The Mw 6.3 Abruzzo, Italy, Earthquake of April 6, 2009
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EERI Special Earthquake Report — June 2009 Learning from Earthquakes The Mw 6.3 Abruzzo, Italy, Earthquake of April 6, 2009 From April 17-24, a team made up versity of Rome La Sapienza; Khalid The earthquake killed 305 people, of representatives of the Earth- Mosalam, Dept. of Civil Engineering, injured ,500, destroyed or dam- quake Engineering Research Insti- UC Berkeley (PEER); H. John Price, aged an estimated 0,000-5,000 tute, the Applied Technology Coun- Curry Price Court, San Diego (ATC); buildings, prompted the temporary cil, and the Pacific Earthquake En- and Marko Schotanus, Rutherford & evacuation of 70,000-80,000 resi- gineering Research Center investi- Chekene, San Francisco. A separate dents, and left more than 24,000 gated the effects of the Abruzzo team from the Geo-engineering Ex- homeless. This event was the earthquake. The team was led by treme Events Reconnaissance Asso- strongest of a sequence that start- Paolo Bazzurro, AIR Worldwide ciation (GEER), led by Jonathan Stew- ed a few months earlier and num- Corp., San Francisco, and included art of UCLA, also contributed to this bered 23 earthquakes of Mw>4 David Alexander, CESPRO, Univer- report. between 03/30/09 and 04/23/09 sity of Florence; Paolo Clemente, (Figure 2), including an Mw 5.6 on The research, publication, and distri- ENEA Casaccia Research Centre, 04/07 and an Mw 5.4 on 04/09. bution of this report were funded by Rome; Mary Comerio, Dept. of the Earthquake Engineering Re- A total of 8 municipalities were Architecture, UC Berkeley (PEER); search Institute Learning from Earth- affected by the earthquake, and 49 Adriano De Sortis, Italian Dept. of quakes project, under grant #CMMI- of them were in the Prime Ministe- Civil Protection, Rome; Filip Filippou, 0758529 from the National Science rial Decree regarding damage of Dept. of Civil Engineering, UC Ber- Foundation. Mercalli-Cancani-Sieberg (MCS) keley (PEER); Agostino Goretti, Ital- intensities VI-X. (Note that in this ian Dept. of Civil Protection, Rome; Introduction range of intensities, the Modi- Mersedeh Jorjani, architectural con- On Monday April 6, 2009 at 3:32 a.m. fied Mercalli Intensity [MMI], more servator, San Francisco (ATC); Fab- widely used in the U.S., can be rizio Mollaioli, Dept. of Structural local time, an Mw 6.3 earthquake with computed as IMMI = 5/6 IMCS, as re- and Geotechnical Engineering, Uni- shallow focal depth (0 km) struck central Italy in the ported by Decanini et al., 995). vicinity of L’Aquila, a The population of L’Aquila includes city of about 73,000 4 surrounding boroughs such as people that is the Onna, Paganica, and Tempera (see capital of the Abruzzo Figure 3). The combined population region (Figure ). of the 48 other towns listed in the Figure 2. Characteristics of 23 earthquakes above Mw4 from Figure 1. The epicenter of the Abruzzo event. 3/30 to 4/23 (Italian National Institute of Geophysics and Vulcanology). EERI Special Earthquake Report — June 2009 basin bounded by predominantly NW-SE-striking and SW-dipping active normal faults (Blumetti, 995; Bagnaia et al., 996). Downtown L’Aquila is set on a fluvial terrace that forms the left bank of the Aterno River (De Luca et al., 2005). The elevation of the terrace reaches 900 m above mean sea level (asl) in the northeastern part of the city and slopes down to 675 m asl in the southwest direction. The terrace ends at the Aterno River, which flows 50 m below. The alluvial deposits constituting the terrace are lower Quaternary in age, and are composed of breccias with limestone boulders and clasts in a The L’Aquila area and the locations of fatalities. Figure 3. marly matrix. Clast dimensions can range from centimeters to meters. official earthquake damage declara- Seismotectonic and Geo- tion is 60,352. The majority range logical Setting and Historical This kind of deposit is common in in size from ,000 to 3,000 people, the Abruzzo region and may be re- with two larger towns of 5,000 and Seismicity lated to catastrophic alluvial events 8,500 inhabitants. The city of L’Aquila is located in the associated with landslides (Blu- metti, 995). The deposits were The earthquake struck when most central part of the Apennine chain, a studied by Demageout (965), who people were sleeping. The loca- thrust-belt that represents a margin of named them “megabrecce.” The tions of the 305 deaths can be seen a plate deformed during continental megabrecce represents a well- in Figure 3. The fatalities were con- collision between the African plate (to defined geological unit with flat top centrated in the 20-29 and over-70 which it belongs) and the Euroasian and bottom surfaces and a thick- age groups, though that does not plate that took place in the Cenozoic ness of some tens of meters, lying reflect the more balanced demo- 65 million years ago (Cavinato and on lacustrine sediments composed graphics of L’Aquila province. The De Celles, 999; Devoti et al., 2008). mainly of silty and sandy layers and peak in the 20-29 group was due to The Apennines belong to a geody- minor gravel beds. The lacustrine the collapse of a student dormitory namic system that migrates from the sediments reach their maximum in downtown L’Aquila. Tyrrhenian to the Adriatic area (from W to E) in response to the flexural re- thickness (around 250 m) in the Many of the region’s cultural sites treat and subsequent sinking of the center of L’Aquila. In contrast, in were badly damaged or destroyed, Apulia platform. This unit is being sub- the Aterno River valley north of including Romanesque churches, ducted westward beneath the Apen- L’Aquila, the thickness of the sedi- palazzi, and other monuments dat- nines (Doglioni et al., 990, 99). ments is never greater than 00 m. ing from the Middle Ages and The city is located on a tectonic Renaissance. The historic centers of villages in the Aterno River valley southwest of L’Aquila — Onna, Paganica, and Castelnuovo — were essentially obliterated, with shaking intensities of up to X on the MCS Scale. Conversely, damage did not exceed MCS intensity VI nearly anywhere to the northwest of L’Aquila. This southeastward elongation of the damage pattern probably reflects a combination of rupture directivity and seismic litostratigraphic amplifi- Figure 4. Historical MCS macroseismic intensity of the major events near cation effects. L’Aquila since 1300. 2 EERI Special Earthquake Report — June 2009 Table 1. Details from five stations with epicentral distance less than 20 km (source: Italian Dept. of Civil Protection). Figure 4 shows the earthquakes Mw 6.3 event one of the best record- based only on Italian data (Scas- that have struck the city of L’Aquila ed earthquakes caused by a normal serra et al., 2009). and the villages around it since fault mechanism. Five stations, all on The 5%-damped spectra of the two 300 (Stucchi et al., 2007; Rovida the hanging wall of the rupture, were horizontal directions produced by et al., 2009). L’Aquila has been se- located within 0 km of the epicen- USGS for the stations in Table verely damaged at least five times ters, and all recorded a horizontal have shown, with the exception of in the last 700 years, specifically peak ground acceleration exceeding the AQK station, that the ground in 35 (M ≈6.7), 1349 (M ≈6.5), 0.35g (Table ). One station (AQM, w w shaking had mostly a high-frequen- 46 (M ≈6.5), 1703 (M ≈6.7), and not included in the table) recorded g w w cy content (Figure 5). The ground 95 (M ≈7.0). The 1461 event or more in the vertical and one horizon- w motion had also relatively short shows a damage distribution simi- tal direction, but went off scale above duration: 95% of the energy was lar to that of the April 6 earthquake, g in the second horizontal direction. released in 0 seconds or less. although it is shifted to the east by Its recordings are still subject to study; Finally, unprocessed data obtained a few km. they have not yet been released. from many stations show perma- Ground Motion Records The stations AQG, AQK, and AQM nent displacements of up to 5 cm. are on rock or stiff high-shear-wave- Fifty-six of the approximately 300 velocity (Vs) material, while AQA and Geotechnical Aspects digital strong-motion stations oper- AQV are on recent alluvium. Prelimi- The Geoengineering Extreme ated by Italian Strong Motion Net nary comparisons of ground motion Events Reconnaissance Association work (RAN) managed by the Italian intensity measures (IMs) from record (GEER) dispatched a team to the Department of Civil Protection (DPC) ings to prediction equations performed affected region within four days of recorded the main shock. In addi- by the GEER team show that high- the main shock. Their field survey tion, 42 broad-band stations re- frequency IMs attenuated more rapid- revealed no surface fault rupture, corded it. Fourteen stations are in ly with distance than is predicted by although they observed numerous the Abruzzo region, while the re- most empirical relations. This faster incidents of ground deformation. It maining ones are scattered in the attenuation is consistent with a recent- is possible that these were associ- Apennines mostly NW and SE of ly published modification of the next ated with co-seismic slip on shallow L’Aquila. This makes the Abruzzo generation of attenuation equations Figure 5. Response spectra for 5% damping for the 5 stations in Table 1 that recorded >0.1 g and comparison with 2008 Italian Code Spectrum.