Dome growth at the Soufriere Hills volcano (1996 to 1998) was frequently accompanied by repetitive cycles of earthquakes, ground deformation, degassing, and explosive eruptions. The cycles reflected unsteady conduit flow of volatile-charged magma resulting from gas exsolution, rheological stiffening, and pressurization. The cycles, over hours to days, initiated when degassed stiff magma retarded flow in the upper conduit. Conduit pressure built with gas exsolution, causing shallow seismicity and edifice inflation. Magma and gas were then expelled and the edifice deflated. The repeat time-scale is controlled by magma ascent rates, degassing, and microlite crystallization kinetics. Cyclic behavior allows short-term forecasting of timing, and of eruption style related to explosivity potential.

Abstract. 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. fault and the equivalent distribution of DC force systems. It also shows the radiation patterns of seismic body waves from a single DC, to which a fault is equivalent in the point source (long wavelength) approximation. For compressional waves, this pattern consists of four symmetrical lobes of alternating polarity. Compressional wave amplitudes vanish in the fault plane and also in an "auxiliary plane" perpendicular to it. The shear wave radiation pattern is also symmetric with respect to these two planes, as are the entire static and dynamic displacement fields, so the fault plane cannot be identified from seismic or geodetic data in the point source approximation.During the early years of seismology, some theories attributed earthquakes to processes other than shear faulting. Ishimoto [1932] in particular, thought that earthquakes resulted from subterranean magma motion, and modeled this process using force systems that produce conical, rather than planar, nodal surfaces for compressional waves. In recent decades, however, the model of an earthquake as a DC force system has underlain most quantitative analysis of seismic waves and has been highly successful in enabling seismologists to use earthquakes to advance our understanding of tectonic processes [e.g., Sykes, 19...

Abstract. Telemetered high-resolution tiltmeters were installed in Montserrat in summer of 1995, in December 1996, and in May 1997. The 1995 installations, several km from the Soufriere Hills vent, were too distant to yield useful data. However, the 1996 and 1997 installations on the crater rim revealed 6-14 h inflation cycles caused by magma pressurization at shallow depths (< 0.6 km below the base of dome). The tilt data correlated with seismicity, explosions, and pyroclastic flow activity, and were used to forecast times of increased volcanic hazard to protect scientific field workers and the general public.

Abstract. Most studies assume that earthquakes have double-couple (DC) source mechanisms, corresponding to shear motion on planar faults. However, many wellrecorded 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. INTRODUCTIONA large body of evidence connecting earthquakes with faulting comes both from field observations and from the study of seismic waves. In theory, the compressional waves radiated by a shear fault have a four-lobed pattern, with adjacent lobes alternating in polarity. This pattern, and the entire static and dynamic field of a shear fault in an isotropic medium, is identical to that produced in an unfaulted medium by a distribution over the fault surface of pairs of force couples, with each pair arranged so that its net torque vanishes. Seismologists usually specify earthquake mechanisms in terms of equivalent force systems, and shear-fault mechanisms are called "double couples" (DCs).The hypothesis that earthquake source mechanisms are DCs has become so widely accepted as to have been treated almost as a fundamental law by many seismologists. To a large extent, however, the success of the DC model has been a consequence of limitations in data quantity and quality. Recent improvements in seismological instrumentation and analysis techniques now identify earthquakes whose radiated waves are beyond•Also at U.S. Geological Survey, Menlo Park, California. doubt incompatible with DC force systems and thus with shear faulting. Well-constrained non-DC earthquakes have been observed in many environments, including particularly volcanic and geothermal areas, mines, and deep subduction zones. A companion paper [Julian et al., this issue, hereinafter referred to as paper 1] summarizes seismic source theory and...

The Montserrat Volcano Observatory (MVO) is a statutory body of the Government of Montserrat and is the organization responsible for volcano monitoring operations on the island. It was formed shortly after the first phreatic explosions from Soufri6re Hills Volcano occurred on 18 July 1995, and evolved from a hastily created, interim entity to a fully established volcano monitoring operation. Participating scientific teams have been drawn mainly from the Seismic Research Unit of the University of the West Indies, the US Geological Survey, the British Geological Survey and universities from various countries including the USA, UK, France and Puerto Rico. Despite its hurried inception, the MVO has been able to provide timely, high quality hazard advice to the civil authorities and has maintained an exceptional documentary record of all scientific aspects of the eruption. Its public education and information efforts have been extensive and there have been unusually high levels of interaction between scientists and the civil authorities, and between scientists and the public, both within Montserrat and outside in the wider world. The experience of setting up and running the MVO, under difficult and stressful conditions, has exemplified the advantages of teamwork and flexibility within monitoring operations and the benefits of openness and clarity in public interactions. Novel techniques have been applied to the appraisal of hazards and advances in scientific understanding have proved invaluable for risk assessment and management.