A Pleistocene Person

Some thoughts on stuff.

Thursday, January 26, 2006

Quaternary Studies are actually useful

Periodically, I'll make mention of a paper in the broad field of Quaternary Science (which I reckon can cover anything from ice cores to stone tools) that strikes me as being particularly interesting. I thought I'd start with that rarity; a study which actually has wider relevance!

In this case it is is work that uses the record of diatoms in Holocene coastal sediments to track events during an earthquake. The authors quantify (using funky maths!) the relationship between a particular assemblage of diatoms and elevation relative to mean high water. This ultimately enables the investigation (albeit indirectly) of subsidence patterns related to a quake event.

Transfer functions are also being used in other contexts, for example in the reconstruction of past temperatures from assemblages of pollen and also midges. Such research isn't without its problems, but is becoming increasingly widespread. In particular, having a quantified estimate of a past climate is especially useful as it enables the testing of climate models. If you want to see how they cope with simulating a different climate from today then check them against the palaeorecord. Without quantification, this would be an imprecise process; now it is less so (though obviously precision doesn't equate to accuracy).

Anyway, the potential importance of this particular study is nicely summed up by the authors first sentence:

"Future earthquake forecasting and reduction of loss require knowing the history of large earthquakes, including their frequency and how patterns of coseismic land movement vary during different earthquakes."

Shennan, I. and Hamilton, S. (2006) Coseismic and pre-seismic subsidence associated with great earthquakes in Alaska. Quaternary Science Reviews, 25, 1-8.

Absract: Alternating beds of peat and mud in sediment sequences on the south-central Alaskan coast record coseismic and inter-seismic relative land and sea-level movements caused by repeated great earthquakes on the Alaska–Aleutian subduction zone. During the AD 1964 Mw=9.2 earthquake, tidal marshes and wetlands around upper Cook Inlet experienced up to 2 m of subsidence, burying peat-forming communities with intertidal mud. Here we use quantitative analyses of fossil diatoms within peat–mud couplets to reconstruct land/sea-level changes for the 1964 and five earlier great earthquakes during the past 3300 years. In contrast to geodetic observations that are limited to the present post-seismic phase, we quantify varying spatial patterns of uplift and subsidence through complete earthquake cycles. Relative land uplift characterises most of the inter-seismic phase of each cycle at our sites, whereas each great earthquake was preceded by a short period of pre-seismic relative land subsidence.


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