Jerome Vergne

Underplating in the Himalaya-Tibet Collision Zone Revealed by the Hi-CLIMB Experiment

Himalayan-Tibetan Underplate The Himalayas formed from the collision of India with Eurasia beginning about 50 million years ago, but the fate and position of the subducted Indian crust was not well defined until the Hi-CLIMB seismic experiment was initiated. The centerpiece of the project is an 800-kilometer-long, closely spaced, linear array of broadband seismographs, extending from the Ganges lowland, across the Himalayas, and onto the central Tibetan plateau. Nábělek et al. (p. 1371) present images of the crust and upper mantle of the Southern Tibetan plateau underthrust northward by the Indian plate, in which they trace the base of the Indian plate to 31°N. The character of the crust-mantle interface in this region suggests that the Indian crust is at least partly decoupled from the mantle beneath. A seismic study delineates the position and local thickening of the Indian plate underlying the Himalayas and southern Tibet. We studied the formation of the Himalayan mountain range and the Tibetan Plateau by investigating their lithospheric structure. Using an 800-kilometer-long, densely spaced seismic array, we have constructed an image of the crust and upper mantle beneath the Himalayas and the southern Tibetan Plateau. The image reveals in a continuous fashion the Main Himalayan thrust fault as it extends from a shallow depth under Nepal to the mid-crust under southern Tibet. Indian crust can be traced to 31°N. The crust/mantle interface beneath Tibet is anisotropic, indicating shearing during its formation. The dipping mantle fabric suggests that the Indian mantle is subducting in a diffuse fashion along several evolving subparallel structures.

Teleseismic imaging of subducting lithosphere and Moho offsets beneath western Tibet

Teleseismic images suggest that the Tarim plate plunges ∼45°S, down to ∼300 km depth, beneath NW Tibet. The 410 km discontinuity shallows by ∼10 km under the plateau, implying ∼100°C cooler upper mantle. The deepest Moho on record (∼90 km) lies under W Qiangtang. It rises abruptly by ∼20 and ∼10 km beneath the Altyn Tagh Fault and Bangong Suture, respectively. Vp/Vs ratios are normal, except in the Yecheng flexural basin and deep under the south Karakax volcanics (∼1.92). W Kunlun’s Neogene tectonics are simply accounted for by oblique subduction of lithospheric mantle beneath an upward-extruding thrust wedge of the Tarim crust.

Seismic evidence for stepwise thickening of the crust across the NE Tibetan plateau

We performed a receiver function analysis on teleseismic data recorded along two 550 km-long profiles crossing the northeastern Tibetan plateau. Results from time to depth migration, grid-search Vp/Vs determination and simulated annealing inversion of waveforms, reveal that the crust thickens from ∼50 km near the northern edge of the plateau to ∼80 km south of the Jinsha suture in the Qiang Tang block. Crustal thickening occurs in staircase fashion with steps located beneath the main, reactivated sutures. The Vp/Vs ratio, close to the global continental average does not suggest widespread partial melting but rather a more usual separation between an upper felsic and a lower mafic part within the northeastern Tibetan crust.

Spectral analysis of seismic noise induced by rivers: A new tool to monitor spatiotemporal changes in stream hydrodynamics

to 20 dB (relative to (m/s) 2 /Hz) for all the stations located along a steep 30-km-long narrow and deeply incised channel of the Trisuli River, a major trans-Himalayan river. The early summer increase in high-frequency energy is modulated by a 24-h periodicity where the minimum of seismic noise level is reached around noon and the maximum is reached late in the evening. A detailed study of seismic noise amplitude reveals a clear correlation with both regional meteorological and hydrological data along the Trisuli River. Seasonal increase in ambient noise coincides with the strong monsoon rainfall and a period of rapid melting of snow and ice in the high elevations. The observed 24-h cyclicity is consistent with the daily fluctuation of the precipitation and river discharge in the region. River-induced seismic noise is partly generated by stream turbulence, but this mechanism fails to explain the observed clockwise hysteresis of seismic noise amplitude versus water level. This pattern is better explained if a significant part of the observed seismic noise is caused by ground vibrations generated by bed load transport. This points out the potential of using background seismic noise to quantify in continuous river bed load and monitor its spatial variations, which remain difficult with classical approaches.

Lithospheric and upper mantle stratifications beneath Tibet: New insights from Sp conversions

[1] We assess the upper mantle structure under the Tibetan plateau from a S-to-P converted waves receiver functions study. Contrary to the Ps receiver functions blurred by multiples from Moho to 350 km depth, the Sp are better suited for imaging at these depths. The Moho is clearly recovered and often exhibits a complex and warped structure. The upper crust is marked by a significant low velocity zone in the southern part of the plateau, probably associated with partial melt, which vanishes north of the Bangong suture. In the upper mantle, between the Moho and the 660 km discontinuity, four stratification levels are identified. The strongest converter at a depth ranging between 120 to 180 km corresponds to the bottom of a low shear-wave velocity layer imaged by surface wave inversion.

The 2015 Gorkha earthquake: A large event illuminating the Main Himalayan Thrust fault

The 2015 Gorkha earthquake sequence provides an outstanding opportunity to better characterize the geometry of the Main Himalayan Thrust (MHT). To overcome limitations due to unaccounted lateral heterogeneities, we perform Centroid Moment Tensor inversions in a 3-D Earth model for the main shock and largest aftershocks. In parallel, we recompute S-toP and P-to-S receiver functions from the Hi-CLIMB data set. Inverted centroid locations fall within a low-velocity zone at 10–15 km depth and corresponding to the subhorizontal portion of the MHT that ruptured during the Gorkha earthquake. North of the main shock hypocenter, receiver functions indicate a north dipping feature that likely corresponds to the midcrustal ramp connecting the flat portion to the deep part of the MHT. Our analysis of the main shock indicates that long-period energy emanated updip of high-frequency radiation sources previously inferred. This frequency-dependent rupture process might be explained by different factors such as fault geometry and the presence of fluids.

Discontinuous low-velocity zones in southern Tibet question the viability of the channel flow model

Abstract Low-velocity zones (‘bright spots’) imaged by the INDEPTH seismic experiment in southern Tibet are extensively interpreted as widespread partial melt within the crust, which has given a strong support for the channel flow model. These suggest that a continuous seismic low-velocity zone underlies Tibet on the large scale. Here we take advantage of the Hi-CLIMB seismic experiment which includes a dense south–north profile and a lateral 2D seismic network to assess the vertical and the horizontal extension of low-velocity zones in southern Tibet. Several approaches including migration, amplitude analysis and waveform inversion of receiver functions are performed to detect crustal low-velocity zones using this new seismological dataset. Our results reveal localized and discontinuous low-velocity zones in Tibet. They indicate that the vertical extension of the low-velocity zones is about 10 km, and their maximum horizontal length appears to be c. 50 km. Our study suggests a partial correlation between the location of these low-velocity zones and the spatial distribution of Tibetan grabens. These results, especially the non-continuity of low-velocity zones, together with the observed regular value of mean crustal VP/VS ratio, question the existence of widespread partial melt of the southern Tibetan crust and, therefore, the viability of the channel flow model.

Observation of deep water microseisms in the North Atlantic Ocean using tide modulations

Ocean activity produces continuous and ubiquitous seismic energy mostly in the 2–20 s period band, known as microseismic noise. Between 2 and 10 s period, secondary microseisms (SM) are generated by swell reflections close to the shores and/or by opposing swells in the deep ocean. However, unique conditions are required in order for surface waves generated by deep-ocean microseisms to be observed on land. By comparing short-duration power spectral densities at both Atlantic shoreline and inland seismic stations, we show that ocean tides strongly modulate the seismic energy in a wide period band except between 2.5 and 5 s. This tidal proxy reveals the existence of an ex situ short-period contribution of the SM peak. Comparison with swell spectra at surrounding buoys suggests that the largest part of this extra energy comes from deep ocean-generated microseisms. The energy modulation might be also used in numerical models of microseismic generation to constrain coastal reflection coefficients.

Uppermost mantle velocity from Pn tomography in the Gulf of Aden

We determine the lateral variations in seismic velocity of the lithospheric mantle beneath the Gulf of Aden and its margins by inversion of Pn (upper mantle high-frequency compressional P wave) traveltimes. Data for this study were collected by several temporary seismic networks and from the global catalogue. A least-squares tomographic algorithm is used to solve for velocity variations in the mantle lithosphere. In order to separate shallow and deeper structures, we use separate inversions for shorter and longer ray path data. High Pn velocities (8.2–8.4 km/s) are observed in the uppermost mantle beneath Yemen that may be related to the presence of magmatic underplating of the volcanic margins of Aden and the Red Sea. Zones of low velocity (7.7 km/s) are present in the shallow upper mantle beneath Sana’a, Aden, Afar, and along the Gulf of Aden that are likely related to melt transport through the lithosphere feeding active volcanism. Deeper within the upper mantle, beneath the Oman margin, a low-velocity zone (7.8 km/s) suggests a deep zone of melt accumulation. Our results provide evidence that the asthenosphere undergoes channelized flow from the Afar hotspot toward the east along the Aden and Sheba Ridges.

Seismic constraints on dynamic links between geomorphic processes and routing of sediment in a steep mountain catchment

Landscape dynamics are determined by interactions amongst geomorphic processes. These interactions allow the effects of tectonic, climatic and seismic perturbations to propagate across topographic domains, and permit the impacts of geomorphic process events to radiate from their point of origin. Visual remote sensing and in situ observations do not fully resolve the spatiotemporal patterns of surface processes in a landscape. As a result, the mechanisms and scales of geomorphic connectivity are poorly understood. Because many surface processes emit seismic signals, seismology can determine their type, location and timing with a resolution that reveals the operation of integral landscapes. Using seismic records, we show how hillslopes and channels in an Alpine catchment are interconnected to produce evolving, sediment-laden flows. This is done for a convective storm, which triggered a sequence of hillslope processes and debris flows. We observe the evolution of these process events and explore the operation of two-way links between mass wasting and channel processes, which are fundamental to the dynamics of most erosional landscapes. We also track the characteristics and propagation of flows along the debris flow channel, relating changes of observed energy to the deposition /mobilization of sediments, and using the spectral content of debris flow seismic signals to qualitatively infer sediment characteristics and channel abrasion potential. This seismological approach can help to test theoretical concepts of landscape dynamics and yield understanding of the nature and e fficie cy of links between individual geomorphic processes, which is required to accurately model landscape dynamics under changing tectonic or climatic conditions and to anticipate the natural hazard risk associated with specific meteorological events.