Michoacan

Michoacán , state (1990 pop. 3,548,199), 23,202 sq mi (60,093 sq km), S Mexico. Morelia is the capital. Dominated by the mountains of the Sierra Madre Occidental and the volcanic chain of central Mexico, Michoacán extends from the Pacific Ocean northeastward into the central plateau. The Lerma River and Lake Chapala form part of its northern boundary with Jalisco; the Río de las Balsas marks the southern border with Guerrero. The climate and soil variations caused by topography and varying elevation make Michoacán a diverse agricultural state, producing temperate and tropical cereals, fruits, and vegetables. The forests yield fine cabinet woods and dyewoods. Mining is a leading industry; gold and silver are most important, but iron, coal, and zinc are also major minerals. Industrial development is modest, centering around iron and steel production. Michoacán, having no important Pacific port, ships its products from the cities of Morelia and Uruapan. Federally sponsored irrigation and hydroelectric power projects have aimed at developing the coastal region, and a reforestation program was instituted in the mountains in the 1990s, where (along the Mexico state border) monarch butterflies overwinter. Lake Pátzcuaro (where UNESCO and the Organization of American States have a training center for Latin American rural teachers) and the Paricutín volcano attract many tourists. Most of the state's inhabitants are native Tarascans ; in recent years the state has seen a large outmigration to the United States. Michoacán played a leading role in Mexico's revolution against Spain and in subsequent struggles.

strong-motion seismology Since the 1970s, a branch of earthquake seismology (see earthquake seismology) has evolved, focusing on the assessment of ground shaking in strong earthquakes. Ground shaking is the most significant hazard generated by earthquakes because of the resulting potentially severe and widespread damage (see earthquake hazards and prediction). As a result of attempts to understand, predict, and mitigate this hazard, strong-motion seismology emerged as a significant application of the science, particularly in the 1990s. It has long been known that (1) the severity of ground shaking increases with increasing earthquake magnitude; (2) ground shaking decreases in strength or attenuates as the distance from the epicentre of the earthquake increases; (3) there are regional differences in the rate at which ground motions attenuate with distance; and (4) geological site conditions (e.g. soil versus rock) can have a significant impact on ground shaking.

Site conditions may be the most significant factor affecting ground motions, at least in terms of earthquake damage. In particular, the amplification of ground motions by soil and unconsolidated sediments has resulted in major earthquake losses. One of the most notable recent examples was the 1985, magnitude (M) 8, Michoacan earthquake, which caused 10 000 deaths and resulted in the collapse of numerous high-rise buildings in Mexico City more than 350 km away from its epicentre in the Pacific Ocean to the west.

In a sense, strong-motion seismology has its roots in earthquake engineering and the strong-motion instrument, or accelerograph. In an attempt to predict future ground shaking, earthquake engineers such as Harry Seed of the University of California at Berkeley began analysing accelerograms (strong-motion recordings) of actual earthquakes in an effort to develop empirical relationships between earthquake magnitude, distance, site geology, and some specified ground-motion parameter.

In the United States, strong-motion instruments are generally abundant in California. A few countries which also have notable strong-motion instrumentation programmes include Japan, Italy, Taiwan, and Mexico. Elsewhere, strong motion instruments are rare. As a result of the recent occurrence of several large California earthquakes, such as the damaging 1994, M 6.7, Northridge event and the 1999 Chi Chi, Taiwan earthquake the strong-motion database has increased dramatically in size and in quality. Strong-motion instruments have not only been useful in providing observations of ground motion in the free-field (away from any structures) but also in showing the structural response of buildings, dams, and bridges. Analyses of such records have thus improved understanding of structural response and, hence, enabled engineers to improve seismic design.

Based on the analyses of the strong-motion database, numerous attenuation relationships have been developed, principally for peak ground acceleration of different geographical regions and, more recently, different styles of earthquake faulting (see earthquake mechanisms and plate tectonics). The attenuation of ground motions is due to the geometrical spreading of seismic waves and the loss of their energy that results from material damping or absorption of energy by the Earth's crust and mantle.

In the 1990s, theoretical approaches were also developed to predict earthquake ground shaking. These approaches, consisting principally of numerical modelling techniques, which have been validated with empirical data, provide an important tool for strong-motion seismologists because of the general lack of strong-motion data, particularly for large earthquakes at short distances. In this regard, ground motion amplification due to soils has been an important issue in strong ground motion prediction and the focus of considerable research. The near-source aspects of ground shaking and two- and three-dimensional effects, such as might be due to topographic features and basin geometry, have also been modelled numerically to assess their contributions to the damage that resulted from earthquakes such as the 1971, M 6.7, San Fernando, California and 1995, M 7.0, Kobe, Japan earthquakes.

Ivan G. Wong