Insight on along-dip fault transition zone rheology through LFE clustering

Active faults present a wide spectrum of slip behaviors, from fast earthquakes, transient slow slip events to steady creep. Although each fault has its own specificities, the distribution of these behaviors is dictated by the evolution of depth-dependent pressure and temperature conditions to first order. In most of the subduction zones, transient slow slip events, accompanied by tectonic tremors and Low Frequency Earthquakes (LFEs), are located in the transition zone, between the updip seismogenic zone and the downdip steadily creeping zone. 

When present, tremors and LFEs are unique tools to monitor in detail the slip behavior in the transition zone. Previous studies analyzed the spatio-temporal clustering of tremors and LFEs (Wech et Creager, 2011, Rubin et Armbruster, 2013, Frank et al., 2013), and revealed that the properties of tremors/LFE bursts evolve with increasing depth: long recurrence intervals and long-lasting bursts occur close to the seismogenic zone while short recurrence intervals and short-lasting bursts happen deeper, close to the continuously creeping zone. These observations were made in various regions and tectonic contexts, including Cascadia (Wech and Creager, 2011), Nankai (Obara et al., 2011) and Mexico (Frank et al., 2013) for subduction zones, and the San Andreas strike-slip fault (Shelly et al., 2017).

In this study, we aim to gain insight on the rheological properties of the transition zone by a systematic analysis of the LFEs clustering properties in different regions and tectonic contexts. To do so, we analyze with similar methods LFE catalogs obtained with template matching, for Mexico (Frank et al., 2013), Nankai (Kato et al., 2020), Cascadia (Sweet et al., 2019) and Parkfield (Shelly et al., 2017).

We analyze the clustering properties of the LFE families (asperities), by computing the auto-correlation spectra of LFE catalogs for single families, and stacking them for bins along depth. This spectrum shows the density of recurrence of LFE bursts, and we observe a clear variation of maximum recurrence intervals between 100 and 10 days along depth for all regions. Additionally, we calculated and compared LFE burst recurrences using the cumulative LFE time series derivative, from which we retrieved prominent bursts, and using other clustering methods such as DBSCAN. Finally, we compile the results from different regions and compare them with the along depth pressure-temperature  conditions of these faults (Behr and Bürgmann, 2021) to have a rheological insight on these observations made using LFEs.