Variable slip mode in the past 3300 years on the fault ruptured in the 2012 M 5.6 Pernik slow earthquake in Bulgaria

The 2012 M5.6 Pernik earthquake in Bulgaria proceeded at slow slip rates and was accompanied with ground failure along the Meshtitsa fault scarp. Our investigation through paleoseismological trenching techniques and electrical resistivity tomography discovered a broad zone with multiple fault cores. In a trench, a 40-m-thick montmorillonite clay stratum is embedded in coarse-grained alluvial deposits along with two narrow gouge zones; together they demonstrate a frictional heterogeneity within the fault zone. The clayey deposits had experienced frictional stability which is recorded in intersecting shear bands interpreted to have formed at slow strain rates. A steep bedding of Oligocene alluvial deposits is interpreted as a result from an earlier phase of strike-slip motion. Since transitioning to normal dip-slip motion in the late Miocene, two gouge zones located at the periphery of the clayey deposits suggest strain localization during surface-rupturing earthquakes. In alluvial sediments deposited 3300 cal BP, localized slip on one of the faults and dispersed tensile cracks in the hangingwall of the other fault likely express failures at different strain rates. We infer that it is likely that the dispersed cracks in the trench, and similarly some of the 2012 ground cracks, resulted from afterslip, which followed ruptures at depth on relatively small seismically coupled fault areas. In contrast, we interpret the slip localized in the fault cores to have occurred when most of fault area was seismically coupled in larger earthquakes. This fault expresses a variability in earthquake sizes and seismic coupling in the past 3300 cal BP.