Slip variability and temporal clustering along the Imperial fault at Mesquite Basin, Imperial Valley, California, and possible through-going rupture to the San Andreas fault

Aron J. Meltzner, Thomas K. Rockwell, Rebecca Y. Tsang, & Paula M. Figueiredo

Submitted August 12, 2016, SCEC Contribution #6728, 2016 SCEC Annual Meeting Poster #124

Paleoseismic trenches across the Mesquite Basin section of the Imperial fault revealed several channels that cross the fault at a high angle and that are displaced in the subsurface. These channels incised into and are embedded within lacustrine strata associated with filling events of Lake Cahuilla. Three-dimensional excavation of these channels has yielded information on slip in the past six surface ruptures. Displacement is well documented for the 1940 and 1979 events, with 15–20 cm of coseismic lateral slip occurring in each event at the site. A small rill, which also corresponds to the feeder channel for older, buried beheaded channels, is deflected by ~60 cm, which we attribute to the 1940 and 1979 events plus creep and afterslip.

In contrast to the modern deflected channel, two subsurface channels, each measuring ~50 cm in width, are completely beheaded by slip on the fault, with no evidence for rounding of the channels or flow along the fault. This relationship argues that displacement in each of the corresponding prehistoric events—the sixth and fifth events back (E6 and E5)—exceeded 50 cm; the channel spacing suggests displacements of 1.4–1.5 m in each of the these events. A younger channel complex contains a sequence of nested channels that suggest two surface ruptures—the fourth and third events back (E4 and E3)—each with smaller displacements than in E6 and E5.

The ages of these past surface ruptures are constrained by their stratigraphic relationship to lacustrine intervals, by local 14C dating, and by analysis of the regional late Holocene lacustrine stratigraphy. The youngest pre-1940 event (E3) occurred when the site (at –32 m elevation) was underwater, likely around the time of the most recent Lake Cahuilla highstand, AD 1711–1714. Events E4–E6 must all have occurred between then and ca. AD 1500. The oldest channel, beheaded by E6, is offset a total of ~5.0 m from the active feeder channel. Hence, the northern Imperial fault has sustained ~5.0 m of slip in the past 500 years, with the majority (~4.4 m) occurring in the four earlier events between about AD 1500 and 1714.

These results imply that: (1) the 1940 and 1979 displacements were not “characteristic” for the northern Imperial fault; (2) slip per event at the site ranges from about 0.15 to 1.5 m; (3) the slip rate for the Imperial fault in Mesquite Basin is about 1 cm/yr for the past five centuries, but the rate is necessarily higher near the International Border; and (4) some paleoseismic displacements are larger than expected for a site so close to the northern terminus of the fault, suggesting that those ruptures may have extended beyond the Imperial fault. Specifically, the size of two large paleoseismic displacements on the Imperial fault and the similarity of their timing with the timing of the two most recent events on the southern San Andreas fault at Coachella argue that through-going rupture scenarios must be considered.

Key Words
Imperial fault, paleoseismology, slip-per-event, recurrence intervals

Meltzner, A. J., Rockwell, T. K., Tsang, R. Y., & Figueiredo, P. M. (2016, 08). Slip variability and temporal clustering along the Imperial fault at Mesquite Basin, Imperial Valley, California, and possible through-going rupture to the San Andreas fault. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Southern San Andreas Fault Evaluation (SoSAFE)