Tom La Touche and the Great Assam Earthquake of 12 June 1897: Letters from the Epicenter

Abstract

Richard Dixon Oldham's classic memoir on the great Mw = 8.1 Assam earthquake of 1897 is seminal for its seismological observations, insights, and conclusions (Oldham 1899). Teleseismic arrivals of waves from Shillong led Oldham (1858–1936) to distinguish the three types of seismic waves and eventually to recognize from them the distinctive presence of the Earth's core. Oldham, however, did not feel the earthquake. He had left the Calcutta office of the Geological Survey of India (GSI) in the care of his colleague Thomas Henry Digges La Touche (1856–1938) two weeks previously (see figure 1). Now recently discovered letters written by La Touche from Calcutta and the epicentral region to his wife, Nancy, provide a firsthand, day-by-day account of the post-earthquake investigation and include a seismogram from an instrument that he constructed in the field at a cost of “less than sixpence” from pieces of tin, a suspended boulder, a glass bead, and a bamboo needle that scratched a glass plate (LaTouche papers, 1880–1913). In his memoir Oldham notes that he focused his team of geologists on the physics of the earthquake. Their reports were submitted “under specific instructions to report only on the facts observed, and to refrain from any expressions of opinion as to the conclusions to be drawn, as this could only be profitably done after a review of the whole of the facts, of which only part could become known to each individually.” (Oldham 1899, p.257). He deduced from their observations that local accelerations in the earthquake had exceeded 1 g, that velocities had exceeded 3 m/s, that electrical currents in the ground had accompanied aftershocks, that postseismic crustal deformation continued to deform the plateau in the year following the earthquake, and that many of the largest aftershocks lay 15 km beneath the plateau. …