In addition to the PGA, PGV, and spectral response maps, we also map estimates of the ground-motion shaking intensity. Seismic intensity has been traditionally used worldwide as a method for quantifying the shaking pattern and the extent of damage for earthquakes. Though derived prior to the advent of today's modern seismometric instrumentation, seismic intensity still provides a useful means of describing information contained in these recordings. Such simplification is helpful for those users who are unfamiliar with instrumental ground motion parameters.
That is not to say that instrumentally derived seismic intensity alone is sufficient for loss estimation. In fact, peak velocity and spectral response may provide a more physical basis for such analyses. However, for the majority of users, we expect that the intensity map will be more readily interpreted than other maps of ground motion parameters.
[Wald et al., 1999a] have recently developed regression relationships between
Modified Mercalli intensity [later revised by Richter, 1958]wood31
and PGA or PGV by comparing the peak ground motions to observed intensities for
eight significant California earthquakes.
For the limited range of Modified Mercalli intensities V 
VIII,
[Wald et al., 1999a] found that for PGA,
and for peak velocity (PGV) within the range
V IX,
Since we are also interested in estimating intensity at lower values,
and our current collection of data from historical earthquakes
does not provide constraints for lower intensity, we have imposed the following relationship
between PGA and :
This basis for the above relationship comes from correlation of TriNet peak ground motions
for recent magnitude 3.5 to 5.0 earthquakes with intensities derived from
voluntary response from Internet users [Wald et al., 1999b] for the same events. We determined
that the boundary between ``not felt'' and ``felt'' ( I and II, respectively) regions corresponds to approximately one to two cm/sec/sec, at least for this range of magnitudes.
We then assigned the slope such that the curve would intersect the relationship in Equation (1)
at V. We plan to refine this relationship as more digital data become available.
The corresponding equation for PGV and is:
By comparing maps of instrumental intensities with for eight significant California
earthquakes []wald99a, we have found that a relationship that follows acceleration for
<VII and follows velocity for >VII works fairly well in reproducing the
observed . In practice, we compute the from
the verses PGA relationship (equations 1 and 3), and
if the intensity value determined from
peak acceleration is 
VII, we then use the value of derived from
the verses PGV relationship (equation 2). If the determined from PGA is between
V and VII, we weight both the PGA and PGV derived-values, ramping from a factor of 1.0 for PGA
at V to 0.0 at VII.
Using peak acceleration to estimate low intensities is intuitively consistent with the
notion that lower (<VI) intensities are assigned based on felt accounts, and people are more
sensitive to ground acceleration than velocity. Higher intensities are defined by the level of
damage; the onset of damage at the intensity VI to VII range is usually characterized by
brittle-type failures (masonry walls, chimneys, unreinforced masonry, etc.) which are
sensitive to higher-frequency accelerations. With more substantial damage (VII and
greater), failure begins in more flexible structures, for which peak velocity is more
indicative of failure []hall96. This practice is consistent with the recent
analysis of [Sokolov and Chernov, 1998] in which they showed that seismic intensities correlate well for
rather narrow ranges of Fourier amplitude spectra of ground acceleration, with 0.7-1.0 Hz being
most representative of > VIII, while the 3-6 Hz range best represents V to VII,
and the 7-8 Hz range best correlates with the lowest range.
In addition, [Boatwright el al., 1999]
have found that for the Northridge earthquake, PGV and the 3-0.3 Hz averaged spectral velocity are
better correlated with intensity (VI and greater) than peak acceleration and their correlation with
intensity and peak spectral velocity is strongest at 0.67 Hz.
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. In assigning integer intensity values
using equations (1-4), the rounding adheres to the convention that, for example,
values between 5.50 and 6.49 round to intensity VI. 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. Further, the range of peak
values for a given intensity is likely to change as additional data sets are included.
Figure 6:
Instrumental intensity map for the 1994 Northridge earthquake derived using the procedure
outlined in the text. Color fill corresponds to the intensity scale in the
legend at the bottom of the figure. The epicenter is shown with a filled
star; blue lines depict highways. Small circles show selected city
locations as labeled. Also given in the scale bar are corresponding peak ground motion
values, one- or two-word damage and perceived shaking descriptors. (Also see the Cover, this volume)
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. The color shading corresponds to the colored intensity scale shown below the figure, and two-word descriptions of both shaking and damage levels are provided (L. Dengler and J. Dewey, written communication, 1998). Given the current TriNet station distribution, we can recover approximately the Northridge intensity map (Fig. 6), which is based on more densely spaced stations, if the CDMG dialup data are included (1/2 hour).
Figure 7 shows a comparison of the instrumental intensity
contour lines and the values of Modified Mercalli intensity observed by
[Dewey et al., 1995]. The instrumental intensity map shares most of the notable
features of the Modified Mercalli map prepared by [Dewey et al., 1995], including the
relatively high intensities near Santa Monica and southeast of the epicenter near
Sherman Oaks. However, in general,
the area of IX on the instrumentally derived intensity map is slightly
larger than on the [Dewey et al., 1995] map. 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. This is a
fundamental difference between observed and instrumentally derived intensities:
Instrumental intensities will show high levels of strong
shaking, independent of the exposure of populations and buildings; observed
intensities only represent intensities where there are structures to damage
and people to experience the earthquake.
Figure 7:
Instrumental intensity map for the 1994 Northridge earthquake derived using
the procedure outlined in the text. Contour lines (thick lines) depict
instrumental intensity values. Numbered circles give the observed Modified Mercalli
intensity values of [Dewey et al., 1995] for comparison. The epicenter is shown with a filled star;
thin lines depict highways.
Though there is some degradation when the 5-minute map is made using only the USGS-Caltech real-time stations, it nonetheless provides for more information than was available in the early morning of January 17, 1994. 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.
