Acquifers and fault lines, oh my!

Earthquakes
The Raymond fault is considered to be the most important fault with regard to the potential for causing surface rupture in the area of the alternatives. In addition, two potentially active faults are present in the SR 710 North Study Area: the Eagle Rock and San Rafael faults. For the purposes of this study, it is assumed that the Eagle Rock and San Rafael faults are also active. The Raymond, Eagle Rock, and San Rafael faultscross the Freeway Tunnel Alternative at tunnel depth. The Raymond and San Rafael faults cross the Light Rail Transit Alternative at tunnel depth, and the BRT Alternative at the surface. Strong ground shaking may occur in theSR 710 North Study Area as the accumulated strain on these and other regional faults is released. Page 3 SR 710 Geologic Hazard Evaluation.pdf
Ground water
Zone 3 (South Pasadena/Pasadena): The Raymond and San Rafael faults are groundwater barriers in this area, and there is one active and two potentially active faults in the area. (EIR page http://www.dot.ca.gov/dist07/710study/pdfs/Executive%20Summary%20SR-710%20Tunnel%20Draft%20Geotechnical%20Summary%20Report-2.pdf
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The purpose of the geotechnical study is to determine the geologic, groundwater, and seismic conditions within the selected study zones to identify factors that affect the geotechnical feasibility of designing and constructing the proposed tunnel, and to provide a basis for a comparison of the geologic conditions with respect to tunneling design and construction.”
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Zone 3 straddles three separate groundwater basins: the Central Sub-basin in the southwest; the San Gabriel Valley Groundwater Basin in the southeast; and the Raymond Groundwater Basin in the north.
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Groundwater levels vary considerably across the study area and occur as deep aquifers and as shallow perched zones. Several of the faults within the study area act as groundwater barriers with different levels on either side of the fault. 
For more info on barrier action read this report:
 
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