Surface faulting earthquakes are known to cluster in time, from historical and palaeoseismic studies, but the mechanism(s) responsible for clustering, such as fault interaction, strain-storage, and evolving dynamic topography, are poorly quantified, and hence not well understood. We present a quantified replication of observed earthquake clustering in central Italy. Six active normal faults are studied using 36Cl cosmogenic dating, revealing out-of-phase periods of high or low surface slip-rate on neighbouring structures that we interpret as earthquake clusters and anticlusters. Our calculations link stress transfer caused by slip averaged over clusters and anti-clusters on coupled fault/shear-zone structures to viscous flow laws. We show that (1) differential stress fluctuates during fault/shear-zone interactions, and (2) these fluctuations are of sufficient magnitude to produce changes in strain-rate on viscous shear zones that explain slip-rate changes on their overlying brittle faults. These results suggest that fault/shear-zone interactions are a plausible explanation for clustering, opening the path towards process-led seismic hazard assessments.
Understanding fault zone evolution is crucial to appreciating how deformation mechanisms may change in time and space. This is of particular importance when considering seismic slip, with its potential for human hazard. Histories of fault evolution and reactivation are recorded as overprinted structures in ancient fault zones, now exhumed from seismogenic depths. Recognition of ancient seismicity is aided by the occurrence of pseudotachylyte, a solidified frictional melt generated at seismic slip speeds along faults. Because pseudotachylytes form on similar timescales to the duration of seismic slip, they capture a snapshot of earthquake parameters such as temperature, depth, strength, magnitude and stress drop.
The Outer Hebrides Fault Zone, UK, was repeatedly reactivated during long-lived collision and bears widespread pseudotachylyte. It is used in this thesis as a case study in which to constrain the seismic history. Slip directions on pseudotachylyte faults are identified using field observations supported by microstructural evidence. The depth and temperature of faulting, and the coseismic temperature rise, are studied using the composition and microstructures in the pseudotachylyte veins, whilst experimentally produced melts further understanding of the control of lithology on coseismic fault strength. Finally, the static strength and the dynamic weakening are derived from further field observations.
Seismicity occurred at ≥ 10 km, somewhat deeper than has been previously thought, and was initially scattered diffusely around the fault zone on small, strong faults. Magnitudes are recorded up to MW 6.3 and static stress drops from 1.5-8.8 MPa. The lithology hosting the fault is shown to control the coseismic strength of the pseudotachylyte-bearing fault both through the melt composition and through the development of fault roughness. Overall, results show that seismicity in the Outer Hebrides occurred throughout a long convergence history because fault weakening, slip localisation and fluid influx were heterogeneously distributed around the fault zone.
Surface faulting earthquakes are known to cluster in time from historical and palaeoseismic studies, but the mechanism(s) responsible for clustering, such as fault interaction, strain-storage, and evolving dynamic topography, are poorly quantified, and hence not well understood. We present a quantified replication of observed earthquake clustering in central Italy. Six active normal faults are studied using 36Cl cosmogenic dating, revealing out-of-phase periods of high or low surface slip-rate on neighboring structures that we interpret as earthquake clusters and anticlusters. Our calculations link stress transfer caused by slip averaged over clusters and anti-clusters on coupled fault/shear-zone structures to viscous flow laws. We show that (1) differential stress fluctuates during fault/shear-zone interactions, and (2) these fluctuations are of sufficient magnitude to produce changes in strain-rate on viscous shear zones that explain slip-rate changes on their overlying brittle faults. These results suggest that fault/shear-zone interactions are a plausible explanation for clustering, opening the path towards process-led seismic hazard assessments.
Recent earthquakes involving complex multi-fault rupture have increased our appreciation of the variety of rupture geometries and fault interactions that occur within the short duration of coseismic slip. Geometrical complexities are intrinsically linked with spatially heterogeneous slip and stress drop distributions, and hence need incorporating into seismic hazard analysis. Studies of exhumed ancient fault zones facilitate investigation of rupture processes in the context of lithology and structure at seismogenic depths. In the Gairloch Shear Zone, NW Scotland, foliated amphibolites host pseudotachylytes that record rupture geometries of ancient low-magnitude (≤M W 3) seismicity. Pseudotachylyte faults are commonly foliation parallel, indicating exploitation of foliation planes as weak interfaces for seismic rupture. Discordance and complexity are introduced by fault segmentation, stepovers, branching and brecciated dilational volumes. Pseudotachylyte geometries indicate that slip nucleation initiated simultaneously across several parallel foliation planes with millimetre and centimetre separations, leading to progressive interaction and ultimately linkage of adjacent segments and branches within a single earthquake. Interacting with this structural control, a lithological influence of abundant low disequilibrium melting-point amphibole facilitated coseismic melting, with relatively high coseismic melt pressure encouraging transient dilational sites. These faults elucidate controls and processes that may upscale to large active fault zones hosting major earthquake activity. Supplementary material : Supplementary Figures 1 and 2, unannotated versions of field photographs displayed in Figures 4a and 5 respectively, are available at https://doi.org/10.6084/m9.figshare.c.4573256 Thematic collection : This article is part of the SJG Collection on Early-Career Research available at: https://www.lyellcollection.org/cc/SJG-early-career-research
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