As seismic and volcanic activity increased, volcanologists working for the USGS in its Vancouver branch prepared to observe any impending eruption. Geologist Don Swanson and others placed reflectors on and around the growing lava domes, and, on May 1, 1980, established the Coldwater I and II observation posts to use laser ranging to measure how the distances to these reflectors changed over time as the domes deformed. Glicken monitored the volcano for two weeks, taking shelter in a trailer at the Coldwater II site located a little more than northwest of the volcano. On May 18, 1980, after working for six days straight, Glicken took the day off to attend an interview for his graduate work with his professor, Richard V. Fisher, in Mammoth, California. His research adviser and mentor David A. Johnston replaced him at his post, despite expressing concerns about its safety given indications of mobile magma within the volcano. After a magnitude 5.1 earthquake centered directly below the north slope triggered that part of the volcano to slide at 8:32 a.m., Mount St. Helens erupted. Johnston was killed after he was enveloped by swift pyroclastic flows that traveled down the mountain's flanks at near supersonic speeds.