Merge pull request #3316 from g-weatherill/ask14_fix

Fixes ASK14 GMPE behaviour to remove ValueError

openquake/hazardlib/gsim/abrahamson_2014.py

@@ -361,10 +361,15 @@ def _get_intra_event_std(self, C, mag, sa1180, vs30, vs30measured,
         phi_al = self._get_phi_al_regional(C, mag, vs30measured, rrup)
         derAmp = self._get_derivative(C, sa1180, vs30)
         phi_amp = 0.4
-        if any(phi_al**2 < phi_amp**2):
-            print('phi_al:' % (phi_al))
-            print('magnitude: %.2f' % (mag))
-            raise ValueError('sqrt argument < 0')
+        idx = phi_al < phi_amp
+        if np.any(idx):
+            # In the case of small magnitudes and long periods it is possible
+            # for phi_al to take a value less than phi_amp, which would return
+            # a complex value. According to the GMPE authors in this case
+            # phi_amp should be reduced such that it is fractionally smaller
+            # than phi_al
+            phi_amp = 0.4 * np.ones_like(phi_al)
+            phi_amp[idx] = 0.99 * phi_al[idx]
         phi_b = np.sqrt(phi_al**2 - phi_amp**2)
         phi = np.sqrt(phi_b**2 * (1 + derAmp)**2 + phi_amp**2)
         return phi