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

Started: Dec. 4, 2018, 8:29 a.m.
Last activity: Dec. 4, 2018, 8:29 a.m.
David Fee
Dec. 4, 2018, 8:29 a.m.
Dear IRIS Special Events,
Did you receive this email and contribution? It does not appear to be on
the special event page.

Thanks

David

--
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>tudelft.nl) wrote:

I make this contribution on behalf of:

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

(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 (
https://github.com/jdassink/beamforming) 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.

Page built 19:51:11 | v.2ac74663