Imt.period() update

src/main/java/gov/usgs/earthquake/nshmp/gmm/Idriss_2014.java

@@ -132,8 +132,12 @@ public class Idriss_2014 implements GroundMotionModel {
   // Aleatory uncertainty model
   private static final double calcStdDev(Coefficients c, double Mw) {
     double s1 = 0.035;
-    Double T = c.imt.period();
-    s1 *= (T == null || T <= 0.05) ? log(0.05) : (T < 3.0) ? log(T) : log(3d);
+    if (c.imt.isSA()) {
+      double T = c.imt.period();
+      s1 *= (T <= 0.05) ? log(0.05) : (T < 3.0) ? log(T) : log(3.0);
+    } else {
+      s1 *= 0.05;
+    }
     double s2 = 0.06;
     s2 *= (Mw <= 5.0) ? 5.0 : (Mw < 7.5) ? Mw : 7.5;
     return 1.18 + s1 - s2;