John W. Schlue

Seismic and acoustic observations at Mount Erebus Volcano, Ross Island, Antarctica, 1994-1998

Volcanic activity at Mount Erebus is dominated by eruptive activity within a phonolitic summit lava lake. Common eruption styles range from passive degassing to Strombolian explosions, which typically occur several times daily, and occasionally in swarms of up to 900 per day. Shallow explosions, although generally the result of steady exsolution of volatiles from depth, can be triggered by surficial input of H 2O through mass wasting of rock, snow and ice from the crater walls. Broadband observations of Strombolian explosions document very-long-period (VLP) signals with strong spectral peaks near 20, 12 and 7 s, which are polarized in the vertical/radial plane. These signals precede lava lake surface explosions by ,1.5 s, are highly repeatable, and persist for up to 200 s. First motions indicate a deflationary source, with any precursory inflation being below the ,30 s passband of our instruments. Particle motions suggest a VLP source residing up to 800 m below the lava lake surface; however, this depth could be exaggerated by near-field radial tilt. Seismic and acoustic signals associated with lava lake explosions commonly show evidence for multiple bubble bursts in corresponding complexity features resulting from varying time delays and relative sizes of superimposed bursts. A systematic decrease in seismic/acoustic ratio for smaller surface explosions suggests that either the seismic energy from the smallest, shallowest bubble bursts experiences much greater seismic attenuation than energy arising from larger events which may involve a deeper, less attenuative portion of the magma column, and/or that the shallowest layer is seismically isolated from deeper parts of a stratified magma column, which are not excited by the smallest explosions due to sharp impedance contrasts across distinct layers. Tremor at Erebus is uncommon, with only a few isolated instances identified in five years of monitoring. Some tremor events are nearly monochromatic, and some exhibit numerous gliding harmonic spectral lines. q 2000 Elsevier Science B.V. All rights reserved.

Broadband recording of Strombolian explosions and associated very‐long‐period seismic signals on Mount Erebus Volcano, Ross Island, Antarctica

In December 1996 and January 1997, broadband seismometers were deployed on the summit plateau of Mount Erebus at radial distances of 0.7, 1.4 and 1.9 km from the central crater and lava lake. Strombolian explosions at Erebus previously have been observed to produce seismic and acoustic energy between 1 and 6 Hz. New observations document significant energy with spectral peaks as grave as 20 s. Nearly identical very-long-period (VLP) signals begin ∼1.5 s prior to explosions, have dilatational onsets and persist for up to 150 s. Similar VLP waveforms were recorded at all three stations, indicating that the seismograms are essentially source-dominated. Particle motions suggest an initial depth for the VLP source of up to several hundred meters, migrating deeper in the course of ∼15 s. Such explosion-associated VLP signals may indicate a nondestructive lossy resonance or nonlinear fluid-flow excitation within the shallow magmatic system.

Broadband Seismic Background Noise at Temporary Seismic Stations Observed on a Regional Scale in the Southwestern United States

Background noise power spectral density (PSD) estimates for 54 PASS- CAL Colorado Plateau/Rio Grande Rift/Great Plains Seismic Transect (LA RISTRA) stations were computed using data from 1999 to 2000. At long periods (0.01-0.1 Hz), typical vertical noise levels are approximately 12 dB higher than the nearby Global Seismic Network (GSN) borehole station ANMO, but horizontal power spec- tral density (PSD) noise levels are approximately 30 dB higher. Long-period noise levels exhibit essentially no spatial correlation along the LA RISTRA transect, indi- cating that local thermal or atmosphere-driven local slab tilt is the dominant source of noise in this band. Between 0.1 and 0.3 Hz, typical noise levels are dominated by naturally occurring microseismic noise and are essentially identical to those observed at ANMO. At short periods, 0.3-8 Hz, typical noise levels along the network exceed ANMO levels by approximately 15 dB, with the highest levels corresponding to proximity to cultural noise sources. No significant day/night variations were observed in the microseismic band; however, both low- and high-frequency noise levels show an increase of up to 8 dB in median midday versus midnight noise levels. We find that the major shortcomings of these shallow PASSCAL-style temporary vaults rela- tive to a GSN-style borehole installation are increased susceptibility to long-period horizontal (20 sec) noise and to surface noise sources above approximately 2 Hz. Although the high-frequency near-surface noise field is unavoidable in shallow vaults, we suggest that increased understanding and mitigation of local tilt effects in shallow vaults offers the possibility of significantly improving the long-period noise environment.

SHEAR-WAVE SPLITTING IN THE RIO GRANDE RIFT

Fast directions and lag times associated with SKS shear-wave splitting are determined for six digital stations in the Rio Grande Rift (RGR). The mean fast direction for three stations in the central RGR is about 45° oblique to the axis of the RGR. In contrast, fast directions approximately parallel to the axis of the RGR are found for two stations in the southern RGR (∼10°), and one station in the northern RGR (∼26°). The fast directions for southern stations are not aligned with the NE-SW extension direction of the Oligocene to early Miocene stage of extension. The significant N-S lateral variation in the SKS shear-wave splitting parameters is difficult to explain by fossil anisotropy because the mantle structure beneath the RGR is underlain by hot material with low seismic-wave velocities. Hence the fast direction of azimuthal anisotropy is probably related to the preferred orientation of olivine created by the present deformation in the upper mantle. Because none of the fast directions are parallel to the present E-W extension direction of the RGR the dominant upper mantle flow direction is not perpendicular to the axis of the RGR. The pattern of the fast directions along the RGR provides evidence for a rising asthenosphere beneath the central to southern RGR and suggests that thermally driven small-scale convection may be faster for flow parallel to the rift than perpendicular to it.

Inferences for the Socorro magma body from teleseismic receiver functions

Receiver functions constructed from teleseisms arriving from the northwest and recorded at station WTX in Socorro, NM show strong P-to-S conversions we interpret as coming from a mid-crustal sill-like magma body. Analysis of these conversions suggest that both the upper and lower surfaces of the magma body are distinct and provide sharp velocity contrasts with the surrounding host rock. Models that provide acceptable fits to these conversions all have magma body thicknesses less than 200 m. The absence of similar phases in data from the south and southeast suggests that the magma body is discontinuous near its edges.

A lower crustal extension to a midcrustal magma body in the Rio Grande Rift, New Mexico

A prominent and spatially localized phase arriving approximately 4 s after the initial compressional arrival, repeatedly observed on seismograms recorded in the central Rio Grande Rift from intermediate- and deep-focus earthquakes at distances near 90°, is identified as a probable compressional to shear wave conversion from a lower crustal root of the Socorro midcrustal magma body (SMB). This phase is inconsistent with previously determined crustal models containing a simple sill-like midcrustal magma body with a negligible conduit system. Finite element synthetic seismogram modeling suggests that a partially molten root with a western boundary dipping steeply to the east extends several kilometers from the 19-km-deep upper surface of the magma body down into the lower crust of the southwestern portion of the Albuquerque-Belen basin. The location and geometry of this feature suggest that the intracrustal intrusion of mantle magmas responsible for the ongoing inflation of the SMB and associated seismicity is occurring via an off-axis conduit system located beneath the active margin of the Albuquerque-Belen basin.