Earthquakes in the Intermountain Seismic Belt (Northern Utah and Vicinity)
The Intermountain seismic belt (ISB) is a prominent north-south-trending zone of recorded seismicity in the Intermountain West, including the Wasatch Front urban corridor. A modern catalog of instrumentally located earthquakes in Utah begins in mid-1962, and historical earthquake records date back to the 1850s. In general, M5 or larger earthquakes occur at an average rate of about one every 10 years and M6 or larger earthquakes about one every 50 years in this area. The largest historical earthquake was the 1934 M6.6 surface-faulting earthquake in Hansel Valley, north of the Great Salt Lake. The ISB in northern Utah includes the Wasatch fault, which has had relatively little historical seismicity despite being Utah’s most active fault based on geological evidence.
Because moderate and large earthquakes are likely, expected levels of strong ground shaking are high. Ground shaking from a M5 earthquake can cause significant damage at distances up to 10 km (6 miles), and a M7 at distances up to 50 km (30 miles) and more. Geologic site conditions such as deep sediment-filled basins may locally amplify and prolong ground shaking.
The ISB in northern Utah coincides with a transition between east-west-directed stretching in the Basin and Range to the west and an area of more stable crust of the Middle Rocky Mountains to the east. The area is characterized by movement along north-trending, east- and west-dipping range-bounding normal faults, resulting in horizontal extension. The ISB in northern Utah contains the Wasatch fault and many of the state’s other most active faults, which commonly show evidence of multiple displacements in the past 10,000 years. Average recurrence intervals for large surface-faulting earthquakes on the most active central segments of the Wasatch fault are 1,300-2,500 years; recurrence intervals are typically longer (thousands to 10,000 years or more) on other faults in the area. ISB faults are inferred to be moderate- to high-angle structures extending to depths of 10-15 km (6-10 miles) and capable of causing large earthquakes and strong ground shaking. However, evidence suggests that some normal faults are gently dipping with down-dip movement on underlying shallow, low-angle, west-dipping thrust faults formed during an earlier Cretaceous/early Tertiary period of east-directed compression. The potential for movement on these faults to cause strong ground shaking is less well understood but likely low.
Surface rupture on the Hansel Valley fault occurred during the 1934 Hansel Valley earthquake. However, most small- to moderate-sized earthquakes in the ISB cannot be associated with recognized geologically young faults. Part of the problem of associating seismicity with faults arises from the superposition of late Cenozoic stretching upon older thrust-belt structure, including buried faults having no surface expression.
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