A ShakeMap (Wald et al., 1999) is a representation of ground shaking produced by an earthquake. Each earthquake produces a range of ground shaking levels at sites throughout the region depending on distance from the hypocenter, the rock and soil conditions at sites, and variations in the propagation of seismic waves from the earthquake due to complexities in the structure of the Earth's crust.
Part of the strategy for generating rapid-response ground motion maps is to determine the best format for reliable presentation of the maps given the diverse audience, which includes scientists, businesses, emergency response agencies, media, and the general public. In an effort to simplify and maximize the flow of information to the public, Wald et al. (1999) developed a means of generating not only peak ground acceleration and velocity maps, but also an instrumentally-derived, estimated Modified Mercalli Intensity map. This map makes it easier to relate the recorded ground motions to the expected felt and damage distribution. The Instrumental Intensity map is based on a combined regression of recorded peak acceleration and velocity amplitudes.
We adopted the ShakeMap version 4.0 (Worden et al., 2018) since 1st January 2020 with the configuration released by the INGV within the OGS-INGV collaboration (tavolo tecnico CRS-ONT). This configuration, calibrated for Italy by Michelini et al. (2020), utilizes the Ground Motion Predictive Equations by Bindi et al. (2011) and the Faenza and Michelini (2010; 2011) relations to compute the Instrumental Intensities. We tuned the standard INGV configuration for Northeastern Italy adopting, for weak events, the empirical relations proposed by Massa et al. (2008).
Limitations of Current Maps
The ShakeMap approach is simple and approximate. It is not considered the direction of rupture nor the peak motions are corrected by a directivity term. The empirical predictive approach only gives average peak ground motions values so it does not account for all the expected variability in motions, other than the aforementioned site amplification variations. Actual ground motions show significant variability for a given distance, magnitude, and site condition and, hence, the scenario ground motions are more uniform than would be expected for an actual earthquake. The true variations are partially attributable to 2D and 3D wave propagation, path effects (such as basin edge amplification and focusing), differences in motions among earthquakes of the same magnitude, and complex site effects are not accounted for by our method.
Note: ShakeMaps are generated automatically following moderate and large earthquakes. These are preliminary ground shaking maps, normally posted within several minutes of the earthquake origin time. The acceleration and velocity values are raw and are at least initially, NOT checked by humans. Further, since ground motions and intensities typically can vary significantly over small distances, these maps are only APPROXIMATE. At small scales, they should be considered unreliable. Finally, the input data is raw and unchecked, and may contain errors. (See Disclaimer)
ShakeMap is a product of the U.S. Geological Survey Earthquake Hazards Program.
For more details also see the ShakeMap wiki.