LBL/UCB team identifies cracks made by LA quake

January 28, 1994

By Mike Woolridge, [email protected]

Flying above the Santa Susana mountains north of the epicenter of the January 17 Southern California earthquake, a team of LBL/UC Berkeley earth scientists located extensive surface fractures that developed along the range's ridges.

The cracks, which measured as much as a foot wide and hundreds of feet in length, give geologists important clues about the little known fault that caused the 6.7 magnitude quake, according to the Earth Sciences Division's Pat Williams, who led the team.

Williams suspects the fractures may also have contributed to the freeway damage on the highways bordering the mountain range: Highway 14 collapsed onto Interstate 5 on the eastern edge of the Santa Susana range; a major bridge along Interstate 5 also collapsed within the range; and the damaged Simi Valley-San Fernando freeway is on the southern edge of the range.

Cal Trans is incorporating the geological data from the fly-over into their post-quake freeway studies.

The Jan. 19 geological flight was a cooperative effort that involved researchers from several institutions, including Williams, Barbara Romanowicz of the UCB Seismographic Station, and Jack Moehle of the UCB Earthquake Engineering Research Center.

Using seismic measurements of the quake recorded at UCB and Caltech, the scientists first pinpointed a 5 km by 20 km area most likely to show new surface features. The team then zig-zagged over the area in a helicopter volunteered by the U.S. Coast Guard.

Considering that Monday's quake involved what Williams called a "shortening" of the earth's crust as faults squeezed together, the cracks came as somewhat of a surprise. "The cracks make it appear that the mountains have been pulled apart," Williams said.

He said the cracking most likely resulted from rumpling of the earth's crust as it was pushed together, much like a fold in a carpet, with the fractures occurring across the rumple's surface.

The findings support the theory that the quake occurred on account of slippage of a poorly known part of the Oak Ridge fault system. The fault that caused the jolt, runs 14 km beneath the San Fernando Valley and projects to the surface at the Santa Susana Mountains, where the cracks were found.

Williams says fault movement at the epicenter was rather coherent, meaning the two slabs of earth slid smoothly against one another like two blocks of balsa wood. The fault's movement closer to the surface, however, was less coherent, producing folding and complex faulting of the Santa Susana Mountains. Such actions combined to shatter the mountain range's surface rocks, Williams said, causing the surface fractures.