Array of 250 Southland Seismic Monitoring Sites Urged

Earthquake experts Monday proposed establishment of a dense array of 250 Global Positioning System stations in urban areas of Southern California to allow more precise determinations of seismic strains accumulating in the region and improve long-range earthquake forecasting.

Representatives of the Southern California Earthquake Center at USC, the U.S. Geological Survey and Jet Propulsion Laboratory declared that the proposed seven-year, $20-million project would probably require a special congressional authorization. The scientists were meeting at JPL.

Just last month, it was disclosed that the National Aeronautics and Space Administration and the Geological Survey had agreed to make grants totaling $400,000 to double the system of stations from the present 15 by the end of the year.

The stations use satellite signals to measure tiny creeping movements within the earth, showing the distribution of annual seismic slip.

But Tom Henyey, executive director of the Earthquake Center, said a larger, more dense network--extending from Santa Barbara east to Palm Springs and from Palmdale south to Orange County--would allow a fault-by-fault measurement of strain that would show in more detail which areas of the Southland are most prone to large earthquakes in the decades ahead.

As a result, Henyey and JPL’s Andrea Donnellan said, building codes could be strengthened in some particularly vulnerable areas, and insurance rates could be adjusted in accord with the quake prospects, with premiums being set higher in those areas and lower in others.

Donnellan said scientists promoting the project have discussed means of funding with the staffs from the offices of Rep. George Brown (D-Colton) and U.S. Sen. Dianne Feinstein (D-Calif.).

Henyey said a number of federal agencies have expressed interest but have also said that it would be difficult to divert money from other projects to fund the network without congressional action.

Hours before Monday’s meeting, a Northridge aftershock of magnitude 3.7, centered five miles north of Chatsworth, awakened people from the San Fernando Valley to Santa Monica, and a few elsewhere, at 1:40 a.m.

Today marks four months since the magnitude 6.8 earthquake, and there have been more than 8,000 aftershocks since then, most of them too small to feel, but several over magnitude 5.

Lucile M. Jones, a seismologist with the Geological Survey, said Monday that there are still about two aftershocks a week that exceed magnitude 3.0, and an average of 1 1/2 every two weeks exceeding magnitude 3.5.

Jones said the odds are that there will be about one magnitude 4 aftershock a month in the months ahead. There is a 56% probability of a magnitude 5 sometime in the next year. The last aftershock greater than 5, a magnitude 5.3 temblor, occurred March 20.