The Inqua scale. An innovative approach for assessing earthquake intensities based on seismically-induced ground effects in the natural environment

The debate originated within the Workshop of the Subcommission on Paleoseismicity held during the XV INQUA Congress in Durban, August 1999, emphasized the importance of developing a multi-proxy empirical database on earthquake ground effects that can be used by, and incorporated into, seismic-hazard assessment practices. The Subcommission selected this task as the primary goal for the past inter-congress period. An interdisciplinary Working Group (WG) was established, including geologists, seismologists and engineers, in order to formalize the collected data into a new scale of macroseismic intensity based only on ground effects: the proposed INQUA scale. This paper illustrates the results of the research conducted by the WG, introduces the proposed INQUA scale, and discusses major issues related to this innovative approach to the intensity assessment. The INQUA scale first draft is due to Leonello Serva, based on the compilation and comparison of the three most commonly used intensity scales, i.e., the Mercalli-Cancani-Sieberg (MCS), Medvedev-Sponhouer-Karnik (MSK) and Mercalli Modified (MM). Eutizio Vittori, Eliana Esposito, Sabina Porfido and Alessandro M. Michetti produced a revised version, after (a) integration with the revised MM scale of Dengler and McPherson (1993) and (b) checking the scale against the description of coseismic ground effects and intensity assessments for several tens of historical and instrumental earthquakes in the world. This version of the INQUA scale, presented during the XVI INQUA Congress in Reno, July 23-30, 2003, is a joint contribution of the WG including new data, editing, comments and scientific discussion from Bagher and Jody Mohammadioun, Eugene Roghozin, Ruben Tatevossian, Aybars Gürpinar, Franck Audemard, Shmulik Marco, Jim McCalpin, Nils-Axel Mörner, and Valerio Comerci. At this stage, the newly revised MM scale for New Zealand (Hancox, Perrin and Dellow, 2002), kindly provided by Graeme Hancox, has been also taken into account. The outstanding progress of paleoseismological and Quaternary geology research in the past decades makes available an entirely new knowledge for understanding the response of the physical environment to seismicity, thereby providing the basis for the proposed INQUA intensity scale. The INQUA scale allows to define the epicentral intensity starting from the VI - VII level, with increasing accuracy going towards the highest levels. In the intention of the WG, the INQUA scale should not be used alone, but in combination with the existing scales. In the intensity range up to IX - X the scale allows a comparison between environmental effects and damage indicators, emphasizing the role of primary tectonic effects, which are independent from the local economy and cultural setting. In the intensity range X to XII, the INQUA scale is arguably the only suitable tool for assessing the epicentral intensity. In summary, we regard the INQUA scale as an unreplaceable addition to all the existing scales up to the IX - X level, while it represent the substance of the epicentral intensity assessment for the highest degrees.