fault2uv: Determine slip vector and fault normal vector from fault orientation

seismolearnpy/mechanism.py

@@ -0,0 +1,52 @@
+import numpy as np
+
+
+def fault2uv(strike, dip, rake):
+    """
+    Determine the slip vector **u** and fault normal vector **v** from fault orientation.
+
+    See Figure 4.20 and Equation 4.83 in Aki & Richards, 1980 for the definitions.
+
+    Parameters
+    ----------
+    strike : float
+        Strike direction of the fault, measured clockwise from North.
+        Valid range is [0, 360) in degrees.
+    dip : float
+        Dip angle of the fault, measured down from the horizontal.
+        Valid range is [0, 90] in degrees.
+    rake : float
+        Angle between the strike direction and slip.
+        Valid range is (-180, 180] in degrees.
+
+    Returns
+    -------
+    u : numpy.ndarray
+        Slip direction vector.
+    v : numpy.ndarray
+        Fault normal vector.
+
+    Examples
+    --------
+    >>> u, v = fault2uv(20, 30, 40)
+    >>> u
+    array([ 0.9102388 , -0.26109644, -0.3213938 ])
+    >>> v
+    array([-0.17101007,  0.46984631, -0.8660254 ])
+    """
+    # convert angles from degrees to radians
+    strike, dip, rake = map(np.radians, (strike, dip, rake))
+
+    sstr, sdip, srak = map(np.sin, (strike, dip, rake))
+    cstr, cdip, crak = map(np.cos, (strike, dip, rake))
+    # slip direction vector
+    u = np.array(
+        [
+            crak * cstr + srak * cdip * sstr,
+            crak * sstr - srak * cdip * cstr,
+            -srak * sdip,
+        ]
+    )
+    # fault normal vector
+    v = np.array([-sdip * sstr, sdip * cstr, -cdip])
+    return u, v