Heriberta Castaños

PSHA: is it science?

Abstract Probabilistic seismic hazard analysis (PSHA) is beginning to be seen as unreliable. The problem with PSHA is that its data are inadequate and its logic is defective. Much more reliable, and more scientific, are deterministic procedures, especially when coupled with engineering judgment.

Unplanned and unforeseen effects of instabilities

The nature-society system is proposed as the relevant analytical unit for the sociological study of disasters. Like other complex systems, this system has emergent properties: its instabilities are the disasters. They often arise as a result of adoption by a community of specific technologies, e.g., housing technologies, that turn out to be unstable in the presence of critical natural or social changes. The following earthquake disasters were caused by unplanned and unforeseen features of housing or siting technologies: Huaxian 1556 (caves in loess), Yungay 1970 (siting in the path of an avalanche), and Mexico 1985 (high-rise buildings on soft ground). Disasters have anarchaeology, in the sense that the instabilities in the nature-society system are not static. This is demonstrated by tracing the 1985 Mexico earthquake disaster back to decisions on urban planning taken after 1521. It is not enough to know the hazard and the vulnerability in order to understand disasters. Technological solutions also have a local history.

Earthquake Disasters in Latin America

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

The Great 1960 Chile Megaquake

Descriptions of witnesses who experienced the great Chile megaquake may support the idea that prograde ground motion contributes to earthquake damage. Coastal changes in megaquakes may be related to a distinctive tectonic framework.

Seismic Hazard of the Uttarakhand Himalaya, India, from Deterministic Modeling of Possible Rupture Planes in the Area

This paper presents use of semiempirical method for seismic hazard zonation. The seismotectonically important region of Uttarakhand Himalaya has been considered in this work. Ruptures along the lineaments in the area identified from tectonic map are modeled deterministically using semi empirical approach given by Midorikawa (1993). This approach makes use of attenuation relation of peak ground acceleration for simulating strong ground motion at any site. Strong motion data collected over a span of three years in this region have been used to develop attenuation relation of peak ground acceleration of limited magnitude and distance applicability. The developed attenuation relation is used in the semi empirical method to predict peak ground acceleration from the modeled rupture planes in the area. A set of values of peak ground acceleration from possible ruptures in the area at the point of investigation is further used to compute probability of exceedance of peak ground acceleration of values 100 and 200 gals. The prepared map shows that regions like Tehri, Chamoli, Almora, Srinagar, Devprayag, Bageshwar, and Pauri fall in a zone of 10% probability of exceedence of peak ground acceleration of value 200 gals.

Comment on “Review: Strong Ground Motions—Have We Seen the Worst?” by Fleur O. Strasser and Julian J. Bommer

Strasser and Bommer (2009) raise some important issues on maximum amplitudes of strong ground motion parameters. Any method of risk assessment operates under uncertainty and there are trade-offs between costs, benefits, and risks to be considered. Peak ground acceleration (PGA) has been shown to be a very poor indicator of building damage, yet it is commonly used to predict earthquake risk for want of a better one. The credibility of a model of risk management that uses PGA as input depends primarily on its performance, meaning its impact on future actions adopted by decision-makers today. Strasser and Bommer conclude that “it is improbable that the worst possible ground motion has been recorded and that in all probability this situation will not change in the foreseeable future.” The factual basis for this statement remains unclear. Earth materials have a finite strength, and it is not clear why this strength should increase with the period of observation or with the quantity of observations. We are skeptical that “the short length of time for which instrumental recordings are available … implies …

The 2010 Chile Earthquake

History is what happens in-between successive disasters. Prograde, long-period surface waves emerged first in the 1960 Chile earthquake and reappeared in 2010. The importance of economic resilience and regional development is discussed. Megaquakes may occur repeatedly in the same region but never exactly in the same place or the same way. Exponentially rising disaster costs are not sustainable.

A List of Significant Earthquakes in Latin America

A review of significant earthquakes in Latin America is provided. The definition of a “significant earthquake” may vary from country to country. Latin America is a highly variegated region of high seismic risk.

The 2010 Haiti Earthquake

What happens when everything goes wrong? Haiti is an example of a disaster with too many causes. Disaster culture in a nation cannot rely exclusively on importing technology from abroad. There must be a local basis for development grounded in education.

The 1985 Mexico Earthquake

The severe damage caused by the 1985 Mexico earthquake must be attributed to a combination of three different factors: (a) some unrecognized features of strong ground motion in sedimentary basins; (b) the enhanced transmission of low-frequency seismic energy from the subduction coast inland; and (c) the unexpected behavior of tall buildings on soft saturated sediments under low-frequency, harmonic oscillations. A critical discussion of available data on the Mexico earthquake is given, and a suggested scenario of the disaster is provided.