Charles Darwin and Captain Moresby on the Drowning of Great Chagos Bank: 19th century discovery of “aseismic” ridge seismicity in the Indian Ocean

Charles Darwin's reading of Lyell's Principles of Geology and his own observations, made during the voyage of the H.M.S. Beagle, on earthquakes, the elevation of the west coast of South America, and the thick sections of marine strata in the Andean Cordillera led him to “. . . reflect much on the effects of subsidence . . .” with the result that he developed his subsidence theory of atoll formation before he ever saw an atoll. In 1836, during his first visit to an atoll, Keeling Island (now Cocos Island) in the Indian Ocean, he observed evidence of recent subsidence. Reports of a severe earthquake at Keeling in 1834 and two lesser ones in the preceding 10 years led him to conclude that small downward movements had recently taken place. However, it was not until he returned to England and became familiar with the hydrographic surveys of Captain Moresby and the latter's accounts of frequent earthquakes on the Great Chagos Bank that Darwin became convinced that the drowned reefs at Chagos, which now lie 6 to 8 fathoms beneath the surface, appeared “ . . . as if they had been carried down by one uniform movement,” the implication being that the earthquakes were accompanied by subsidence of the Great Chagos Bank. The observations of Darwin and Moresby on earthquakes and tectonics on an “aseismic” ridge are in accord with modern interpretations of “intraplate tectonics” of the Indian Ocean. In particular, earthquakes up to magnitude 7, which occur on the Chagos‐Laccadive Ridge, show normal faulting on an east‐west fault plane that intersects the Great Chagos Bank and can cause vertical slip of several meters. The work of Darwin and Moresby 150 years ago showed that atolls and their associated drowned banks are sensitive recorders of “intraplate tectonics.”