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.

We develop a rate- and state-dependent friction (RSF) model to investigate a compendium of recent experiments performed in the laboratory. In the documented experiments, a fault was sheared until macroscopic stick-slip frictional failure. Before macro-failure, small precursor seismicity nucleated from regions that also experienced aseismic slow slip. This behavior requires heterogeneity and is defined in our model as local variation in frictional parameters inferred from the roughness. During sliding wear introduced a smooth-polished surface onto a previously rough surface and was quantified using a bimodal Gaussian distribution of surface heights. We used spatial distribution of the smooth and rough sections to impose binary partitioning in critical slip distance $D_{c}$ to a planar frictional model. Simulations revealed that local seismicity nucleated on the “smooth’ sections, while the larger “rough’ section hosted aseismic slip. As the level of heterogeneity between smooth and rough sections increased, the model transitioned from a predominantly stick-slip to creeping. The simulations produced a dominant asperity, which appeared to control aspects of rupture nucleation: ($i$) weak heterogeneity caused the dominant asperity to generate foreshocks but also “ignite’ cascade-up fault-wide event, while ($ii$) strong heterogeneity led to constrained repeaters. Seismic source properties: average slip $\delta$, seismic moment $M_{0}$, stress drop $\Delta \tau$ and fracture energy $G^{’}$, were determined for each event and agreed with separate kinematic estimates made independently from seismic measurements. Our numerical calculations provide insight into rate-dependent cascade-up nucleation theory where frictional heterogeneity here was associated with wear of solid frictional contacts in the laboratory.

A high-resolution 3-D crustal and upper-mantle shear-wave velocity model of Northeast China is established by joint inversion of receiver functions and Rayleigh wave group velocities. The teleseismic data for obtaining receiver functions are collected from 107 CEA permanent sites and 118 NECESSArray portable stations. Rayleigh wave dispersion measurements are extracted from an independent tomographic study. Our model exhibits unprecedented detail in S-velocity structure. Particularly, we discover a low S-velocity belt at 7.5-12.5 km depth covering entire Northeast China (except the Songliao basin), which is attributed to a combination of anomalous temperature, partial melts and fluid-filled faults related to Cenozoic volcanism. Localized crustal fast S-velocity anomaly under the Songliao basin is imaged and interpreted as late-Mesozoic mafic intrusions. In the upper mantle, our model confirms the presence of low velocity zones below the Changbai mountains and Lesser Xing’an mountain range, which agree with models invoking sub-lithospheric mantle upwellings. We observe a positive S-velocity anomaly at 50-90 km depth under the Songliao basin, which may represent a depleted and more refractory lithosphere inducing the absence of Cenozoic volcanism. Additionally, the average lithosphere-asthenosphere boundary depth increases from 50-70 km under the Changbai mountains to 100 km below the Songliao basin, and exceeds 125 km beneath the Greater Xing’an mountain range in the west. Furthermore, compared with other Precambrian lithospheres, Northeast China likely has a rather warm crust (~480-970 °C) and a slightly warm uppermost mantle (~1200 °C), probably associated with active volcanism. The Songliao basin possesses a moderately warm uppermost mantle (~1080 °C).

Understanding the origin of seismic swarms can be controversial, especially when they occur near volcanic areas. Here, we investigate a seismic sequence which is steadily active in a non-volcanic area close by the volcanic field of Harrat Lunayyir in the western shield of Arabia. Our results unveil a planar zone of seismicity with ~5 km long E-W, sub-vertically ~9 km south-dipping structure, which is characterized by a dominant tensional focal mechanism. Independent evidence for the tectonic style dominance came from assessing the ground deformation images using the InSAR technique. This local seismicity might be attributed to a reactivated structure along a regional weakness zone of the Najd Fault System, which dominates the Precambrian structure of our area. Comparing the effects of high- and low-frequency datasets for the moment tensor inversion conclude a consistency of our solution. The frequency index analysis for P- and S- waves spectral datasets, does not suggest fluid-driven processes. We observe average stress drop of ~5.40 MPa with corner frequency of ~2.75 Hz. Our study confirms a localized reactivation of a brittle crustal seismogenic zone in the area of interest. This interpretation relies on the integration of several analysis methods, including spatial and magnitude-frequency distributions statistics.

We investigate experimental results from a direct shear friction apparatus, where a fault was formed by pressing mature, worn surfaces of two polymethyl methacrylate (PMMA) samples on top of each other in a dry environment. The fault was sheared until macroscopic stick-slip frictional failure occurred. Before the macro-failure small precursory seismicity nucleated from regions that also experienced aseismic slow slip. These precursory events did not cascade-up into gross fault rupture and arrested locally. Reasons as to why ruptures arrested are investigated using a 1-D rate and state friction (RSF) model. Surface profilometry of the fault surface taken \textit{a posteriori} revealed wear in the form of a bimodal Gaussian distribution of surface height. In our model, this unique distribution of surface roughness is determined to be a proxy for the heterogeneous spatial description of the critical slip distance $D_{c}$. We assume that smooth (polished) sections of fault exhibited lower $D_{c}$ than rougher sections of the bimodal Gaussian roughness profile. We used a quasi-dynamic RSF model that determined localized seismicity initiated at the smooth sections. Source properties: average slip $\delta$, seismic moment $M_{0}$, stress drop $\Delta \tau$ and fracture energy $G^{’}$, were determined for each event. We compare the numerically modeled source properties to experimental source characteristics inferred from seismological estimates using an array of acoustic emission sensors from a concerted study. We discuss similarities, discrepancies and assumptions between these two independent models (kinematic and dynamic) used to study earthquakes for the first time in the laboratory.