Geology topics

Research about south-central Alaska

Source-linked reports with geographic coverage including south-central Alaska.

4 recordsLinked to original sources

USGS2025

Refining the earthquake history of south-central Alaska through lake records

The Alaska–Aleutian subduction zone (AASZ) is one of the world's most seismically active plate boundaries and the source of the 1964 M w 9.2 Great Alaska earthquake–the second largest instrumentally recorded earthquake in the world. Understanding the nature and frequency of such earthquakes is necessary for seismic and tsunami hazard assessment, but instrumental and historical records that span less than 150 years are too short to allow a statistically reliable analysis of earthquake recurrence times. This calls for studies of evidence of past earthquakes, extending the earthquake catalog further back in time. Subduction-zone paleoseismology in south-central Alaska is predominantly based on coastal evidence of land-level changes and tsunamis generated by megathrust earthquakes and preserved in the geological record. A complementary approach is lacustrine paleoseismology, which is still a relatively young discipline in Alaska. However, globally, lake basins are well-established high-resolution and continuous recorders of paleoseismic activity along subduction zones, relying on the identification of underwater landslide deposits and turbidites generated by seismic shaking. As a result, lake basins not only register ground shaking from megathrust earthquakes, but also from intraslab and crustal earthquakes, which are typically not accompanied by significant land-level changes. In this review paper, we combine coastal and lacustrine paleoseismology approaches to refine the south-central Alaskan earthquake history by comparing the paleoseismic records from two lakes (i.e., Eklutna Lake, located in the Chugach Mountain Range, and Skilak Lake, situated on the Kenai Peninsula) with the coastal and crustal earthquake catalog in Alaska. The resulting age ranges of all known megathrust earthquakes involving the Alaskan megathrust between the Kodiak and Prince William Sound (PWS) sections are more precise and accurate for the last 1.3 kyrs BP than the previously published age ranges from coastal records. As a result, this study supports the following key conclusions: (1) The 1964 CE earthquake was an exceptionally strong and unique event in the last 2000 years, rupturing the PWS, Kenai, Barren Islands, and Kodiak sections simultaneously. (2) The high-resolution and seasonal markings of the varved lake records now disentangle for the first time closely timed earthquakes, which was not possible based on the coastal evidence alone. (3) No persistent megathrust rupture boundaries exist. So, the possibility of a full rupture of the entire eastern AASZ, from PWS to Semidi cannot be excluded. (4) The rupture pattern in the eastern AASZ reveals superimposed cycles of multi-asperity ruptures (1964 earthquake) and clustered complementary partial ruptures, or rupture cascades. (5) The PWS section hosts the largest asperity in the eastern AASZ. (6) The shaking record of megathrust earthquakes indicates a time-dependent (quasiperiodic) behavior for the study area, but the observation of complementary clusters means that the hazards will not drop to zero but instead may even increase for a neighboring section. (7) The time-independent behavior of intraplate earthquakes implies that the intraslab hazard did not decrease following the 2016 and 2018 earthquakes. This study utilizes an integrated approach for subduction zone paleoseismology as a solution for unraveling recurrence and rupture patterns in Alaska, which can be applied worldwide.

Alaska↗

USGS1976

Preliminary survey of modern glaciolacustrine sediments for earthquake-induced deformational structures, south-central Alaska

Bottom sediments of four lakes in South-Central Alaska were studied in the summer of 1975 for evidence of earthquake-induced deformation. The lakes are: Summit, Upper Trail, and Skilak, all on the Kenai Peninsula, and Eklutna, northeast of Anchorage (fig. 1). Interest in these lakes was stimulated by hypotheses developed from a study of sediments in Van Norman Reservoir, California, after the 1971 San Fernando earthquake (Sims, 1973). During that study three zones of deformational structures were found and correlated with moderate earthquakes that shook the San Fernando area in 1930, 1952, and 1971. Results of that study, coupled with the experimental formation of deformational structures similar to those from Van Norman Reservoir, led to a search for similar structures in Pleistocene and Holocene lakes and lake sediments in other seismically active areas. The lakes for this study were chosen specifically because of their location within the area affected by the 1964 Prince William Sound earthquake, and the probability of obtaining varved sediments which would allow counting of years between sedimentologic events.

Alaska↗

USGS1974

Catalog of earthquakes in south-central Alaska, April - June 1972

The National Center for Earthquake Research of the U.S. Geological Survey (USGS) installed a network of eleven seismograph stations in south-central Alaska in the summer of 1971 to collect seismological data for investigating seismic and tectonic processes in the Cook Inlet region, for evaluating the seismic hazard in the Cook Inlet region and in the Chugach Mountains along the southern end of the route of the proposed trans-Alaska oil pipeline, and for studying the structure of the crust and upper mantle in south-central Alaska. The first recordings from the network were obtained in late September 1971. The first earthquakes to be located with data from the network occurred in October 1971. This earthquake catalog, which is the first to be compiled for the USGS Alaska network, presents origin times, focal coordinates and magnitudes for 210 shocks occurring in the second quarter of 1972 (except from June 15 to 23) in south-central Alaska, primarily in the Cook Inlet region. Readings from a total of 18 stations were used to locate the shocks. To supplement the readings from the 10 USGS stations that were operative during this quarter, arrival times were obtained from 6 stations operated by the Palmer Seismological Observatory of the National Oceanic and Atmospheric Administration (NOAA) and 2 stations operated by the Geophysical Institute of the University of Alaska (U of A). The data in this catalog are the most detailed and precise earthquake data compiled to date for earthquakes within the Cook Inlet region. Earthquakes in south-central Alaska as small as magnitude 3.0 have been routinely located by the National Earthquake Information Center of NOAA (currently the National Earthquake Information Service of USGS) since the great Alaska earthquake of 1964 and published in the reports "Preliminary Determination of Epicenters" (PDE). In contrast the shocks included in this catalog are as small as magnitude 1.0 and most are smaller than magnitude 3.0. Data for the larger historic earthquakes in south-central Alaska have been tabulated by Davis and Echols (1962) and U.S. Coast and Geodetic Survey (1966). The locations of the eleven stations of the NCER seismograph network are plotted in Figure 1 and listed in Table 1 along with the additional stations from which readings were obtained. The USGS stations have single, vertical-component seismometers except for NKA and SKL, which also have two horizontal seismometers. The station ERN was installed in the summer of 1971, however the telemetry circuit was not available until September 1972.

Alaska↗

USGS1972

Floods in the summer of 1971 in south-central Alaska

Floods and high water occurred throughout the summer of 1971 in south-central Alaska. Snow cover, 150 percent of average, in the mountains on May 1 caused local snowmelt floods from mid-May to mid-July. The peak discharge of 265,000 cfs (cubic feet per second) on July 15 at Copper River near Chitina was the highest in 22 years of record. However, the major flood period was August 8-11. Precipitation totals of 3 to 9 inches during August 5-11 were recorded in an area extending northeastward from Iliamna Lake to Palmer, Talkeetna, and Paxson Lake. The principal flood areas were the upper Copper, Matanuska, Susitna, Chakachatna, and Kvichak River basins. Flooding in the Anchorage area was not severe. Total damage caused by the August flood is estimated to be 8 to 10 million dollars of which 6 million dollars occurred in the Matanuska Valley. Extreme floods occurred in the Matanuska Valley. A lake near Sutton on an unnamed tributary to Granite Creek breached its embankment, which released a peak discharge in Granite Creek on August 10 of 58,600 cfs, 23.4 times the probable 50-year flood. Other streams near Sutton, tributary to the Matanuska River, had peak discharges 1.8 to 8.9 times the probable 50-year flood. The Matanuska River at Palmer had a peak discharge of 82,100 cfs, 1.2 times the probable 50-year flood. Downstream near Bodenburg Butte, an area was inundated when the Matanuska River overtopped a dike. Another extreme flood occurred at the Chakachatna River near Tyonek on August 11 where the peak discharge was estimated to be 470,000 cfs. The peak was caused by lateral erosion of a channel constriction formed by Barrier Glacier at the outlet of Chakachamna Lake. Hydrologic data for planning, discussions of antecedent conditions, and meteorology along with a description of the floods and flood damage are included. Tables of storm precipitation, peak discharge data, sediment data, and discharge for the June-August period are also included.

Alaska↗