How do we determine recurrence interval? (continued: dig trenches on active faults. Look at movements that have taken place in the past. Step 2: determine absolute age for fault movement using carbon. 60,000: use other radiometric processes for faults in solid rock years ago) measured decay. It is a subjective determination to decide if a particular movement event was a big earthquake or just a small one. We can turn to other evidence of big earthquake activity that is less subjective. C: look for evidence of ancient tsunami, and of ground subsidence, in soils: 1. Ground elevation and ground subsidence: use carbon 14 dating on soil materials that contain dead, downward shift of land resulting from earthquake, each layer represents big earthquakes trees and plants. Radiocarbon dates suggest that the recurrence interval is 160 years: 9 major events in 1400 years. Next one is predicted to be soon.