Fix psmeca failure when P and T axes have equal dips

src/seis/utilmeca.c

@@ -914,7 +914,9 @@ void meca_axe2dc (struct SEIS_AXIS T, struct SEIS_AXIS P, struct SEIS_NODAL_PLAN
 	NP2->dip = d2; NP2->str = p2;
 
 	im = 1;
-	if (P.dip > T.dip) im = -1;
+	/* Logic: Use Normal fault (im=-1) if P dip > T dip*/
+	/* OR if dips are equal (e.g., 45 deg) BUT not zero (Strike Slip case P=T=0) */
+	if (P.dip > T.dip || (fabs(P.dip - T.dip) < SEIS_EPSILON && P.dip > SEIS_EPSILON)) im = -1;
 	NP1->rake = meca_computed_rake2 (NP2->str, NP2->dip, NP1->str, NP1->dip, im);
 	NP2->rake = meca_computed_rake2 (NP1->str, NP1->dip, NP2->str, NP2->dip, im);
 }

test/seis/seis_equal_dip.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+#
+# # Check psmeca for cases where P and T axes have equal dips (e.g., vertical dip-slip)
+gmt begin seis_equal_dip ps
+gmt basemap -R0/10/0/6.5 -JM15c -B0 -BWSen+t"Focal Mechanisms Comparison"
+
+echo 3.0  5.0 10.0 0 90 90 5.0 0 0 | gmt meca -Sa3.2c+m -Gred       -W0.8p,black
+echo 7.0  5.0 10.0 0 90 0  5.0 0 0 | gmt meca -Sa3.2c+m -Gblue      -W0.8p,black
+
+echo 3.0  1.8 5   1 360 45 1 90 0 1 180 45.01 1 45.01 | gmt meca -Sa3.2c+m -Gpurple   -W0.8p,black
+echo 7.0  1.8 5   1 360 45 1 90 0 1 180 45.1  1 45.1  | gmt meca -Sa3.2c+m -Gdarkgreen -W0.8p,black
+
+# Labels 
+gmt text -F+f8p,Helvetica-Bold,black+jCB -N << EOF
+3.0  3.6 Vertical dip-slip
+7.0  3.6 Strike-slip
+3.0  0.50 Equal dip 45
+7.0  0.50 Near-equal 45.1
+EOF
+
+gmt end