The legend below Figure 6 gives the peak ground motions that correspond to each unit Modified Mercalli intensity value according to our regression of the observed peak ground motions and intensities for California earthquakes. By relating recorded ground motions to Modified Mercalli intensities, we can now generate instrumental intensities within a few minutes of the event based on the recorded peak motions made at strong motion stations that will correspond approximately to the existing scale. Note that the estimated intensity map is derived from ground motions recorded by accelerographs and represents intensities that are likely to have been associated with the ground motions. However, unlike conventional intensities, the instrumental intensities are not based on observations of the earthquake effects on people or structures. Color fill corresponds to the intensity scale in the legend at the bottom of the figure.
Small circles show selected city locations as labeled.
Figure 6 shows the instrumental intensity map derived using the procedure outlined here using strong motion data recorded during the Northridge earthquake. The epicenter is shown with a solid star and blue lines depict highways. This likely reflects the fact that although much of the Santa Susanna mountains, north and northwest of the epicenter, were very strongly shaken, the region is also sparsely populated, hence, observed intensities were not determined there. In that case it took over one half hour to provide just the epicenter and magnitude. Had this map been available in the minutes following the Northridge earthquake, much more would have been understood about the scope of the disaster and the variations in damage over the Los Angeles metropolis.