Preliminary Report on the Seismological and Geotechnical Aspects of the April 6 2009 L'Aquila Earthquake in Central Italy (Version 2.0)

Abstract

The L’Aquila earthquake occurred on April 6 2009 at 03:32:39 local time. The earthquake was located in the central Italy region of Abruzzo. Much of the damage occurred in the capital city of L’Aquila, a city of approximate population 73000, although many small villages in the surrounding regions were significantly damaged including Paganica, Castelnuovo, and Onna. Collapsed and damaged structures in L’Aquila included both older masonry buildings and relatively modern reinforced concrete structures. At the time of this writing, 307 people are known to have died from the earthquake, most in collapsed structures, making this the deadliest earthquake to strike Italy since the 1980 Irpinia earthquake. A number of reconnaissance teams were mobilized to the affected region in the weeks following the earthquake. The national institute of geophysics and volcanology (Istituto Nazionale di Geofisica e Vulcanologia, INGV) mobilized a team of geologists (EMERGEO Working Group) to look for evidence of surface rupture and other effects; some of their findings are discussed in this report. The GEER team was assembled to investigate geological, seismological, and geotechnical engineering aspects of the event. The international GEER team is comprised of members from Italy, Austria, Switzerland, Greece, and the United states. Team members were selected to provide needed expertise in geology, engineering geology, GIS applications, earthquake ground motions, and geotechnical earthquake engineering. The team includes individuals highly experienced in post-earthquake reconnaissance and relatively young professionals investigating their first earthquake. The GEER team did not focus on structural engineering or lifeline aspects of the event, which were investigated by an EERI team. The GEER and EERI activities were closely coordinated to optimize resources in the documentation of the valuable, perishable data associated with the earthquake effects. The GEER team employed a number of innovative technologies to facilitate effective reconnaissance. All teams mobilized for field work had a common GPS unit and laptop with a Google Earth (GE) GIS database activity maintained over the course of the work. The GE database was used to keep track of visited locations, but also contained maps of surface geology, locations of aftershocks, strong motion stations, and other information relevant to investigators in the field. Another valuable use of technology involved LIDAR mapping of a site having significant incidents of ground failure (Lake Sinizzo). This report presents the GEER findings. Following this introduction, Chapter 2 describes the geologic and tectonic setting, moment tensor solutions for the mainshock and several triggered events, analysis of aftershock patterns, and analysis of GPS and InSAR data. Included in Chapter 2 is a preliminary model of the ruptured fault. Chapter 3 describes the ground motions recorded during the mainshock by a digital instrument array. Metadata associated with the recordings is presented, trends in the recorded ground motions are presented, and preliminary comparisons to ground motion prediction equations are made. Chapter 4 presents damage patterns, both within L’Aquila and through comparisons of damage intensities in adjacent villages with similar construction. The results provide valuable insights into possible site effects on ground motion in regions where recordings are not available. Chapter 5 presents our findings on ground failure, defined as permanent ground deformations induced by the earthquake. Observed ground failure included several rockfalls, seismic compression of fill materials, and apparent strength loss of soil materials leading to inward movement of the banks of a lake. Chapter 6 reviews the performance of earth dams and earth retaining structures, both of which generally performed well.