PROJECT DESCRIPTION N 100100 kmkm ew 100100 kmkm S p an or An existing dam in San Diego County is t A In nd g re le 75 km SSS a w 75 km SSSaaa s 25 0 25 being raised to increase its water storage o aaan fa od JJJ u Eart h Consult ants I nternati onal f aaaccc lt a Elsinore fault zone ccciiin z M i l e s u nttt o lt ttto ne capacity. was zo f ffaa n 50 km aau e lllttt E zzo C ar onn o th ee retained as part of the engineering team to S ro q a n u n a ak C d e o 2525 kmkm V le R m B al develop the recommended seismic design o le e a y n n s f t k e a e u f fa C lt a S a u a u L criteria for the project. We conducted the l n a lt n t y z N z D o o Site o Site n n a I n ie e m e g c f i p o a o following tasks 1) prepared a report u e T n r l i r t a o f l z a f u o u a g n l u h t describing the tectonic setting of the site; 2) e lt

f z a o u L n a l g t e u na compiled a fault database that summarizes S al Ca ad B la a Map Explanation a D ba fa Cerro Prieto fault the current state-of-knowledge for all known h e VallecitosVallecitos faultfaults u Historic fault displacement i s a lt a c s f S a au Holocene fault displacement o n l S s t an le o faults within 100 km (62 miles) of the site that Late Quaternary fault displacement I d s i a fa dr d u Quaternary fault, age not differentiated o f l S f a t a au u z n are thought to have moved at least once in Simplified fault, age not differentiated l l o M t t n z z ig on o e u e n e e l f au the past 35,000 years (the California Division lt

Source: Reproduced with permission, California Division of Mines and Geology, CD-ROM 2000-006 (2000), Digital Database of Faults from the Fault Activity Map of California and Adjacent Areas, Jennings, 1994. calculated peak ground accelerations using a Figure 1 (above): Site Location Map showing the fa ults, both onshore and offshore, that have deterministic analysis, and 4) developed the been mapped in the area. The circles illustrate th e area within 25, 50, 75, and 100 km o f t h e acceleration time histories. site, respectively. Our study also considered thos e faults south of the international border that could also pose a seismic hazard to the site.

SOLUTION As part of our study, we compiled all known seismic sources within 100 km of the site (Figure 1) into a database. Each of the faults in the database was classified according to style of faulting, segment length, maximum magnitude earthquake, and closest distance to the dam site. Once these data were developed, we computed the peak ground accelerations that could be experienced at the site for each of the fault segments considered. Our analysis showed that the La and Rose Canyon faults, as well as the Julian segment of the Elsinore fault, are the principal contributors to the seismic hazard at the site. Specifically, rupture of the La fault during an earthquake of moment magnitude (Mw) 6.7 could produce horizontal ground accelerations at the site of about 0.16g (50th 0.3 percentile) and 0.26g (84th percentile), whereas a Mw 7.1 1992 Landers Earthquake Adjusted Horizontal Component (Whitewater Station) earthquake on the Rose Canyon fault, or a Mw 7.5 earthquake on the Julian segment of the Elsinore fault, could produce horizontal peak ground accelerations at the site in excess of 0.13g (50th percentile) and 0.21g (84th percentile). We then selected two natural earthquake records as reasonable 0 analogues to the design earthquakes on the La and Elsinore faults: the Gilroy array from the 1989 Loma Prieta earthquake, and the Whitewater Trout Farm station from the 1992 Landers earthquake (Figure 2). These natural earthquakes were used to model the maximum ground motions expected at -0.3 the site. 0 5 10 15 20 25 30 35 40 Time (sec) Figure 2 (above): Adjusted horizontal time history from the 1992 Landers earthquake used to model the ground motion expected at the site from the Julian segment of the Elsinore fault.

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