UR Reports - II Phase

Agreement INGV-DPC 2007-2009 Project S5 : “High-resolution multi-disciplinary monitoring of active fault test-site areas in Italy” UR Reports - II phase May 1st, 2009 – May 31st, 2010 Task1. Test site Alto Tiberina Fault (ATF). UR1 -INGV: Lauro Chiaraluce (CNT-INGV). The works carried out and finalised over the past two years have been devoted to the foundation of the permanent Alto Tiberina Fault test site. During this period of time the INGV Airplane working group finalised the monitoring infrastructure composed of a dense network of co-located seismological and geodetic instruments. There was then a need to recover complementary information to turn on both the observing system and to immediately start planning its future implementation. The S5 project allowed us to provide many of these necessities. With the Working Package 1 (Raffaele Di Stefano; CNT-INGV) we answered the question: how can we observe the seismicity pattern that characterises the activity of a normal fault system for a long period of time with the least manual work? That is why we set up a modular and semi-automatic procedure that goes from continuous waveforms to high resolution event location, computation of magnitude and focal mechanism. The process comprises independent codes that allow us to monitor with time a series of parameters such as network detection threshold versus the completeness of magnitude of our catalogues, the rate of seismic release, b-values and VP/VS ratio at single station and the level of noise. A final modulus analyses the generated datasets to produce maps and graphs for interpretation. The whole procedure runs on the on-purpose setup Linux machine. Results obtained in the project framework. The terrible occurrence of the L’Aquila earthquake in April 2009 forced us to implement, calibrate and test our procedure on the aftershock sequence. Examples of the result from automatic 1D locations and magnitude completeness are shown in the following two figures. Figure 1.1: Map and set of vertical cross sections showing the outputs of the location 4 steps procedure run on the L’Aquila seismic sequence subset. These are all the events with ML≥1.9 present in the INGV National Network catalogue we picked also at the stations of the local and temporary networks. The last column of Figure 1.1 shows the great enhancement we achieved in focusing the fault system geometry through the automatic detection and location of seismicity which hit the L’Aquila region. All the P- and S-wave arrival times catalogue have been generated by using a semi-automatic procedure: we did not hand pick a single seismogram. The great improvement in location resolution is mainly due to the homogeneity of the weighted scheme we were able to provide for the whole catalogue. The total machine time for the elaboration of 3000 events recorded at a mean of 50 three component seismic stations is ~3 days. Despite the need for a sometimes elaborated calibration procedure of the picking code, we believe the availability of this procedure represents a high innovation with regards to time consuming procedures. Figure 1.2: MC versus time for two catalogues containing the seismicity occurred about 40 km far from the L’Aquila city from the 6th to the 16th of April, 2009. The red line in Figure 1.2 represents the completeness versus time of the catalogue we generated by applying our triggering algorithm and location procedure on the continuous recordings of the permanent and local networks. A comparison is made with the INGV Iside catalogue, shown with a blue line. In this Figure we can appreciate the high performance of our automatic procedure that once the noise related to the mainshock occurrence (6th of April) decreases, the triggering-location modules are able to reach MC ~1.3. The availability of these modules will allow us to perform analysis of a very large dataset composed of tens of thousands of earthquakes opening new ways to look at seismicity patterns. Problems and difficulties. As mentioned above we run our procedure on the 2009 L’Aquila sequence instead of testing and calibrating it on the test site data which is available from the beginning of 2010. This means that both the organisation of the data storage and results dissemination through dedicated web site pages were provided by the ING National Earthquakes Centre. Further on- line information will be available for information produced on the ATF. The dedicated web server is hosted on the same computer that performs all the daily analysis and the modules devoted to the synthesis of the results is a simple web page architecture builder written in Linux bash shell that daily rebuild web pages with updated maps and graphs for interpretation, based on the elaboration and analyses of the newly generated datasets. With the Working Package 2 (Resp.: Luigi Improta; Roma1-INGV) we performed a high resolution seismic exploration survey aimed at obtaining reflectivity and VP shallow images to provide insights both on the shallow architecture of the Quaternary Tiber basin and the Alto Tiberina Fault (ATF) system, together with information to optimize the location of shallow drillings to install borehole seismometers planned in the framework of the Project MIUR AIRPLANE. Details on the survey design, data acquisition and processing are available in a technical report where we also describe Deliverables D2-D3. The first phase of research was devoted to the sites for surveying that could target both the structure of the Tiber basin and shallow splays of the ATF. Finding suitable survey sites was a challenging task because of significant logistic and environmental difficulties. We investigated the sector of the basin between Umbertide and Perugia, about 20 km long, by analyzing aerial photos, geologic maps, industry seismic reflection profiles and by carrying out field surveys (Figure 2.1). Figure 2.1. Structural map of the Tiber basin with outlined the traces of commercial reflection profiles (grey lines) and the area of the seismic experiment (red box). The blue line is the commercial reflection profile L7 located 3km to the south of the survey area, which provides information on the large scale structure of the basin and ATF (after Mirabella et al., 2004). As a result, we identified a suitable area located just 3 km to the north of a commercial profile, where we collected two HR SW-trending seismic profiles that extend from the central portion of the basin to the western and eastern ridges (Line A and Line B in Figure 2.2). Figure 2.2. Simplified geologic map of the investigated sector of the basin, with outlined the two acquired HR profiles (A and B). The total length of Line A and Line B is 1635 and 2050 m, respectively. Data were acquired with a dense, wide-aperture geometry. A high-resolution Vibroseis source was recorded by a geophone spreading 835 m long. Source and receiver spacing was 10 and 5 m. This geometry allowed the simultaneous collection of both multifold, common-mid-point, reflection data and dense refraction data. Overall, common shot gathers display a good signal to noise ratio. First arrivals are clear even for far offset traces, while most of the record sections show high- frequency near-vertical reflections and large amplitude, low-frequency wide-angle reflections. Results obtained in the project framework. Seismic imaging aimed at defining HR reflectivity images of the sub-surface by standard CDP processing of reflection data, complemented by P-wave velocity images obtained by a refraction multiscale tomography. The two seismic stack sections show interpretable events down to 0.4 s TWT corresponding to 400-500 m depth. The western Line (A) shows coherent and laterally continuous reflectors, which define the internal geometry of the Quaternary continental infill along the western side of the basin (Figure 2.3). A noticeable result is the detection of an eastward dipping extensional fault along the western hill-slope, which did not appear in published geologic maps. This structure is interpreted as a high-angle synthetic splay of the low-angle ATF that can be traced along the entire seismic profile down to 0.4 s following gently eastward dipping subtle reflectors. The tilting and growth of fluvio- lacustrine strata towards this high-angle fault evidence its syn-sedimentary activity, while truncation of near-surface reflectors (<30-50 m depth) referable to Late-Pleistocene–Holocene terraced deposits documents recent slip along this structure. Figure 2.3. Stack section with schematic line drawing of Line A. The red dashed Line A. line outlines the low dipping ATF. Note the synthetic high angle extensional splay positioned at 600 m. The eastern Line (B) well defines the westward thickening of the Tiber basin (Figure 2.4). West-dipping fluvio-lacustrine Quaternary deposits on-lap onto an articulated substratum, which rapidly rise to only 0.15 s (~150 m depth) beneath the central portion of the profile. This feature is confirmed by VP models obtained by multiscale refraction tomography, which consistently image a broad high-VP (3.0-3.4 km/s) anti-formal structure. This structure can be interpreted as the prolongation of the fold developed within Miocene rocks, which crops out ~1 km to the north of the profile. The substratum and overlying Lower-Middle Pleistocene fluvio-lacustrine deposits appears truncated by a westward dipping fault below the eastern side of the profile. This structure could represent an antithetic extensional splay of the ATF system. Figure 2.4. Stack section with schematic line drawing of Line B. Note the articulated geometry of the pre- Quaternary sub-stratum (brown line). And the SW-dipping extensional structure on the eastern side. The analysis of the acquired data will allow us to greatly improve the imaging resolution of the shallower portion of the system complementarily to the industrial reflection profiles quite poor in the upper 0.2-0.4 s TWT.

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