DEGASSING EXPLOSIONS AT KARYMSKY VOLCANO, KAMCHATKA

During the summer of 1997, Karymsky Volcano produced summit explosions about six times each hour. Typical explosive episodes lasted between 30 seconds and three minutes, produced gas and ash columns several hun dred meters high, and ejected some incandescent material. To better un derstand the physical source mechanisms responsible, we recorded hundr eds of explosions with a three component broad-band seismometer and mi crophone located 1650 meters from the active vent. Nearly every explos ion is recorded as an emergent yet identical seismic wavelet which is followed 4.15 s later by an impulsive acoustic arrival. We interpret t he signals as a near-surface gas volume burst which fractures the vent 'plug,' lowers the lithostatic pressure within the magma column, and often induces further degassing. When degassing continues, it is gener ally manifested as either a series of regular one second 'chugging' ex plosions, steady higher frequency 'jetting', or a hybrid combination. We believe that the seismic signature for 'chugs,' short duration harm onic tremor with integer overtones, is the result of repeated gas volu me bursts at the vent. In contrast, seismograms for jetting are non-ha rmonic and contain higher frequencies. We believe that the competing d egassing behaviors are influenced by the gas flux as well as the plug/ conduit characteristics. We propose that a plug exists due to a viscos ity gradient caused by volatile depletion in the upper conduit.