Annual Report, 1997

Characterization of Active Faults
in East Los Angeles

Kerry Sieh, Michael Oskin
California Institute of Technology

Young folds in East Los Angeles represent a system of parasitic structures on the south limb of the active Elysian Park Anticlinorium (EPA.) Through compilation of geologic maps, geotechnical investigation, and geomorphology, we document systematic, active uplift of the south limb of the EPA from Whittier Narrows to Hollywood. Altogether, we interpret the EPA as an active, south-vergent, fault-propagation-style fold.

Recently, we have worked to quantify the growth rate of the anticlinorium and the source characteristics of a hypothesized, kinematically linked reverse fault. Our studies indicate that a "characteristic" rupture of the entire fault underlying the 18 km length of the anticlinorium is capable of producing a Mw 6.5-6.8 earthquake with an average recurrence interval of 1300 3000 years. The size and location of this structure beneath the metropolitan area represents an important, new component in future seismic hazard models.

In East Los Angeles, the parasitic folds constitute a southward-propagating system of imbricate, south-vergent structures. The topographic expression of these anticlines increases southward, whereas the bedrock expression increases northward. The northern two anticlines are characterized by well-developed, asymmetric fold limbs in bedrock, but little or no topographic expression, which suggests that these structures are no longer active. By contrast, the southern two anticlines are characterized by pronounced surface deflection, but comparatively minor bedrock flexure. Using the position and deformation of geomorphic surfaces, we construct a history of the interaction of the fluvial system and the EPA that reflects the foreland (southward) migration of parasitic folding through the Los Angeles River fan [Fig 1.]. Currently, the Los Angeles River occupies a SW-directed fan bounded by uplifted sediments at Boyle Heights and the Wilshire Arch. By restoring surfaces and drainages at Qp, Q1 and Q3 time, we document southward translation of the LA river fan head and SE to SW migration of the fan axis in response to fold growth.

By comparing structures in East Los Angeles with youthful features to the east and west, a consistent pattern of uplift of the south limb of the EPA emerges [fig 2.] Eastward, the south limb of the EPA forms a single, uplifted homocline, capped by folded terrace deposits. Westward, the Elysian Hills are a similar, youthful uplift of the south limb of the EPA. Uplifted terraces and narrow, topographically expressed zones of bedrock flexure in
the Elysian Hills correlate with folds in East Los Angeles. Borehole transects through the Los Angeles River fan confirm a connection between two topographically prominent zones of flexure--the Coyote Pass and Mac Arthur Park escarpments. This flexure trends parallel to bedding along the entire length of the south limb of the anticlinorium. Where the EPA axis reverses plunge in the Elysian Hills, the Coyote Pass-Mac Arthur Park flexure mimics this change by bending sharply to the northwest. Throughout it's length, youthful parasitic folds and flexures are characteristic of the southern limb of the EPA. This deformation style probably reflects shortening of the forelimb of a fault-propagation fold. We interpret the presence and continuity of youthful, compressive structures along the entire, 18kmlength of the southern limb of the EPA to suggest that the entire structure is an active, faultpropagation fold.

To quantify the growth rate of the EPA, we use the deflection of the widely preserved, 60+10ka Q1 surface by the active, parasitic anticlines. In doing so, we assert that the contraction rate of parasitic structure represents a significant component of the total contraction rate of the anticlinorium. From detailed geotechnical study of the Coyote Pass Escarpment in East Los Angeles, we directly measure 17.2+2.1m of uplift and 9.8+0.5m contraction from deformed bedding. By extrapolating the ratio of contraction / uplift to the maximum deflections of the Q1 surface, we estimate 36+10m of contraction by parasitic folding. This corresponds to 0.6iO.3mm/yr of contraction. r

Our results are consistent with published cross-sections by Davis and Namson (1989) and Wright (1991) which suggest that the EPA is produced as a fault propagation fold. Using Davis and Namson's cross-section through downtown Los Angeles as a basis, we hypothesize a 48+15 north-dipping blind reverse fault (herein referred to as the Elysian Park Fault) beneath the western San Gabriel Valley that is kinematically linked to growth of the EPA. We estimate a slip rate of 0.8+0.2mm/yr upon this structure, which is consistent with the geomorphically-derived contraction rate and the long-term uplift of the Puente-Fernando contact.

To estimate a characteristic earthquake for the Elysian Park Fault, we estimate a reasonable fault plane area and use the magnitude-rupture area-displacement regressions for Southern California earthquakes published in Dolan, et al., 1995. The strike length of the Elysian Park Anticlinorium axis is 18km, extending from Whittier Narrows to the Hollywood Fault. Using a range of potential down-dip fault widths, we estimate a fault plane area of 200-400 square kilometers. From the Southern California regression, this range of rupture areas corresponds to Mw 6.5-6.8. We can also crudely estimate a recurrence interval for a characteristic event on the Elysian Park Fault. Using the same set of regressions, we calculate that an event in the M(w) 6.5-6.8 range would display a
characteristic offset of 1.3 to 1.8m per event. Using our estimated slip rate of
0.8+0.2rnm/yr, the characteristic offset represents 1300 to 3000 years of accumulated
strain

In the past year, we have made significant progress towards publication of our

research on active folding in Downtown and East Los Angeles. Also, by expanding our
analysis of parasitic structures in Northern Los Angeles Basin, we have broadened our
understanding of the Elysian Park Anticlinorium. We now conclude that this structure is an
active fault-propagation fold. We propose the Elysian Park Fault, a blind reverse fault, is
kinematically linked to growth of the anticlinorium. This structure, which is comparable to
the rupture plane of the 1994 Northridge Earthquake, represents a sign)ficant seismic
hazard to the Los Angeles Metropolitan region in general, and to the Downtown, East
L.A., and Hollywood areas in particular.

Selected References:

Bullard, T. F. and Lettis, W. R., 1993, Quaternary Fold Deformation Associated With Blind Thrust Faulting, Los Angeles Basin, California: Journal of Geophysical Research, v. 98, n. B5, p. 8349-8369.

Davis, T. L., et al., 1989, A Cross Section of the Los Angeles Area: Seismically Active Fold and Thrust Belt, The 1987 Whittier Narrows Earthquake, and Earthquake Hazard: Journal of Geophysical Research, v. 94, n. B7, p. 9644-9664.

Dibblee, T. W., Jr., 1989, Geologic Map of the Los Angeles Quadrangle, Dibblee Geological Foundation, Santa Barbara, California.

Dolan, J. F., et. al., 1995, Prospects for larger or more frequent earthquakes in the Los Angeles Metropolitan Region: Science, v. 267, p. 199-205.

Miller, G., et al., 1996, Fault Investigation for: Engineering Design Program, Eastside Extension, Metro Red Line Project: Geotransit Consultants, Long Beach, California.

Quarles, M., Jr., 1941, Geology of the Repetto and Montebello Hills: California Institute of Technology Master's Thesis.

Soper, E.K., and Grant, U.S., 1932, Geology and paleontology of a portion of Los Angeles, California: Bulletin of the Geological Society of America, v. 43, p. 1041-1068.

Wright, T. L., 1991, Structural Geology and Tectonic Evolution of the Los Angeles Basin, California: Active Margin Basins, K. Biddle, ea., American Association of Petroleum Geologists Memoir 52, p. 35-134.