Abstract:
Mount Erebus is a 3794m high phonolitic volcano on Ross Island with an active lava lake in its summit crater. Study of teleseismic waves recorded by the telemetry network between Novmber 1988 and December 1990, indicates a P-wave velocity between 1651 (±680) m/s for the flanks and 3930 (±441) m/s for the core for the volcano. A similarly low near-surface velocity (V ≤2000 m/s) was resolved by shallow seismic refraction surveys on the Erebus summit plateau, away from areas of permafrosting. Shots in the warm ground of the Side crater suggest an average seismic velocity of 3000m/s at depth of about 100m. Synthetic ray tracing of explosion earthquakes in the lava lake suggests seismic velocities between 3750 m/s and 4400 m/s at about 2 km below the volcano surface.
Extrapolation of reflection and refraction seismic survey results in McMurdo Sound suggests that between 4 and 9 km of pre-Cenozoic sediments on 6.60 km/s basement dip to the NE below Mount Erebus. This is supported by seismic network records of an explosion experiment which define a 6.60 km/s refractor at 4.5 km below sea level in southern McMurdo Sound, dipping to a depth of about 9km below sea level at Mount Terror.
A homogeneous plane layer model was developed for use by HYPO71PC earthquake location routine, and tested using travel time data of strombolian eruptions and explosion experiments. Non-explosion events located in the upper 1 km of the volcano showed a vertical pipe-like distribution. These events had no quadrantal fault plane solution and are possibly associated with fluid injection into the magma column beneath the summit lava lake. Earthquakes located below 1.3 km showed almost pure reverse faulting, with a near-vertical tensional stress axis trending ENE to NE. The presence of an aseismic zone between 2 and 3 km immediately below the summit supports the formation of a sill through vertical extension by bounding reverse faults.
During winter 1990, a five to ten metre thick lava cap developed over the northern half of the Inner crater (at least) with two small pit craters indicating the active surface area of the lava lake. This compares with an earlier lava lake morphology of a thin (less than 2m) lava crust and greater areas of visibly active convection. All explosions recorded on video during the study period occurred from these active surface areas. A possible relationship between the waveform of these explosions and the changes in crater morphology supports Fadeli's(1984) claim that various vent types have distinct waveforms, and that the explosion earthquake originates at or just inside the vent (Dibble et al, 1989). A low frequency onset recorded by stations within 2.5 km of the vent can be explained by either (i) a critically refracted wave from the explosion event passing through a lava cap to the near stations (Dibble et al, 1989), or (ii) elastic waves caused by resonance of this lava cap due to the vibration of an unstable bubble near the surface of the lava lake.
