WorldWide Drilling Resource

Tales from the Field by Jeremy C. Wire Geoconsultants, Inc. Water Wells in Earthquake Country The recent pair of magnitude 6.4 and 7.1 earthquakes that rocked Southern California, once more focuses our attention on these destructive events, after a period of relatively low earthquake activity in the region. Surface damage, which occurred in recent events, is shown in the news media, but little atten- tion is devoted to water facilities, except maybe in videos showing waves of sloshing water overflowing swimming pools. How- ever, water wells and related infrastructure can sustain significant damage from ground shaking during a large magnitude earthquake. An earlier “Tales” ( Two Water Wells and an Earthquake , November 2018 WWDR ) related the effect on two deep wells at least 80 miles from an earthquake epicenter, resulting in the collapse of the bottom open hole section of one of the wells, but caused absolutely no damage to the nearly identical well relatively close by. Other examples of damage to wells and infrastructure from a strong earthquake were associated with the magnitude 7.1 Loma Prieta earthquake, which hit the San Francisco Bay area in October 1989. On a local ridge occupied by country homes a few miles north of the epicenter, domestic water wells and tanks were particularly affected by the energy of the shaking along the crest. The bottom sections of some partially empty steel tanks were compressed in wrinkles much like an accordion; other poly tanks were thrown off their founda- tions and rolled down nearby slopes into neighboring properties, to be retrieved by their owners. Most domestic wells with PVC casings were damaged, with submersible pumps parting from their drop pipes and safety cables then dropping to the bot- tom of the well, in some cases never to be retrieved again. Concurrently, water lev- els dropped in many wells because of the opening and extension of fractures in the bedrock underlying the ridge. Springs appeared lower down on the flank of the ridge where none had existed before. In several cases, it took many months for water levels to recover anywhere near the pre-earthquake levels. In a lowland area, further removed from the epicenter, a test hole over 1000 feet deep had just been reamed to a diameter of 26 inches in unconsolidated materials and was waiting to be cased. We felt for sure the borehole had col- lapsed during the strong shaking and aftershocks that occurred in the area. However, the next day the drill operator was able to trip back into the borehole to total depth, as if nothing had happened. The occurrence of damage during strong shaking can vary, depending on local subsurface conditions. Considering the most recent events, it will be inter- esting to learn of any reported damage to wells and related infrastructure caused by the strong shaking and aftershocks which affected Southern California. Jeremy michele@ worldwidedrillingresource.com 23 WorldWide Drilling Resource ® SEPTEMBER 2019 WTR