def _check_seismogenic_depths(self, upper_depth, lower_depth):
        '''
        Checks the seismic depths for physical consistency
        :param float upper_depth:
            Upper seismogenic depth (km)

        :param float lower_depth:
            Lower seismogenic depth (km)
        '''
        # Simple check on depths
        if upper_depth:
            if upper_depth < 0.:
                raise ValueError('Upper seismogenic depth must be greater than'
                                 ' or equal to 0.0!')
            else:
                self.upper_depth = upper_depth
        else:
            self.upper_depth = 0.0

        if not lower_depth:
            raise ValueError('Lower seismogenic depth must be defined for '
                             'simple fault source!')
        if lower_depth < self.upper_depth:
            raise ValueError('Lower seismogenic depth must take a greater'
                             ' value than upper seismogenic depth')

        self.lower_depth = lower_depth
Checks the seismic depths for physical consistency
        :param float upper_depth:
            Upper seismogenic depth (km)

        :param float lower_depth:
            Lower seismogenic depth (km)