THE 1983 BORAH PEAK, IDAHO EARTHQUAKE—A MODEL FOR SURFACE-RUPTURING EVENTS IN THE WESTERN U.S

The Mw 6.9, 1983 Borah Peak, Idaho, earthquake is the most recent normal-faulting earthquake to produce large scarps in the western U.S. It produced about 36 km of surface rupture on the central part of the 127-km-long Lost River fault (LRF), which can be divided into six segments based on the geometry of the range front and the distribution and characteristics of late Quaternary fault scarps. The earthquake nucleated at about 16 km depth, downdip from a prominent range-front salient that marks the boundary between the 24-km-long Thousand Springs segment (TSS) and the 23-km-long Mackay segment (MS) to the southeast. The rupture propagated unilaterally to the northwest and produced the biggest scarps (maximum throw of 2.7 m) and zones of complex ruptures on the TSS. Near the northwestern end of the TSS, the rupture bifurcates and formed a 14-km-long west-trending splay of small, simple scarps across the Willow Creek Hills (WCH). The WCH is an intra-valley bedrock ridge that separates TSS from the Warm Springs segment (WSS) to the northwest. Along the range-front fault, simple scarps, less than or equal to 1 m high, formed along 8 km of the WSS.

The LRF and surrounding area was largely aseismic in the decades prior to 1983, so precursory seismicity did not signal the impending event. The earthquake ruptured a 45°–50°-dipping, planar fault that extends through the entire seismogenic crust. Virtually no aftershocks occurred on the MS, suggesting that the structure at the MS–TSS boundary allowed the two parts of the fault to act with complete independence. In contrast, the northwestward propagating rupture on the TSS was able to trigger a small amount of slip on the WSS and create ruptures on western splay in the WCH. The steep range front suggests that the LRF has a history of sustained late Cenozoic slip, and limited paleoseismic studies indicate that the Holocene recurrence for surface-faulting earthquakes is several thousand years. The Borah Peak earthquake provided valuable insight into the three-dimensional fault geometry and the coseismic rupture on long Basin-and-Range normal faults. This insight shows us what we might expect when the next large earthquake ruptures a major range-front fault in the Intermountain West such as the Utah’s Wasatch fault.