Finite source analysis of small earthquakes using the fault isochrone back-projection method: examples from the Alto Tiberina fault.

Small earthquakes are frequently modeled as point sources or as ruptures with a simple circular geometry. While these representations are sufficiently accurate within a specific range of frequencies, the actual rupture processes of these earthquakes are inherently more complex.

Earthquake waveforms represent a convolution of source and propagation effects, requiring their separation to enable independent analysis of each component. To investigate the earthquake source, isolating the source time function is crucial. Kinematic rupture models are commonly constructed using Theoretical Green's Functions (TGFs), which rely on simplified one-dimensional (1-D) velocity models that incorporate anelastic attenuation and wave propagation. However, for small earthquakes, this method requires highly detailed structural models, which are often unavailable.

An alternative approach utilizes deconvolution with the Empirical Green’s Function (EGF), obtained from a smaller, co-located event recorded by the same instruments. In this study, we employed the EGF method to extract the source function for small earthquakes (Mw ~3.5) that occurred in the Alto Tiberina fault area. The Landweber deconvolution technique (Bertero et al., 1998) was applied, with a semi-automated selection of parameters, including the signal window and the maximum duration of the apparent source time function (ASTF), the latter based on the methodology proposed by Meng et al. (2020). When automated selection was not possible, we performed a parametric analysis to map the uncertainty on the final results corresponding to the different choice of possible parameters.

Additionally, we used the fault isochrone back-projection method outlined in Király-Proag et al. (2019) to investigate the kinematic source process of these events.

The findings show that this approach allows resolving finite fault properties and rupture directivity of small earthquakes, along with their related uncertainty.