Nuisance flooding (NF) is defined as minor, nondestructive flooding that causes substantial, accumulating socioeconomic impacts to coastal communities. While sea-level rise is the main driver for the observed increase in NF events in the United States, we show here that secular changes in tides also contribute. An analysis of 40 tidal gauge records from U.S. coasts finds that, at 18 locations, NF increased due to tidal amplification, while decreases in tidal range suppressed NF at 11 locations. Estuaries show the largest changes in NF attributable to tide changes, and these can often be traced to anthropogenic alterations. Limited long-term measurements from estuaries suggest that the effects of evolving tides are more widespread than the locations considered here. The total number of NF days caused by tidal changes has increased at an exponential rate since 1950, adding ~27% to the total number of NF events observed in 2019 across locations with tidal amplification.
The Chandler wobble (CW) and annual wobble (AW) are the two main components of polar motion, which are difficult to separate because of their very close periods. In the light of Fourier dictionary and basis pursuit method, a Fourier basis pursuit (FBP) spectrum is developed, which can reduce spectral smearing and leakage caused by the finite length of the time series. Further, a band-pass filtering method based on FBP spectrum (FBPBPF), which can effectively suppress the edge effect, is proposed in this paper. The simulation test results show that the FBPBPF method can effectively suppress the edge effect caused by spectral smearing and leakage and that its reconstruction accuracy at the boundary is approximately three times higher than the Fourier transform band-pass filtering method, which is based on Hamming windowed FFT spectrum, in extracting quasi-harmonic signals. The FBPBPF method is then applied to Earth's polar motion data during 1900-2015. Through analyzing the amplitude and period variations of CW and AW, and calculating the eccentricity variation of the AW, we found that: (1) the amplitude of the CW is currently at a historic minimum level, and it is even possible to diminish further until a complete stop; and (2) the eccentricity of the AW has a gradually decreased fluctuation during the last 116 years.
How the initial rupturing of the magma-poor rifts started remains a controversial scientific issue due to the lack of definitive studies on the lithospheric structure and mantle flow beneath the lithosphere from these rift systems. We constrain the anisotropic structure of the mantle in the magma-poor Malawi rift zone (MRZ) by observing quasi-Love (QL) waves, which are abnormal waveforms with Rayleigh wave polarization characteristics formed by the scattering of Love waves through lateral gradients in anisotropic structures. Here, we observed clear QL waves at stations in the western MRZ, indicating that there are significant lateral gradients in anisotropy beneath the western MRZ. The stations that recorded QL waves are close to the eastern edge of an unexposed Niassa craton revealed by the latest high-resolution tomographic model AF2019. Comparing the observations of QL waves with recently published shear-wave splitting measurements in the MRZ, we infer that the cratonic lithosphere of the Niassa induced edge-driven convection (EDC) of the asthenospheric mantle on its eastern boundary, resulting in lateral gradients in seismic anisotropy beneath the western MRZ. As the EDC is a small-scale, relatively weak form of mantle convection that is easily overwhelmed by strong active upwelled asthenosphere, its presence indicates the lack of large-scale mantle convection associated with asthenospheric upwelling beneath the MRZ. The magma-poor MRZ could be an example of passive rifting due to the local asthenospheric mantle flow.
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