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Seismic Imaging Beneath Iceberg Lake, Alaska: Sediment Characteristics and Fundamental Site Response Parameters Beneath a Drained Lake With an Alaskan-Aleutian Subduction Zone Paleoseismic Record

Description: 

The emerging field of lacustrine paleoseismology requires an understanding of a lake’s response to earthquake ground shaking. We acquired seismic data across the drained Iceberg Lake, Alaska valley floor to characterize basin sediments and ground motions associated with earthquakes along the eastern portions of the Alaskan-Aleutian subduction zone. The glacier-dammed lake, located in the Wrangell-St. Elias National Park, drained in 1999. From exposed and cored lake-bottom sediments, the lake shows a 1500+ year history. Our hammer seismic profiles along three transects provide basin thickness and seismic velocity (Vs30) estimates for lake sediments and surrounding bedrock. Coupled with microtremor measurements from 12 three-component Nodal geophones, we calculate horizontal-to-vertical (H/V) spectral ratios that provide a predominant site frequency (F0) at each measured location.

We identify ~50 m of saturated sediments on top of bedrock at the basin depocenter, suggesting a long post-glacial (~14 ka?) record. Vs30 values are consistent with site D-Class stiff soils beneath the Iceberg Lake basin margins and interfluve, and E-Class soft soils within the central basin. F0 estimates from our Nodal data range from less than 2 Hz at sites that lie on the valley floor to more than 50 Hz at our bedrock station without sediment and therefore no low frequency basin resonance. Sites along the basin margins recorded intermediate F0 values, consistent with shallower bedrock depths compared to the basin center. We find a match between F0 derived from microtremor analyses and from Vs30 and bedrock depths derived from hammer seismic data. Through empirical relationships, we correct Vs30 for overburden pressure from past lake levels and compare lake floor slopes with Vs30 values to develop a basin-wide site response model. These data provide inputs to assess ground motions along subduction zones from megathrust, intraslab, and crustal earthquakes using lake sediments. By coupling our results with the lake’s paleoseismic record, we improve our understanding of seismic shaking in subduction zone coastal environments.

Session: From Faults to Fjords: Earthquake Evidence in Terrestrial and Subaqueous Environments - II

Type: Oral

Date: 5/1/2024

Presentation Time: 10:30 AM (local time)

Presenting Author: Lee

Student Presenter: No

Invited Presentation: 

Authors