Ground motion and site response

The M sub w 8.4 Southern Peru earthquake affected a widespread area (see area map in the Preface, page xi), including several important population centers in southern Peru and northern Chile. Unfortunately, the region is sparsely instrumented, and only a limited number of strong motion instruments recorded the main shock of 23 June 2001. Although the lack of recorded ground motions precludes an extensive analysis of site response issues, the documented damage at various population centers (e.g. Tacna, Moquegua, and Ilo) constitute an important database for future study of site effects in subduction zone earthquakes, particularly for the types of soil deposits common to southern Peru, such as stiff alluvial deposits and weathered volcanic rocks. This chapter reports the main shock ground motions recorded in the event, as well as observations regarding site and topographic effects in the main population centers affected by the earthquake. A special emphasis is placed on damage documented in the city of Tacna, where significant data was gathered. Seismological issues are discussed in detail in Chapter 1, Seismicity and Tectonics, and ground failure-related observations are presented in Chapter 3, Ground Failure. The National Science Foundation (NSF)-sponsored U.S.-Peruvian geotechnical reconnaissance team collected the majority of the information presented in this chapter. The initial reconnaissance trip was undertaken from 7 July through 13 July 2001. One team member made a second visit to the city of Tacna within three months of the earthquake. Additional information on damage evaluation and subsurface conditions was obtained during this second trip. Information was also obtained from a literature review of microzonation studies performed prior to and immediately following the 23 June 2001 earthquake (e.g. Cotrado and Sina 1994 in Tacna; Sato and Konagai 2002 in Moquegua; Kosaka-Masuno et al. 2001 and Lermo et al. 2002 for the entire affected region). The geotechnical team adopted a reconnaissance philosophy consistent with that of teams studying previous earthquakes (e.g. Bardet et al. 2000; Stewart et al. 2001); that is, to accurately and efficiently collect as much high quality field data as possible. Interpretation of the data was not made in the field; rather this task will be completed only after comprehensive analyses of the collected data. Accordingly, this article does not present conclusive interpretations of the field data.