EARTHQUAKE HAZARDS OF ACTIVE BLIND-THRUST FAULTS UNDER THE CENTRAL LOS-ANGELES BASIN, CALIFORNIA

We document several blind-thrust faults under the Los Angeles basin th at, if active and seismogenic, are capable of generating large earthqu akes (M = 6.3 to 7.3). Pliocene to Quaternary growth folds imaged in s eismic reflection profiles record the existence, size, and slip rates of these blind faults. The growth structures have shapes characteristi c of fault-bend folds above blind thrusts, as demonstrated by balanced kinematic models, geologic cross sections, and axial-surface maps. We interpret the Compton-Los Alamitos trend as a growth fold above the C ompton ramp, which extends along strike from west Los Angeles to at le ast the Santa Ana River. The Compton thrust is part of a larger fault system, including a decollement and ramps beneath the Elysian Park and Pales Verdes trends. The Cienegas and Coyote Hills growth folds overl ie additional blind thrusts in the Elysian Park trend that are not clo sely linked to the Compton ramp. Analysis of folded Pliocene to Quater nary strata yields slip rates of 1.4 +/- 0.4 mm/yr on the Compton thru st and 1.7 +/- 0.4 mm/yr on a ramp beneath the Elysian Park trend. Ass uming that slip is released in large earthquakes, we estimate magnitud es of 6.3 to 6.8 for earthquakes on individual ramp segments based on geometric segment sizes derived from axial surface maps. Multiple-segm ent ruptures could yield larger earthquakes (M = 6.9 to 7.3). Relation s among magnitude, coseismic displacement, and slip rate yield an aver age recurrence interval of 380 years for single-segment earthquakes an d a range of 400 to 1300 years for multiple-segment events. If these n ewly documented blind thrust faults are active, they will contribute s ubstantially to the seismic hazards in Los Angeles because of their lo cations directly beneath the metropolitan area.