The western Hellenic subduction zone (WHSZ) exhibits well‐documented along‐strike variations in lithosphere density (i.e., oceanic versus continental), subduction rates, and overriding plate extension. Differences in slab density are believed to drive deformation rates along the WHSZ; however, this hypothesis has been difficult to test given the limited seismic constraints on the structure of the WHSZ, particularly beneath northern Greece. Here, we present high‐resolution seismic images across northern and southern Greece to constrain the slab composition and mantle wedge geometry along the WHSZ. Data from two temporary arrays deployed across Greece in a northern line (NL) and southern line (SL) are processed using a 2D teleseismic migration algorithm based on the Generalized Radon Transform. Images of P‐ and S‐wave velocity perturbations reveal N60E dipping low‐velocity layers beneath both NL and SL. The ∼8 km thick layer beneath SL is interpreted as subducted oceanic crust while the ∼20 km thick layer beneath NL is interpreted as subducted continental crust. The thickness of subducted continental crust inferred within the upper mantle suggests that ∼10 km of continental crust has accreted to the overriding plate. The relative position of the two subducted crusts implies ∼70–85 km of additional slab retreat in the south relative to the north. Overall, our seismic images are consistent with the hypothesis that faster sinking of the denser, oceanic portion of the slab relative to the continental portion can explain the different rates of slab retreat and deformation in the overriding plate along the WHSZ.

Broadband data from the Meso‐America Subduction Experiment (MASE) line in central Mexico were used to image the subducted Cocos plate and the overriding continental lithosphere beneath central Mexico using a generalized radon transform based migration. Our images provide insight into the process of subducting relatively young oceanic lithosphere and its complex geometry beneath continental North America. The converted and reverberated phase image shows complete horizontal tectonic underplating of the Cocos oceanic lithosphere beneath the North American continental lithosphere, with a clear image of a very thin low‐velocity oceanic crust (7–8 km) which dips at 15–20 degrees at Acapulco then flattens approximately 300 km from the Middle America Trench. Farther inland the slab then appears to abruptly change from nearly horizontal to a steeply dipping geometry of approximately 75 degrees underneath the Trans‐Mexican Volcanic Belt (TMVB). Where the slab bends underneath the TMVB, the migrated image depicts the transition from subducted oceanic Moho to continental Moho at ∼230 km from the coast, neither of which were clearly resolved in previous seismic images. The deeper seismic structure beneath the TMVB shows a prominent negative discontinuity (fast‐to‐slow) at ∼65–75 km within the upper mantle. This feature, which spans horizontally beneath the arc (∼100 km), may delineate the top of a layer of ponded partial melt.

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[1] It is well known that soil sites have a profound effect on ground motion during large earthquakes. The complex structure of soil deposits and the highly nonlinear constitutive behavior of soils largely control nonlinear site response at soil sites. Measurements of nonlinear soil response under natural conditions are critical to advancing our understanding of soil behavior during earthquakes. Many factors limit the use of earthquake observations to estimate nonlinear site response such that quantitative characterization of nonlinear behavior relies almost exclusively on laboratory experiments and modeling of wave propagation. Here we introduce a new method for in situ characterization of the nonlinear behavior of a natural soil formation using measurements obtained immediately adjacent to a large vibrator source. To our knowledge, we are the first group to propose and test such an approach. Employing a large, surface vibrator as a source, we measure the nonlinear behavior of the soil by incrementally increasing the source amplitude over a range of frequencies and monitoring changes in the output spectra. We apply a homodyne algorithm for measuring spectral amplitudes, which provides robust signal-to-noise ratios at the frequencies of interest. Spectral ratios are computed between the receivers and the source as well as receiver pairs located in an array adjacent to the source, providing the means to separate source and near-source nonlinearity from pervasive nonlinearity in the soil column. We find clear evidence of nonlinearity in significant decreases in the frequency of peak spectral ratios, corresponding to material softening with amplitude, observed across the array as the source amplitude is increased. The observed peak shifts are consistent with laboratory measurements of soil nonlinearity. Our results provide constraints for future numerical modeling studies of strong ground motion during earthquakes.