On the evening of December 22,, 1972 two small earthquakes were felt at about 9:30 and 10:15 in Managua. But earthquakes were not a novelty to the inhabitants of the city, and no one could foresee the disastrous quake that would follow within a few hours. Sixty-eight earthquakes large enough to be located by the World-Wide Standard Seismograph Network had occurred in Nicaragua in the period January 1961 through August 1972. These were quakes generally having a magnitude of four or higher. (The re gion included a coastal zone extending fifteen kilometers from the country's western shore.) Experts believed that many additional quakes, smaller in magnitude and not located by the Seismograph Network, had also occurred in the area.
The earthquakes of Nicaragua are confined almost entirely to a belt lying along the western border of the country. In fact, ninety per cent of the quakes plotted during the eleven-year, eight-month period occurred within forty-five miles of the coastline . Ten of the quakes were within twenty-two miles of Managua itself. The eastern region of the country has been essentially free of quakes. Unfortunately, this is the less populated area of Nicaragua.
Experts determined that the December 23 earthquake was centered beneath the southern edge of Lake Managua, which extends from the northern rim of the city. It is precisely in this northern section that the downtown area lies, the area that was devastated . In a scale whose maximum was XII, experts assigned IX to the intensity of the quake in that portion of the city.
The tremors did not end with the December 23 quake. This writer found and moved into an empty house that still stood not far from the downtown area. On the night of December 24 he was awakened -- and frightened -by windows that were rapidly vibrating and a general shaking that-resembled a vast subway train moving somewhere nearby. Experts of the University of Texas' Marine Biomedical Institute set up seismographic equipment to measure aftershocks in the general Managua area. During the period January 10 to 29 the number of aftershocks varied between fifty and 200 per day. During the twenty-day period there were thus well over 1,000 quakes. The magnitudes of these varied between 0.0 and 3.0. Approximately ten of the aftershocks were large enough to be felt by Managuans.
The experts reported that the underground "'stress system" which caused the earthquakes was "still active." As a result they concluded: "... We must assume that the probability of future earthquakes in the Managua region is high. If a reconstructed city is to survive where the ruins of Managua now stand, it must be built to withstand earthquakes of at least moderate intensity."
(Earthquakes are not the only danger to Managua. The University of Texas experts also noted: "'The volcanoes of Nicaragua are noted for the explosive character of their eruptions. It is estimated that outpourings of ash from these volcanoes over the past 200 years have been sufficient to cover the entire region of the seismic belt to an average depth of one meter. In fact, most of the ash falls to the west of the volcanic belt owing to the prevailing easterly winds. Thus, over long periods of time, the danger of widespread destruction in western Nicaragua may be as great from volcanism as it is from earthquakes.")