Small Earthquakes Matter in Injection‐Induced Seismicity

Abstract

A simplistic triggering mechanism, pore pressure increase from injection, has been the focus of injection‐induced seismicity studies for decades. Research into other possible mechanisms, like poroelastic stress changes, is ongoing, but there has been relatively little focus on earthquake interaction. While studies have looked at how moderate‐magnitude events (M ≥ 3.0) may trigger larger magnitude‐induced seismicity, research into the cumulative effect of the hundreds to thousands of small‐magnitude (M ≤ 3.0) events is lacking. Here we use generic models to compare the possible stress changes from pore pressure increase and from earthquake interactions of small‐magnitude events. We find that the area of increased pore pressure is much larger than that of positive Coulomb static stress transfer; however, maximum Coulomb static stress change is larger than maximum pore pressure increase. We argue that, yes, small earthquakes do matter, and their interaction may be an important triggering mechanism to consider.

Source Complexity of the 2015 Mw 4.0 Guthrie, Oklahoma Earthquake

“…High‐resolution earthquake relocations and focal mechanisms reveal complex basement fault systems that show poor correlation with previously mapped faults (Chen et al, ; McNamara et al, ; Schoenball & Ellsworth, ). In addition to the stress changes from injection, stress interactions between earthquakes, even at small magnitudes (M < 3), contribute to spatiotemporal evolution of seismicity by static and/or dynamic stress triggering (Brown & Ge, ; Pennington & Chen, ; Qin et al, ; Sumy et al, ). Investigation of the detailed source processes of these induced earthquakes is needed to understand the underlying physics governing rupture growth and triggering processes.…”

Causal mechanism of injection-induced earthquakes through the Mw 5.5 Pohang earthquake case study

“…Once initial seismicity is induced, Coulomb stress transfer can play a role in the stability of faults in affected areas 17,19,20,41 . With seismicity occurring on a fault, static Coulomb stress transfer will follow, which may bring the fault closer to failure as indicated by the 66-71% of events that had Coulomb static stress transfer modeling occurring in areas of positive Coulomb stress change.…”

“…Research is on-going to understand the mechanism of why and how post-injection earthquakes occur, particularly at the wastewater injection induced seismicity area. Poroelastic stress transfer 14 , aseismic creep 15,16 , density-driven pressure transients 52 , and static Coulomb stress change 17,18,20 are some of the mechanisms proposed. This study adds to the ongoing research to better understand the phenomena that largest events can occur after the injection ceases.…”

“…In-situ experiments have demonstrated that aseismic slips can create stresses that could trigger earthquakes beyond the spatial extent of pore pressure diffusion 15 and such aseismic slip stress could propagate faster than pore pressure diffusion 16 . Cumulative Coulomb stress changes from smaller earthquakes on a fault can match or exceed the values of pore pressure increase and therefore become the triggering mechanisms 17,18 . Coulomb static stress can be modified to take into consideration pore pressure effects by replacing the friction coefficient with an effective friction coefficient 19,20 , in which pore pressures were estimated based on mean volumetric strain on faults but do not change with time.…”

Seismic and Microseismic Signatures of Fluids in Rocks: Bridging the Scale Gap

“…Monitoring MS (field scale) or AE (laboratory scale) activity has been successfully used to remotely characterize rock fracturing and slip events (e.g., hydraulic fracturing, shear fracturing/faulting, and reactivation), as well as fluid migration through porous and/or fractured rocks when local pore pressure perturbations impact the in situ stress field. Most recently, numerous studies focused on the interplay between fluids, fractures, and seismicity have been published (e.g., Barbosa et al, ; Brown & Ge, ; Cueto‐Felgueroso et al, ; De Barros et al, ; Dempsey et al, ; Dempsey & Suckale, ; Diehl et al, ; Fazio et al, ; Goodfellow et al, ; Improta et al, ; Jeanne et al, ; Levandowski et al, ; Noël et al, ; Rivet et al, ; Segall & Lu, ; Zhang et al, ). This highlights the growing interest for such issues in the geoscience community, which is clearly driven by an increased need to better address actual problems encountered during fluid injection/withdrawal operations.…”