Study objective: Over large expanses, the risk for hospital damage from an earthquake attenuates as the distance from the epicenter increases, which may not be true within the immediate disaster zone (near field), however. The following study examines the impact of epicenter distance and ground motion on hospital evacuation and closure for those structures near the epicenter of the 1994 Northridge Earthquake and the implications for patient evacuation.
Methods: This is a retrospective case-control study of all hospitals reporting off-site evacuations or permanent closure because of damage from the January 17, 1994, earthquake in Northridge, CA. Control hospitals were randomly identified from those facilities that did not evacuate patients. Distances from the epicenter and peak ground accelerations were calculated for each hospital from Trinet ShakeMap data and compared.
Results: Eight hospitals evacuated patients (study group); 4 of these hospitals were condemned. These were compared to 8 hospitals that did not evacuate patients (control group). The median epicenter-to-hospital distance for evacuated facilities was 8.1 miles (interquartile range [IQRs] 4.0 to 17.2 miles), whereas that for nonevacuated facilities was 14.1 miles (IRQ 10.5 to 17.0 miles). The difference in the median distances was 6.0 miles (95% confidence interval -4.8 to 11.9 miles). The peak ground acceleration had a median of 0.77 g (IQR 0.53 to 0.85 g) for study hospitals and a median of 0.36 g (IQR 0.24 to 0.50 g) for control hospitals, where 1 g equals the force of gravity. The difference in median acceleration of 0.41 g (95% CI 0.14 to 0.55 g) was significant (P=.009).
Conclusion: The distances from the epicenter for evacuated or condemned facilities and control hospitals do not appear to differ in the near field. Peak ground acceleration is a superior indicator of the risk for hospital damage and evacuation. Physicians can obtain these data in real time from the Internet and should transfer patients to facilities in areas of lower recorded peak ground acceleration regardless of distance from the epicenter.