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Seismology I GEOL0029: HW1
you wrote for your homework.
Note the plots produced in SAC can be saved to image file (i.e., pdf, png)
using the SAC command (saveimg filename.format).
1. Typical very broadband (VBB) sensor has a response I(ω) = X(ω)−iωY (ω) =
Gω2
ω2−2hiω0ω−ω20
. This response links sensor displacement (X(ω)) to ground veloc-
ity (−iωY (ω)).
(a). Discuss the asymptotic behavior of the amplitude response |I(ω| varies
against angular frequency ω, or |I(ω| ∼ ωα. Discuss α at low frequency
(ω ω0) and high frequency (ω ω0), respectively. [5%].
(b) write down the response I(ω) that links sensor velocity to ground veloc-
ity and repeat (a). [5%].
(c) write down the response I(ω) that links sensor acceleration to ground
velocity and repeat (a). [5%].
2. Typical mechanical sensor has a response I(ω) = X(ω)Y (ω) =
Gω2
ω20−2hiω0ω−ω2
,
where f0 = 1.0 Hz, h=0.707, G=1. This response links sensor displacement
X(ω) to ground displacement Y (ω) (note ω = 2pif).
(a) plot the amplitude (log-log plot) and phase response (semilogx plot)
against frequency f [in Hz]. [10%]
(b) calculate and plot the locations of zeros and poles of the instrument
response I(ω). Change f0 = 0.0083 Hz, and repeat the task. Use the graphic
method in the complex s domain (s = σ + iω) and quantify the difference in
their amplitude response at f = 0.1 Hz in dB [10%]
(c) download seismogram data file (example.conv.SAC) from Moodle page.
Implement the time-domain recursive filter method to remove the instrument
response. Plot the original seismogram and seismogram with instrument re-
sponse I(ω) removed. Briefly discuss their difference (Note you can download
and use the subroutine rsac.m to read the data file). [15%]
(USAGE : data = rsac(’example.conv.SAC’); time = data(:,1); amp = data(:,2);).
3. Following the handout and demonstration in the practical session, with
FetchData and FetchSyn scripts, you will download and analyze 30 mins data/synthetics
1
recorded at the station ANMO (network IU, channel BHZ) from the 2020 North
Chile earthquake (click here: Link to North Chile event info) (Note You will
need the event origin time and eventID to download the data/synthetics).
(a) Compare Data and Synthetics with SAC: First comment on the no. of
zeros and poles in the polezero files (be careful that you may have more than
one sensor in the same station). perform data processing to obtain displace-
ment; bandpass filter the data and synthetics at 0.006-0.2 Hz. Plot the vertical
displacement data and synthetic waveforms; observe and briefly discuss their
difference (focusing on the time window within 3 minutes of the first P wave).
[10%]
(b) Repeat (a), but filter the data at 0.006-0.025 Hz. [10%]
(Hint: to filter the data or synthetics, you can use the sac command with the
syntax: bp co 0.006 0.2 p options. Here the option co defines the frequency
corners of the filter and the option p is used to specify either one-way causal
filter (i.e., p 1) or two-way acasual filter (i.e., p 2).
4 Compare seismic background noise on Earth and Mars
Here you may use FetchData to download 1-hour long recording at the station
ANMO (network IU, channel BHZ) at 14:00:00 - 15:00:00, 2021-02-06. Perform
the data processing to obtain the acceleration, you can use the SAC command
fft to compute acceleration amplitude spectra in the frequency domain. To plot
the amplitude spectra, you can use SAC command psp am. Perform the same
task against Mar seismic station ELYSE (network XB, channel BHV).
(a) Plot and compare the instrument response (amplitude and phase re-
sponse) between the station ANMO (Earth) and ELYSE (Mars). [10%]
(b) Plot and compare the acceleration amplitude spectra of ANMO and EL-
YSE between 0.003 Hz and 5 Hz. [10%]
(c) Following (b), discuss their differences and possible noise sources. Try to
be specific about the noise source against the spectra feature. [10%]