A document by Vanille Ritz. Click on the document to view its contents.

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.

We propose two new methods to calibrate the parameters of the epidemic-type aftershock sequence (ETAS) model based on expectation maximization (EM) while accounting for temporal variation of catalog completeness. The first method allows for model calibration on earthquake catalogs with long history, featuring temporal variation of the magnitude of completeness, mc. This extended calibration technique is beneficial for long-term probabilistic seismic hazard assessment (PSHA), which is often based on a mixture of instrumental and historical catalogs. The second method jointly estimates ETAS parameters and high-frequency detection incompleteness to address the potential biases in parameter calibration due to short-term aftershock incompleteness. For this, we generalize the concept of completeness magnitude and consider a rate- and magnitude-dependent detection probability – embracing incompleteness instead of avoiding it. Using synthetic tests, we show that both methods can accurately invert the parameters of simulated catalogs. We then use them to estimate ETAS parameters for California using the earthquake catalog since 1932. To explore how the newly gained information from the second method affects earthquakes’ predictability, we conduct pseudo-prospective forecasting experiments for California. Our proposed model significantly outperforms the base ETAS model, and we find that the ability to include small earthquakes for simulation of future scenarios is the main driver of the improvement. Our results point towards a preference of earthquakes to trigger similarly sized aftershocks, which has potentially major implications for our understanding of earthquake interaction mechanisms and for the future of seismicity forecasting.