The effects of any earthquake depend on a number of widely varying factors. These factors are all of:

• Intrinsic to the earthquake - its magnitude, type, location, or depth;

• Geologic conditions where effects are felt - distance from the event, path of the seismic waves, types of soil, water saturation of soil; and

• Societal conditions reacting to the earthquake - quality of construction, preparedness of populace, or time of day (e.g., rush hour).

One can count the number of deaths caused by large earthquakes to compare the results of all these disparate factors in combination. The Oct. 17, 1989 Loma Prieta earthquake occurred in the least-populated area of the generally urban San Francisco peninsula. Construction standards in the area are relatively high, and the populace relatively prepared. However, soft, highly-saturated soils near San Francisco Bay caused some spectacular failures of large highway structures unusually far away from the event. Even though it was rush hour, many fewer cars were on the roads due to the start of the opening game of the World Series, being played locally. Thus deaths were limited to about 75.

On the other hand, the same year an earthquake of nearly identical energy struck the war-torn country of Armenia, between Russia and Turkey. It was located much closer to the major cities of the region, where poorly-engineered houses of unreinforced concrete collapsed on their occupants during the night. The number of fatalities passed 25,000.

The worst earthquake catastrophe in years occurred on western Honshu Island early in 1995. More than 5000 people perished in southern Hyogo prefecture, most in the city of Kobe, Japan's most important port. The loss of so many lives, in a country where so much effort had been made to prepare for earthquakes, shocked observers worldwide. However, the magnitude of this catastrophe is probably due to a terrible coincidence of a few simple seismological and societal factors, which may become clear in the photos below.

There are two classes of earthquake effects: direct, and secondary. Direct effects are solely those related to the deformation of the ground near the earthquake fault itself. Thus direct effects are limited to the area of the exposed fault rupture. Many earthquake faults (such as at Northridge) never break the surface, ruling out direct effects. In the Hyogo-Ken Nanbu event, surface rupture of the fault was observed only in a rural area of Awaji Island, with displacements of up to 3 meters. Few structures were near enough the fault to be damaged by the displacement, although underground utilities, fences, and irrigation ditches were cut. Rice paddies were thrown far out of level.

Elastic rebound, the permanent deformation of the ground due to the fault rupture, will extend many kilometers from the fault itself, and is often measurable even where the rupture itself remains buried. In the past geodesists have had to make painstaking and expensive surveys, visiting hundreds of field sites, to measure deformation of an area struck by an earthquake.

(Analyzed by the Geographical Survey Institute of Japan and the National Space Development Agency of Japan; used by permission.)

Lately planetary geophysicists have developed a quick way to make a map image of deformation using satellite radar interferometry. They compare satellite microwave-radar images of a region taken before and after the event. Any area displaced toward or away from the spacecraft's positions will form an interference pattern, tracing out contours of equal displacement. The map image above of Awaji Island shows eight or more colored interference fringe lines approaching the fault, at 11 cm of vertical displacement per fringe contour, demonstrating almost 1 meter of uplift by the earthquake. At left, two fringes parallel the coast through the city of Kobe, showing about 20 cm of displacement across the city from the buried fault.

Most of the damage done by earthquakes is due to their secondary effects, those not directly caused by fault movement, but resulting instead from the propagation of seismic waves away from the fault rupture. Secondary effects result from the very temporary passage of seismic waves, but can occur over very large regions, causing wide-spread damage. Such effects include: seismic shaking; landslides; liquefaction; fissuring; settlement; and the triggering of aftershocks and additional earthquakes.

(from Bardet et al., 1995; used by permission)

The columns above show a failure typical of somewhat older reinforced concrete structures throughout the world. The vertical steel rods can hold the weight of the structure just fine when that weight is exerted straight down, as usual. During seismic shaking much more steel wound around the rods horizontally can keep the column from breaking apart under the shear forces. Stronger columns are more expensive to build.

(from Japanese TV)

The destruction of lifelines and utilities made it impossible for firefighters to reach fires started by broken gas lines. Large sections of the city burned, greatly contributing to the loss of life.

(from Bardet et al., 1995; used by permission) The Kobe port, having been constructed on two artificial islands made of relatively loose fill, and always water saturated, suffered widespread liquefaction and settlement, and was incapacitated for two months. Shipping was disrupted worldwide.

(from Bardet et al., 1995; used by permission) On the port islands settlement was so pervasive that any structure built on deep pilings, like this elevated roadway, appeared to have risen a full meter. The world's longest suspension bridge, under construction but having such foundations, was hardly damaged at all.

Next: Earthquake Intensity