Linking Historical Earthquake Records to Long Term Fault Slip Rates Using Cosmogenic 36Cl: Evidence for Migrating Earthquake Activity on a Millenial Timescale Across the Central Italian Apennines
Wednesday, 17 December 2014: 10:20 AM
Where deformation is accommodated by complex arrays of interacting faults, individual fault strands often show fluctuating levels of activity through time with intervals of high, or low, slip rate corresponding to earthquake clusters or quiescence, respectively. Thus earthquakes may occur on structures other than those defined as active based on historical events and paleoseismic records, confounding attempts to map seismic hazard with such datasets. A major challenge is to identify which faults are ahead of their long-term slip rate, which faults are behind their long term rate but still active, and which faults are no longer active. We show that cosmogenic 36Cl depth profiles measured systematically along the fault plane in trenches excavated below the base of bedrock scarps as well as along the exhumed scarp formed by extensional fault slip can be used to estimate both the average Holocene slip rate and the elapsed time since the last earthquake. Specifically, the increase in 36Cl concentration with height immediately above and below the ground surface at the scarp base reveals whether the elapsed time is anomalously long (or short) compared to the expected mean earthquake recurrence. For an area of active extension in Italy, we demonstrate that independent constraints on Holocene slip rates and elapsed times of historical earthquakes show a close correspondence with estimates derived from the 36Cl profiles. However, some faults slipped at rates higher than their Holocene-averaged rate from ~5 ka to the Middle Ages, indicating greater strain accumulation on the SW flank of the central Apennines in a zone that is now quiescent. The historical catalogue shows that, since the earthquake of 1349 A.D., activity has been concentrated in a 50km wide zone of faults further to the NE. In contrast, when viewed over the whole Holocene, strain is symmetrically distributed across the whole array. When taken together, along with documented earthquake shaking since Roman times, these data suggest that the locus of strain accumulation has migrated from one crustal scale fault system to another over a distance of several 10’s of km over a time scale of several thousand years and that deformation in this area is more variable in both space and time than the geodetically-determined interseismic strain accumulation would predict.