Slip variability and temporal clustering along the Imperial fault at Mesquite Basin, Imperial Valley, California

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

Submitted September 11, 2022, SCEC Contribution #12568, 2022 SCEC Annual Meeting Poster #114

Paleoseismic trenches across the Mesquite Basin section of the Imperial fault revealed channels that cross the fault at high angle and that are displaced in the subsurface. These channels incised into and are embedded within lacustrine strata associated with fillings of Lake Cahuilla. Three-dimensional excavation of these channels 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 in each event at the site. A small rill, co-located with 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 slip of 1.4–1.5 m in each event. A younger channel complex contains a sequence of nested channels that suggest two additional surface ruptures—the fourth and third events back (E4 and E3)—with smaller displacements than in E6 and E5.

The ages of these past surface ruptures are constrained by local 14C dating, by the stratigraphic relationship of each event horizon to lacustrine intervals, and by the regional late Holocene lake history. The youngest pre-1940 event (E3) occurred when the site (at –32 m elevation) was underwater, around the time of the most recent Lake Cahuilla highstand, ca. 1732 CE. Events E4–E6 all occurred between then and ca. 1520 CE. The oldest channel, beheaded by E6, is offset a total of ~5.0 m. 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 ~1520 and ~1732 CE. These results imply that slip per event at the site ranges from about 0.15 to 1.5 m, and the slip rate for the Imperial fault in the Mesquite Basin is about 1 cm/yr for the past five centuries (but it would be higher near the International Border).

We also identified evidence for 6–7 earlier events between ~920 and ~1490 CE. Evidence includes upward termination of fault strands, surface fissures, folding, angular unconformities, and massive liquefaction events.

Key Words
earthquake recurrence, earthquake clustering, slip per event

Citation
Meltzner, A. J., Rockwell, T. K., Tsang, R. Y., & Figueiredo, P. M. (2022, 09). Slip variability and temporal clustering along the Imperial fault at Mesquite Basin, Imperial Valley, California. Poster Presentation at 2022 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology