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.

Damage in Armenia, Colombia, for the 25 January 1999 (M_w=6.2, peak ground acceleration (PGA) 580 Gal) event was disproportionate. We analyze the damage report as a function of number of stories and construction age of buildings. We recovered two vulnerability evaluations made in Armenia in 1993 and in 2004. We compare the results of the 1993 evaluation with damage observed in 1999 and show that the vulnerability evaluation made in 1993 could have predicted the relative frequency of damage observed in 1999. Our results show that vulnerability of the building stock was the major factor behind damage observed in 1999. Moreover, it showed no significant reduction between 1999 and 2004.

Damage in Armenia, Colombia, for the 1999 (Mw6.2) event was disproportionate. We analyse the damage report as a function of number of storeys and construction age. We recovered two vulnerability evaluations made in Armenia in 1993 and in 2004. We compare the results of the 1993 evaluation with damages observed in 1999 and show that the vulnerability evaluation made in 1993 could have predicted the relative frequency of damage observed in 1999. Our results show that vulnerability of the building stock was the major factor behind damage observed in 1999. Moreover, it showed no significant reduction between 1999 and 2004.

We present the results of the analysis of vertical component from earthquake and seismic noise records obtained in the Sabana de Bogotá to determine subsoil structure in that region. Rayleigh waves present in the earthquake data were processed to estimate to estimate group velocities. In the case of seismic noise, we used seismic interferometry to estimate Green’s functions between stations from average cross-correlation. Rayleigh wave pulses were recovered between stations from seismic noise. We processed the results to estimate group velocities in all cases. The results show the impact of geology on group velocities in the studied region. Different station pairs show varying group velocities depending on the age of the rocks along the paths. Our estimated impedance contrast between surficial deposits and underlying bedrock suggest possible amplification of ground motion larger than a factor of 10. The poor quality and the limited quantity of our data limited the comprehensiveness of our results. The frequency ranges of the results showed poor overlap, hindering comparisons among paths. In spite of their limitations, our results highlight the importance of site effects in the Sabana de Bogotá and the need to take them into account to estimate ground motion for future events in that region. Se presentan resultados del análisis de componentes verticales de registros de sismos y ruido sísmico en la Sabana de Bogotá con el objetivo de explorar la estructura del subsuelo. En el caso de los registros de sismos, se analizaron directamente las ondas de Rayleigh presentes en los registros para estimar velocidades de grupo. Para los registros de ruido sísmico utilizamos el método de interferometría sísmica que permite estimar funciones de Green a partir de la correlación cruzada. El promedio de esas correlaciones cruzadas permitió recuperar pulsos de ondas Rayleigh entre pares de estaciones. En todos los casos estimamos curvas de dispersión de velocidad de grupo. Los resultados muestran el impacto de la geología en los valores de velocidad de grupo en la región de estudio. Observamos diferencias entre trayectorias contenidas en los sedimentos cuaternarios que cubren la Sabana de Bogotá y aquellas que incluyen parte de su trayecto por rocas del Cretácico. El contraste de impedancia estimado entre los sedimentos superficiales y los estratos subyacentes sugiere que la amplificación del movimiento sísmico excede un factor de 10. La deficiente cantidad y calidad de los datos limitaron los alcances del este trabajo. Los rangos de frecuencia de los resultados no coincidieron para las distintas fuentes de datos, lo que obstaculizó las comparaciones entre trayectorias. A pesar de esas limitaciones, estos resultados contribuyen a mostrar la relevancia de los efectos de sitio en la Sabana de Bogotá y la necesidad de tomarlos en cuenta para estimar el movimiento durante sismos futuros en esta zona.

Seismic noise and earthquake records were used to determine group velocity distribution in the region. The analysis was restricted to vertical components to avoid ambiguity in identifying Rayleigh waves. 47 small earthquake records were processed using the multiple filter technique. We also used six months of continuous records of seismic noise in 12 broad band stations from Servicio Geológico Colombiano to estimate Green’s functions from seismic interferometry (long-term average of cross-correlation between station pairs). In this analysis we have shown that a non-linear stack of individual cross-correlation windows allows improving signal-to-noise ratios in those Green’s functions. These results were also processed with the multiple filter technique to estimate group velocities. In total, 111 group velocity dispersion curves were computed, which is not enough to compute a seismic tomography in the region of study, especially considering its size and the complexity of its geology. In addition, the period range for which results were obtained using small earthquakes is significantly different from that using seismic noise. However, we are able to estimate a reliable average group velocity of the region and show the variability of group velocities due to its complex geology.