def _get_site_classification(self, vs30):
        """
        Define the site class categories based on Vs30. Returns a
        vector of site class values and a dictionary containing logical
        vectors for each of the site classes
        """
        site_class = np.ones(vs30.shape, dtype=int)
        idx = {}
        idx[1] = vs30 > 600.
        idx[2] = np.logical_and(vs30 > 300., vs30 <= 600.)
        idx[3] = np.logical_and(vs30 > 200., vs30 <= 300.)
        idx[4] = vs30 <= 200.
        for i in [2, 3, 4]:
            site_class[idx[i]] = i
        return site_class, idx
Define the site class categories based on Vs30. Returns a
        vector of site class values and a dictionary containing logical
        vectors for each of the site classes