Fluids trigger earthquakes as they flow upward along the subducted slab and escape into the overlying rocks.

The Western Hellenic Subduction Zone is characterized by a transition from oceanic to continental subduction. In the southern oceanic portion of the system, abundant seismicity reaches depths of 100 km to 190 km, while the northern continental portion rarely exhibits deep earthquakes. Our study investigates how this oceanic‐continental transition affects fluid release and related seismicity along strike. We present results from local earthquake tomography and double‐difference relocation in conjunction with published images based on scattered teleseismic waves. Our tomographic images recover both subducting oceanic and continental crusts as low‐velocity layers on top of high‐velocity mantle. Although the northern and southern trenches are offset along the Kephalonia Transform Fault, continental and oceanic subducting crusts appear to align at depth. This suggests a smooth transition between slab retreat in the south and slab convergence in the north. Relocated hypocenters outline a single‐planed Wadati‐Benioff Zone with significant along‐strike variability in the south. Seismicity terminates abruptly north of the Kephalonia Transform Fault, likely reflecting the transition from oceanic to continental subducted crust. Near 90 km depth, the low‐velocity signature of the subducting crust fades out and the Wadati‐Benioff Zone thins and steepens, marking the outline of the basalt‐eclogite transition. Subarc melting of the mantle is only observed in the southernmost sector of the oceanic subduction, below the volcanic part of the arc. Beneath the nonvolcanic part, the overriding crust appears to have undergone large‐scale silica enrichment. This enrichment is observed as an anomalously low Vp/Vs ratio and requires massive transport of dehydration‐derived fluids updip through the subducting crust.

Teleseismic receiver functions (RFs) provide a simple means to image structure in the Earth’s subsurface. Despite its ease of use, the method has traditionally required considerable user input to ensure that key processing operations run in a stable manner and yield robust results. As a consequence, large‐scale applications to global datasets have been limited. Here, we present preliminary results from a global database of RF data products dedicated to imaging structure in the crust and upper mantle. Earthquake signals recorded at seismic stations worldwide are downloaded, quality controlled, and processed using a range of deconvolution approaches to generate robust RFs. We describe our automated workflow and apply it to the entire data holding of the Incorporated Research Institutions for Seismology, producing 1,300,424 individual radial and transverse Ps RFs computed at 8258 stations. A series of tools designed to visualize the results will be made publicly available on a website. These include a map‐based interface that allows users to inspect RFs at individual stations, and interactive programs that can generate cross sections through 3D volumes of RFs.

Earthquakes in subduction zones occur in the slab mantle, in the subducting crust, on the subduction plate interface, and, in some cases, in the mantle wedge–regions that are separated by strong seismic discontinuities. These discontinuities are typically imaged with techniques using teleseismic waves, while local earthquakes are located based on arrival times. While this combination of imaging and earthquake location provides a good initial overview of where the earthquakes are located, the uncertainties associated with the two approaches are too large (i.e., few kilometers) to robustly identify on which side of a discontinuity (with thickness 100 m) the earthquakes occurred. Here we investigate how the waveforms of local earthquakes, which contain secondary phases arising from wave scattering at discontinuities, can be exploited to determine the source region of subduction zone earthquakes more robustly. Our investigation involves a three‐step approach and includes an application to data from western Greece. First, to identify characteristic secondary phases, we analyzed synthetic seismograms from a generic 2‐D subduction zone. Second, to enhance the visibility of secondary phases in field data, we implemented a workflow to process three‐component seismograms. Third, to identify individual secondary phases in the data, we matched their timing to arrivals computed in a 3‐D velocity model. We identified on average two to three secondary arrivals per station. These include P‐ and S‐reflections from the plate interface which indicate hypocenters in the mantle wedge, and P‐reflections from the slab Moho which indicate hypocenters on the plate interface and in the subducting crust.

The 21 March 2022 $Mw$ 5.1 Tampen Spur earthquake is the largest event in the North Sea over the last 33 yr. The earthquake was recorded by a 10,708 sensors permanent reservoir monitoring system (PRM) deployed at the Snorre field only a few kilometers away from the epicenter. The event was also recorded on regional networks, as well as at teleseismic distances. Here, we take advantage of this large number of observations to relocate this seismic event, estimate a moment tensor, and refine the depth of the earthquake. The different data sets and location methods yield similar locations, suggesting a well‐constrained earthquake location with a depth of 22 ± 5 km. Furthermore, a moment tensor is estimated, which indicates a thrust fault mechanism. Deep crustal earthquakes (>20 km) have been documented within several continental rifts, for example, in Basin and Range in the United States and in northern Germany. Studying deep intraplate earthquakes such as the event observed here can offer insights into the rheology and structures in the deeper portions of the crust. This thrust earthquake implies the reactivation of an older fault in the lower crust consistent with the ridge‐push forces from the mid‐Atlantic ridge. Such brittle failure at depth indicates a lower crust with little to no fluids present at this depth within the Tampen Spur region.