Extraction of tsunami source coefficients via inversion of DART \circledR^{\circledR} buoy data

The ability to accurately forecast potential hazards posed to coastal communities by tsunamis generated seismically in both the near and far field requires knowledge of so-called source coefficients, from which the strength of a tsunami can be deduced. Seismic information alone can be used to set the source coefficients, but the values so derived reflect the dynamics of movement at or below the seabed and hence might not accurately describe how this motion is manifested in the overlaying water column. We describe here a method for refining source coefficient estimates based on seismic information by making use of data from Deep-ocean Assessment and Reporting of Tsunamis (DART ^\circledR^{\circledR}) buoys (tsunameters). The method involves using these data to adjust precomputed models via an inversion algorithm so that residuals between the adjusted models and the DART ^\circledR^{\circledR} data are as small as possible in a least squares sense. The inversion algorithm is statistically based and hence has the ability to assess uncertainty in the estimated source coefficients. We describe this inversion algorithm in detail and apply it to the November 2006 Kuril Islands event as a case study.     

SrCa and MgCa ratios in polygenetic carbonate allochems from a Michigan marl lake

Rapid accumulation of CaCO₃ is occurring in Littlefield Lake, a marl lake located in central Michigan. The sediment, which is 95% CaCO₃, primarily consists of eight different genetic groups of carbonate allochems. These include calcite muds, sands, algal oncoids and Chara encrustations, as well as the dominant aragonitic gastropods Valvota tricarinota. Gyraulus deflectus and Amnicola integra. and the dominant aragonitic pelecypod Sphaerium partumeium. Samples of each of these groups were analyzed for Ca, Sr and Mg. Molar $\text{Mg}\text{Ca}$ ratios are primarily controlled by allochem mineralogy, with calcitic forms having $\text{Mg}\text{Ca}$ ratios 5–10 times larger than aragonitic (shelled) forms. The $\text{Sr}\text{Ca}$ ratios are primarily controlled by biochemical fractionation, and are significantly lower than $\text{Sr}\text{Ca}$ ratios of inorganically precipitated aragonite from other settings. Partition coefficients were determined for both Sr and Mg for each carbonate allochem group and, based on comparisons with results reported by other workers, the partition coefficients determined here are generally considered ‘typical’ or representative values for biogeneous freshwater carbonates. An analysis of variance of the data indicates that most genera and species of carbonate-secreting organisms in marl lakes have highly characteristic $\text{Sr}\text{Ca}$ and $\text{Mg}\text{Ca}$ ratios. These ratios can potentially serve as geochemical tracers in future investigations of lacustrine carbonate diagenesis. Both Sr and Mg are influenced by grain size and/or surface area, probably due to the presence of these elements in non-lattice-held (exchangeable) positions.