[Shake-dev] "Ringing" near intensity IV [SEC=UNCLASSIFIED]

[Trevor.Allen at ga.gov.au]

Hi folks,

I've recently stumbled upon a strange feature in the intensity grids being produced by my ShakeMap installation, and also the installation of colleagues in Indonesia.  We are both using V3.5 - as yet, I have not implemented the latest update Bruce described a month or so ago.  We also both use topo-Vs30 site corrections as is applied in GSM.  Taking this approach, Vs30 values in oceans are typically set to 360 m/s.

When contouring the mi.grd files, I noticed intensity appears to oscillate about intensity level IV.  At first I thought this was limited to particular GMPE-GMICE combinations since I was originally smoothing the contours.  However, taking a closer look at unsmoothed contours, I noticed the effect was observable (to varying degrees) for all the GMPE-GMICE combinations I tried.  The attached figure shows four scenarios for a hypothetical earthquake of MW 5.7.  As can be observed in all but the bottom-left subplot, the "ringing" about intensity IV is most apparent in oceans where the Vs30 is set to 360 m/s.

Hacking into "topo2grd" and modifying the variable "$VS30_OCEAN" to 686 m/s, I reran the scenario and got the bottom-left subplot, in which this ringing effect seems to have disappeared.  Aesthetically, this looks much better, but it does not solve the potential problem for onshore soft-soil sites, which, due to the irregularity of the topo-based approach for determining Vs30, would not be as apparent on the contoured map.

It's fair to say that at this point, I'm a little confused as to what might be going on.  I was wondering whether anybody else has encountered this issue?  And if so, is there any easy fix (other than setting $VS30_OCEAN to a higher velocity)?

Thanks in advance for any advice,
Trev

--
Trevor Allen
Earthquake Seismologist
Risk and Impact Analysis & Earth Monitoring Groups
Geospatial & Earth Monitoring Division
Geoscience Australia

T +61 2 6249 9359 | F +61 2 6249 9986
Physical Address: Cnr Jerrabomberra Ave & Hindmarsh Drv Symonston ACT 2609 Australia
Postal Address: GPO Box 378 Canberra ACT 2601 Australia

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