The seismic ground motion of a test area in the eastern district of Naples is computed with a hybrid technique based on the mode summation and the finite difference methods. Synthetic seismograms are validated with the 1980 Irpinia earthquake, a good example of strong shaking for Naples, recorded 10km from the modeled sites. Realistic SH and P-SV wave seismograms are computed along geological cross sections representative of the area. As expected, the sedimentary cover causes an increase of the signal's amplitudes along the radial and transverse components. The spectral amplification of the vertical motion is generally negligible; instead, that of the horizontal motion ranges between 2 and 5 in the 0.7-2.2 Hz frequency range. Average and maximum spectral amplifications are considered along the different geological cross sections and acceleration response spectra for the Irpinia earthquake are evaluated in order to formulate appropriate design spectra and improvements to the building code.

The special geological conditions in the Xiji-Langfu area are the main reason for the anomalous high macroseismic intensity caused by the Tangshan, 1976 earthquake. The area is formed by deep deposits-mainly alluvium sands and clays poorly consolidated and with high water content -that have been trapped by the Xiadian fault. From simulated ground motion we have computed quantities commonly used for engineering purposes like the acceleration maximum amplitude (AMAX) and the total energy of ground motion (W), which is related to the Arias Intensity. The thick low velocity deposits are responsible for the large increment of the values of AMAX and W inside the basin. On the two sides of the Xiadian fault AMAX and W can vary by 200% and 700% respectively, and these variations are quite stable with varying thickness of the sedimentary deposit used in the models. With the existing relationships between acceleration (AMAX) and macroseismic intensity (I) our results can explain the large values of I observed in the Xiji-Langfu area, in connection with the Tangshan earthquake.

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