Using LOCTOPO_SURFACE in a marine setting

[ev-anderson]

Hello,

I'm attempting to locate earthquakes in a marine setting and have tried using LOCTOPO_SURFACE with a (negative-valued) bathymetry grid as the masking topography, however I am locating events within the water column.

I was curious if you've had success with using LOCTOPO in such an environment?

[alomax]

Hi,

I see no reason why topo below sea level should fail. As long as the topo/bathy values are in meters and positive/negative above/below sea level.

LOCTOPO_SURFACE is complex and difficult to use since, unfortunately, it relies on the complex GMT grid header file format.

Do you have a way of confirming that LOCTOPO_SURFACE is working or not for topography above sea-level?

Recently, the LOCTOPO_SURFACE masking did not work for me for a binary grd + hdr file I generated with GMT 6.
It looks like the GMT format for the output of grdinfo has changed at some point (???). I have adjusted the code to fix this.
If this is the problem you have, you may be able to fix the problem with the current code by replacing "n_rows:" in the line like:
DEM_GMT_ETNA/Etna.nc: y_min: 37.4045171091 y_max: 37.9789897333 y_inc: 0.00044915764206 name: y n_rows: 1279
with "ny:".

You can also get more information on the processing of LOCTOPO_SURFACE if you use:
CONTROL 2 54321
or
CONTROL 3 54321

If the above does not help0, can you provide the header lines in your .asc grd header file and any relevant information messages?

PS - I created a bin/hdr topo grd pair with:

    gmt grdinfo DEM_GMT_ETNA/Etna.nc > Etna_topo.bin.hdr
    gmt grd2xyz -ZTLd DEM_GMT_ETNA/Etna.nc > Etna_topo.bin

    > gmt --version
    6.4.0

Anthony

[ev-anderson]

Hi Anthony,

Thank you for the quick reply and apologies for my delayed response.

My slowness grid starts at sea level, so I do not have a way to check if LOCTOPO is working above sea level.

I previously used an xyz grid of my bathymetry, and then tried your suggestion of using a bin/hdr and adjusting the hdr file. Implementing these changes yielded the same results as before. However, upon considering it further LOCTOPO seems to be working for the most part with minimal locations still resolving within the water column as in the profile slice below.

[alomax]

Locations above the topo may be accepted if their epicenter is outside the topo grid. Otherwise, I suppose something is still wrong.

Sometimes it is useful to allow locations above the topo at a constant elevation or depth, this makes it easy to flag and ignore or correct poorly constrained locations. You could set the top of the NLL LOCGRID to just above the shallowest topo.

But if events are generally well constrained, it is probably preferable to mask the topo/bathy.

Anthony