Geology topics

A.D. Miller

Publications and source records attributed to A.D. Miller.

9 recordsLinked to original sources

USGS1998

Soufrière Hills eruption, Montserrat, 1995 - 1997: volcanic earthquake locations and fault plane solutions

A total of 9242 seismic events, recorded since the start of the eruption on Montserrat in July 1995, have been uniformly relocated with station travel-time corrections. Early seismicity was generally diffuse under southern Montserrat, and mostly restricted to depths less than 7 km. However, a NE-SW alignment of epicentres beneath the NE flank of the volcano emerged in one swarm of volcano-tectonic earthquakes (VTs) and later nests of VT hypocentres developed beneath the volcano and at a separated location, under St. George's Hill. The overall spatial distribution of hypocentres suggests a minimum depth of about 5 km for any substantial magma body. Activity associated with the opening of a conduit to the surface became increasingly shallow, with foci concentrated below the crater and, after dome building started in Fall 1995, VTs diminished and repetitive swarms of ‘hybrid’ seismic events became predominant. By late-1996, as magma effusion rates escalated, most seismic events were originating within a volume about 2 km diameter which extended up to the surface from only about 3 km depth - the diminution of shear failure earthquakes suggests the pathway for magma discharge had become effectively unconstricted. Individual and composite fault plane solutions have been determined for a few larger earthquakes. We postulate that localised extensional stress conditions near the linear VT activity, due to interaction with stresses in the overriding lithospheric plate, may encourage normal fault growth and promote sector weaknesses in the volcano.

Geophysical Research Letters↗

USGS1998

Seismicity associated with dome growth and collapse at the Soufriere Hills Volcano, Montserrat

Varied seismicity has accompanied growth and collapse of the lava dome of the Soufriere Hills Volcano, Montserrat. Earthquakes have been classified as either volcano-tectonic, long-period or hybrid, and daily variations in the numbers of events have mapped changes in the style of eruption. Repetitive hybrid earthquakes were common during the first months of dome growth. In July 1996 the style of seismicity changed and regular short-lived hybrid earthquake swarms became common. This change was probably caused by an increase in the magma flux. Earthquake swarms have preceded almost all major dome collapses, and have accompanied cyclical deformation, thought to be due to a built-up of pressure in the upper conduit which is later released by magma moving into the dome.Varied seismicity has accompanied growth and collapse of the lava dome of the Soufriere Hills Volcano, Montserrat. Earthquakes have been classified as either volcano-tectonic, long-period or hybrid, and daily variations in the numbers of events have mapped changes in the style of eruption. Repetitive hybrid earthquakes were common during the first months of dome growth. In July 1996 the style of seismicity changed and regular, short-lived hybrid earthquake swarms became common. This change was probably caused by an increase in the magma flux. Earthquake swarms have preceded almost all major dome collapses, and have accompanied cyclical deformation, thought to be due to a build-up of pressure in the upper conduit which is later released by magma moving into the dome.

Geophysical Research Letters↗

USGS1998

Observations of hybrid seismic events at Soufriere Hills Volcano, Montserrat: July 1995 to September 1996

Swarms of small repetitive events with similar waveforms and magnitudes are often observed during the emplacement of lava domes. Over 300,000 such events were recorded in association with the emplacement of the lava dome at Soufriere Hills Volcano, Montserrat, from August 1995 through August 1996. These events originated < 2–3 km deep. They exhibited energy ranging over ˜ 1.5–4.5 Hz and were broader band than typical long-period events. We term the events “hybrid” between long-period and volcano-tectonic. The events were more impulsive and broader band prior to, compared with during and after, periods of inferred increased magma flux rate. Individual swarms contained up to 10,000 events often exhibiting very similar magnitudes and waveforms throughout the swarm. Swarms lasted hours to weeks, during which inter-event intervals generally increased, then decreased, often several times. Long-duration swarms began about every two months starting in late September 1995. We speculate that the events were produced as the magma column degassed into adjacent cracks.

Geophysical Research Letters↗

USGS1998

Non-double-couple earthquakes. 1. Theory

Historically, most quantitative seismological analyses have been based on the assumption that earthquakes are caused by shear faulting, for which the equivalent force system in an isotropic medium is a pair of force couples with no net torque (a “double couple,” or DC). Observations of increasing quality and coverage, however, now resolve departures from the DC model for many earthquakes and find some earthquakes, especially in volcanic and geothermal areas, that have strongly non-DC mechanisms. Understanding non-DC earthquakes is important both for studying the process of faulting in detail and for identifying nonshear-faulting processes that apparently occur in some earthquakes. This paper summarizes the theory of “moment tensor” expansions of equivalent-force systems and analyzes many possible physical non-DC earthquake processes. Contrary to long-standing assumption, sources within the Earth can sometimes have net force and torque components, described by first-rank and asymmetric second-rank moment tensors, which must be included in analyses of landslides and some volcanic phenomena. Non-DC processes that lead to conventional (symmetric second-rank) moment tensors include geometrically complex shear faulting, tensile faulting, shear faulting in an anisotropic medium, shear faulting in a heterogeneous region (e.g., near an interface), and polymorphic phase transformations. Undoubtedly, many non-DC earthquake processes remain to be discovered. Progress will be facilitated by experimental studies that use wave amplitudes, amplitude ratios, and complete waveforms in addition to wave polarities and thus avoid arbitrary assumptions such as the absence of volume changes or the temporal similarity of different moment tensor components.

Reviews of Geophysics↗

USGS1998

Non-double-couple earthquakes 2. Observations

Most studies assume that earthquakes have double-couple (DC) source mechanisms, corresponding to shear motion on planar faults. However, many well-recorded earthquakes have radiation patterns that depart radically from this model, indicating fundamentally different source processes. Seismic waves excited by advective processes, such as landslides and volcanic eruptions, are consistent with net forces rather than DCs. Some volcanic earthquakes also have single-force mechanisms, probably because of advection of magmatic fluids. Other volcanic earthquakes have mechanisms close to compensated linear vector dipoles and may be caused by magmatic intrusions. Shallow earthquakes in volcanic or geothermal areas and mines often have mechanisms with isotropic components, indicating volume changes of either explosive or implosive polarity. Such mechanisms are consistent with failure involving both shear and tensile faulting, which may be facilitated by high-pressure, high-temperature fluids. In mines, tunnels are cavities that may close. Deep-focus earthquakes occur within zones of polymorphic phase transformations in the upper mantle at depths where stick-slip instability cannot occur. Their mechanisms tend to be deviatoric (volume conserving), but non-DC, and their source processes are poorly understood. Automatic global moment tensor services routinely report statistically significant non-DC components for large earthquakes, but detailed reexamination of individual events is required to confirm such results.

Reviews of Geophysics↗

USGS1998

Three-dimensional seismic structure and moment tensors of non-double-couple earthquakes at the Hengill-Grensdalur volcanic complex, Iceland

The volcanic and geothermal areas of Iceland are rich sources of non-double-couple (non-DC) earthquakes. A state-of-the-art digital seismometer network deployed at the Hengill–Grensdalur volcanic complex in 1991 recorded 4000 small earthquakes. We used the best recorded of these to determine 3-D V P and V P /V S structure tomographically and accurate earthquake moment tensors. The V P field is dominated by high seismic wave speed bodies interpreted as solidified intrusions. A widespread negative (−4 per cent) V P /V S anomaly in the upper 4km correlates with the geothermal field, but is too strong to be caused solely by the effect of temperature upon liquid water or the presence of vapour, and requires in addition mineralogical or lithological differences between the geothermal reservoir and its surroundings. These may be caused by geothermal alteration. Well-constrained moment tensors were obtained for 70 of the best-recorded events by applying linear programming methods to P - and S -wave polarities and amplitude ratios. About 25 per cent of the mechanisms are, within observational error, consistent with DC mechanisms consistent with shear faulting. The other 75 per cent have significantly non-DC mechanisms. Many have substantial explosive components, one has a substantial implosive component, and the deviatoric component of many is strongly non-DC. Many of the non-DC mechanisms are consistent, within observational error, with simultaneous tensile and shear faulting. However, the mechanisms occupy a continuum in source-type parameter space and probably at least one additional source process is occurring. This may be fluid flow into newly formed cracks, causing partial compensation of the volumetric component. Studying non-shear earthquakes such as these has great potential for improving our understanding of geothermal processes and earthquake source processes in general.

Geophysical Journal International↗

USGS1997

Non-double-couple earthquake mechanisms at the Hengill-Grensdalur volcanic complex, southwest Iceland

The Hengill-Grensdalur area in Iceland generates frequent small non-double-couple earthquakes with explosive volumetric components. We collected high quality three-component digital recordings of 4,000 earthquakes on a purpose-designed, 32-station network in 1991, and determined focal mechanisms for 100 of the best-recorded earthquakes by inverting amplitude ratios. Many of the mechanisms are consistent, within the errors, with simultaneous shear and tensile faulting, with tensile faults parallel to the local spreading ridge, and shear faulting similar to that in the South Iceland transform-fault zone. Some events cannot be explained by this model, however, and require other processes, such as crack closing and partial compensation of tensile cracks by fluid flow.

Geophysical Research Letters↗

USGS1996

Crustal shear-wave splitting from local earthquakes in the Hengill triple junction, southwest Iceland

The Hengill region in SW Iceland is an unstable ridge-ridge-transform triple junction between an active and a waning segment of the mid-Atlantic spreading center and a transform that is transgressing southward. The triple junction contains active and extinct spreading segments and a widespread geothermal area. We evaluated shear-wave birefringence for locally recorded upper-crustal earthquakes using an array of 30 three-component digital seismographs. Fast-polarization directions, ϕ, are mostly NE to NNE, subparallel to the spreading axis and probably caused by fissures and microcracks related to spreading. However, there is significant variability in ϕ throughout the array. The lag from fast to slow S is not proportional to earthquake depth (ray length), being scattered at all depths. The average wave-speed difference between qS1 and qS2 in the upper 2–5 km of the crust is 2–5%. Our results suggest considerable heterogeneity or strong S scattering.

Geophysical Research Letters↗

USGS1993

Non-shear focal mechanisms of earthquakes at The Geysers, California and Hengill, Iceland, geothermal areas

Several thousand earthquakes were recorded in each area. We report an initial investigation of the focal mechanisms based on P-wave polarities. Distortion by complicated three-dimensional crustal structure was minimized using tomographically derived three-dimensional crustal models. Events with explosive and implosive source mechanisms, suggesting cavity opening and collapse, have been tentatively identified at The Geysers. The new data show that some of these events do not fit the model of tensile cracking accompanied by isotropic pore pressure decreases that was suggested in earlier studies, but that they may instead involve combination of explosive and shear processes. However, the confirmation of earthquakes dominated by explosive components supports the model that the event are caused by crack opening induced by thermal contraction of the heat source.

Conference Paper↗