Quake Was ‘Textbook’ Except for the Numerous Aftershocks : Seismology: Wednesday’s temblor is part of the continuing geological tug-of-war that is reshaping the state.

For millions of years, earthquakes such as the one that struck Southern California on Wednesday evening have been pushing and tugging at the Earth in a process that gradually created the San Bernardino and San Gabriel mountains.

Wednesday’s earthquake, which was given a preliminary magnitude of 5.5 by Caltech and the U.S. Geological Survey, struck in rocky, hilly terrain that is highly fractured by various faults. The quake, centered three miles northwest of the San Bernardino County community of Upland, was the most common type of earthquake in Southern California.

Seismologist Lucile Jones of the U.S. Geological Survey’s Pasadena office said the quake struck in an area near the juncture of two faults, the Cucamonga and the Sierra Madre, but it could not be immediately determined which fault it was on.

Caltech staff seismologist Kate Hutton described it as a “textbook earthquake.”

It was a left lateral strike-slip quake, meaning the earth moved in a horizontal rather than vertical direction and a person standing near the fault would see objects on the other side of the fault move to the left. The epicenter was about five miles below the surface.

Earthquakes in California are triggered by a complex pattern of stresses in the Earth’s crust caused by the movement of two giant chunks of crust, called tectonic plates. The Los Angeles Basin is riding north on the Pacific Plate, which extends all the way across the Pacific Ocean. The boundary between the Pacific Plate and the North American Plate is marked by the San Andreas Fault, the largest fault in California and one of the most famous in the world.

Earthquakes occur here because the two plates do not move smoothly along the San Andreas. They catch, like two pieces of sandpaper being rubbed against each other, and then suddenly slip from a few feet to a few miles, causing an earthquake. The amount of movement is the primary factor that determines the size of the quake.

Wednesday’s earthquake hit more than 20 miles away from the San Andreas on one of many lesser faults along the edge of the basin.

The area near Upland had been seismically quiet until a 4.7 quake struck there in June, 1988. Since then it has been quite active with smaller quakes, including one foreshock at 12:39 p.m. Wednesday that measured 3.6.

The main shock hit almost exactly three hours later--at 3:43 p.m.--and was followed by an unusually active period of small aftershocks. Most measured below 2 in magnitude, but a 4.8 aftershock struck at 7:24 p.m.

Scientists said they expect aftershocks to continue for several days, and they could not rule out the possibility of a larger quake. Jones, of the USGS, said she believes faults in the area are capable of generating earthquakes as great as 6.5 to 7, but she does not “think it’s likely” that a quake that large will follow Wednesday’s temblor.

The earthquake tossed this huge boulder from a nearby hillside onto Angeles Crest Highway, about 25 miles north of La Canada Flintridge, moments before a school bus was scheduled to pass by.

QUAKES AND THE LAND--The push and tug of earthquakes--over the course of millions of years--gradually created mountain ranges.

Wednesday’s quake occurred in a complex seismological area at the juncture of two faults, the Cucamonga and the Sierra Madre. The quake was centered three miles northwest of Upland. The epicenter was about five miles below the surface.