Paleoseismology of Utah Volume 2

Paleoseismic analysis of the Wasatch fault zone at the Brigham City trench site, Brigham City, Utah and the Pole Patch trench site, Pleasant View, Utah

Special Study 76, 1991, 39 pages, $6.00

ABSTRACT

In September 1986, trench BC- I was excavated across an 8-m-high (26 ft) fault scarp on the Brigham City segment of the Wasatch fault zone near Brigham City, Utah. The trench exposed a complex, 4-m-wide (I 3 ft) zone of "stepped" normal faults that offset a sequence of lower and middle Holocene alluvial-fan deposits about 6 m (20 ft). A series of fault-scarp colluvial wedges and several apparent mean-residence-time (AMRT) 14C ages on buried soils provide evidence for three normal-slip surface-faulting events.

The first (oldest) event consisted of about 2.5 m (8.2 ft) of vertical displacement; although undated, this event probably occurred 5 to 7 ka. The second event consisted of about 2.5 m (8.2 ft) of vertical displacement; AMRT 14C ages suggest that this event occurred about 4.7 t 0.5 ka. The third (youngest) event consisted of about 1.0 m (3.3 ft) of vertical displacement; AMRT 14C ages suggest that this event occurred about 3.6 t 0.5 ka.

These data yield an average displacement of 2 m (6.5 ft) per event, a single recurrence interval between the second and third events of I 100 t 1000 years, and an elapsed time since the third event of 3600 t 500 years. The timing of the second and third events yields a post-middle-Holocene slip rate of 0.75 t 0.3 mm/ yr (.03 t.0 I in/ yr). Empirical analysis of historic earthquakes on normal faults suggests that the surface displacements of 1.0 to 2.5 m (3.3 - 8.2 ft) on the Brigham City segment were associated with earthquakes of magnitude (Ms) 6.8 to 7. 1.

The long elapsed time since the third event and the apparent decrease in displacement associated with this event suggest that the Brigham City segment may have experienced a decline in strain accumulation in late Holocene time. Alternatively, the segment may have entered a quiescent phase of strain accumulation between earthquake "clusters," or the segment may be continuing to accumulate strain at its middle-Holocene rate and is therefore overdue for another surface-faulting earthquake.

Longer term recurrence data will be required to choose which of these scenarios best explains the pattern of recurrence of Holocene surface-faulting earthquakes on the Brigham City segment.