Earthquake Source Mechanisms Research Articles

Long-Period P Waveforms and the Source Mechanism of Intermediate Earthquakes

Long-period P waveforms have been analyzed to interpret the source mechanism of four intermediate earthquakes with magnitudes of 6.0-6.5 and focal depths between 100 and 200km. The synthetic seismograms appropriate to each recording station have been constructed, to compare with observed records, on the basis of moving dislocation models with various parameters, including the fault length and width, the amount of dislocation, its time dependence and the fracture velocity, taking into account the combined effects of wave propagation in the earth and of a recording instrument. General features of the observed waveforms do not differ greatly from those for a double-couple point source, but the comparison with synthesized waveforms at a number of stations indicated seismic moment of order of 5-9×1026 dyne·cm, and also probable ranges for some other source parameters on tne assumed source. The bounds of the stress-strain drop, the released strain energy, and of efficiency of seismic wave radiation at the source were also discussed.

Seismic moment, stress, and source dimensions for earthquakes in the California-Nevada region

The source mechanism of earthquakes in the California-Nevada region was studied using surface wave analyses, surface displacement observations in the source region, magnitude determinations, and accurate epicenter locations. Fourier analyses of surface waves from thirteen earthquakes in the Parkfield region have yielded the following relationship between seismic moment, M0 and Richter magnitude, ML: log M0 = 1.4 ML + 17.0, where 3 < ML < 6. The following relation between the surface wave envelope parameter AR and seismic moment was obtained: log M0 = log AR300 + 20.1. This relation was used to estimate the seismic moment of 259 additional earthquakes in the western United States. The combined data yield the following relationship between moment and local magnitude: log M0 = 1.7 ML + 15.1, where 3 < ML < 6. These data together with the Gutenberg-Richter energy-magnitude formula suggest that the average stress multiplied by the seismic efficiency is about 7 bars for small earthquakes at Parkfield and in the Imperial Valley, about 30 bars for small earthquakes near Wheeler Ridge on the White Wolf fault, and over 100 bars for small earthquakes in the Arizona-Nevada and Laguna Salada (Baja California) regions. Field observations of displacement associated with eight Parkfield shocks, along with estimates of fault area, indicate that fault dimensions similar to the values found earlier for the Imperial earthquake are the rule rather than the exception for small earthquakes along the San Andreas fault. Stress drops appear to be about 10% of the average stress multiplied by the seismic efficiency. The revised curve for the moment versus magnitude further emphasizes that small earthquakes are not important in strain release and indicate that the zone of shear may be about 6 km in vertical extent for the Imperial Valley and even less for oceanic transform faults.

Elastic radiation from a propagating phase boundary

The elastic effects which occur when a solid undergoes a phase change from a metastable to a stable structure are considered. In such a change, stored energy is released, and some of this energy may appear in elastic form. Equations describing the elastic effects of the process are derived from an examination of internal strain and an assumption of linearity for frequencies in the seismic range. According to concepts presented in this paper, a solid can be strained internally by the presence of dislocations (real internal strain), and also by possessing a potential to deform at every point through a change in crystal structure (frozen internal strain). Equations for elastic effects are derived by using these concepts, and they are solved for plane phase boundaries in steady motion. These results are valid only for times sufficiently short that the linear approximation still applies. The speed of boundary motion cannot be determined from macroscopic classical theory, but the equations have solutions that correspond to two different mechanisms of boundary propagation. The first occurs if a stress pulse is able to induce the transformation. The plane phase boundary then propagates at the speed of sound, with a growing elastic wave tied to it, using up some of the energy released in the transformation. The second propagation mechanism occurs when externally applied stress exerts a force on the real strain field associated with the phase boundary. In this case, the speed of propagation is different from the sound velocity, but the process again involves an increase in elastic energy at the boundary due to the release of internal strain under phase transition. The amount of internal strain release depends on the thermodynamic nature of the process. If the reaction is sudden, strain release is small, and the transformation essentially diffusionless, the initial process is considered to occur at constant entropy. If the reaction continues, it becomes non-linear and the entropy is expected to increase. The non-linear problem is not treated in this paper. An estimate of energy release for the linear problem is given for the purpose of considering the propagating phase boundary as a possible earthquake source mechanism.

Note on the aseismicity of Antarctica

Abstract Compared with the other continents Antarctica is remarkably free from earthquakes, especially in view of the active volcanism and other signs of instability. On the hypothesis that earthquake activity is affected by variations in the thickness of the ice-cap, any future earthquake of even moderate magnitude may be expected to yield useful information about the source mechanism of earthquakes and also about ice-cap dynamics.

Source Mechanism of Volcanic Earthquake of the Volcano Aso

Source Mechanism of Volcanic Earthquake of the Volcano Aso

Radiation pattern of mantle Rayleigh waves and the source mechanism of the Hindu Kush earthquake of July 6, 1962

abstract The source mechanism of the Hindu Kush earthquake of July 6, 1962 (magnitude 634-7, focal depth 218 km) was studied by comparing the observed amplitude and phase radiation patterns of mantle Rayleigh waves of 150 sec and 200 sec period with theoretical radiation patterns of Rayleigh waves from single- and double-couple point sources, and by considering evidence from Love waves and the shape of P and S pulses. The solution for the source mechanism, which is consistent with all the body wave and surface wave data available for this earthquake, is a double couple acting as a step function in time, with nodal planes oriented as determined from P wave data. Since for waves with periods greater than about 5 sec, the source appears to be an ideal point source, the radius of the equivalent source volume is estimated to be less than 10 km. For Rayleigh waves of 150 sec period, the agreement between observed and theoretical phases (for the above source model) is greatly improved by assuming regional phase velocities instead of a uniform phase velocity for all areas. It is concluded that with the accuracy currently attainable, a study of Rayleigh waves alone cannot determine the source mechanism of an earthquake uniquely.

On the general solution of elastic wave equations in a homogeneous medium with applications to the study of earthquake source mechanisms

Abstract The elastic wave equation in an isotropic homogeneous semi-infinite elastic medium has been transformed by a combination of Fourier and Laplace-transforms and the surface displacements due to general force-system in three mutually perpendicular directions have been directly obtained. By suitably specifying the force-system, the surface displacements for all possible types of point sources so far used, have been calculated, obtaining in particular, the results previously derived by Haskell for a dipolar point source of arbitrary direction. It has been shown that now the surface displacement for finite size of the source can be calculated for both body and surface waves. Expressions for the displacements at any point of the medium have been derived, giving in particular the displacements associated with incident wave in the medium, separately.

An application of statistical discrimination to classify seismic events

Abstract A basic statistical method is presented for the classification of observations into one of two multivariate, normal populations. This theory is applied to the identification of seismic events by considering one of the “populations” to consist of measurements on seismic recordings of earthquakes, the other of explosions. The measurements, or parameters, consist of ratios of the “energies” contained within predetermined “velocity windows” on the seismograms. The choice of velocity windows is guided by the assumption that earthquake source mechanism is extended both in time and space and generates a larger fraction of energy in shear waves as compared to explosion source mechanism. The best separation of twenty earthquakes and twenty-seven explosions is achieved when only seven of the nine ratios calculated are used in the statistical discriminator. Based on this data we have about 85% probability of correctly classifying a given event either as an explosion or as an earthquake.

Earthquake Source Mechanisms

Earthquake Source Mechanisms

Seismic model studies of first motions produced by an actual fault

In an effort to discover the expectable source mechanism of earthquakes due to faulting in the earth, two-dimensional seismic models were investigated. The models were constructed with faults whose surfaces are locked together in the rest position. There was a provision for straining the fault region until the locking strength was overcome, allowing a sudden, spontaneous release of the accumulated strain energy and consequent generation of elastic waves. A number of models made of Plexiglas and of steel plus Plexiglas were constructed with different-length faults, ranging from a small fraction of the P wavelength to a fault length equal to the P wavelength. The amplitudes of the first motions of P and S waves were recorded and plotted. The amplitude distributions from all models approximated that of the double-couple source mechanism and were in good agreement with Knopoff and Gilbert's dislocation theory of first motions. The effects of a theoretical shear-strain dislocation could not be detected in the amplitude distributions even though theory suggested that the steel and Plexiglas models should have yielded the amplitude patterns of shear-strain dislocation.