Expected ground motion in the south-east of Spain due to an earthquake in the epicentral area of the 1910 Adra earthquake

To study the ground motion level associated with historical earthquakes located in Southern Spain, we have chosen a scenario placed in the Poniente Almeriense (Southeast Spain). In this zone, some relevant historical earthquakes have occurred, such as those of 1522, 1804 and 1910. In particular, the earthquakes of 1804 and 1910 the estimated and calculated magnitudes are of M = 6.3. Those earthquakes took place near the epicentral zone of a seismic series happened in 1993–94. As part of this series, two earthquakes with M∼5 were recorded by strong ground motion instruments on 23rd December 1993, and 4th January, 1994 at Adra, Almería and Motril. We have used the acceleration records as empirical Green functions in order to simulate the expected ground motion associated with a hypothetical earthquake of magnitude M = 6.3 like those of 1804 and 1910. The simulations have been carried out for three sites (Almería, Adra and Motril) using three different approaches. A total of 30 simulations, for each approach, have been carried out for each ground motion component in each site. The peak ground acceleration (PGA) and the response spectra are compared with the values obtained through empirical relationships for the distances and soil conditions corresponding to the three chosen sites. The results of the simulations show that the horizontal PGA could exceed the values observed in 23/XII/93 and 4/I/94 by a factor of 5–8, surpassing in some cases the value of 140 gals. Besides, some of the peak spectral accelerations simulated reach Samax = 400 gals, Adra being the location where the highest values of amax and Samax are reached, due to the nearness of this station to the epicentres of 23 /XII/93 and 4/I/94. At Almeria, the PGA values reach 40 gals, which may be considered as input in the bedrock. In Motril, the PGA surpass a value of 130 gals, considering as due to a strong local site effect. Finally, the peak ground acceleration (PGA) and the response spectra obtained with the simulations have been compared with other values estimated through empirical relationships for similar conditions. The conclusions about the expected ground motion levels have an important application aimed at the revision of the maximum acceleration and response spectra of the Spanish building Code, NCSE-94.