Kusala Rajendran

Characteristics of Deformation and Past Seismicity Associated with the 1819 Kutch Earthquake, Northwestern India

The 1819 earthquake in Kutch, northwestern India, is one of the most significant events to have occurred in a plate-interior setting. Despite being the second largest among the stable continental region (SCR) earthquakes, this event has not been analyzed within the context of present-day understanding of earthquake seismology. Coseismic changes related to this earthquake include massive ground deformation in a wide low-lying tidal-flat area. Although detailed historic accounts of this earthquake exist, many questions regarding the mode of deformation and the seismic history of the region remain unresolved. We explored the region nearly 180 years after the earthquake, and the information gathered adds to our understanding of this event and provides a fresh perspective on this unique intraplate seismogenic zone. A 90-km-long tract of elevated land with a peak height of 4.3 m is the most visible surface expression of this earthquake. We surveyed and analyzed the morphological features of this scarp and also carried out exploratory trenching in this region. The scarp morphology is suggestive of a growing fold related to a buried north-dipping thrust rather than a discrete fault that could have resulted from a surface rupture. The extensive liquefaction field associated with the earthquake offered an ideal setting to explore the paleoearthquake history. Age data of liquefaction features suggest that a previous event of comparable size must have occurred 800–1000 years ago. Seismic activity appears to be related to the reactivation of an ancient rift in a stress regime that is dominated by nearly north–south compression.

Crustal Deformation and Seismic History Associated with the 2004 Indian Ocean Earthquake: A Perspective from the Andaman–Nicobar Islands

The Indian Ocean earthquake of 26 December 2004 led to significant ground deformation in the Andaman and Nicobar region, accounting for ~800 km of the rupture. Part of this article deals with coseismic changes along these islands, observable from coastal morphology, biological indicators, and Global Positioning System (GPS) data. Our studies indicate that the islands south of 10° N latitude coseismically subsided by 1–1.5 m, both on their eastern and western margins, whereas those to the north showed a mixed response. The western margin of the Middle Andaman emerged by >1 m, and the eastern margin submerged by the same amount. In the North Andaman, both western and eastern margins emerged by >1 m. We also assess the pattern of long-term deformation (uplift/subsidence) and attempt to reconstruct earthquake/tsunami history, with the available data. Geological evidence for past submergence includes dead mangrove vegetation dating to 740 ± 100 yr B.P., near Port Blair and peat layers at 2–4 m and 10–15 m depths observed in core samples from nearby locations. Preliminary paleoseismological/tsunami evidence from the Andaman and Nicobar region and from the east coast of India, suggest at least one predecessor for the 2004 earthquake 900–1000 years ago. The history of earthquakes, although incomplete at this stage, seems to imply that the 2004-type earthquakes are infrequent and follow variable intervals

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.

Seismogenesis in the stable continental interiors: an appraisal based on two examples from India

Characteristics of two recent moderate earthquakes from India (1993 Killari, Mw 6.1 and 1997 Jabalpur, Mw 5.8) are presented in this paper to illustrate the influence of geologic and tectonic setting on the nature of seismicity in stable continental regions (SCRs). The Killari event, which occurred in the middle of a cratonic region of low background seismicity, long recurrence interval and poorly developed neotectonic features, may be qualified as a common type of SCR event. This earthquake is also characterized by a shallow focal depth and a long sequence of aftershocks. In contrast, the Jabalpur earthquake shows a spatial association with a well-defined tectonic structure with significant background seismicity. A remarkable feature of this earthquake is its deep focus, not commonly observed in SCR seismicity. This event was associated with fewer aftershocks unlike most other SCR earthquakes. Analogous events are found in other shield regions as well. Based on their general characteristics, a broad classification of moderate SCR earthquakes is attempted here. Two broad groups have been identified in this study: (1) a set of deep focus earthquakes that are related to intracratonic paleorifts, and (2) shallow-focus earthquakes associated with discrete faults in the less deformed Precambrian terrains. A synthesis of earthquake data from different geologic environments will improve our understanding of the seismogenesis in the shield regions.

Assessing the previous activity at the source zone of the 2001 Bhuj earthquake based on the near‐source and distant paleoseismological indicators

[1] The M w 7.7 2001 Bhuj (Kachchh) earthquake was not associated with any primary surface rupture, but it produced secondary faulting, folding and liquefaction. This study highlights the potential of a secondary rupture and proxies like lateral spreads and sandblows in unraveling the past activity related to the 2001 source. Chronological constraints of an older lateral spread and far-field paleoliquefaction features, combined with archeological data, provide evidence for occurrences of two previous earthquakes at the 2001 source zone about 4000 and 9000 years, ago. Distinct stratigraphic evidence for at least one previous offset dated at 4424 ± 656 years could be detected at a stepover zone associated with a dextral secondary fault, reactivated during the 2001 earthquake. The studies imply longer interseismic intervals for the 2001 source zone, in comparison with the source zone of the 1819 earthquake located toward the northwestern part of the Rann of Kachchh. The spatial and temporal correlation of previous events derived on the basis of the available paleoseismic data from the region suggest not only repeated activity at the 2001 source, but possibility for additional potential sources in parts of Kachchh and Cambay basins. Although we infer a longer recurrence interval for the 2001 Bhuj earthquake source, our study points to the fact that these additional sources may have the potential to rupture in the future, considering the long elapsed time.

The June 2010 Nicobar Earthquake: Fault Reactivation on the Subducting Oceanic Plate

The similar to 1300-km-long rupture zone of the 2004 Andaman-Sumatra megathrust earthquake continues to generate a mix of thrust, normal, and strike-slip faulting events. The 12 June 2010 M(w) 7.5 event on the subducting plate is the most recent large earthquake on the Nicobar segment. The left-lateral faulting mechanism of this event is unusual for the outer-rise region, considering the stress transfer processes that follow great underthrusting earthquakes. Another earthquake (M(w) 7.2) with a similar mechanism occurred very close to this event on 24 July 2005. These earthquakes and most of their aftershocks on the subducting plate were generated by left-lateral strike-slip faulting on north-northeast-south-southwest oriented near-vertical faults, in response to north-northwest-south-southeast directed compression. Pre-2004 earthquake faulting mechanisms on the subducting oceanic plate are consistent with this pattern. Post-2004, left-lateral faulting on the subducting oceanic plate clusters between 5 degrees N and 9 degrees N, where the 90 degrees E ridge impinges the trench axis. Our study observes that the subducting plate off the Sumatra and Nicobar segments behaves similarly to a chip of the India-Australia plate, deforming in response to a generally northwest-southeast oriented compression, an aspect that must be factored into the plate deformation models.

Geological investigations at Killari and Ter, central India and implications for palaeoseismicity in the shield region

Abstract The 1993 Killari earthquake occurred in the central part of the Indian shield, an area generally believed to be of low seismogenic potential. This rare event provided an opportunity to understand the seismogenesis within the shield regions. Trench excavations in the rupture zone as well as other natural exposures in the vicinity of the epicentral region of the Killari earthquake indicate episodic activity separated by long periods. Deep drilling in the epicentral area recorded a maximum of ∼6 m of slip in the deeper strata ( Gupta et al., 1998a ). Assuming an average of about 1 m slip for a moderate earthquake, as revealed by the 1993 event, accumulated slip indicates at least six moderate earthquakes at Killari. While the timing of the penultimate event may work out to be hundreds of thousands of years or more, evidence for at least one moderate earthquake ∼1500 yr ago was obtained from Ter, about 40 km northwest of Killari. Observations at Killari and Ter indicate the localization of deformation along a preexisting shear zone that are being reactivated in the current stress field. Because of factors like low background seismicity, long interseismic intervals and smaller ruptures compounded by low preservation of surface exposures, behaviour of seismogenic structures in the shield regions may appear to be random in space and time. Observations presented in this paper suggest that the perceived randomness probably holds only for individual patches in the current time window. Other parts of the seismogenic fault may behave independently, showing a different temporal pattern. Comprehensive investigations of the entire structure are needed to understand the long-term behaviour of such faults, and thereby improve the seismic hazard assessment in the cratonic interiors.

Studying earthquake recurrence in the Kachchh region, India

The occurrence of two large earthquakes (M>7.5) in the Kachchh basin within a short span of ∼200 years is quite unusual for an intracratonic region (Figure l). What factors could possibly make this region different from other, mid-plate settings? Could some of these factors be responsible for the relatively shorter recurrence times? Does the tectonic history provide us with any clues about the uniqueness of this intraplate region?

Three dimensional P velocity image of the Oroville Reservoir Area, California, from local earthquake tomography

This study images the P velocity structure of a region in the vicinity of the large artificial reservoir at Oroville, California. The velocity structure of the upper crust (to a depth of 12 km) was obtained by simultaneous inversion of P-arrivals of 168 aftershocks of the 1975 Oroville earthquake. A decrease in P-wave velocity by 7 to 8% was observed across a known fault zone. The hypocenters of the 1975 sequence are associated with a southwest dipping structure characterized by low velocity, with most of the deeper activity occurring in adjacent regions of higher velocity. Resolution of the velocity structure was examined through inversions of synthetic travel times using different starting velocity models. Although the role of the reservoir in generating the low velocities remains speculative, the structure is comparable to that of some active segments of the San Andreas fault where fluid pressures are thought to be high.