We address the ground-motion characterization of three damaging earthquakes that occurred in the low-to-moderate seismicity region of Murcia (southeast Spain) and compare our results with current earthquake-resistant provisions to establish whether those provisions are consistent with the available data or need to be revised.
The analyzed series are the 1999 Mula (mbLg 4.8, IEMS = VI), the 2002 southwest Bullas (mbLg 4.8, IEMS = VI), and the 2005 La Paca (mbLg 4.7, IEMS = VII). Recorded peak ground accelerations are low (pga < 0.025 g), partly because of the low magnitude of the events and the relatively large epicentral distance of the stations (Rep >20 km). We find that local factors control the distribution of ground motions, possibly in combination with propagation effects. Characteristic spectral shapes, representative of ground motions at each station, are observed. Several ground-motion predictive models and a simulation method are used for estimating accelerations and response spectra in the most damaged towns, where no records exist. The different methods consistently predict similar response spectra for the epicentral areas.
Recorded and predicted normalized spectral shapes exceed the design spectral shape of the Spanish Building Code ncse-02 for all soil categories for intermediate– high frequencies (above about 3 Hz). Moreover, the ncse-02 absolute response spectra are possibly exceeded by the corresponding predicted spectra for the epicentral areas in the same frequency range. Predominant frequencies of common soils and critical periods of most conventional buildings in the Region of Murcia also lie in this frequency range. These factors could explain, in part, the observed damage.
The results raise questions about the definition of the ncse-02 design spectra, and specifically about the design spectral shape used. The approach followed in this work may be used to calibrate other national seismic codes, especially of regions with similar characteristics: moderate seismicity and limited availability of ground-motion data.