[Shake-dev] Varying GMPE bias correction parameters

[David Wald]

OopsŠfired off that too quickly. Switched my norms thereŠ :)

Distance issues still apply!

David

From:  David Wald <wald at usgs.gov>
Date:  Thursday, October 16, 2014 at 8:23 PM
To:  Bruce Worden <cbworden at gmail.com>, Nick Horspool
<n.horspool at gns.cri.nz>
Cc:  <shake-dev at geohazards.usgs.gov>
Subject:  Re: [Shake-dev] Varying GMPE bias correction parameters

> Hi Nick,
> 
> Good to hear from you. Thanks for sharing this query with the group. Let me
> add to Bruce's thoughts on this.
> 
> Bruce implemented both an L1 & L2 norms for the bias correction. L1 is the
> default. L2, however, will weight larger amplitudes over lower ones thus
> ³favoring" close-in stations (if the number of stations is comparable). This
> behavior needs to be validated. If the L2 is effective for this purpose, you
> could increase the maximum distance routinely.
> 
> That said, I¹m not sure I would want to rely on a distant-only data set to
> compute an inter-event bias term, hence the cut off. A GMPE misfit in the
> attenuation with distance at distant sites by could be incorrectly absorbed in
> a bias term that then incorrectly resets estimated amplitudes at near-in
> distances. Thus a reasonable cut-off distance (bias_max_range) seems like a
> good precaution. 
> 
> All this gets back to the very real concern that a singe inter-event term in
> the GMPE business is probably too simple for some events. For instance most
> GMPE¹s underestimate close in stations for recent M6.0 Napa event yet
> over-predict distance sites. A single bias term mostly fixes one yet degrades
> the other. We know better than trying to invert for too many parameters on the
> fly, however. 
> 
> Cheers,
> 
> David
> 
> ‹
> David Wald, Ph.D.
> U.S. Geological Survey, Golden, CO
> wald at usgs.gov, (303) 273 8441
> 
> Adjunct Associate Professor of Geophysics
> Colorado School of Mines
> 
> From:  Bruce Worden <cbworden at gmail.com>
> Date:  Thursday, October 16, 2014 at 6:42 PM
> To:  Nick Horspool <n.horspool at gns.cri.nz>
> Cc:  <shake-dev at geohazards.usgs.gov>
> Subject:  Re: [Shake-dev] Varying GMPE bias correction parameters
> 
>> 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 don¹t 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
>>> ___________________________
>>> Nick Horspool 
>>> Natural Hazard Risk Specialist
>>> Risk and Society Department
>>> GNS Science 
>>> TE PU AO 
>>> 1 Fairway Drive, Avalon, P O Box 30 368, Lower Hutt 5040, New Zealand
>>> E n.horspool at gns.cri.nz, T +64 4 570 4282, F +64 4 570 4600
>>> www.gns.cri.nz <http://www.gns.cri.nz/>
>>> 
>>> 
>>> 
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