Ivan Koulakov

VELOCITY STRUCTURE AROUND THE BAIKAL RIFT ZONE FROM TELESEISMIC AND LOCAL EARTHQUAKE TRAVELTIMES AND GEODYNAMIC IMPLICATIONS

Abstract We present new results on the velocity structure of the Baikal rift zone, Asia, deduced from a comparative teleseismic and local tomography analysis. The aim of this paper is to better identify the role of deep mantle processes versus that of far-field tectonic effects on the occurrence of extensional tectonics within a continental plate. We use 36000 traveltimes of P-refracted waves from the ISC catalogues and Pg and Pn traveltimes of 578 earthquakes recorded by the Russian regional network to determine a velocity model by the use of local and teleseismic inversion procedures. The models show that some velocity patterns are continuous from the surface down to at least 400 km. Among them, a narrow negative anomaly goes through Mongolia and follows the southern and eastern margins of the Siberian craton: this structure is interpreted as a thin mantle plume rising beneath the rift axis. However, our results do not evidence any wide asthenospheric upwarp at this place. Other velocity anomalies observed near the surface are not deeply rooted. In particular, a negative anomaly is observed at shallow levels (48 km) beneath the northern third of Lake Baikal, which is disconnected from deeper structures. It may be explained by the existence of underplated magmatic material at the bottom of the crust. By comparing the geometry of deep-rooted anomalies to the present-day stress field patterns, we conclude that the sub-lithospheric mantle dynamics is not the main factor controlling extensional processes in the Baikal rift. However, it does contribute to a thermal weakening of the lithosphere along a mechanical discontinuity bounding the Siberian shield. We finally conclude that three favourable conditions are gathered in the Baikal area to generate extension: far-field extensional stress field, mechanical inherited lithospheric weakness and heat supply. Further studies should help to precise the genetic link between these three factors.

Structure and dynamics of the upper mantle beneath the Alpine–Himalayan orogenic belt, from teleseismic tomography

Abstract The upper mantle structure beneath the Alpine–Himalayan orogenic belt (AHB) at depths from 100 to 500 km has been studied by ITS tomographic inversion of P-wave travel times from earthquakes that occurred in the region and were recorded by the worldwide seismological network at teleseismic distances (from the ISC catalogues). The resulting maps were obtained as a sum of independent inversions in about 60 overlapping blocks 600–800 km in diameter. This approach is similar to high-frequency spatial filtration, and allows investigation of much subtler structures than in global inversion. High-velocity linear patterns along the AHB have been interpreted as traces of continental or oceanic lithosphere sinking to the upper mantle under the effect of active regional compression. The tomographic maps show well-pronounced traces of subduction in the region of Cretan arc, Hindukush, and Burma, also confirmed by independent data, as well as other less certain evidence of subduction. Positive anomalies in the western part of the AHB are attributed to subduction in the area of Cyprus and along the Caucasus–Kopet-Dagh–Lut plate. In the tomographic maps one can see traces of the subducted Indian slab under western part of Himalayas. Beneath eastern Himalayas, the upper mantle is rather hot. Zones of sinking lithosphere are traceable around the Tarim block. A strong low-velocity anomaly in Mongolia is apparently related to the Hangai plume whose existence was inferred from geological and geophysical evidence.

Tomography on PP-P waves and its application for investigation of the upper mantle in central Siberia

Abstract We develop the method of tomographic inversion, called RR-R scheme, which is based on joint use of teleseismic P or S refracted rays and corresponding PP or SS rays with bounce points located within a study region. This scheme allows imaging the deep seismic structure beneath “blank” areas where there are neither recording stations nor earthquakes. Utilization of differential travel times makes it possible to avoid the difficulty of source and station corrections, which cause problems in teleseismic tomography. The RR-R scheme has been applied to more than 10xa0000 ray pairs from the ISC database to investigate a large region from the North Arctic Ocean to the northern part of China and Mongolia. Inversion was carried out in several separate windows covering the study area down to the depth of 530 km. Velocity anomalies were computed in grid nodes distributed in the study 3D area according to ray density. The tomography results from different windows were combined into one general map of the lithospheric structure in central Siberia. Principal positive P-wave velocity anomalies (+(1–3)% down to 400 km) are associated with the Precambrian Siberian craton. Negative velocity anomalies are related to the thin lithosphere of the West Siberian plate, the Altai Mountains and the Hangai plateau (−3–1% in the depth range 30 to 430 km). A strong negative anomaly beneath the Precambrian Hangai block is apparently associated with a mantle plume. Positive velocity anomalies in sedimentary basins at Southern Siberia are believed to have the crustal origin.

Breathing of the Nevado del Ruiz volcano reservoir, Colombia, inferred from repeated seismic tomography

Nevado del Ruiz volcano (NRV), Columbia, is one of the most dangerous volcanoes in the world and caused the death of 25,000 people in 1985. Using a new algorithm for repeated tomography, we have found a prominent seismic anomaly with high values of the Vp/Vs ratio at depths of 2–5u2009km below the surface, which is associated with a shallow magma reservoir. The amplitude and shape of this anomaly changed during the current phase of unrest which began in 2010. We interpret these changes as due to the ascent of gas bubbles through magma and to degassing of the reservoir. In 2011–2014, most of this gas escaped through permeable roof rocks, feeding surface fumarole activity and leading to a gradual decrease of the Vp/Vs ratio in the reservoir. This trend was reversed in 2015–2016 due to replenishment of the reservoir by a new batch of volatile-rich magma likely to sustain further volcanic activity. It is argued that the recurring “breathing” of the shallow reservoir is the main cause of current eruptions at NRV.

The feeder system of the Toba supervolcano from the slab to the shallow reservoir

The Toba Caldera has been the site of several large explosive eruptions in the recent geological past, including the worlds largest Pleistocene eruption 74,000 years ago. The major cause of this particular behaviour may be the subduction of the fluid-rich Investigator Fracture Zone directly beneath the continental crust of Sumatra and possible tear of the slab. Here we show a new seismic tomography model, which clearly reveals a complex multilevel plumbing system beneath Toba. Large amounts of volatiles originate in the subducting slab at a depth of ∼150u2009km, migrate upward and cause active melting in the mantle wedge. The volatile-rich basic magmas accumulate at the base of the crust in a ∼50,000u2009km3 reservoir. The overheated volatiles continue ascending through the crust and cause melting of the upper crust rocks. This leads to the formation of a shallow crustal reservoir that is directly responsible for the supereruptions.

Full Aftershock Sequence of the M w 6.9 2003 Boumerdes Earthquake, Algeria: Space???Time Distribution, Local Tomography and Seismotectonic Implications

We present a detailed analysis of the aftershocks of the May 21, 2003 Boumerdes earthquake (Mwxa0=xa06.9) recorded by 35 seismological stations and 2 OBS deployed in the epicentral area. This network recorded the aftershock activity for about 1 year and resulted in locating about 2500 events. The five main aftershocks (4.7xa0<Mxa0<5.8) display thrust faulting consistent with the main shock, except for the second event (M5.8, 29/05/2003) which depicts a strike-slip focal solution at the western tip of the rupture zone. Most aftershocks are clustered near the main rupture plane, in the footwall or at the westernmost tip of the 2003 Boumerdes rupture area. Many aftershocks last over the whole seismic crisis ahead (north) of the main rupture zone, forming a diffuse, low-angle surface within the footwall where the coseismic static stress change is predicted to increase. At the SW tip of the rupture, short-lived clusters locate at intersections of faults near the contact between the inner (Kabylia) and outer (Tell) zones. The tomographic inversion depicts high-velocity P- and S-wave anomalies coinciding with Miocene magmatic intrusive bodies in the upper crust, partially hidden by surrounding basins. The area of the main shock is associated with a large low-velocity body subdivided into sub-domains, including Neogene basins on land and offshore. Our results support a rupture model strongly controlled by geological inhomogeneities and extending as ramp–flat–ramp systems upward, favoring heterogeneous slip and segmentation in the fault plane with strong afterslip toward the surface. The diffuse aftershock activity in the footwall evidences an inherited discontinuity at mid-crustal depth that we interpret as the contact of Kabylian and African (Tethyan) continental crusts that were stacked during the Upper Miocene collision.

Focused magmatism beneath Uturuncu volcano, Bolivia: Insights from seismic tomography and deformation modeling

We have carried out a tomographic inversion for seismic velocity in the vicinity of Uturuncu volcano (Bolivia) based on a 33-station temporary seismic network deployment. We combine travel times from earthquakes in the shallow crust with those from earthquakes on the subducting Nazca plate to broadly constrain velocities throughout the crust using the LOTOS tomography algorithm. The reliability and resolution of the tomography is verified using a series of tests on real and synthetic data. The resulting three-dimensional distributions of Vp, Vs, and Vp/Vs reveal a large tooth-shaped anomaly rooted in the deep crust and stopping abruptly 6 km below the surface. This feature exhibits very high values of Vp/Vs (up to 2.0) extending to ~80 km depth. To explain the relationship of this anomaly with the surface uplift observed in interferometric synthetic aperture radar (InSAR) data, we propose two scenarios. In the first, the feature is a pathway for liquid volatiles that convert to gas, due to decompression, at ~6 km depth, causing a volume increase. This expansion drives seismicity in the overlying crust. In the second model, this anomaly is a buoyant pulse of magma within the batholith, ascending due to gravitational instability. We propose a simplified numerical simulation to demonstrate how this second model generally supports many of the observations. We conclude that both of these scenarios might be valid and complement each other for the Uturuncu case. Based on joint analysis of the tomography results and available geochemical and petrological information, we have constructed a model of the Uturuncu magma system that illustrates the main stages of phase transitions and melting.

Geophysical Studies of the Lithosphere Along the Dead Sea Transform

In this chapter we report on the deep structure of the Dead Sea Transform (DST) as derived from geophysical observations and numerical modelling, calibrated by geological and geodynamic evidence.