Green’s functions databases for Icy Ocean Worlds

The ETH Zurich stores precomputed Green’s functions databases produced for structural background models of Icy Ocean Worlds. These databases were computed using AxiSEM (Nissen-Meyer et al., 2014), an axi-symmetric spectral element method which runs within a 2D computational domain. The synthetic seismograms for any source/event pair can be calculated using Instaseis (van Driel et al., 2015), a python library built with a fast extraction algorithm for waveforms. Instaseis makes heavy use of Obspy Python package.

To see a list of models for computing the Green’s functions databases, click on the Green’s functions databases tab below. Instructions on extracting waveforms from any of these databases are given under the How to download waveforms tab. The databases are hosted at http://instaseis.ethz.ch/icy_ocean_worlds/, and can be accessed using a database name from the table below combined with this URL. For convenience, complete connection URLs are also provided.

This work was supported by computational grants from Leibniz-Rechenzentrum Garching (grant pr63qo "3D wave propagation and rupture: forward and inverse problem"), Deutsche Forschungsgemeinschaft (DFG, grant SI1538/4-1) and the Swiss National Science Foundation (SNF-ANR project 157133 "Seismology on Mars"). This work was partially supported by strategic research and technology funds from the Jet Propulsion Laboratory, Caltech, and by the Icy Worlds node of NASA's Astrobiology Institute (13-13NAI7_2-0024).

If you use our web services, please cite the study below:

Stähler, S. C., M. P. Panning, S. D. Vance, M. van Driel, R. Lorenz, T. Nissen-Meyer, and S. Kedar, 2017. Seismic wave propagation in icy ocean worlds, submitted to J. Geoph. Res.

You might also consider citing the ObsPy, AxiSEM, and Instaseis papers:

Krischer, L., T. Megies, R. Barsch, M. Beyreuther, T. Lecocq, C. Caudron, and J. Wassermann, 2015. ObsPy: a bridge for seismology into the scientific Python ecosystem, Computational Science & Discovery, 8(1), 014003, doi: 10.1088/1749-4699/8/1/014003.

Nissen-Meyer, T., van Driel, M., Stähler, S. C., Hosseini, K., Hempel, S., Auer, L., Colombi, A., and Fournier, A., 2014. AxiSEM: broadband 3-D seismic wavefields in axisymmetric media, Solid Earth, 5, 425-445, doi:10.5194/se-5-425-2014.

van Driel, M., Krischer, L., Stähler, S. C., Hosseini, K., and Nissen-Meyer, T., 2015. Instaseis: instant global seismograms based on a broadband waveform database, Solid Earth, 6, 701-717, http://dx.doi.org/10.5194/se-6-701-2015.

Other studies we use:

Summary of the Green's functions databases:

The model names are defined as: AAABBBkm-CCpDQ-EQ_X
with AAA: The planetary body name (Eur(opa), Tit(an), ...) BBB: The thickness of the surface ice in km CC: Salt content in water in per mille. D: high or low Q model X: Additional parameters
Eur005km-00pMS-hQ_hyd30km_2s
The columns in the nd files are depth(km), Vp(km/s), Vs(km/s), density(g/cm^3), Qmu and Qkappa. The deck format includes radius(m), density(kg/m^3), Vp(m/s), Vs(m/s), Qkappa, Qmu, V1(m/s), V2(m/s), and iso/aniso columns in order. V1 and V2 are other velocities which we do not use; therefore, we duplicate the values of Vp and Vs.

Object	Ice thickness	Salt	Salt amount (in 1e-3)	Q-model	Extra features	Min. period (s)	Record length(min.)	Database name	Model data	Complete URL for database	Current status
Enceladus	53 km	MgSO4	0	high	30km hydr. mantle layer	1	~60	Enc053km-00pMS-hQ_hyd30km_1s	png, nd	http://instaseis.ethz.ch/icy_ocean_worlds/Enc053km-00pMS-hQ_hyd30km_1s	Active
Ganymede	126 km	MgSO4	0	high	30km hydr. mantle layer	2	~60	Gan126km-00pMS-hQ_hyd30km_4s	png, nd	http://instaseis.ethz.ch/icy_ocean_worlds/Gan126km-00pMS-hQ_hyd30km_4s	Active
Titan	46 km	NH3	33	high	IceVI layer removed	2	~20	Tit046km-33pNH-hQ_noiceVI_2s	png, nd	http://instaseis.ethz.ch/icy_ocean_worlds/Tit046km-33pNH-hQ_noiceVI_2s	Active
Titan	124 km	NH3	33	high		2	~60	Tit124km-33pNH-hQ_2s	png, nd	http://instaseis.ethz.ch/icy_ocean_worlds/Tit124km-33pNH-hQ_2s	Active
Europa	5 km	MgSO4	0	high	30km hydr. mantle layer	2	~60	Eur005km-00pMS-hQ_hyd30km_2s	png, nd	http://instaseis.ethz.ch/icy_ocean_worlds/Eur005km-00pMS-hQ_hyd30km_2s	Active
Europa	5 km	MgSO4	0	low	30km hydr. mantle layer	2	~60	Eur005km-00pMS-lQ_hyd30km_2s	png, nd	http://instaseis.ethz.ch/icy_ocean_worlds/Eur005km-00pMS-lQ_hyd30km_2s	Inactive
Europa	5 km	MgSO4	0	low	30km hydr. mantle layer, scattering	2	~60	Eur005km-00pMS-lQ_hyd30km_2s	png, nd	http://instaseis.ethz.ch/icy_ocean_worlds/Eur005km-00pMS-lQ_hyd30km_scatter_2s	Active
Europa	20 km	MgSO4	0	high	30km hydr. mantle layer	2	~60	Eur020km-00pMS-hQ_hyd30km_2s	png, nd	http://instaseis.ethz.ch/icy_ocean_worlds/Eur020km-00pMS-hQ_hyd30km_2s	Active
Europa	20 km	MgSO4	0	low	30km hydr. mantle layer	2	~60	Eur020km-00pMS-lQ_hyd30km_2s	png, nd	http://instaseis.ethz.ch/icy_ocean_worlds/Eur020km-00pMS-lQ_hyd30km_2s	Active
Europa	20 km	MgSO4	0	high	30km hydr. mantle layer, 10 hours long	5	~600	Eur020km-00pMS-hQ_hyd30km_5s	png, nd	http://instaseis.ethz.ch/icy_ocean_worlds/Eur020km-00pMS-hQ_hyd30km_5s	Active

Use your own Python scripts [example code]:

We recommend developing your own Python scripts for generating synthetics. If you choose to use your own scripts, simply import Instaseis and connect to one of our databases remotely. Note that our Instaseis servers compute the waveforms in displacement units by default. Most of the default parameters can be overridden in your scripts according to your needs. Consult to Instaseis webpage for more info.

			/usr/bin/python
			>>> import obspy
			>>> import instaseis
			>>> 
			>>> db = instaseis.open_db("http://instaseis.ethz.ch/marssynthetics/C30VH-BFT13-1s")
			>>> print(db)
			RemoteInstaseisDB reciprocal Green's function Database (v7) generated with these parameters:
				components           : vertical and horizontal
				velocity model       : external
				attenuation          : True
				dominant period      : 1.000 s
				dump type            : displ_only
				excitation type      : dipole
				time step            : 0.230 s
				sampling rate        : 4.346 Hz
				number of samples    : 7823
				seismogram length    : 1799.9 s
				source time function : gauss_0
				source shift         : 1.611 s
				spatial order        : 4
				min/max radius       : 3289.0 - 3389.5 km
				Planet radius        : 3389.5 km
				min/max distance     : 0.0 - 180.0 deg
				time stepping scheme : symplec4
				compiler/user        : gfortran 4.9.2 20141030 (Cray Inc.) by vamartin on dora22
				directory/url        : http://instaseis.ethz.ch/marssynthetics/C30VH-BFT13-1s
				size of netCDF files : 564.9 GB
				generated by AxiSEM version v1.2-11-g8461-dirty at 2016-03-11T12:50:33.000000Z 
			>>>

Define a source/receiver pair using the moment tensor solution and location parameters. Use a whole ObsPy.Catalog for this purpose in a loop as you wish.

			>>> receiver = instaseis.Receiver(

			                 latitude=4.5, longitude=136.0, network="7J", station="SYNT1")

			

			>>> source = instaseis.Source(

			                 latitude=30.0, longitude=30.0, depth_in_m=50000,

			                 m_rr=1.710000e+24,

			                 m_tt=1.810000e+22,

			                 m_pp=1.740000e+24,

			                 m_rt=1.990000e+23,

			                 m_rp=1.050000e+23,

			                 m_tp=1.230000e+24,

			                 origin_time=obspy.UTCDateTime(2020, 1, 2, 0, 0, 0))

Retrieve the waveforms.

			>>> seismogram = db.get_seismograms(source=source, receiver=receiver)

Do some more processing as needed...

			
			>>> print(seismogram)
			>>> seismogram.plot()

Use REST-like API of Instaseis:

Instaseis currently supports nine back-end commands. Check http://www.instaseis.net/routes/seismograms_raw.html and http://www.instaseis.net/routes/seismograms.html webpages for a list of parameters needed to compute waveforms. You can use these commands within your Python scripts, or use wget to download the data.

Use Instaseis GUI:

You also have the option to view synthetics for any source type using the Instaseis GUI. This tool is useful especially for exploring the waveforms before generating a set of them. To start the GUI, simply type the command below in your command screen.

			
			> python -m instaseis.gui