2472, ANNABELLA GRABEN

Structure number: 2472.

Comments: Hecker's (1993) fault number 9-32.

Structure name: Annabella graben.

Comments:

Synopsis: Poorly understood zone of latest Pleistocene or early Holocene faulting near Annabella at the north end of the northern Sevier fault (2355).

Date of compilation: 10/99.

Compiler and affiliation: Bill D. Black, Utah Geological Survey, and Suzanne Hecker, U.S. Geological Survey.

State: Utah

County: Sevier.

1° x 2° sheet: Richfield.

Province: Colorado Plateaus.

Reliability of location: Good.

Comments: Mapped or discussed by Anderson and Bucknam (1979) and Anderson and Barnhard (1992). Mapping from Anderson and Bucknam (1979).

Geologic setting: Normal and strike-slip faults forming a wide graben in a structurally complex bend at the north end of the northern Sevier fault (2355) east of Annabella. The faults bound the western edge of the Sevier Plateau and separate a weakly deformed, east‑tilted Sevier Plateau block bounded by the northern Sevier fault from a highly deformed series of west‑tilted blocks downfaulted toward the plateau. The deformation appears younger than the Sevier River Formation, dated as young as 5.6 Ma in this area.

Sense of movement: N, NS, and ND.

Comments:

Dip: No data.

Comments:

Dip direction: Varies.

Geomorphic expression: Faults within the upthrown and downthrown blocks have diverse orientations and slip directions, although dip‑ and oblique‑slip faults predominate over strike‑slip faults. Normal‑fault dip directions are mostly incompatible with the presence of a major, range‑front fault system. Late Quaternary faulting and historical seismicity are concentrated within the Annabella graben, indicating high stress accumulation within a possible structural juncture. The deformation may also be related to flowage of the Arapien Shale, exposed northeast of the graben, and growth of the south end of the Sanpete‑Sevier Valley anticline. In addition to faulting, youthful tectonism takes the form of closed basins on mountain flanks, deflections of major drainages, and aligned inflections of parallel ridge crests. Individual fault scarps within the graben are less than 5 kilometers long and record spatial differences in rates of late Quaternary faulting. The highest scarp, which is on the main strand of the northern Sevier fault, has a cumulative displacement of about 109 meters in late Quaternary(?) deposits and may be the highest alluvial scarp in Utah. The short lengths of both the zone of young faulting and of individual faults argue against causative earthquakes with large magnitudes.

Age of faulted deposits: Late Quaternary.

Paleoseismology studies: None.

Timing of most recent paleoevent: (2) Latest Quaternary (<15 ka).

Comments: Single-event scarp morphology suggests the most-recent event has an age comparable to the Bonneville shoreline. A 1982 magnitude 4.0 earthquake and aftershock sequence, associated with internally inconsistent faulting kinematics, was centered in the Annabella graben in a historically rare association of seismicity with mapped Quaternary faults.

Recurrence interval: No data.

Comments: The 90‑meter‑long, 35° midslope of the 109-meter-high scarp implies numerous faulting events closely spaced in time. The age and rate of deformation within the structural juncture is likely not characteristic of deformation along the rest of the northern and southern Sevier faults, where larger, longer return‑period earthquakes are likely.

Slip rate: Unknown, probably <0.2 mm/yr.

Comments: Hecker (1993) estimates 4.7 to 5.2 meters displacement per event, though slip rate is unknown.

Length: End to end (km): 13

Cumulative trace (km): 10

Average strike (azimuth): N38°W

REFERENCES

Anderson, R.E., and Barnhard, T.P., 1992, Neotectonic framework of the central Sevier Valley area, Utah, and its relationship to seismicity, in Gori, P.L., and Hays, W.W., editors, Assessment of regional earthquake hazards and risk along the Wasatch Front, Utah: U.S. Geological Survey Professional Paper 1500-F, 47 p.

Anderson, R.E., and Bucknam, R.C., 1979, Map of fault scarps in unconsolidated sediments, Richfield 1° x 2° quadrangle, Utah: U.S. Geological Survey Open‑File Report 79‑1236, 15 p., scale 1:250,000.

Hecker, Suzanne, 1993, Quaternary tectonics of Utah with emphasis on earthquake-hazard characterization: Utah Geological Survey Bulletin 127, 2 plates, scale 1:500,000, 257 p.