lahar observed during the 1989/1990 eruption. Smaller lahars that did not impact DROT have also occurred. Several of Redoubt's recent explosions have resulted in measurable ash fall over populated areas as distant as Delta Junction (340 miles northeast of Redoubt), with more significant ash fall in more proximate areas, including the Susitna Valley, the Kenai Peninsula and the Anchorage bowl. On the afternoon of March 28, ash fall in Anchorage closed the airport from 17:00 until 07:00 the next morning (March 29). The maximum ash fall measured so far in a populated area is about 1/16 inch (1.5 mm) near Seldovia following the April 4 explosion. A measurement of about 1/8 inch (3 mm) following the explosion on March 26, was recorded near Silver Salmon Creek Lodge, about 30 miles (48 km) south of the volcano. Trace ash fall has also been observed between explosive events during times when the volcano is emitting a continuous low altitude (< 15,000 feet ASL) gas and ash plume. A rich variety of seismic signals have been recorded at Redoubt throughout the eruption and in the preceding months of unrest. These seismic events include: (1) typical "rock-breaking" earthquakes; (2) volcanic "tremor", indicative of steam and other volcanic gases or fluids vigorously propagating through cracks; (3) large, far-reaching, "cigar-shaped" signals resulting from volcanic explosions; (4) ground shaking from lahars as they pass nearby seismometers; and (5) small, repetitive, self-similar events associated with the slow extrusion of lava (this kind of extrusion is often referred to as "dome building"). More than 1700 earthquakes have been located since mid-January, and many more occurred that were either too small or of such a character as to make location impossible. Typically, the magnitudes of these events are small, the average being around M0.5. However, on April 9 a magnitude 3.3 earthquake occurred about 2.5 miles (4 km) E/NE of Redoubt's summit, possibly resulting from a crustal adjustment to the ongoing withdrawal from Redoubt's subterranean magma reservoir. Since the fall of 2008, AVO has flown 13 gas measurement flights, and of these, 5 have occurred since the eruption began on March 15, 2009. Elevated carbon dioxide (CO2) emission rates (up to 2000 tonnes/day) were measured in October–November, 2008. Starting in late January 2009, and coincident with a strong increase in seismicity, gas emission rates rose to a level (> 5000 tonnes/d) suggesting significant unrest at the volcano, and emissions stayed