Juan de Fuca subduction zone from a mixture of tomography and waveform modeling

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 P velocity structure beneath the Midwestern United States derived from triplicated waveforms

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

Determination of earthquake focal depths and source time functions in central Asia using teleseismic P waveforms

We developed a new method to determine earthquake source time functions and focal depths. It uses theoretical Green's function and a time-domain deconvolution with positivity constraint to estimate the source time function from the teleseismic P waveforms. The earthquake focal depth is also determined in the process by using the time separations of the direct P and depth phases. We applied this method to 606 earthquakes between 1990 and 2005 in Central Asia. The results show that the Centroid Moment Tensor solutions, which are routinely computed for earthquake larger than M5.0 globally using very long period body and surface waves, systematically over-estimated the source depths and durations, especially for shallow events. Away from the subduction zone, most of the 606 earthquakes occurred within the top 20 km of crust. This shallow distribution of earthquakes suggests a high geotherm and a weak ductile lower crust in the region

Lithospheric waveguide beneath the Midwestern United States; massive low-velocity zone in the lower crust

Variations in seismic velocities are essential in developing a better understanding of continental plate tectonics. Fortunately, the USArray has provided an excellent set of regional phases from the recent M5.6 Oklahoma earthquake (6 November 2011, Table 1) that can be used for such studies. Its strike-slip mechanism produced an extraordinary set of tangential recordings extending to the northern edge of the USArray. The crossover of the crustal slow S to the faster S_n phase is well observed. S_mS has a critical distance of around 2° and its first multiple, SmS^2, reaches critical angle near a distance of about 4°, and so on, until S_mS^n merges with the stronger crustal Love waves. These waveforms are modeled in the period band of 2–100 s by assuming a simple three-layer crust and a two-layer mantle, which allows a grid-search approach. Our results favor a 15 km thick low-velocity zone (LVZ) in the lower crust with an average shear velocity of less than 3.6 km/s. The short-period Lg waves (S waves, at periods of 0.5–2 s) travel with velocities near 3.5 km/s and decay with distance faster than high-frequency S_n (>5.0 Hz) which travels at a velocity of 4.6 km/s and persists to large distances. Although these short-period waveforms are not modeled, their amplitude and travel times can be explained by adding a small velocity jump just below the Moho with essentially no attenuation. P_n is equally strong but is complicated by the interference produced by the depth phase sP, but well modeled. The P velocities appear normal with no definitive LVZ. While these observations of S_n and P_n are common beneath most cratons, the lower crustal LVZ appears to be anomalous and maybe indicative of hydrous processes, possibly caused by the descending Farallon slab

Source Parameters of the Shallow 2012 Brawley Earthquake, Imperial Valley

Resolving earthquake parameters, especially depth, is difficult for events occurring within basins because of issues involved with separating source properties from propagational path effects. Here, we demonstrate some advantages of using a combination of teleseismic and regional waveform data to improve resolution following a bootstrapping approach. Local SS‐S differential arrivals from a foreshock are used to determine a local layered model which can then be used to model teleseismic depth phases: pP, sP, and sS. Using the cut‐and‐paste (CAP) method for which all strike (θ), dip (δ), rake (λ), and depth variations are sampled for several crustal models. We find that regional data prove the most reliable at fixing the strike, whereas the depth is better constrained by teleseismic data. Weighted solutions indicate a nearly pure strike‐slip mechanism (θ=59°±1°) with a centroid depth of about 4.0 km and an M_w of 5.4 for the mainshock of the 2012 Brawley earthquake

gCAPjoint, A Software Package for Full Moment Tensor Inversion of Moderately Strong Earthquakes with Local and Teleseismic Waveforms

Earthquake moment tensors and focal depths are crucial to assessing seismic hazards and studying active tectonic and volcanic processes. Although less powerful than strong earthquakes (M 7+), moderately strong earthquakes (M 5–6.5) occur more frequently and extensively, which can cause severe damages in populated areas. The inversion of moment tensors is usually affected by insufficient local waveform data (epicentral distance <5°⁠) in sparse seismic networks. It would be necessary to combine local and teleseismic data (epicentral distance 30°–90°) for a joint inversion. In this study, we present the generalized cut‐and‐paste joint (gCAPjoint) algorithm to jointly invert full moment tensor and centroid depth with local and teleseismic broadband waveforms. To demonstrate the effectiveness and explore the limitations of this algorithm, we perform case studies on three earthquakes with different tectonic settings and source properties. Comparison of our results with global centroid moment tensor and other catalog solutions illustrates that both non‐double‐couple compositions of the focal mechanisms and centroid depths can be reliably recovered for very shallow (⁠<10  km⁠) earthquakes and intermediate‐depth events with this software package

Hidden hotspot track beneath the eastern United States

Hotspot tracks are thought to be the surface expressions of tectonic plates moving over upwelling mantle plumes, and are characterized by volcanic activity that is age progressive. At present, most hotspot tracks are observed on oceanic or thin continental lithosphere. For old, thick continental lithosphere, such as the eastern United States, hotspot tracks are mainly inferred from sporadic diamondiferous kimberlites putatively sourced from the deep mantle. Here we use seismic waveforms initiated by the 2011 M_w 5.6 Virginia earthquake, recorded by the seismic observation network USArray, to analyse the structure of the continental lithosphere in the eastern United States. We identify an unexpected linear seismic anomaly in the lower lithosphere that has both a reduced P-wave velocity and high attenuation, and which we interpret as a hotspot track. The anomaly extends eastwards, from Missouri to Virginia, cross-cutting the New Madrid rift system, and then bends northwards. It has no clear relationship with the surface geology, but crosses a 75-million-year-old kimberlite in Kentucky. We use geodynamical modelling to show that an upwelling thermal mantle plume that interacts with the base of continental lithosphere can produce the observed seismic anomaly. We suggest that the hotspot track could be responsible for late Mesozoic reactivation of the New Madrid rift system and seismicity of the eastern United States

Fault-Plane Determination of the 18 April 2008 Mount Carmel, Illinois, Earthquake by Detecting and Relocating Aftershocks

We developed a sliding-window cross-correlation (SCC) detection technique and applied the technique to continuous waveforms recorded by the Cooperative New Madrid Seismic Network stations following the 18 April 2008 Illinois earthquake. The technique detected more than 120 aftershocks down to M_L 1.0 in the 2 week time window following the mainshock, which is three times more than the number of aftershocks reported by the seismic network. Most aftershocks happened within 24 hrs of the mainshock. We then relocated all events by the double-difference relocation algorithm. Accurate P- and S-wave differential arrival times between events were obtained by waveform cross correlation. After relocation, we used the L1 norm to fit all located events by a plane to determine the mainshock fault plane. The best-fit plane has a strike of 292°±11° and dips 81°±7° to the northeast. This plane agrees well with the focal mechanism solutions of the mainshock and four largest aftershocks. By combining the aftershock locations and focal mechanism solutions, we conclude that the 18 April earthquake occurred on a nearly vertical left-lateral strike-slip fault orienting in the west-northwest–east-southeast direction. The fault coincides with the proposed left-stepping Divide accommodation zone in the La Salle deformation belt and indicates reactivation of old deformation zone by contemporary stresses in the Midcontinent

Upper mantle surprises derived from the recent Virginia earthquake waveform data

Recent high resolution regional waveform modeling reveals that the lithosphere beneath the North American craton is subdivided into an upper nearly uniform layer and a lower layer with a high velocity gradient. The boundary occurs at about a depth of 115 km and is responsible for 8° discontinuity in seismic record sections that is often observed in craton environments. Unexpectedly, we find seismic velocities in the lower layer significantly reduced along a corridor from the New Madrid rift zone to Virginia. This reduced velocity in the lower lithosphere may be associated with a possible historic hotspot activity. We also find a well developed X-discontinuity that we model as a ∼3% increase in P velocity starting at a depth of ∼290 km. These anomalous features transition into a nearly 1D craton structure to the north with a strong low-velocity anomaly just above the 410 discontinuity along an east–west boundary. The latter two features may be relics of structures formed from the descending Farallon plate between Late Cretaceous and Early Tertiary