EARTHQUAKE MAGNITUDE AND RECURRENCE FROM SCARP MORPHOLOGY, EUREKA VALLEY FAULT ZONE, EASTERN CALIFORNIA

The Eureka Valley fault zone (EVFZ) is a normal-oblique fault zone located in northern Death Valley National Park, eastern California. Geodetic models infer the northeast-striking EVFZ is a significant structure in a complex area of the western Basin and Range Province, transferring regional slip from the Owens Valley fault zone to the Fish Lake Valley fault zone with a slip rate of 3-3.5 mm/year. The EVFZ is the source of a 1993 M_w 6.1 earthquake with an epicenter in the western valley near the base of the Inyo Range; however, limited or insignificant ground rupture was reported. Maps and imagery display discontinuous faults offsetting Quaternary deposits in the eastern part of the valley along the west side of the Last Chance Range. In the southeastern portion of Eureka Valley, fault scarps offset landslide and alluvial-fan deposits composed mainly of Paleozoic limestones. The offset alluvial fans have well developed desert pavements and pitted limestone clasts consistent with ~70 ka alluvial fans in Death Valley, The lateral levee of the landslide is the oldest unit (profile EV-3) and is offset 18.7 m vertically. Two profiles across the ~70 ka alluvial fan showed two events. Profile EV-1 has offsets of 5.5 m and 4.0 m (9.5 m total). Profile EV-4 has offsets of 6.4 m and 5.3 m (11.7 m total). A third profile (EV-3) shows only one 10.4 m event. All three profiles were completed in close proximity and it is unclear whether two events were discernible at EV-3. Younger deposits morphologically correlative with ~30 ka fan surfaces of Death Valley are not faulted. The cross-cutting relations show that faulting occurred between ~70 ka and ~30 ka with no faulting in the last 30 ka. The measured offsets yield a 70 ka-30 ka slip rate of 0.2-0.3 mm/yr. Regression analysis yields event magnitudes of 7.14-7.63 for normal faults. The cross cutting relations also indicate that this portion of the EVFZ is inactive. Considering orientation of the observed surface rupture and aftershock pattern from the 1993 event, it appears that the southern EVFZ is being cut off by a developing cross valley fault in this extensional basin.

Scarp	Event A	Mw Event A	Event B	Mw Event B
EV-1	5.5	7.24	4.0	7.14
EV-2	18.7	7.63
EV-3	10.4	7.44
EV-4	6.4	7.29	5.3	7.23