updated gulf ref site scaling

src/main/java/gov/usgs/earthquake/nshmp/gmm/ChapmanGuo_2021.java

@@ -34,7 +34,7 @@ class ChapmanGuo_2021 {
       Imt.mprsImts().toArray(new Imt[0]));
 
   static final double VS_REF = 1000.0;
-  static final double Z_CUT = 0.5;
+  static final double Z_CUT = 0.2;
 
   private static final double[] Z = {
       0.0, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8,
@@ -81,12 +81,12 @@ class ChapmanGuo_2021 {
 
   /**
    * Return the depth scaling factor for application of reference site scaling.
-   * Curently set to 0.0 at z = 0.0 km and 1.0 at z = 0.5 km.
    *
    * @param z sediment depth (in km)
    */
   static double zSiteScale(double z) {
-    return Math.min(1.0, z / Z_CUT);
+    double s = 1.0 - Math.exp(-z / Z_CUT);
+    return s * s * s * s;
   }
 
   /**