Thread: Re: Special Event: North of Anchorage, Alaska

Started: 2018-12-04 16:29:33
Last activity: 2018-12-04 16:29:33
David Fee
2018-12-04 16:29:33
Dear IRIS Special Events,
Did you receive this email and contribution? It does not appear to be on
the special event page.



David Fee
Associate Professor
Wilson Alaska Technical Center
Alaska Volcano Observatory
Geophysical Institute - UAF
2156 Koyukuk Drive - Rm 506G
Fairbanks AK 99775-7320
cell: 907.347.8599 <//907.347.8599>
office: 907.474.7564 <//907.474.7564>

On November 30, 2018 at 5:10:28 PM, Shahar Shani-Kadmiel - CITG (
s.shanikadmiel<at> wrote:

I make this contribution on behalf of:

Shahar Shani-Kadmiel(1,2), Jelle Assink(2), David Fee(3), and Läslo

(1) Department of Geoscience and Engineering, Faculty of Civil Engineering
and Geosciences, Delft University of Technology.

(2) R&D Department of Seismology and Acoustics, Royal Netherlands
Meteorological Institute (KNMI).

(3) Wilson Alaska Technical Center and Alaska Volcano Observatory,
University of Alaska Fairbanks, Fairbanks, Alaska, USA.

Attached are array-processing (Figure 1) and back-projection (Figure 2)
results using data from International Monitoring System (IMS) array
IM.I53US near Fairbanks, Alaska approximately 390 km away from the
epicenter. This is an array of 8 microbarometers that are sensitive to
mechanical vibrations as well as pressure fluctuations so both seismic and
acoustic arrivals are detected.

The raw waveforms are bandpass filtered between 0.3 and 5 Hz and the
instrument response is removed. A time-domain Fisher detector ( is used to beamform the data.
Array processing results show seismic detections with celerity (average
propagation velocity calculated as epicentral distance divided by travel
time) range of 6 to 2 km/s and similar apparent velocity (horizontal
propagation velocity over the array) range. Between 800 seconds and 1100
seconds after origin time, a coherent (high SNR) acoustic signal (apparent
velocity between 340 and 380 m/s) traverses the array with a stable
back-azimuth of ~212°. This signal has a peak-to-peak (pop) amplitude of
1.7 Pa. Between 1100 seconds and 1600 seconds the back-azimuth points to
~180° and the coherency is lower. Between 1600 seconds and 2500 seconds the
back-azimuth points to ~208°.

The travel time and back-azimuth information from beamforming is used to
back project the detections on a 0.05° by 0.05° grid and illuminate source
patches that radiate infrasound. A constant seismic propagation velocity of
4 km/s is assumed and is in agreement with the celerity value associated
with the arrival of the peak amplitude of the seismic wavetrain. Acoustic
propagation velocity is around 0.33 km/s and is corrected to account for
along- and cross-track wind. The coherent signal between 800 and 1100
seconds traces the Alaska Range north-west of the epicenter. The less
coherent signal between 1100 and 1600 seconds illuminates the steep
topography of the peninsula south-south-east of the epicenter. Finally, the
signal between 1600 and 2500 seconds illuminates the south-western flanks
of the Alaska Range.

Figures and figure captions:
Figure 1: Array-processing results. The frames show the following wavefront
parameters as a function of time: Spectrogram of the best beam, best beam,
back-azimuth, apparent velocity (log scale above 450 m/s and linear below),
Fisher ratio. Scatter points colored by SNR of the detection. Celerity is
indicated on the top frame.
Figure 2: Backprojection result overlaid on a
topography/bathymetry-shaded relief (the GEBCO_2014 30 arcsec
grid, Weatherall et al., 2015). The epicenter, is marked by a star; the
I53US array location is marked by a triangle and array configuration is
presented in the top right inset. Color coding by the number of detection
that originate in each grid cell.

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