Preservation of amorphous ultrafine material: A proposed proxy for slip during recent earthquakes on active faults.

Tetsuro Hirono,Shunya Kaneki,Akihiro Ito,Satoru Asayama

doi:10.1038/srep36536

Abstract

The criteria for designating an “Active Fault” not only are important for understanding regional tectonics, but also are a paramount issue for assessing the earthquake risk of faults that are near important structures such as nuclear power plants. Here we propose a proxy, based on the preservation of amorphous ultrafine particles, to assess fault activity within the last millennium. X-ray diffraction data and electron microscope observations of samples from an active fault demonstrated the preservation of large amounts of amorphous ultrafine particles in two slip zones that last ruptured in 1596 and 1999, respectively. A chemical kinetic evaluation of the dissolution process indicated that such particles could survive for centuries, which is consistent with the observations. Thus, preservation of amorphous ultrafine particles in a fault may be valuable for assessing the fault’s latest activity, aiding efforts to evaluate faults that may damage critical facilities in tectonically active zones.

Highlights

Established the mineralogical and crystallographic nature of these particles
We evaluated the duration of these particles, in both the ATTL and the Chelungpu fault, by using the chemical kinetics of the dissolution reaction to establish an indicator for recent fault activity
The results showed that nanometric particles (100 nm radius) of kaolinite, amorphous silica and muscovite would disappear within approximately 1000 years whereas quartz and montmorillonite particles would survive longer (Fig. 4a) and that micrometric particles (10 μm radius) of all components would hardly dissolve at all within 1000 years (Fig. 4b)

Summary

Methodological procedure

We tested a new approach consisting of the following four steps: (1) Determine the amount of the amorphous component both in and around the fault and conduct microscopic observations of the ultrafine particles. (2) Determine whether the ultrafine particles are crystalline or amorphous and analyse their atomic composition. (3) Characterize the environment in which the ultrafine particles formed (near-surface or hydrothermal) on the basis of their nanometric structure and in-situ environmental conditions such as temperature and pH. (4) Perform chemical kinetic modelling of the dissolution process to estimate the lifetime of amorphous ultrafine particles, constraining the most recent time that the fault slipped. The standard deviation of total mineral amounts (and unfitted components) from these RockJock results was reported as ±5.9 wt.%6 To test this attribution and attempt to quantify the amount of amorphous material in the samples, we obtained XRD spectra of prepared mixtures of amorphous silica (Kanto Chemical, Japan) and quartz powder (Wako Pure Chemical Industries, Japan) by using a zero-diffraction plate (made from a single silicon crystal). Sharp peaks in the XRD spectra of these mixtures were attributed to crystalline quartz and the internal standard (α-alumina), but wider, more subdued peaks were observed around 20–30° 2θ, indicating amorphous material[21,22], in samples with smaller quartz contents (Supplementary Figure 2a) Most silicate minerals, such as feldspar, show similar broad bump in their amorphous state[23]. Triplicate multivariate analyses of the XRD spectra from two separate samples (six analyses in total) showed that determinations of the amount of the amorphous component had a standard deviation of ±1.2 wt.%

Results

Discussion

Methods