A magnitude 7 earthquake strikes Alaska on the recently discovered conductor fault

The earthquake that struck a remote area of ​​Alaska on December 6, 2025 serves as an example of a successful seismic risk assessment.

Written by Ross Stein and Vulkan Civilgen, Tumbler Corporation

Citation: Stein, R.S. and Sevilgen, V., 2025, Magnitude 7 earthquake hits Alaska on recently discovered conductor fault, Temblor, http://doi.org/10.32858/temblor.370

On December 6, 2025, a magnitude 7 earthquake struck along a fault beneath Hubbard Glacier, located amid inhospitable mountainous terrain near the remote Alaska-Yukon border (Figure 1). As a result of the isolated epicenter, the earthquake was not widely felt and did not cause any damage. So why does such a shock matter so much, regardless of its size?

Figure 1. Glacial valley basins in the landscape indicate transient faulting. North of the epicenter of the magnitude 7.0 quake, some stream valleys appear to be facing right laterally. Fault traces and names taken from Alaska Science Center (2021). Credit: Tumbler, CC BY-NC-ND 4.0

Newly discovered error

Identifying the fault causing this earthquake – the Mosul Fault – over the past few years is a landmark achievement for academic and government geologists. Since 1970, seismologists have been searching for a connection between the main active Tochunda fault to the northwest and the Fairweather fault 200 kilometers to the southeast.

The Tochunda (as well as the Western Denali Fault) ruptured in the 2002 magnitude 7.9 Denali earthquake (Eberhart-Phillips et al., 2003), and the Fairweather Fault ruptured in the 1958 magnitude 7.8 shock (Rollins et al., 2021) (Figure 2). These faults are very similar to the San Andreas in terms of slip, slip rate, and magnitude of associated earthquakes. It would be almost impossible not to know 200 kilometers from San Andreas. But in Alaska, bug detection is much more difficult due to geographic inaccessibility.

Figure 2. The magnitude 7.0 shock struck near the southern end of the 200 km gap between the 1958 Fairweather mainshock rupture and the 2002 Denali earthquake (dark green lines with stars outlined in green at the epicenters). Until recently, no error between these two errors had been detected. Earthquakes since the 2002 Denali earthquake (green rupture to the north) highlight their own aftershocks. Relative to the rupture zone of the 1958 magnitude 7.8 earthquake, current seismic activity is much quieter. A more notable group of earthquakes is associated with the Alaska subduction zone, which in 1964 produced a 9.2-magnitude earthquake in Prince William Sound. Because the fault is tilted relative to the surface, the green fault trace is located far south of its center and the bulk of its seismic activity. Brown polygons are fault zones (named), which are diffuse areas of concentrated seismic activity without discrete, defined faults; The Pamplona fault zone, closest to the magnitude 7.0 shock of 2025, was investigated by Doser et al. (1997). Credit: Tumbler, CC BY-NC-ND 4.0

In 2008, Kalbas et al. He suggested that the area between the Tochunda and Fairweather faults should be called the Toot Fair fault. Further research by Bender and Haeussler (2021), Biegel et al. (2024), and Hoesler et al. (2025) were renamed the “Mosul Fault,” where it was harnessed as a seismic source in the USGS 2023 seismic hazard model for Alaska (Powers et al., 2024). This means that the models used by the USGS to predict the strength of shaking across the United States included this fault as the source. Presumably, their inclusion led to more realistic assessments of the likelihood of strong shaking.

How likely is this shock?

Temblor's seismic hazard model is not based on errors, but rather on a combination of strain rate measured by GPS and earthquakes that have occurred within the past 120 years. Ideally, this alternative approach should yield results similar to those of the USGS and, unlike the USGS model, can be applied consistently throughout the world, where error inventories are often inadequate.

Our model gives a 50/50 chance over a typical human lifetime of about 85 years of an impact of magnitude greater than or equal to 6.8 at the site of the December 6 event. So, a 7.0 magnitude shock would have an approximate recurrence time of 100 to 150 years.

In this sense, a 7.0 magnitude shock is not surprising. About 45 kilometers to the west of the magnitude 7.0 shock, a magnitude 6.0 event occurred in 2014, another indication of the high seismic potential in the region.

Bigell et al. (2024) Seismic transmission for a high-resolution image of the conductor and its associated faults (Figure 3). What emerged was a patchwork of seismic activity with many distinct directions, which they interpreted as short fault segments with many directions within a wide area. Interestingly, the slip sensation of larger earthquakes corresponds to a magnitude 7.0 shock, which has right-lateral shear components in addition to compression.

Figure 3. Beagle et al. (2024) found intermittent and clustered seismicity along the Mosul Fault, which they interpreted as short fault segments with many directions within a wide area. Credit: Modified from Biegel et al. 2024

Although Beigel et al. If the explanation is possible, the Mosul fault may instead be simpler and more continuous than can be inferred from seismicity alone, as suggested by a magnitude 7.0 shock on December 6 parallel to the main fault direction.

Did previous earthquakes reinforce this shock?

We must consider whether the 1958 magnitude 7.8 earthquake and the 2002 magnitude 7.9 earthquake played a role in making the latter quake more likely.

We can almost rule out the 2002 Denali shock, because its southeastern tip is 200 km from the 7.0 magnitude event in 2025. Our calculations suggest that the pressure change was too small to change seismic rates (Figure 4).

Figure 4. The transfer of Coulomb stress from the 2002 Denali earthquake to the site of the magnitude 7.0 shock is negligible, or less than 0.05 bar, so we do not believe these two shocks interacted. Credit: Tumbler, CC BY-NC-ND 4.0

On the other hand, the much-delayed uplift of the 1958 Fairweather earthquake is possible because the magnitude 7.0 shock is located only about 30 km north of the tip of the Fairweather fault, where the pressure increase may have been closer to 1 bar. But if so, why the 67-year delay? In general, we expect aftershocks near the fault heads to continue for 30 to 50 years.

Aftershocks and major aftershocks in the future

The aftershock zone is about 50 kilometers long, and the largest aftershock until December 8 was a 5.8 magnitude earthquake. Both of these features—the length of the aftershock and the largest aftershock size—are typical of a magnitude 7.0 rupture with slip and thrust components (Figure 5).

Figure 5. In the 24 hours following the 7.0 magnitude earthquake, the aftershock zone extended 50 km. The largest aftershock as of this writing is a 5.8 magnitude earthquake. These features are typical for an event of this size. Earthquakes from the USGS ANSS catalog. Credit: Tumbler, CC BY-NC-ND 4.0

However, there remains an unbroken 150 km section between the Mosul and Tochunda faults, from the southern end of the 2002 magnitude 7.9 Denali rupture to the 2025 magnitude 7.0 rupture. If this discontinuous section failed in a single event, it could result in a 7.5 magnitude shock.

South of the 2025 magnitude 7.0 earthquake, there is an uninterrupted 30-kilometre extension to the Fairweather Fault, which could rupture in a single event with a magnitude of approximately 6.8.

The last word

Most of the effects of the December 6 earthquake are hidden under glaciers. The area is very mountainous and remote, making it difficult for field mapping by geologists and the deployment of seismic and Global Positioning System (GPS) stations.

Against all odds, geologists have discovered a 250-kilometre-long Great American Fault and included it in the US Geological Survey's current seismic risk assessment. This is a rare and welcome case where a major new fault is discovered before the main shock, rather than after.

References

Alaska Science Center, USGS (2021). Geological inputs for the 2023 Alaska update of the US National Earthquake Hazard Model (NSHM). Metadata updated: November 27, 2025.

Bender, A. M., and Haeussler, P. J. (2021). Alaska Fault Tracking Mapping, 2021, Data Release, doi: 10.5066/P9H02FXB.

Bigell, K. M., Joslin, J. M., Detmer, J., Colbrun, M., Enkelman, E., & Cain, J. S. (2024). Seismic transports define a discrete fault network and deformation corridor throughout southeastern Alaska and southwestern Yukon, Tectonics, 43, e2023TC008140, https://doi.org/10.1029/2023TC008140.

Doser, D., Pelton, Jr., & Viviello, A. M. (1997). Earthquakes in the Pamplona area, Yakutat block, south-central Alaska, J. Geophys. Res., 102(B11), 24499–24511, DOI:10.1029/97JB0.

Eberhardt-Phillips, D., et al. (2003). The 2002 Denali fault earthquake, Alaska: a large-scale, slip-slip event, Science, 300, 1113, doi: 10.1126/science.1082703.

Heusler, B. J., Bender, A. M., Powers, P. M., Kohler, R. D., & Bro, D. S. (2025). Crustal fault model update for the 2023 Alaska National Seismic Hazard Model in Tectonics and Seismic Structure of Alaska and Northwest Canada: EarthScope and Beyond, Geophysical Monograph 290, edited by Ruppert, N.A., Jadamec, M.A., and Freymueller, J.T., American Geophysical Union.

Kalbas, J. L., Farid, A. M., & Ridgway, K. D. (2008). Contemporary Fault Mechanics in Southern Alaska, in Geophysical Monographs Series, edited by Freymueller, J. T., Haeaussler, P. J., West, R. L., and Ekström, G., pp. 321–3336, American Geophysical Union, https://doi.org/10.1029/179GM1

Powers, PM, Altekruse, JM, Llenos, AL, Michael, AJ, Haynie, KL, Haeussler, PJ, Bender, AM, Rezaeian, S., Moschetti, MP, Smith, JA, Briggs, RW, Witter, RC, Mueller, CS, Zeng, Y., Girot, DL, Herrick, JA, Shumway, AM, and Petersen, MD (2024). 2023 Alaska National Earthquake Hazard Model, Earthquake Spectra, 40(4), 2545–2597, https://doi.org/10.1177/87552930241266741.

Rollins, C., Freimüller, J. T., and Sauber, J. M. (2021). Stress enhancement of the 1958 Mw∼7.8 Fairweather Fault earthquake and others in southeastern Alaska by glacial isostatic modulation and interseismic stress transfer, J. Geophys. Res., 126, e2020JB020411, https://doi.org/10.1029/2020JB020411.

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Text © 2025 Tumbler. CC BY-NC-ND 4.0

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