Alberto Michelini

Variations of crustal elastic properties during the 2009 L'Aquila earthquake inferred from cross‐correlations of ambient seismic noise

[1] We retrieve seismic velocity variations within the Earth’s crust in the region of L’Aquila (central Italy) by analyzing cross‐correlations of more than two years of continuous seismic records. The studied period includes the April 6, 2009, Mw 6.1 L’Aquila earthquake. We observe a decrease of seismic velocities as a result of the earthquake’s main shock. After performing the analysis in different frequency bands between 0.1 and 1 Hz, we conclude that the velocity variations are strongest at relatively high frequencies (0.5– 1H z) suggesting that they are mostly related to the damage in the shallow soft layers resulting from the co‐seismic shaking. Citation: Zaccarelli, L., N. M. Shapiro, L. Faenza, G. Soldati, and A. Michelini (2011), Variations of crustal elastic properties during the 2009 L’Aquila earthquakeinferred from cross‐correlations of ambient seismic noise, Geophys. Res. Lett., 38, L24304, doi:10.1029/2011GL049750.

Collecting, digitizing, and distributing historical seismological data

The digital preservation of the unique seismological heritage consisting of historical seismograms and earthquake bulletins, and of related documentation (e.g., observatory logbooks, station books, etc.), is critically important in order to avoid deterioration and loss overtime [Kanamori, 1988]. Dissemination of this seismological material in digital form is of equal importance, to allow reanalysis of past earthquakes using modern techniques and the reevaluation of seismic hazard. This is of particular interest for those areas where little or no earthquake activity has occurred since the last significant historical earthquake. In 2001, the Istituto Nazionale di Geofisica e Vulcanologia (INGV) started an innovative project, Progetto SISMOS (i.e., SISMOgrammi Storici), to scan (i.e., convert into digital form for storage on a computer), at very high resolution,and archive seismological paper records and related material. The Italian Ministry for the Environment originally funded the project to encompass the digitization of seismogram records of the Italian seismic observatories and of associated bulletins

Rapid determination of earthquake size for hazard warning

The 26 December 2004 M9 Sumatra-Andaman Islands earthquake caused a tsunami that devastated Indian Ocean coasts within three hours after the earthquake. Improved tsunami warning and emergency response for future great earthquakes require knowing an earthquakes size within minutes after the event. Although the hypocenter of a distant earthquake is routinely determined from the first seismic P waves within about 15 min, several hours may pass before a reliable size determination for very large earthquakes is available (e.g., for the 2004 Sumatra-Andaman earthquake see Menke and Levin [2005]). Seismologists specify the size of an earthquake using seismic scalar moment (M0), a physical quantity proportional to the product of fault area and mean slip on the fault. Currently, analyses of long-period seismograms (period >100 s) provide reliable estimates of M0 for very large events [e.g., Harvard Centroid-Moment Tensor (CMT), Dziewonski et al., 1981], but these seismograms are not available until an hour or more after the event.

Man‐induced low‐frequency seismic events in Italy

Unconventional seismic events in Italy are detected by scanning three years of continuous waveforms recorded by the Italian National Seismic Network. Cross correlation of signal templates with continuous seismic records has evidenced unusual events with similar low-frequency characteristics in several Italian regions. Spectral analysis and spatiotemporal distribution of these events, some of which are previously interpreted as tectonic long-period transients, suggest that they are not natural, but produced by huge cement factories. Since there are at least 57 full-cycle cement plants operating in Italy, each affecting areas of about 1250 to 2800 km2, we argue that significant portions of the Italian territory (23% to 51%) can be affected by this man-made noise. Seismic noise analyses, such as those used for microzonation or crustal structure investigations, as well as data mining techniques used to retrieve anomalous transient signals, should thus take into account this peculiar and pervasive source of seismic waves.