How to Reinforce a Wood-Framed House in a Seismic Zone

Some info here which gives some info about securing the two floors together under the heading, “Anchoring systems of shear walls.”

http://www.huettemann-holz.de/application/files/2114/4663/3792/Earthquake_Flyer.pdf

Hold down straps available from here (UK company.) 
https://www.tradecounteronline.co.uk/simpson-strong-tie-stainless-steel-timber-frame-hold-down-strap/