Ground-Motion Observations at Hotel Montana during the M 7.0 2010 Haiti Earthquake: Topography or Soil Amplification?

Abstract

Unusually severe structural damage was reported during the 2010 M 7.0Haiti earthquake in the vicinity of Hotel Montana, located on top of a ridge in the district of Pétionville. Prompted by the observations, U.S. Geological Survey seismic stations were deployed, and aftershock recordings indicated ground-motion amplification on the top of the hill compared to adjacent stations on reference site conditions. The presence of topographic relief has been shown to significantly aggravate the consequences of strong ground motion during past events, and topographic effects were brought forward to explain the observations. In this paper, we test the hypothesis of topographic amplification as the dominant factor that contributed to the damage concentration in the vicinity of Hotel Montana. We initially conduct numerical simulations of the ridge seismic response assuming elastic homogeneous site conditions, and show that numerical predictions of topographic amplification disagree with the field data both in amplitude and in frequency. Conversely, while 1D ground-response analyses for the site conditions at the hilltop predict amplification in the same frequency range as the field data, they significantly underestimate the recorded amplitude. We then conduct numerical simulations of the foothill ridge response to seismic motion while accounting for soil layering, and qualitatively demonstrate that the recorded amplification is most likely attributed to coupled site-topographic amplification effects, namely to seismic waves trapped in the soft soil layers of the near surface, amplified as a consequence of reverberations, and further modified due to diffraction and scattering upon incidence on the irregular ground surface. Parametric investigations of the topography-soil amplification coupling effects are then conducted, and our results show that when accounting for a hypothetical soil-bedrock interface at 100 m depth, predictions are in excellent agreement with the observed motion.

Site Effect Assessment of the Gros-Morne Hill Area in Port-au-Prince, Haiti, Part A: Geophysical-Seismological Survey Results

“…As shown above and reproduced in Figure 16 below for comparison, our spectral ratios using Delmas 60 as the reference station, as well as S-wave HVSR obtained for the Hotel Montana site also present peaks at similar frequencies with slightly smaller amplitudes. However, as indicated The main reason for our lower computed spectral ratios, compared to these previous studies (especially with respect to the results of [6], is probably related to the selection of the reference site. Both studies were carried out considering HCEA as the reference station.…”

“…Here, for Gros-Morne hill, the frequency of topographical amplification should be~3 Hz and~1.8 Hz for a bedrock velocity of respectively 2286 m/s and 1300 m/s. We observed clearly higher amplitudes between 1 and 2 Hz on the N-S SSR with respect to the E-W SSR, which are more likely due to topographic effects than what has been assumed by [6], who considered a 7-Hz peak as the one due to topographic amplification (as the hill section has a minimum length of 700 m and not of 600 m and as the Vs of the hill rocks at a depth is certainly well below 2000 m/s and probably even below 1300 m/s). Second, they considered that the 600 m-wide hill (in the N-S direction) produces an amplification for a wavelength that is half of the hill width, i.e., 300 m. Taking the average Vs = 2286 m/s for the hill material, the topographic resonance frequency would then be 2286/300~7.5 Hz.…”

“…We observed clearly higher amplitudes between 1 and 2 Hz on the N-S SSR with respect to the E-W SSR, which are more likely due to topographic effects than what has been assumed by [6], who considered a 7-Hz peak as the one due to topographic amplification (as the hill section has a minimum length of 700 m and not of 600 m and as the Vs of the hill rocks at a depth is certainly well below 2000 m/s and probably even below 1300 m/s). The main reason for our lower computed spectral ratios, compared to these previous studies (especially with respect to the results of [6], is probably related to the selection of the reference site. Both studies were carried out considering HCEA as the reference station.…”

“…Results from [5] outlined two main resonance peaks at 2.3 and 6.8 Hz, with respective amplitudes of three and four ( Figure 15b) and which they mainly attributed to topographic effects. Assimaki et al [6] computed much higher amplitudes of about 10-12 for those two frequency ranges (see Figure 15c) using the same dataset and consequently concluded that the Gros-Morne hill crest must be affected by very strong site effects. Moreover, they attributed the 7-Hz peak to a coupling of topographical and stratigraphic amplification.…”

“…They attributed part of the amplification to topographic effects and part to soft soil amplification. To better determine the origin of this amplification, Assimaki et al [6] reanalyzed the same dataset of recorded aftershocks and also completed numerical simulations. They presented spectral ratios for the Hotel Montana site, which they explained as being related to both topographical and soil amplification, but the amplitudes of the computed spectral ratios were larger by a factor of two than those produced by [5].…”