Research ArticleGEOPHYSICS

Seismic observations, numerical modeling, and geomorphic analysis of a glacier lake outburst flood in the Himalayas

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Science Advances  16 Sep 2020:
Vol. 6, no. 38, eaba3645
DOI: 10.1126/sciadv.aba3645
  • Fig. 1 Region of study.

    Lugge Tsho was the source of the 1994 GLOF event, while Raphstreng Tsho and Thorthormi Tsho are also considered high risk for future GLOFs. Locations of the five seismometers used in the study are denoted by the yellow circles in (A). (B) Location of the Punakha and Wangdue villages downstream of the GLOF source area. (C) Unstable glacial lakes in the Lunana region. (D) Mid-Holocene glacier moraines that were left largely intact by the GLOF. (E) Lugge Tsho and moraine breach zone.

  • Fig. 2 Key seismic observations on 7 October 1994.

    (A) Example of a seismic trace (station BB20 BHZ) band-pass filtered between 1 and 5 Hz and spectrogram from 1 to 5 Hz during the GLOF duration. Black arrows mark seismic events that correlate in time with events in the USGS catalog, and gray arrows mark potential local earthquakes that appear on all five stations (fig. S2) but are not in the catalog. mb, body wave magnitude; Mw, moment magnitude. (B) Twelve-hour time series of signal coherence for the day containing the GLOF event (red; 1994-10-07) and the day before the GLOF event (gray; 1994-10-06). The top and bottom bounds of each shaded region represent the 95th and 75th percentile of coherence values across all lag times, respectively. The blue shaded regions mark 90 min of most significant GLOF signal relative to background levels. (C) Migrations of the CCFs during the two intervals using a velocity of 3.0 km s−1 (likely indicating regional short-period Rayleigh waves; fig. S3) for several station pairs is summed together to form these final coherence maps (see Materials and Methods and fig. S3). These two images illustrate the downstream progression of seismic energy generated by the GLOF. The coherence map on the left corresponds to seismic energy generated during the initial GLOF breach, while the map on the right corresponds to seismic energy generated approximately 4 hours later and ~70 km downstream. Times are Asia/Thimphu local time [universal time coordinated (UTC) +6].

  • Fig. 3 Results from the HEC-RAS 2D unsteady flood model.

    (A) Elevation profile of the river valley, with hourly locations of flood arrival time from the best-fit model run. (B) Distance from the moraine breach versus time. The gray square symbols and brackets are independent observations from seismic, eyewitness, and gauge station sources. The color-shaded regions represent the range of model outputs that match observations within ±30 min, and the colored curves represent the single best-fit model run. The orange curve is the simulated arrival of the flood wave, the blue curve is the peak flow, and the yellow curve indicates when the flow has subsided and reached 1/e (~37%) of the peak. The horizontal separation of the orange and blue curves indicates the duration between flood arrival and peak flow for a given location. The gray (dashed) boxes indicate the intervals in Fig. 2 during which the peaks in coherent seismic energy were detected. (C) Map view of region also with modeled flood arrival times. Times are Asia/Thimphu local time (UTC+6).

  • Fig. 4 Geomorphic map of the Lunana area.

    The lower (red) moraines were deposited by the glacier more than 4000 years ago during the mid-Holocene, as indicated by the 10Be ages of three sampled boulders given with 1-s analytical error. The younger lake-fringing (purple) moraines are late-Holocene in age.

  • Table 1 Range of modeled flood arrival and peak flow times at populated villages.

    Flood arrivalPeak flow
    TimeHours from breachTimeHours from breach

Supplementary Materials

  • Supplementary Materials

    Seismic observations, numerical modeling, and geomorphic analysis of a glacier lake outburst flood in the Himalayas

    J. M. Maurer, J. M. Schaefer, J. B. Russell, S. Rupper, N. Wangdi, A. E. Putnam, N. Young

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    • Figs. S1 to S13
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