How does a shakemap perform with varying degrees of station coverage?

To illustrate this we compare in the left panel a shakemap computed with the numbers of stations that may have been expected in very near-real-time following the Northridge earthquake. The contours in all maps are in g and the numbers are given in tenths of a g. This map defines a region of 0.4 g PGA and an area of the northern San Fernando Valley where PGA exceeded 1 g.  The center illustrates a case with 40 km  station spacing as may be anticipated in many areas of California. With a station located to the north of the fault plane an area of high ground motion is defined, but note that the region with greater than 0.4 g is no longer well defined.  The right panel illustrates a case with 40 km station spacing without a fortuitously located station.  In this case the region of strong shaking is not defined.