A series of small earthquakes that began April 19th, 1999, continued April 21st near the central Utah town of Elsinore, several miles southwest of Richfield in Sevier County, the University of Utah seismograph station reported. The station has recorded 63 minor earthquakes since the 19th. All are about three miles north of Elsinore. Seismography station official Sue Nava said the largest of Wednesday's tremors were a magnitude 3.3 shock that occurred at 4:51 a.m. and a 3.2 quake that occurred at 5:22 a.m.
Although earthquake sequences of this type are not unusual in central and southwestern Utah, the particular location of this sequence calls attention to history. In late September and early October 1921, following 2 1/2 weeks of minor earthquakes, the Elsinore area was struck by two damaging earthquakes of magnitude 6. The first came on Sept. 29, 1921, and the second was two days later. The small earthquakes preceding the main shocks in 1921 proved to be foreshocks, which can be recognized as such only in hindsight, said Nava.
A major past earthquake caused liquefaction and significant displacement of sediments all the way up to the ground surface in the Salt Lake City Area. Liquefied Lake Bonneville sediments blew through to the surface, causing geysers of liquefied sand and gravel of immense proportions over widespread areas. The soil is clearly sheared with major continuous displacements, much like any fault would.
These exposures show beyond any doubt that ground-shaking from an earthquake has caused catastrophic rupture and displacement of sediments throughout the Salt Lake City area. It is logical to assume this could happen during any large earthquake today. Scientists around the world all agree that this is a geologic hazard which should be incorporated into the building code. To not consider the high probability of the liquefaction and ground-rupture potential of the sediments in this area of Salt Lake City would constitute extreme negligence on the part of any designer. If the Utah politicians keep digging a bigger hole, at least they will have a place to push all the high-rise buildings when they topple predictably during the inevitable large earthquake in the not-so-distant future.
(Peter Shaffner, an engineering geologist who lives in Evergreen, Colo. )
