What is the test for "reasonably reasonable" liability for damages caused by natural causes?

I think, however, that it is unfortunate that this salutary principle of tort law which is consistent with, but an extension, of our holding in Rowland v. Christian (1968) 69 Cal.2d 108, 70 Cal.Rptr. 97, 443 P.2d 561, is expressed within the circumstances alleged in this case. I find it exceedingly difficult to imagine what respondents reasonably could have done to prevent or reduce the damage caused by the natural condition here present.