diff --git a/src/main/java/gov/usgs/earthquake/nshmp/gmm/ChapmanGuo_2021.java b/src/main/java/gov/usgs/earthquake/nshmp/gmm/ChapmanGuo_2021.java
index bc4ddc13426fcc495cd113f091db9f80eb9d202d..955704ea96539cba6bcbbef9d21f2825d89dccf8 100644
--- a/src/main/java/gov/usgs/earthquake/nshmp/gmm/ChapmanGuo_2021.java
+++ b/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 = 590.0;
+  static final double VS_REF = 1000.0;
 
   private static final double[] Z = {
       0.0, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8,