def calculate_z2pt5_ngaw2(vs30):
    '''
    Reads an array of vs30 values (in m/s) and
    returns the depth to the 2.5 km/s velocity horizon (in km)
    Ref: Campbell, K.W. & Bozorgnia, Y., 2014.
    'NGA-West2 ground motion model for the average horizontal components of
    PGA, PGV, and 5pct damped linear acceleration response spectra.'
    Earthquake Spectra, 30(3), pp.1087–1114.

    :param vs30: the shear wave velocity (in m/s) at a depth of 30 m
    '''
    c1 = 7.089
    c2 = -1.144
    z2pt5 = numpy.exp(c1 + numpy.log(vs30) * c2)
    return z2pt5
Reads an array of vs30 values (in m/s) and
    returns the depth to the 2.5 km/s velocity horizon (in km)
    Ref: Campbell, K.W. & Bozorgnia, Y., 2014.
    'NGA-West2 ground motion model for the average horizontal components of
    PGA, PGV, and 5pct damped linear acceleration response spectra.'
    Earthquake Spectra, 30(3), pp.1087–1114.

    :param vs30: the shear wave velocity (in m/s) at a depth of 30 m