Francisco José Chávez-García

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.

Full waveform inversion (FWI) has proved to be a reliable tool for high-resolution imaging of lithospheric structures at various depths down to the upper mantle. However, when the size of the model is large, the computational burden is significative and applications are restricted to low frequencies. To tackle this issue, we developed a new 2D time-domain hybrid method to simulate high-frequency teleseismic body waves propagating through a local heterogeneous elastic Earth model: The frequency-wavenumber (FK) integration method is coupled with the staggered grid finite difference method (SGFD). The FK method is used to compute the wavefield due to obliquely incident plane P and SV waves in a 1D multilayered half space that excites a local heterogeneous region. Inside this region, the velocity-stress staggered grid FD method (SGFD) is used to accurately deal with wave propagation in heterogeneous media. Spurious waves that might be generated at the boundaries of the local region are avoided using convolutional perfectly matched layers (CPML). This new hybrid method inherits the low-memory requirements of the FK method and the accuracy, efficiency and easy implementation of the SGFD. The new hybrid method is benchmarked against the analytical FK method for some canonical models and shows good agreement with analytical solutions. Subsequently, our modeling tool is incorporated into a full waveform inversion algorithm adapted for teleseismic configurations to invert the incident P wave and its coda. The inversion is carried out using a gradient approach that is efficiently implemented via the adjoint-state method. The results suggest that our hybrid method and FWI algorithm represent a valuable tool for 2D forward and inverse regional applications using teleseismic data sets.

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.

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.

An important object of study in geotechnical engineering is the interface between structures and soil. It is through this interface that earthquakes affect man-made structures. Thus, geotechnical engineering lies at the interface between seismology and structural dynamics. Ideally, experts from both fields should work together to estimate the maximum seismic demand that may affect a structure during its lifetime. This assessment must take into consideration site effects (the amplification of seismic waves due to the surface geology conditions at the site of interest). The estimation of site effects would be improved if the data used to design the foundation of the structure were considered. On the other side, geotechnical design would be enhanced if the uncertainties and limitations that affect site effect estimation were considered. The necessary feedback between seismology and engineering requires fostering the communication between disciplines and a better understanding of the way in which similar problems are attacked by our colleagues of the complementary discipline. This work aims to contribute to that exchange. The state of the practice of site effect estimation in seismology is presented. We address commonly used techniques used to assess site effects due to soft soil deposits from seismic records or ambient noise. In addition, 3-D simulations of site effects are discussed. The application of these methods is described using the small sedimentary basin of Parkway (New Zealand) as our example. Our main objective is to discuss the limitations and uncertainties associated to site effect estimation and their impact on the reliability of the structures we build. We hope to stimulate increased exchanges between seismology and geotechnical engineering.

Determination of the shear-wave velocity structure at shallow depths is a common necessity for engineering or environmental projects. To this end, subsoil structure exploration using surface waves is frequently used. Methods such as SASW or MASW determine phase velocity dispersion from surface waves generated by a source recorded on a line of geophones. Using MASW, it is important that the array of receivers be as long as possible to increase the precision at low frequencies. However, this implies that possible lateral variations are disregarded. Hayashi and Suzuki (2004) proposed a different way of stacking gather shots to increase lateral resolution. They combined strategies used in MASW with the CMP (Common Mid-Point) summation current in reflection seismology. In their method (CMPCC), they cross-correlate traces sharing common mid-point locations before determining phase velocity dispersion. Another recent approach to subsoil structure exploration is based on seismic interferometry. It has been shown that cross-correlation of a diffuse field such as seismic noise allows determining the Green's function between two receivers. Thus, a virtual source seismic section may be constructed from the cross-correlation of records of seismic noise obtained in a line of receivers. In this paper, we use seismic interferometry to process seismic noise records obtained in seismic refraction lines of 24 geophones, and analyze the results using CMPCC to increase lateral resolution. Cross-correlation of the noise records allows reconstructing seismic sections with virtual sources at each receiver location. The Rayleigh wave component of the Green's functions is obtained with a high signal-to-noise ratio. Using CMPCC analysis of the virtual source seismic lines, we are able to identify lateral variations of phase velocity inside the seismic line, increasing the resolution obtained previously with those data.