Earthquake Time-Slip History of the Kaparelli Fault (Greece) from in situ Chlorine-36 Cosmogenic Dating

Direct cosmogenic dating of exhumed earthquake scarps provide a new way to determine the slip-rate and the earthquake history of a fault. Here, we have applied in situ ³⁶Cl cosmic ray exposure dating to a limestone fault scarp produced by the Kaparelli fault. The Kaparelli Fault is located at the eastern extremity of the Gulf of Corinth. In 1981, a sequence of three earthquakes with magnitude greater than 6 struck this region (Jackson et al. E.P.S.L. 57, 1982, King et al. G.J.Int, 80, 1985). The Kaparelli fault was responsible for the third event (March, 4, 1981, Ms ~6.4) with an associated rupture of 12 km and a maximum vertical displacement of 70 cm. For about 5 km, the surface break followed a 3-4 m high limestone fault scarp along which the 1981 event exhumed 40 cm of new scarp. On the best preserved section of this scarp we sampled a continuous 3.5 m vertical profile, with a 10 cm resolution. ³⁶Cl is produced at shallow depths in calcite predominantly by spallation of calcium. This production decreases exponentially with depth beneath the surface but varies little with height once a section of the scarp has been tectonically exhumed. The concentration of ³⁶Cl and of stable chlorine in the 35 samples has been measured by accelerator mass spectrometry (AMS) at the LLNL Center for Accelerator Mass Spectrometry. The ³⁶Cl concentration increases at an approximately constant rate with the height of the scarp, suggesting that the slip-rate of the fault is constant with time. The ³⁶Cl concentrations indicate that the scarp is probably more than ten thousand years old. Moreover, the ³⁶Cl pattern suggests that this scarp might have formed as a result of 6 events. Further analysis of our data and refinement of ³⁶Cl production rates will allow us to further test these hypotheses. This work was performed under the auspices of the U.S. Department of Energy by University of California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.