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Description
The Imperial fault is the only fault in southern California to have ruptured during two major earthquakes in the 20th century. In 1940, it ruptured end-to-end in an Mw 6.9 earthquake, and in 1979, the northern segment of the fault ruptured again in an Mw 6.4 event. Little is known about the northern Imperial fault prior to 1940, although paleoseismic investigations on the central Imperial fault have revealed that the penultimate event there occurred around A.D. 1680. I excavated a trench across the northern Imperial fault south of Harris Road, adjacent to Mesquite Basin, where the fault has both dextral and normal slip components. On the downdropped side of the fault, a laminated clay unit (inferred to be the most recent Lake Cahuilla clay, at ca. A.D. 1680) was exposed and is overlain by more than 1.5 m of younger overbank and colluvial deposits. A series of fissures and flower structures adjacent to the main fault in the hangingwall block permits the distinction of individual earthquakes. There is evidence for four events on the northern Imperial fault since the most recent Lake Cahuilla highstand, including the 1979 and 1940 earthquakes. I see no evidence in the trench for regularly repeating 1979-type events as suggested by the slip-patch model of non-characteristic earthquakes with characteristic slip. The Brawley fault zone (BFZ) and the Brawley Seismic Zone comprise the principal transfer zone accommodating strain between the San Andreas and Imperial faults in southernmost California. The BFZ ruptured along with the Imperial fault in the 1979 Mw 6.4 and the 1940 Mw 6.9 earthquakes, although in each case only minor slip apparently occurred on the BFZ; several other episodes of slip and creep have been documented on the BFZ historically. Until this study, it has been unclear whether the past few decades reflect average behavior of the fault. I opened two trenches and a series of auger boreholes across three strands of the BFZ at Harris Road, and I compared the amount of slip observed historically with the displacements observed in the paleoseismic record. I present evidence that the vertical slip rate documented in the 1970s is significantly higher than the long-term average; the long-term vertical rate across the BFZ at Harris Road is 2.8 (+4.1/–1.4) mm/yr. During initial reconnaissance work, I observed a subtle, modified scarp continuing northward from the mapped northern terminus of the BFZ, which may project toward the San Andreas fault at Bombay Beach. Ground-penetrating radar (GPR) profiles at one site 3 km north of the mapped terminus appear to agree with the surface geomorphology and support the presence of a fault, and a seismic refraction line 7 km north of the terminus is also consistent with the presence of a fault zone but is inconclusive. Seismic reflection lines and cone penetrometer tests (CPTs) at sites farther north were inconclusive. If the BFZ continues more than 7 km north of its previously mapped northern terminus, it does not have an appreciable dip component of slip.
