diff --git a/src/main/java/gov/usgs/earthquake/nshmp/gmm/UsgsPrviBackbone2025.java b/src/main/java/gov/usgs/earthquake/nshmp/gmm/UsgsPrviBackbone2025.java
index 6e0e2b239b1042189836125ddb59b87c8f1fd6ac..aa37b1eb46e6a880fecb67909602876cc3b298dc 100644
--- a/src/main/java/gov/usgs/earthquake/nshmp/gmm/UsgsPrviBackbone2025.java
+++ b/src/main/java/gov/usgs/earthquake/nshmp/gmm/UsgsPrviBackbone2025.java
@@ -81,6 +81,10 @@ public abstract class UsgsPrviBackbone2025 implements GroundMotionModel {
     final double a, b;
     final double Ï†s2sPrvi, Ï†s2sNga;
     final double Ï„Sub, Ï†Sub, Ï†SubSS, Ï†SubS2S;
+    final double Ï„SubPga, Ï†SubPga, Ï†SubSSpga, Ï†SubS2Spga;
+
+    final double bnl, cnl, vlin;
+    final double bnlSub, vlinSub, ÏB, ÏW;
 
     Coefficients(Imt imt) {
 
@@ -103,6 +107,24 @@ public abstract class UsgsPrviBackbone2025 implements GroundMotionModel {
       Ï†Sub = coeffsSubSigma.get("phi_mean");
       Ï†SubSS = coeffsSubSigma.get("PhiSS_PRVI");
       Ï†SubS2S = coeffsSubSigma.get("phiS2S_PRVI_smooth");
+
+      Map<String, Double> coeffsSubSigmaPga = COEFFS_SIGMA_SUBDUCTION.get(Imt.PGA);
+      Ï„SubPga = coeffsSubSigmaPga.get("tau_mean");
+      Ï†SubPga = coeffsSubSigmaPga.get("phi_mean");
+      Ï†SubSSpga = coeffsSubSigmaPga.get("PhiSS_PRVI");
+      Ï†SubS2Spga = coeffsSubSigmaPga.get("phiS2S_PRVI_smooth");
+
+      Map<String, Double> coeffsAsk14 = AbrahamsonEtAl_2014.COEFFS.get(imt);
+      bnl = coeffsAsk14.get("b");
+      cnl = coeffsAsk14.get("c");
+      vlin = coeffsAsk14.get("Vlin");
+
+      Map<String, Double> coeffsAg20 = AbrahamsonGulerce_2020.COEFFS.get(imt);
+      bnlSub = coeffsAg20.get("b");
+      vlinSub = coeffsAg20.get("vlin");
+      ÏB = coeffsAg20.get("rhoB");
+      ÏW = coeffsAg20.get("rhoB");
+
     }
   }
 
@@ -221,6 +243,8 @@ public abstract class UsgsPrviBackbone2025 implements GroundMotionModel {
         CB_14_PRVI, 0.25,
         CY_14_PRVI, 0.25);
 
+    private final AbrahamsonEtAl_2014 ÏƒPgaGmm;
+
     ActiveCrust(Imt imt) {
       this(imt, NAME);
     }
@@ -231,18 +255,40 @@ public abstract class UsgsPrviBackbone2025 implements GroundMotionModel {
 
     ActiveCrust(Imt imt, String name, Map<Gmm, Double> gmms) {
       super(imt, name, gmms, getPrviEpi(ACTIVE_CRUST, imt));
+      this.ÏƒPgaGmm = (AbrahamsonEtAl_2014) Gmm.ASK_14_BASE.instance(imt);
     }
 
     @Override
     double[] calcSigmas(GmmInput in) {
       Coefficients c = super.coeffs;
       double Ï„ = calcTau(in.Mw, c);
+      double saRock = calcSaRock(in);
+      double dAmp_p1 = AbrahamsonEtAl_2014.get_dAmp(c.bnl, c.cnl, c.vlin, in.vs30, saRock) + 1.0;
+      double Ï„nl = Ï„ * dAmp_p1;
       double Ï†SS = calcPhiSS(in.Mw, c);
-      double ÏƒNga = sqrt(Ï„ * Ï„ + c.Ï†s2sNga * c.Ï†s2sNga + Ï†SS * Ï†SS);
-      double ÏƒPrvi = sqrt(Ï„ * Ï„ + c.Ï†s2sPrvi * c.Ï†s2sPrvi + Ï†SS * Ï†SS);
+      double Ï†SqNga = Ï†SS * Ï†SS + c.Ï†s2sNga * c.Ï†s2sNga;
+      double Ï†SqPrvi = Ï†SS * Ï†SS + c.Ï†s2sPrvi * c.Ï†s2sPrvi;
+
+      double Ï†NlNgaSq = calcPhiSSNonLinSq(Ï†SqNga, dAmp_p1);
+      double Ï†NlPrviSq = calcPhiSSNonLinSq(Ï†SqPrvi, dAmp_p1);
+
+      double ÏƒNga = sqrt(Ï„nl * Ï„nl + Ï†NlNgaSq);
+      double ÏƒPrvi = sqrt(Ï„nl * Ï„nl + Ï†NlPrviSq);
+
       return new double[] { ÏƒNga, ÏƒPrvi };
     }
 
+    private double calcSaRock(GmmInput in) {
+      // ASK Vs30 rock = 1180 m/s
+      // Also disable basin term.
+      GmmInput inRock = GmmInput.builder().fromCopy(in)
+          .vs30(AbrahamsonEtAl_2014.VS_RK)
+          .z1p0(Double.NaN)
+          .build();
+
+      return Math.exp(ÏƒPgaGmm.calc(inRock).get(0).value().mean());
+    }
+
     // active crust tau
     static double calcTau(double Mw, Coefficients c) {
       if (Mw <= 4.5) {
@@ -262,6 +308,11 @@ public abstract class UsgsPrviBackbone2025 implements GroundMotionModel {
     static double calcPhiSS(double Mw, Coefficients c) {
       return Mw <= 5.0 ? c.a : Mw > 6.5 ? c.b : c.a + (Mw - 5.0) * ((c.b - c.a) / 1.5);
     }
+
+    static double calcPhiSSNonLinSq(double Ï†, double dAmp_p1) {
+      double phiBsq = Ï† * Ï† - AbrahamsonEtAl_2014.PHI_AMP_SQ;
+      return phiBsq * dAmp_p1 * dAmp_p1 + AbrahamsonEtAl_2014.PHI_AMP_SQ;
+    }
   }
 
   /* PRVI 2025 Active Crust - sigma NGA */
@@ -387,6 +438,8 @@ public abstract class UsgsPrviBackbone2025 implements GroundMotionModel {
         KBCG_20_PRVI_INTERFACE, 0.3333,
         PSBAH_20_PRVI_INTERFACE, 0.3334);
 
+    private final AbrahamsonGulerce_2020 ÏƒPgaGmm;
+
     Interface(Imt imt) {
       this(imt, NAME);
     }
@@ -397,14 +450,12 @@ public abstract class UsgsPrviBackbone2025 implements GroundMotionModel {
 
     Interface(Imt imt, String name, Map<Gmm, Double> gmms) {
       super(imt, name, gmms, getPrviEpi(SUBDUCTION_INTERFACE, imt));
+      ÏƒPgaGmm = (AbrahamsonGulerce_2020) Gmm.AG_20_GLOBAL_INTERFACE.instance(Imt.PGA);
     }
 
     @Override
     double[] calcSigmas(GmmInput in) {
-      Coefficients c = super.coeffs;
-      double ÏƒNga = sqrt(c.Ï„Sub * c.Ï„Sub + c.Ï†Sub * c.Ï†Sub);
-      double ÏƒPrvi = sqrt(c.Ï„Sub * c.Ï„Sub + c.Ï†SubS2S * c.Ï†SubS2S + c.Ï†SubSS * c.Ï†SubSS);
-      return new double[] { ÏƒNga, ÏƒPrvi };
+      return calcSubductionSigmas(super.coeffs, ÏƒPgaGmm, in);
     }
   }
 
@@ -531,6 +582,8 @@ public abstract class UsgsPrviBackbone2025 implements GroundMotionModel {
         KBCG_20_PRVI_SLAB, 0.3333,
         PSBAH_20_PRVI_SLAB, 0.3334);
 
+    private final AbrahamsonGulerce_2020 ÏƒPgaGmm;
+
     Slab(Imt imt) {
       this(imt, NAME);
     }
@@ -541,14 +594,12 @@ public abstract class UsgsPrviBackbone2025 implements GroundMotionModel {
 
     Slab(Imt imt, String name, Map<Gmm, Double> gmms) {
       super(imt, name, gmms, getPrviEpi(SUBDUCTION_SLAB, imt));
+      ÏƒPgaGmm = (AbrahamsonGulerce_2020) Gmm.AG_20_GLOBAL_SLAB.instance(Imt.PGA);
     }
 
     @Override
     double[] calcSigmas(GmmInput in) {
-      Coefficients c = super.coeffs;
-      double ÏƒNga = sqrt(c.Ï„Sub * c.Ï„Sub + c.Ï†Sub * c.Ï†Sub);
-      double ÏƒPrvi = sqrt(c.Ï„Sub * c.Ï„Sub + c.Ï†SubS2S * c.Ï†SubS2S + c.Ï†SubSS * c.Ï†SubSS);
-      return new double[] { ÏƒNga, ÏƒPrvi };
+      return calcSubductionSigmas(super.coeffs, ÏƒPgaGmm, in);
     }
   }
 
@@ -683,4 +734,42 @@ public abstract class UsgsPrviBackbone2025 implements GroundMotionModel {
     return tree.build();
   }
 
+  private static double[] calcSubductionSigmas(
+      Coefficients c,
+      GroundMotionModel ÏƒPgaGmm,
+      GmmInput in) {
+
+    double pgaRock = calcSubPgaRock(in, ÏƒPgaGmm);
+    double dSite = AbrahamsonGulerce_2020.calc_dSite(c.bnlSub, c.vlinSub, in.vs30, pgaRock);
+    double Ï„NlSq = calcNonLinTermSq(c.Ï„Sub, c.Ï„SubPga, dSite, c.ÏB);
+
+    double Ï†Prvi = sqrt(c.Ï†SubS2S * c.Ï†SubS2S + c.Ï†SubSS * c.Ï†SubSS);
+    double Ï†PrviPga = sqrt(c.Ï†SubS2Spga * c.Ï†SubS2Spga + c.Ï†SubSSpga * c.Ï†SubSSpga);
+
+    double Ï†NlSqNga = calcNonLinTermSq(c.Ï†Sub, c.Ï†SubPga, dSite, c.ÏW);
+    double Ï†NlSqPrvi = calcNonLinTermSq(Ï†Prvi, Ï†PrviPga, dSite, c.ÏW);
+
+    double ÏƒNga = sqrt(Ï„NlSq + Ï†NlSqNga);
+    double ÏƒPrvi = sqrt(Ï„NlSq + Ï†NlSqPrvi);
+    return new double[] { ÏƒNga, ÏƒPrvi };
+
+  }
+
+  private static double calcSubPgaRock(GmmInput in, GroundMotionModel ÏƒGmm) {
+    // ASK Vs30 rock = 1180 m/s
+    // Also disable basin term.
+    GmmInput inRock = GmmInput.builder().fromCopy(in)
+        .vs30(AbrahamsonGulerce_2020.VS30_ROCK)
+        .z2p5(Double.NaN)
+        .build();
+
+    // branch index 1 = middle branch of epistemic logic tree in AG_20
+    return Math.exp(ÏƒGmm.calc(inRock).get(1).value().mean());
+  }
+
+  /* Used for both Ï„ and Ï† calculations; v = variable. */
+  private static double calcNonLinTermSq(double v, double vPga, double dSite, double Ï) {
+    return v * v + dSite * dSite * vPga * vPga + 2.0 * dSite * v * vPga * Ï;
+  }
+
 }