Today's 2017 April 26. An active earthquake sequence is arising since a few days offshore Valparaiso in Central Chile. So I choose to recall the famous earthquake which occurred there in 1906. An earthquake that made huge destruction in Valparaiso, initiated fires downtown, and killed thousands. On 1906 August 17th, two major earthquakes hit the Aleutian islands and the Chilean margin. Those earthquakes were separated by half an hour only, which made identification and study difficult on seismograms. Famous japanese seismologist F. Omori tried to do such a study with the few recording he had. He eventually published his paper in 1907. Nearly a century later, in a 2005 paper, Émile Okal made a modern reevaluation of these two earthquakes. Okal gave a definitive demonstration that the Valparaiso event was a relatively shallow subduction megathrust rupture.
Above is one of the seismograms reported by Omori (Here the record in Osaka, Japan). Below are the earthquake locations he calculated from the records he studied.
I'm very impressed by Omori's study who managed to have good localisations, and to write a paper just 6 months after the earthquakes (his paper was written in January 1907). Even now, with publish or perish pressure, and rush for NATURE / SCIENCE first publication after a big event, it's difficult to be more rapid. And just think to the time to get the seismograms and information from observatories spread around the world.
Below are illustrations of Émile Okal results: an evaluation of the depth and shallow dip thrust mechanism for the Valparaiso earthquake (for the Aleutian event, he concluded to an unusual "torque" deformation of the slab). The figure also shows the vertical deformation models (= source models) he took to calculate the tsunami waves that crossed the Pacific Ocean.
- F. Omori F. (1907), Notes on the Valparaiso and Aleutian Earthquakes of August 17, 1906. Bull. Imperial Earthquake Investigation Committee (Japan), vol. I, no. 2, p. 75-113. Get PDF from IRIS web page.
- Okal, E. A. (2005). A re-evaluation of the great Aleutian and Chilean earthquakes of 1906 August 17. Geophysical Journal International, 161(2), 268-282. PDF from GJI
