Blasts to Help Pinpoint Faults

The U.S. Geological Survey and Caltrans begin a joint project to delineate earthquake faults along Interstate 215 in San Bernardino today with the first of more than 200 subterranean explosions over the next three weeks.

Scientist Rufus Catchings of the Geological Survey said the blasts at depths of 15 feet would be used to generate “a CAT-scan image of the Earth’s subsurface in the region” as a means of helping Caltrans design the retrofitting of several freeway bridges.

The blasts both day and night alongside the east side of the freeway from the I-10 junction to north of Highland Avenue may be heard by nearby residents, Catchings said, but they will not be powerful enough to cause any damage.

The San Jacinto fault lies in the vicinity of I-215, but there may also be a host of unknown faults transcending the freeway that are capable of generating a damaging earthquake as well, Catchings said.

Portable seismometers will record the seismic energy generated by the blasts and their reverberations as far as two miles under the surface.

Using blasts of small quantities of explosives to help locate faults and determine their potential power has been done with little or no disruption to residents in the Los Angeles area in recent years.

But this is believed to be the first time such research has been done before retrofittings bridges, Catchings said.

Both the 1989 Loma Prieta earthquake in the San Francisco Bay Area and the 1994 Northridge earthquake caused the collapse of portions of freeway bridges, inspiring retrofitting projects on freeways throughout the state. About two-thirds of the 63 deaths in the magnitude 7.1 Loma Prieta quake occurred when elevated portions of the Nimitz Freeway collapsed in the East Bay.