High-resolution passive seismic imaging of shallow subsurface structures is often challenged by the scarcity of coherent body-wave energy in ambient noise recorded at surface stations. We show that autocorreleation (AC) of teleseismic P-wave coda extracted from just 1-month of continuous recording at 5 Hz geophones can overcome this limitation. We apply this method to investigate the longitudinal subsurface structure of Unaweep Canyon, a paleovalley in western Colorado (US) with complex evolution. Both fluvial and glacial processes have been proposed to explain the canyon’s genesis and morphology. The teleseismic P-wave coda AC retrieves zero-offset reflections from the shallow (200 – 500 m depth) basement interface at 120 stations along a 5 km long profile. Additionally, we invert interferometrically retrieved surface wave dispersion for the shear-wave structure of the sedimentary fill. Combined interpretation of these results and other geophysical and well data suggests an overdeepened basement geometry due to glacial processes.

We combine earthquake spectra from multiple studies to investigate whether the increase in stress drop with depth often observed in the crust is real, or an artifact of decreasing attenuation (increasing Q) with depth. In many studies, empirical path and attenuation corrections are assumed to be independent of the earthquake source depth. We test this assumption by investigating whether a realistic increase in Q with depth (as is widely observed) could remove some of the observed apparent increase in stress drop with depth. We combine event spectra, previously obtained using spectral decomposition methods, for over 50,000 earthquakes (M0 to M5) from 12 studies in California, Nevada, Kansas and Oklahoma. We find that the relative high-frequency content of the spectra systematically increases with increasing earthquake depth, at all magnitudes. By analyzing spectral ratios between large and small events as a function of source depth, we explore the relative importance of source and attenuation contributions to this observed depth dependence. Without any correction for depth-dependent attenuation, we find a systematic increase in stress drop, rupture velocity, or both, with depth, as previously observed. When we add an empirical, depth-dependent attenuation correction, the depth dependence of stress drop systematically decreases, often becoming negligible. The largest corrections are observed in regions with the largest seismic velocity increase with depth. We conclude that source parameter analyses, whether in the frequency or time domains, should not assume path terms are independent of source depth, and should more explicitly consider the effects of depth-dependent attenuation.

To better quantify how injection, prior seismicity and fault properties control rupture growth and propagation of induced earthquakes, we perform finite-fault slip inversion on a $M_{w}$ 4.1 earthquake that occurred in April 2015, which is the largest earthquake of an induced sequence near Guthrie, Oklahoma. The slip inversion reveals a complex rupture with multiple slip patches that are anti-correlated to the cumulative slip distributions of prior seismicity. This indicates that the $M_{w}$ 4.1 earthquake likely ruptured relatively strong asperities, while earlier seismicity driven by pore pressure occurred in weaker area. Compared to similar magnitude events in swarms from other regions, intraplate earthquakes in Oklahoma have higher number of well separated slip patches, indicating a difference in fault characteristics between regions. These observations suggest that both pore pressure perturbations, earthquake interactions, and fault characteristics control rupture propagation in moderate size earthquakes in Oklahoma, with the latter likely the dominant factor.

Exploring the connections between injection wells and seismic migration patterns is key to understanding processes controlling growth of fluid-injection induced seismicity. Numerous seismic clusters in Oklahoma have been associated with wastewater disposal operations, providing a unique opportunity to investigate migration directions of each cluster with respect to the injection-well locations. We introduce new directivity migration parameters to identify and quantify lateral migration toward or away from the injection wells. We take into account cumulative volume and injection rate from multiple injection wells. Our results suggest a relationship between migration patterns and the cluster-well distances, and unclear relationship with injected volume and equivalent magnitudes. Migration away from injection wells is found for distances shorter than 5-13 km, while an opposite migration towards the wells is observed for larger distances, suggesting an increasing influence of poroelastic stress changes. This finding is more stable when considering cumulative injected volume instead of injection rate.

Earthquake stress drop is an important source parameter that directly links to strong ground motion and fundamental questions in earthquake physics. Stress drop estimations may contain significant uncertainties due to factors such as variations in material properties and data limitations, which limits the applications of stress drop interpretations. Using a high-resolution borehole network, we analyze 4537 earthquakes in the Parkfield area in Northern California between 2001 and 2016 with spectral decomposition and an improved stacking method. To evaluate the influence of spatiotemporal variations of material properties on stress drop estimations, we apply six different strategies to account for spatial variations of velocity and attenuation changes, and divide earthquakes into three separate time periods to correct temporal variations of attenuation. These results show that appropriate corrections can significantly reduce the scatter in stress drop estimations, and decrease apparent depth and magnitude dependence. We further investigate the influence of data limitations on stress drop estimations, and show that insufficient bandwidth may cause systematic underestimation and increased stress drop scatter. The stress drop measurements from the high-frequency borehole recordings exhibit complex stable spatial patterns with no clear correlation with the nature of fault slip, or the slip distribution of the 2004 M6 earthquake. In some regions with the largest numbers of earthquakes, we can resolve temporal variations that indicate stress drop decrease following the 2004 earthquake, and gradual recovery. These temporal variations do not affect the long-term stress drop spatial variations, suggesting local material properties may control the spatial heterogeneity of stress drop.

We investigate the directivity of three well-recorded repeating sequences of earthquakes (M2-3, 2001–2016) on the San Andreas Fault at Parkfield (California) that are well-recorded by a borehole network. We calculate rupture directivity and velocity from P waves using the empirical Green’s function method. The individual events in each sequence all show the same directivity; those in the largest magnitude sequence (M~2.7, 8 events) rupture unilaterally to the NW (at ~0.8Vs), those in the second sequence (M~2.3, 9 events) rupture unilaterally to the SE, and those of smallest magnitude sequence (M~2, 11 events) are less well resolved. The source spectra of the M~2.7 sequence exhibit no detectable temporal variation. The M~2 sequence exhibits a decrease in high frequency energy following the M6 event, that recovers with time, implying a decrease in stress drop then gradual healing. The third sequence (M~2.3) shows a similar, less well resolved response to the M6.

The 2011 Mw 5.7 Prague earthquake is the second largest induced earthquake in Oklahoma, and occurred after decades of wastewater disposal. The local geological structure that led to the triggering of this large earthquake is not well understood. In this study, tomographic inversion of seismic data recorded by a dense local seismic network resulted in a high-resolution 3D velocity model with three major layers. The model clearly illuminates the geometry and characteristics of the Meeker-Prague Fault that hosted the 2011 Prague sequence. A conceptual model is proposed to link the tomographic structure to the triggering process of the sequence. The low-permeability second layer at ~1.5-3.5 km may be the key that delays the occurrence of the first sizeable earthquake after decades of wastewater injection. However, a low-shear-velocity zone within this layer at the intersection of two major faults could have provided a fluid pathway to facilitate downward fluid propagation.

Earthquake stress drop is an important source parameter that directly links to strong ground motion. However, estimating stress drops is often challenging due to many factors, such as data limitations, methodology, and attenuation. Different studies may yield highly inconsistent stress drop values for the same earthquakes, leading to different interpretations of stress drop scaling and spatial patterns. Stress drop is also important for interpretation of earthquake triggering processes. In particular, the roles of foreshocks have continuous debate, and some recent studies show that detailed source parameter analysis is the key. In this study, we combine analyses of three sequences in Southern California using different methods to investigate the resolution of stress drops and the roles of foreshocks. The three sequences include the M7 El-Mayor Cucapah, the 2012 M5 Brawley swarm, and the 2019 M7 Ridgecrest sequence. The Ridgecrest sequence will participate in the Community Stress Drop Validation Study. For each sequence, we apply an improved stacking method and a spectral ratio method. We will use different types of waves: P-wave, S-wave, and Coda-wave. Stress drop results from this study will be compared with available previous studies. We will first discuss the influence of wave type and methodology on stress drop estimations, then we will investigate detailed stress interactions between foreshocks and the mainshock for different types of earthquake sequences (i.e., mainshock-aftershock and swarms that involve aseismic slip or fluid).