Joint resolving of the fault plane ambiguity and anisotropic earthquake
triggering in Southern California
Abstract
The inversion of the focal mechanism (FM) provides an estimate of the
fault plane orientation and the direction of slip of an earthquake,
giving us valuable insights into the mechanical processes involved in
the occurrence of an earthquake. Given the recorded first motion
polarities at a set of stations, there are always two possible planes
that explain the observations equally well. This so-called fault plane
ambiguity is often resolved based on expert judgment, considering
knowledge about the local geology and the locations of fore- or
aftershocks. With seismic networks and inversion algorithms continuously
improving, we can obtain large numbers of inverted FMs, even for events
of low magnitudes, which calls for an automated procedure to resolve the
fault plane ambiguity. Using an enhanced epidemic-type aftershock
sequence (ETAS) model, we jointly invert the plausibility of each of the
two fault planes specified by the inverted FM and a magnitude-dependent
shape of elliptic aftershock triggering oriented in the direction of
strike, based on FMs of M≥2.5 earthquakes in Southern California since
1981. Results of this inversion do not only provide an approach to
resolve fault plane ambiguity but also an ETAS model which goes beyond
the common assumption of spatially isotropic triggering. Preliminary
results suggest that aftershocks occur predominantly in strike direction
relative to their triggering events and that the shape of the ellipse
describing this behaviour is magnitude-independent. We conduct
pseudo-prospective forecasting experiments to compare our novel
anisotropic ETAS model based on fault plane plausibility estimates to
the current state-of-the-art isotropic ETAS model to test the utility of
understanding source anisotropy for earthquake forecasting.