As Temblors Subside, Scientists Seek Answers

The rate and size of the weekend’s earthquakes tailed off Monday, and scientists settled down to what will be a protracted study of exactly where the quakes were, what faults were involved and what it might mean for the future.

The most recent of a total of 13 quakes of at least magnitude 3.0, during the likely Northridge aftershock spasm, hit at 6:20 p.m. Sunday, when a 3.0 rattled the epicentral area west of the Santa Clarita Valley.

The strongest quake by Monday evening was only a 2.4, which was 500 times smaller than the 5.0 and 4.9 quakes that rocked the area early Saturday and Sunday.

But presuming that the latest sequence is not over, the Jet Propulsion Laboratory moved one of its global positioning instruments from Santa Barbara County to Granada Hills, and said it would install another near Castaic.

“What we’re trying to find out is basically how the Earth’s crust is made up and how does it behave,” explained the JPL’s Andrea Donnellan.

“We’ve been getting data since Northridge (Jan. 17, 1994), and these aftershocks complicate the picture. We’re trying to find out if the strain in the Ventura Basin [extending from Magic Mountain through (the city of) Ventura] has increased.”

Meanwhile, several scientists said that better knowledge of what fault was involved in the latest temblors must await a more precise calculation of where and at what depth the quakes occurred. That task is being undertaken by Caltech seismologist Egill Hauksson.

It is known that the quakes were generally located on the northernmost edge of the Northridge rupture zone and very likely in a region of north-dipping faults such as the Santa Susana.

The 6.7 Northridge main shock of January 1994 was on a south-dipping thrust fault, so this would mean the quakes were on a different system, which is not uncommon with such occurrences.

One scientist who has studied the region intensively, Oregon State geology professor Robert Yates, raised the possibility Monday that the quakes might not actually be aftershocks. But he said it was premature to reach such a conclusion.

Caltech’s Kerry Sieh said he felt “it would be very chancy” to consider the weekend’s quakes not to be aftershocks, although, he observed, the precise relationship between the weekend’s seismic events and the original Northridge quake remains to be worked out.