Seismic activity has decreased slightly since yesterday. The frequent small long-period earthquakes recorded at the Gages seismic station have continued, although the rate of occurrence has decreased since this morning. There have been fewer episodes of broadband tremor. Four (4) earthquakes were located, three of them beneath the southern end of St George's Hill at depths of 4-7 km, and one east of Garibaldi Hill at a depth of 4.5 km. A mudflow signal was recorded by the Gages station at 20:46 on 16 Dec.
EDM measurements were made of the Castle Peak, Gages Wall and Chances Peak reflectors. Initial results show only small changes since these lines were last measured. The Gages Wall reflector was cleaned this afternoon. The GPS sites at St George's Hill, Reid's Hill and Harris' Lookout were occupied, and processing of GPS data from 14 Dec has been completed. The line from Chances Peak to Harris' Lookout has lengthened by 10 cm since it was last measured in October. This change is consistent with the lengthening of the Tar River to Chances Peak line that was reported previously.
Visual observations were made from the helicopter this morning and from the crater rim this afternoon. A major new spine has appeared close to Castle Peak, just to the south of the spine that emerged in early December. This new spine was not visible on December 15, and thus has grown to about 30 metres high in less than two days. The new dome in the west of the crater that was first seen on 15 Dec appears to have grown slightly, and material is falling from it into the moat. The crater area was steaming vigourously, especially from the vent to the west of Castle Peak. Several small ash eruptions were observed during the day.