Use alternate VS_REF value of 2000 m/s

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

@@ -33,7 +33,7 @@ class ChapmanGuo_2021 {
       PGV,
       Imt.mprsImts().toArray(new Imt[0]));
 
-  static final double VS_REF = 1000.0;
+  static final double VS_REF = 2000.0;
   static final double Z_CUT = 0.2;
 
   private static final double[] Z = {
@@ -51,6 +51,7 @@ class ChapmanGuo_2021 {
   private static final Map<Imt, double[][][]> scales;
 
   static {
+    System.out.printf(">>>>>>>>> ChapmanGuo2021 VS_REF = %.1f\n", VS_REF);
     scales = new EnumMap<Imt, double[][][]>(Imt.class);
     for (Imt imt : IMTS) {
       scales.put(imt, loadTable(imt));