Biju John

The 1993 Killari (Latur), central India, earthquake: An example of fault reactivation in the Precambrian crust

The September 30, 1993, Killari event in central India is a rare incidence of an earthquake occurring within a Precambrian craton. A pertinent question concerning seismicity in such regions is whether a preexisting fault exists and, if so, what is its reactivation interval?Ourstudiesinthe1993rupturezonesuggestthattheKillariearthquakeoccurred in a region of previous seismic activity. Older thrust sheets and fault gouge, presumably formedduringpreviousepisodes,wereexposedinadeeptrench.Thestudiesalsoindicated an obsequent fault-line scarp, aligned with the current rupture zone. The morphological features in the area suggest mass removal of the upper part of the hanging wall on the southwestern side of the rupture. Existence of a prominent northwest-striking structure passing through the epicentral zone is revealed in the digital Landsat data. These data, together with the spatial trend of historic earthquakes along the northwest-striking structure, reinforce the argument that the earthquake at Killari is related to the reactivation of a preexisting fault.

Medieval pulse of great earthquakes in the central Himalaya: Viewing past activities on the frontal thrust

The Himalaya has experienced three great earthquakes during the last century1934 Nepal-Bihar, 1950 Upper Assam, and arguably the 1905 Kangra. Focus here is on the central Himalayan segment between the 1905 and the 1934 ruptures, where previous studies have identified a great earthquake between thirteenth and sixteenth centuries. Historical data suggest damaging earthquakes in A.D. 1255, 1344, 1505, 1803, and 1833, although their sources and magnitudes remain debated. We present new evidence for a great earthquake from a trench across the base of a 13m high scarp near Ramnagar at the Himalayan Frontal Thrust. The section exposed four south verging fault strands and a backthrust offsetting a broad spectrum of lithounits, including colluvial deposits. Age data suggest that the last great earthquake in the central Himalaya most likely occurred between A.D. 1259 and 1433. While evidence for this rupture is unmistakable, the stratigraphic clues imply an earlier event, which can most tentatively be placed between A.D. 1050 and 1250. The postulated existence of this earlier event, however, requires further validation. If the two-earthquake scenario is realistic, then the successive ruptures may have occurred in close intervals and were sourced on adjacent segments that overlapped at the trench site. Rupture(s) identified in the trench closely correlate with two damaging earthquakes of 1255 and 1344 reported from Nepal. The present study suggests that the frontal thrust in central Himalaya may have remained seismically inactive during the last similar to 700years. Considering this long elapsed time, a great earthquake may be due in the region.

Electron spin resonance dating of fault gouge from Desamangalam, Kerala: Evidence for Quaternary movement in Palghat gap shear zone

The field investigations in the epicentral area of the 1994 Wadakkancheri (Desamangalam), Kerala, earthquake (M 4.3) indicate subtle, but clearly recognizable expressions of geologically recent fault zone, consisting of fracture sets showing brittle displacement and a gouge zone. The fracture zone confines to the crystalline basement, and is spatially coincident with the elongation of the isoseismals of the 1994 mainshock and a 10-km-long WNW-ESE trending topographic lineament. The preliminary results from the electron spin resonance (ESR) dating on the quartz grains from the fault gouge indicate that the last major faulting in this site occurred 430 ± 43 ka ago. The experiments on different grain sizes of quartz from the gouge showed consistent decrease in age to a plateau of low values, indicating that ESR signals in finer grains were completely zeroed at the time of faulting due to frictional heat. The results show a relatively young age for displacement on the fault that occurs within a Precambrian shear zone. Discrete reactivated faults in such areas may be characterized by low degree of activity, but considering the ESR age of the last significant faulting event, the structure at Desamangalam may be categorized as a potentially active fault capable of generating moderate earthquakes, separated by very long periods of quiescence.