Northeastern Italy and Western Slovenia lies in the region of moderate earthquake activity. In the past, rare strong earthquakes struck the region, namely 1511 Idrija M 6.9, Friuli 1976 series of M 6+ and 1998 and 2004 M 5.6 and 5.2 Ravne fault earthquakes. Multiple geodetic, geomorphologic and geologic studies show, that IFS is an active fault system, with slip rates up to 3.8 mm/yr over all the faults. Since 2006 area of IFS is covered by relatively dense network of broadband stations recording in continuous mode. To better understand the IFS itself, a detailed seismologic study was performed in this study. To gain the knowledge of fault system geometry, physics of faulting and temporal behavior, the data of all the stations in the region was analyzed. First, time series were analyzed using classic STA/LTA detection algorithm for the detection of earthquakes. After the initial scan of the database, arrival times of phases of detected earthquakes were manually repicked to obtain as precise arrival times of phases as possible. The magnitude of completeness for the catalog obtained in such way was 0.9. To obtain lower magnitude of completeness, additional manual inspection of waveforms was performed. New micro earthquakes were added to the existing database. For the detection of even lower magnitude earthquakes, all the earthquakes of magnitudes above 0.8 were selected as a template earthquakes. Matched filter detection of earthquakes was performed on the selected stations with simultaneous picking of precise phases as obtained from cross correlation around the P or S arrival times between template earthquake and the detected earthquake. The newly obtained earthquakes were added to the database lowering the magnitude of completeness to -0.7. Manually relocated earthquakes and earthquakes obtained from matched filter detection clearly illuminated active faults of IFS, showing activity along all the faults of the system. From cross sections perpendicular to the IFS clear geometry was defined for the central part of the system. Cross section showed that earthquakes along Ravne fault are much more shallow than on other faults of the system and illuminate a SW dipping fault which probably connects to Idrija fault at depth. Idrija fault was only active in its northern extend in the analyzed period, with deeper earthquakes illuminating vertical fault plane. Predjama and Selce fault were mostly active in the central and southern part of the system. Earthquakes along these two faults show vertically dipping faults. Finally, earthquake activity along Raša fault is mostly present in its central and southern extend and show a fault plane dipping towards NE. High resolution detection of the earthquakes also gave a better understanding of temporal earthquake evolution along the IFS. Temporal clustering was observed in the years 2009-2010 and in the late 2017. In 2009-2010 majority of the system shows high activity, with swarms taking place in northern and central part of the system and multiple mainshock-aftershock series in southern part. Similar, in 2017 multiple swarms and mainshock series happened in the southern part of the system. Combining the detected temporal clustering with the datasets of extensometer mounted on Predjama fault we observed that in the same time period as the 2009-2010 elevated activity happened, also a surface deformation was present. We modeled the surface deformation and earthquake activity using Coulomb stress changes. Best fit was given to the changes of normal stresses along the Idrija fault, which led to the unclamping of the faults parallel to Idrija fault, increasing the earthquake rates along these faults and causing the surface deformations due to the extraction and migration of fluids at depth.

High resolution earthquake relocation along Idrija fault system in Western Slovenia and its application in understanding the deformation along the active faults of NW External Dinarides

VIČIČ, BLAŽ
2018-03-23

Abstract

Northeastern Italy and Western Slovenia lies in the region of moderate earthquake activity. In the past, rare strong earthquakes struck the region, namely 1511 Idrija M 6.9, Friuli 1976 series of M 6+ and 1998 and 2004 M 5.6 and 5.2 Ravne fault earthquakes. Multiple geodetic, geomorphologic and geologic studies show, that IFS is an active fault system, with slip rates up to 3.8 mm/yr over all the faults. Since 2006 area of IFS is covered by relatively dense network of broadband stations recording in continuous mode. To better understand the IFS itself, a detailed seismologic study was performed in this study. To gain the knowledge of fault system geometry, physics of faulting and temporal behavior, the data of all the stations in the region was analyzed. First, time series were analyzed using classic STA/LTA detection algorithm for the detection of earthquakes. After the initial scan of the database, arrival times of phases of detected earthquakes were manually repicked to obtain as precise arrival times of phases as possible. The magnitude of completeness for the catalog obtained in such way was 0.9. To obtain lower magnitude of completeness, additional manual inspection of waveforms was performed. New micro earthquakes were added to the existing database. For the detection of even lower magnitude earthquakes, all the earthquakes of magnitudes above 0.8 were selected as a template earthquakes. Matched filter detection of earthquakes was performed on the selected stations with simultaneous picking of precise phases as obtained from cross correlation around the P or S arrival times between template earthquake and the detected earthquake. The newly obtained earthquakes were added to the database lowering the magnitude of completeness to -0.7. Manually relocated earthquakes and earthquakes obtained from matched filter detection clearly illuminated active faults of IFS, showing activity along all the faults of the system. From cross sections perpendicular to the IFS clear geometry was defined for the central part of the system. Cross section showed that earthquakes along Ravne fault are much more shallow than on other faults of the system and illuminate a SW dipping fault which probably connects to Idrija fault at depth. Idrija fault was only active in its northern extend in the analyzed period, with deeper earthquakes illuminating vertical fault plane. Predjama and Selce fault were mostly active in the central and southern part of the system. Earthquakes along these two faults show vertically dipping faults. Finally, earthquake activity along Raša fault is mostly present in its central and southern extend and show a fault plane dipping towards NE. High resolution detection of the earthquakes also gave a better understanding of temporal earthquake evolution along the IFS. Temporal clustering was observed in the years 2009-2010 and in the late 2017. In 2009-2010 majority of the system shows high activity, with swarms taking place in northern and central part of the system and multiple mainshock-aftershock series in southern part. Similar, in 2017 multiple swarms and mainshock series happened in the southern part of the system. Combining the detected temporal clustering with the datasets of extensometer mounted on Predjama fault we observed that in the same time period as the 2009-2010 elevated activity happened, also a surface deformation was present. We modeled the surface deformation and earthquake activity using Coulomb stress changes. Best fit was given to the changes of normal stresses along the Idrija fault, which led to the unclamping of the faults parallel to Idrija fault, increasing the earthquake rates along these faults and causing the surface deformations due to the extraction and migration of fluids at depth.
COSTA, GIOVANNI
30
2016/2017
Settore GEO/10 - Geofisica della Terra Solida
Università degli Studi di Trieste
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11368/2922574
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