Intensity evaluation attempts to quantify a qualitative description of the effects observed in an earthquake ranked in terms of how strong it was felt and the amount of damage. Despite its qualitative nature, intensity data are essential to study both historical and recent earthquakes such as in earthquake hazard studies and in which dense accelerometric data are missing. The magnitude of historical earthquakes in Brazil has been determined with relations involving magnitude and felt area. Intensity attenuation equations (decay of intensity as a function of magnitude, depth, and distance) determined in other regions of the world are intrinsically dependent on the crustal tectonic characteristics and cannot be applied to Brazil without proper evaluation. We determined an intensity attenuation equation using 20 earthquakes in Brazil and neighboring intraplate areas, in the magnitude range 3.5–6.2 and hypocentral distances up to 720 km.
The best attenuation model was (standard deviation = 0.778), in which is the magnitude () and is the hypocentral distance (in kilometers). The uncertainties of the estimated magnitudes using this equation are around for events with 30 or more intensity points.
We tested the new equation with intensity data from two important events: the 1939 offshore earthquake ( 6) in southern Brazil and the 2018 Venezuela earthquake ( 6.9). The fit was reasonable for both earthquakes, with a root mean square residual of 1.0 and 1.1 intensity units, respectively.
We also re‐evaluated the 1861 earthquake in southeast Brazil. The new epicenter was 23.1° S 44.7° W with a magnitude of 5. The epicenter was located onshore, but the estimated uncertainties include a large offshore region. Given that, the location of the 1861 event remains uncertain.