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Seismic tomography images of the upper mantle structures beneath the Pacific Northwestern United States display a maze of high-velocity anomalies, many of which produce distorted waveforms evident in the USArray observations indicative of the Juan de Fuca (JdF) slab. The inferred location of the slab agrees quite well with existing contour lines defining the slab's upper interface. Synthetic waveforms generated from a recent tomography image fit teleseismic travel times quite well and also some of the waveform distortions. Regional earthquake data, however, require substantial changes to the tomographic velocities. By modeling regional waveforms of the 2008 Nevada earthquake, we find that the uppermost mantle of the 1D reference model AK135, the reference velocity model used for most tomographic studies, is too fast for the western United States. Here, we replace AK135 with mT7, a modification of an older Basin-and-Range model T7. We present two hybrid velocity structures satisfying the waveform data based on modified tomographic images and conventional slab wisdom. We derive P and SH velocity structures down to 660 km along two cross sections through the JdF slab. Our results indicate that the JdF slab is subducted to a depth of 250 km beneath the Seattle region, and terminates at a shallower depth beneath Portland region of Oregon to the south. The slab is about 60 km thick and has a P velocity increase of 5% with respect to mT7. In order to fit waveform complexities of teleseismic Gulf of Mexico and South American events, a slab-like high-velocity anomaly with velocity increases of 3% for P and 7% for SH is inferred just above the 660 discontinuity beneath Nevada.

Upper mantle seismic velocity structures in both vertical and horizontal directions are key to understanding the structure and mechanics of tectonic plates. Recent deployment of the USArray Transportable Array (TA) in the Midwestern United States provides an extraordinary regional earthquake data set to investigate such velocity structure beneath the stable North American craton. In this paper, we choose an M_w5.1 Canadian earthquake in the Quebec area, which is recorded by about 400 TA stations, to examine the P wave structures between the depths of 150 km to 800 km. Three smaller Midwestern earthquakes at closer distance to the TA are used to investigate vertical and horizontal variations in P velocity between depths of 40 km to 150 km. We use a grid-search approach to find the best 1-D model, starting with the previously developed S25 regional model. The results support the existence of an 8° discontinuity in P arrivals caused by a negative velocity gradient in the lithosphere between depths of 40 km to 120 km followed by a small (∼1%) jump and then a positive gradient down to 165 km. The P velocity then decreases by 2% from 165 km to 200 km, and we define this zone as the regional lithosphere-asthenosphere boundary (LAB). Beneath northern profiles, waves reflected from the 410 discontinuity (410) are delayed by up to 1 s relative to those turning just below the 410, which we explain by an anomaly just above the discontinuity with P velocity reduced by ∼3%. The 660 discontinuity (660) appears to be composed of two smaller velocity steps with a separation of 16 km. The inferred low-velocity anomaly above 410 may indicate high water concentrations in the transition zone, and the complexity of the 660 may be related to Farallon slab segments that have yet to sink into the deep mantle.

We determined the location, size, mechanism, and the frequency content of the first 4.0 s of the 2011 Tohoku–Oki earthquake. Since the beginning of this earthquake is very small, we develop a comparative approach against a near-by reference earthquake, the master event. We first determined the water depth near the master event using the differential timing between the water phase pwP reflected from the air–water interface and the depth phase pP reflected from the water–crust interface. Then we located the master event using the well-known ocean bathymetry in the area. After calibrating teleseismic arrays (Δ=30° to 90°) at short periods for timing and amplitude with respect to the master event, we were able to determine the initiation of themain event. It began as a small (M_w=4.9) thrust event located at 38.19°N, 142.68°E at a depth of 21 km, and, a few seconds later, evolved into a slower extremely large slip event up-dip.

A recent prospective on the Yellowstone Caldera discounts its explosive potential based on inferences from tomographic studies which suggests a high degree of crystallization of the underlying magma body. In this study, we show that many of the first teleseismic P-wave arrivals observed at seismic stations on the edge of the caldera did not travel through the magma body but have taken longer but faster paths around the edge. After applying a number of waveform modeling tools, we obtain much lower seismic velocities than previous studies, 2.3 km/sec (V_p) and 1.1 km/sec (V_s). We estimate the physical state of the magma body by assuming a fluid-saturated porous material consisting of granite and a mixture of rhyolite melt and water and CO_2 at a temperature of 800°C and pressure at 5 km (0.1 GPa). We found that this relatively shallow magma body has a volume of over 4,300 km^3 and is about 32% melt saturated with about 8% water plus CO_2 by volume.

The back projection method is a tremendously powerful technique for investigating the time dependent earthquake source, but its physical interpretation is elusive. We investigate how earthquake rupture heterogeneity and directivity can affect back‐projection results (imaged location and beam power) using synthetic earthquake models. Rather than attempting to model the dynamics of any specific real earthquake, we use idealized kinematic rupture models, with constant or varying rupture velocity, peak slip rate, and fault‐local strike orientation along unilateral or bilateral rupturing faults, and perform back‐projection with the resultant synthetic seismograms. Our experiments show back‐projection can track only heterogeneous rupture processes; homogeneous rupture is not resolved in our synthetic experiments. The amplitude of beam power does not necessarily correlate with the amplitude of any specific rupture parameter (e.g., slip rate or rupture velocity) at the back‐projected location. Rather, it depends on the spatial heterogeneity around the back‐projected rupture front, and is affected by the rupture directivity. A shorter characteristic wavelength of the source heterogeneity or rupture directivity toward the array results in strong beam power in higher frequency. We derive an equation based on Doppler theory to relate the wavelength of heterogeneity with synthetic seismogram frequency. This theoretical relation can explain the frequency‐ and array‐dependent back‐projection results not only in our synthetic experiments but also to analyze the 2019 M7.6 bilaterally rupturing New Ireland earthquake. Our study provides a novel perspective to physically interpret back‐projection results and to retrieve information about earthquake rupture characteristics., Plain Language Summary: With the deployment of continental scale seismic arrays, seismologists can quickly locate the high‐frequency seismic radiation sources and track the earthquake rupture propagation using a technique called back‐projection. It is a signal beamforming technique application in seismology, and similar applications can be found in fields such as radar, wireless communication, and radio astronomy. Recent studies have proposed multiple advancements in improving the back‐projection location. However, the physical interpretation of the amplitude of stacked high‐frequency source radiations, which is commonly referred to as beam power, is still challenging since the analysis is not based on a forward model. In this article, we conduct a set of synthetic experiments to investigate the physical significance of back‐projection beam power. We find that beam power is mainly controlled by the spatial heterogeneity wavelength near the rupture front, rupture directivity, and the seismogram frequency. It is in contrast with some previous studies that link the beam power to the maximum slip rate (acceleration) amplitude near the rupture front. Based on the results, we develop a novel theoretical framework that can quantitatively interpret the frequency‐ and array‐dependent back‐projection results not only in our synthetic experiments, but also the 2019 bilateral rupture M7.6 New Ireland earthquake., Key Points: We use kinematic forward models to investigate the relation between back‐projection beam location, power and earthquake source properties. Frequency‐dependent back‐projection peak beam power depends on the spatial heterogeneity near the rupture front, and rupture directivity. We develop a novel framework to analyze frequency‐ and array‐dependent back‐projection results, including the 2019 M7.6 New Ireland Event., European Union's Horizon 2020 (ChEESE), European Research Council (ERC), German Research Foundation (DGF), KAUST‐CRG, Leon and Joann V.C. Knopoff, NSF EAR, https://doi.org/10.7914/SN/AK, https://doi.org/10.7914/SN/AT, https://doi.org/10.7914/SN/AV, https://doi.org/10.7914/SN/BW, https://doi.org/10.7914/SN/CC, https://doi.org/10.7914/SN/CI, https://doi.org/10.7914/SN/CN, https://doi.org/10.7914/SN/II, https://doi.org/10.7914/SN/IU, https://doi.org/10.7914/SN/TA, https://doi.org/10.7914/SN/UW