Anomalies of Recordings of Spitsbergen Earthquakes A.V. Fedorov, V.E. Asming, S.V. Baranov Kola Branch of Geophysical Survey of RAS

Tectono-geological conditions Spitsbergen archipelago is an extremely heterogeneous object which has a very complicated geological structure and composed of various sedimentary, metamorphic and intrusive rocks of different ages. Large number of faults cleaves this area in sub-meridional and sub-latitudinal directions The complicated geology and large number of different sources of seismicity caused a variety of wave forms of events recordings.

Outline 1.Earthquakes with multiple P and S onsets Reasons of unusual recordings Its influence on location 2.Low-Frequency seismic events Sources location Time distribution Possible nature

Map of the region Knipovich ridge SFZ Mohn ridge Storfjord SPIBRB KBS HSPB

Example of an earthquake with multiple P wave onsets We suppose that the same multiple S onsets are masked by P coda

Comparison of 4 North-East Land earthquakes recorded by KBS Tp2-Tp1 are near the same. Thus, the weak first P are due to the medium properties, not properties of the sources

Location of the event by first visible arrivals is inconsistent Lines P-S, P-P do not cross at a point, st.deviation of T0 is large

Location by recovered S onsets times seems to be more consistent TS recovered=TS2-1.7*(TP2-TP)

Location by strongest arrivals assumed to be Pg, Sg Origin times and coordinates are close to previous ones

Places where events with anomalous recordings were observed Anomaly at SPI Anomaly at KBSNo anomaly

Differences in events location due to the anomalies NORSAR RRB NORSAR GBF KB GS RAS

Conclusions The weak first P onsets are due to the medium properties on the path of propagation Almost all events with anomalies occurred in intraplate area Weak first P onsets can result in large uncertainty of events location

Low-frequency events Features: Almost all spectral amplitude of the events lies in a narrow frequency band Hz P and S-waves have almost the same spectra Magnitudes of these events do not exceed 0.5 P S

Low-Frequency Events

Detection scheme 1.SNR detector in a target frequency band (2-5 Hz) with some threshold 2.Filtrating in higher frequency band (5-8 Hz) 3.Comparison of average amplitudes. If the low band amplitude is 1.5 times greater than the high band one then the data fragment is saved 4.Manual inspection of results. In order to exclude not target events

Low-Frequency events preliminary location Location based only on SPI array data for 2011

Question What is the nature of this events?

LF events zones and glaciers Olav Liestol: Glaciers of Svalbard,Norway // U.S. Geological Survey Professional Paper 1386-E-5 Kronebreen Monacobreen Broggerbreen Lovenbreen Hinlopenbreen Hochstetterbreen Sotrbreen Hornsbreen Svalisbreen

Time distribution All Events North SouthEast

Conclusions There are at least 3 zones of low- frequency events activity. Preliminary location shows that almost all epicenters hit the land The active zones are in agreement with areas of large Svalbard glaciers Maximal number of low-frequency events occurred in autumn-winter period

Thank you for attention!

Spectral content of events at different distances ~30 km ~18 km ~136 km ~132 km HSPB KBS SPI SPI Dist ~ 30 km Dist ~ 18 km Dist ~ 132 km Dist ~ 136 km

Similar P- and S-wave spectra HSPB SPI KBS HSPB SPI KBS P-wave spectraS-wave spectra