Geologists for years watched stress build along the Wasatch Fault as its adjacent sides — one at the base of the mountain range, the other the beginning of the valley — stretched apart in reaction to earlier, prehistoric compression. For example, recent scientific measurements showed the "valley side" of the fault being stretched to the west by about 2 millimeters a year, or 0.8 inches a decade.

University of Utah seismologist Robert Smith describes such fault stress as being "slowly loaded like a stretched rubber band that suddenly breaks when its strength is exceeded."

Also, earthquakes of about magnitude 7.0 occur on average every 200 to 300 years somewhere on the broad Wasatch Front area, according to Walter Arabasz, director of the University of Utah Seismograph Stations.

He also said they occur on average about once every 1,300 years on the Salt Lake City segment of the Wasatch fault.

When did that last happen? "About 1,300 years ago," Arabasz said back in 2006.

But such quakes are not uniformly spaced and could occur any time enough pressure has built.

Fault rupture

The fault ruptures, releasing 1,300 years of built-up pressure, about 10 miles deep. Arabasz said that is typical for a 7.0 quake. This quake begins far beneath a point about the middle of the Salt Lake Valley. It is a worst-case scenario site at the middle of the Wasatch Front, so the quake within seconds is felt along all the urban area there.

The fault slippage rips upward at a 60-degree angle toward the surface at six times the speed of sound, reaching the surface in two seconds. On the surface, the ground on the "mountain side" of the fault line is suddenly shoved upward, while the "valley side" slips downward.

The movement soon leaves a new 10-foot-tall ledge or "scarp" — a height that varies at different points along the fault — as scientists had predicted. The scarp is 29 miles long and now divides areas west and east of it like a great wall. Roads that were once level now have a 10-foot drop-off. Power, water and gas lines there snap.

Some expensive homes had been built atop the old fault scarp left by ancient earthquakes — which is now suddenly 10 feet higher in spots. The view of the valley from such points just above the old scarp cliffs had been spectacular.

Smith, in fact, had observed jokingly during a 2006 presentation, when he showed photos of such precariously perched homes, that, "Fault scarp provides wonderful homesites." Some of those homesites are now newly vacant, except for rubble.

Such earth breakage and scarp formation is occurring all along the main branches of the Salt Lake City segment of the Wasatch fault — one of 10 independent segments of the larger fault. This segment runs from Corner Canyon in Draper along the base of the Wasatch Mountains to about 4500 South.