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Seismology I GEOL0029
Fetch Data/Synthetics
Task 1. Download Seismic Data and synthetics with IRIS FetchData and FetchSyn
service
(Note: command-line inputs in aristotle are shown with italic font).
(Note: the term synthetics is defined as simulated data from a given Earth model, either 1-D,
spherically symmetric or 3-D model. In this module, we are dealing with synthetics from 1-D
model)
Before we start,
(1) Login to Aristotle, Copy scripts FetchData, FetchSyn and mseed2sac in my directory.
(2) load the modules in aristotle:
module load gcc-libs/4.9.2
module load perl/5.22.0
module load python3/recommended
Without loading these modules, you will not be able to run the following commands.
Now we can download data/synthetics from IRIS Fetch data/syn service:
(a) Follow IRIS Fetch Data usage:Link to IRIS Fetch Data. To run the command in aristotle,
use perl FetchData . Make sure use option -m to obtain metadata, option -sd to save polezero
files in the designated directory.
Example: Download 1 hour vertical BHZ component data from the network IU, station MAJO.
perl FetchData -N IU -S MAJO -C BHZ -s 2018-09-28,10:02:43 -e 2018-09-28,11:02:43 -o
test.mseed -m test.metadata -sd testpole (note you must create directory testpole first).
once data are downloaded, use ./mseed2sac to convert the data to SAC format. Make sure to
use option -m to include metadata file in the data format conversion.
Example: ./mseed2sac test.mseed -m test.metadata
(b) From IRIS Fetch synthetics usage:Link to IRIS FetchSyn. To run the command in aristotle,
use python FetchSyn (options). To see available options, use python FetchSyn -u.
Example: Download Z,R,T components velocity synthetics from the network IU, station MAJO
using CMT earthquake solution and eventID. Save the archive to the file test.zip. Synthetics are
computed with 1D iasp91 model and CMT solution (accurate down to 2 sec period). Note there
are multiples option for synthetics (synthetics model options).
python FetchSyn -N IU -S MAJO -C ZRT -model iasp91 2s -units v -outfile test -evid
GCMT:M201809281002A
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Task 2. Seismic Analysis Code (SAC): data visualization and processing
SAC is one of the most commonly used software in global seismology. In this module, we will
make use of the tools available in SAC and perform visualization and data processing. The
online tutorial can be referred here (click here: Link to sac overview and tutorial). Before the
practical session, make sure you check this overview.
To use SAC in Aristotle, we will have to load the software with the following commands:
module load gcc-libs/4.9.2
module load xorg-utils/X11R7.7
module load curl/7.47.1/gnu-4.9.2
module load matlab/full/r2019b/9.7
module load sac/102.0
You can then access SAC by typing sac. The usage of individual command can then be retrieved
by typing help command (e.g., help plot1 ).
Following Task 1, we aim to visualize the downloaded data and synthetics and perform basic
processing. As discussed in the lecture, the downloaded data represent the sensor motion and
therefore it is necessary to remove the instrument response and recover the ground motion. Here
we will use the SAC command transfer to remove the instrument response.
The following example illustrates how you may read the downloaded data (or synthetics) into
the memory in SAC and perform processing and basic visualization (see video here).
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FURTHER TASK:
I. Using FetchData and FetchSyn scripts, you will download and analyze 1-hour data/synthetics
recorded at the station GSC (network CI) from the 2019 Peru earthquake (click here: Link
to Peru event info) (Note You will need the event origin time and eventID to download the
data/synthetics).
II. Compare Data and Synthetics: perform data processing to obtain displacement data. Low-
pass filter the data and synthetics at 0.05 Hz. Plot and overlay the vertical displacement data
and synthetic waveforms (sac command plot2 )