Abstract
The city of Armenia, Colombia has been repeatedly subjected to moderate magnitude earthquakes. Damage in that city for the 1999 (Mw6.2) event was disproportionate (maximum observed EMS-92 intensity of IX), even considering the small epicentral distance (18 km). Two main factors have been invoked: Site effects and vulnerability of the building stock. We re-analyze available data on site effects, including: Records of aftershocks of the 1999 event, ambient noise records obtained using standalone stations, array records of ambient noise, and available shear wave profiles from seismic cone measurements. We estimate local amplification from spectral ratios of earthquake records relative to a reference site, the horizontal relative to the vertical component (HVSR, Horizontal-to-Vertical Spectral Ratios) of earthquakes and ambient noise records, and ratios of response spectra relative to a reference site or to simulated ground motion. These estimates are compared to amplification functions computed for 1D soil models, inverted from microtremor array observations. Our estimates of site effects for Armenia are therefore robust and bring together results previously available only in internal reports. We show that spectral ratios relative to a reference site may fail to estimate the amplification level. Site effects in Armenia are relatively homogeneous. Although site amplification is very significant and contributed to the observed damage, it does not account for the irregular damage distribution observed in 1999.
Highlights
The coffee-growing region in Colombia has been repeatedly subjected to moderate magnitude earthquakes
We evaluated effects from aftershock using different techniques: ratios relative to asite reference site our (SSR), spectral records ratios of the three horizontal relative to the Spectral vertical ratios relative to a reference site (SSR), spectral ratios of the horizontal relative to the vertical component component (HVSR), and response spectral ratios (RSR)
Efforts were made to contrast the different estimates of the site amplification and relate them to available data on the subsoil structure
Summary
The coffee-growing region in Colombia has been repeatedly subjected to moderate magnitude earthquakes. Two main factors were detected: Site effect amplification and the exceptionally high vulnerability of the building stock Our purpose, in this and its companion paper, is to examine the contribution of these two factors to the exceptional consequences of the 1999 event in the city of Armenia. Data on shear wave velocity structure are available from shallow seismic cone measurements at 21 sites within the city, and from the inversion of dispersion curves retrieved from the analysis of ambient noise data recorded by two linear arrays This allows us to compare the performance of different standard techniques used to estimate local amplification, with expected site effects predicted from the inverted subsoil structure. In general, very good agreement among the different techniques
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