Research ArticleGEOLOGY

Monsoonal control on a delayed response of sedimentation to the 2008 Wenchuan earthquake

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Science Advances  12 Jun 2019:
Vol. 5, no. 6, eaav7110
DOI: 10.1126/sciadv.aav7110
  • Fig. 1 Map of the core site at the Zipingpu reservoir, the Min Jiang River draining the Longmen Shan mountains, and the landslides triggered by the 2008 Mw 7.9 Wenchuan earthquake.

    (A) Study area in the regional context of the Tibetan Plateau and the Yangtze Basin. (B) Map of the Min Jiang catchment (red-outlined polygon) showing earthquake-triggered landslides (21) (yellow polygons), earthquake epicenter (red star), peak ground acceleration (PGA) contours (dashed lines), the Zipingpu reservoir (white circle), and the sites of the Weizhou, Shaba, and Guojiaba hydrological stations (white squares). (C) The core site at the Zipingpu reservoir (white circle) and landslides (21) (yellow polygons). (D) The locations of cores A and B at the Zipingpu reservoir.

  • Fig. 2 Chronology of the Zipingpu reservoir sediment core based on the correlation between reservoir water level and sedimentary MS.

    The red line marks the boundary of the 2008 Wenchuan earthquake (EQ) in the core; two blue arrows show the dates of river interception (23 November 2002) and water impoundment in the reservoir (30 September 2004). The blue-shaded bars indicate annual high water levels (note inverted scale) that are associated with low MS values. Large variation in MS observed before reservoir impoundment is likely due to water discharge (Qw) changes determining the transport and deposition of heavy magnetic minerals. The role of Qw appears to have become highly muted from 2005 onward, with MS instead controlled by reservoir water level. We use this as the basis for our chronology while acknowledging that Qw may still have had minor effects, for example, influencing secondary features in the MS record and potentially introducing some uncertainties especially at seasonal time scales.

  • Fig. 3 Core sedimentary data from the Zipingpu reservoir and water discharge (Qw) in the Min Jiang River before and after the 2008 Wenchuan earthquake.

    Daily mean river Qw (blue), median grain size D50 (black), and Rb/Sr ratios (green). Red line marks the time of the 2008 Wenchuan earthquake based on independent chronology (Fig. 2). Qw data are from the main stem of the Min Jiang at the Weizhou hydrological station (Fig. 1) that accounts for more than 70% of the discharge flux to the Zipingpu reservoir. Both Rb/Sr ratio and D50 show remarkable changes after the earthquake, with the most pronounced shifts in 2010 associated with very high Qw. A slight offset in the timing of changes in sediment properties in 2010 appears to precede the rise in Qw, potentially because of uncertainties in the chronology related to seasonally varying sedimentation rates; the chronology defined by linear interpolation between the low-MS tie points is likely to introduce some uncertainties within a year (Fig. 2) and thus accurately captures the timing of changes at the annual but not at the seasonal scales. CPS, counts per second.

  • Fig. 4 Sedimentological versus hydrological parameters before and after the 2008 Wenchuan earthquake.

    Relationships between (A) annual water discharge and annual depositional thickness and (B) the proportion of total runoff of >3 mm/day and the percentage of coarse grains (>32 μm). Black lines show the best-fit relationship from linear least-squares regression, and the yellow-shaded regions show 95% confidence intervals, excluding the data from 2012 in (A). The outlier from 2012 may result from high Qw dominated by heavy rainfall in the upstream portions of the Min Jiang River catchment with limited earthquake-triggered landslides (fig. S9). In (B), the relationship may reflect a shift from a supply-limited regime (gray-shaded region) to a transport-limited regime (yellow-shaded region) after the Wenchuan earthquake.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/6/eaav7110/DC1

    Supplementary Materials and Methods: Magnetic mineralogy

    Fig. S1. χ-T curves of representative samples.

    Fig. S2. Hysteresis loops of representative samples from the Zipingpu sediment core.

    Fig. S3. Seasonal water level difference of the Zipingpu reservoir.

    Fig. S4. Difference in MS of topmost 60-cm sediments of cores A and B from the Zipingpu reservoir.

    Fig. S5. Grain size distributions of the Zipingpu core sediments around the earthquake.

    Fig. S6. Correlation of Rb/Sr ratio versus Sr content.

    Fig. S7. Interannual variations of daily runoff (>3 mm/day; orange crosses), daily (gray dots), and annual (blue dots) suspended particulate material yield at the Guojiaba hydrological station on the Shouxi River before and after the Wenchuan earthquake.

    Fig. S8. Remaining seismic landslides within the catchment after ten years since the earthquake.

    Fig. S9. Difference in suspended sediment fluxes versus river water discharge (Qw) between two typical tributaries within the Min Jiang Basin during the period of 2006–2012.

    References (4146)

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Materials and Methods: Magnetic mineralogy
    • Fig. S1. χ-T curves of representative samples.
    • Fig. S2. Hysteresis loops of representative samples from the Zipingpu sediment core.
    • Fig. S3. Seasonal water level difference of the Zipingpu reservoir.
    • Fig. S4. Difference in MS of topmost 60-cm sediments of cores A and B from the Zipingpu reservoir.
    • Fig. S5. Grain size distributions of the Zipingpu core sediments around the earthquake.
    • Fig. S6. Correlation of Rb/Sr ratio versus Sr content.
    • Fig. S7. Interannual variations of daily runoff (>3 mm/day; orange crosses), daily (gray dots), and annual (blue dots) suspended particulate material yield at the Guojiaba hydrological station on the Shouxi River before and after the Wenchuan earthquake.
    • Fig. S8. Remaining seismic landslides within the catchment after ten years since the earthquake.
    • Fig. S9. Difference in suspended sediment fluxes versus river water discharge (Qw) between two typical tributaries within the Min Jiang Basin during the period of 2006–2012.
    • References (4146)

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