The effective elastic thickness of the India Plate from receiver function imaging, gravity anomalies and thermomechanical modelling

The range and the meaning of the effective elastic thickness (EET) in continental areas have been subject to controversy over the last two decades. Here we take advantage of the new data set from the Hi-CLIMB seismological experiment to re-estimate the EET of the India Plate along a south-north profile extending from the Ganges basin to central Tibet. Receiver functions give a high-resolution image of the base of the foreland basin at similar to 5 km depth and constrain the crustal thickness, which increases northwards from similar to 35 km beneath the indo-gangetic plain to similar to 70 km in southern Tibet. Together with available data sets including seismic profiles, seismological images from both INDEPTH and HIMNT experiments, deep well measurements and Bouguer anomaly profiles, we interpret this new image with 2-D thermomechanical modelling solutions, using different type of crustal and mantle rheologies. We find that (1) the EET of the India Plate decreases northwards from 60-80 to 20-30 km as it is flexed down beneath Himalaya and Tibet, due to thermal and flexural weakening; (2) the only resistant layer of the India Plate beneath southern Tibet is the upper mantle, which serves as a support for the topographic load and (3) the most abrupt drop in the EET, located around 200 km south of the MFT, is associated with a gradual decoupling between the crust and the mantle. We show that our geometrical constraints do not allow to determine if the upper and lower crust are coupled or not. Our results clearly reveal that a rheology with a weak mantle is unable to explain the geometry of the lithosphere in this region, and they are in favour of a rheology in which the mantle is strong

Seismic velocities in Southern Tibet lower crust: a receiver function approach for eclogite detection

Beneath the Tibet plateau, the deficit of crustal thickening with respect to what is expected from the plate tectonic constraints is thought to be absorbed either by lateral extrusion or by vertical rock-mass transfer. To nourish the unsettled debate of the relative importance of these two processes, we propose a new approach, based on the S-to-P and the P-to-S wave conversions, enabling the precise determination of the seismic velocities. The weighted amplitudes of the direct conversion and of reverberations are stacked at their predicted arrival times for various values of layer thickness and v(P)/v(S) ratio separately for two sets of P- and S-receiver functions. For each set of receiver functions, coherent stack gives the v(P)/v(S) ratio and thickness for the considered layer (the grid search stacking method). The values of v(P)/v(S) ratio and layer thickness are functions of the velocity used for stacking the set of receiver functions, but using the P- and S-receiver functions allows us to solve this indetermination and to find the effective parameters of the layer: velocity v(S), v(P)/v(S) ratio and thickness. We use a bootstrap resampling of the receiver function data sets to estimate the parameters uncertainties. For the Southern Lhasa Block, the migrated sections of both P- and S-receiver functions (Hi-CLIMB experiment data) show a layer in the lower crust that may be related to the lower Indian crust underplated beneath Tibet. With the grid search stacking method, high shear wave velocities (v(S) similar to 4.73 km s(-1)) and low v(P)/v(S) ratios (similar to 1.69) are detected in this layer. Such values are typical for high-grade eclogites, and the low v(P)/v(S) ratio precludes the confusion with mafic granulites. There is no evidence for partial eclogitization near and south of the Yarlung-Tsangpo Suture, and the about 19 km thick eclogitic layer extends northwards only to about the middle of the Lhasa terrane

Development of a Conceptual Chum Salmon Emergence Model for Ives Island

The objective of the study described herein was to develop a conceptual model of chum salmon emergence that was based on empirical water temperature of the riverbed and river in specific locations where chum salmon spawn in the Ives Island area. The conceptual model was developed using water temperature data that have been collected in the past and are currently being collected in the Ives Island area. The model will be useful to system operators who need to estimate the complete distribution of chum salmon emergence (first emergence through final emergence) in order to balance chum salmon redd protection and power system operation

Dislocation modeling of blind thrusts in the eastern Los Angeles basin, California

The East and West Coyote Hills in the eastern Los Angeles Basin are the surface expression of uplift accompanying blind reverse faulting. Folded Quaternary strata indicate that the hills are growing and that the faults underlying them are active. Detailed subsurface mapping in the East Coyote Oil Field shows that a previously mapped, reverse separation fault is predominantly an inactive, left‐lateral, strike‐slip fault that is not responsible for the uplift of the East Coyote Hills. The fault responsible for folding and uplift of the Coyote Hills does not cut wells in either the East or West Coyote Oil Fields. To characterize the geometry of the blind fault responsible for folding, we employ dislocation modeling. The dip and upper fault tip depths obtained from modeling suggest that the thrust fault beneath the Coyote Hills may be an extension of the Puente Hills blind thrust fault that continues westward beneath the Santa Fe Springs Oil Field. Modeling results suggest that the segment of the thrust fault responsible for folding the Coyote Hills would have accumulated 1500 m of reverse displacement over the last 1.2 Myr, yielding an average slip rate of 1.3 ± 0.5 mm/yr. The Santa Fe Springs segment of the fault has a slip rate of 1.5 ± 0.4 mm/yr for the last 1.2 Myr. The estimated moment magnitude for a reverse displacement earthquake on the Puente Hills blind thrust ranges from 6.6 to 7.2, depending on the length of the rupture. The estimated average recurrence interval for these earthquakes is 1700–3200 years.Keywords: Southern California, Blind thrust faults, Earthquake

Spatiotemporal sequence of Himalayan debris flow from analysis of high-frequence seismic noise

During the 2003 summer monsoon, the Hi‐CLIMB seismological stations deployed across the Himalayan Range detected bursts of high‐frequency seismic noise that lasted several hours to days. On the basis of the cross correlation of seismic envelopes recorded at 11 stations, we show that the largest transient event on 15 August was located nearby a village partially destroyed on that day by a devastating debris flow. This consistency in both space and time suggests that high‐frequency seismic noise analysis can be used to monitor debris flow generation as well as the evacuation of the sediment. A systematic study of one year of seismic noise, focusing on the detection of similar events, provides information on the spatial and temporal occurrence of mass movements at the front of the Himalayas. With a 50% probability of occurrence of a daily event, a total of 46 debris flows are seismically detected. Most of them were generated in regions of steep slopes, large gullies, and loose soils during the 2003 summer monsoon storms. These events are compared to local meteorological data to determine rainfall thresholds for slope failures, including the cumulative rainfall needed to bring the soil moisture content to failure capacity. The inferred thresholds are consistent with previous estimates deduced from soil studies as well as sediment supply investigations in the area. These results point out the potential of using seismic noise as a dedicated tool for monitoring the spatiotemporal occurrence of landslides and debris flows on a regional scale

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

Analysis of continuous seismic data recorded by a dense passive seismological network (Hi‐CLIMB) installed across the Himalayas reveals strong spatial and temporal variations in the ambient seismic energy produced at high frequencies (>1 Hz). From June to September 2003, the high‐frequency seismic noise is observed to increase up 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

Teleseismic body wave analysis of the 1988 Armenian earthquake

Long‐period and broadband body waves from 14 digital seismic stations are used to investigate the rupture process of the December 7, 1988 earthquake near Spitak, Armenia, USSR. The inversion of these data gives the following centroidal source parameters: strike 299°, dip 64°, rake 151°, depth 6.3 km and seismic moment 1.5×10¹⁹ Nm, indicating that on average the earthquake had a strike‐slip mechanism with a substantial reverse component. The broadband waveforms, however, show significant complexity; they are best fit with a source model that includes three sub‐events, very similar in size, but with distinct focal mechanisms and locations. Rupture apparently initiated as a shallow reverse fault at a point of maximum bending on a right‐lateral strike‐slip fault, and then extended bilaterally, first towards the southeast and then towards the west. This interpretation agrees with the aftershock distribution and fault lineations observed on LANDSAT images

Thermal and tectonic consequences of India underthrusting Tibet

The Tibetan Plateau is the largest orogenic system on Earth, and has been influential in our understanding of how the continental lithosphere deforms. Beneath the plateau are some of the deepest ( ~ 100 ) earthquakes observed within the continental lithosphere, which have been pivotal in ongoing debates about the rheology and behaviour of the continents. We present new observations of earthquake depths from the region, and use thermal models to suggest that all of them occur in material at temperatures of ≲600 °C. Thermal modelling, combined with experimentally derived flow laws, suggests that if the Indian lower crust is anhydrous it will remain strong beneath the entire southern half of the Tibetan plateau, as is also suggested by dynamic models. In northwest Tibet, the strong underthrust Indian lower crust abuts the rigid Tarim Basin, and may be responsible for both the clockwise rotation of Tarim relative to stable Eurasia and the gradient of shortening along the Tien Shan

Lower edge of locked Main Himalayan Thrust unzipped by the 2015 Gorkha earthquake

Large earthquakes are thought to release strain on previously locked faults. However, the details of how earthquakes are initiated, grow and terminate in relation to pre-seismically locked and creeping patches is unclear ^1-4. The 2015 Mw 7.8 Gorkha, Nepal earthquake occurred close to Kathmandu in a region where the prior pattern of fault locking is well documented ^5. Here we analyze this event using seismological records measured at teleseismic distances and Synthetic Aperture Radar imagery. We show that the earthquake originated northwest of Kathmandu within a cluster of background seismicity that fringes the bottom of the locked portion of the Main Himalayan Thrust fault (MHT). The rupture propagated eastwards for about 140 km, unzipping the lower edge of the locked portion of the fault. High-frequency seismic waves radiated continuously as the slip pulse propagated at about 2.8 km s-1 along this zone of presumably high and heterogeneous pre-¬seismic stress at the seismic-aseismic transition. Eastward unzipping of the fault resumed during the Mw 7.3 aftershock on May 12. The transfer of stress to neighbouring regions during the Gorkha earthquake should facilitate future rupture of the areas of the MHT adjacent and up-dip of the Gorkha earthquake rupture.This is the author accepted manuscript. The final version is available from Nature Publishing Group via http://dx.doi.org/10.1038/ngeo251