def _read_moment_tensor_from_ndk_string(ndk_string, system='USE'):
    """
    Reads the moment tensor from the ndk_string representation
    ndk_string = [Mrr, sigMrr, Mtt, sigMtt, Mpp, sigMpp, Mrt, sigMrt, Mrp,
                  sigMrp, Mtp, sigMtp]
    Output tensors should be of format:
        expected = [[Mtt, Mtp, Mtr],
                    [Mtp, Mpp, Mpr],
                    [Mtr, Mpr, Mrr]]
        sigma = [[sigMtt, sigMtp, sigMtr],
                 [sigMtp, sigMpp, sigMpr],
                 [sigMtr, sigMpr, sigMrr]]
    Exponent returned in Nm

    :param str ndk_string:
        String of data in ndk format (line 4 of event)
    :param str system:
        Reference frame of tensor Up, South, East {USE} or North, East, Down
        (NED)
    """
    exponent = float(ndk_string[0:2]) - 7.
    mkr = np.array([2, 9, 15], dtype=int)
    vector = []
    for i in range(0, 6):
        vector.extend([
            float(ndk_string[mkr[0]:mkr[1]]),
            float(ndk_string[mkr[1]:mkr[2]])])
        mkr = mkr + 13
    vector = np.array(vector)
    mrr, mtt, mpp, mrt, mrp, mtp = tuple(vector[np.arange(0, 12, 2)])
    sig_mrr, sig_mtt, sig_mpp, sig_mrt, sig_mrp, sig_mtp = \
        tuple(vector[np.arange(1, 13, 2)])

    tensor = utils.COORD_SYSTEM[system](mrr, mtt, mpp, mrt, mrp, mtp)
    tensor = (10. ** exponent) * tensor

    sigma = utils.COORD_SYSTEM[system](sig_mrr, sig_mtt, sig_mpp,
                                       sig_mrt, sig_mrp, sig_mtp)
    sigma = (10. ** exponent) * sigma

    return tensor, sigma, exponent
Reads the moment tensor from the ndk_string representation
    ndk_string = [Mrr, sigMrr, Mtt, sigMtt, Mpp, sigMpp, Mrt, sigMrt, Mrp,
                  sigMrp, Mtp, sigMtp]
    Output tensors should be of format:
        expected = [[Mtt, Mtp, Mtr],
                    [Mtp, Mpp, Mpr],
                    [Mtr, Mpr, Mrr]]
        sigma = [[sigMtt, sigMtp, sigMtr],
                 [sigMtp, sigMpp, sigMpr],
                 [sigMtr, sigMpr, sigMrr]]
    Exponent returned in Nm

    :param str ndk_string:
        String of data in ndk format (line 4 of event)
    :param str system:
        Reference frame of tensor Up, South, East {USE} or North, East, Down
        (NED)