How Strong Does an Earthquake Have to Be to Feel It
Please Report Felt Earthquakes
HVO needs your help! When you feel an earthquake, please share your experience to help the USGS map the shaking. Even if you did not feel the earthquake, your 'not-felt' response provides important information. Your reports fill in gaps between instruments that record shaking. They also contribute to more complete earthquake assessments and provides valuable data for further earthquake research.
- Go to the USGS Earthquake Hazards program "Did You Feel It?" webpage.
- Under the "DYFI, Past 24 Hours" heading, select the earthquake you felt. If it's not there, click the "Report an Unknown Event" button above the earthquake list.
- Fill out the DYFI questionnaire and click "Submit." It's that simple!
Both "magnitude" and "intensity" describe earthquake size.
There are two ways to describe the size of an earthquake. Magnitude characterizes the amount of energy released by an earthquake at its source. The energy released is determined by measuring the amplitude or duration of earthquake waves recorded by a seismometer. Magnitude numerical values are calculated using a modern formulation of the Richter Scale. These values are assigned to an earthquake, independent of location.
Intensity describes the effects of shaking on people and structures. An earthquake's intensity is determined by collecting felt reports or by measuring the actual shaking of the ground. Intensity values are assigned using the Modified Mercalli Intensity Scale. Unlike magnitude, intensity may vary greatly from one place to another for a given earthquake. Intensity values are usually, but not always, highest near an epicenter and decrease with distance from the source.
Learn more from the USGS Earthquake Hazards Program.
The Modified Mercalli Intensity (MMI) Scale assigns intensities as Roman numerals based on observed effects. (Public domain.)
Earthquake intensity can be shown with different types of maps.
USGS Community Internet Intensity Map for the Kīholo Bay earthquake (star marks location) on October 15, 2006, beneath the northwest coast of the Island of Hawai'i. (Public domain.)
USGS Community Internet Intensity Map for the Kīholo Bay earthquake (star marks location) on October 15, 2006, beneath the northwest coast of the Island of Hawai'i.
Community Internet Intensity Maps (CIIM) are color-coded maps reflecting intensity—the effects of an earthquake—as described by reports submitted by people through the Did You Feel It? webpage. CIIM show average intensity values for each zip code region from which a report is received. The more felt reports received for your ZIP code, the more reliable will be the average intensity assigned to that ZIP code. Information in CIIMs is particularly useful to fill in geographic gaps where there are few seismic stations. Learn more about DYFI.
ShakeMaps are another type of earthquakeintensity map made by the USGS. These maps show a combined ground motion and shaking intensity for earthquakes with magnitudes of 3.5 or higher in Hawaii. They incorporate measurements of ground acceleration and velocity recorded by seismic instruments and felt reports submitted through DYFI. ShakeMaps, therefore, offer more complete pictures of the shaking produced by earthquakes. They are used by federal, state, and local organizations for post-earthquake response and recovery efforts and public and scientific information, as well as for preparedness exercises and disaster planning. Learn more about ShakeMap.
How Strong Does an Earthquake Have to Be to Feel It
Source: https://www.usgs.gov/observatories/hawaiian-volcano-observatory/felt-earthquakes