Instrumental and historical records have proved too short to estimate the potential magnitudes an... more Instrumental and historical records have proved too short to estimate the potential magnitudes and recurrence intervals of rare events such as the 2004 Indian Ocean and 2011 Tohoku-Oki great earthquakes and tsunamis. Paleoseismology improves our understanding of subduction zone hazards by extending earthquake histories thousands of years into the past. Through paleoseismic investigations at subduction zones, we scrutinize coastal sediments in low-energy depositional environments to reconstruct relative sea-level (RSL) changes related to upper-plate deformation from past earthquakes and tsunami inundation. Microfossils—long recognized as valuable RSL indicators—in coastal sediment provide an independent test of earthquake related RSL change and tsunami deposition inferred from coastal stratigraphy. Here, we illustrate the value and potential of paleoseismic methods at megathrust sites in Sumatra, Chile, and the eastern Aleutian Islands to identify both coseismic subsidence and uplift...
Although metropolitan central Chile (32-35oS) has a 470-year-long written seismic history, the co... more Although metropolitan central Chile (32-35oS) has a 470-year-long written seismic history, the country’s longest, no attempt has been made to match this history with the geological record. Doing so would extend back in time, perhaps in thousands of years, the seismic story of the Chile’s most populated area. Here, we use historical, sedimentary and biological evidence to reconstruct the paleoseismic record of central Chile. A coastal lowland site north of Valparaíso contains geological and biological evidence for three instances of coseismic uplift, with accompanying paleotsunami deposits. Each earthquake is marked by a buried soil sharply overlain by a distinct, tabular, and laterally continuous sheet of pale sand. The inferred earthquakes and tsunami deposits are dated to 4,300, 3,800, 3,500 yrs BP from in situ plant remains. This record of paleoseismicity in central Chile is typical of a subduction zone coastline that has experienced falling relative sea-levels during the late Ho...
The~500-year historical record of seismicity along the central Chile coast (30e34 S) is character... more The~500-year historical record of seismicity along the central Chile coast (30e34 S) is characterized by a series of~M 8.0e8.5 earthquakes followed by low tsunamis (<4 m) occurring on the megathrust about every 80 years. One exception is the AD 1730 great earthquake (M 9.0e9.5) and high tsunami (>10 m), but the frequency of such large events is unknown. We extend the seismic history of central Chile through a study of a lowland stratigraphic sequence along the metropolitan coast north of Valparaíso (33 S). At this site, higher relative sea level during the mid Holocene created a tidal marsh and the accommodation space necessary for sediment that preserves earthquake and tsunami evidence. Within this 2600-yr-long sequence, we traced six laterally continuous sand beds probably deposited by high tsunamis. Plant remains that underlie the sand beds were radiocarbon dated to 6200, 5600, 5000, 4400, 3800, and 3700 cal yr BP. Sediment properties and diatom assemblages of the sand bedsdfor example, anomalous marine planktonic diatoms and upward fining of silt-sized diatom valvesdpoint to a marine sediment source and high-energy deposition. Grain-size analysis shows a strong similarity between inferred tsunami deposits and modern coastal sediment. Upward fining sequences characteristic of suspension deposition are present in five of the six sand beds. Despite the lack of significant lithologic changes between the sedimentary units under-and overlying tsunami deposits, we infer that the increase in freshwater siliceous microfossils in overlying units records coseismic uplift concurrent with the deposition of five of the sand beds. During our mid-Holocene window of evidence preservation, the mean recurrence interval of earthquakes and tsunamis is~500 years. Our findings imply that the frequency of historical earthquakes in central Chile is not representative of the greatest earthquakes and tsunamis that the central Chilean subduction zone has produced.
On February 27, 2010 a great M 8.8 earthquake and accompanying tsunami struck the region between ... more On February 27, 2010 a great M 8.8 earthquake and accompanying tsunami struck the region between Constitución and Concepción in south-central Chile. In the year immediately preceding this event, we described and surveyed deposits from previous tsunamis at several sites in the Concepción region (36.5°-38.5° S. Lat). This research positioned us to document the geomorphic and tectonic effects of the
Instrumental and historical records have proved too short to estimate the potential magnitudes an... more Instrumental and historical records have proved too short to estimate the potential magnitudes and recurrence intervals of rare events such as the 2004 Indian Ocean and 2011 Tohoku-Oki great earthquakes and tsunamis. Paleoseismology improves our understanding of subduction zone hazards by extending earthquake histories thousands of years into the past. Through paleoseismic investigations at subduction zones, we scrutinize coastal sediments in low-energy depositional environments to reconstruct relative sea-level (RSL) changes related to upper-plate deformation from past earthquakes and tsunami inundation. Microfossils—long recognized as valuable RSL indicators—in coastal sediment provide an independent test of earthquake related RSL change and tsunami deposition inferred from coastal stratigraphy. Here, we illustrate the value and potential of paleoseismic methods at megathrust sites in Sumatra, Chile, and the eastern Aleutian Islands to identify both coseismic subsidence and uplift...
Although metropolitan central Chile (32-35oS) has a 470-year-long written seismic history, the co... more Although metropolitan central Chile (32-35oS) has a 470-year-long written seismic history, the country’s longest, no attempt has been made to match this history with the geological record. Doing so would extend back in time, perhaps in thousands of years, the seismic story of the Chile’s most populated area. Here, we use historical, sedimentary and biological evidence to reconstruct the paleoseismic record of central Chile. A coastal lowland site north of Valparaíso contains geological and biological evidence for three instances of coseismic uplift, with accompanying paleotsunami deposits. Each earthquake is marked by a buried soil sharply overlain by a distinct, tabular, and laterally continuous sheet of pale sand. The inferred earthquakes and tsunami deposits are dated to 4,300, 3,800, 3,500 yrs BP from in situ plant remains. This record of paleoseismicity in central Chile is typical of a subduction zone coastline that has experienced falling relative sea-levels during the late Ho...
The~500-year historical record of seismicity along the central Chile coast (30e34 S) is character... more The~500-year historical record of seismicity along the central Chile coast (30e34 S) is characterized by a series of~M 8.0e8.5 earthquakes followed by low tsunamis (<4 m) occurring on the megathrust about every 80 years. One exception is the AD 1730 great earthquake (M 9.0e9.5) and high tsunami (>10 m), but the frequency of such large events is unknown. We extend the seismic history of central Chile through a study of a lowland stratigraphic sequence along the metropolitan coast north of Valparaíso (33 S). At this site, higher relative sea level during the mid Holocene created a tidal marsh and the accommodation space necessary for sediment that preserves earthquake and tsunami evidence. Within this 2600-yr-long sequence, we traced six laterally continuous sand beds probably deposited by high tsunamis. Plant remains that underlie the sand beds were radiocarbon dated to 6200, 5600, 5000, 4400, 3800, and 3700 cal yr BP. Sediment properties and diatom assemblages of the sand bedsdfor example, anomalous marine planktonic diatoms and upward fining of silt-sized diatom valvesdpoint to a marine sediment source and high-energy deposition. Grain-size analysis shows a strong similarity between inferred tsunami deposits and modern coastal sediment. Upward fining sequences characteristic of suspension deposition are present in five of the six sand beds. Despite the lack of significant lithologic changes between the sedimentary units under-and overlying tsunami deposits, we infer that the increase in freshwater siliceous microfossils in overlying units records coseismic uplift concurrent with the deposition of five of the sand beds. During our mid-Holocene window of evidence preservation, the mean recurrence interval of earthquakes and tsunamis is~500 years. Our findings imply that the frequency of historical earthquakes in central Chile is not representative of the greatest earthquakes and tsunamis that the central Chilean subduction zone has produced.
On February 27, 2010 a great M 8.8 earthquake and accompanying tsunami struck the region between ... more On February 27, 2010 a great M 8.8 earthquake and accompanying tsunami struck the region between Constitución and Concepción in south-central Chile. In the year immediately preceding this event, we described and surveyed deposits from previous tsunamis at several sites in the Concepción region (36.5°-38.5° S. Lat). This research positioned us to document the geomorphic and tectonic effects of the
Uploads
Papers by L. Ely