We perform inversions for the shear-wave velocity structure of the southeastern United States (SEUS) using Rayleigh-wave phase and amplitude data from the broadband stations of the South Eastern Suture of the Appalachian Margin Experiment (SESAME) and EarthScope Transportable Array (TA). Our tomographic images of shear-wave velocities in the upper mantle beneath the SEUS provide new constraints on the evolution of mantle lithosphere, both from the inheritance of structures from repeated Wilson cycles and from processes that have occurred while at a passive margin setting. Our images also allow us to correlate these structures to evidence of Eocene to recent tectonism observed at the surface. We find evidence for both inherited structures and more recently evolved structures, both of which bear some correlation to observations of ongoing tectonism. Our results suggest that lithospheric mantle continues to evolve while in a passive margin setting and that even relatively "stable" continental mantle lithosphere is subject to episodes of delamination, foundering, and erosion due to processes that are still not well understood. Our results provide structural constraints on the types of processes that may be ongoing and on possible explanations for the numerous observations of comparatively recent tectonic activity occurring along this passive margin setting.

The 30 November 2017 Delaware earthquake with magnitude M-w 4.2 occurred beneath the northeastern tip of the Delmarva Peninsula near Dover, Delaware. The earthquake and its aftershocks provide an opportunity to evaluate seismicity in a passive margin setting using much improved coverage by high-quality permanent broadband seismometers at regional distance ranges in the central and eastern United States. This is the largest instrumentally recorded earthquake in Delaware, and it triggered a collaborative rapid-response effort by seismologists at five institutions along the mid-Atlantic. As a result of this effort, 18 portable seismographs were deployed in the epicentral region within 24 hrs of the mainshock. High-quality seismic recordings at more than 380 permanent regional broadband seismographic stations in the eastern United States show a remarkably small decrease in amplitude with distance between 800 and 2000 km. The mainshock focal mechanism shows predominantly strike slip with a significant thrust component. The orientation of the subhorizontal P axis is consistent with that of earthquakes in the nearby Reading-Lancaster seismic zone in Pennsylvania, but the trend is rotated counterclockwise about 45 degrees from that of the M-w 5.8 Mineral, Virginia, earthquake. We detected small aftershocks below the normal event detection threshold using a waveform cross-correlation detection method. This demonstrated the effectiveness of this approach for earthquake studies and hazard evaluation in the eastern United States. Based on their waveform similarities, repeating earthquakes with magnitudes greater than 1.5 are detected in 2010, 2015, and 2017. Although there is a large time interval between events, 5 and 2.2 yrs, respectively, the events occur within a spatially tight cluster located near the 2017 Dover, Delaware, earthquake mainshock.

The 30 November 2017 Delaware earthquake with magnitude M-w 4.2 occurred beneath the northeastern tip of the Delmarva Peninsula near Dover, Delaware. The earthquake and its aftershocks provide an opportunity to evaluate seismicity in a passive margin setting using much improved coverage by high-quality permanent broadband seismometers at regional distance ranges in the central and eastern United States. This is the largest instrumentally recorded earthquake in Delaware, and it triggered a collaborative rapid-response effort by seismologists at five institutions along the mid-Atlantic. As a result of this effort, 18 portable seismographs were deployed in the epicentral region within 24 hrs of the mainshock. High-quality seismic recordings at more than 380 permanent regional broadband seismographic stations in the eastern United States show a remarkably small decrease in amplitude with distance between 800 and 2000 km. The mainshock focal mechanism shows predominantly strike slip with a significant thrust component. The orientation of the subhorizontal P axis is consistent with that of earthquakes in the nearby Reading-Lancaster seismic zone in Pennsylvania, but the trend is rotated counterclockwise about 45 degrees from that of the M-w 5.8 Mineral, Virginia, earthquake. We detected small aftershocks below the normal event detection threshold using a waveform cross-correlation detection method. This demonstrated the effectiveness of this approach for earthquake studies and hazard evaluation in the eastern United States. Based on their waveform similarities, repeating earthquakes with magnitudes greater than 1.5 are detected in 2010, 2015, and 2017. Although there is a large time interval between events, 5 and 2.2 yrs, respectively, the events occur within a spatially tight cluster located near the 2017 Dover, Delaware, earthquake mainshock.

Subduction systems play a key role in plate tectonics, but the deformation of the crust and uppermost mantle during continental subduction remains poorly understood. Observations of seismic anisotropy can provide constraints on dynamic processes in the crust and uppermost mantle in subduction systems. The subduction zone beneath Peru and Bolivia, where the Nazca plate subducts beneath South America, represents a particularly interesting location to study subduction-related deformation, given the along-strike transition from flat to normally dipping subduction. In this study we constrain seismic anisotropy within and above the subducting slab (including the overriding plate) beneath Pent and Bolivia by examining azimuthal variations in radial and transverse component receiver functions. Because anisotropy-aware receiver function analysis has good lateral resolution and depth constraints, it is complementary to previous studies of anisotropy in this region using shear wave splitting or surface wave tomography. We examine data from long-running permanent stations NNA (near Lima, Peru) and LPAZ (near La Paz, Bolivia), and two dense lines of seismometers from the PULSE and CAUGHT deployments in Pent and Bolivia, respectively. The northern line overlies the Peru flat slab, while the southern line overlies the normally dipping slab beneath Bolivia. We applied harmonic decomposition modeling to constrain the presence, depth, and characteristics of dipping and/or anisotropic interfaces within the crust and upper mantle. We found evidence for varying multi-layer anisotropy, in some cages with dipping symmetry axes, underneath both regions. The presence of multiple layers of anisotropy with distinct geometries that change with depth suggests a highly complex deformation regime associated with subduction beneath the Andes. In particular, our identification of depth-dependent seismic anisotropy within the overlying plate crust implies a change in deformation geometry, dominant mineralogy, and/or theology with depth, shedding light on the nature of deep crustal deformation during orogenesis.

Subduction systems play a key role in plate tectonics, but the deformation of the crust and uppermost mantle during continental subduction remains poorly understood. Observations of seismic anisotropy can provide constraints on dynamic processes in the crust and uppermost mantle in subduction systems. The subduction zone beneath Peru and Bolivia, where the Nazca plate subducts beneath South America, represents a particularly interesting location to study subduction-related deformation, given the along-strike transition from flat to normally dipping subduction. In this study we constrain seismic anisotropy within and above the subducting slab (including the overriding plate) beneath Pent and Bolivia by examining azimuthal variations in radial and transverse component receiver functions. Because anisotropy-aware receiver function analysis has good lateral resolution and depth constraints, it is complementary to previous studies of anisotropy in this region using shear wave splitting or surface wave tomography. We examine data from long-running permanent stations NNA (near Lima, Peru) and LPAZ (near La Paz, Bolivia), and two dense lines of seismometers from the PULSE and CAUGHT deployments in Pent and Bolivia, respectively. The northern line overlies the Peru flat slab, while the southern line overlies the normally dipping slab beneath Bolivia. We applied harmonic decomposition modeling to constrain the presence, depth, and characteristics of dipping and/or anisotropic interfaces within the crust and upper mantle. We found evidence for varying multi-layer anisotropy, in some cages with dipping symmetry axes, underneath both regions. The presence of multiple layers of anisotropy with distinct geometries that change with depth suggests a highly complex deformation regime associated with subduction beneath the Andes. In particular, our identification of depth-dependent seismic anisotropy within the overlying plate crust implies a change in deformation geometry, dominant mineralogy, and/or theology with depth, shedding light on the nature of deep crustal deformation during orogenesis.