Effect of azimuthal coverage of an earthquake on moment tensor solutions estimated by waveform inversion

The purposes of this seismological investigation are to understand and describe the effect of decrease in the azimuthal coverage of an earthquake on moment tensor solution estimated by waveform inversion. It will be very useful and worthwhile as mostly seismological networks are sparse and in case when only one or two station data are available. In this work, we have used two moderate earthquakes, 21 September 2009 (near Uttarakashi) and 3 May 2010 (near Ghansali). The waveform inversion has been carried using ISOLA software. The moment tensor is first estimated by using all station data and then by removing the stations so that the azimuthal coverage changes. The results show that strike of both nodal planes is varying with the change in azimuthal coverage. However, the slip and dip of both nodal planes remain quite stable against the variation in azimuthal coverage for these two earthquakes analyzed. The effect of decrease in the azimuthal coverage showed increase in double-couple percentage (DC %) and decrease in compensated linear vector decomposition (CLVD %). The other focal parameters such as T-axis azimuth, P-axis azimuth, T-axis plunge, and P-axis plunge have been found stable against the variation in azimuth coverage. The study also demonstrates that the moment tensor solutions obtained from waveform inversion using single station are almost similar to those estimated using maximum azimuthal coverage data and by polarity inversion.     

The ‘humped’ soil production function: eroding Arnhem Land,Australia

We report erosion rates and processes, determined from in situ‐produced beryllium‐10 (¹⁰Be) and aluminum‐26 (²⁶Al), across a soil‐mantled landscape of Arnhem Land, northern Australia. Soil production rates peak under a soil thickness of about 35 cm and we observe no soil thicknesses between exposed bedrock and this thickness. These results thus quantify a well‐defined ‘humped’ soil‐production function, in contrast to functions reported for other landscapes. We compare this function to a previously reported exponential decline of soil production rates with increasing soil thickness across the passive margin exposed in the Bega Valley, south‐eastern Australia, and found remarkable similarities in rates. The critical difference in this work was that the Arnhem Land landscapes were either bedrock or mantled with soils greater than about 35 cm deep, with peak soil production rates of about 20 m/Ma under 35–40 cm of soil, thus supporting previous theory and modeling results for a humped soil production function. We also show how coupling point‐specific with catchment‐averaged erosion rate measurements lead to a better understanding of landscape denudation. Specifically, we report a nested sampling scheme where we quantify average erosion rates from the first‐order, upland catchments to the main, sixth‐order channel of Tin Camp Creek. The low (～5 m/Ma) rates from the main channel sediments reflect contributions from the slowly eroding stony highlands, while the channels draining our study area reflect local soil production rates (～10 m/Ma off the rocky ridge; ～20 m/Ma from the soil mantled regions). Quantifying such rates and processes help determine spatial variations of soil thickness as well as helping to predict the sustainability of the Earth's soil resource under different erosional regimes. Copyright © 2009 John Wiley & Sons, Ltd.     

Dynamic Site Characterization and Correlation of Shear Wave Velocity with Standard Penetration Test ‘N’ Values for the City of Agartala,Tripura State,India

Seismic site characterization is the basic requirement for seismic microzonation and site response studies of an area. Site characterization helps to gauge the average dynamic properties of soil deposits and thus helps to evaluate the surface level response. This paper presents a seismic site characterization of Agartala city, the capital of Tripura state, in the northeast of India. Seismically, Agartala city is situated in the Bengal Basin zone which is classified as a highly active seismic zone, assigned by Indian seismic code BIS-1893, Indian Standard Criteria for Earthquake Resistant Design of Structures, Part-1 General Provisions and Buildings. According to the Bureau of Indian Standards, New Delhi (2002), it is the highest seismic level (zone-V) in the country. The city is very close to the Sylhet fault (Bangladesh) where two major earthquakes (M _w > 7) have occurred in the past and affected severely this city and the whole of northeast India. In order to perform site response evaluation, a series of geophysical tests at 27 locations were conducted using the multichannel analysis of surface waves (MASW) technique, which is an advanced method for obtaining shear wave velocity (V _s) profiles from in situ measurements. Similarly, standard penetration test (SPT-N) bore log data sets have been obtained from the Urban Development Department, Govt. of Tripura. In the collected data sets, out of 50 bore logs, 27 were selected which are close to the MASW test locations and used for further study. Both the data sets (V _s profiles with depth and SPT-N bore log profiles) have been used to calculate the average shear wave velocity (V _s30) and average SPT-N values for the upper 30 m depth of the subsurface soil profiles. These were used for site classification of the study area recommended by the National Earthquake Hazard Reduction Program (NEHRP) manual. The average V _s30 and SPT-N classified the study area as seismic site class D and E categories, indicating that the city is susceptible to site effects and liquefaction. Further, the different data set combinations between V _s and SPT-N (corrected and uncorrected) values have been used to develop site-specific correlation equations by statistical regression, as ‘V _s’ is a function of SPT-N value (corrected and uncorrected), considered with or without depth. However, after considering the data set pairs, a probabilistic approach has also been presented to develop a correlation using a quantile–quantile (Q–Q) plot. A comparison has also been made with the well known published correlations (for all soils) available in the literature. The present correlations closely agree with the other equations, but, comparatively, the correlation of shear wave velocity with the variation of depth and uncorrected SPT-N values provides a more suitable predicting model. Also the Q–Q plot agrees with all the other equations. In the absence of in situ measurements, the present correlations could be used to measure V _s profiles of the study area for site response studies.     

Strength matters: Resisting erosion across upland landscapes

Soil-covered upland landscapes comprise a critical part of the habitable world and our understanding of their evolution as a function of different climatic, tectonic, and geologic regimes is important across a wide range of disciplines. Soil production and transport play essential roles in controlling the spatial variation of soil depth and therefore hillslope hydrological processes, distribution of vegetation, and soil biological activity. Field-based confirmation of the hypothesized relationship between soil thickness and soil production is relatively recent, however, and here we quantify a direct, material strength-based influence on variable soil production across landscapes. We report clear empirical linkages between the shear strength of the parent material (its erodibility) and the overlying soil thickness. Specifically, we use a cone penetrometer and a shear vane to determine saprolite resistance to shear. We find that saprolite shear strength increases systematically with overlying soil thickness across three very different field sites where we previously quantified soil production rates. At these sites, soil production rates, determined from in situ produced beryllium-10 (¹⁰Be) and aluminum-26 (²⁶Al), decrease with overlying soil thickness and we therefore infer that the efficiency of soil production must decrease with increasing parent material shear strength. We use our field-based data to help explain the linkages between biogenic processes, chemical weathering, hillslope hydrology, and the evolution of the Earth's surface. © 2019 John Wiley & Sons, Ltd.     

Landscape disequilibrium on 1000–10,000 year scales Marsyandi River, Nepal, central Himalaya

In an actively deforming orogen, maintenance of a topographic steady state requires that hillslope erosion, river incision, and rock uplift rates are balanced over timescales of 10⁵–10⁷ years. Over shorter times, <10⁵ years, hillslope erosion and bedrock river incision rates fluctuate with changes in climate. On 10⁴-year timescales, the Marsyandi River in the central Nepal Himalaya has oscillated between bedrock incision and valley alluviation in response to changes in monsoon intensity and sediment flux. Stratigraphy and ¹⁴C ages of fill terrace deposits reveal a major alluviation, coincident with a monsoonal maximum, ca. 50–35 ky BP. Cosmogenic ¹⁰Be and ²⁶Al exposure ages define an alluviation and reincision event ca. 9–6 ky BP, also at a time of strong South Asian monsoons. The terrace deposits that line the Lesser Himalayan channel are largely composed of debris flows which originate in the Greater Himalayan rocks up to 40 km away. The terrace sequences contain many cubic kilometers of sediment, but probably represent only 2–8% of the sediments which flushed through the Marsyandi during the accumulation period. At 10⁴-year timescales, maximum bedrock incision rates are 7 mm/year in the Greater Himalaya and 1.5 mm/year in the Lesser Himalayan Mahabarat Range. We propose a model in which river channel erosion is temporally out-of-phase with hillslope erosion. Increased monsoonal precipitation causes an increase in hillslope-derived sediment that overwhelms the transport capacity of the river. The resulting aggradation protects the bedrock channel from erosion, allowing the river gradient to steepen as rock uplift continues. When the alluvium is later removed and the bedrock channel re-exposed, bedrock incision rates probably accelerate beyond the long-term mean as the river gradient adjusts downward toward a more “equilibrium” profile. Efforts to document dynamic equilibrium in active orogens require quantification of rates over time intervals significantly exceeding the scale of these millennial fluctuations in rate.     

Quantifying sediment transport across an undisturbed prairie landscape using cesium-137 and high resolution topography

Soil erosion is a global environmental problem, and anthropogenic fallout radionuclides offer a promising tool for describing and quantifying soil redistribution on decadal time scales. To date, applications of radioactive fallout to trace upland sediment transport have been developed primarily on lands disturbed by agriculture, grazing, and logging. Here we use ¹³⁷Cs to characterize and quantify soil erosion at the Konza Prairie Long-Term Ecological Research (LTER) site, an undisturbed grassland in northeastern Kansas. We report on the small scale (< 10 m) and landscape scale (10 to 1000 m) distribution of fallout ¹³⁷Cs, and show significant variability in the concentrations and amounts of ¹³⁷Cs in soils at our site. ¹³⁷Cs soil concentrations and amounts typically vary by 10% to 30% on small scales, which most likely represents the spatial heterogeneity of the depositional processes. Landscape scale variability of soil ¹³⁷Cs was significantly higher than small scale variability. Most notably, soils collected on convex (divergent) landforms had ¹³⁷Cs inventories of 2500 to 3000 Bq m^− 2, which is consistent with the expected atmospheric inputs to the study area during the 1950s and 1960s. Concave landforms, however, had statistically lower inventories of 1800 to 2300 Bq m^− 2. The distribution of ¹³⁷Cs on this undisturbed landscape contrasts significantly with distributions observed across disturbed sites, which generally have accumulations of radioactive fallout in valley bottoms. Because the upslope contributing area at each sampling point had a significant negative correlation with the soil inventory of ¹³⁷Cs, we suggest that overland flow in convergent areas dominates soil erosion at Konza on time scales of decades. Very few points on our landscape had ¹³⁷Cs inventories significantly above that which would be predicted from direct deposition of ¹³⁷Cs on the soil surface; we conclude therefore that there is little net sediment storage on this undisturbed landscape.