Clalculating dip

Hi,

I have 2D grid file for the subducting slab in terms of latitude, longitude in degrees and depth in km. My slab grid file (slabsurf2.grd) has 345639 data points in it and extended between 10-18 degree south and 66-78 degree west. I want to determine the gradient at some user defined latitude and longitude (example: 74.567 degree west, 12.345 degree south), which falls with in the range of extension of the slab but doesn't coincide exactly with any specific point present in the slab grid file. I use the following commands as per previous suggestions from the GMT-HELP:


 grdgradient slabsurf2.grd -Dn -Sslabgradient.grd -M -V


grdmath slabgradient.grd 1000 MUL ATAN = dip.grd


I used grd2xyz to look at the dip at various grid points as follows:


grd2xyz dip.grd > dip.xyz


I checked the dip.xyz file and it has dip in the third column as:


-71.4326        -16.0458        0.44211
-71.3581        -16.0458        0.425007
-71.2837        -16.0458        0.428402
-71.2093        -16.0458        0.44172
-71.1349        -16.0458        0.460254
-71.0605        -16.0458        0.478551


I was expecting the dip to be in the range of 20-30 degree since the subduction is normal and slab is roughly subducting at 30 degree from the horizontal. If I take inverse tan of the last column than the dip angle falls in the expected range. I would be extremely grateful for any suggestions in this regard.


Abhash kumar

University of North Carolina at Chapel Hill

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