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Understanding subduction zone earthquakes: The 2004 Sumatra earthquake example

New Geology articles posted online ahead of print June 23, 2015

Geological Society of America

Boulder, Colo. -- The 26 December 2004 Mw ~9.2 Indian Ocean earthquake (also known as the Sumatra-Andaman or Aceh-Andaman earthquake), which generated massive, destructive tsunamis, especially along the Aceh coast of northern Sumatra, Indonesia, clearly demonstrated the need for a better understanding of how frequently subduction zone earthquakes and tsunamis occur. Toward that end, Harvey M. Kelsey of Humboldt State University and colleagues present a study of earthquake history in the area.

Using subsidence stratigraphy, the team traced the different modes of coastal sedimentation over the course of time in the eastern Indian Ocean where relative sea-level change evolved from rapidly rising to static from 8,000 years ago to the present day.

Kelsey and colleagues discovered that 3,800 to 7,500 years ago, while sea level was gradually rising, there were seven subduction zone earthquakes recorded in coastal deposits. This was determined in part by the fact that each earthquake caused burial of a mangrove soil by sediment and/or deposition of tsunami sand at the time of the earthquake.

The team also discovered that sea level gradually stopped rising about 3,800 years ago, which meant that buried soils no longer formed. Thus, detecting subduction zone earthquakes required a different approach. They found a record of successive earthquakes in a sequence of stacked tsunami deposits on the coastal plain. Individual tsunami deposits were 0.2 to 0.5 m thick. Based on this information, Kelsey and colleagues determined that in the past 3,800 years there were between four and six tsunamis caused by Andaman-Aceh-type earthquakes.

The authors conclude that knowing the relative sea-level record for a coastal region on a subduction zone margin is the initial step in investigating paleoseismic history. For mid-latitude coasts that border subduction zones, sequences of buried soils may provide a long-duration, subsidence stratigraphic paleoseismic record that spans to the present, but in other settings such as the Aceh coastal plain, joint research approaches, for example targeted foraminiferal analyses and palynology, are required to both exploit the changing form of the relative sea-level curve and characterize coastal evolution in the context of the diminishing importance of accommodation space.

Featured article: Accommodation space, relative sea level, and the archiving of paleo-earthquakes along subduction zones. Harvey M. Kelsey et al., Humboldt State University, Arcata, California, USA. Published online ahead of print on 23 June 2015; http://dx.doi.org/10.1130/G36706.1.

Other recently posted Geology articles (see below) cover such topics as:

  1. The 1 April 2014 M 8.1 Chile earthquake;
  2. The Sept. 2013 flooding along the Front Range in Colorado, USA;
  3. A new dataset from the EarthScope USArray project

Upper plate reverse fault reactivation and the unclamping of the megathrust during the 2014 northern Chile earthquake sequence. Gabriel González et al., National Research Center for Integrated Natural Disaster Management, Universidad Católica del Norte, Antofagasta, Chile. Published online ahead of print on 23 June 2015; http://dx.doi.org/10.1130/G36703.1.

The 1 April 2014 magnitude 8.1 earthquake offshore Iquique, Chile, was the largest earthquake in the region since 1877. The earthquake, which occurred on the Andean subduction zone interface, was preceded by more than two weeks of foreshocks, most notably a magnitude 6.7 earthquake on 16 March. This foreshock did not occur on the subduction zone but on a fault within the South American tectonic plate, and its geometry is similar to that of numerous faults documented onshore in northern Chile. In this study, Gabriel González and colleagues show that this event was the primary trigger for subsequent foreshocks that preceded the magnitude 8.1 earthquake, as it unclamped the subduction zone interface, facilitating slip that led to the main event.

Aspect-dependent soil saturation and insight into debris-flow initiation during extreme rainfall in the Colorado Front Range. Brian A. Ebel et al., Cooperative Institute for Research in Environmental Sciences (CIRES), and Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309, USA, and U.S. Geological Survey, National Research Program, Denver, Colorado 80225, USA. Published online ahead of print on 23 June 2015; http://dx.doi.org/10.1130/G36741.1.

Extreme rainfall events in interior continental areas can cause widespread slope failures, leading to threats to life and property. These events can be especially threatening because emergency preparation for slope failure is minimal and knowledge of the landscape conditions that make particular locations vulnerable is poorly characterized. In September of 2013, a storm event in the Colorado Front Range, USA, produced rainfall totals with a 1,000-year recurrence interval, causing flooding and debris flows that took eight lives and damaged nearly 19,000 homes, spreading over 320 km and 17 counties. Here, Brian A. Ebel and colleagues show aspect-dependent hydrologic behavior may result from (i) a larger gravel/stone fraction, and hence lower soil-water storage capacity, on south-facing slopes, and (ii) lower weathered-bedrock permeability on south-facing slopes, because of lower tree density and associated deep roots penetrating bedrock as well as less intense weathering, inhibiting soil drainage.

Quantifying the post-tectonic topographic evolution of closed basins: The Ebro basin (northeast Iberia). Daniel Garcia-Castellanos and Juan Cruz Larrasoaña, Instituto de Ciencias de la Tierra Jaume Almera, ICTJA-CSIC, Barcelona, Spain, and Instituto de Ciencias de la Tierra Jaume Almera, ICTJA-CSIC, Barcelona, Spain, and Instituto Geológico y Minero de España, Unidad de Zaragoza, Zaragoza, Spain. Published online ahead of print on 23 June 2015; http://dx.doi.org/10.1130/G36673.1.

Basins formed within mountainous regions often become perfect sedimentary traps that do not drain to the sea but to internal evaporitic lakes instead. When they do, their sediment layers ideally record the climatic, topographic, and tectonic history of the surroundings. And when these basins eventually overtop or overfill with sediment, they are rapidly excavated by the new outflowing fluvial network, exposing excellent stratigraphic outcrops. However, this erosion often removes the uppermost basin infill, and essential information about the late basin history is lost. In this study, Daniel Garcia-Castellanos and Juan Crus Larrasoaña estimate the timing and elevation of the maximum infill of the Ebro basin (NE Spain) by computing the rebound of the basin in response to erosion, adopting the common idea that Earth's rigid outer shell (the lithosphere) rests on a fluid magmatic asthenosphere in an Archimedes-type equilibrium. They combine these calculations with existing paleomagnetic ages of the sediment basin infill. Their results show that the basin became overfilled between 12 and 7.5 million years ago, and that it reached a maximum elevation of up to 750 m above present sea level. The basin has been ever since incised at a rate close to 0.1 mm/yr and has been isostatically uplifted by up to 630 m at its center. This uplift may explain why the Ebro River, opposite to other large Mediterranean rivers, did not excavate a deep gorge within its own basin during the desiccation of the Mediterranean (Messinian salinity crisis, 5.5 million years ago).

USArray shear wave splitting shows seismic anisotropy from both lithosphere and asthenosphere. Sutatcha Hongsresawat et al., University of Florida, Gainesville, Florida, USA. Published online ahead of print on 23 June 2015; http://dx.doi.org/10.1130/G36610.1.

North America provides an important test for assessing the coupling of large continents with heterogeneous lithosphere to underlying mantle flow. Here we show a dense new dataset of measurements of a seismic observation called splitting intensity (at more than 1400 seismic stations from the EarthScope USArray project) that has a potential to solve a long-standing debate regarding the source of upper mantle fabrics. These fabrics cause seismic anisotropy (dependence of velocity of seismic waves on direction of propagation and polarization of waves) resulting from alignment of minerals in either lithospheric mantle or underlying asthenosphere, and our measurements of splitting intensity are sensitive to this anisotropy. Shear in the asthenosphere due to viscous coupling at the base of lithospheric plates or vertical coherent deformation through the crust and lithosphere during tectonic interactions are both invoked as mechanisms for this upper mantle fabric development, but should result in different characteristic measurements of splitting intensity. We conclude that upper mantle seismic anisotropy may be sourced in both the asthenosphere and lithosphere, and variations in measurements are due to lithospheric anisotropy developed during deformation over long timescales.

Recognition and importance of amalgamated sandy meander belts in the continental rock record. Adrian J. Hartley et al., University of Aberdeen, Aberdeen, UK. Published online ahead of print on 23 June 2015; http://dx.doi.org/10.1130/G36743.1.

The deposits of ancient rivers form important aquifers, reservoirs for hydrocarbons, and mineral accumulations, such as uranium and copper deposits. Understanding how extensive and connected the sandstones are that are deposited by these ancient rivers is important for predicting the volume and extent of these resources. Sandy meandering rivers are a common and important feature of modern landscapes yet are not widely recognized in the rock record. Adrian J. Hartley and colleagues use examples of exceptional exposures of ancient meander belts in Jurassic rocks from Utah to explain why geoscientists haven't recognized this type of river deposits in the past and to outline the new approaches needed to fully understand and exploit resources contained within these ancient river deposits.

Latitudinal temperature gradients and high-latitude temperatures during the latest Cretaceous: Congruence of geologic data and climate models. Garland R. Upchurch Jr. et al., Texas State University, San Marcos, Texas, USA. Published online ahead of print on 23 June 2015; http://dx.doi.org/10.1130/G36802.1.

A major challenge in paleoclimatology is disagreement between data and models for periods of warm climate. Data generally indicate equable conditions and reduced latitudinal temperature gradients, while models generally produce colder conditions and steeper latitudinal gradients except when using very high CO2. Here we show congruence between temperature indicators and climate model output for the cool greenhouse interval of the latest Cretaceous (Maastrichtian) using a global database of terrestrial and marine indicators and fully coupled simulations with the Community Climate System Model version 3. In these simulations, we explore potential roles of greenhouse gases and properties of pre-anthropogenic liquid clouds in creating warm conditions. Our model simulations successfully reproduce warm polar temperatures and the latitudinal temperature gradient without overheating the tropics. Best fits for mean annual temperature are simulations that use 6-times preindustrial levels of atmospheric CO2, or 2-times preindustrial levels of atmospheric CO2 and liquid cloud properties that may reflect pre-anthropogenic levels of cloud condensation nuclei. The Siberian interior is problematic, but this may relate to reconstructed elevation and the presence of lakes. Data and models together indicate tropical sea-surface temperatures approx. 5 degrees Celsius above modern, an equator-to-pole temperature difference of 25-30 degrees Celsius, and a mid-latitudinal temperature gradient of approx. 0.4 degrees Celsius per one degree latitude, similar to the Eocene. Modified liquid cloud properties allow successful simulation of Maastrichtian climate at the relatively low levels of atmospheric CO2 indicated by proxies and carbon cycle modeling. This supports the suggestion that altered properties of liquid clouds may be an important mechanism of warming during past greenhouse intervals.

Chlorine in mantle-derived carbonatite melts revealed by halite in the St.-Honoré intrusion (Québec, Canada). Vadim S. Kamenetsky et al., University of Tasmania, Hobart, Tasmania, Australia. Published online ahead of print on 23 June 2015; http://dx.doi.org/10.1130/G36843.1.

To understand how planet Earth formed and changed over time researchers need to know how the composition of the deepest Earth has evolved with time. Carbonatite magmas, which are extremely rich in CO2, derive from Earth's mantle and provide important information about the composition of the deep Earth. This study by Vadim S. Kamenetsky and colleagues reveals that carbonatite magmas have different composition than previously thought and indicates that unexpectedly larger concentrations of elements like chlorine are present in carbonatites and, by implication, in Earth's interior. This study shows that carbonatite magmas are very enriched in Na and Cl, contrarily to what inferred from previous studies of carbonatites (with the notable exception of the Oldoynio Lengai volcano). This discovery has important implications for past and potentially future evolution of the mantle-crust-atmosphere system.

Europium anomalies constrain the mass of recycled lower continental crust. Ming Tang et al., University of Maryland, College Park, Maryland 20742, USA. Published online ahead of print on 23 June 2015; http://dx.doi.org/10.1130/G36641.1.

While nature creates the crust, it destroys the crust from beneath. Using available crustal samples and geochemical modeling, we found that about four times by mass the continental crust has been created in the entire Earth history, 75% of which has sunk into the mantle (deep part of the silicate Earth). Such large scale crustal recycling processes have significant impacts on the evolution of the silicate Earth.

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