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It is well known that large earthquakes often exhibit significant rupture complexity such as well separated subevents. With improved recording and data processing techniques, small earthquakes have been found to exhibit rupture complexity as well. Studying these small earthquakes offers the opportunity to better understand the possible causes of rupture complexities. Specifically, if they are random or are related to fault properties. We examine microearthquakes (M < 3) in the Parkfield, California, area that are recorded by a high-resolution borehole network. We quantify earthquake complexity by the deviation of source time functions and source spectra from simple circular (omega-square) source models. We establish thresholds to declare complexity, and find that it can be detected in earthquakes larger than magnitude 2, with the best resolution above M2.5. Comparison between the two approaches reveals good agreement (>90 per cent), implying both methods are characterizing the same source complexity. For the two methods, 60–80 per cent (M 2.6–3) of the resolved events are complex depending on the method. The complex events we observe tend to cluster in areas of previously identified structural complexity; a larger fraction of the earthquakes exhibit complexity in the days following the Mw 6 2004 Parkfield earthquake. Ignoring the complexity of these small events can introduce artefacts or add uncertainty to stress drop measurements. Focusing only on simple events however could lead to systematic bias, scaling artefacts and the lack of measurements of stress in structurally complex regions.

To better quantify how injection, prior seismicity, and fault properties control rupture growth and propagation of induced earthquakes, we perform a finite‐fault slip inversion on aM_w4.0 earthquake that occurred in April 2015, the largest earthquake in an induced sequence near Guthrie, Oklahoma. The slip inversion reveals a rupture with slip patches that are anti‐correlated to the locations of prior seismicity. The prior seismicity driven by low pore pressure changes and static stress changes occurred on weaker portions of the fault, while theM_w4.0 earthquake likely ruptured relatively stronger portions of the fault. To resolve if pore pressure changes or the initial underlying stress distribution and fault strength controlled the final slip distribution of the GuthrieM_w4.0 earthquake, we compare strike‐slip events of similar magnitude from tectonically active regions and previously inactive regions. Earthquakes on reactivated faults exhibit different slip distributions than active regions, they have more prominent and well separated slip patches, a behavior often associated with faults of lower fault maturity. Pore pressure shows little effect on the distributions. These observations suggest that the initial underlying stress distribution and fault strength of reactivated faults in low deformation regions is the primary controlling factor of the slip distribution with pore pressure perturbations and earthquake interactions being secondary. Therefore, GuthrieM_w4.0 earthquakes slip distribution was enhanced by pore‐pressure perturbations and earthquake interactions by creating an optimal stress state for its failure, but the slip distribution itself is controlled by its fault's initial stress and strength state.

We compare source parameter estimates for earthquakes in the 2011 Prague Mw 5.7, Oklahoma, sequence to investigate random uncertainty and systematic bias, and resolve reliable relative variations in stress drop. Source parameters provide insight into the earthquake rupture processes but large variations between studies occur. The Prague earthquake sequence is a prime example of this, with different studies reaching contrasting interpretations of the effects of injection on source parameters. We examine the Prague earthquake sequence using a single coherent catalog for all the events detected by the Oklahoma Geological Survey (OGS) and McMahon et al. (2017). We use three principal approaches to estimate stress drop in order to understand the biases of each: a spectral decomposition method based on stacking, individual event spectral modeling, and a spectral ratio method based on highly correlated events. We also compare our results with previous studies for the Prague sequences aftershocks, as well as past results for the Mw 4.8 foreshock and Mw 4.8 aftershock and Mw 5.7 mainshock. The absolute values of stress drop vary significantly between methods, but the relative patterns remain consistent, except when low quality or low bandwidth data are included. The consistent relative patterns reveal that the stress drops of aftershocks are dependent on the fault orientation and the proximity of the events to the mainshocks slip. These results indicate that fault structure as well as past events play an important role in stress drop patterns.

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 (increasingQ) 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 inQwith 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.