- © 2008 by the Seismological Society of America
Investigating the remains of past human activity in search of evidence of ancient earthquakes is the principal objective of archaeoseismology. In this regard, it sits within a continuum of overlapping and complementary subdisciplines that span the broad research realm of earthquake science, specifically bridging the gap between instrumental and historical seismology on one side and paleoseismology and earthquake geology on the other. Each of these research fields focuses on a particular source of data, applies appropriate analytical methods and techniques, and targets a specific time window, which in the case of archaeoseismology tends to be cultural material data spanning the last few millennia. Ultimately, however, each of these discrete subdisciplines converges on a common goal: to reduce the seismic hazard within a region through a better understanding of its earthquake history.
“Ancient accounts of earthquakes do not help us much; they are incomplete, and accuracy is usually sacrificed to make the most of a good story.”
— Charles Richter
Half a century ago, Charles Richter (1958) complained that “ancient accounts of earthquakes do not help us much; they are incomplete, and accuracy is usually sacrificed to make the most of a good story.” Those remarks were directed at documentary records of antique earthquakes, but in the intervening 50 years a concerted effort to rigorously extract meaningful quantitative seismic parameters from textual archives has ensured that historical seismology is now an important facet of modern seismic-hazard assessment. Archaeological data, however, remains aloof from such inclusion. Why exactly is that? To us, it seems that the main reason is that many earthquake scientists still question the basic principles and practices of the discipline. In short, they are skeptical as to whether cultural phenomena—destruction layers, structural damage to man-made constructions, displaced manmade structures, indications of repair and abandonment, and inscriptions—can reliably be used as earthquake indicators at all.
It is important to acknowledge that archaeoseismology is plagued by many of the same ambiguities that geologists encounter when interpreting earthquake indicators in the landscape (e.g., fault scarps) or in the stratigraphic layers of a paleoseismological trench (e.g., colluvial wedges, liquefaction features). All are prone to naturally disruptive processes that can mimic the expression of seismic rupture or shaking. But cultural phenomena bear the additional vagaries of uncertain human actions, from the questionable quality of construction to the potential for manmade destruction (e.g., war, pillage, vandalism). The result is that it is difficult (some might argue impossible) to irrefutably distinguish between damage caused by an earthquake and that caused by man or competing natural agents (e.g., ground instability, erosion, fire). Typologies of earthquake-characteristic damage have been proposed but when subjected to critical appraisal—in particular through numerical and analogue modeling—most of these typologies do not pass the test. Even if they could, it remains uncertain how the seismic traces in destruction layers and dislocated buildings can be meaningfully translated into earthquake parameters such as intensity, peak ground acceleration, magnitude, distance to epicenter, etc.
Often this ambiguity is conveniently ignored, in particular when archaeologists indiscriminately use earthquakes to explain the otherwise inexplicable, such as the sudden desertion of a site or its destruction at a time when no marauders are known. Earthquakes, in this context, simply add drama and conjecture to a site's history. But the danger is that such “rogue” earthquakes, supposedly proven by archaeologists, will be used by seismologists as real events in a seismic-hazard analysis or be invoked to explain abandonment or damage at other sites. With the poor spatial and/or temporal resolution of the inherently incomplete archaeological record, a closely timed burst of modest earthquakes can all too easily become amalgamated into a great seismic paroxysm, a catastrophic event whose reach seems to dwarf the extent of seismic events known from the historical and/or instrumental record and implies an underestimation of the real long-term seismic threat.
There is another unfortunate side effect of working with an archaeological record that is not evenly distributed through time. Like slip on a fault, ancient history is characterized by long periods of stability interrupted by moments when “all hell breaks loose.” During times of flourishing economies and cultural stability, the physical signs of earthquakes are readily covered up, and seismic events live on predominantly in written records. In contrast, during instances of social and political upheaval, when written records perhaps become more fitful, there is little impetus or funds for repair, reconstruction, and recovery. The result is that earthquake effects that would otherwise have been little noted in history are instead left extant, though they may be indistinct from the accompanying signs of human disruption. Nevertheless, when such effects are discovered in the wreckage of ancient settlements, it is all too easy to assume that earthquakes were the catalyst for that cultural collapse.
Rather than simply deriving crude parameters for past earthquakes, archaeoseismology instead becomes a more holistic and interdisciplinary research field concerned with establishing the essential earthquake culture of a region.
Given the all too obvious limitations and constraints of the archaeological record, perhaps it is timely to reconsider what archaeoseismology is all about. Can we legitimately claim that it is a potential contributor to probabilistic seismic hazard studies? Or does the true value of archaeoseismological research lie elsewhere?
The International Geoscience Programme IGCP 567, Earthquake Archaeology: Archaeoseismology along the Alpine-Himalayan Seismic Zone (http://ees.kuleuven.be/igcp567/), provides an opportunity to reappraise the role and scope of this research field. First and foremost, while stressing the need for coherent principles and common practices, it advocates that archaeological sites have a potentially unique value for earthquake science. Rather than simply augmenting earthquake catalogs with possible events gleaned from archaeological sites, ancient cultural sites and remains can be used strategically to examine specific earthquake scenarios. Key targets could be those major events that appear from historical accounts to be atypically destructive, but whose excessive reach and intensity warrant careful appraisal, e.g., events such as the A.D. 21 July 365 Crete earthquake and tsunami. In this context, archaeological sites become “seismoscopes”—the testing ground for predicted site effects of paleo-earthquake rupture models.
IGCP 567 also encourages a broadening of the scope and goals of archaeoseismological studies. Rigorous and systematic studies of past seismic effects at ancient sites might potentially contribute directly to seismic-hazard assessment at earthquake-prone cultural heritage sites. But such studies need to be robustly underpinned by more intimate collaborations between earthquake geologists and archaeologists in deciphering the precise role of earthquakes in the cultural history of a site. A side benefit of this closer union would be a better appreciation of the complex dynamics by which ancient cultures dealt with and responded to damaging earthquakes, which in turn might shed light on the resilience of past societies and their relative capacity to withstand seismic shocks. By highlighting how their ancient ancestors coped with earthquakes, archaeoseismological studies could play an important role in fostering better earthquake preparedness in modern local communities that are equally threatened. Rather than simply deriving crude parameters for past earthquakes, archaeoseismology could instead become a more holistic and interdisciplinary research field concerned with establishing the essential earthquake culture of a region. In establishing such an earthquake culture, “telling a good story” may be good enough!