Estimation of Velocity Structure Using Microtremor Recordings from Arrays: Comparison of Results from the SPAC and the F-K Analysis Methods

Microtremor recordings have become a useful tool for microzonation studies in countries with low to moderate seismicity and also in countries where there are few seismographs or the recurrence time for an earthquake is quite long. Microtremor recordings can be made at almost any time and any place without needing to wait for an earthquake. The measurements can be made using one station or an array of stations. Microtremor recordings can be used to estimate site response directly (e.g. by using Nakamura's technique), or they can be used to estimate shear-wave velocities, from which site response can be calculated. A number of studies have found that the direct estimation of site response may be unreliable, except for identifying the fundamental resonant period of a site. Obtaining shear-wave velocities requires inverting measurements of Rayleigh wave phase velocities from microtremors, which are obtained by using the Spatial Autocorrelation (SPAC) (Aki, 1957) or the Frequency-Wave Number (F-K) (Horike, 1985) methods. Estimating shear-wave velocities from microtremor recordings is a cheaper alternative than direct methods, such as the logging of boreholes. In this work we use simultaneous microtremor recordings from triangular arrays located at two sites in Mexico City, Mexico, one ("Texcoco") with a lacustrine sediment layer of about 200 m depth, and the other one ("Ciudad Universitaria") underlain by 2,000 year old basaltic flows from Xitle volcano. The data are analyzed using both the SPAC method and by the standard F-K method. The results obtained from the SPAC method are more consistent with expectations from the geological conditions and an empirical transfer function (Montalvo et al., 2001) than those from F-K method. We also analyze data from the Hollister Municipal Airport in California. The triangular array at this site is located near a borehole from which seismic velocities have been obtained using a downhole logging method (Liu et al., 2000). We compare results from the microtremor recordings analyzed using both the SPAC and F-K methods with those obtained from the downhole logging.