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.

We investigate the influence of local site effects on earthquake source parameter estimates using the LArge-n Seismic Survey in Oklahoma (LASSO). The LASSO array consisted of 1825 stations in a 25 km x 32 km region with extensive wastewater injection and recorded more than 1500 local events (M < 3) during spring 2016. We analyze the site amplification dependence on earthquake corner frequency (fc), seismic moment (M0), and stress drop estimated by modeling individual spectra. We evaluate and correct these site effects and compare the effectiveness of the correction to results using the spectral ratio method. We estimate local site amplification at each station using the average Peak-Ground-Velocity (PGV) of 14 regional earthquakes (~130 km away). The fc from the single spectrum method negatively correlates with site amplification, whereas M0 from the single spectrum method positively correlates with site amplification. This suggests the source parameters calculated by modeling individual spectra are biased by the local site effects. The high amplifications are typically located on young alluvial sedimentary deposits. We correct site effects by removing the trend between PGV and these two parameters in the regression analysis, which reduces the standard deviation of these parameters across the array and makes the calculated stress drop less site dependent. We compare corrections using other site-effect proxies such as the Root-Mean-Square (RMS) amplitude, surface geological formation, P-arrival-delay, and topographic slope. The PGV and the RMS corrections provide the greatest reduction of the spatial deviation of source parameters. In comparison, the spectral ratio method effectively removes the site effects using the Empirical Green’s Function (EGF) approach. The trends being removed by EGF are close to the apparent trends between the single spectrum estimated parameters and the PGV, which suggests the consistency of these different correction approaches. Our results provide a potential way to remove the site effects when only the main event spectrum is available and demonstrates the effectiveness of using the EGF approach for removing site effects. The resulting inter-station variability provides an estimate of the likely uncertainty in source parameters estimated from smaller numbers of stations.

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.

We calculate high-precision absolute and relative earthquake relocations to investigate the relationship between seismicity and major active faults, and to explore variation in seismogenic depths across the Northern Walker Lane. We first compute datum-adjusted and station-residual-corrected absolute relocations, before relocating events using waveform cross-correlation. Of 40,581 routinely located earthquakes between 2002 and 2018, we relocate 27,132 (66.9%) with resulting median horizontal and vertical location uncertainties less than ~100 m. We then compute 95thpercentile depths as a proxy for seismogenic depth and compare to published Moho depths. Microseismicity occurs in large highly clustered source areas, often consisting of many short, distinct fault structures. Activity concentrates near the ends of mapped Quaternary faults rather than along them. Microseismicity-defined structures in transition zones between major surface faults may identify active fault networks that link faults at the depth. Seismogenic depth shallows away from the Sierra Nevada to the east-northeast over approximately 80 km, from an approximate depth of 17 km to 13 km. This follows, to scale, the decrease in Moho depth across the same region from about 35 km to 30 km. We compare seismogenic and Moho depths to topographic relief and heat flow measurements to discuss controls on the depth of seismicity in the region. Heat flow increases smoothly over the same region of the decreasing seismogenic and Moho depth, increasing by as much as 20 mW/m2.

Seismicity along transform faults provides important constraints for our understanding of the factors that control earthquake ruptures. Oceanic transform faults are particularly useful due to their relatively simple structure in comparison to continental counterparts. The seismicity of several fast-moving transform faults has been investigated by local networks, but as of today there have not been many studies of slower spreading centres. Here we present the first local seismicity catalogue based on event data recorded by a temporary broadband network of 39 ocean bottom seismometers located around the slow-moving Chain Transform Fault (CTF) along the Mid-Atlantic Ridge (MAR) from March 2016 to March 2017. Locations are constrained by simultaneously inverting for a 1-D velocity model informed by the event P- and S-arrival times. Depths and focal mechanisms of the larger events are refined using deviatoric moment tensor inversion. We find a total of 972 events in the area. Most of the seismicity is located at the CTF (700) and Romanche transform fault (94) and the MAR (155); a smaller number (23) can be observed on the continuing fracture zones or in intraplate locations. The ridge events are characterised by normal faulting and most of the transform events are characterised by strike slip faulting, but with several reverse mechanisms that are likely related to transpressional stresses in the region. CTF events range in magnitude from 1.1 to 5.6 with a magnitude of completeness around 2.3. Along the CTF we calculate a b-value of 0.81 ± 0.09. The event depths are mostly shallower than 15 km below sea level (523), but a small number of high-quality earthquakes (16) are located deeper, with some (8) located deeper than the brittle-ductile transition as predicted by the 600˚C-isotherm from a simple thermal model. The deeper events could be explained by the control of seawater infiltration on the brittle failure limit.