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Coseismic Water Level Changes Due to November 2002 Denali Earthquake - Abstract
The Mw 7.9 Denali earthquake on November 3, 2002 was the strongest event of that year. Water level changes due to this earthquake have been reported from Seattle, New Orleans, Pennsylvania and Canada. We have studied the water level changes of 23 continuously monitored wells due to this earthquake. Most of these wells are located near Fairbanks, in the same unconsolidated aquifer system, but 3 wells are in a confined consolidated aquifer. Coseismic water level changes of those 3 wells can be explained by poroelastic theory alone. That is, the water level changed by an amount proportional to the volumetric strain (water level increased in the compression zone and decreased in the dilatation zone). The rest of the wells showed an increase in water level, opposite in sign to the prediction of poroelastic theory. We found most of these anomalous wells showed step like increases in water level due to the earthquake. The coseismic step like changes in water levels can be estimated using a combination of linear and Heaviside function. The magnitude of estimated water level rises can not be explained purely by liquefaction/ground shaking theory too.
After Denali earthquake, strong ground liquefaction was observed in different parts of Alaska. We have found that liquefaction pattern of Denali earthquake has a very good correlation with our calculated volumetric strain map. Areas of high liquefaction fall in the zone of compression and areas of low liquefaction fall in the dilatation zone. It appears from the correlation that, magnitude of liquefaction is related to poroelastic water level changes.
To explain the observed step like water level changes in the wells, we made the hypothesis that observed increment in water level is a combined effect of poroelasticity and ground shaking liquefaction. To test our hypothesis, we corrected the observed water level rises for the changes predicted by poroelasticity. These corrected water level changes show a good correlation with distance between wells and point of maximum slip on the fault. Residual water level changes were inversely proportional to the square of these distances. This is commonly observed in the case of water level changes caused by ground shaking. So we conclude that the observed ground water level rise due to Denali earthquake was caused by a combination of poroelastic and ground shaking liquefaction phenomena.
Acknowledgments and Associated Footnotes
Copyright © 2014 by the Alaska Geological Society