W Griffith

Mechanical Validation of the Three-Dimensional Intersection Geometry between the Puente Hills Blind-Thrust System and the Whittier Fault, Los Angeles, California

The sensitivity of fault interaction to alternative, kinematically plausible intersection geometries of the Puente Hills blind-thrust system and the Whittier fault is modeled under geodetically constrained horizontal contraction. Comparisons of modeled slip rates to available geologic rates (1) suggest that the Coyote Hills seg- ment of the Puente Hills system extends to the base of the seismogenic crust and (2) give slight preference for extension of the active Whittier fault to the base of the seismogenic crust rather than limiting active slip to the hanging wall of the Coyote Hills fault. Furthermore, analysis of strain energy density demonstrates that the pre- ferred model has the greatest mechanical efficiency. This correlation of fit to geologic slip rates and mechanical efficiency supports the effectiveness of this method for evaluating among alternative geometries in the absence of geologically constrained slip rates. The methodology implemented in this study may be effectively used in future studies of deformation within fault systems. Model-generated slip rates along the Puente Hills faults show significant strike slip, implying that seismic hazard of these faults may be underestimated by consid- ering reverse-slip rates alone. Contraction at 036 may not be appropriate in the Puente Hills region because of resulting sinistral slip on the Whittier fault, which disagrees with paleoseisimc observations; however, Whittier strike-slip rates fit pa- leoseismic rates under 006.5 contraction. Because strike-slip rates are more sensitive to contraction direction than fault intersection geometry, contraction direction should be further constrained in order to accurately assess seismic hazard on these faults.

Off-fault tensile cracks: A link between geological fault observations, lab experiments, and dynamic rupture models

[1] We examine the local nature of the dynamic stress field in the vicinity of the tip of a semi-infinite sub-Rayleigh (slower than the Rayleigh wave speed, cR) mode II crack with a velocity-weakening cohesive zone. We constrain the model using results from dynamic photoelastic experiments, in which shear ruptures were nucleated spontaneously in Homalite-100 plates along a bonded, precut, and inclined interface subject to a far-field uniaxial prestress. During the experiments, tensile cracks grew periodically along one side of the shear rupture interface at a roughly constant angle relative to the shear rupture interface. The occurrence and inclination of the tensile cracks are explained by our analytical model. With slight modifications, the model can be scaled to natural faults, providing diagnostic criteria for interpreting velocity, directivity, and static prestress state associated with past earthquakes on exhumed faults. Indirectly, this method also allows one to constrain the velocity-weakening nature of natural ruptures, providing an important link between field geology, laboratory experiments, and seismology.

Elastic Deformation due to Polygonal Dislocations in a Transversely Isotropic Half‐Space

Based upon the fundamental solution to a single straight dislocation segment, a complete set of exact closed‐form solutions is presented in a unified manner for elastic displacements and strains due to general polygonal dislocations in a transversely isotropic half‐space. These solutions are systematically composed of two parts: one representing the solution in an infinite transversely isotropic medium and the other accounting for the influence of the free surface of the half‐space. Numerical examples are provided to illustrate the effect of material anisotropy on the elastic displacement and strain fields associated with dislocations. It is shown that if the rock mass is strongly anisotropic, surface displacements calculated using an isotropic model may result in errors greater than 20%, and some of the strain components near the fault tip may vary by over 200% compared with the transversely isotropic model. Even for rocks with weak anisotropy, the strains based on the isotropic model can also result in significant errors. Our analytical solutions along with the corresponding MATLAB source codes can be used to predict the static displacement and strain fields due to earthquakes, particularly when the rock mass in the half‐space is best approximated as transversely isotropic, as is the case for most sedimentary basins. Online Material: MATLAB scripts to calculate rectangular and triangular dislocations in a transversely isotropic half‐space.

Microscopic evolution of laboratory volcanic hybrid earthquakes

Characterizing the interaction between water and microscopic defects is one of the long-standing challenges in understanding a broad range of cracking processes. Different physical aspects of microscopic events, driven or influenced by water, have been extensively discussed in numerical calculations but have not been accessible in micro-scale experiments. Through the analysis of the emitted ultrasound excitations during the evolution of individual dynamic microcracking events, we show that the onset of a secondary instability – known as hybrid events in coda part of the recorded waveforms occurs during the fast equilibration phase of the system, which leads to (local) sudden increase of pore water pressure in the process zone. As a result of this squeezing-like process, a secondary induced instability akin to the long period event occurs. This mechanism is consistent with observations of hybrid earthquakes found in volcanic settings. Introduction Critical challenges remain in the study of dynamic interactions between in-situ liquids and moving defects (fractures and micro-defects such as dislocations and other topological defects) and in triggering the nucleation and/or movement of defects [1-3]. Such interactions might emit broadband phononic excitations which mirror the complexity of the source dynamics from which they are derived as well as the environment in which the sources propagate. An important manifestation of these excitations in the geosciences is the study of the seismic response of rocks in the presence of pore fluids. Seismological observations of earthquakes associated with active volcanism have exposed a wide variety of physical phenomena that are manifested as seismic activity [4-5]. In particular, Low-Frequency (LF) earthquakes have been associated with so-called slow slip events in subduction zones and as a consequence of fluid movement during volcanic unrest [6-7]. LF events also known as longperiod (i.e., dominant low frequency component in the energy spectrum) and very long-period events, are observed on all types of active volcanoes, often in swarms preceding eruption. Such LF events differ from Volcanotectonic seismicity (VT events) in terms of both their characteristic frequency range and extended harmonic (coda) signature [4,6-7] and have been postulated to be generated from fluid flow and resonance in fractures and conduits within the edifice. Finally, the third type of seismicity shows features of both HF seismicity and also LF harmonic tremor. Known as hybrid events, this type of seismicity is characterized by a high frequency, VT-like onset and a LF-like coda, suggesting that hybrid generation is stimulated by stress regimes leading to both rock failure, and also where fluids are present in order to generate LF and tremor [4,5-8]. Laboratory manifestations of LF, VT, and hybrid seismicity are accessible by recording the Acoustic (phonon) Emissions (AEs) – the laboratory analogue of seismic events in Earth’s crust and a commonly-used proxy in laboratory rock physics [9]. The high resolution andCharacterizing the interaction between fluids and microscopic defects is one of the long-standing challenges in understanding a broad range of cracking processes, in part because they are so difficult to study experimentally. We address this issue by reexamining records of emitted acoustic phonon events during rock mechanics experiments under wet and dry conditions. The frequency spectrum of these events provides direct information regarding the state of the system. Such events are typically subdivided into high frequency (HF) and low frequency (LF) events, whereas intermediate “Hybrid” events, have HF onsets followed by LF ringing. At a larger scale in volcanic terranes, hybrid events are used empirically to predict eruptions, but their ambiguous physical origin limits their diagnostic use. By studying acoustic phonon emissions from individual microcracking events we show that the onset of a secondary instability–related to the transition from HF to LF–occurs during the fast equilibration phase of the system, leading to sudden increase of fluid pressure in the process zone. As a result of this squeezing process, a secondary instability akin to the LF event occurs. This mechanism is consistent with observations of hybrid earthquakes.