Quantification of Hanging-Wall Effects on Ground Motion: Some Insights from the 1999 Chi-Chi Earthquake

Abstract

Accelerometric records of the Chi-Chi earthquake from sites on the hanging wall exhibit larger acceleration than those from the footwall. Based on ground accelerations recorded at 79 near-field stations (10 hanging-wall stations and 69 footwall stations, respectively) and precise mapping of fault-rupture traces, the hanging-wall/footwall effects of the Chi-Chi earthquake have been fully studied. We show that the hanging-wall effects cannot be simply accounted for by a proper choice of distance metric. The closest distance to the rupture plane (D rup ) is then selected to develop an empirical ground-motion model by using the data collected during the mainshock of the Chi-Chi earthquake that struck Taiwan. With the exception of some sites immediately next to the rupture traces (D rup Յ 5 km), the acceleration residuals between this empirical model and the recorded data at the footwall stations are close to zero for stations in the distance range from 5 to 50 km. On the other hand, the average acceleration amplification on the hanging wall is equal to the natural logarithmic values of 0.64 ‫ע‬ 0.4 for all hanging-wall sites within 20 km of D rup . The hanging-wall/footwall effects have also been evaluated for several response spectral periods. It is observed that both the horizontal and vertical components of spectral acceleration are apparently amplified for sites on the hanging wall at a distance from 5 to 20 km for spectral periods 0.02 to 0.5 sec, whereas the vertical component has less amplification than the horizontal in all the spectral periods considered. The horizontal component of spectral acceleration at the hanging-wall sites also shows a larger value for the long-period motion, relative to the footwall, for periods larger than 1.0 sec. The hanging-wall effects are relatively constant, at low frequencies, as the distance extends to about 20 km. This observation cannot be explained by the simplified empirical model. Rather, it suggests that waves trapped in the hangingwall wedge may have been involved.

Improving seismotectonics and seismic hazard assessment along the San Ramón Fault at the eastern border of Santiago city, Chile

“…Results from kinematic rupture scenarios for an M w 6.9 earthquake in the San Ramón Fault show that the maximum PGVH and PGAH mean values obtained from different rupture models always fall on the hanging wall of the structure, with expected maximum mean values in the order of 0.7-0.8 g, which is consistent with hanging-wall effects observed in previous studies (e.g., Chang et al 2004). Modeling near-fault broadband strong ground motions, in particular, in the vicinity of active faults are always critic to finite-source effects that are rather well simulated with the kinematic fractal k -2 source model (Ruiz et al 2011) used in this study.…”

“…Based on the kinematic rupture scenarios simulated in this study, the maximum PGVH and PGAH values obtained in the different models coincide with the fact that these are always on the hanging wall of the San Ramón Fault, which is rather coherent with results from Chang et al (2004). The overall results obtained here from kinematic rupture scenarios given an M w 6.9 earthquake in the San Ramón Fault support the kinematic composite fractal k -2 source model like a tool for ground-motion parameter prediction in the near-fault region.…”

“…Based on empirical observations, recent studies have proposed that peak ground motions from thrust earthquakes can be 20-30 % larger than from strike-slip earthquakes. It has also been shown that the hanging wall systematically exhibits amplified near-field ground motion in comparison to the footwall (e.g., the 1971 San Fernando earthquake, the 1980 El Asnam, Algeria earthquake and the 1994 Northridge earthquake; Chang et al 2004). …”

Spectral element analysis on the characteristics of seismic wave propagation triggered by Wenchuan M s8.0 earthquake

“…Whole area of the seismic wave propagations around the epicenter is divided into four quadrants along south-north and east-west direction. Displacements in the northwest quadrant are greater than displacements in the southeast quadrant; this may because that the northwest part of the thrust fault is a hanging-wall, can be influenced by the hanging-wall effects [35,36] . Displacements in the northeast quadrant are greater than displacements in the southwest quadrant, because the earthquake rupture propagated from southwest to northeast.…”

Effect of the free surface on the earthquake energy: A case study of reverse faulting

“…Owing to fault asymmetric property in the spatial domain and the coupling characteristics between the fault and free surface, the large thrust earthquake with or without surface breaks was generally associated with abnormally strong ground motion, especially for the low-angle thrusting. Two mostly representative examples are the 1999 M w 7.6 Chi-Chi earthquake in Taiwan and the great 2004 M w 9.3 Sumatra-Andaman earthquake [5] . The former creates gigantic near-field permanent surface displacement, while the latter causes the most appalling tsunami beyond the memory of men [6] .…”