Earthquake focal mechanism data provide information about the stress state at the origin of these earthquakes. The inversion methods that are commonly used to infer the stress tensor from focal mechanisms have varying complexity but always rely on a number of assumptions. We present an iterative method built upon a classic linear stress tensor inversion that allows to relax the assumption on shear stress magnitudes while preserving the computational simplicity of the linear problem. Every iteration of our method computes the least-squares solution of the problem, which makes the method fast enough to estimate the inverted parameter errors with non-parametric resampling methods such as bootstrapping. Following previous studies, this method removes the fault plane ambiguity in focal mechanism data by selecting the nodal plane that best satisfies the Mohr-Coulomb failure criterion. We first test the performance and the robustness to noise of the proposed method on synthetic data sets, and then apply it to data from Southern California and from the Geysers geothermal field. We focus the study on investigating the consequences of relaxing the assumption of constant shear stress magnitudes. Our variable shear method successfully generalizes its constant shear counterpart: it is able to perform similarly when the constant shear assumption is a good approximation and provides more accurate results when it is not. We provide the Python package ILSI to implement the proposed method.

The 17 August 1999 $M_{w}$7.4 Izmit earthquake ruptured the western section of the North Anatolian Fault Zone (NAFZ) and strongly altered the fault zone properties and stress field. Consequences of the co- and post-seismic stress changes were seen in the spatio-temporal evolution of the seismicity and in the surface slip rates. Thirteen years after the Izmit earthquake, in 2012, the dense seismic array DANA was deployed for 1.5 years. We built a new catalog of microseismicity (M < 2) by applying our automated detection and location method to the DANA data set. Our method combines a systematic backprojection of the seismic wavefield and template matching. We analyzed the statistical properties of the catalog by computing the Gutenberg-Richter b-value and by quantifying the amount of temporal clustering in groups of nearby earthquakes. We found that the microseismicity mainly occurs off the main fault and that the most active regions are the Lake Sapanca step-over and near the Akyazi fault. Based on previous studies, we interpreted the b-values and temporal clustering \textit{i}) as indicating that the Akyazi seismicity is occurring in high background stresses and is driven by the Izmit earthquake residual stresses, and \textit{ii}) as suggesting evidence that intricate seismic and aseismic slip was taking place on heterogeneous faults at the eastern Lake Sapanca, near the brittle-ductile transition. Geodesy shows enhanced north-south extension around Lake Sapanca following the Izmit earthquake, therefore, the seismicity supports the possibility of slow slip at depth in the step-over.

A better understanding of Earth’s core-mantle boundary (CMB) region is required to address major questions about our planet’s internal dynamics, magnetic field, and thermal evolution. Valuable constraints have come from observations of (CMB-) Stoneley modes, a class of seismic free oscillation whose displacement decreases away from the solid-fluid boundary. The high-frequency modes that are most sensitive to the CMB region are too localized there to be observed at Earth’s surface. Here we clarify why some higher-frequency Stoneley modes can be detected: via ‘mixing’ with surface-localized Rayleigh-type modes of similar frequency. We examine the concept of mixed Rayleigh-Stoneley modes analytically and with a finite-element method. Our calculations show that mixed modes are a sensitive probe of radial and lateral variations in material properties near the CMB. More generally, ‘seismic waveguide coupling’ could help to characterize systems ranging from cell membranes to Pluto’s lithosphere.

The continental lithospheric mantle plays an essential role in stabilizing continents over long geological time scales. Quantifying spatial variations in compositional and thermochemical properties of the mantle lithosphere is crucial to understanding its formation and its impact on continental stability; however, our understanding of these variations remains limited. Here we apply the Whole-rock Interpretive Seismic Toolbox For Ultramafic Lithologies (WISTFUL) to estimate thermal, compositional, and density variations in the continental mantle beneath the contiguous United States from MITPS_20, a joint body and surface wave tomographic inversion for Vp and Vs with high resolution in the shallow mantle (60‒100 km). Our analysis shows lateral variations in temperature beneath the continental United States of up to 800–900°C at 60, 80, and 100 km depth. East of the Rocky Mountains, the mantle lithosphere is generally cold (350–850°C at 60 km), with higher temperatures (up to 1000°C at 60 km) along the Atlantic coastal margin. By contrast, the mantle lithosphere west of the Rocky Mountains is hot (typically >1000°C at 60 km, >1200°C at 80–100 km), with the highest temperatures beneath Holocene volcanoes. In agreement with previous work, we find that the predicted chemical depletion does not fully offset the density difference due to temperature. Extending our results using Rayleigh-Taylor instability analysis, implies the lithosphere below the United States could be undergoing oscillatory convection, in which cooling, densification, and sinking of a chemically buoyant layer alternates with reheating and rising of that layer.