A proposal for the physical modeling of ghost geodetic transients

Faults showcase a wide range of possible slip behaviours. After the occurrence of a large earthquake, once the coseismic offset takes place, the slip rates keep high for months or even years as a response to stress redistribution. An exponential fit (or, equivalently, a logarithmic one) is usually performed to reproduce geodetic measurements during the post-seismic phase (Sobrero et al., 2020). However, compelling evidence has been provided recently that the fault slip cycle is more complicated than previously avowed (e.g., Hammond et al., 2021). Long-term post-seismic attenuations have been detected and they are not compatible with classical viscous relaxations (Hearn et al., 2013; Carafa et al., 2024). Here, we would like to face this issue and try to propose a possible physical explanation. We show that in the low-strain regime, i.e., after several weeks after the mainshock, the structural and rheological heterogeneities determine a dynamic slowing-down of the mechanical stabilization of the system. We derive a possible analytical solution for the postseismics also valid in the long run. The result is a long-term “ghost contribution” to geodetic measurements that can last for decades and affecting wide regions. We apply our model to available recent and past GNSS recordings both in Italy and in other countries after appropriate pre-processing to remove spurious and short-term components of geodetic data series. References Carafa M. M., Bird P., Verdecchia A., Taroni M. and Doglioni C.; 2024: Empirical evidence for multi-decadal transients affecting geodetic velocity fields and derived seismicity forecasts in Italy. Sci. Rep., 14(1), 19941. Hammond W. C., Blewitt G., Kreemer C., Koehler R. D. and Dee S.; 2021: Geodetic observation of seismic cycles before, during, and after the 2020 Monte Cristo Range, Nevada earthquake. Seismol. Res. Lett., 92(2A), 647-662. Hearn E. H., Pollitz F. F., Thatcher W. R. and Onishi C. T.; 2013: How do “ghost transients” from past earthquakes affect GPS slip rate estimates on southern California faults? Geochem. Geophys. Geosyst., 14(4), 828-838. Sobrero F. S., Bevis M., Gómez D. D. and Wang F.; 2020: Logarithmic and exponential transients in GNSS trajectory models as indicators of dominant processes in postseismic deformation. J. Geod., 94(9), 84.