Exploring the Rheology of a Seismogenic Zone by Applying Seismic Variation

Although the study of spatiotemporal variation of a subsurface velocity structure is a challenging task, it can provide a description of the fault geometry as well as important information on the rheological changes caused by fault rupture. Our main objective is to investigate whether rheological changes of faults can be associated with the seismogenic process before a strong earthquake. For this purpose, a 3D tomographic technique is applied to obtain P- and S-wave velocity structures in central Taiwan using travel time data. The results show that temporal variations in the Vs structure in the source area demonstrate significant spatiotemporal variation before and after the Chi-Chi earthquake. We infer that, before the mainshock, Vs began to decrease (and Vp/Vs increased) at the hanging wall of the Chelungpu fault, which may be induced by the increasing density of microcracks and fluid. However, in the vicinity of the Chi-Chi earthquake’s source area, Vs increased (and Vp/Vs decreased), which may be attributed to the closing of cracks or migration of fluid. The different physical characteristics at the junctional zone may easily generate strong earthquakes. Therefore, seismic velocity changes are found to be associated with a subsurface evolution around the source area in Taiwan. Our findings suggest that monitoring the Vp and Vs (or Vp/Vs) structures in high seismic potential zones is an important ongoing task, which may minimize the damage caused by future large earthquakes.

Challenges in the Preservation of Disaster Remains – Example of the Chelungpu Fault Preservation Park

Taiwan is located along the boundary of the Eurasian and the Philippine Sea plates and experiences tens of thousands of earthquakes each year. Based on historical records, Taiwan has had several earthquakes of magnitude greater than 7.0. Notable and deadly quakes occurred in 1906 (Meishan Earthquake), 1935 (Hsinchu-Taichung Earthquake), and 1999 (Chi-Chi Earthquake). Statistically, Taiwan has had a major earthquake every 30–60 years. Therefore, earthquake museums are needed for long-term earthquake education and geoheritage exhibitions. Earthquake museums highlight disaster risks and preparedness information. The purpose of preserving earthquake remains is to educate visitors about Taiwan’s natural disasters and provide a memorable experience that inspires earthquake preparedness. The Chushan trench across the Chelungpu fault is a good example of Chi-Chi Earthquake rupture. This trench has recorded the five most important earthquake events on the Chelungpu fault. Although the Chelungpu Fault Preservation Park (CFPP) has worked to preserve these earthquake remains, they have been threatened due to seepage over the years. The aim of this paper is to analyze trench seepage and explore the development of an anti-seepage model, to provide a reference for the preservation of earthquake remains and museum development worldwide.

Discrete Element Modeling on Deformation Pattern of Composite Strata Induced by Repeated Thrust Faulting: Case Study of Chushan Site, Central Taiwan

<p>In 1999, Chi-Chi earthquake hit Taiwan and caused severe damage to the infrastructures along the Chelungpu fault because of overburden deformation. Previous study excavated several trenches near the Chelungpu fault to study the fault characteristics and the fault deformation zone. The most important trench, Chushan site, records the Chi-Chi earthquake with 1.7m vertical offset and other four large paleoseismic events. This fault trench was now retained in the Chelungpu Fault Preservation Park, Taiwan that greatly contributes to observing the deformation pattern of overburden layer induced by repeated thrust faulting. For the north wall of the Chushan trench, the east-dipping basal thrust with a dip angle of 24° splits into two branches and the sedimentary layer, which consists of silt layer and gravel layer, is deformed into an asymmetric anticline fold. This observation indicates that the overburden layer in natural is the composite strata and the presence of gravel layer in the composite strata could be an indicator for the coseismic deformation.</p><p>In this study, three-dimensional DEM simulations are conducted to identify the deformation pattern of composite strata under repeated thrust faulting. The numerical model was constructed based on the Chushan trench. Silt layers are made by balls and the gravel layer is compose of balls and ellipsoid particles. Results show that a fault-propagation fold forms during the initial stage of the deformation, and an asymmetric anticline fold with one limb slightly overturned forms in the Chi-Chi earthquake. The rotation of ellipsoid particles in the numerical model indicates the evolution of folding, which conduces to understand the deformation progress in the full faulting process.</p>

Strong‐Motion Directionality and Evidence of Rupture Directivity Effects during the Chi‐Chi Mw 7.6 Earthquake

This article investigates the spatial distribution, predominant direction, and variations in the intensity measures (IMs) with orientation for classified pulse‐like and nonpulse motions during Chi‐Chi Mw 7.6 earthquake. The results show evidence of high polarization for long‐period spectral accelerations at relatively large source‐to‐site distances (50–100 km) north of the Chelungpu fault. The polarization of long‐period motions shows a clear correlation with the directivity parameters’ isochrone directivity predictor and ξ, indicating a connection between directionality and rupture directivity. The variation in strong‐motion directionality with the period is also studied. The discrepancy in directionality caused by strong directivity increases with the period from 1 to 10 s, which confirms a clear correlation of period‐dependent directionality with directivity effects. This study finds stronger directionality of pulse‐like motions than nonpulse motions for long periods over 3 s with higher maximum‐to‐median and maximum‐to‐minimum IM ratios. For periods over 3 s, the maximum‐to‐median ratios of pulse‐like motions are higher than the mean prediction of the Shahi and Baker (2014a) model, whereas those of nonpulse motions are lower than the prediction. However, this study does not find simple and clear results for the directions of the maximum component at different periods for pulse‐like and nonpulse motions. Despite clear differences between the unidirectional fling‐step and bidirectional forward directivity pulses, the effects of fling‐step and forward directivity are actually coupled in the waveforms.

Detection of a fracture zone using microtremor array measurement

We have conducted microtremor array measurements to estimate shallow S-wave velocity ([Formula: see text]) structures at two sites (the 921 Earthquake Museum of Taiwan and the Taiwan Provincial Consultative Council) located near surface ruptures of the Chelungpu Fault. Ten stations, consisting of three different-aperture triangles and a central station, are adopted for each array deployment. Using the array data, we calculate dispersion curves of Rayleigh waves using the frequency-wavenumber spectrum method and then estimate [Formula: see text] structures by the surface-wave inversion technique. The obtained 2D [Formula: see text] profiles could clearly show compressive and flexural deformation structures with the surface ruptures located at relatively weak (low [Formula: see text]) zones. This indicates compressive buckling as the most likely mechanism for surface rupturing along these low [Formula: see text] zones. Importantly, this study successfully depicts strata disturbances in a fault fracture zone using microtremor array measurements and forward numerical modeling of trishear fault-propagation folds.

Surface Folds Hint at Magnitude of Slip Along Thrust Faults

The shape of deformed sediments at the surface may allow researchers to estimate the cumulative slip along thrust faults such as the Chelungpu fault in Taiwan.