The shallow structure of a surface-rupturing fault in unconsolidated deposits from multi-scale electrical resistivity data: The 30 October 2016 Mw 6.5 central Italy earthquake case study

Abstract

We report the results of a shallow electrical resistivity investigation performed across a normal fault that ruptured the surface displacing with average ~0.05m vertical offset alluvial fan deposits (<23kyr old) within an intermontane fault-bounded basin following the 30 October 2016 Mw 6.5 earthquake in central Italy. We adopted a multi-scale geophysical approach, by acquiring three 2-D electrical resistivity tomography (ERT) profiles centred on the coseismic ruptures, and characterized by different spatial resolution and investigation depth. Below the fault scarp, the ERT models show a narrow (~10m wide) and steeply-dipping moderately-resistive region (100–150Ωm), which we interpret as the electrical response of the fault zone displacing layers of relatively high-resistivity (300–700Ωm) values. We explain the electrical signature of the retrieved fault zone as due to an increment of permeability caused by coseismic fracturing, and to the subsequent water migration from adjacent shallow aquifers squeezed by compaction induced by seismic waves. By using a statistically-based classification of electrical units, we estimate that the shallowest alluvial fan layer is affected by 2.7±0.9m vertical offset, which is consistent with the measured 2.3–2.8m morphologic offset of the top fan surface, and suggesting a post-12 kyr throw-rate of 0.23±0.08mm/yr. Similarly, we evaluate a post-23 kyr throw of 5.1±1.7m, indicating a Late Pleistocene throw-rate of 0.22±0.07mm/yr, in accordance with available paleoseismic data. We further hypothesize a minimal total fault throw >30m, which likely accrued since the Middle Pleistocene (possibly in the last 350–500kyr). The investigated fault structure is therefore an important splay characterized by a thick and highly permeable damage zone in unconsolidated deposits, and which ruptured the surface during several tens of strong (M>6) earthquakes.