diff --git a/src/main/java/gov/usgs/earthquake/nshmp/www/gmm/GmmCalc.java b/src/main/java/gov/usgs/earthquake/nshmp/www/gmm/GmmCalc.java
index 49355944a9a17fb2cd4d8bb492b79d98aa716063..ef285d34066e82be80aa31bc54e82c70f08d8a81 100644
--- a/src/main/java/gov/usgs/earthquake/nshmp/www/gmm/GmmCalc.java
+++ b/src/main/java/gov/usgs/earthquake/nshmp/www/gmm/GmmCalc.java
@@ -44,7 +44,6 @@ class GmmCalc {
   /* Compute ground motion response spectra. */
   static Map<Gmm, GmmSpectraData> spectra(Request request) {
     Set<Imt> saImts = Gmm.responseSpectrumImts(request.gmms);
-    List<Double> periods = Imt.periods(saImts);
 
     Map<Gmm, GmmSpectraData> gmmSpectra = new EnumMap<>(Gmm.class);
 
@@ -58,7 +57,7 @@ class GmmCalc {
           new GmmSpectraData(
               treeToDataGroup(gmm, Imt.PGA, request.input),
               treeToDataGroup(gmm, Imt.PGV, request.input),
-              treesToDataGroup(periods, saImtTrees)));
+              treesToDataGroup(Imt.periods(saImts), saImtTrees)));
     }
 
     return gmmSpectra;