Introduction

1. Response of the Groundwater Wells in Alaska to Near and Distant Large Earthquakes Samik Sil (1) and Jeffrey T. Freymueller (2)University Of Alaska Fairbanks, Geophysical Institute, 903 Koyukuk Drive, Fairbanks, AK-99775 • Conclusions • We observed groundwater level changes in Alaska following the Nenana Mountain (October, 2002, Mw 6.7), the Denali fault (November, 2002, Mw 7.9) and the Sumtra-Andaman earthquakes (December, 2004, Mw 9.1-9.2). • All the wells responded to the Denali earthquake, Group-I wells of the Tanana basin area responded to all three earthquakes and Group-II responded to the Denali and the Nenana earthquakes. • 3. Response of EDOP wells and KB6 well to the Denali earthquake can be explained by poroelastic theory. • Response of Group-I wells were similar (coseismic step-like water level increase and gradual postseismic decrease) to all the earthquakes. The average water level steps is proportional to horizontal ground velocities. So dynamic strain from ground shaking caused the water level changes in this group of wells. We also fit the postseismic decay of data using equation 2. A low value of decay constant indicates the pore pressure sources are very near to the wells. • Group-II wells showed gradual increases or decreases in water level following the Nenana and Denali earthquakes. Fitting of postseismic response to equation 2 obtained higher value of z. So we proposed near by fracture formation/cleaning due to ground shaking changed the fluid pressure gradient and thus the chages occurred. • 6. Multiple mechanisms are responsible for the water level changes in Alaska due to near and distant large earthquakes. The Sumatra-Andaman Earthquake Introduction We observed water level changes in 22 groundwater wells in Alaska following the large 2002 Nenana Mountain, the Denali fault and the Sumatra-Andaman earthquakes. Multiple mechanisms are responsible for the variable temporal pattern and the magnitude of the observed water level changes. For the wells in a consolidated confined aquifer system, diffusion of earthquake induced pore pressure is the main cause of water level changes [Roeloffs, 1998]. Whereas for the wells in partially confined unconsolidated aquifer, fracture formation due to an earthquake [Brodsky et al., 2003] and consolidation of unconfined aquifer by earthquake induced dynamic strain (liquefaction) [Manga et al., 2003] are both important mechanisms. For each well the dominant mechanisms are the same for all earthquakes. Though more than 10000 km away, the Sumatra earthquake induced water level changes are very much consistent with the changes due to the local large earthquakes. Along with the determination of the tectonics mechanics of the well water level changes in Alaska, we also determined general hydrological parameters of the wells which can be helpful for future studies. • Basic Theory • 1)Poroelastic Theory of water level changes: • (1) • For a poroelastic material water level change (h) is directly proportional to the volumetric strain( ).B is the Skempton’s coefficient and νu is the undrained Poisson’s ratio. B is estimated from the response of atmospheric pressure changes. • 2)Shaking induced changes: • Dynamic strain: Water level changes are directly proportional to the horizontal ground velocities • Fracture formation: Ground-shaking induced fracture formation can develop a new pore pressure gradient, which diffuses according to the following equation: • where (2) • Here p is the pore pressure and c is the hydraulic diffusivity. Higher value of decay constant (τ) indicates a large distance of pore pressure source (z). Sil and Freymueller [2006] detected step-like water level changes in Group-I wells following the 2004 Sumatra-Andaman earthquake. The changes occurred during the arrival of largest surface waves from Sumatra to Fairbanks. No other wells showed any detectable changes in water level. Fig. 6. Step-like changes are observed in Group-I wells following the Sumatra-Andaman earthquake Discussions Using equation 1 and the calculated volumetric strain (Fig. 4), expected water level changes are calculated for the EDOP and KB6 wells. The expected water level changes from poroelastic theory matches the observed water level changes excellently. This indicates poroelasticity is the cause of water level changes in EDOP and KB6 wells. The Denali Fault Earthquake Fig. 2. Step-like water level drops were observed and estimated at the EDOP wells. Step-like rises were observed and estimated in Group-I and KB6 wells. For the estimation of steps following equation is used to fit the time series: Well locations and aquifers The EDOP wells are drilled into an upland aquifer system contained within highly fractured bedrock consisting of metamorphic and igneous rocks. The well MCGR is situated almost 15 km east of the EDOP wells, and is drilled into a confined aquifer system that consists of Quartz-mica schist of pre-Jurassic age. A cluster of 18 wells are situated southwest of the city of Fairbanks, in the Tanana Valley area of Alaska, which is covered by thick deposits of alluvium and loess. They are all drilled into Quaternary Chena alluvial deposits. The aquifer system is unconsolidated and is considered to be confined during the winter because of the presence of a permafrost layer. During summer seasons the aquifer is not confined and the hydrographs of the wells vary systematically with the variation of the water levels of the near by Chena (Group I wells of the figure) and Tanana rivers (Group II wells). Another well, KB6, is situated 29 km north of Anchorage, drilled into Quaternary sand and gravel. Water level data were collected at an interval of one hour in all of the wells using a submersible pressure transducer at the time of the Denali earthquake. At present, the water level are monitored at 15 minute intervals for the Tanana basin wells. Resolutions of water level measurement are 0.3 mm and 3 mm, for the Tanana basin and other wells respectively. Fig. 7. Comparison between predicted and observed water level changes from the EDOP and KB6 wells using eqn.1. Literature cited Brodsky, E., E. Roeloffs, D. Woodcock, I. Gall, and M Manga, A mechanism for sustained groundwater pressure changes induced by distant earthquake. J. Geophys. Res.,108, 2390, 2003. Manga, M., E. Brodsky, and M, Boone, Response of streamflow to multiple earthquakes, GRL,30,2003. Roeloffs, E., Poroelastic techniques in the study of earthquake related hydrologic phenomena, Advances in Geophysics, 37, 135-189,1996. Sil, S., and J. T. Freymueller. Well water level changes in Fairbanks, Alaska, due to the great Sumatra-Andaman earthquake, EPS, 58, 181-184, 2006. Group-I wells always showed a water level rise after all 3 earthquakes. The average water level rise from all the wells of this group is proportional to the horizontal ground velocities due to 3 studied earthquakes. This indicates dynamic strain is the cause of water level changes in Group-I wells. Fitting of postseismic water level changes with an error function and decay constant (equation. 2) returned a small value of τ, which indicates a highly localized pore pressure source. Fig. 3. Transient changes were observed in Group-II wells after the Nenana Mountain and Denali Fault earthquakes . Fig. 1. Studied well locations with names. RHS is the zoomed view of the red box. The Tanana basin wells are divided into two groups according to their seasonal behaviors. Fig. 7. Comparison between average water level rises and ground velocities for the Group-I wells for all 3 earthquakes. Fig. 4. We determine the volumetric strain due to the Denali fault earthquake. Only EDOP wells and the KB6 well showed the direction of water level changes consistent with poroelastic theory. Water level rise in Group-I wells and transient changes in Group-II wells are not being explained by this theory. Fig. 8. Postseismic water levels of the Group-II wells are modeled with an error function with a decay constant for the Nenana (left) and the Denali (right) earthquakes. Acknowledgments We thank Ms. Heather R. Best of USGS and Dr. David Barnes of UAF for providing us water level data. We are also thankful to Dr. Michael Manga of University of California, Berkeley and Dr. Emily Brodsky of University of California, Los Angeles, for several technical discussion which help to improve this work. This project is supported by National Science Foundation EAR-03/04/0 For further information, contact (1)ftss@uaf.edu (2) jeff@giseis.alaska.edu. The Nenana Mountain Earthquake Fig. 5. After the Nenana Mountain earthquake step-like water level rises and transient changes (Fig.3) were observed in Group-I and Group-II wells respectively. The similar pattern of water level changes from both the earthquakes indicates mechanisms of water level changes are same as for the Nenana Mountain earthquake. Postseismic gradual changes of water level following the Nenana mountain and the Denali earthquake for Group-II wells are fitted with a error function and decay constant. The very high value of the decay constant indicates a larger value of z (equation. 2). We proposed fracture cleaning/formation near well sites and thus changing the hydraulic gradient is the cause of water level changes in this group of wells.