By developing a tomographic model using measurements of surface waves derived from noise rather than seismic waves, as has traditionally been done by seismologists, a National Taiwan University (NTU) research team has announced new findings about mountain formation in Taiwan.
NTU doctoral student Huang Tzu-yin (黃梓殷) co-authored the research with NTU and Academia Sinica researchers, analyzing noisy waveform from the nation’s 85 seismic stations to explain the formation of the Taiwan mountain belt, which has long been debated.
Huang said the research did without the analysis of seismic waves generated by earthquakes, instead turning to surface waveform, which is usually regarded as irrelevant noise detected on a seismometer. The team found that the pattern of noisy waveform is extremely similar to seismic waves and therefore could be taken as a convenient substitute to analyze geological fabrics kilometers deep into the Earth’s crust beneath Taiwan.
Photo: Tang Chia-ling, Taipei Times
It requires only two seismic stations measuring noisy waveforms to produce signals similar to those generated by earthquakes, without a real quake needing to happen, Huang said.
Comparing her methodology with computed tomography scans, Huang said that a geological structure can be “scanned” by analyzing the different directions and speeds a noisy waveform travels at to map out rock formation and arrays of rock strata deep in the Earth’s crust and even the lithosphere.
The advantage of “noise seismology” is that it is free from the uneven temporal and spatial distribution of earthquakes, enabling an understanding of a geological structure at any given place and time.
Based on their findings, Huang and her team proposed a new theory — layered deformation — to explain the mountain formation in the Taiwan orogen, disputing two major contesting theories in the field: The “thin-skinned” and “thick-skinned” models.
The thin-skinned model hypothesizes that crustal deformation occurs mainly in the upper crust and only involves sedimentary cover, with a detachment surface separating the deforming upper layers from the deeper crustal rocks. In the thick-skinned model, the deformation is accommodated to basement rocks’ crust as well as its shallow portions, with the basement crustal rocks and the upper crust undergoing appreciable and consistent deformation.
The theory of layered deformation disputes both models by contending that both the upper crust and basement rock crust undergo deformation, but that the upper crust is dominated by collision-related compressional deformation, whereas the lower crust is dominated by convergence-parallel shear deformation, Huang said.
There is a nearly 90o rotation of anisotropic fabrics across a 10km to 20km depth of the Taiwan orogen, she said.
The research was published in the US journal Science this month.
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