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Late Holocene rupture history of the Ventas de Zafarraya Fault in Southern Spain

My latest paper deals with the Holocene activity of the Ventas de Zafarraya Fault in Southern Spain. It was published some days ago in the most recent issue of Cuaternario y Geomorfología. The Ventas de Zafarraya Fault (VZF) west of the Granada basin (36.96° N, 4.14°W) has a beautiful morphologic expression and an exciting history. The fault bounds the Zafarraya polje to the south, with Quaternary sediments to the north (hanging wall) and limestones of the Internal Subbetics in the footwall (Fig. 1).

Fig. 1: Simplified geology of the Betic Cordilleras in Andalucia and geological and structural map of the southern Granada Basin and the Zafarraya Polje.

Fig. 2: Morphological expression of the Ventas de Zafarraya Fault

The E-W trending normal fault has an average dip of 60° to the north and is visible for at least 15 km. A nice limestone fault scarp with up to 2 m height can be found in the central area. This scarp marks the surface rupture of the “Andalusian Earthquake” which occurred on Christmas Day 1884 (Udias & Munoz, 1979). Several villages were destroyed back then and hundreds of people died. Epicentral intensities reached MSK X. First data on the paleoseismicity of this fault were published by Reicherter (2001) and Reicherter et al. (2003). In order to complete their record, we continued field work which included geomorphological analyses (Fig. 2), trenching (Fig. 3) and geophysical investigations (Fig. 4).

Fig. 3: Photo mosaic of trench 1, the fault plane is at the left side of the trench where the slope begins to steepen.

We started with georadar (GPR) investigations (200 MHz antenna) to find a suitable trench site and then opened three trenches. Georadar data allowed us to prove that the features which were found in the trenches can be found all along the fault line within the polje. Therefore, we can exclude the possibility of local slope movements. Also, we found sets of colluvial wedges and secondary faults imaged by GPR. Trenching allowed us to estimate a magnitude of ~6.5-6.7 for the Christmas event. Four events are proposed within the last 10 ka. We determined the slip rate to be between 0.3-0.45 mm/a and found that the recurrence intervall is in the order of 2,000 a.

Fig. 4: Paleoseismological trench and GPR data, the lower radargram was collected at the trench floor.

A sketch illustrates the sediment features found with GPR measurements:

Fig. 5: Sediment deformation associated with the fault movement, derived from GPR data.

Fig. 6: Timing of earthquake events found along the Ventas de Zafarraya Fault, including the data and interpretation of Reicherter et al. (2003).

The Ventas de Zafarraya Fault significantly contributes to the seismic hazard in Southern Spain. Future research focus should now be drawn to other faults of similar length and activity, as those faults are posing significant seismic hazards, too.

References

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Christoph Grützner

Christoph Grützner

works at the Institute of Geological Sciences, Jena University. He likes Central Asia and the Mediterranean and looks for ancient earthquakes.

See all posts Christoph Grützner

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