Insights on the rheology of the transition zone and its along-dip variation using low-frequency earthquakes clustering

Active faults exhibit a broad spectrum of slip modes, primarily governed by depth-dependent pressure and temperature conditions. These transitions manifest as fast earthquake ruptures at shallow, seismogenic depths, gradually evolving into transient slow slip and steady creep with increasing depth. In most subduction zones, the transition zone—situated between the updip seismogenic zone and the downdip steadily creeping region—is the locus of slow slip events, tectonic tremors, and Low-Frequency Earthquakes (LFEs). Within this transition zone, the recurrence patterns of tremors and LFEs display depth-dependent variations. Recurrence times decrease with depth, transitioning from low-recurrence, long-lasting bursts near the seismogenic zone to high-recurrence, short-duration bursts near the steadily creeping limit (e.g. Wech & Creager, 2011).

In this study, we perform a comparative analysis across multiple plate boundaries, focusing on the along-dip spatio-temporal clustering of tremors and LFEs in Cascadia, Nankai, Mexico, and the San Andreas Fault. We developed a robust method to systematically analyze LFE catalogs from Mexico (Frank et al., 2014), Nankai (Kato et al., 2020), Cascadia (Sweet et al., 2019), and Parkfield (Shelly, 2017).

Our method consists in examining the autocorrelation function of LFE occurrence time series to estimate the periodicity and duration of LFE bursts at various depths. Across all studied regions, we observe a consistent trend: recurrence intervals and burst durations of LFE activity decrease with increasing depth. Finally, we connect these depth-dependent behaviors to the thermodynamic conditions specific to each region, and to the plate convergence rates, providing insights into the rheological properties governing LFE activity within the transition zone.

References

Wech, A.G., Creager, K.C.: A continuum of stress, strength and slip in the Cascadia subduction zone. Nature Geoscience 4(9), 624–628 (2011) https://doi.org/10.1038/ngeo1215

Frank, W.B., Shapiro, N.M.: Automatic detection of low-frequency earthquakes (LFEs) based on a beamformed network response. Geophysical Journal International 197(2), 1215–1223 (2014) https://doi.org/10.1093/gji/ggu058

Kato, A., Nakagawa, S.: Detection of deep low-frequency earthquakes in the Nankai subduction zone over 11 years using a matched filter technique. Earth, Planets and Space 72(1), 128 (2020) https://doi.org/10.1186/s40623-020-01257-4

Shelly, D.R.: A 15 year catalog of more than 1 million low-frequency earthquakes: Tracking tremor and slip along the deep san andreas fault. Journal of Geophysical Research: Solid Earth 122(5), 3739–3753 (2017) https://doi.org/10.1002/2017JB014047