Volcanic eruptions progress by co-evolving fluid and solid systems. The fluid mechanics can be observed through the plumes and ejecta produced, but how does the solid system evolve? When does the conduit open? When does it close? Seismology can potentially tell us about these processes by measuring the failure of the solid rock. Such inferences require the detection of earthquakes during an explosive eruption. Standard earthquake detection methods often fail during this time as the eruption itself produces seismic waves that obscures the earthquake signals. We address this problem by applying supervised and unsupervised search techniques to the existing catalog of the 2008 Okmok eruption to find brittle failure signals during the continuous eruptive sequence. The interaction between fluid pathways and seismicity is reinforced by high precision earthquake relocations that highlight a ring-fault structure, which may be acting as a conduit for fluids to the surface. The timing of the earthquakes during the eruption reveal that the seismicity gradually increases during the vent-opening stage (July 12-July 24), peaks during the vent-widening stage (July 24-August 1) which culminates in a large burst of earthquakes, and then gradually decrease until the end of the eruptive period. Seismic bursts during the eruption are not synchronized with the exhalation of large ash and steam plumes. In other words, when the system is closed, the rock breaks. We call this scenario clog and crack.

Oceanic internal gravity waves propagate along density stratification within the water column and are ubiquitous. They can propagate thousands of kilometers before breaking in shoaling bathymetry and the ensuing turbulent mixing affects coastal processes and climate feedbacks. Despite their importance, internal waves are intrinsically difficult to detect as they result in only minor amplitude deflection of the sea surface; the need for global detection and long time series of internal waves motivates a search for geophysical detection methods. The pressure coupling of a propagating internal wave with the sloping seafloor provides a potential mechanism to generate seismically observable signals. We use data from the South China Sea where exceptional oceanographic and satellite time series are available for comparison to identify internal wave signals in an onshore passive seismic dataset for the first time. We analyze potential seismic signals on broadband seismometers in the context of corroborating oceanographic and satellite data available near Dongsha Atoll in May-June 2019 and find a promising correlation between transient seismic tilt signals and internal wave arrivals and collisions in oceanic and satellite data. It appears that we have successfully detected oceanic internal waves using a terrestrial seismometer. This initial detection suggests that the seismic detection and amplitude determination of oceanic internal waves is possible and can potentially be used to expand the historical record by capitalizing on the existing terrestrial seismic network.

Rate-and state-friction is an empirical framework that describes the complex velocity-, time-, and slip-dependent phenomena observed during frictional sliding of rocks and gouge in the laboratory. Despite its widespread use in earthquake nucleation and recurrence models, our understanding of rate-and state-friction, particularly its time-and/or slip-dependence, is still largely empirical, limiting our confidence in extrapolating laboratory behavior to the seismogenic zone. While many microphysical models have been proposed over the past few decades, none have explicitly incorporated the effects of strain hardening, anelasticity, or transient viscous rheology. Here we present a new model of rock friction that incorporates these phenomena directly from the microphysical behavior of lattice dislocations. This model of rock friction exhibits the same logarithmic dependence on sliding velocity (strain rate) as rate-and state-friction and predicts a dependence on the internal backstress caused by long-range interactions among geometrically necessary dislocations. Changes in the backstress evolve exponentially with plastic strain of asperities and are dependent on both the current backstress and previous deformation, which give rise to phenomena consistent with interpretations of the ‘critical slip distance,’ ‘memory effect,’ and ‘state variable’ of rate- and state-friction. Fault stability in this model is controlled by the evolution of backstress and temperature. We provide several analytical predictions for RSF-like behavior and the ‘brittle-ductile’ transition based on 2 microphysical mechanisms and measurable parameters such as the geometrically necessary dislocation density and strain-dependent hardening modulus.

Damage zones are important to the rupture dynamics, evolution and fluid coupling of earthquakes. However, information about the damage zone at depth is limited. It is unclear if damage zones increase or decrease in intensity with depth. Here we use marine 3-D seismic surveys and modern fault detection methods to address the depth-dependent structure of damage zones. We use two overlapping legacy industry seismic volumes collected offshore of Los Angeles span approximately 20 km of the Palos Verdes strike-slip fault. The data here allows visibility of the damage zone in the sedimentary formations to 2,200 meters depth, which is comparable to the constraints provided by SAFOD and other studies. Using both interpreted mapped primary fault strands and seismic attributes to identify subsidiary faults, we map and quantify spatial variations in damage zone size and intensity. The damage zone consists of subsidiary faults, or linked discontinuities in the seismics selected within assigned ranges of geometries to the primary strands. Damage was identified using a variation of the seismic attribute semblance, or multi-trace similarity. This method allows interrogation of damage zone in response to changes sedimentary lithology and fault geometry. Subsidiary faults delineate the damage zone to approximately 1 km in width and fracture density decays with distance from the primary fault strands for all sedimentary lithologies in the study area. The damage zone narrows with depth, but fracture density increases because the intensity of fracturing more than compensates for the decreased width. In the thickest formation we find that fracture density increases as Z1.8, where Z is depth in meters. These results are then compared to resolution changes with depth. The damage intensity increase and localization potentially provides a strong constraint for efforts to determine an appropriate rheology for producing damage zones and studying their effects.