Classical focal mechanism techniques for body waves

Abstract

Abstract. The initial motion of primary body waves and polarization directions of secondary body waves have been applied successfully to the study of the mechanism operating at the earthquake focus. Equal area plots of these body wave characteristics resulted in radiation patterns that were compared to patterns due to theoretical focal mechanism sources. Such an approach indicated that a double couple force is the source mechanism operating at earthquake foci. This can be physically represented by faulting at the focus. This seems likely because of the relation between earthquakes and fault motion in shallow earthquakes. The possibility of other mechanisms operating in deep focus earthquakes has not been ruled out.The technique of solving for fault plane orientation/motion in equal area plots of P-wave first motion and S-wave polarization, has been a powerful tool in areas where fault motions cannot be directly observed in studying tectonics. Such an approach has been used to test the theory of global tectonics. This approach has resulted in the confirmation of the suggested mechanics of plate motion, and the results of plate motion, such as spreading of the sea floor from mid-oceanic ridges and underthrusting of lithospheric plates at the sites of oceanic trenches.

Intraplate earthquakes and the state of stress in oceanic lithosphere

“…The classical method of inferring teleseismic earthquake source mechanisms from P-wave first motions [e.g., Brumbaugh, 1979] [Bergman and Solomon, 1980]. In particular, the fault strike of the ubiquitous intraplate thrust fault event is often completely undetermined by the first motion data, since at teleseismic distances every station records a compressional arrival [e.g., Sykes and Sbar, 1974].…”

Focal Mechanism Determination of Event S1222a and Implications for Tectonics Near the Dichotomy Boundary in Southern Elysium Planitia, Mars

“…On Earth, moment tensors are routinely calculated following significant seismic events (e.g., Dziewonski & Woodhouse, 1983) and have played a crucial role in understanding global patterns of seismicity as well as deformation across tectonic settings (e.g., Ekström et al, 2012). Techniques to constrain source characteristics of teleseismic events typically rely on data from globally distributed seismic stations, and optimal focal mechanisms are found either by fitting body wave first motions (e.g., Brumbaugh, 1979) or by fitting waveforms of intermediate period body wave (Langston & Helmberger, 1975) or surface wave phases (e.g., Arvidsson & Ekström, 1998), or complete long period waveforms (e.g., Dziewonski et al, 1981). Information about source depth comes from the lag-time of depth phases (e.g., Basham & Ellis, 1969) and/or by the frequency dependence of surface wave excitation (e.g., Tsai & Aki, 1970).On Mars, moment tensor inversions are challenging for several reasons: (a) only a single seismic station is available, limiting the sampling of the focal sphere; (b) depth phases are only rarely unequivocally identified, rendering source depth estimates inaccurate; (c) contribution of along-path structure to waveforms is difficult to estimate…”