def get_centroids(self, ridx):
        """
        :returns: array of centroids for the given rupture index
        """
        centroids = []
        with h5py.File(self.source_file, "r") as hdf5:
            for idx in ridx:
                trace = "{:s}/{:s}".format(self.idx_set["sec"], str(idx))
                centroids.append(hdf5[trace + "/Centroids"].value)
        return numpy.concatenate(centroids)
:returns: array of centroids for the given rupture index