UR of the Università of Napoli - Federico II

Objectives: The main objective of this workpackage is the investigation of the robustness and the properties of the Green functions extracted from cross-correlation of data recorded by the seismic network ISNet. The network is equipped with 5 broad-band seismometers and 20 short-period velocimeters, with central frequency of 1Hz. The average distance between the stations is 10 km at the center of the network and 20 km for the stations located on the outskirts. The first task will be the stability analysis of the noise at periods shorter than 2s and the extension of the technique at high frequency. The derived S velocity models, will be compared with the P tomographic models derived by passive seismic analysis.

Activities: Ambient noise data will be collected for the single station and gathered in one day blocks. After filtering data in several frequency ranges and performing a 1 bit normalization, they will be cross- correlated with data coming from other stations, to build up a real-time stack. A dispersion analysis on the Green function database will provide the trend of the group velocity as a function of the frequency. The dispersion curves will be inverted with the Hermann algorithm (Herrmann and Al-Eqabi, 1991), to achieve a tomographic image below the ISNet network.
The technique will be initially applied to the data recorded by the broad-band seismometers. In a second step, we will investigate the possibility to extend the cross-correlation to the data from short period seismometers. The Green function database will be initially built up on a stack of 6 months cross-correlation traces, then the resolution will be increased with the following records.

Methodologies:

1. Real Time noise data management, archiving and automatic preliminary processing;
2. methods for noise processing and stacking analysis;
3. method for dispersion curve analysis and estimation;
4. method for tomographic inversion of surface waves.

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Objectives: The main objective of this workpackage is the achievement of high resolution images of the Irpinia active fault system through the accurate determination of location, size and fault mechanisms of microearthquakes in the magnitude range 1<M<3. As concerns the earthquake location, refined re-picking techniques will be implemented based on massive waveform cross-correlation while source parameters are estimated by non-linear inversion of displacement spectra.

Activities: In the first part of this work package, we will analyze a large dataset of earthquakes recorded at the Irpinia Seismic Network (ISNet) and the surrounding INGV stations during last 3 years. This data set will be integrated with new events recorded during the project duration. The original dataset consists of more than 400 events (1<Mw<3) which have been recently hand-picked. These phases will be quantittively reviewed through cross-correlation techniques. The resulting refined picks will be used to determine both an accurate velocity model for the Irpinia region and an improved image of the fault system through precise relocation. The obtained locations will be used to estimate source parameters by the automated, non-linear inversion of P- and S-wave displacement spectra. Assuming an omega-square source model, the
Downhill simplex optimization technique is used to retrieve the low frequency spectral level, corner frequency and attenuation quality factor. A preliminary investigation of attenuation and site amplification effects is needed in order to correct for path/site effects the spectral shapes.

Methodologies:

1. Real-Time and off-line earthquake data management, archiving and preliminary processing (automatic picking, event binding, spectral analysis);
2. method for refined re-picking based on waveform cross-correlation;
3. method for double-difference tomography and earthquake re-location;
4. method for source parameter estimation by non-linear inversion of displacement spectra.

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Objectives: The aim of this study is to image crustal P- and S-velocities, and subsurface discontinuities beneath the ISNet seismic network using local earthquake data recorded by this network and possibly other stations in the area. Of interest are subhorizontal discontinuities such as the boundary between the seismic basement and the sedimentary cover, and also steeply-dipping reflectors that may be related to faults. The final model may lead to more accurate event locations and may assist in the interpretation of secondary phases detected in seismograms of future events. Another aspect of this study is the development of a reflection processing scheme for local earthquake data.

Activities: The first step in this study is to select appropriate waveform data from the available ISNet database, i.e. recordings of well-located events with a high signal-to-noise ratio and preferably a short, impulsive source signal. Then the selected traces will be corrected for the different origin times to facilitte the application of standard reflection seismic processing techniques and the gathering of traces into e.g. common reflection point (CRP) gathers. Additionally, the source time functions of different events must be equalized e.g. by deconvolution or at least approximately by a polarity correction. The most challenging task of this study is the identification and phase association of reflected and converted arrivals in the seismograms. As in previous investigations, this task involves move-out correction with subsequent stacking or waveform coherency analysis as well as visual inspection of CRP gathers. Since the stations are equipped with three-component sensors, polarization analyses can provide additional information on the wave type, e.g. an arriving P-wave would show a dominant polarization in vertical direction. In addition, seismic migration will support the identification of reflected phases, because it can provide a direct image of reflecting zones in the subsurface.
Once major reflected phases (PP, PS, or SS) are identified, their travel times will be picked manually, accompanied by 1-D kinematics ray tracing for quality control and a consistent association of travel times and their corresponding reflectors. Finally, the picked reflection and conversion travel times shall be inverted for a 3-D tomographic interface model below the study region. Depending on the number and spatial distribution of travel time picks, a joint inversion of reflection and first-arrival travel times for a unified velocity model may be feasible.

Methodologies:

1. Earthquake data gathering and standard seismic reflection processing (filtering, trace equalization, move-out and stack analysis);
2. methods for identification and picking of reflection events on earthquake data seismic sections (polarization analysis, beam forming techniques);
3. method for kinematic ray modelling of reflected/converted phase travel times in a 3D medium ;
4. method for linearized inversion of reflected/converted phase travel times to infer 3D tomographic and interface models of the structure beneath the ISNet network.

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UR INGV

Objectives:: The general aim of the RING network is not only devoted to understand long-term deformation in the Eurasia and Africa plate boundary, but also, more locally, to detect strain accumulation on single faults or faulting structures. For this types of studies, the 30s sampling rate is enough. GPS offers several advantages over seismic instrumentation. Estimation of earthquake-generated co- seismic offsets and strain fields requires measurement of ground displacements. The processing required to estimate this from seismic data inherently enhances noise. In GPS, displacements are the basic measurement and, thus, estimates do not suffer from this noise source. In addition, seismic wave amplitudes vary over many orders of magnitude and, although the dynamic ranges of the best seismometers can capture most of these, many saturate for the largest, and most interesting, earthquakes. The accuracy of GPS data actually improves as the magnitude increases and there is no saturation. High-rate GPS: The first goal of this working package concerns the beginning of the acquisition and storage of the HRGPS data acquired at the RING permanent GPS stations located around the Irpinia test area. Those data will be processed by using a geodetic-quality software. The main goal of this working package is represented by the development of a procedure which allows the computation of the mean displacement up to a few tens of minutes after the occurrence of an earthquake. This procedure will be tested with appropriate scenarios to evaluate the sensibility of the HRGPS to detect low to moderate earthquakes signal associated with the arrival seismic waves.

Activities: High-rate GPS acquisition: The scientific interest for the potential of the GPS seismology will allow an increasing number of continuous GPS stations acquiring with 1-Hz sampling rate. The Irpinia represents one of the test site for the HRGPS data acquisition, transmission and storage.

High-rate GPS processing: The 1-Hz GPS data acquired at the continuous GPS sites in the Irpinia test sites will be processed by using Gipsy software, a noncommercial geodetic-quality software developed at JPL (Zumberge et al., 1997), and by using the ambiguity resolution approach described by Blewitt et al. (1989, 2006a).

High-rate GPS procedure for alert systems: One of the goal of this project deals with the development of a procedure to compute the mean displacement related to the occurrence of the an earthquake (Blewitt et al., 2006b).

Methodologies:High-rate GPS: Within the framework of this project, we propose to acquire at 1-Hz sampling rate the data of the RING CGPS stations within the Irpinia test site. These high-rate GPS data will be processed, by using Gipsy software, in a few minutes moving time windows to determine the GPS site position time series and then to enhance possible mean displacements related to the occurrence of an earthquake. The static offsets carried out in this way will then be used to rapidly model the earthquake and, then, to contribute effectively to the computation of a realistic magnitude moment Mw and source parameters. Using this method, Blewitt et al. (2006b) showed, for the case of Sumatra Mw=9.2-9.3 event, that the earthquake's true size and tsunami potential could have been determined by using GPS data to only 15 minutes after the earthquake initiation, by tracking the mean displacement of the Earth's surface associated with the arrival seismic waves. By implementing the GPS displacement method as an operational real-time system, GPS could be incorporated into earthquake warning systems. Even if the Blewitt et al. (2006b) study concerned such a big earthquake, we propose to apply this method to more moderate seismogenic structures monitored by a dense GPS network.

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MAIN REFERENCES

Blewitt, G., (1989) - Carrier Phase Ambiguity Resolution for the Global Positioning System Applied to Geodetic Baselines up to 2000 km, J. Geophys. Res., 94(B8), 10187-10203.

Blewitt, G., (2006a) - The fixed point theorem of ambiguity resolution for precise point positioning of GPS networks: Theory and applications, Eos Trans. AGU 87(52), Fall Meet. Suppl., Abstract G43A-0977.

Blewitt, G., C. Kreemer, W. C. Hammond, H.-P. Plag, S. Stein, and E. Okal (2006b) - Rapid determination of earthquake magnitude using GPS for tsunami warning systems, Geophys. Res. Lett., Vol. 33, L11309, doi:10.1029/2006GL026145.

Herrmann R. B. and G. Al-Eqabi (1991) - Surface waves: Inversion for shear wave velocity, in Shear Waves in Marine Sediments, edited by J. M. Hovem, M. D. Richardson, and R. D. Stoll, pp. 545Ð 556, Springer, NewYork.

Zumberge, J. F., M. B. Heflin, D. C. Jefferson, M. M. Watkins, and F. H. Webb (1997) - Precise point positioning for the efficient and robust analysis of GPS data from large networks, J. Geophys. Res., 102, 5005Ð5018.

Weaver R. L. and O. I. Lobkis (2001) - Ultrasonics without a Source: Thermal Fluctuation Correlations at MHz Frequencies, Phys. Rev. Lett., 87, 134301.

Weber E., Convertito V., Iannaccone I., Zollo A., Bobbio A., Cantore L., Corciulo M., Di Crosta M., Elia L., Martino C., Romeo A., and Satriano C. (2007) - An advanced seismic network in the Southern Apennines (itly) for seismicity investigations and experimentation with earthquake early warning, Seismological Reserach Letters, Vol.78, N.6.

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Task 3 Objectives

Staff