Variations in slip-rate and earthquake occurrence across 3D structural complexities on active normal faults

Iezzi, Francesco (2020) Variations in slip-rate and earthquake occurrence across 3D structural complexities on active normal faults. Doctoral thesis, Birkbeck, University of London.

Abstract

This PhD thesis provides a series of studies on the relationship between the non-planar geometry and the seismic behaviour of active normal faults. Herein, several examples show that throw and fault dip increase within along-strike fault bends in order to preserve the horizontal strain-rate within the bend and along the fault. This has been demonstrated for a variety of normal faults (a) located in different geodynamic domains and (b) for measurements of throw taken over different time periods. Furthermore, throw enhancement within fault bends has been observed on (1) immature faults, where fault bends are still propagating up to the surface and are not yet fully established, (2) well-established single fault segments, where fault bends affect one continuous fault segment, and (3) densely-spaced fault systems arranged across strike (with fault spacing < 5 km), where a change in strike across several fault segments creates an overall bend in the system. The results presented in this thesis suggest that the relationship between the non-planar geometry and the distribution of throw is scale-independent, and can act across systems of faults if they are closely spaced across strike. Moreover, 36Cl-cosmogenic dating of tectonically-exposed fault planes on faults spaced > 5 km across-strike shows that these faults are clustered, with a non-systematic alternance of periods of rapid slip accumulation (i.e. earthquake clustering) and periods of quiescence (i.e. earthquake anti-clustering); this suggests that parallel faults interact in terms of sharing the regional strain-rate, with switching activity that affects the slip-rate on a single fault. The results have implications for numerous and diverse aspects of the earthquake geology, such as interpretation of palaeoseismology studies including at trench sites and cosmogenic dating of fault planes, empirical scaling relationships, PSHA, and off-fault deformation.

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