Abstract. Seismic arrays provide tools for the localization of events without clear phases or events outside of the network, where the station coverage prohibits classical localization techniques. Beamforming allows the determination of the direction (backazimuth) and the horizontal (apparent) velocity of an incoming wavefront. Here we combine multiple arrays to retrieve event epicenters from the area of intersecting beams without the need to specify a velocity model. The analysis is performed in the time-domain, which allows to select a relatively narrow time window around the phase of interest while preserving frequency bandwidth. This technique is applied to earthquakes and hybrid events in the region of Fogo and Brava, two islands of the southern chain of the Cape Verde archipelago. The results show that the earthquakes mainly originate near Brava whereas the hybrid events are located on Fogo. By multiple-event beam-stacking we are able to further constrain the locations of the hybrid events in the north-western part of the collapse scar of Fogo. In previous studies, these events were attributed to shallow hydrothermal processes. However, we obtain relatively high apparent velocities at the arrays, pointing to either deeper sources or to complex ray paths. For a better understanding of possible errors of the multi-array analysis, we also compare slowness values obtained from the array analysis with those derived from earthquake locations from classical (local network) localizations. In general, the results agree well, however, the arrays also show some aberrations that can be quantified for certain event locations.
Abstract The Earth's mantle exhibits a layered structure seismically characterized by sudden velocity changes or strong gradients. Several seismic boundaries have been identified in the mantle, and a large number of theoretical calculations and laboratory experiments have contributed to the debate on their origin. We analyze P ‐to‐ S converted phases generated at such interfaces to image the velocity structure within the sublithospheric mantle beneath Indonesia. Our study confirms the existence of various layer boundaries in the upper and lower mantle revealing up to 11 consecutive discontinuities down to ~1,700‐km depth. We detect Ps phases from the Lehmann and the X discontinuities originating at ~245 and ~294 km, respectively, followed by the top of a low‐velocity layer (LVL‐410) at ~368 km. The transition zone discontinuities are imaged at average depths of 408 and 665 km, respectively, which indicates the absence of significant temperature anomalies. In the midmantle we find vague indications for another interface at ~970‐km depth. At ~1,220 km a negative phase is observed followed by a sequence of converting structures of unknown origin at ~1,320, ~1,460, and ~1,500 km. We interpret these interfaces as compositional anomalies related to persisting fragments of subducted lithosphere. A further boundary is observed at ~1,700 km. Even though different causes exist to explain the observed seismic discontinuities including mineral phase transitions, partial melt, and chemical changes, most of them require additional mineral components. Thus, our findings provide clear evidence for significant compositional alteration of the mantle beneath Indonesia as a result of recurring subduction.
