def source_model(freq, M0, fc, n=2, gamma=1):
    """Model for source displacement spectrum (Abercrombie 1995)

    :param freq: frequencies
    :param M0: seismic moment (Nm)
    :param fc: corner frequency
    :param n: high frequency fall-of
    :param gamma: corner sharpness
    """
    return M0 * (1 + (freq / fc) ** (n * gamma)) ** (-1 / gamma)