An authoritative study on this earthquake has been published by Byrne et al., [1992] and its detailed findings will not be repeated here. This page summarizes a problem with the late arrival of the 1945 tsunami and provides links to texts that relate both to this earthquake and to earlier ones mentioned by Byrne et al. [1992]. The figure below illustrates the approximate locations of great ruptures discussed in their text. Note, however, that no historical evidence for great earthquakes to the west or east of the 1945 rupture currently exists (see below).
1765 Possibly not an earthquake and almost certainly not in 1765
An earthquake is alleged to have caused a landslide on the Makran Coast 150 km west of Karachi at some time in the 18th century. Ambraseys and Melville [1982] list the year as c.1765 with no implied or stated uncertainty. The source of this information is the director of the Makran telegraph line, who wrote a note to the Government of Bombay indicating that " a smart shock of an earthquake" had been felt in Gwadar at 00:45 on the morning of the 25th August 1864 and speculates that large earthquakes, if they occurred along the Makran coast, could jeopardize telegraph communications [Walton, 1864]. From its brief mention in telegrams from Gwadar and not from other telegraph posts along the Makran coast, the 1864 earthquake was presumably quite small (M<5). Walton’s letter describes the possible occurrence of an earthquake remembered by local people: " As the entire coast of Mekran (sic) is volcanic, I often enquired of the Baluchees regarding the occurrence of earthquakes, and the only phenomenon of this sort, of which I could obtain any information, was said to have happened about 100 years ago, when, as my informant assured me, an entire hill, with men and camels on it, disappeared into the sea. I imagine this must have been a landslip caused by some submarine disturbance. The spot was pointed out to me and is known as Ras Koocheree on the chart." (Walton’s characterization of the Makran coast as volcanic was linked to the mud volcanoes he describes in a preceding sentence. Ras Koocheree is shown on Snead's 1967 map of the coast.)
The precise identification of 1765 as the year of a great earthquake is obviously inappropriate based on the vague mention of the timing of the earlier event. Byrne et al. [1992] identify this earthquake as a great rupture beneath the westernmost end of the Makran subduction zone. However, a landslide could have also been triggered by a large strike-slip earthquake on the southernmost Ornach-Nal fault system. It is even possible that no large earthquake occurred around 1765; the landslide could have been spontaneous, or the result of unusually heavy rain.
19 April 1851 at 5 pm: Not a great earthquake
Three shocks caused minor damage in Gwadar in 1851. Byrne et al. [1992] cite Quittmeyer and Jacob [1979] as a reference to this event. They cite Oldham's [1883] catalog who like Ambraseys and Melville [1982] cites Merewether [1851]. The actual account is barely two lines "19th April 1851-At 5 PM, three shocks felt at Gwadir in Mekran - several houses destroyed". Quittmeyer and Jacob [1979] do not include the earthquake on their list of documented south-central Asian earthquakes that produced maximum intensity of VIII or greater. Walton’s 1864 letter from Gwadar discussed above, written just 13 years after 1851 makes no mention of a large earthquake.
28 November 1945 Makran Mw 8.1 Earthquake (Section above shows inferred location of 1945 rupture)
According to Byrne et al [1992] rupture terminated near the coast or slightly offshore. This follows both from waveform modeling and from a consideration of coastal uplift at Ormara [Snead, 1966; VitaFinzi, 1975]. An unresolved problem with this earthquake, however, is that although a minor tsunami was caused by the earthquake, the damaging tsunami occurred 2-3 hours later both near the epicenter and at Karachi [Pendse, 1946; Ambraseys and Melville, 1982]. It is not possible to envisage a wave reflection that would cause this delay. The Makran submarine telegraph cable was severed in eight places, and four temporary islands were formed attesting to considerable sea-floor changes offshore [Ambraseys & Melville, 1982]. Thus the Makran tsunami that features so prominently in post 2004 hazard discussions was caused by one or more submarine slumps and not by primary sea floor uplift. The implication is that a tsunami in the region could be generated by a relatively modest earthquake. A tsunami could also be triggered by a marine slide without an earthquake. One such orphan tsunami (a tsunami without a known earthquake) caused consternation among the ships of Vasco da Gama' s fleet in 1524 near Dabul, south of Bombay [Bendick and Bilham, 1999].
The formation of the temporary islands was accompanied by loud sounds, and in one case by the spontaneous combustion of gas followed by a small tsunami onshore. Ambraseys observes that the delicate features of the solid-earth waves and fractured surfaces frozen into the surface of the islands (see picture from Sondhi [1947] below) should have been eroded had the island appeared before the main tsunami. The negligible effects of waves in this photo, however, and Sondhi's description of the hardness of the materials on their surface suggests that the islands may have preceeded (or accompanied) the tsunami.
This study developed from a workshop held at NED University, Karachi, in January 2007 attended and reported by the authors in the Seism.Res.Lett article below. Partial support for the work came from L’Institute de Physique du Globe de Paris and National Science Foundation Grant No. 0229690. Any opinions, findings, and conclusions or recommendations expressed in this paper are those of the authors and do not necessarily reflect the views of the National Science Foundation.
References