L'Aquila, Italy Earthquake Clearinghouse » Earth Sciences

GEER findings – damage patterns

stewart — Sun, 19 Apr 2009 21:25:43 +0000

We examined damage patterns at two scales – damage within L’Aquila and regional damage distribution based on impact in small villages (figure below). The figure below shows our track logs and call out places we visited as of Wednesday April 15. In most cases we have approximate damage statistics for those villages.

Within L’Aquila, we see two interesting patterns. First, on the hanging wall side of the Pettino fault, we see a 100-200 m wide zone of heavy damage. Footwall side is steep terrain so no damage observations. Below the 100-200 m wide zone is relatively light damage. Second pattern has to do with site effects. North of the historic center of L’Aquila, we see light damage on the Pleistocene terraces, but relatively heavy damage (but no collapses) in younger Holocene swales cut into the terrace. We have pretty good documentation of both of these damage patterns. We have not mapped damage in the historic center, which will be an area of emphasis for the EERI team.

Track logs and villages visited

Regarding villages, we did not do detailed mapping, but we did visit many villages and made estimates of percentages of structures with different damage states for the predominant construction style (often masonry). Villages visited as of April 15 are marked on figure above. Comparing one village to another can indicate site effects. For example, we have several cases of side by side villages where one is heavily damaged and others are fine, usually with the damaged ones being on Pleistocene soils.

GEER findings – source description (very preliminary)

stewart — Sun, 19 Apr 2009 21:02:00 +0000

The faulting associated with this earthquake is complex and is not yet well understood. We hope to have a working hypothesis for the faulting mechanism by the time we publish our preliminary report in early May. The relevant data sources include the aftershock pattern and its variation with time, moment tensor solutions of the mainshock and aftershocks, and SAR data.

We do know that the faulting is shallow (around 10 km and less). The mainshock moment tensor indicates normal faulting on approximatley a 45 degree dip (in contrast to approximately 65 degree dips of well-expressed normal faults in the region). There was a large April 7 aftershock (Ml 5.3) SE of L’Aquila. For awhile we thought this might be a triggered event, but we now think it is probably an aftershock given significant aftershock activity in that area prior to the April 7 event. Whether the large April 9 event (Ml 5.2) was an aftershock or a triggered event remains under debate.

INGV’s working hypothesis is that the main shock occurred on a relatively shallow dipping blind normal fault. Under this hypothesis, the faults that are expressed at the surface (e.g., Pettino fault) are secondary features that terminate at depth on the main fault.

GEER findings – ground motions

stewart — Sun, 19 Apr 2009 20:11:30 +0000

We have not looked at individual records yet, but we have the motions for the mainshock, largest aftershock, and triggered event from a DPC website. The mainshock data have been forwarded to PEER and CESMD for processing. We have 56 mainshock recordings, including 4 in the immediate vicinity of L’Aquila. Three of those are west of the city in a sparsely developed valley (however, this is near the hospital). The fourth is near the historic center of L’Aquila. The center sits on a Pleistocene terrace. There is a swale east of the center, and the instrument is above a pedestrian tunnel portal on the edge slope of the Pleistocene terrace (nearby structure is a car park). We were not able to walk up to the instrument because access to the car park is denied. This instrument recorded about 0.35 g. The instruments W of L’Aquila recorded 0.5-0.7 g.

GEER findings – ground failure

stewart — Sun, 19 Apr 2009 20:04:06 +0000

We did not found evidence for surface fault rupture. Scott Kieffer, Teddy Button, and students looked in the area of the Pettino and Paganica faults. Both faults are very well expressed at the surface, but we did not find evidence of displacement from this earthquake. However, recent reports from others indicate some ground failure in vicinity of Paganica fault, and some geologists are going into the field to look more carefully at this. Therefore, it is too early to conclude that there was no surface rupture.

Ground failure occurred at Lake Sinizzo, where slumping occurred around the rim of this natural lake (no obvious dam). We have performed detailed mapping of the ground fissures and measured crack widths so that displacement vectors can be developed. Surface soils are gravelly clay, but we don’t know what materials are at depth. Hence, we don’t know if this is a liquefaction issue or slope failure in clay. No sediment boils. Displacement range from few cm to about 15 m. No structures were affected. A photograph along the margin of the lake is attached below.

Lake Sinizzo

Ground failure involving fissures but no sediment boils was also observed by George Mylonakis near Onna. There is a creek nearby, and this may have involved displacements towards the creek. The ground fissures passed around the foundation of a building in the area.

There are a number of other cases of slope displacements and rockfalls, some of which impacted structures. These were in the vicinity of Onna and Paganica. Additional reports continue to come in, so this information will change with time.

GEER team – overview of activities

stewart — Sun, 19 Apr 2009 19:44:22 +0000

The GEER team deployed on April 10 and worked through April 18. Additional observations will continue to come in as additional teams go into the field. We have made every effort to provide our findings to the leadership of those teams to optimize their efficiency.

Our findings are synthesized into a Google Earth (GE) database maintained by Rob Kayen. All of the locations visited by our team are marked on the map along with notes on our observations and photographs. The GE database also includes geologic maps, aftershock distributions, locations of strong motion stations, and SAR data. The GE database also contains the results of detailed mapping of damage distributions in building structures within L’Aquila. This database has been provided to Paolo Bazzurro, the EERI team leader. After some clean up, it will be posted to the GEER web site.

While some individual team members continue with field work, mostly to assist the recently deployed EERI and PEER teams, for the most part our efforts now have shifted to documentation of the results obtained to date. We hope to have a preliminary report ready for posting to the GEER web site by May 6. This report will discuss the seismological aspects, ground motions, damage patterns, ground failure, and performance of dams.

Main Shock Time Histories from station AQV (provided by M.Celebi, USGS)

EERI Admin — Sun, 19 Apr 2009 14:19:33 +0000

Largest peak accelerations of the main shock from station AQV and their amplitude spectra :

Local report on new faults

silviocalvi — Thu, 16 Apr 2009 06:08:59 +0000

The attached website reports pictures of new faults in the area:

It’s published in the local newspaper “Il Centro”

http://ilcentro.gelocal.it/multimedia/home/5466682/4

Silvio Calvi

GEER Update Apr 12 2009

stewart — Sun, 12 Apr 2009 20:52:33 +0000

Ground motions: uncorrected mainshock data obtained (56 triaxial recordings) & will be forwarded Brian Chiou shortly for processing at PEER.

Fault: No surface rupture found, but likely scarp of normal fault identified. Aftershock pattern suggests two-segment rupture in mainshock. Possible triggered second event several days after mainshock on separate fault with its own aftershock distribution.

Tentative evidence of site effects identified in LAquila, mapping forthcoming.

Ground failure around lake, mapping forthcoming.

Some rockfalls that damaged structures.

Basic Earth Science Information

EERI Admin — Fri, 10 Apr 2009 00:41:39 +0000

USGS Moment Tensor
Shakemap
Italian seismic sources