Surface faulting along the Superstition Hills fault zone and nearby faults associated with the earthquakes of 24 November 1987

Abstract

The M 6.2 Elmore Desert Ranch earthquake of 24 November 1987 was associated spatially and probably temporally with left-lateral surface rupture on many northeast-trending faults in and near the Superstition Hills in western Imperial Valley. Three curving discontinuous principal zones of rupture among these breaks extended northeastward from near the Superstition Hills fault zone as far as 9 km; the maximum observed surface slip, 12.5 cm, was on the northern of the three, the Elmore Ranch fault, at a point near the epicenter. Twelve hours after the Elmore Ranch earthquake, the M 6.6 Superstition Hills earthquake occurred near the northwest end of the right-lateral Superstition Hills fault zone. Surface rupture associated with the second event occurred along three strands of the zone, here named North and South strands of the Superstition Hills fault and the Wienert fault, for 27 km southeastward from the epicenter. In contrast to the left-lateral faulting, which remained unchanged throughout the period of investigation, the right-lateral movement on the Superstition hills fault zone continued to increase with time, a behavior that was similar to other recent historical surface ruptures on northwest-trending faults in the Imperial Valley region. We measured displacements over 339 days at as many as 296 sites along the Superstition Hills fault zone, and repeated measurements at 49 sites provided sufficient data to fit with a simple power law. Data for each of the 49 sites were used to compute longitudinal displacement profiles for 1 day and to estimate the final displacement that measured slips will approach asymptotically several years after the earthquakes. The maximum right-lateral slip at 1 day was about 50 cm near the south-central part of the North strand of Superstition Hills fault, and the predicted maximum final displacement is probably about 112 cm at Imler Road near the center of the South strand of the Superstition Hills fault. The overall distributions of right-lateral displacement at 1 day and the estimated final slip are nearly symmetrical about the midpoint of the surface rupture. The average estimated final right-lateral slip for the Superstition Hills fault zone is about 54 cm. The average left-lateral slip for the conjugate faults trending northeastward is about 23 cm. The southernmost ruptured member of the Superstition Hills fault zone, newly named the Wienert fault, extends the known length of the zone by about 4 km. The southern half of this fault, south of New River, expressed only vertical displacement on a sinuous trace. The maximum vertical slip by the end of the observation period there was about 25 cm, but its growth had not ceased. Photolineaments southeast of the end of new surface rupture suggest continuation of the Superstition Hills fault zone in farmland toward Mexico.

Characteristic Slow-Slip Events on the Superstition Hills Fault, Southern California

“…In this case, lower slip during the interseismic period should be compensated by higher coseismic slip. However, the observed surface slip due to the 1989 Superstition Hills earthquake was essentially the same on the northern and southern sections of the SHF (Sharp et al, 1989). Another possibility is that the observed smaller slip on the northern SHF during "system-size" SSEs (Figures 3 & 5) is compensated by smaller SSEs that rupture only the northern SHF.…”

Characteristic Slow‐Slip Events on the Superstition Hills Fault, Southern California

“…In this case, lower slip during the interseismic period should be compensated by higher coseismic slip. However, the observed surface slip due to the 1989 Superstition Hills earthquake was essentially the same on the northern and southern sections of the SHF (Sharp et al., 1989). Another possibility is that the observed smaller slip on the northern SHF during “system‐size” SSEs (Figures 3 and 5) is compensated by smaller SSEs that rupture only the northern SHF.…”

Surface slip variability on strike‐slip faults

“…Progressive landscape evolution in the interseismic period may increase interpretation error for historical earthquakes. For example, reconstructing the original offset geometry for historical earthquakes becomes more difficult as the preservation of the fault and offset markers degrades due to erosion, deposition, and widening of the geomorphic fault zone (Lienkaemper & Strum, 1989; Noriega et al, 2006; Reitman et al, 2019b; Sharp et al, 1989). Image correlation of numerical models illustrates landscape change after one earthquake (Figures 6 and 7).…”