Paleoearthquake reconstruction on an impure limestone normal fault scarp, Greece

Reliable reconstructions of paleoseismicity are useful for understanding, and mitigating, seismic hazard risks. In this study, we apply ³⁶Cl exposure-age dating and measurement of concentrations of rare-earth elements and yttrium (REY) to unravelling the paleoseismic history of the Sparta fault, Greece, which is a range-bounding normal fault developed in limestone. From our ³⁶Cl data and ³⁶Cl data we re-modelled from a previously published study we can reasonably constrain two earthquakes; one at ~2.1-2.5 kyr BP, which correlates with the 464 BCE event that devastated Spartan society, and the other at ~3.2-4.2 kyr BP. In addition, cumulative uplift rates have increased to ~1.0-1.2 mm a^-1 over the past 4.2 kyr, from ~0.6-0.9 mm a^-1 over the preceding 4.4 kyr. From these data we can tentatively infer a recurrence interval of 1.6-2.0 kyr, which should be viewed as a maximum, because the ³⁶Cl data indicate a lower frequency of earthquakes earlier in the record. The Sparta fault scarp is composed of fault breccia, which contains quartz and clay-lined pores, in addition to host rock-derived clasts of calcite and microcrystalline calcite cement. The vertical distribution of REY elements is highly correlated with the locations of the pores. The exchange of REY between the hanging wall colluvium and the fault scarp calcite, which has been successfully applied to the study of paleoseismicity on other limestone normal faults, is overwhelmed on this fault scarp by REYs attached to the breccia pore clays. Holocene earthquakes and their magnitudes, inferred from fault slip lengths, therefore cannot be inferred from REY data for impure limestone faults such as the Sparta fault but, rather, these data may indicate processes of fault evolution in the Earth's near surface.