Although our San Andreas Fault is formidable, the fault that triggered Wednesday's deadly earthquake in Peru demands even greater respect, said geophysicist Wayne Thatcher of the U.S. Geological Survey in Menlo Park.

The two faults are caused by different kinds of geological growing pains in the maturation of the Earth - and geologists like Thatcher say we're generally better off here.

The Peruvian quake was caused by what is called a subduction fault, which creates the largest earthquakes. One giant slab of the Earth's crust, the oceanic Nazca Plate, is creeping beneath the South American Plate. (This same South American Plate, lifted during the subduction, created the towering Andes Mountains.)

When pieces break off the plates, they lock up the natural slide, building ferocious energy that eventually creates a jolt - and some of the strongest earthquakes in the world. In the Pacific Northwest, oceanic crust is being pushed beneath the North American continent along a major boundary parallel to the coast of Washington, Oregon and Northern California.

In contrast, Bay Area quakes - such as those triggered along the San Andreas and Hayward faults - are caused when two slabs of crust slide past one another. Known as strike-slip faults, the sliding of the Pacific Plate and the North American Plate creates a horizontal rupture.

These Bay Area-based strike-slip earthquakes tend to be deep and up to 800 miles long - but they are narrow in width, generally limiting the worst damage to a 10-mile wide stretch.

The Peruvian subduction earthquakes are not as deep, but far wider - 150 to 200 miles - and the impact can be huge, Thatcher said.

"The very largest earthquakes are known to originate in subduction zones and can be as large as 9.5" in magnitude, Thatcher said. "For earthquakes like those on the San Andreas Fault, a magnitude of 8 is the upper limit."

Coastal Peru has a history of very large earthquakes. Wednesday's massive temblor originated near the epicenter of two earthquakes, both in the magnitude 8 range, that occurred in 1908 and 1974. It was south of the source of a magnitude 8.1 earthquake that occurred in 1966, and north of a magnitude- 8.4 earthquake that occurred in 2001. The largest earthquake along the coast of Peru was a magnitude 9 that occurred in 1868, producing a tsunami that killed thousands of people along the South American coast.

It also was a subduction fault that triggered the huge tsunami that killed about 300,000 people along the coast of the Indian Ocean in 2005.

It is unlikely that the

Peruvian quake would have any direct impact on California geology, because the regions' tectonic plates are not part of the same picture.

But geology does not deserve full blame for high death tolls, Thatcher said. Human factors also are involved. Densely populated communities of poor-quality homes, many of them perched on the edges of steep mountains, can send death tolls soaring.

"It is the proximity of the strong ground shaking to areas of population and structures, rather than the magnitude of an earthquake, that is the most important feature," he said.

lkrieger@mercurynews.com