title: Understanding seismic properties of fault zones
Creators: name: Kelly, Christina
Publisher: name: University of Liverpool Repository
Year: 2014

description: Fault zone properties at depth are often inferred from seismic properties such as seismic velocities and attenuation. An understanding of how fault zone properties and processes influence seismic measurements is required for successful interpretations to be made. As fault zones are heavily fractured and often fluid-rich areas, a knowledge of the influences of cracking and fluid content on seismic measurements is needed. This will allow better interpretation of fault zone properties and how they may change at the time of an earthquake. Research presented in this thesis is concentrated on two regions of strike-slip faulting: the Parkfield area of the San Andreas fault and the exhumed Carboneras fault zone region of SE Spain. Well-preserved exhumed faults allow observation of fault structure at seismogenic depths. The structure of the exhumed Carboneras fault has previously been suggested as an analogue for the Parkfield area at depth. Laboratory measurements can help us to determine what processes occur at seismogenic depths in active faults. They can also aid in interpretation of seismic studies. In this thesis laboratory and seismic studies are brought together in order to gain a greater understanding of fault zone seismic properties at depth and how to interpret them. In order to characterise the properties of the Carboneras fault, laboratory experiments of velocities through fault gouge and fault zone rocks are performed. The influences of fracture damage and local geological fabric on velocities are investigated. Gouge velocities are measured to be less than those of the mica schist rock through which the fault cuts. Velocity changes due to variations in crack damage in cyclic loading experiments are less than 5% of the original rock velocity. Strong velocity anisotropy is observed in the mica schist, with velocities of the order of 30% less when measured perpendicular to the strong foliation present in the rock. The consequences in terms of seismically imaging the fault zone are discussed. The effects of this strong velocity anisotropy need to be considered for specific source-receiver geometries and the local geological fabric in the locations of seismic experiments. Surface wave tomography and ambient noise analysis of the Carboneras fault zone region shows that faults are imaged as low velocity features at depth. Results suggest that velocities are reduced by approximately 10% at depths close to 3 km. The strong anisotropy observed in laboratory experiments of mica schist may also have implications for seismic imaging of this region as this rock crops out widely. This is discussed in terms of a potentially strong crustal component to shear-wave splitting observations in the region. In the second part of the thesis, temporal changes in seismic attenuation at the time of the 2004 M6.0 Parkfield earthquake are investigated. Seismic attenuation can give indications of fracture damage and healing. Spectral ratios between earthquakes within repeating clusters are calculated. A sharp increase in attenuation is observed immediately after the earthquake, which then decays over the next 2 years. The postseismic decay is fit by a logarithmic function. The timescale of the decay is found to be similar to that in GPS data and ambient seismic noise velocities following the 2004 M6.0 Parkfield earthquake. The amplitude of the attenuation change corresponds to a decrease of approximately 10% in QP at the time of the earthquake. The greatest changes are recorded to the northeast of the fault trace, consistent with preferential damage in the extensional quadrant behind a north-westerly propagating rupture tip. Our analysis suggests that significant changes in seismic attenuation and hence fracture dilatancy during co-seismic rupture are limited to depths of less than about 5 km.