2 nd INQUA-IGCP-567 International Workshop on Active Tectonics, Earthquake Geology, Archaeology and Engineering, Corinth, Greece (2011) INQUA PALEOSEISMOLOGY AND ACTIVE TECTONICS EARTHQUAKE ARCHAEOLOGY 198 REGIONAL STRAIN-RATES ON ACTIVE NORMAL FAULTS AND VARIABILITY IN THE SEISMIC CYCLE: AN EXAMPLE FROM THE ITALIAN APENNINES Roberts, Gerald (1), Joanna Faure Walker (1), Patience Cowie (2), Richard Phillips (3), Ken McCaffrey (4), Ioannis Papanikolaou (5), Max Wilkinson (4), Alessandro Michetti (6), Peter Sammonds (1). (1) Department of Earth and Planetary Sciences, Birkbeck/UCL, University of London, Gower Street, WC1E 7HX. UK. gerald.roberts@ucl.ac.uk (2) Department of Earth Sciences, University of Bergen, Postboks 7800, NO-5020 BERGEN (3) Institute of Geophysics and Tectonics, School of Earth and Environment, University of Leeds, LS2 9LT, UK (4) Department of Earth Sciences, Durham University, Science Labs, Durham DH1 3LE. UK. (5) Mineralogy-Geology Lab of the Agricultural University of Athens, Greece. (6) Università degli Studi dell'Insubria - Sede di Como, Facoltà di Scienze MM. FF. NN., Dipartimento di Scienze Chimiche e Ambientali, Via Valleggio, 11 - 22100 Como – Italy. Abstract(Regional strain-rates on active normal faults and variability in the seismic cycle: an example from the Italian Apennines): The rate at which a fault slips fundamentally determines the seismic hazard because average earthquake recurrence intervals tend to decrease as slip rates increase. Slip-rates on different faults within a growing fault system are not distributed randomly in space and time, because slip-rates must accommodate the regional strain-rate. Here we show how slip-rates vary temporally and spatially along the Italian Apennines, constrained by offsets across fault scarps dated with 36 Cl in situ cosmogenic dating. We map regional variations in strain-rate, investigating natural variability in the seismic cycle. Key words: Fault scarps, regional strain-rates, cosmogenic dating, seismic-cycle. INTRODUCTION Palaeoseismologists and earthquake geologists should try to move forward from simply characterising slip on single faults. Instead, they should try to characterise slip across systems of faults as it is clear that faults interact through stress transfer, and this controls regional patterns of slip-rate (the geography of seismic hazard), and the recurrence intervals for earthquakes on specific faults. The regional approach is needed to complement the regional strain-rate databases provided through GPS geodesy and instrumental seismicity. We should attempt to constrain how the geography of seismic hazard implied over 10 2 -10 3 years (palaeoseismology and earthquake geology) compares with that over 10- 100 years (geodesy and instrumental seismicity). We expect differences between data from these different timescales to reveal temporal and spatial variability in the seismic cycle (e.g. Faure Walker et al. 2010). Fault scarps and 36 Cl cosmogenic dating The Italian Apennines are characterised by fault scarps that offset and deform deposits and landforms formed during the last glacial maximum. The offsets have accumulated since 15 ±3 ka shown by studies of tephrachronology and 14 C dating (Giraudi and Frezzotti 1995, 1997) and this is confirmed by 36 Cl in situ cosmogenic dating (Palumbo et al. Schlagenhauf 2009, Schlagenhauf et al. 2010), and our own ongoing cosmogenic dating. The scarps are widespread and allow a regional study of strain rates and natural variability in the seismic cycle. Figure 1. A regional strain-rate map for the Italian Apennines constrained by offsets of 15 ±3 ka features across fault scarps. Regional Strain rates A key unknown is how far a fault can stray from its long-term slip rate, both at a timescale equivalent to the interseismic period (10 2 -10 3 years), and over timescales equivalent to several seismic cycles (10 3 - 10 4 years). The lack of such knowledge impedes our ability to perform probabilistic seismic hazard