Non‐double‐couple earthquakes 2. Observations

Abstract

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...

Non-double-couple microearthquakes at Long Valley caldera, California, provide evidence for hydraulic fracturing

“…Furthermore, similar observations, including moment tensors with large volumetric components, have since been obtained from dense seismic networks at volcanic and geothermal areas in many other parts of the world (e.g. Arnott and Foulger, 1994;Console and Rosini, 1998;Foulger, 1988;Foulger and Long, 1984;Foulger et al, 1989;Hung and Forsyth, 1996;Kubas and Sipkin, 1987;Miller et al, 1998a;Sarao ' et al, 2001;Shimizu et al, 1988;Vavrycuk, 2002).…”

“…These were the M w 5.5 Wheeler Crest earthquake of 4 October 1978 (Ekstro « m andDziewonski, 1983) and two of the four M w V6 earthquakes of 25 and 27 May 1980 (Barker and Langston, 1983;Julian, 1983;Julian and Sipkin, 1985). Since that time, non-DC microearthquakes have been identi¢ed at many other volcanic areas (Miller et al, 1998a), but the physical processes that cause them remain imperfectly understood, and it is not clear even that non-DC events of widely di¡ering magnitudes are genetically similar. Julian (1983) and Julian and Sipkin (1985) suggested that the large 1978 and 1980 Long Valley earthquakes involved tensile failure caused by £uid pressure, but the seismic data available for these events could not resolve the volume changes that would accompany tensile faulting, so alternative explanations such as simultaneous slip on di¡erently oriented shear faults have remained tenable.…”

A Source Model for Induced Earthquakes at the Geysers Geothermal Reservoir

“…In addition, MILLER et al (1998) summarize results for several types of earthquakes, including those in geothermal areas, where the observational data indicate a source mechanism that is fundamentally different from a simple shear failure.…”

The 1998 Earthquake Sequence South of Long Valley Caldera, California: Hints of Magmatic Involvement

“…The evidence, however, remains ambiguous. A non-double-couple component was inferred for two of the largest events within the 1980 sequence, events that occurred within the SNB (Julian and Sipkin, 1985;Miller et al, 1998). However, the volumetric component of these events was unresolvably small , leaving open the possibility that the apparent non-double-couple component reflected complex shear faulting (Wallace, 1985).…”