Properties and Processes of Crustal Fault Zones: Volume II

Abstract

Active faults and earthquake rupture zones are largely buried under miles of rocks. Due to the effects of attenuation, typical seismological and other geophysical data lack detailed information on faulting processes. For these reasons as well as the inherent complexity of faulting, many fundamental questions on the structure and processes of fault zones in relation to earthquake properties and generated ground motion remain unanswered. To discuss the state of the art in the field, the 40th Workshop of the International School of Geophysics, titled ‘‘Properties and Processes of Crustal Fault Zones’’, was held May 18‐24, 2013, in the Ettore Majorana Foundation and Centre for Scientific Culture of Erice, Sicily. The papers in a previous volume I and the present volume II discuss observational and theoretical results based on presentations at the workshop and additional contributions that advance the understanding of earthquakes and faults. Topics covered in this volume include imaging of fault zones and the crust with seismic and other signals, microstructural analyses of fault zone rocks, long paleoseismic records of large earthquakes, inferences on stress, stress drops and fault geometries from seismological and geological data, properties of dynamic ruptures, generation and healing of rock damage, temporal changes of seismic properties, postseismic deformation, and scaling of earthquake rupture areas with moment. In the first paper of the volume, Zigone et al. use Rayleigh and Love waves constructed from the ambient seismic noise to obtain a 3D model of S wave velocities in the southern California plate-boundary region. The results show clear damage zones in the top few kilometres around the San Andreas, San Jacinto, and Elsinore faults, and velocity contrasts across various fault sections, which augment earthquake tomography images at greater depth. Additional findings include 2-h azimuthal anisotropy with fast directions parallel to geometrically simple fault sections and mixed directions in complex areas. Bennington et al. develop a method for joint inversion of P wave velocity and electrical resistivity using a normalized cross-gradient constraint, and apply it to obtain a structural profile across the San Andreas Fault Observatory at Depth (SAFOD). The results provide strong constraints on several key geological units and are used to infer possible fluid-rich regions in the fault zone structure. Bradbury et al. describe the microstructure and chemical composition of faultrelated rocks from the SAFOD borehole and surface outcrops that may be useful analogs for San Andreas Fault rocks. The observed rock textures are consistent with largely aseismic deformation punctuated by seismic slip. The mineralogy and whole-rock geochemistry indicate that the fault zone experienced transient fluid‐rock interactions. Ferrarini et al. constrain the fault segmentation pattern and regional stress tensor in the intra-Apennine area of central Italy using geological and seismological fault-slip data associated with the Colfiorito 1997 Mw 6.0 and L’Aquila 2009 Mw 6.1 seismic sequences. The results indicate a prevailing tensional regime involving a subhorizontal NE‐SW minimum stress axis and only minor strike-slip regimes. The findings are at odds with results of other studies indicating wide areas characterized by strike