Spacial and energetic trends of the microearthquakes activity in the Central Betics

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Abstract
Since early 1983, when the Andalusian Seismic Network began to operate, to late 1987, 4198 microearthquakes have been located in the Central Betics. This activity is quite nonuniform in space and time. The most important fault systems drawn by the geologic cartography and the observed lineaments by Landsat images are N20-40E N60-80E and N120-140E. Those fault systems which have shown activity during the period of study have been identified through the alignment of microearthquake epicentres and the diagrams of consecutive earthquakes relative azimuths. The most active systems in the whole region have been N70-80E and N90-100E. In the south zone appear lineaments N0-30E. There seems to be a gap zone in the area near Arenas del Rey, where the earthquake of December 24, 1984 (I0 = X.M.S.K.) occurred, and during the studied period only one earthquake of m1 = 5.0 has been detected, with neither precursors nor aftershocks. In the Granada Basin, the wellknown fracture system cabia-Santa Fé-Pinos Puente have shown little activity, while the N30-35E of the east border of Sierra Elvira and that of the N60-70E and the N80-90E have been more active; this latter has been identified through gravimetric data as well. In the west side of the Granada Basin and in the Axarquía of Málaga, lineaments of direction N40-60E, N70-80W and N70-90E have been observed. In the southern Central Betics there occur subcrustal earthquakes with depths up to 120 Km, with a trend NE-SW from Sierra Tejeda to the east of Fuengirola, whereas on the surface the trend E-W is dominant. The linear fitting of the magnitude-frequency law shows two different slopes for magnitudes under and above approximately m1 = 3. This discontinuity may be related to another in the seismic moment-spectral corner frequency relation for about M0 = 1021 dyn.cm (corresponding approximately to m1 = 3). The value of coefficient « b » is of 0.95 for earthquakes greater than m1 = 3, of the same order as those obtained for broader regions including the studied zone.

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