Vulnerability to natural disasters is increasing globally1-3. In parallel, the responsibility for natural hazard preparedness has shifted to communities and individuals4. It is therefore crucial that households increase their preparedness, yet adoption of household preparedness measures continues to be low, even in high-risk regions5-8. In addition, few hazard-preparedness interventions have been evaluated longitudinally using observational measures. Therefore, we conducted a controlled intervention with a 12-month follow-up on adults in communities in the United States and Turkey that focused on improving household earthquake and fire preparedness. We show that this Fix-it intervention, involving evidence-based, face-to-face workshops, increased multihazard preparedness in both cultures longitudinally. Compared to baseline, the primary outcome-overall preparedness-increased significantly in the intervention groups, with more improvement in earthquake preparedness in the Turkish participants and more improvements in fire preparedness in the US participants. High baseline outcome expectancy and home ownership predicted overall preparedness change in both intervention groups longitudinally, implying that a sense of agency influences preparedness. An unintended consequence of observation is that it may increase preparedness, as even the control groups changed their behaviour. Therefore, observation of home preparatory behaviours by an external source may be a way to extend multihazard preparedness across a population.