Non-reporting 'Did You Feel It?' areas can be used to improve earthquake intensity maps

Since its launch in the late 1990s, the DYFI website (https://earthquake.usgs.gov/data/dyfi/) has collected millions of reports from people who log on to the site to share information when they feel an earthquake in their ZIP code. With these data, seismologists can prepare maps within minutes that outline the intensity of the earthquake -- where and how strongly the quake was felt and what kind of damage may have occurred.

Earthquake intensity is a measure of ground shaking, usually ranging from "not felt" to "extreme." Intensity differs from earthquake magnitude or size, which measures the energy released by a quake.

In the past, DYFI earthquake intensity maps have ignored ZIP codes with no DYFI reports from the public. But in the SRL study, USGS researchers John Boatwright and Eleyne Phillips suggest that these "non-reporting" ZIP codes represent real data -- that is, they indicate that no earthquake was felt in the area.

Use of the DYFI website is so widespread -- "the envy of every scientific website in the world," -- said Boatwright -- that it is more reasonable to assume that "no report" means "no shaking," rather than a lack of participation by the public in that particular ZIP code.

"Feeling an earthquake is a powerful inducement for submitting a DYFI felt report, but there is no similar stimulus for submitting a 'not felt' report. In fact, 'not felt' reports make up less than 1% of DYFI reports," Boatwright added.

In their study, Boatwright and Phillips included information from non-reporting areas to develop new intensity mapping for two California earthquakes: a January 2011 magnitude 4.5 urban earthquake that occurred near San Juan Bautista, and the February 2012 magnitude 5.6 Weitchpec earthquake that occurred in a more rural area in Humboldt County.

The approach allowed the researchers to better delineate the "felt area" for these earthquakes, particularly where the intensities were lowest. For the two quakes that they analyzed, they found that there were no non-reporting ZIP codes located near the earthquake epicenter, overlapping reporting and non-reporting ZIP codes at intermediate distances of 80 to 180 kilometers away from the centers, and few reporting ZIP codes located at even further distances.

After including the non-reporting ZIP codes in their analysis, Boatwright says, the researchers' DYFI-based maps of seismic intensity "resemble very well the older, historical maps that seismologists like [Charles] Richter and others published for these areas."