Source mechanism of the deep Colombian earthquake of 1970 July 31 from the free oscillation data

Abstract

The method propos$d by Mendiguren to determine the source parameters from free oscillation data is applied to the 1970 July 31 deep Colombian earthquake. The results indicate a source propagating horizontally for about 150 km along the lithosphere and cutting across its width. The slab behaves as a guide for source propagation. The horizontal propagation velocity is determined as 3.8 km/s. The intensity of the source grew proportionately to the second power of the propagation distance. This rate of source intensity growth may be interpreted either by a fan-shaped fault model or by a cone-shaped volume source. The average slip and stress drop are estimated as 360 cm and 300 bar for the fault model. For the volume source model the transformational shear strain and stress are estimated as 11 x lo-' and 160 bar. There is no evidence of a double couple radiation preceding the P origin time. It is shown that the isotropic and deviatoric components of the moment tensor cannot be uniquely resolved when only observations of a single mode are available. It is observed that, from a statistical basis, the available oSn data for Colombian shock can be equally well explained by a pure deviatoric source model or by a source model including an isotropic component. Numerical experiments indicate that the inclusion of higher mode data does not change this situation. But, on the other hand, numerical experiments show that the available data and the scheme used for the inversion would not result in a solution including an artificial implosive component if the actual source were pure deviatoric. If the departure from a pure deviatoric source is produced by noise, it hasto be non-random, as it could be produced by lateral heterogeneities not included in the inversion scheme.

Determination of the isotropic component of deep focus earthquakes by inversion of normal-mode data

“…As has been suggested by previous studies (Mendiguren & Aki 1978;Kawakatsu 1991Kawakatsu , 1996, the difficulty in identifying the isotropic component primarily comes from the fact that the radiation of seismic waves due to the isotropic component is very similar to that due to the vertical CLVD component (Randall & Knopoff 1970). There are two data sets that make it possible to resolve these components.…”

Focal depths and moment tensor representations of shallow earthquakes associated with the Great Sumba Earthquake

“…The ill-conditioned nature of the inner product matrix results from (1) The resolution of M• is avoided by setting the trace of th e moment tensor to zero: a constraint that is linear and therefore preserves the linearity of the inversion for the moment tensor [Mendiguren and Aki, 1978]. A zero trace implies no monopole component in the source, which may be reasonable for earthquakes but surely is inapplicable for underground explosions [North and Fitch, 1981].…”

Observability of the isotropic component of a moment tensor

“…Since then, many researchers have attempted to either prove or disprove their result (e.g. Geller 1974;Hart & Kanamori 1975;Kennett & Simons 1976;Mendiguren & Aki 1978;Okal & Geller 1979), but no definitive conclusion seems to have emerged. More recent attempts to observe the isotropic component of deep earthquakes do not yet seem to have ended this debate (e.g.…”