code: def _get_site_class(self, sites):
          
          vs30 = sites.vs30
          S = np.zeros_like(vs30)
          S[vs30 <= 760] = 1
  
          return S
docs: Return site class flag (0 if vs30 > 760, that is rock, or 1 if vs30 <=
          760, that is deep soil)