Cinna Lomnitz

Creep Measurements in Igneous Rocks

The Benioff torsion apparatus for creep testing of slender cylindrical specimens is described. Continuous creep and creep recovery curves for granodiorite and gabbro at room temperature and atmospheric pressure are given. For constant-torque tests of up to 1 weeks duration, the results are closely represented by the equation \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage{wasysym} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document}

Earthquake Disasters in Latin America

\epsilon (t) = \frac{\sigma}{\mu} [1 + q\ {\mathrm{ln}}\ (1 + at)],

Networks and marginality : life in a Mexican shantytown

\end{document}, which was found valid for low stresses, up to 0.05 per cent of the rigidity modulus. The results are compared with early torsion creep experiments by A. A. Michelson. The strain behavior of rocks at low stresses is discussed.

Fundamentals of earthquake prediction

An attempted use of seismic gap observations to predict a large earthquake in Oaxaca, Mexico is discussed. The observations were initially published in a scientific journal and were subsequently distorted by nonscientists, who predicted a major earthquake and tsunami to take place at Pinotepa Nacional, Oaxaca on 23 April 1978. Public reactions and property losess sustained by individuals and communities were comparable to those expected from an actual earthquake. A revision of epicenter locations from the NOAA data file revealed that a number of earthquakes did occur in the alleged gap but had been excluded because their reported focal depth was in excess of 60 km. It is shown that the probability that the number of earthquakes in two consecutive time intervals of a stationary Poisson process differs by an amount which would be reported as a ‘seismic gap’ is of the order of 5% or more for Oaxaca. This means that spurious ‘seismic gaps’ would be observed in one out of 20 data runs. The possibility of detecting a true interval of abnormal quiescence in a random earthquake sequence appears to be fairly remote in this case.

Global tectonics and earthquake risk

Charles Darwin felt and described the great 1835 Chile earthquake which destroyed Concepción and raised the coastline. He was the first scientist to suggest that earthquake waves could be similar to ocean waves. He also thought earthquakes might be caused by tectonic deformation of the earth. The physics of earthquakes is introduced and it is shown how prograde ground motion might destabilize buildings. Tsunami warning systems are critically examined. 1.1 Darwin Feels an Earthquake His Majesty’s Ship Beagle, a ten-gun, two-mast, square-rigged brig, sailed from Plymouth Sound two days after Christmas of 1831, or 4 years before this story begins (Fig. 1.1). It was late summer in the Southern hemisphere and the Beagle was strenuously wending her way northward along a little-explored stretch off the Pacific coast of South America. The frail vessel encountered unseasonably poor weather. Captain Robert FitzRoy, the moody and aristocratic commander of the Beagle, guided the small craft beating to windward. They finally reached the small Chilean outpost of Valdivia on the night of February 8, 1835. During the following days, FitzRoy busied himself with hydrographic measurements while the young naturalist on board, Mr Darwin, spent his 26th birthday exploring the rain forests with a local guide. On February 20, shortly before noon, he was resting in a forest clearing when a major earthquake occurred. The epicenter was some 300 km to the north. Charles Darwin describes the experience as follows. It came on suddenly, and lasted two minutes, but the time appeared much longer. The rocking of the ground was very sensible There was no difficulty in standing upright, but the motion made me almost giddy: it was something like the movement of a vessel in a H. Castaños and C. Lomnitz, Earthquake Disasters in Latin America, SpringerBriefs in Earth Sciences, DOI: 10.1007/978-94-007-2810-3_1, The Author(s) 2012 1 little cross-ripple, or still more like that felt by a person skating over thin ice, which bends under the weight of his body. A bad earthquake at once destroys our oldest associations: the earth, the very emblem of solidity, has moved beneath our feet like a thin crust over a fluid (Darwin 1845) Darwin’s reference to giddiness was significant. He suffered from seasickness: years on board the Beagle had done nothing to overcome this infirmity. Seasickness is a kind of motion sickness or kinetosis which affects the vestibular system of the inner ear. Three semi-circular canals in each ear contain a fluid that moves against tiny sensors to detect rotations of the head. This system will react to some types of motion by inducing a feeling of vertigo or giddiness. In one short paragraph, Darwin made short shrift of age-old prejudice. He made four separate references to fluid motion, yet he knew he was on solid ground. But Valdivia was located inside a deep bay where three major rivers converged. The main streets of Valdivia were canals, as in Venice. The settlement was on soft, partly swampy terrain. And soft soils are recognized as intermediate materials between solids and liquids. The state transition between solid and fluid soil has been explored chiefly by soil mechanics. It is not as clear-cut as it would be in water. Before a granular material like sand or clay can flow like a liquid it must first traverse a borderline state known as a mesophase. During the mesophase and before reaching the point of liquefaction the material is a solid: it supported Darwin’s weight. But the intergranular cohesion dropped to a point where it would no longer propagate elastic waves. Instead, gravity took over as the restoring force. As the Beagle sailed into the harbor of Concepción on March 4th 1835, the British explorers were confronted with a dire spectacle. They were amazed to find the town totally destroyed by the earthquake. The port area had been washed away by huge tsunami waves and there were many dead in town. It had not been the first megaquake to afflict the locality. Concepción is the second-largest city of Chile today, but it continues to suffer severe damage from successive megaquakes. The epicenter of the 1835 megaquake was probably just Fig. 1.1 H.M.S. Beagle at anchor off the coast of Patagonia (after a contemporary drawing) 2 1 Darwin and Plate Tectonics

A Model for the Occurrence of Large Earthquakes

Thanks to the many readers who have sent their comments and suggestions to me over the past year. I plan to use your suggestions when I return to those topics in future columns. This issues column features a rapid return to the subject of earthquake location, which generated an unusual level of correspondence. Here, I mention two contributions. First, Prof. Jose Pujol (University of Memphis) has created graphics related to the classic geometric techniques of earthquake location, and links to his figures can be found via the EduQuakes Web page. Second, Prof. Cinna Lomnitz (UNAM) has provided a guest column that further discusses the problems of hypocenter determination. Here is the quite engaging column from Cinna Lomnitz. Earthquake location is not just one of the most basic and important activities in seismology, it is still an unsolved problem. Ruff (“EduQuakes”, SRL 72 , p. 197, 2001) lifts the lid off an ancient bucket …