A magnitude 4.0 earthquake near the Sonoma County town of Healdsburg on Tuesday night has illustrated the growing popularity of a U.S. Geological Survey Web site that allows people to describe how they felt a quake at their homes and businesses.
By midafternoon Wednesday, 835 people, including 170 in San Francisco, which is 70 miles from the quake centered five miles northwest of Healdsburg, had recorded their impressions of the 9:50 p.m. temblor, which caused goods to fall off shelves at local stores. There were no injuries.
Citizens going to the Web site, https://quake.wr.usgs .gov, can make comments in a category labeled “Shake Maps” under the subtitle, “Did You Feel It?”
The system was originated nearly five years ago by scientist Dave Wald, who is now at the National Earthquake Information Center in Golden, Colo.
The 835 respondents who submitted data Tuesday night and Wednesday morning on the Healdsburg quake represented 85 postal ZIP Codes ranging as far away from the epicenter as Sunnyvale, near San Jose. Their entries showed that the quake was felt as moderately intense close to the epicenter but very lightly in San Francisco and the East Bay.
Respondents use the Modified Mercalli Intensity Scale,which lists physical descriptions of 12 levels of sensitivity.
The strongest shaking Tuesday night, according to this scale, meant that sleeping people were awakened, liquids were spilled, and small, unstable objects were displaced or upset. Doors also opened or closed and pictures moved.
Wald said Wednesday that since the system became operational, there have been more than 350,000 submissions of data. The system is catching on nationwide.
When a magnitude 4.6 quake shook an Alabama-Georgia border area on April 29, 17,000 people from 15 states submitted data, Wald said.
A map depicting the nationwide pattern of responses shows that the three states with by far the strongest shaking in the five-year period have been California, Washington and Alaska.