[Shake-dev] Varying GMPE bias correction parameters

[Bruce Worden]

Hi Nick,

This issue has come up for us recently, too. I'm thinking of adding an optional parameter that sets the maximum number of stations to use in the bias in addition to the minimum. I'm not sure if that would solve your problem (I'm not sure it would solve ours, either), but it is something we're thinking about.

In the meantime, there currently isn't a particularly elegant way to do what you want. The best I can offer would be to use zone_config and define geographic polygons where events would have adequate numbers of near stations (these polygons are defined with "box" statements in zone_config.conf). If you name these boxes "NEAR," for example, then in the .../config/zone directory you would have "grind.NEAR" with the 100km config. The default .../config/grind.conf would have the 200km config. You would then run zone_config as the first process after .../data/<event_id>/input/event.xml is created. zone_config would look at the epicenter to determine if it was inside one of the polygons: If it was, .../config/zone/grind.NEAR would be copied (as grind.conf) to .../data/<event_id>/config and used in preference to the default .../config/grind.conf. If the epicenter was not inside one of your polygons, then .../config/grind.conf would be used.

You could also do the opposite -- have the polygons for the more distant stations. Or you could have three or more zones.

If you go this route, one thing to keep in mind is that the events that have event-specific grind.conf (in .../data/<event_id>/config) will always use that file  (until you delete it) in preference to the default config file. So if you change grind.conf in .../config and re-run the event, it won't see the change. You'd have to change (or delete) .../data/<event_id>/config/grind.conf.

Hope this helps,
Bruce


On Oct 16, 2014, at 2:37 PM, Nick Horspool <n.horspool at gns.cri.nz> wrote:

> Hi All
> 
> I was wanting to know if there is a way to vary the GMPE bias correction parameters. I would like to set the bias_max_distance parameter to a default value (say 100km), but if the conditions for bias correction is not met, then revert to another value (e.g. 200km). The min_number_stations would be set to a fixed value (e.g. 6). 
> 
> The reason for this, is that for most of our events, our network is dense enough to have many stations within 100km to perform bias correction. However, for a small proportion of events our network does not have enough stations within 100km to run the bias correction. We don’t want to default to much more than 100km as we would like to have the near source observations dominating the bias correction. 
> 
> Thanks in advance
> 
> Nick Horspool
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