Abstract
In the past, several destructive earthquakes have occurred in the North African Atlas Mountain ranges located along the Africa–Eurasia plate boundary. Although the region is rich with impressive archaeological sites, including those in modern Tunisia, few comprehensive archaeoseismological studies have been conducted. Historic sources account at least three damaging earthquakes in the Kairouan area in central Tunisia between AD 859 and 1041. Little is known about which faults triggered these earthquakes or the size of these events. The water supply of the city of Kairouan depended on a 32-km-long aqueduct with a large bridge (now partially collapsed) at the confluence of the de Mouta and Cherichira rivers. The original bridge of Roman construction was retrofitted twice during the Aghlabid period (AD 800–903) and probably in AD 995 during the Fatimid period. The ruined section of the bridge shows damage which might be related to the AD 859 earthquake shaking. Here, we present a detailed study of the history, the status and the damage of the Cherichira aqueduct bridge using previous historic accounts and written works, a 3D laser scan model, local geological and seismological characteristics, and include results of radiocarbon dating and a timeline of events. In addition to earthquake ground motions, we consider severe flash floods on the bridge as a potential cause of the damage. We estimate the severity of such flash floods and develop a model with 18 earthquake scenarios on local reverse and strike-slip faults with magnitudes between MW 6.1 and 7.2. While a few damage patterns might be indicative of flooding, most damage can be attributed to earthquakes. It is highly probable that the earthquake in AD 859 caused enough damage to the Aghlabid bridge to render it dysfunctional; however, to resolve the question of whether another earthquake in AD 911 or 1041 caused the complete destruction of the previously retrofitted aqueduct by the Fatimids requires dating of additional sections of the bridge.
Highlights
Characterizing pre-instrumental earthquakes can be accomplished in part by the study of their effects on manmade structures from historical sources (e.g. Fréchet et al 2008; Albini et al 2013), archeological sites (Stiros 1988; Stiros1 3 Vol.:(0123456789)K.-G
We develop a model to estimate ground motions at the Cherichira aqueduct bridge (CAB) and the Kairouan region to test the plausibility that local earthquake sources caused the observed damage at the bridge and correlate with the damage described in historic records
Originating in Roman times, it must have undergone a long period of degradation and damage either during or after the Roman era ended before its first retrofitting during the Aghlabid period (AD 800–903)
Summary
Characterizing pre-instrumental earthquakes can be accomplished in part by the study of their effects on manmade structures from historical sources (e.g. Fréchet et al 2008; Albini et al 2013), archeological sites Aqueducts often extend tens or even hundreds of kilometers and are susceptible to earthquake ground motions that cross geologic environments and can even be caused by multiple active faults In their neo-tectonic and seismotectonic investigation of seismically active regions in Tunisia, Bahrouni et al (2014) describe deformations of the Cherichira aqueduct bridge (CAB), a neuralgic spot of the aqueduct west of Kairouan (Fig. 2). They report that this aqueduct was built by the Romans and restored twice by Arabs, first during the Aghlabid period (AD 800–903) and second by the Fatimid (AD 909–969). We will (1) document the current status of the CAB based on 3D terrestrial laser scans combined with digital photography and total station measurements; (2) describe damage patterns and repair phases; (3) discuss the amount of water in possible flash floods that could have affected the CAB; (4) show results of HVSR (Horizontal to Vertical Spectral Ratio) measurements at the CAB; (5) based on a new tectonic model of the Cherichira area, we use 18 earthquake scenarios to calculate synthetic seismograms for the site and intensity maps for a larger area including villages and the cities of Kairouan and Sousse affected by the AD 859 earthquake and possibly later historical earthquakes (Bahrouni et al 2020b); and (6) use historic reports and additional physical C14 dating to establish a timeline of events from the middle of the 8th to the thirteenth century AD
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