Effects of Loma Prieta Earthquake on Surface Waters in Waddell Valley1

ABSTRACT: Waddell Creek is a perennial stream near Santa Cruz, California, which discharges into the Pacific Ocean about 38 km from the epicenter of the Loma Prieta earthquake (October 17, 1989, M 7.1). The earthquake caused no significant changes or damage to structures or surface features in the area, but was followed by an order of magnitude transient increase in discharge of Waddell Creek and nearby springs. The transients approximate a step rise followed by an exponential recession with time constants of from 37 to 59 days. The flow recession pattern of Waddell Creek was obscured by heavy rains after about 50 days; however, the springs maintained an exponential recession with minimal rain interference for periods of several time constants, then abruptly ceased flowing. A comparison of the pre‐quake and post‐quake chemical signature of one of the springs shows only minor changes in water chemistry indicating a common source for pre‐ and post‐quake waters. Increased permeability of the subsurface ground material appears to be the most probable cause of the hydrologic changes. Post‐seismic reduction of permeability has not been observed or inferred.