Interpreting the style of faulting and paleoseismicity associated with the 1897 Shillong, northeast India, earthquake: Implications for regional tectonism

Abstract

The 1897 Shillong (Assam), northeast India, earthquake is considered to be one of the largest in the modern history. Although Oldham's [1899] classic memoir on this event opened new vistas in observational seismology, many questions on its style of faulting remain unresolved. Most previous studies considered this as a detachment earthquake that occurred on a gently north dipping fault, extending from the Himalayan front. A recent model proposed an alternate geometry governed by high‐angle faults to the north and south of the plateau, and it suggested that the 1897 earthquake occurred on a south dipping reverse fault, coinciding with the northern plateau margin. In this paper, we explore the available database, together with the coseismic observations from the region, to further understand the nature of faulting. The geophysical and geological data examined in this paper conform to a south dipping structure, but its location is inferred to be in the Brahmaputra basin, further north of the present plateau front. Our analyses of paleoseismic data suggest a 1200‐year interval between the 1897 event and its predecessor, and we identify the northern boundary fault as a major seismic source. The Shillong Plateau bounded by major faults behaves as an independent tectonic entity, with its own style of faulting, seismic productivity, and hazard potential, distinct from the Himalayan thrust front, a point that provides fresh insight into the regional geodynamics.

Comment on “Interpreting the style of faulting and paleoseismicity associated with the 1897 Shillong, northeast India, earthquake: Implications for regional tectonism” by C. P. Rajendran et al.

“…Their article includes speculation based on indirect evidence, limited field, and laboratory measurements (chronology constraints), supported by the selective and misleading presentation of sparse seismological and gravity data. Lest future investigators are mislead by the data and conclusions presented by Rajendran et al [2004], we discuss the absence of appropriate data to support the existence of a fault near the Brahmaputra river and support the occurrence of a large earthquake of 1450-1650 A.D. and a 500 years recurrence interval for major earthquakes in the Shillong Plateau.…”

“…[2] Rajendran et al [2004] propose the location of BF somewhere about 26.25°N in Brahmaputra Valley and justify it using geomorphological changes, seismicity and gravity data. They attempt to constrain the rupture model of the 1897 earthquake with their hypothetical fault, but ignore the Survey of India geodetic data [Bilham and England, 2001].…”

“…They believe it was responsible for the great 1897 earthquake and have questioned the location of Oldham Fault (OF) computed from repeated triangulation data [Bilham and England, 2001]. Further, Rajendran et al [2004] have concluded that the return period of great earthquakes in the Shillong Plateau area is about 1200 years, thus questioning the return period of 500 years estimated by Sukhija et al [1999aSukhija et al [ , 1999b, who have identified a large earthquake 1450 -1650 A.D. (besides 1897 A.D., 700 -1050 A.D. and earlier than 600 A.D. seismic events) based on liquefaction and other deformational features in the Shillong Plateau. With only three 14 C dates from two obscure liquefaction features from a single location (Dilma) and some additional archaeological and historic evidences, Rajendran et al [2004] correlated this (1450 -1650 A.D.) event with another earthquake (felt at Sadiya in 1697 A.D.) distal from the Shillong Plateau and infer an earlier event (1200 years B.P.)…”

“…as the penultimate one to the 1897 earthquake. We discuss the weaknesses in the interpretation and conclusions by Rajendran et al [2004]. We are concerned that their article ignores the precise parameters inferred from triangulation data and revised intensity data and the published 14 C chronology of available liquefaction features.…”

“…[1] In their recent paper, Rajendran et al [2004] have proposed the existence of a Brahmaputra Fault (BF), north of Shillong Plateau in northeast India. They believe it was responsible for the great 1897 earthquake and have questioned the location of Oldham Fault (OF) computed from repeated triangulation data [Bilham and England, 2001].…”