Water level fluctuations induced by tidal strains can be analyzed to estimate the elastic properties, porosity, and transmissivity of the surrounding aquifer material. We review underutilized methods for estimating aquifer properties from the confined response to earth tides. The earth tide analyses are applied to an open well penetrating a confined carbonate aquifer. The resulting range of elastic and hydraulic aquifer properties are in general agreement with that determined by other investigators for the area of the well. The analyses indicate that passive monitoring data from wells completed in sufficiently stiff, low porosity formations can provide useful information on the properties of the surrounding formation.

The pool in Devils Hole is a sensitive indicator of crustal strain and fluctuates in response to changes in atmospheric pressure, earth tides, earthquakes, large-scale tectonic activity and ground-water development. Short-term and cyclic water-level fluctuations caused by atmospheric pressure and earth tides were found to be on the order of millimeters to centimeters. The 1992 Landers/Little Skull Mountain earthquake sequence and the 1999 Hector Mine earthquake induced water-level offsets of greater than )12 and )3.6 cm, respectively. The results of a dislocation model used to compute volumetric strain for each earthquake indicates that the coseismic water-level offsets are consistent in magnitude and sense with poroelastic responses to earthquake-induced strain. Theoretical postseismic fluid-flow modeling indicates that the diffusivity of the system is on the order of 0.03 m 2 sec )1 , and identified areas of anomalous water-level fluctuations. Interpretation of model results suggests that while the persistent post-Landers rise in water-level can be attributed to deformation-induced channeling of fluid to the Devils Hole fault zone, the cause of the pre-Hector Mine water-level rise may be related to postseismic excess fluid pressures or preseismic strain accumulation.

[1] The thickness of the Barbados accretionary prism varies along strike. The increased thickness of the prism to the south is believed to affect the transport of heat by fluids, resulting in anomalously high heat flow at the deformation front. We utilized a coupled three-dimensional numerical fluid flow and heat transport model to investigate the effect of variable accretionary prism thickness on fluid flow patterns and heat transport in the décollement zone. The variation in sediment thickness produced along-strike fluid flow in the more permeable décollement. Although surface heat flow is higher for simulations where flow occurs along both the décollement and underthrust sediments, the resultant advective heat transport was not sufficient to appreciably raise steady state surface heat flow at the latitude of the Ocean Drilling Program transect. Simulated temperatures in the décollement were consequently much lower than in situ borehole measurements, suggesting an additional source of warmer fluid. We then investigated the transient hydrologic and thermal response of the system to enhanced fault permeability. Model results provide insight into the degree to which episodic faulting may drive fluid flow and heat transport within subduction complexes. A transient scenario in which décollement permeability was increased two orders of magnitude with fluid flow along both the décollement and underthrust sediments caused the temperature gradient at the location of the drill sites to increase by a factor of two within 7100 years. The results of the threedimensional numerical simulations indicate that while both along-strike advective heat transport and transient fluid flow events should be considered in the heat flow distribution of the northern Barbados accretionary complex, the effects of episodic fluid flow are more significant.

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