Anomalies of Recordings of Spitsbergen Earthquakes

A.V. Fedorov, V.E. Asming,

S.V. Baranov

Kola Branch of Geophysical Survey of RAS

www.krsc.ru

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

Kn

ipovi

ch

rid

ge

Kn

ipovi

ch

rid

ge

SFZSFZ

Mohn ridgeMohn ridge

StorfjordStorfjord

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 mediumproperties, 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 KBS No 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 2.5-5 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

KronebreenMonacobreenBroggerbreenLovenbreen

HinlopenbreenHochstetterbreen

SotrbreenHornsbreenSvalisbreen

Time distribution

All Events

North South East

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 04.02.10

KBS 03.07.10

SPI 04.02.10

SPI 03.07.10

Dist ~ 30 km

Dist ~ 18 km

Dist ~ 132 km

Dist ~ 136 km

3

3

3

3

Similar P- and S-wave spectra

HSPB

SPI

KBS

HSPB

SPI

KBS

P-wave spectra S-wave spectra