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<p class=MsoNormal><font size=2 face=Calibri><span style='font-size:11.0pt;
font-family:Calibri'>Hi folks,<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Calibri><span style='font-size:11.0pt;
font-family:Calibri'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Calibri><span style='font-size:11.0pt;
font-family:Calibri'>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 <st1:country-region w:st="on"><st1:place w:st="on">Indonesia</st1:place></st1:country-region>.
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.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Calibri><span style='font-size:11.0pt;
font-family:Calibri'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Calibri><span style='font-size:11.0pt;
font-family:Calibri'>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.
<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Calibri><span style='font-size:11.0pt;
font-family:Calibri'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Calibri><span style='font-size:11.0pt;
font-family:Calibri'>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.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Calibri><span style='font-size:11.0pt;
font-family:Calibri'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Calibri><span style='font-size:11.0pt;
font-family:Calibri'>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)?<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Calibri><span style='font-size:11.0pt;
font-family:Calibri'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Calibri><span style='font-size:11.0pt;
font-family:Calibri'>Thanks in advance for any advice,<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Calibri><span style='font-size:11.0pt;
font-family:Calibri'>Trev<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Calibri><span style='font-size:11.0pt;
font-family:Calibri'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color="#4b4b4b" face=Calibri><span
style='font-size:10.0pt;font-family:Calibri;color:#4B4B4B'>--</span></font><o:p></o:p></p>
<p class=MsoNormal><strong><b><font size=2 color="#4b4b4b" face=Calibri><span
style='font-size:10.0pt;font-family:Calibri;color:#4B4B4B'>Trevor Allen</span></font></b></strong><o:p></o:p></p>
<p class=MsoNormal><font size=2 color="#4b4b4b" face=Calibri><span
style='font-size:10.0pt;font-family:Calibri;color:#4B4B4B'>Earthquake
Seismologist</span></font><o:p></o:p></p>
<p class=MsoNormal><font size=2 color="#4b4b4b" face=Calibri><span
style='font-size:10.0pt;font-family:Calibri;color:#4B4B4B'>Risk and Impact
Analysis & Earth Monitoring Groups</span></font><o:p></o:p></p>
<p class=MsoNormal><font size=2 color="#4b4b4b" face=Calibri><span
style='font-size:10.0pt;font-family:Calibri;color:#4B4B4B'>Geospatial &
Earth Monitoring Division</span></font><o:p></o:p></p>
<p class=MsoNormal><font size=2 color="#4b4b4b" face=Calibri><span
style='font-size:10.0pt;font-family:Calibri;color:#4B4B4B'>Geoscience <st1:country-region
w:st="on"><st1:place w:st="on">Australia</st1:place></st1:country-region></span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color="#4b4b4b" face=Calibri><span
style='font-size:10.0pt;font-family:Calibri;color:#4B4B4B'>T +61 2
6249 9359 | F +61 2 6249 9986</span></font><o:p></o:p></p>
<p class=MsoNormal><font size=2 color="#4b4b4b" face=Calibri><span
style='font-size:10.0pt;font-family:Calibri;color:#4B4B4B'>Physical Address: <st1:Street
w:st="on"><st1:address w:st="on">Cnr Jerrabomberra Ave</st1:address></st1:Street>
& Hindmarsh Drv Symonston ACT 2609 <st1:country-region w:st="on"><st1:place
w:st="on">Australia</st1:place></st1:country-region></span></font><o:p></o:p></p>
<p class=MsoNormal><font size=2 color="#4b4b4b" face=Calibri><span
style='font-size:10.0pt;font-family:Calibri;color:#4B4B4B'>Postal Address: GPO
Box 378 <st1:City w:st="on">Canberra</st1:City> ACT 2601 <st1:country-region
w:st="on"><st1:place w:st="on">Australia</st1:place></st1:country-region></span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
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