Japan is really geared up for earthquakes I must say. I spent around 1.5 years there and experienced around 4 tremors, two of them quite big/long. Big to me, water off a ducks back to the locals!
I understand your relatives concern, but the issue is much smaller than she might think. Much of Tokyo was levelled in WW2 (and afterwards), and most modern buildings are designed to be earthquake resistant.
- Example, the block of flats I lived in was built as a 20m*20m 'square cylinder' around a central courtyard, 20-30 floors high. Like a big tube. Much stronger than a standard building.
- My office was more of a 'standard building' design, but on the top floor (circa 30th). The whole floor was filled with huge metal dampers that would oscillate in the opposite way to any ground/movements and hence cancel them out. Amazing really.
At my flat. The Fire Brigade came down and we did a statutory annual 'earthquake evacuation drill'. We had to put on hoods and stuff and they sprayed out loads of imitation smoke, it was actually quite a laugh.
Anyway, you get the picture, to the Japanese it is not really a big deal.
What you relative might do, is wherever her family members might be, ensure all of them have practised the fire exit routes, and know where the 'muster points' are. If a young child, check that day-care teachers can explain this (to show they know...(but you can assume they will, it is so drilled into people)).
At home, keep a torch on hand and some emergency rations, esp. water. One of those torches that works my shaking, and does not rely on normal batteries that might go flat is a much better idea.
Other than that, don't worry too much. Japan is a fantastic place, and the people are lovely. 8 months there .... oh I am green with envy! Long enough to get a hang of the place, not too long to get jaded :-)
HTH