Detection of earthquake clusters on the basis of waveform similarity: An application in the monferrato region (Piedmont, Italy)

In August 2000 and July 2001 two seismic sequences, characterized by mainshocks with Ml (local magnitude) respectively 5.1 and 4.8, occurred in the Monferrato region (Italy). The regional seismic network of North-western Italy (RSNI) recorded more than 250 foreshocks and aftershocks. The routine locations, obtained from the Hypoellipse code, show a seismic activity concentrated in a circular area, of a radius of about 15 km, located near Acqui Terme, and randomly distributed in depth. Location errors, due to an uneven azimuthal station distribution of the regional seismic network, prevent recognition of the geometry of the active zone. Waveform analysis revealed the presence of several multiplets. In order to discriminate and successively relocate them, an automatic cross-correlation procedure was applied. Normalized cross-correlation matrixes, for the RSNI stations which recorded almost 90% of considered events, on the basis of a signal to noise ratio analysis, were computed using only S wave time windows. The use of a relocation procedure, based on the double-difference method which incorporates ordinary absolute travel-time measurements and/or cross-correlation differential travel-times, allowed us to overcome the constraints of the uneven distribution of stations giving a quite different frame of seismicity. The improved locations showed that the seismic activity is mainly arranged along a NE-SW oriented volume, at a depth range of 8–20 km, involving the basement crystalline units. This orientation is confirmed by the analysis of the focal mechanisms: most focal solutions show a strike slip component with one of the nodal planes consistent with the main orientation of the seismic events.

[1]  Nicholas Deichmann,et al.  Rupture geometry from high-precision relative hypocentre locations of microearthquake clusters , 1992 .

[2]  Mikhail Zhizhin,et al.  Identification of a geological region for earthquakes using syntactic pattern recognition of seismograms , 1994 .

[3]  Joan S. Gomberg,et al.  The effect of S-wave arrival times on the accuracy of hypocenter estimation , 1990, Bulletin of the Seismological Society of America.

[4]  Jean-Luc Chatelain,et al.  A fractal approach to the clustering of earthquakes: Applications to the seismicity of the New Hebrides , 1987 .

[5]  R. Di Giovambattista,et al.  Local earthquake relative location by digital records , 1987 .

[6]  William L. Ellsworth,et al.  Monitoring velocity variations in the crust using earthquake doublets: An application to the Calaveras Fault, California , 1984 .

[7]  H. Langer,et al.  High-Precision Relative Locations of Two Microearthquake Clusters in Southeastern Sicily, Italy , 2003 .

[8]  Manfred Joswig,et al.  Automated classification of local earthquake data in the BUG small array , 1995 .

[9]  R. Geller,et al.  Four similar earthquakes in central California , 1980 .

[10]  Hansruedi Maurer,et al.  Microearthquake cluster detection based on waveform similarities, with an application to the western Swiss Alps , 1995 .

[11]  Stephen D. Malone,et al.  High precision relative locations of earthquakes at Mount St. Helens, Washington , 1987 .

[12]  Fred W. Klein,et al.  Deep fault plane geometry inferred from multiplet relative relocation beneath the south flank of Kilauea , 1994 .

[13]  A. Lomax,et al.  The effect of velocity structure errors on double‐difference earthquake location , 2004 .

[14]  A. Frankel.  Precursors to a magnitude 4.8 earthquake in the Virgin Islands: Spatial clustering of small earthquakes, anomalous focal mechanisms, and earthquake doublets , 1982 .

[15]  Marco Cattaneo,et al.  Seismic multiplets analysis and its implication in seismotectonics , 1995 .

[16]  Masaru Tsujiura,et al.  Characteristic frequencies for earthquake families and their tectonic implications: Evidence from earthquake swarms in the kanto district, Japan , 1983 .

[17]  H. Kanamori,et al.  OF THE 1979 IMPERIAL VALLEY, CALIFORNIA, EARTHQUAKE: EVIDENCE FOR FAULT HETEROGENEITY? , 1982 .

[18]  P. Augliera,et al.  A Waveform Similarity Approach to Investigate Seismicity Patterns , 1999 .

[19]  David Oppenheimer,et al.  FPFIT, FPPLOT and FPPAGE; Fortran computer programs for calculating and displaying earthquake fault-plane solutions , 1985 .

[20]  Marco Cattaneo,et al.  Reconstruction of seismogenetic structures by multiplet analysis: An example of Western Liguria, Italy , 1997, Bulletin of the Seismological Society of America.

[21]  L. Marini,et al.  Fluid geochemistry of the Acqui Terme-Visone geothermal area (Piemonte, Italy) , 2000 .

[22]  Dino Bindi,et al.  An ML Scale in Northwestern Italy , 2002 .

[23]  Felix Waldhauser,et al.  hypoDD -- A Program to Compute Double-Difference Hypocenter Locations , 2001 .

[24]  J. Haase,et al.  Constraints on temporal variations in velocity near Anza, California, from analysis of similar event pairs , 1995, Bulletin of the Seismological Society of America.

[25]  CLUSTERING STRUCTURE AND FRACTURE PROCESS OF MICROEARTHQUAKE SEQUENCES , 1987 .

[26]  K. Aki.  Characterization of barriers on an earthquake fault , 1979 .

[27]  F. Waldhauser,et al.  A Double-Difference Earthquake Location Algorithm: Method and Application to the Northern Hayward Fault, California , 2000 .

[28]  J. C. Pechmann,et al.  Waveform analysis of two preshock-main shock-aftershock sequences in Utah , 1990 .

[29]  Luca Malagnini,et al.  Ground-Motion Scaling in the Western Alps , 2006 .

[30]  William H. Press,et al.  Numerical Recipes in FORTRAN - The Art of Scientific Computing, 2nd Edition , 1987 .

[31]  J. Lahr.  HYPOELLIPSE; a computer program for determining local earthquake hypocentral parameters, magnitude, and first-motion pattern , 1979 .