Seismic Moment tensors of western U.S. earthquakes and implications for the tectonic stress field

Patton, Howard J.; Zandt, G.

doi:10.1029/91jb01838

Cited by 56 publications

(23 citation statements)

References 24 publications

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“…An ML 3.5 earthquake on May 14, 1981, at the Gentry Mountain mining region, Utah, had unusually small surface waves with spectra similar to those of explosions in the area [Taylor, 1994]. Surface wave amplitudes and initial phases require a volume decrease [Patton and Zandt, 1991…”

Section: Earthquakes At Minesmentioning

confidence: 95%

Non‐double‐couple earthquakes 2. Observations

Miller¹,

Foulger²,

Julian³

1998

Reviews of Geophysics

191

106

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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...

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Section: Earthquakes At Minesmentioning

confidence: 95%

Non‐double‐couple earthquakes 2. Observations

Miller¹,

Foulger²,

Julian³

1998

Reviews of Geophysics

191

106

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“…The isotropic moment of the smallest explosion was estimated from near-field data by Stump et al [unpublished data, 1992]. For the earthquakes, we use the deviatoric moments from the surface wave study of Patton and Zandt [1991]. A plot of Mo versus mt•(Pn) for both earthquakes and explosions is shown in Figure la.…”

Section: Regional Mo:mb Observationsmentioning

confidence: 99%

Regional moment:magnitude relations for earthquakes and explosions

Patton

Walter

1993

Geophysical Research Letters

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We present Mo:mb relations using mb(Pn) and mb(Lg) for earthquakes and explosions occurring in tectonic and stable areas. The observations for mb(Pn) range from about 3 to 6 and show excellent separation between earthquakes and explosions on Mo:mb plots, independent of the magnitude. The scatter in Mo:mbobservations for NTS explosions is small compared to the earthquake data. The Mo:mb(Lg) data for Soviet explosions overlay the observations for U. S. explosions. These results, and the small scatter for NTS explosions, suggest weak dependence of Mo:mb relations on emplacement media. A simple theoretical model is developed which matches all these observations. The model uses scaling similarity and conservation of energy to provide a physical link between seismic moment and a broadband seismic magnitude. Three factors, radiation pattern, material property, and apparent stress, contribute to the separation between earthquakes and explosions. This theoretical separation is independent of broadband magnitude. For U.S. explosions in different media, the material property and apparent stress contributions are shown to compensate for one another, supporting the observations that Mo:mb is nearly independent of source geology.

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“…Intermediate‐period surface waves exhibit high signal‐to‐noise ratios, especially at local and regional distances (e.g. Fukushima et al 1989; Patton & Zandt 1991; Ritsema & Lay 1993; Romanowicz 1995; Thio & Kanamori 1995). Until recently these waves have typically been excluded from source studies, as their propagation characteristics were poorly predicted by the available upper mantle and crustal velocity models.…”

Section: Introductionmentioning

confidence: 99%

Shallow seismic source parameter determination using intermediate-period surface wave amplitude spectra

Fox

Selby²,

Heyburn³

et al. 2012

Geophysical Journal International

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SUMMARY Estimating reliable depths for shallow seismic sources is important in both seismo‐tectonic studies and in seismic discrimination studies. Surface wave excitation is sensitive to source depth, especially at intermediate and short‐periods, owing to the approximate exponential decay of surface wave displacements with depth. A new method is presented here to retrieve earthquake source parameters from regional and teleseismic intermediate period (100–15 s) fundamental‐mode surface wave recordings. This method makes use of advances in mapping global dispersion to allow higher frequency surface wave recordings at regional and teleseismic distances to be used with more confidence than in previous studies and hence improve the resolution of depth estimates. Synthetic amplitude spectra are generated using surface wave theory combined with a great circle path approximation, and a grid of double–couple sources are compared with the data. Source parameters producing the best‐fitting amplitude spectra are identified by minimizing the least‐squares misfit in logarithmic amplitude space. The F‐test is used to search the solution space for statistically acceptable parameters and the ranges of these variables are used to place constraints on the best‐fitting source. Estimates of focal mechanism, depth and scalar seismic moment are determined for 20 small to moderate sized (4.3 ≤Mw≤ 6.4) earthquakes. These earthquakes are situated across a wide range of geographic and tectonic locations and describe a range of faulting styles over the depth range 4–29 km. For the larger earthquakes, comparisons with other studies are favourable, however existing source determination procedures, such as the CMT technique, cannot be performed for the smaller events. By reducing the magnitude threshold at which robust source parameters can be determined, the accuracy, especially at shallow depths, of seismo‐tectonic studies, seismic hazard assessments, and seismic discrimination investigations can be improved by the application of this methodology.

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Seismic Moment tensors of western U.S. earthquakes and implications for the tectonic stress field

Cited by 56 publications

References 24 publications

Non‐double‐couple earthquakes 2. Observations

Non‐double‐couple earthquakes 2. Observations

Regional moment:magnitude relations for earthquakes and explosions

Shallow seismic source parameter determination using intermediate-period surface wave amplitude spectra

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