Andrey Jakovlev

Deep crustal earthquakes in North Tanzania, East Africa: Interplay between tectonic and magmatic processes in an incipient rift

In this study, we explore the origin of lower crustal seismicity and the factors controlling rift propagation using seismological data recorded within the youngest part of the East African Rift System, the North Tanzanian Divergence (NTD). Most earthquakes below Lake Manyara occur at depth ranging between 20 and 40 km and have a swarm-like distribution. Focal mechanisms of 26 events indicate a combination of strike-slip and normal faulting involving Archaean basement structures and forming a relay zone. The derived local stress regime is transtensive and the minimum principal stress is oriented N110°E. Crustal seismic tomography reveals low-velocity anomalies below the rifted basins in the NTD, interpreted as localized thermomechanical perturbations promoting fluid release and subsequent seismicity in the lower crust. SKS splitting analysis in the NTD indicates seismic anisotropy beneath 17 stations most likely due to aligned magma lenses and/or dikes beneath the rift and to the lithospheric fabrics. Our results favor a strain pattern intermediate between purely mechanical and purely magmatic. We suggest that melt products arising from a large asthenospheric thermal anomaly enhance lithospheric weakening and facilitate faulting and creeping on critically oriented inherited structures of the Precambrian lower crust. Although the crust is unlikely weakened at a point comparable to other parts of the East African Rift System, this deep-seated thermomechanical process is efficient enough to allow slow rift propagation within the eastern Tanzanian cratonic edge.

Three different types of plumbing system beneath the neighboring active volcanoes of Tolbachik, Bezymianny, and Klyuchevskoy in Kamchatka

The Klyuchevskoy group of volcanoes (KGV) in Kamchatka includes three presently active volcanoes (Klyuchevskoy, Bezymianny, and Tolbachik) located close together in an area of approximately 50 × 80 km. These three volcanoes have completely different compositions and eruption styles from each other. We have analyzed new data recorded by a temporary seismic network consisting of 22 seismic stations operated within the area of Tolbachik in 2014–2015 in conjunction with the data from the permanent network and the temporary PIRE network deployed at the Bezymianny volcano in 2009. The arrival times of the P and S waves were inverted using a local earthquake tomography algorithm to derive 3-D seismic models of the crust beneath the KGV as well as accurate seismicity locations. High-resolution structures beneath the Tolbachik volcanic complex were identified for the first time in this study. The tomography results reveal three different types of feeding system for the main KGV volcanoes. The basaltic lavas of the Klyuchevskoy volcano are supplied directly from a reservoir at a depth of 25–30 km through a nearly vertical pipe-shaped conduit. The explosive Bezymianny volcano is fed through a dispersed system of crustal reservoirs where a lighter felsic material separates from the mafic component and ascends to the upper crust to form andesitic magma sources. For Tolbachik, low-viscosity volatile-saturated basalts ascend from two deep reservoirs following a system of fractures in the crust associated with the intersections of regional faults. Plain Language Summary The Klyuchevskoy group of volcanoes (KGV) in Kamchatka includes three presently active volcanoes (Klyuchevskoy, Bezymianny, and Tolbachik) located close together in an area of approximately 50 × 80 km. These three volcanoes are among the most active volcanoes in the world, and they have completely different compositions and eruption styles from each other. We have analyzed new data recorded by a temporary seismic network consisting of 22 seismic stations installed within the area of Tolbachik in 2014–2015 in harsh natural conditions. Based on these data, we have derived high-resolution structures beneath the Tolbachik volcanic complex and surrounding areas. The tomography results reveal three different types of feeding system for the main KGV volcanoes. The basaltic lavas of the Klyuchevskoy volcano are supplied directly from a reservoir at a depth of 25–30 km through a nearly vertical pipe-shaped conduit. The explosive Bezymianny volcano is fed through a dispersed system of crustal reservoirs where a lighter felsic material separates from the mafic component and ascends to the upper crust to form andesitic magma sources. For Tolbachik, low-viscosity volatile-saturated basalts ascend from two deep reservoirs following a system of fractures in the crust associated with the intersections of regional faults.

Three‐dimensional seismic anisotropy in the crust and uppermost mantle beneath the Taiwan area revealed by passive source tomography

We present a 3-D anisotropic seismic model of the crust and uppermost mantle beneath the Taiwan region based on the tomographic inversion of traveltime data from regional earthquakes. In the crust beneath eastern Taiwan, we observe coast-parallel anisotropy that perfectly delineates the major geological structures. In westernmost Taiwan, we distinguish a crustal block corresponding to the Peikang High at the margin of the Eurasian Plate, where coast-perpendicular anisotropy within a high-velocity anomaly is observed. In the uppermost mantle, the direction of anisotropy beneath central Taiwan turns perpendicular to the coast, which may indicate eastward underthrusting of the Peikang Block that was induced by collisional processes. To the NE of Taiwan, the anisotropy forms circular patterns coinciding with the shape of the Ryukyu arc, which may reflect the distribution of the deformations and fractures in the accretion and arc complex.

Growth of mountain belts in central Asia triggers a new collision zone in central India

Several unusual strong earthquakes occurred in central India along the Narmada-Son Lineament (NSL) zone, far from active plate boundaries. To understand the role of collisional processes in the origin of this seismicity, we develop a numerical thermomechanical model of shortening between the Indian Plate and Asia. We show that at the final stage of collision, the shortening rate of the high mountain areas slows. The continuing convergence of India and Asia triggers the initiation of a new collision zone in continental part of India. Various geological and geophysical observations indicate that the NSL is a weakest zone with northward thrusting of the thinner central Indian lithosphere underneath the thicker northern part of the Indian Plate. We hypothesize that the NSL was reactivated during the final stage of the India Asia convergence and it will possibly form a new mountain belt within the Indian continent.