I feel the house is really a safe place to be in an earthquake. As a solid redwood built home, it flexed and absorbed the shock of earthquakes when I lived there. Which was very reassuring as I had moved to Eureka after horrid experiences with earthquakes in the early 80's Mammoth Lakes, California. After leaving Eureka I moved to the San Francisco/Bay Area and was there during the big quake - which was scary. The construction of the S street house is optimal for safety in earthquakes, but there are probably a few inexpensive improvements which could be made. I hope this offers you some comfort related to your Grandmother's safety. :)
On another note, however, I must take issue with anyone who suggests that a 100 year old home is safer than homes built according to the standards imposed after the 1994 Northridge Earthquake. We've been providing seismic retrofitting services on old homes from bungalows to mansions from San Diego to Santa Barbara since the mid 80's and I can tell you first hand that many if not most homes built from circa '40's and older were never properly affixed to their foundation and can and often times do fall off their foundation resulting in a total loss.
If there are homes built during that period that are still standing it's not because of their redwood superiority albeit full sized heart redwood is by far one of the best ways to build. But a structure is only as strong as its weakest point and that point is where the plate meets the stem wall is the most vulnerable if not bolted. Additionally if the home is built on a cripple wall and it is not shear paneled it will fall over like a house of cards.
However, if you give me some basic information and possibly email some photos of the home I would be happy to give you some suggetions that may help you. My email address is email@example.com. I would be happy to do a visual and offer up some ideas at no cost to you.
However, bear in mind that there is no such thing as "Earthquake Proofing" any structure and I would not assume any responsibiity for any damages your home might incur if in the path of an Earthquake. These are only suggestions. For a more accurate study you will want to contact an earthquake engineering company in your area for a thorough evaluation of the property.
Yes, living in that region in California you are more susceptible to earthquakes but everyone there is taking that risk. It is your decision whether or not you wish to live in an area that experiences earthquakes.
Do make sure that the hot water heater is strapped to the walls.
Good luck to you!
Prudential Connecticut Realty