The Hellenic Seismological Network of Crete (HSNC): Monitoring results and the new strong motion network

Transcription

1 The Hellenic Seismological Network of Crete (HSNC): Monitoring results and the new strong motion network Georgios Chatzopoulos 1, Ilias Papadopoulos 1 and Filippos Vallianatos 1 1 Technological Educational Institute of Crete, School of Applied Sciences, Laboratory of Geophysics & Seismology gechat@chania.teicrete.gr Keywords: Seismic network, seismic acceleration, strong motion network. Abstract The Crete Island is located in the front of the Hellenic Arc, where the subduction of African plate under the Eurasian takes place resulting in a very seismic active environment. The constant need for monitoring such a complex geotectonic setting is the main reason for establishing the Hellenic Seismological Network of Crete. Since the HSNC started its operation, a large amount of data has been recorded in the vicinity of the front Hellenic Arc. Aside from the continuous monitoring of seismic activity, HSNC is also interested in the seismic hazard in the area. An area of great interest is the Western Crete, where the city of Chania resides, built in a sedimentary basin with different geological formations (Mountrakis et al, 2012). A strong motion network is established to monitor and measure the Peak Ground Velocity (PGV) and Acceleration (PGA) of the different geological formations. The monitoring results after a shock occurred are published in G.I.S. maps which are available for public, authorities and scientists in SAFER CHANIA webpage. Introduction The Hellenic arc is created by the subduction of the Mediterranean ocean plate under the Aegean microplate (McKenzie, 1972). The convergence of these plates has a velocity rate about 4cm/y which makes the vicinity of Crete a very seismic active environment (Pichon and Angelier, 1979). It is estimated that more than 60% of the Europe s total amount of seismic energy is being released to South Aegean area (Papazachos, 1990). The monitoring of HSNC started working in 2004 with few short period seismometers and nowadays it has 13 broadband seismometers. The stations are equipped with Reftek digitizers ( recording data at 100 Hz sps coupled with Guralp CMG ESPC 60 and 120 sec sensors s. The HSNC collaborates with three neighboring seismic networks and exchanges data in real time. These networks are the GEIN-NOA (Athens), AUTH (Thessaloniki) and KOERI (Istanbul) : (Fig. 1). The seismic signals of more than 30 stations are checked by three Earthquake Monitoring Systems (SeisComp3, Earthworm and SNDA). Since 2011, near the network operation area, HSNC has recorded 8 large earthquakes M 6.0. One of these earthquakes with magnitude M=6.4, occurred in 12/10/2013 shook the west part of Crete. This event gave origin to the idea of installing a strong motion seismic network (Fig. 2). We installed seven Reftek 130-SM instruments in different geological formations in the general area of Chania basin. An automated system calculates ground shaking values (Peak Ground Velocity and Acceleration) and creates GIS maps using prior information regarding the geological formations, such as the shear velocity of surface layers and topography. Interpolation of data values fills the gaps between stations.

2 The Hellenic Seismological Network of Crete (HSNC): Monitoring results and the new strong motion network Figure 1. The HSNC and neighbor networks Figure 2. The SAFER Chania network. Geology of the general area is after Mountrakis et al. (2012)

3 Monitoring results The HSNC The Hellenic Seismological Network of Crete (HSNC) is a continuous expanding local seismological network with main purpose to monitor the seismicity in the front of Hellenic Arc. The last 5 years the complex regime of Crete region provided 8 large earthquakes (M 6.0) and an enormous amount of events with M 5.0. The network provided detailed results for the large earthquakes and their aftershock sequences. A good example of HSNC analyses are the two large events occurred in 2013 in southern (15/06) and western (12/10) part of Crete region. Another example is the 16/04/2015 event in eastern area of Crete. The location of main shocks and their aftershocks are products of Hypoinverse relocation software (Klein, 2002). Figure 3. The 12/10/2013 Mainshock and its aftershocks The SAFER CHANIA strong motion network The SAFER Chania strong motion network focuses to monitor the city of Chania and its Southern basin - a cultural monument in Mediterranean area - and provide all the useful information to Civil Protection authorities. The ground shaking recording in terms of pick ground acceleration and velocity (PGA and PGV) is calculated for every seismic event recorded by sensors. The response spectrum of the sensors is also calculated and all these information are illustrated at the SAFER Chania website. In 24/06/2015 a small earthquake is recorded from HSNC s network West of Chania city, with Magnitude ML=2.3 (Fig. 4). The distance from the city is ~50 Km. The SAFER Chania strong motion network recorded the event. After processing of data, the Peak Ground Acceleration is calculated (PGA) and placed on the map below. Interpolation is done by using an Inverse Distance Weighting algorithm.

4 The Hellenic Seismological Network of Crete (HSNC): Monitoring results and the new strong motion network Figure 4. A small earthquake example Conclusions The HSNC operation covers the monitoring need of the Hellenic Arc front. It has a dense network of sensors and in the future new stations will be installed to strategic places. The new strong motion network called SAFER CHANIA provides some good results but it is still under development. The plan is to expand the strong motion network by installing a few more sensors to have better resolution of ground shaking. Acknowledgements This work was implemented through the project IMPACT-ARC in the framework of action ARCHIMEDES III Support of Research Teams at TEI of Crete (MIS380353) of the Operational Program "Education and Lifelong Learning" and is co-financed by the European Union (European Social Fund) and Greek national funds. References Klein F.W., 2002 User's Guide to HYPOINVERSE-2000, a Fortran program to solve for earthquake locations and magnitudes, Open File Report , u.s. Geological Survey. Le Pichon X., and Angelier J., 1979, The Hellenic Are and Trench System: a key to the neotectonic evolution of the eastern Mediterranean area. Tectonophysics, 60, pp Mountrakis D., Killias A., Pavlaki A., Fassoulas C., Thomaidou E., Papazachos C., Papaioannou C., Roumeliozi Z., Benetatos C. and Vamvakaris D., 2012, Neotectonic study of

5 Western Crete and implications for seismic hazard assessment, Journal of the Virtual Explorer, 42. McKenzie D., 1972, Active Tectonics of the Mediterranean Region. The Geophysical Journal of the Royal Astronomical Society 30: Papazachos B.C., 1990, Seismicity of the Aegean and surrounding area, Tectonophysics, 178,