Klaus-G. Hinzen’s research while affiliated with University of Cologne and other places

The Cherichira aqueduct, originating from Roman times, supplied the city of Kairouan, Tunisia, with water and has had alternating phases of damage and repair after the Roman and during the Aghlabid and Fatimid era. A crucial section of the lifeline is the Cherichira aqueduct bridge (CAB), and scholars have discussed the possibility that earthquake ground motions caused damage which disrupted the water supply of Kairouan. However, little was known about the dynamic behavior of the bridge and its vulnerability to earthquake ground motions. Computer-aided design based on a detailed laser scan model of the remains of the bridge and published data were used to reconstruct the CAB of the Aghlabid period. Subsequently converted into a discrete element model, the digital version of the CAB was subjected to analytic ground motion signals and full 3D simulations of local earthquakes. The CAB model shows a fundamental eigenfrequency close to 1 Hz in the direction transverse to its trend, and single-component ground motions in this direction with peak particle velocities above 1.0 m/s cause damage to the top of the CAB. Among the earthquake scenarios with full 3D ground motions applied, only the activation of a nearby thrust fault caused distinct damage. While fractures in the ruins of the CAB cutting through the upper part of the bridge which includes the water canal are a likely cause for disrupting the water flow and are similar to the damage pattern produced in the model calculations, a solely seismogenic cause of the total collapse of some parts of the CAB cannot be verified by the simulations.

Candlestick stalagmites exhibit resonance in the frequency-band of regional earthquake ground motions. An earthquake can break such elongated structures if the ground movement is strong enough. Therefore, the existence of intact stalagmites in caves would indicates that a certain level of ground movement has not been exceeded since they exist. Field surveys were carried out in the Han-sur-Lesse Cave (Belgian Ardennes) to estimate the eigenfrequencies of stalagmites of different sizes and shapes (e.g., Minaret Stalagmite, “Verviétois” gallery) and to explore their reactions to external events such as quarry blasts or earthquakes. The eigenfrequencies of the stalagmites are obtained from direct measurements of ambient seismic noise caused by human activities, microseisms, etc. Seismic sensors were placed on the stalagmites and on the nearby cave floor as well as at various other locations inside and outside the cave. Noise and transient events (e.g., quarry blasts) were recorded during weeks of continuous measurements. 3D laser scans of the stalagmites and Finite Element Modelling allowed constraining the link between their complex shape and their eigenfrequencies. https://uis-speleo.org/wp-content/uploads/2022/09/ACTES_CONGRES_UIS_WEB_VOLUME_5.pdf

A steadily increasing number of citizen seismological stations, often located in populated areas, record a plethora of man-made events. These events are especially of interest, when they are caused by criminal activity or man-made explosions. On 4 December 2021, during an attempted robbery of an automated teller machine (ATM) in Bonn-Röttgen, Germany, the burglars used explosions. The seismic effects of the explosions were recorded with a Raspberry Shake (RS) station at a distance of 580 m from the site. While working on the analysis of this signal, another attack on an ATM on 23 February 2022 in Kürten-Dürscheid was recorded by another RS station, this time at 830-m distance with an instrument that also included an air pressure channel. The seismic signatures of both events indicate similar procedures in both cases whereby a larger explosion was quickly followed a smaller explosion after 21 s and 49 s, respectively. An estimate of the charge weight of the explosions shows that ratios of the strength of the first to second explosion were 21:1 and 9.4:1 in the Röttgen and Dürscheid attacks, respectively.

The Kairouan city, capital of the Aghlabides Dynasty (800–909 AD), and surrounding region were affected by a large earthquake in 8 Rajab 245 Hijri (9 October 859). Contemporaneous accounts by local travellers to the Abbasides Caliphate (750–1258 AD) and reported by the chroniclers and historians Al-Baghdadi (died 860) and Al-Tabari (838–923 AD) have described the damage to the city and more than 13 villages that suffered severe destructions and left a large number of people homeless. In Kairouan city, the dome and other holy places (Minbar and Mihrab) of the great mosque, the houses, fortifications and bridges suffered damage. The aqueduct that used to supply Kairouan with fresh water from western mountains was badly damaged at about 20 km west of the city. Field investigations of the aqueduct using geodesy and archeoseismology with radiocarbon dating characterize the damage features. Recent field investigations in the region taking into account the construction typology and inferred damage distribution suggest a macroseismic intensity reaching IX–X MSK. The seismotectonic context suggests a seismic source along the major Sbiba E–W trending transpressive fault that includes the Cherichira, Sfaia, El Baten folding system near Kairouan. The active fault system affecting late Quaternary units, made of 30-km-long thrust-related “en-echelon” folds related to the E–W trending fault, is characterized and identified as seismogenic ~20 km west of Kairouan.

Our knowledge of pre-instrumental earthquakes is mainly based on historic reports and palaeoseismological evidence. Under favorable conditions, damage to archaeologically documented structures may quantify earthquake-related parameters, such as the type of ground motions that caused damage. The Mediterranean Region, with a plethora of archaeological sites and one of the highest global seismicity rates, is an ideal place for archaeoseismological research. In addition to basic questions, e.g. whether observed damage is of a co-seismic nature and bracketing ground motion levels, advanced surveying and modelling techniques can also provide additional insights. We show that models of precariously balanced archaeological structures provide natural limits to past ground motion levels and that structures built on active faults may give information on the slip velocity during earthquakes. Quantitative models also provide perspectives for discussions about historical building techniques, such as the existence of ancient earthquake-resistant construction. The results of such investigations can improve efforts to preserve the cultural heritage of a region and can be utilized to avoid future construction mistakes.

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 M W 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.

Archaeological structures built across active faults and ruptured by earthquakes have been used as markers to measure the amount of displacement caused by ground motion and thus to estimate the magnitude of ancient earthquakes. The example used in this study is the Crusader fortress at Tel Ateret (Vadum Iacob) in the Jordan Gorge, north of the Sea of Galilee, a site which has been ruptured repeatedly since the Iron Age. We use detailed laser scans and discrete element models of the fortification walls to deduce the slip velocity during the earthquake. Further, we test whether the in-situ observed deformation pattern of the walls allows quantifi-cation of the amount both sides of the fault moved and whether post-seismic creep contributed to total displacement. The dynamic simulation of the reaction of the fortification wall to a variety of earthquake scenarios supports the hypothesis that the wall was ruptured by two earthquakes in 1202 and 1759 CE. For the first time, we can estimate the slip velocity during the earthquakes to 3 and 1 m/s for the two events, attribute the main motion to the Arabian plate with a mostly locked Sinai plate, and exclude significant creep contribution to the observed displacements of 1.25 and 0.5 m, respectively. Considering a minimum long-term slip rate at the site of 2.6 mm/year, there is a deficit of at least 1.6 m slip corresponding to a potential future magnitude 7.5 earthquake ; if we assume~5 mm/year geodetic rate, the deficit is even larger.

Intact speleothems can be used as indicators for an upper limit of the level of horizontal ground motions of past earthquakes which affected the cave. Candlestick stalagmites have eigenfrequencies well in the frequency-band of regional earthquake ground motions. An earthquake can therefore break such elongated structures if the ground movement is strong enough. In the study of the response of speleothems to earthquakes, eigenfrequencies are a fundamental parameter. A study was carried out at Han-sur-Lesse (Belgium Ardennes) to estimate these frequencies for the so called Minaret stalagmite, an imposing 4.5 m tall structure. Three-component seismic sensors were used to record the ambient noise during a period of 22 days on the stalagmite, at its base on the cave floor, and at Earth’s surface. This technique allows a precise determination of the first eigenfrequencies of the stalagmite (two firsts mode shapes) and the polarization of the motions associated with the frequencies. The use of three-component seismic sensors is a precondition to identifying these polarizations. Moreover, the horizontal motions recorded on the stalagmite show significant amplification (4 and 14 times depending on the orientation) compared to those recorded at the free surface outside the cave. The long recording period allows the measurement of transient events like earthquakes or quarry blasts. In addition, a 3D laser scan of the stalagmite’s shape has been used to construct numerical models. The dynamic behaviour of the models is in good agreement with the measured parameters. The use of the 3D scans clearly increased accordance between model and measurements compared to simply shaped approximations of the stalagmite. The combination of measured and modelled data clearly show that the shape of the stalagmite (elliptical cross-section and shape irregularities) influences the eigenfrequencies and the polarization of the modes while also causing a near-orthogonal split of the natural frequencies. Knowing that the shape and the height of the stalagmites evolve over time, further steps in this study will be to date the candlestick stalagmites in order to have an approximation of their height (and therefore their eigenfrequencies) during their history and to model their eigenfrequency evolution with time.

Since the mid-1990s, the local seismic network of the University of Cologne has produced digital seismograms. The data all underwent a daily routine processing. For this study, we re-processed data of almost a quarter century of seismicity in the Northern Rhine Area (NRA), including the Lower Rhine Embayment (LRE) and the Eifel Mountain region (EMR). This effort included refined discrimination between tectonic earthquakes, mine-induced events, and quarry blasts. While routine processing comprised the determination of local magnitude ML, in the course of this study, source spectra-based estimates for moment magnitude MW for 1332 earthquakes were calculated. The resulting relation between ML and MW agrees well with the theory of an ML ∝ 1.5 MW dependency at magnitudes below 3. By applying Gutenberg-Richter relation, the b-value for ML was less (0.82) than MW (1.03). Fault plane solutions for 66 earthquakes confirm the previously published N118° E direction of maximum horizontal stress in the NRA. Comparison of the seismicity with recently published Global Positioning System–based deformation data of the crust shows that the largest seismic activity during the observation period in the LRE occurred in the region with the highest dilatation rates. The stress directions agree well with the trend of major faults, and declining seismicity from south to north correlates with decreasing strain rates. In the EMR, earthquakes concentrate at the fringes of the area with corresponding the largest uplift.