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ETAS With Anisotropy in the Spatial Distribution of Aftershocks

Description: 

In the standard Epidemic-Type Aftershock Sequence (ETAS) model, the spatial aftershock density depends on the distance to the triggering event but does not incorporate directional information. However, well-located aftershocks generally exhibit directional patterns, primarily occurring along fault lines. For specific faults, detailed fault geometries and stress patterns have been used to refine aftershock forecasts (Bach and Hainzl, 2012; Field et al., 2017; Savran et al., 2020). However, in contexts where fault-specific information is unavailable or where data spans multiple fault systems, we aim to adopt a purely data-driven approach. We modify the ETAS formulation and the associated parameter inversion algorithm by incorporating an elliptical model that better captures the directional nature of aftershock triggering. In the expectation step of the Expectation-Maximization (EM) procedure, each triggering event is assigned an ellipse that best describes its aftershocks’ locations. This adaptation influences the optimized triggering function resulting from each maximization step, as it depends on the evolving properties of the ellipses. Overall, our method yields a self-consistent estimation of ETAS parameters and event-wise aftershock ellipses.

Aiming to improve earthquake forecasting models for Europe, we apply this method to the ESHM20 dataset (Danciu et al., 2021), and compare forecasts, but also parameter trends (e.g. productivity law), to models for the same area which assume an isotropic spatial kernel. Preliminary retrospective tests demonstrate a positive information gain of the elliptical model versus its isotropic counterpart. We further evaluate the model in a pseudoprospective framework and test it across different regions, comparing the results against synthetic best-case scenarios. Our goal is to investigate the best approach to apply this modified aftershock modelling to real-time forecasting, resulting in more precise forecasts of spatial distribution and productivity of events: this is especially important during sequences with large-magnitude mainshocks and potentially destructive impact.

Session: Improving the State of the Art of Earthquake Forecasting Through Models, Testing and Communication - I

Type: Oral

Date: 4/15/2025

Presentation Time: 02:00 PM (local time)

Presenting Author: Marta

Student Presenter: Yes

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