The Importance of Assessing the Geological Site Effects of Ancient Earthquakes from the Archaeoseismological Point of View

Abstract

Earthquakes have and continue to, occur worldwide, though some places are affected more than others by earthquake-induced ground shaking and the same earthquake can cause more damage in one area than in nearby locations due to site-specific geological site conditions, also known as local site effects. Depending on the chronology of the earthquakes, various disciplines of seismology include instrumental and historical seismology, archaeoseismology, palaeoseismology and neotectonics, each focusing on using specific sources of information to evaluate recent or ancient earthquakes. Past earthquakes are investigated to expand the pre-instrumental and instrumental earthquake catalog and better evaluate a region’s seismic hazard. Archaeoseismology offers a way to achieve these goals because it links how ancient civilizations and their environment might have interacted and responded to past earthquake-induced ground motion and soil amplification. Hence, archaeoseismology explores pre-instrumental (past) earthquakes that might have affected sites of human occupation and their nearby settings, which have left their co-seismic marks in ancient manufactured constructions exhumed by archaeological excavations. However, archaeoseismological observations are often made on a limited epicentral area, poorly constrained dated earthquakes and occasionally on unclear evidence of earthquake damage. Archaeological excavations or field investigations often underestimate the critical role that an archaeological site’s ancient geological site conditions might have played in causing co-seismic structural damage to ancient anthropogenic structures. Nevertheless, the archaeological community might document and inaccurately diagnose structural damage by ancient earthquake shaking to structures and even estimate the size of past earthquakes giving little or no consideration to the role of geological site effects in addressing the causative earthquake. This mixture of factors frequently leads to imprecise estimates of the size of ancient earthquakes and unlikely earthquake environmental impacts, leaving unexplained the location and the moment magnitude of the causative earthquake. Hence, it is essential not to rely solely on earthquake intensities based on archaeologically documented co-seismic damage without assessing the nature of the observed structural damage and the contribution of the geological site effects. This paper explains the geological site effects concept to archaeologists unfamiliar with the notion. It clarifies its role in assessing ground shaking, soil amplification and earthquake intensity by past earthquakes and how and why the geological site effects can be estimated when a site is thought to have been struck by an earthquake. Hence, the geological site effects must be considered when archaeological excavations describe and interpret destruction layers. Conversely, engineers and seismologists dealing with seismic hazard risk assessment must pay close attention to archaeological investigations assessing earthquake intensities and locations based on field evidence of damage to structures attributed to past earthquakes, because the geological site effects might have been factored in inaccurately or not at all.

Unraveling the Seismic Source in Archaeoseismology: A Combined Approach on Local Site Effects and Geochemical Data Integration

“…Its role is crucial in addressing gaps within historical seismic catalogs and in verifying or enhancing existing seismic historical records. Furthermore, it can also incorporate local site effects and geochemical analysis, as shown in the flowchart represented in [11,15,25]. As previously mentioned, local site effects provide insights into variations in ground shaking due to geological conditions specific to a given site.…”

“…To study the local site effects and identify the causative faults, the integration of geophysical techniques and geochemical data analysis becomes crucial (Figure 2). Archaeoseismologists can combine these methodologies to precisely determine the location of seismic causative sources, particularly in sites where the evidence is unclear [11,15,16]. This, in turn, fosters a deeper understanding of earthquake hazards and their influence on archaeological structures.…”

“…The evolution from qualitative [7,10] to quantitative approaches [8,11,15] in archaeoseismology underscores the need for interdisciplinary collaboration. While qualitative methods involve subjectivity and individual interpretation, quantitative approaches employ analytical and numerical modeling procedures [11].…”

“…Estimating the surface ground motion in archaeoseismic studies can be achieved by using empirical methods, such as analyzing recordings of real earthquakes [28], or numerical techniques like stochastic methods or Green's function, which represents the response of a site to a point seismic source [15,31]. Field tests and analytical/numerical models are employed to evaluate seismic site amplification by studying the dynamic responses of sites using active sources, ambient noise, and real earthquakes [28,32].…”