Ground-Motion Similarities in the Valley of Mexico from Subduction Earthquakes

ABSTRACT Since the eighties, several subduction earthquakes on the Mexican Pacific coast have been recorded at accelerometric networks in the Valley of Mexico (VM). Such are the cases of the 19 September 1985 M 8.1 Michoacán earthquake and the recent 7 September 2021 M 7.1 Guerrero earthquake. These events have caused from minor damages to widespread structural collapses in the VM. The vertical ground motion recorded at several sites in the VM clearly show recurrent Rayleigh wavetrains, which present similar characteristics in period, amplitude, and duration among stations. These characteristics are very stable in space and time. On the other hand, the corresponding horizontal ground motion are strongly affected by the surface geology of the valley because of the shear wave-energy trapped at the uppermost soft layers. In addition, the horizontal response at several soft-ground sites displays significant azimuthal effects. Based on these observations, we analyze thrust-faulting subduction earthquakes that occurred along the Mexican Pacific coast from Michoacán to Chiapas states, all recorded at the VM accelerometric networks. We identified similarities in the vertical and horizontal ground motion not only among stations but also between seismic events recorded at the same station. The local generation of surface waves is studied using a kinematic analysis in the frequency–wavenumber domain (f-k), and sundry techniques in frequency and time domains. As a preliminary explanation, our analysis suggests that azimuthal effects related to thrust faulting subduction earthquakes strongly influence the local generation of surface waves affecting the 3D response of the valley. Moreover, for all earthquakes in all stations, our results show that while the vertical ground motion is similar and its amplifications are small and governed by earthquake size, the two horizontal components reveal strong site effects.