Recent and Long-Term Behavior of the Brawley Fault Zone, Imperial Valley, California: An Escalation in Slip Rate?

Abstract

The Brawley fault zone (BFZ) and the Brawley Seismic Zone constitute 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 1940 M w 6.9 and the 1979 M w 6.4 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. Two trenches were opened and a series of auger holes were bored across three strands of the BFZ at Harris Road to compare the amount of slip observed historically with the displacements observed in the paleoseismic record. Evidence is presented, across the westernmost strand of the BFZ and across the entire BFZ at Harris Road, to show that both the average vertical slip rate observed in modern times (since 1970) and the vertical creep rate (excluding coseismic slip) observed during the 1970s are significantly higher than the long-term average. Across the westernmost strand, the longterm vertical rate is 1.2 ‫)5.0מ/5.1ם(‬ mm/yr, and the average rate since about A.D. 1710 is determined to be no greater than 2.0 mm/yr; in contrast, the average vertical rate between 1970 and 2004 across that strand was at least 4.3 mm/yr, and the 1970s vertical aseismic creep rate was 10 mm/yr. Likewise, across the entire BFZ, the longterm vertical rate is 2.8 ‫)4.1מ/1.4ם(‬ mm/yr, whereas the rate between 1970 and 2004 was at least 7.2 mm/yr, and the 1970s aseismic creep rate was 10 mm/yr. The long-term strike-slip rate cannot be determined across any strands of the BFZ but may be significant. In contrast to the commonly accepted higher sedimentation rates inferred for the entire Imperial Valley, we find that the average sedimentation rate on the downthrown side of the BFZ adjacent to Mesquite Basin, in the millennium preceding the onset of agricultural influences, was at most 3.5 mm/yr. Finally, a creep event occurred on the BFZ during our study in 2002 and is documented herein.

Geodetic investigation into the deformation of the Salton Trough

“…We estimate that the extension within the Mesquite Basin corresponds to 13.8 ± 3.0 mm/yr of subsidence (using a 75° dip angle estimated from Chen and Shearer []), which is roughly in line with estimates of vertical creep since the 1970s along the Brawley Fault from fault trenching by Meltzner et al . [].…”

Subsidence rates at the southern Salton Sea consistent with reservoir depletion

“…Fault scarps are not visible at the surface, so we assume that sedimentation rates are equivalent to or greater than slip rates on high‐angle normal faults averaged over geologic time, which implies that subsidence rates associated with faulting cannot be greater than the highest sediment accumulation rate. While sedimentation rates across the ST generally vary from 2 to 11 mm yr −1 [ Van De Kamp , ; Meltzner et al , ; Dorsey , ], robust late‐Holocene rates of ∼20 mm yr −1 have been observed [ Brothers et al , ] at the depocenter, underneath the Salton Sea, and ∼10 km to the northwest of Obsidian Butte. Slip rates on potentially active structures southeast of the depocenter, toward our model faults, cannot be determined through marine seismic methods because of active CO 2 degassing, as we mentioned before and are wholly unconstrained by geodetic data.…”

San Andreas Fault Earthquake Chronology and Lake Cahuilla History at Coachella, California

“…18) are potentially consistent with the Coachella lake chronology, given the dating uncertainties. Incomplete and less well-dated lake chronologies have been reported from the Superstition Mountain (Gurrola and Rockwell, 1996), International Border (Thomas and Rockwell, 1996), and Brawley (Meltzner et al, 2006) paleoseismic sites (see locations on Fig. 1b), at the 12-m highstand shoreline, 9 m elevation, and 37 m, respectively.…”