Improving confidence and revealing fault complexity using multiple sampling sites in 36Cl Quaternary slip rate studies
Abstract
The cosmogenic isotope chlorine-36 (36Cl) is an increasingly popular tool for investigating normal fault slip rates over the last 10-20 thousand years. These slip rates are a key component in understanding earthquake hazard and the behaviour of normal faults. There has been little work done validating these 36Cl derived slip histories due to the expensive and time-consuming nature of the measurements. Here we test if 36Cl derived slip histories are repeatable and therefore reliable estimates of normal fault slip rates.As normal fault planes are progressively exhumed from the surface they accumulate cosmogenically-produced 36Cl. Our methodology is to collect 36Cl sample ladders at multiple locations along strike on a fault and compare between sites, as they should have similar exposure histories if all of the exhumation is a result of earthquake slip. We estimate the history of exposure and fault slip rate by modelling 36Cl concentrations using an Bayesian approach. We then compare these slip histories to test our hypothesis.
We present results from two faults in the Italian Apennines, where we have collected samples from 3 new sites on the Campo Felice fault and 2 new sites on the Roccapreturo fault. We have also remodelled data from 1 site on each fault from published studies. Our modelling indicates good agreement between sites where the geomorphology has been carefully considered whilst selecting sampling location. Where the geomorphology is not appropriate there is a large variation in models between sites on the same fault. This is possibly because exhumation of the fault scarp at these sites is not only due to earthquake slip but also due to the removal of material by slope processes. These results indicate that given proper site selection 36Cl derived slip histories are a consistent and therefore reliable record of fault activity. We also discuss the uncertainties in the modelled datasets, as this affects the degree to which we can reasonably interpret the slip histories. Finally, we see evidence for how fault relay zones behave over the 10-20 thousand year time scales.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.S44A..06G
- Keywords:
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- 4314 Mathematical and computer modeling;
- NATURAL HAZARDSDE: 7215 Earthquake source observations;
- SEISMOLOGYDE: 7221 Paleoseismology;
- SEISMOLOGYDE: 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICS