Seismic nests (or clusters) are typically defined as regions of high seismic activity at intermediate depths that do not move over time and are not associated with volcanic activity at the surface (e.g. Zarifi and Hayskov, 2003; Zarifi et al., 2007; Prieto et al., 2012). In this study, we focus on a seismic nest located in central Peru beneath the Amazon basin city of Pucallpa. The location of this nest is just beyond the eastern-most extent of the Peruvian flat slab, similar to the settings of two other South American clusters, the Bucaramanga and Pipanaco nests. While the Pucallpa nest is visible in the figures of many earlier papers on intermediate depth South American seismicity, it has not to our knowledge been described as a seismic nest before. We present the first detailed description of the Pucallpa nest, compare it to other established nests, and discuss its implications for our understanding of the Peruvian flat slab. Our results indicate that the Pucallpa nest demarcates the northern margin of a sag in the horizontally subducting Nazca plate in central Peru. The position of the nest along the projected location of the downgoing Mendana fracture zone is consistent with local variations in b-values and could help to explain both the nest's genesis and the co-located change in slab geometry. The nest is also spatially well correlated with the northern margin of the thick-skinned Shira Mountains and with the high heat flow associated with the Agua Caliente dome (Hermoza et al., 2006; Navarro Comet, 2018). Further study is needed to understand the effects of the complex Peruvian slab geometry on the formation of thick skinned deformation and heat flow anomalies on the overriding South American continent.

Seismic nests (or clusters) are typically defined as regions of high seismic activity at intermediate depths that do not move over time and are not associated with volcanic activity at the surface (e.g. Zarifi and Hayskov, 2003; Zarifi et al., 2007; Prieto et al., 2012). In this study, we focus on a seismic nest located in central Peru beneath the Amazon basin city of Pucallpa. The location of this nest is just beyond the eastern-most extent of the Peruvian flat slab, similar to the settings of two other South American clusters, the Bucaramanga and Pipanaco nests. While the Pucallpa nest is visible in the figures of many earlier papers on intermediate depth South American seismicity, it has not to our knowledge been described as a seismic nest before. We present the first detailed description of the Pucallpa nest, compare it to other established nests, and discuss its implications for our understanding of the Peruvian flat slab. Our results indicate that the Pucallpa nest demarcates the northern margin of a sag in the horizontally subducting Nazca plate in central Peru. The position of the nest along the projected location of the downgoing Mendana fracture zone is consistent with local variations in b-values and could help to explain both the nest's genesis and the co-located change in slab geometry. The nest is also spatially well correlated with the northern margin of the thick-skinned Shira Mountains and with the high heat flow associated with the Agua Caliente dome (Hermoza et al., 2006; Navarro Comet, 2018). Further study is needed to understand the effects of the complex Peruvian slab geometry on the formation of thick skinned deformation and heat flow anomalies on the overriding South American continent.

The eastern North American margin community seismic experiment (ENAM-CSE) was conceived to target the ENAM Geodynamic Processes at Rifting and Subducting Margins (GeoPRISMS) primary site with a suite of both active- and passive-source seismic data that would shed light on the processes associated with rift initiation and evolution. To fully understand the ENAM, it was necessary to acquire a seismic dataset that was both amphibious, spanning the passive margin from the continental interior onto the oceanic portion of the North American plate, and multiresolution, enabling imaging of the sediments, crust, and mantle lithosphere. The ENAM-CSE datasets were collected on- and offshore of North Carolina and Virginia over a series of cruises and land-based deployments between April 2014 and June 2015. The passive-source component of the ENAM-CSE included 30 broadband ocean-bottom seismometers (OBSs) and 3 onshore broadband instruments. The broadband stations were deployed contemporaneously with those of the easternmost EarthScope Transportable Array creating a trans-margin amphibious seismic dataset. The active-source portion of the ENAM-CSE included several components: (1) two onshore wide-angle seismic profiles where explosive shots were recorded on closely spaced geophones; (2) four major offshore wide-angle seismic profiles acquired with an airgun source and short-period OBSs (SPOBSs), two of which were extended onland by deployments of short-period seismometers; (3) marine multichannel seismic (MCS) data acquired along the four lines of SPOBSs and a series of other profiles along and across the margin. During the cruises, magnetic, gravity, and bathymetric data were also collected along all MCS profiles. All of the ENAM-CSE products were made publicly available shortly after acquisition, ensuring unfettered community access to this unique dataset.

The eastern North American margin community seismic experiment (ENAM-CSE) was conceived to target the ENAM Geodynamic Processes at Rifting and Subducting Margins (GeoPRISMS) primary site with a suite of both active- and passive-source seismic data that would shed light on the processes associated with rift initiation and evolution. To fully understand the ENAM, it was necessary to acquire a seismic dataset that was both amphibious, spanning the passive margin from the continental interior onto the oceanic portion of the North American plate, and multiresolution, enabling imaging of the sediments, crust, and mantle lithosphere. The ENAM-CSE datasets were collected on- and offshore of North Carolina and Virginia over a series of cruises and land-based deployments between April 2014 and June 2015. The passive-source component of the ENAM-CSE included 30 broadband ocean-bottom seismometers (OBSs) and 3 onshore broadband instruments. The broadband stations were deployed contemporaneously with those of the easternmost EarthScope Transportable Array creating a trans-margin amphibious seismic dataset. The active-source portion of the ENAM-CSE included several components: (1) two onshore wide-angle seismic profiles where explosive shots were recorded on closely spaced geophones; (2) four major offshore wide-angle seismic profiles acquired with an airgun source and short-period OBSs (SPOBSs), two of which were extended onland by deployments of short-period seismometers; (3) marine multichannel seismic (MCS) data acquired along the four lines of SPOBSs and a series of other profiles along and across the margin. During the cruises, magnetic, gravity, and bathymetric data were also collected along all MCS profiles. All of the ENAM-CSE products were made publicly available shortly after acquisition, ensuring unfettered community access to this unique dataset.

The border between Georgia and South Carolina has a moderate amount of seismicity typical of the Piedmont Province of the eastern United States and greater than most other intraplate regions. Historical records suggest on average a M-w 4.5 earthquake every 50 yr in the region of the J. Strom Thurmond Reservoir, which is located on the border between Georgia and South Carolina. The M-w 4.1 earthquake on 15 February 2014 near Edgefield, South Carolina, was one of the largest events in this region recorded by nearby modern seismometers, providing an opportunity to study its source properties and aftershock productivity. Using the waveforms of the M-w 4.1 mainshock and the only cataloged M w 3.0 aftershock as templates, we apply a matched-filter technique to search for additional events between 8 and 22 February 2014. The resulting six new detections are further employed as new templates to scan for more events. Repeating the waveform-matching method with new templates yields 13 additional events, for a total of 19 previously unidentified events with magnitude 0.06 and larger. The low number of events suggests that this sequence is deficient in aftershock production, as compared with expected aftershock productivities for other mainshocks of similar magnitudes. Hypocentral depths of the M-w 4.1 mainshock and M-w 3.0 aftershock are estimated by examining the differential time between a depth phase called sPL and P-wave arrivals, as well as by modeling the depth phase of body waves at shorter periods. The best-fitting depths for both events are around 3-4 km. The obtained stress drops for the M-w 4.1 mainshock and M-w 3.0 aftershock are 3.75 and 4.44 MPa, respectively. The corresponding updated moment magnitude for the aftershock is 2.91.

The border between Georgia and South Carolina has a moderate amount of seismicity typical of the Piedmont Province of the eastern United States and greater than most other intraplate regions. Historical records suggest on average a M-w 4.5 earthquake every 50 yr in the region of the J. Strom Thurmond Reservoir, which is located on the border between Georgia and South Carolina. The M-w 4.1 earthquake on 15 February 2014 near Edgefield, South Carolina, was one of the largest events in this region recorded by nearby modern seismometers, providing an opportunity to study its source properties and aftershock productivity. Using the waveforms of the M-w 4.1 mainshock and the only cataloged M w 3.0 aftershock as templates, we apply a matched-filter technique to search for additional events between 8 and 22 February 2014. The resulting six new detections are further employed as new templates to scan for more events. Repeating the waveform-matching method with new templates yields 13 additional events, for a total of 19 previously unidentified events with magnitude 0.06 and larger. The low number of events suggests that this sequence is deficient in aftershock production, as compared with expected aftershock productivities for other mainshocks of similar magnitudes. Hypocentral depths of the M-w 4.1 mainshock and M-w 3.0 aftershock are estimated by examining the differential time between a depth phase called sPL and P-wave arrivals, as well as by modeling the depth phase of body waves at shorter periods. The best-fitting depths for both events are around 3-4 km. The obtained stress drops for the M-w 4.1 mainshock and M-w 3.0 aftershock are 3.75 and 4.44 MPa, respectively. The corresponding updated moment magnitude for the aftershock is 2.91.

Between 1965 and 2003, the Carnegie Institution of Washington's Department of Terrestrial Magnetism operated a continuous network of nine broadband seismographs with locations in South America, Japan, Iceland, Papua New Guinea, and Washington, D.C. The Carnegie seismographs designed in the 1960s by Selwyn Sacks were among the earliest broadband instruments, sensing between at least 30 s and similar to 30 Hz. Given the scarcity of historic seismic data of comparable bandwidth and dynamic range prior to the widespread shift to force-feedback instruments and digital recording around the mid-1980s, this dataset is still of high scientific value today.

Processes related to eruptions at arc volcanoes are linked by structures that transect the entire crust. Imaging the mid- to lower-crustal portions (here, similar to 5-15km and>15km respectively) of these magmatic systems where intermediate storage may occur has been a longstanding challenge. Tomography, local seismic source studies, geodetic, and geochemical constraints, are typically most sensitive to shallow (<5km) storage and/or have insufficient resolution at these depths. Geophysical methods are even further limited at frequently-erupting volcanoes where well-developed trans-crustal magmatic systems are likely to exist, due to a lack of deep seismicity. Here we show direct evidence for mid-crustal magma storage beneath the frequently erupting Cleveland volcano, Alaska, using a novel application of seismic receiver functions. We use P-s scattered waves from the Moho as virtual sources to investigate S-wave velocities between the Moho and the surface. Our forward modeling approach allows us to provide direct constraints on the geometry of low velocity regions beneath volcanoes despite having a comparatively sparse seismic network. Our results show clear evidence of mid-crustal magma storage beneath the depths of located volcanic seismicity. Future work using similar approaches will enable an unprecedented comparative examination of magmatic systems beneath sparsely instrumented volcanoes globally.

The nature and cause of deep earthquakes remain enduring unknowns in the field of seismology. We present new models of thermal structures of subducted slabs traced to mantle transition zone depths that permit a detailed comparison between slab pressure/temperature (P/T) paths and hydrated/carbonated mineral phase relations. We find a remarkable correlation between slabs capable of transporting water to transition zone depths in dense hydrous magnesium silicates with slabs that produce seismicity below similar to 300-km depth, primarily between 500 and 700 km. This depth range also coincides with the P/T conditions at which oceanic crustal lithologies in cold slabs are predicted to intersect the carbonate-bearing basalt solidus to produce carbonatitic melts. Both forms of fluid evolution are well represented by sublithospheric diamonds whose inclusions record the existence of melts, fluids, or supercritical liquids derived from hydrated or carbonate-bearing slabs at depths (similar to 300-700 km) generally coincident with deep-focus earthquakes. We propose that the hydrous and carbonated fluids released from subducted slabs at these depths lead to fluid-triggered seismicity, fluid migration, diamond precipitation, and inclusion crystallization. Deep focus earthquake hypocenters could track the general region of deep fluid release, migration, and diamond formation in the mantle. The thermal modeling of slabs in the mantle and the correlation between sublithospheric diamonds, deep focus earthquakes, and slabs at depth demonstrate a deep subduction pathway to the mantle transition zone for carbon and volatiles that bypasses shallower decarbonation and dehydration processes.

Seismic tomography of the crust is an essential tool for studying the three-dimensional structure of magmatic plumbing systems feeding active volcanoes, but it is often limited in resolution by the absence of deep local seismicity. Teleseismic receiver functions can be used to illuminate local structural variations, but typically do not account for the effects of three-dimensional velocity heterogeneities. Here we harness the complementary strengths of both techniques by processing Ps-P delay times derived from teleseismic receiver functions in a tomographic S wave inversion. Using our inversion technique, we produce the first tomographic crustal velocity model beneath Cleveland Volcano, identifying a vertically extensive high V-P/V-S anomaly beneath the volcano that likely signifies a middle-to-lower crustal magma reservoir. The observation is the first of its kind in the central Aleutians, illustrating the potential of our technique to advance our understanding of crustal magmatic systems without broad seismic networks or distributed local seismicity.