Analysis of the 29th May 2008 Ölfus earthquake and aftershock sequence using three-component t processing on ICEARRAY

Transcription

1 Analysis of the 29th May 2008 Ölfus earthquake and aftershock sequence using three-component t processing on ICEARRAY Benedikt Halldórsson Steven J. Gibbons International Symposium on Strong-motion Earthquake Effects University of Iceland 29 May 2009

2 NORSAR -a bi briefhistory Original NORSAR array built by the United States and Norway Aim: to detect and locate Underground Nuclear Tests 132 sensors in 22 sub-arrays with an array aperture over 100 km. In 1976, the array was reduced to 7 sub-arrays (42 instruments).

3 Beamforming is used in detection seismology to detect weaker signals than would be possible using only a single instrument. Traces are delayed and summed according to theoretical arrival times. Noise is suppressed and signal-to-noise noise ratio (SNR) improved. The signal shown was observed on the NORSAR array in the early hours of Monday morning (May 25, 2009)

4 Beamforming can be performed over a grid of parameter space to indicate the direction an incoming wavefront arrives from. For teleseismic P, this amounts to a projection of (part of) the Earth's surface onto a slowness grid. Slowness analysis for this signal indicates that the origin was likely to be South- Eastern China or the northernmost Korean peninsula. The resolution for any given The resolution for any given phase, at any frequency is determined by the geometry of the array and the nature of the underlying rock.

5 Nordic seismic i stations ti of the IMS The need to detect and locate seismic events of lower magnitudes meant that large aperture teleseismic arrays were no longer sufficient. A global network of 3-C stations and small aperture arrays was necessary to detect and locate events at regional distances.

6 Why 3-component array processing? At local and regional distances, detection and identification of secondary phases is essential for event location. Teleseismic P usually results in the largest amplitudes on the vertical components - often not the case with regional (especially S) phases.

7 Using the 3-C array to distinguish (confidently!) between local Pg and regional Sn. Novaya Zemlya Sn f-k (vertical) Unknown signal f-k (vertical) Signal to the left is of vital importance in explosion monitoring. Signal to the right is completely uninteresting... but they look similar to the automatic processing.

8 Using the 3-C array to distinguish (confidently!) between local Pg and regional Sn. Novaya Zemlya Sn f-k (horizontal) Unknown signal f-k (horizontal) Using the horizontal components in the slowness analysis easily identifies the phase on Using the horizontal components in the slowness analysis easily identifies the phase on the left as an S-phase... and associates it with a P phase from a similar direction to generate an automatic event location near Novaya Zemlya.

9 Using the 3-C array to distinguish (confidently!) between local Pg and regional Sn. Novaya Zemlya Sn f-k (horizontal) Unknown signal f-k (horizontal) The signal on the right is identified as a local P phase.

10 What about processing at event shorter distances? ICEARRAY has a similar aperture to the regional arrays operated by NORSAR - we can anticipate a similar coherence between sensors. However, the epicentral distances to the events of interest are very different.

11 Vespagram analysis is a way of measuring the direction of arrival of a wavefront with time. The plot shows the ICEARRAY recordings of one of the first aftershocks (16:05.35) after the Ölfus main event. It is clear that the event was to the North of the array (backazimuth ~30 o ) because of very coherent P-energy from this direction. The vespagram is calculated with fixed P-velocity on the vertical seismograms.

12 We can then rotate all of the seismograms by the same backazimuth (30 degrees) and perform VESPA analysis to identify coherent energy between sensors in the transverse direction. This should give us a good estimate for the S-phase arrival... and a high level of confidence that the phase we observe is in fact an S-phase from the right direction. This is a huge advantage of seismic arrays! Note the transience of the Note the transience of the coherent energy!!

13 Vertical components (above)... Horizontal components (below)... Coherence as a function of time and backazimuth (left)... Waveforms - station IS605 (right). In principle, we can locate the event with this information alone.

14 What are our goals with array-processing on ICEARRAY? One primary goal is to map out as accurately as possible the distribution of seismicity following the main event. The locations of events will provide probably our best delineation of fault lines. In addition to traditional array processing we can also exploit almost repeating seismic i events to obtain highly hl accurate relative travel times and perform double-difference location.

15 Repeating seismicity The similarity of the waveforms constrains the events to be VERY close to each other.

16 Repeating seismicity Event 2 is measurably further away from our sensor than Event 1... Note how much more accurately the differences between arrival times can be read, than the arrival times themselves...

17 What are our goals with array-processing on ICEARRAY? One primary goal is to map out as accurately as possible the distribution of seismicity following the main event. The locations of events will provide probably our best delineation of fault lines. Another goal is to examine as best we can the rupture pattern from the 29th May 2008 main event.

18 The event to the right is believed to be located quite close to the start of the rupture for the main event. The vespagrams are quite difficult to interpret - the ground motion of the secondary wavefield is large and energy appears to arrive from a broad range of angles.

19 One problem encountered by traditional f-k analysis is that it can depend highly upon the frequency content of the signal... and be heavily influenced by the secondary wavefield. Maybe narrow-band f-k analysis using multitaper bi-spectral estimation and Principal Maybe narrow band f k analysis using multitaper bi spectral estimation and Principal Component Analysis is an answer??

20 One problem encountered by traditional f-k analysis is that it can depend highly upon the frequency content of the signal... and be heavily influenced by the secondary wavefield. Maybe narrow-band f-k analysis using multitaper bi-spectral estimation and Principal Maybe narrow band f k analysis using multitaper bi spectral estimation and Principal Component Analysis is an answer??

21 Multitaper narrow-band f-k analysis? The f-k plots in the previous slides use only a single dominant eigenvector of the spectral covariance matrix. The dominant eigenvector often corresponds to an incoming wavefront - the remaining energy in the observed wavefield is then distributed over the other eigenvectors. Using this technique we may observe patterns which evolve with time under the main event which resemble patterns observed for smaller and better constrained events... and allow us to understand better the spatial and temporal evolution of the wavefield for the 15:45 29th May 2008 event.

22 Summary ICEARRAY has produced superb recordings of the 15:45 main shock and an extensive aftershock sequence. Traditional array processing methods - especially when utilizing the horizontal components - provide excellent constraints upon the location of aftershocks. Waveform correlation, cluster analysis, and double-difference location techniques ("precision seismology") may help to provide excellent illumination of the fault geometries. The ground-motion for the main shock is difficult to assess due to the large wavelengths involved. Calculating narrow-band spectral covariance matrices as a function of time - both for the main shock and aftershocks - may allow us to place additional constraints upon the source mechanism for the 29th May event.