Recurrent giant earthquakes in South-Central Chile revealed by lacustrine sedimentary records
Moernaut, J.; de Batist, M. A.; Heirman, K.; van Daele, M.; Brümmer, R.; Pino, M.; Urrutia, R.; Wolff, C.; Brauer, A.; Roberts, S.; Kilian, R. (2009). Recurrent giant earthquakes in South-Central Chile revealed by lacustrine sedimentary records. Eos, Trans. (Wash. D.C.) 90(52): NH43B-1319 In: Eos, Transactions, American Geophysical Union. American Geophysical Union: Washington. ISSN 0096-3941; e-ISSN 2324-9250, more |
| Authors | | Top | | - Brümmer, R.
- Pino, M.
- Urrutia, R.
- Wolff, C.
| - Brauer, A.
- Roberts, S.
- Kilian, R.
|
| Abstract | Megathrust (‘giant’) earthquakes in the South-Central Chilean subduction zone (e.g. 1960 earthquake; Mw: 9.5) cause landslides, tsunamis, soil liquefaction, coastal uplift/subsidence, volcanic eruptions, all of which pose a major threat to society. A reliable seismic hazard assessment requires establishing if such mega-events occurred in the past and determining their recurrence pattern. The Lake District (39-42°S) in South-Central Chile, located in the northern half of the 1960 earthquake rupture zone, contains several large, steep-sloped glacigenic lakes with high sedimentation rates, and whose sedimentary deposits are highly susceptible to earthquake-triggered slope instability. To establish the reoccurrence interval of earthquakes during the Late Holocene, we mapped the spatial distribution of seismically-induced sedimentary ‘event’ deposits and structures in each lake using very-high resolution seismic data, and collected a series of short gravity cores and long piston cores. Multi-proxy sedimentary analyses (color, magnetic susceptibility, density, geochemistry, grain size), radiocarbon dating, varve-counting, and tephro-stratigraphy were used to identify ‘event’ deposits in each core and correlate paleoseismic horizons across basins. The sediment sequences investigated contain four main types of earthquake-induced structures: 1) multiple mass-wasting deposits on a single stratigraphic level, which are relicts of a basin-wide, subaqueous slope instability ‘event’; 2) homogenite deposits in the deepest parts of steep basins indicative of lake seiches and tsunamis; 3) fluid-escape structures (e.g. sediment volcanoes), which reflect sudden liquefaction in buried mass-wasting deposits and subsequent vertical fluidization flow; 4) in-situ deformed units (e.g., contorted bedding) in nearly-flat layers, which reflect strong horizontal ground acceleration events. Comparison with historical earthquakes suggests the spatial extent, thickness, nature of event deposits provide key quantitative information about local earthquake intensity. For example, deposits associated with the giant earthquakes of 1575 and 1960 earthquakes are well-defined in all records, but event deposits associated with the smaller 1737 and 1837 earthquakes are more fragmentary. In summary, we found up to 25 paleoseismic ‘events’ with a sedimentary signature comparable with that of the giant 1960 earthquake. Our data enables reliable recurrence rates for ‘giant’ seismic events to be calculated, and will help improve earthquake hazard assessments for this part of Chile. |
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