Earthquake activity in the Rana region recorded by a local seismic network and a seismic array

1. Earthquake activity in the Rana region recorded by a local seismic network and a seismic array Mathilde B. Sørensen, Steven Gibbons(*) and Jens Havskov * NORSAR Nordic Detection Seminar, August 2006

2. Earthquakes 1980-2004

3. Historical seismicity and structure Regional seismicy up to 1998 Uplift gradient is indicated by the red color 1819 event within square Figure from Hicks et al. (2000)

4. NORSAR Rana network Earthquakes located by NORSAR network August 1997 to january 1999. Triangles are temporary stations and MOR8 is the permanet Norwegian National Seismic Network station. STOK was installed in 2003 and STOK1 and 2 in July 2005. Figure modfied from Hicks et al. (2000)

5. STOK2 seismic station

6. Technical setup Sensors are 4.5 Hz geophones 24 bit digitizer Recording under Windows Seislog Data mailed to Bergen every few months using a memory stick

7. Local event within network

8. 2005 seismicity Seismicy recorded by the the local network in the second half of 2005, a total of 750 events.Most are from the second half of 2005.

9. Summary of seismicity STOK2 swarm area Seismicity at 5 km depth, right over the station Magnitudes from –1.0 2-10 events detected per day NORSAR network 180 events/year New network 1200 - 1500 events per year

10. Improved location Normal location Double difference location

11. Fault plane solutions Horizontal stresses from earthquake focal mechanisms and overcoring measurements. Figure from Hicks et al. (2000)

12. cience University of Bergen Detection by correlation • Select master event • Cross correlate traces of master event at each array station with all data, a high correlation indicates a similar event • Sum correlation function from each array element • Use sum of correlations (correlation beam) to detect event Detection beam using 2 differnt window lengths of the master event templates. Detection level is about Ml = 0.5

13. Department of Earth Science University of Bergen Rana events, 2005 All events Ml > 2 recorded conventionally by NORSAR arrays in 2005. Signals are bandpass-filtered between 2 and 5 Hz.

14. Department of Earth Science University of Bergen Epicenter of the 24 June, 2005 Ml=3.5 event

15. Detection by correlation method 32 events detected 2005 1 false 1 master 2 of m>2 events did not correlate A total of 28 new events, all confirmed by the local network

16. Bergen STOK recordings Seismograms at STOK for events detected by correlation. Left:P, Right: Complete seismogram

17. STOK1 recordings Seismograms at STOK1 for events detected by correlation

18. STOK2 recordings Seismograms at STOK2 for events detected by correlation

19. Department of Earth Science University of Bergen Location of main event White circle: Array location Star: Location with local stations Location by array: In progress

20. Summary Rana region one of the most active in Noway Local network potentiall provide –1 detection threshold Very shallow seismicity Swarm activity Clusters of events in the Rana area seem to locate within a small volume Abnormal local stress conditions DD location shows promising results, to be improved when further data are available in the future Local network provide unique calibration data Array detection threshold reduced significantly by using cross correlation of array data, potentially down to 0.5 Cross correlation provides a good suplement to STA/LTA trigger systems