Velocity structure under the Trans-Mexican Volcanic Belt: preliminary results using correlation of noise

Abstract

SUMMARY Prediction of ground motion in central Mexico is of great consequence not only for Mexico City but also for several other cities in the region. However, the geometry and velocity distribution of the geological structures in central Mexico is unknown. In this paper, the velocity structure in central Mexico is investigated using seismic noise records from three different arrays, one permanent and two temporary. Distances between stations span 3–300 km. Seismic noise is analysed using time domain cross-correlation to estimate the Green's function between stations. Only the fundamental mode of Rayleigh waves is recovered in the cross-correlation of vertical component records, between 5 and 10 s period. At smaller periods, we obtain no correlation, even between closely spaced stations. This suggests that the heterogeneity of the Trans-Mexican Volcanic Belt (TMVB) hampers the propagation of Rayleigh waves smaller than 9 km. We observe a large velocity contrast between the Guerrero terrain and the TMVB. This velocity contrast, measured here for the first time at this scale, explains the appearance of regional amplification in central Mexico and the origin of the diffracted waves behind the long duration of ground motion observed in Mexico City. In the large scale, average Rayleigh wave group velocity at 5 s period is about 2.5 km s−1 in the TMVB and larger than 3.6 km s−1 outside it. Significant lateral heterogeneity is evident. In the stations of the dense Meso-American Subduction Experiment array we analysed, we observe in addition a significant increase in velocity from north to south in the central portion of the TMVB. Our results are part of ongoing studies with the final objective of building a model of TMVB that may be used to simulate ground motion in central Mexico from subduction zone earthquakes.