19-Jul-2022: Grain shape influences liquefaction of sand, a major earthquake hazard
Scientists have found that the shape of sand grains influences the liquefaction of sand, one of the major factors behind the collapse of structures during earthquakes. Liquefaction of sand is a phenomenon in which the strength and stiffness of a soil is reduced by earthquake shaking or other rapid loading and leads to the collapse of structures resting on the liquefied ground.
As natural sand with regular shape liquefies easily, the scientists have concluded that natural sand used in structures like slopes and retaining walls can be replaced with irregular manufactured sand to improve the stability and sustainability.
Though the qualitative effects of grain size and grain shape on the resistance of sand to liquefaction are well established, quantitative correlations between them are elusive. Most of the studies in this direction used conventional methods to quantify the grains' size and shape, including sieve analysis and visual observations.
In a breakthrough study, researchers at the Indian Institute of Science (IISc) used digital image analysis for grain shape characterizations and related them to the liquefaction potential of the sands. They found a strong relation between the two. This is because the shear force (unaligned forces pushing one part of a body in one specific direction and another part of the body in the opposite direction) required to break the inter-particle locking is more for the grains with relatively irregular shapes.
Microscopic images of sand particles were analyzed through computational algorithms developed in MATLAB (MATrix LABoratory), which is a high-performance computing platform for analyzing data to determine their shape parameters. Cyclic simple shear tests in which specimens are subjected to simulated earthquake conditions of alternate cycles of tension and compression were carried out on sand samples to determine their potential to liquefy under specific earthquake conditions. For these tests, the scientists used the cyclic simple shear test setup (GCTS USA make) procured through Department of Science and Technology – Fund for Improvement of S&T Infrastructure in Universities and Higher Educational Institutions (FIST) funding. The study has been accepted for publication in Indian Geotechnical Journal, for carrying out cyclic simple shear tests.
The research team found that glass beads, which have regular shape with higher roundness and sphericity, liquefied first in the cyclic shear tests, while river sand, whose roundness and sphericity (how much of an even sphere it is) fall between glass beads and manufactured sand, liquefied next, followed by manufactured sand, whose shape is relatively irregular. These tests clearly highlighted the important effects of grain shape on the liquefaction potential of granular soils.
As the shape of the particles becomes irregular, with their overall form deviating from that of a sphere and their corners becoming sharper, they get interlocked with each other during shearing. Interlocking provides additional resistance to shear, and hence the tendency to get separated from each other to float in the fluid becomes lesser for particles with irregular shapes.
Further, tortuosity, or the deviation in the fluid path, increases with the irregular shape of the particles. Greater tortuosity decreases water flow through the pore network and decreases the chance for water to separate the particles.
1-Jun-2022: Study attributes tectonic linkage in the northeast edge of the Indian plate to great Assam Earthquake
Researchers have traced the great Assam Earthquake to complex tectonics of the North Eastern fringe of the Indian Plate in the Eastern Himalaya and the Indo-Burma Ranges (IBR) and the interactions between the two, which can produce deeper earthquakes in IBR and crustal ones in the Eastern Himalaya.
The north-eastern fringe of the Indian Plate in the Eastern Himalaya has been found to be seismically active up to about 40 km depth, while the seismicity in the Indo-Burma Ranges (IBR) is observed up to a depth of around 200 km.
They have suggested that this seismic structure forms a complex tectonics which produced the great Assam earthquake of 1950 (M 8.6) and maybe building up stress for a future earthquake. –The Great Assam Earthquake is the largest intra-continental earthquake ever recorded, which was located at the India-China border near the Mishmi Hills of Arunachal Himalaya.
The Eastern Himalayan Syntaxis (EHS) in Arunachal Pradesh and bordering regions of Assam is acknowledged as one of the most seismically active regions in the world. The northeast corner of the Indian Plate in the EHS belongs to the seismic zone V of the national zoning map of India and does have a potential to trigger major earthquakes in the future.
In contrast to several studies carried out in the EHS and adjoining SE Tibetan plateau, extremely less studies have been done in the north-eastern fringe of the Indian Plate in the Eastern Himalayan Syntaxis (Tidding-Tuting Suture, TTSZ) for understanding seismogenesis and its tectonic linkage. After the 1950 great Assam earthquake, the region between the upper Assam and the Mishmi Block is not producing any large earthquakes and is considered as a seismic gap region. A previous study has suggested a locked zone in the Mishmi Thrust (MT) zone, which may suggest building up of stress for a future earthquake.
Moderate magnitude earthquakes in the region are rarely reported by global seismological networks. To obtain information of moderate and microearthquakes, the Wadia Institute of Himalayan Geology, Dehradun, an autonomous institute of the Department of Science and Technology, established 11 broadband seismological stations in the Lohit Valley and 8 stations in the Siang Window of Arunachal Himalaya.
A study conducted by a team led by Dr. Devajit Hazarika from Wadia Institute of Himalayan Geology with the help of Micro and moderate magnitude earthquakes in the region, recorded at the local seismological stations emphasized the overall seismicity pattern in the EHS and adjoined Indo-Burma Ranges (IBR) with special emphasis on the seismotectonic of the Lohit Valley region.
The study published in Tectonophysics Journal reveals that the TTSZ is seismically active up to around 40 km depth. In contrast, the seismicity in the Indo-Burma Ranges (IBR) is observed up to a depth of around 200 km suggesting the active subduction process of the Indian plate beneath the IBR. It suggests that the IBR is more susceptible to deeper earthquakes, while crustal-scale earthquakes are more likely to occur in the TTSZ.
This research suggests that the subduction process terminates north of around 270 N Latitude and the indentation process of the rigid Indian plate into south-east Asia predominantly controls the seismicity north of the IBR.
The results reveal that the closely spaced Mishmi, Tidding, and Lohit faults along the Lohit and Dibang River Valleys of eastern Arunachal Pradesh are steeply dipping thrust sheets that accommodate the large crustal shortening owing to the indentation process and clockwise rotation tectonics. The Walong fault, in the upper part of the Lohit River Valley of Arunachal Pradesh, is characterized by strike-slip motion with a thrust component that facilitates the clock-wise rotation of crustal material around the syntaxis. Significant strain partitioning is anticipated from the variation of pressure (P) axes orientations indicating the effect of complex syntaxial tectonics.
13-Jan-2022: Spectacular landscape changes detected in Gujarat’s Kachchh region due to major earthquakes in recent geological past
Major earthquake events in last 30,000 years resulted in spectacular changes in landscape of the Katrol Hill Fault in the Kachchh region in Gujarat, a study conducted on sediment samples revealed. These surprising geological facts about the seismic history of the fault in the recent geological past necessitate a revised seismic hazard assessment and mitigation strategies in Kachchh Basin, owing to its close proximity to industrial corridor and major settlements, including Bhuj city.
Earthquakes are one of the natural hazards that geologists are still grappling with its complex nature. The complexity is attributed to its widespread occurrence through space and time. Seismicity in Kachchh region is highly complex as it is characterized by multiple seismic sources in the form of several East-West trending fault lines, which release continuously accumulating tectonic stresses at intervals producing earthquakes. Real-time monitoring of earthquakes since the occurrence of devastating 2001 Bhuj earthquake indicate that most of the faults in the region, viz., Kachchh Mainland Fault (KMF), South Wagad Fault (SWF), Gedi Fault (GF), and Island Belt Fault (IBF) are seismically active. However, seismic activity along other faults like the Katrol Hill Fault (KHF) is not apparent, thus making the task of seismic hazard estimation and mitigation in the region a scientifically complex process.
Geologists from the Department of Geology, the Maharaja Sayajirao University of Baroda, Vadodara, have been trying to decode the seismic activity in Kachchh using geological methods. Recent focused studies of this research group led earlier by Prof. L. S. Chamyal and later by Prof. D. M. Maurya on the not so well understood Katrol Hill Fault (KHF) have estimated the length of surface rupture produced by three large magnitude earthquakes during the last ~30,000 years as nearly 21 km. This study was carried out by field mapping and using sophisticated field instruments like Ground Penetrating Radar and laboratory equipments like Scanning Electron Microscope for analyzing sediment samples.
This research published in the journal ‘Engineering Geology’ and ‘Earth Surface Processes and Landforms’ was made possible through high-end scientific equipments funded mainly under the FIST Programme of the Department of Science & Technology, Govt. of India. The equipment, housed in the Department of Geology, The Maharaja Sayajirao University of Baroda, Vadodara, are being actively used for advanced research in geological and allied sciences.
The team of geologists at the university carried high magnification Scanning Electron Microscope (SEM) study of the surface of sediment samples collected along the Faultline, which showed features indicative of surface faulting. Based on various fault parameters deduced like length of surface rupture, displacement, and slip rate, the study shows that the Katrol Hill Fault (KHF) has produced high magnitude seismic events during the past ~30,000 years and is, therefore, a credible seismic source capable of generating surface rupture hazard in the Kachchh Basin.
Further, field-based geomorphological studies revealed that the events resulted in spectacular changes in landscape, as evidenced by the disruption and reorganization of the channel of the Gunawari River in the fault zone. It is interesting that these events produced surface rupture, whereas the 2001 Bhuj earthquake (Mw 7.7) did not rupture the surface. The Palaeo-earthquakes along the Katrol Hill Fault produced surface rupture probably because they originated at relatively shallow depths. However, these events show a much longer recurrence interval for the KHF on the scale of thousands of years as compared to other seismically active faults in the Kachchh Basin.