By David Jacobson, Temblor
At 12:30 a.m. local time early this morning, a M=5.8 earthquake struck western Montana, and based on felt reports from the USGS was felt as far away as Seattle and Portland, which are both over 730 km away. By 8:30 a.m. in Montana, over 13,000 people had filled out a felt report on the USGS website, highlighting how widely felt this quake was. Despite this, no reports of damage or injuries have been reported, though there were power outages in the small town on Lincoln 11 km from the epicenter. Additionally, a gas leak was reported in the state capital of Helena 53 km away. According to the ShakeMap, very strong shaking was felt close to the epicenter, with seismic energy quickly dissipating. So far, there have been 14 aftershocks with magnitudes ranging from 2.5-4.9. What should also be pointed out in the map below is that in the last two weeks, there were two small magnitude quakes in this area. One of these was only a day ago. While not necessarily foreshocks, these small quakes show that the area is indeed active.
This morning’s earthquake occurred at a depth of 13.6 km, and had a focal mechanism suggesting strike-slip motion. According to the USGS, this is consistent with right-lateral faulting within the Lewis and Clark line. This is a zone that extends for approximately 400 km from northern Idaho to east of Helena, Montana. Faults within the Lewis and Clark line have strike-slip, dip-slip, and/or oblique-slip motion. In close proximity to today’s earthquake is the St. Mary’s-Helena Valley Fault, a large right-lateral strike-slip structure. However, this fault is to the east of the epicenter, meaning today’s quake was likely on an unmapped structure.
In the Temblor map above, which shows the USGS Quaternary Fault Database, one can see that there are very few mapped faults in the immediate vicinity of the quake. However, we believe that this mapping is incomplete, as is evident by the additional faults added in. These faults were added based on the geomorphology, and old structural maps of the region. Despite this, based on strain accumulation and the last 40 years of earthquakes, which the Global Earthquake Activity Rate (GEAR) uses, today’s quake is an extremely rare event, and the largest event in western Montana since the 1959 M=7.3 Hebgen Lake earthquake. In the Temblor map below, the GEAR model is shown, highlighting the rarity of the earthquake. Furthermore, USGS geophysicist Robert Sanders said in an interview to the Los Angeles Times, “It’s not impossible, but it is a very rare event.”
University of Utah Seismograph Stations
C. A. WALLACE, D. J. LIDKE, R. G. SCHMIDT, Faults of the central part of the Lewis and Clark line and fragmentation of the Late Cretaceous foreland basin in west-central Montana, GSA Bulletin, August 1990.
R.G. Schmidt, J.S. Loen, C.A. Wallace, and H.M. Mehnert, Geology of the Elliston Region, Powell and Lewis and Clark Counties, Montana, USGS Bulletin 2045, , 1994