Alexei Gorbatov

Three‐dimensional visualization of a near‐vertical slab tear beneath the southern Mariana arc

The use of a three-dimensional ray-tracing inversion algorithm has greatly enhanced the resolution of gradients and strong variations in wave speeds to create improved P wave tomographic images of the Mariana arc region. The images obtained from the Mariana arc region show relatively low amplitudes of heterogeneity due to the limited number of seismic stations in the area. Despite these limitations, detailed interpretations of the three-dimensional geometry and morphology of the Pacific Plate subducting beneath the Philippine Sea Plate have provided a three-dimensional model of the steep dip of the Pacific plate and the curvature of the slab beneath the Mariana arc in unprecedented detail. The new P wave tomography and seismicity depict a previously unidentified E-W trending near-vertical tear in the subducting plate at the southern end of the Mariana arc that divides the arc into two distinct segments: a steeply dipping curved slab penetrating the lower mantle and a short (∼250 km depth) slab along the Challenger Deep segment of the arc. The slab tear is likely to be the result of the need to accommodate the reduced volume the slab must occupy as it is subducted beneath the Philippine Sea plate along a curved arc.

Tracking high‐frequency seismic source evolution: 2004 Mw 8.1 Macquarie event

The 2004 Mw 8.1 event on 23 December 2004 near the Macquarie Ridge is a very large intraplate event that has been overshadowed by the Mw 9.3 Sumatra-Andaman event only 3 days later. We are able to track the progress of source evolution by estimating the progression of the points of energy emission, exploiting the good azimuthal distribution of available stations. The results indicate that this event ruptured on two nearby fault systems reactivating fossil fracture zones, with the second subevent to the west triggered by the first. The total duration of high-frequency radiation is quite short, about 60 s, for such a large event. Much of the high-frequency radiation occurs on a fault subparallel to that inferred from long-period studies. This composite fault behavior with displaced triggered failure appears to be a characteristic of large intraplate events beneath the oceans.

Spiral-arm seismic arrays

Abstract Seismic arrays have many uses for signal enhancement, from surface‐wave characterization of the near surface to teleseismic detection in the context of monitoring nuclear tests. Many variants of the geometrical configuration of stations have been used with the objective of maximizing potential resolution of the incoming wavefronts direction of arrival. A versatile class of array configurations, with good resolution properties, can be constructed with multiple spiral arms. The array response is comparable with the same number of full circles, but with far fewer stations and is robust to minor position changes in emplacement. The desirable properties of the spiral‐arm arrays are illustrated for a permanent array in the Precambrian Pilbara craton in northwestern Australia and for a temporary array on ancient sediments in southern Queensland, Australia. In each case, the practical array response is very good and matches the theoretical expectations. The spiral‐arm configuration allows the deployment of relatively large aperture arrays with a limited number of stations, which is advantageous in a broad range of seismic applications, including near‐surface characterization. Online Material: Figures illustrating the relation between spiral‐arm and multiring circular arrays.