Aftershock of April Quake Jolts North Coast : Seismology: Magnitude 5.0 temblor does little damage. Strength of main shock is upgraded to 7.1.

A magnitude 5.0 aftershock, the largest in more than a month, struck an area south of Eureka on Friday, shaking some items from shelves but doing little other damage.

By coincidence, the temblor occurred on the day that the U.S. Geological Survey upgraded the strength of the April quake that leveled buildings in the area to magnitude 7.1. The agency also raised the magnitude on two aftershocks, based on more complete reports from 400 seismographic stations around the world.

Originally, the strength of the April 25 main shock in Humboldt County had been put at 6.9, and the two aftershocks, early the next day, at 6.2 and 6.6.

But based on an examination of all the data, Stuart Koyanagi of the Geological Survey’s National Earthquake Information Center at Golden, Colo., said the magnitudes are 6.6 and 6.7 for the two aftershocks. Such revisions are common several weeks after major quakes.

At magnitude 7.1, the main quake was as strong as the Loma Prieta earthquake that rocked the San Francisco Bay Area and Santa Cruz Mountains on Oct. 17, 1989. That quake did much more damage and caused 63 deaths because it affected urban areas.

The Humboldt County earthquakes most severely affected the small towns of Ferndale, Petrolia, Scotia and Rio Dell, and caused no deaths. Some damage was suffered in Eureka.

Koyanagi said that Friday’s aftershock, shortly before 3 p.m., was centered 35 miles southwest of Eureka in the same area where the main shock was centered, and was felt through much of Humboldt and Mendocino counties.

A spokeswoman for the Humboldt County Sheriff’s Department said it caused fairly strong shaking in several communities but that only minor damage was reported.