The level of seismicity during the day was high compared to the past few days. A total of 50 long-period signitures were recorded by the network during the day, about half of which were of the type associated with rock falls. A signal recorded for 8 minutes starting at 08:23 this morning (23 March) was correlated with the major ash fall in Plymouth - this signal is similar to those that were associated with phreatic explosions during the earlier phase of the volcanic activity. The ash came from an area west of the old Casthe Peak dome which has been venting vigorously for the past few days and may have been caused by the opening of a new vent or the extension of an old one. Another possible explanation is that it could have been generated by a major rock avalanche from the dome, possibly associated with small explosions. Several other light ash falls occurred overnight associated with other rock fall signals. Low to medium amplitude, broadband tremor has been recorded on the Gages and Chances Peak seismometers since 10:00 this morning; this level of tremor has not been recorded for some considerable time.
The eastern EDM triangle and also the line between Tar River and Castle Peak were measured today - all of these measurements suggest continued very slow movement of the Castle Peak reflector. The Tar River to Castle Peak line has shortened 8 mm in the past 3 days, suggesting a slightly higher rate of movement than of late.
Visual observations were made today from Chances Peak and from the east side. During the morning, frequent rock falls were seen occurring from the south and east faces of the dome, generating small amounts of ash which drifted westwards over Plymouth. Due to technical problems, no helicopter flights were made today. The scientists were therefore not able to confirm what caused the ash cloud that drifted over Plymouth at about 08:23 this morning and fell for 5 to 10 minutes. Observations in the afternoon from Chances Peak suggested that rock falls had lessened significantly in frequency. Steam production remains high, although the main steam vent has been covered by new rock fall material so that steam production is now more dispersed.