def california_basin_model(vs30):
    """
    Returns the centred z1.0 (mu_z1) based on the California model
    (equation 11)
    """
    coeff = 570.94 ** 4.0
    model = (-7.15 / 4.0) * np.log(
        ((vs30 ** 4.0) + coeff) / ((1360.0 ** 4.0) + coeff)
        ) - np.log(1000.)
    return np.exp(model)
Returns the centred z1.0 (mu_z1) based on the California model
    (equation 11)