diff --git a/src/main/java/gov/usgs/earthquake/nshmp/netcdf/reader/StaticHazard.java b/src/main/java/gov/usgs/earthquake/nshmp/netcdf/reader/StaticHazard.java
index 28dc0f0262274f1232ae9c8205ed8a54feac219f..fa202d9d019e233979ab9e88de95d4e3d0922a47 100644
--- a/src/main/java/gov/usgs/earthquake/nshmp/netcdf/reader/StaticHazard.java
+++ b/src/main/java/gov/usgs/earthquake/nshmp/netcdf/reader/StaticHazard.java
@@ -2,18 +2,21 @@ package gov.usgs.earthquake.nshmp.netcdf.reader;
 
 import static com.google.common.base.Preconditions.checkState;
 
-import java.util.HashMap;
+import java.util.EnumMap;
 import java.util.Map;
 
 import gov.usgs.earthquake.nshmp.data.XySequence;
 import gov.usgs.earthquake.nshmp.gmm.Imt;
 
 @SuppressWarnings("serial")
-public class StaticHazard extends HashMap<Imt, XySequence> {
+public class StaticHazard extends EnumMap<Imt, XySequence> {
 
-  public StaticHazard() {}
+  public StaticHazard() {
+    super(Imt.class);
+  }
 
-  private StaticHazard(Map<Imt, XySequence> staticHazard) {
+  private StaticHazard(EnumMap<Imt, XySequence> staticHazard) {
+    super(Imt.class);
     putAll(staticHazard);
   }
 
@@ -26,10 +29,10 @@ public class StaticHazard extends HashMap<Imt, XySequence> {
   }
 
   public static class Builder {
-    Map<Imt, XySequence> staticHazard;
+    EnumMap<Imt, XySequence> staticHazard;
 
     private Builder() {
-      staticHazard = new HashMap<>();
+      staticHazard = new EnumMap<>(Imt.class);
     }
 
     public Builder put(Imt imt, XySequence xySequence) {
diff --git a/src/main/java/gov/usgs/earthquake/nshmp/netcdf/reader/StaticHazards.java b/src/main/java/gov/usgs/earthquake/nshmp/netcdf/reader/StaticHazards.java
index 0bba7f00946e615ea23549487d967e946d8c886a..a634c22d60a5f53656ec858718273ba9ceec69f5 100644
--- a/src/main/java/gov/usgs/earthquake/nshmp/netcdf/reader/StaticHazards.java
+++ b/src/main/java/gov/usgs/earthquake/nshmp/netcdf/reader/StaticHazards.java
@@ -2,17 +2,19 @@ package gov.usgs.earthquake.nshmp.netcdf.reader;
 
 import static com.google.common.base.Preconditions.checkState;
 
-import java.util.HashMap;
-import java.util.Map;
+import java.util.EnumMap;
 
 import gov.usgs.earthquake.nshmp.netcdf.SiteClass;
 
 @SuppressWarnings("serial")
-public class StaticHazards extends HashMap<SiteClass, StaticHazard> {
+public class StaticHazards extends EnumMap<SiteClass, StaticHazard> {
 
-  public StaticHazards() {}
+  public StaticHazards() {
+    super(SiteClass.class);
+  }
 
-  private StaticHazards(Map<SiteClass, StaticHazard> staticHazards) {
+  private StaticHazards(EnumMap<SiteClass, StaticHazard> staticHazards) {
+    super(SiteClass.class);
     this.putAll(staticHazards);
   }
 
@@ -21,10 +23,10 @@ public class StaticHazards extends HashMap<SiteClass, StaticHazard> {
   }
 
   public static class Builder {
-    Map<SiteClass, StaticHazard> staticHazards;
+    EnumMap<SiteClass, StaticHazard> staticHazards;
 
     private Builder() {
-      staticHazards = new HashMap<>();
+      staticHazards = new EnumMap<>(SiteClass.class);
     }
 
     public Builder put(SiteClass siteClass, StaticHazard staticHazard) {