Geophysical investigation of the Denali fault and Alaska Range orogen within the aftershock zone of the October–November 2002, M = 7.9 Denali fault earthquake

Michael A. Fisher; Warren J. Nokleberg; N. A. Ratchkovski; Louise Pellerin; Jonathan M.G. Glen; Thomas M. Brocher; J. Booker

doi:10.1130/g20127.1

Home/Archive/USGS

USGS · 70210586

Geophysical investigation of the Denali fault and Alaska Range orogen within the aftershock zone of the October–November 2002, M = 7.9 Denali fault earthquake

Michael A. Fisher·Warren J. Nokleberg·N. A. Ratchkovski·Louise Pellerin·Jonathan M.G. Glen·Thomas M. Brocher·J. Booker

View original source ↗Export metadata ↓

Abstract

The aftershock zone of the 3 November 2002, M = 7.9 earthquake that ruptured along the right-slip Denali fault in south-central Alaska has been investigated by using gravity and magnetic, magnetotelluric, and deep-crustal, seismic reflection data as well as outcrop geology and earthquake seismology. Strong seismic reflections from within the Alaska Range orogen north of the Denali fault dip as steeply as 25°N and extend to depths as great as 20 km. These reflections outline a relict crustal architecture that in the past 20 yr has produced little seismicity. The Denali fault is nonreflective, probably because this fault dips steeply to vertical. The most intriguing finding from geophysical data is that earthquake aftershocks occurred above a rock body, with low electrical resistivity (>10 Ω·m), that is at depths below ∼10 km. Aftershocks of the Denali fault earthquake have mainly occurred shallower than 10 km. A high geothermal gradient may cause the shallow seismicity. Another possibility is that the low resistivity results from fluids, which could have played a role in locating the aftershock zone by reducing rock friction within the middle and lower crust.

Explore related subjects

Geology Article

Source-reported bounding extent: 59.987997631212224° to 63.966318784384235° latitude; -153.544921875° to -141.328125° longitude. This indicates report coverage, not an exact sampling location. View area on OpenStreetMap.

Keep this discovery

Explore connections, maps & timelines

BibTeX RIS

Michael A. Fisher, Warren J. Nokleberg, N. A. Ratchkovski, Louise Pellerin, Jonathan M.G. Glen, Thomas M. Brocher, J. Booker. 2004. Geophysical investigation of the Denali fault and Alaska Range orogen within the aftershock zone of the October–November 2002, M = 7.9 Denali fault earthquake. https://doi.org/10.1130/g20127.1

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related USGS reports

USGS2026

Zircon petrochronology fingerprints mantle sources of magmatic rare earth element deposits

The mantle sources of carbonatite and alkaline magmatic systems that host rare earth element (REE) deposits are difficult to constrain because whole-rock compositions are commonly altered. Alteration-resistant zircon trace-element compositions are used to fingerprint mantle sources at Mountain Pass, California, the largest REE deposit in the United States, and extend this approach to a global carbonatite−alkaline rock dataset. Autocrystic zircon samples from the Mountain Pass intrusive suite record subduction-like U-Sc-Nb-Yb compositions, oxidized fO2 (0.8−2.4 relative to fayalite-magnetite-quartz buffer [ΔFMQ]), and Ti-in-zircon temperatures up to ∼970 °C, indicating a lithospheric mantle source enriched and oxidized by Paleoproterozoic subduction and subsequently tapped during Mesoproterozoic postcollisional lithospheric thinning. This contrasts with the ocean island basalt−like enriched mantle recorded by Mesoproterozoic zircon from the Bayan Obo carbonatite complex, China, demonstrating that world-class REE deposits of comparable age can reflect fundamentally different mantle sources. New classifiers separate subduction-metasomatized from non-subduction-enriched mantle domains with ∼90% accuracy, providing a zircon-based tool for fingerprinting the mantle source regions of magmatic REE deposits.

Geology↗

USGS2026

Deformed submarine terraces in Puget Sound, Pacific Northwest, indicate only one M >~7.5 earthquake on the Seattle fault zone in the past 11,000 yr

Submerged marine terraces in Puget Sound, deformed across the Seattle fault zone (SFZ), indicate that only one earthquake as large as M~7.5 has occurred in at least the past 11 kyr. Previous paleoseismic studies document a M~7.5 earthquake between 923–4 CE, which uplifted coastal marine terraces by as much as 8 m. We demonstrate that this earthquake was the only such event since ~11 ka by mapping and quantifying deformation of older marine terraces, now submerged in Puget Sound. The submerged terraces, attributed to a late-glacial sea-level lowstand, record both glacial isostatic rebound and tectonic deformation. Vertical offset of the ~11 ka terraces within the SFZ is comparable to that of the marine terraces uplifted in 923 CE, implying no additional large (M>~7.5) earthquake on the SFZ since ~11 ka. This result implies a longer recurrence interval than current hazard estimates, which assumes recurrence of M>7.1 events every 5 kyr. Our mapping of SFZ deformation since ~11 ka also supports fault segmentation and contiguous block uplift between the Seattle and Tacoma fault zones.

Washington↗

USGS2025

Widespread anhydrite saturation in Laramide-age arc magmas of southwestern USA

Anhydrite is considered a rare mineral phase in magmas, with only ∼33 documented occurrences worldwide. However, anhydrite readily decomposes in the near-surface environment, making it difficult to recognize its former presence in rocks collected at or near Earth’s surface. In such samples, only small anhydrite inclusions fully shielded within other minerals can have survived. During a recent field trip to the southwestern USA, we sampled 17 Laramide-age (ca. 40−80 Ma) magma systems, most of which are associated with porphyry copper deposits. A systematic search for anhydrite inclusions preserved within apatite, amphibole, plagioclase, and quartz phenocrysts in ∼100 rock samples by optical microscopy and Raman spectroscopy revealed that each of these 17 magma systems was at least temporarily anhydrite-saturated. Also, most previously identified magmatic anhydrite-bearing intrusions are associated with porphyry copper deposits, and both intrusive and volcanic rocks containing magmatic anhydrite show high Sr/Y ratios. These observations suggest that anhydrite saturation and porphyry copper formation are linked via magma fractionation at high pressure. Compared to average arc magmas, anhydrite-bearing magmas are unusually oxidized and sulfur-rich and seem to also be unusually water-rich. Hence, our preferred interpretation is that magma generation and/or fractionation at high pressure promotes the formation of superhydrous and oxidized magmas, which in turn promotes high sulfur contents and ultimately the precipitation of anhydrite. The high mineralization potential of these magmas does not need to result from their high sulfur content but could be caused by other properties of high-pressure magmas.

Arizona, New Mexico↗