Timing and slip for prehistoric earthquakes on the Superstition Mountain Fault, Imperial Valley, southern California

Abstract

Trenches excavated across the Superstition Mountain fault in the Imperial Valley, California, have exposed evidence for four prehistorical earthquakes preserved in displaced lacustrine stratigraphy associated with ancient Lake Cahuilla. The presence of shoreline peat accumulations along with abundant detrital charcoal allows for high-precision age determination of some stratigraphic units, thereby providing constraints on the timing of three of the palcoearthquakes. These three events occurred within a 480-to 820-year interval during the past 1200 years. The most recent earthquake (event 1) occurred during a fluvial phase of deposition between A.D. 1440-1637, immediately prior to the inundation of the Cahuilla basin at about A.D. 1480 and 1660. A channel margin was offset 2.2 +0.4/-0.15 m in this rupture, suggesting an earthquake with a magnitude & 7. The penultimate event (event 2) also occurred during fluvial deposition after A.D. I280 but before another lakestand at A.D. 1440-1640. Lateral slip could not be resolved for event However, based on juxtaposition of dissimilar units and the amount of deformation producedby this event, it is presumed that this was also a large earthquake. The timing of event 3 is constrained to have occurred between about A.D. 820 and 1280. This event is represented by several fractures and small displacements that rupture up to a distinct stratigraphic level or event horizon. Slip was not resolved for this event. Finally, the timing of event 4 is very poorly constrained to between A.D. 964 and 4670 B.C. Undoubtedly, many events may have occurred during this period. Notably, the past three earthquakes occurred within a period of less than 820 years, and it has been over 350 years since the last earthquake.

Late Quaternary slip rate gradient defined using high‐resolution topography and 10Be dating of offset landforms on the southern San Jacinto Fault zone, California

“…Combining slip rates across a transect from the southern Santa Rosa Mountains locality (1.5 ± 0.4 mm/yr) with previously determined Holocene slip rates for the Coyote Creek fault (∼1-5 mm/yr) implies a southward slip rate decrease across the southern SJFZ, from ∼10-14 mm/yr in its central portion to ∼2-7 mm/yr near the fault at the latitude of the Borrego and Fish Creek badlands ( Figures 1 and 2); however, the residual strain in this region is presumably accommodated through folding and thrusting in the adjacent Borrego Badlands basin [Belgarde and Janecke, 2006]. Farther south, the entire 8-15 mm/yr may be accommodated by slip along the Superstition Hills and Superstition Mountain faults [Gurrola and Rockwell, 1996;Hudnut and Sieh, 1989], at least in the Holocene. Comparing these results with the 15.9 ± 3.4 mm/yr slip rate determined over the same late Quaternary time interval for the Indio segment of the San Andreas fault [van der Woerd et al, 2006] suggests that the SJFZ is probably subordinate to the southern San Andreas fault zone, although it is also possible (within uncertainties) that deformation is partitioned fairly evenly between the two.…”

“…Inset shows the index map for major faults in southern California. References are labeled as follows: a, Clark [1972], Sharp [1981], and Pollard and Rockwell [1995]; b, Hudnut and Sieh [1989]; and c, Gurrola and Rockwell [1996]. Figure 2.…”

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

“…Although there is evidence for minor scouring and local erosion of some of the lake deposits of the past few thousand years, there is no evidence for widespread erosion (i.e., scouring that is wider than the length of the trench) of significant portions of the lake deposits. Furthermore, we consider it unlikely for there to have been significant erosion on the downdropped side of the BFZ. We know from trenches at the Lake Cahuilla shoreline (13 m above sea level) that there were four Lake Cahuilla highstands in fairly rapid succession between A.D. 1440 and approximately A.D. 1680 (Gurrola and Rockwell, 1996). Nearby, at sea level, there is evidence for four distinct lakes between A.D. 1630 and about A.D. 1680 (Orgil, 2001).…”

“…At least five times during the past 1200 years, the Colorado River has switched from its present course (emptying southward into the Gulf of California) to flowing northward into the Salton Trough. Each time the Colorado River followed a northward course, it inundated much of the below-sea level Coachella and Imperial Valleys, producing the freshwater Lake Cahuilla that typically rose to elevations of between 9 and 13 m above modern sea level, the altitude of the lowest point on the Colorado River delta (Stanley, 1963(Stanley, , 1966Thomas, 1963;Van de Kamp, 1973;Waters, 1983;Sieh, 1986;Sieh and Williams, 1990;Rockwell and Sieh, 1994;Gurrola and Rockwell, 1996;Thomas and Rockwell, 1996;Orgil, 2001). After filling to an elevation of 13 m, excess input to the lake flowed south over the delta to the Gulf of California; eventually, the Colorado River would revert to a southward course, and because of the hot, dry climate, Lake Cahuilla would desiccate over about 60-70 years (Sieh and Williams, 1990).…”

The Seismogenic Thickness of the Southern California Crust

“…8). The entire fault zone has a high seismicity rate and has ruptured in several major earthquakes during the past 2000 years (Gurrola and Rockwell, 1996). Two areas of significant, off-themain-trace, fault-parallel seismicity (Hot Springs and Buck Ridge faults) are not included in the CDMG/USGS fault database.…”