Spectral analysis is widely used to estimate and refine earthquake source parameters such as source radius, seismic moment, and stress drop. This study aims to quantify the precision of the single spectra and empirical Green's function spectral ratio approach using the Large-n Seismic Survey in Oklahoma (LASSO) array. The dense station coverage in an area of local saltwater disposal offers a unique opportunity to observe and quantify radiation pattern effects and subsequent precision of spectral estimates of small earthquakes (M < 3). The results suggest that the precision of source properties estimated from direct phase arrivals for arrays with less than 20 stations should be assumed to be not less than 30% and could be as high as 150% if less than five stations are used. Furthermore, we do not see clear evidence for, or against, a scaling of stress drop with magnitude of small earthquakes (M < 3) as observed by other studies. Plain Language Summary Seismologists use ground motion recordings of seismic waves (seismograms) to infer details about the earthquake rupture process. While large earthquakes often generate a physical imprint on the earth's surface through surface rupture, small earthquakes can often only be studied from seismograms. Nevertheless, small earthquakes are of particular interest to learning about the rupture process for many reasons. For example, they are much more numerous than larger magnitude earthquakes and might rupture by the same physical process(es). While seismic arrays are usually restricted to a few to tens of stations, here we use a very large seismic array with >1,800 temporary stations to study small earthquakes and how station resolution may bias source property estimates. Source properties include the physical size of the rupture surface and the corresponding slip on that surface, which relate to the amount of stress released by the earthquake. The large number of stations allows us to estimate the source properties in unique detail and test the variability in measurements using different numbers of stations to estimate the precision. We find that the estimation of source properties is highly biased when using a small number of stations (<20), which should be taken under consideration in future studies.

The estimation of earthquake static stress drop values over a wide range of magnitudes generates considerable recent interest due to the related implications for frictional models (e.g., Abercrombie, 1995), ground motion predictions (e.g., Baltay et al., 2013), and the physical differences between large and small earthquakes (e.g., Prieto et al., 2004). The average difference in shear stress on a fault (static stress drop) caused by an earthquake typically ranges between 0.1 and 100 MPa (e.g., Hanks, 1977). It can be estimated from the radiated ground motion spectrum of an earthquake by assuming a particular model for fault geometry and rupture dynamics (e.g.

ABSTRACT The clustering behavior of injection-induced earthquakes is examined using one month of data recorded by the LArge-n Seismic Survey in Oklahoma (LASSO) array. The 1829-node seismic array was deployed in a 25  km×32  km area of active saltwater disposal in northern Oklahoma between 14 April and 10 May 2016. Injection rates in the study area are nearly constant around the time of the deployment. We develop a local magnitude (ML) equation for the region and estimate magnitudes for 1104 earthquakes recorded by the deployment. The determined earthquake magnitudes range from ML 0.01 to 3.0. The majority of earthquakes occurred between 1.5 and 5.5 km depth, and the shallowest earthquake depths overlap with the base of injection wells at depths between 1.5 and 2.5 km. We compute focal mechanisms of the largest events (ML&gt;2.0), and find a mix of normal- and strike-slip-faulting types. Earthquakes occur regularly in time during the deployment, but are not evenly distributed in space across the study area, that is, they are spatially clustered. Analysis of the nearest-neighbor distances in the space–time–magnitude domain shows the seismicity is dominated by single-event clusters (i.e., independent events). This high proportion of single-event clusters compared with multievent clusters has been previously noted for induced events at geothermal sites. When clustering occurs, the number of events in a cluster is typically small. We observe only four clusters with 10 or more events. For these larger clusters, we find equivalent numbers of foreshocks and aftershocks; however, the foreshock sequences are significantly longer in duration lasting days to tens of days, while aftershock sequences are observed only on the order of one day. The minimal clustering observed for events in the LASSO array suggests that the majority of events are being directly driven by stress changes due to local saltwater disposal.

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This work presents a high resolution source property study of hydraulic fracturing induced earthquakes in the Montney Formation, a low‐permeability tight shale reservoir in the Kiskatinaw area, northeast British Columbia, Canada. We estimate source parameters, including focal mechanism solutions (FMSs), seismic moment, spectral corner frequency, and static stress drop values of earthquakes recorded between July 2017 and July 2020. Waveform‐similarity‐based event classification of 8,283 earthquakes yields 52 event families (clusters) and 1,014 isolated events (individuals). We calculate a total of 64 FMSs of events ML > 2.5 with high‐quality waveforms. Of the 64 solutions, 54 come from events within families, and are used to infer an additional 3,500 focal mechanisms of smaller‐magnitude events with similar waveforms. The other 10 are isolated events. The dominant faulting style inferred from FMSs highlights multiple cascading, shallow, strike‐slip events and generally isolated, larger‐magnitude reverse‐style events in close proximity to the Fort St. John Graben system. Inferred nodal planes of strike‐slip events are at low‐angles to regional SHmax, suggesting optimally oriented, left‐lateral faults. Reverse faulting nodal planes are roughly perpendicular to SHmax and have an orientation consistent with the reactivation of pre‐existing normal basement faults. Source spectral analysis using three approaches, including spectral‐ratio fitting, suggests a constant stress drop of 1–10 MPa and self‐similarity for induced events. Constant stress drop scaling breaks down at magnitudes smaller than ∼ML 2.0, likely due to observational bandwidth limitations.