle Peak. Growth in this area appears to be mainly vertical with some lateral movement towards the crater wall at Farrells. The south whale back consists of a horizontal slab of solid rock which is actively growing towards the crater wall in the Upper Galways area. Scree deposits from the front face of both slabs has filled in the moat area and is now piling up against the crater wall.