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Hello, greetings. My name is Sean Ahdi.
It is snowing here in Denver, Colorado, but it’s sunny in
San Francisco, as my background shows. I would like to introduce
myself and thank you for inviting me to this workshop.
I am a research geophysicist newly hired at the Geological Hazards
Science Center in Golden, Colorado, and also a lecturer at the UCLA
Department of Earth, Planetary, and Space Sciences. And today I’m going to
talk about why we are still using Vs30 for seismic site characterization.
Like to acknowledge my USGS colleagues Alan Yong, Grace Parker,
my supervisor Morgan Moschetti, the National Seismic Hazard Model
Group, and the Near-Real-Time Products Group, and other folks
in the Earthquake Science Center with whom I’ve collaborated over
the years and discussed this work. And a special shout-out to
Jack Boatwright, who very graciously invited me to give a presentation
at this workshop a few years ago when I was still just a student.
I’d like to acknowledge my UCLA Shear Wave Velocity
Profile Database project team. Professor, PI, and my adviser
Jonathan Stewart and Professor Scott Brandenberg.
Dong Youp Kwak, Paolo Zimmaro, Pengfei Wang, and Professor Yousef
Bozorgnia, and many others who helped with this effort.
Numerous individuals and agencies provided their data and their time to
input this data into our databases that I will talk about briefly at the end.
And support from the USGS California State Geologic Survey,
COSMOS, PEER, UCLA, PG&E, SoCal Edison, and BC Hydro
at various times throughout the projects.
A quick outline of my talk. I will talk about the history of use of
Vs30 for seismic site characterization. Arguments for and against Vs30
and other site parameters used for site characterization.
Ongoing efforts supported by Vs30 data collection in NorCal and beyond
and the products that exist that show shear wave velocity data.
How about some odes to Vs30, if you will?
Some quotes I’ve heard over the years.
It is what everyone loves to hate.
It’s a curious dilemma. We don’t really like to use Vs30,
but using anything better introduces other challenges.
Are there any other things we can look into using?
With a laugh, watching people who try to correlate Vs30 with Z2.5, which is
the depth to the 2-1/2 kilometer per second shear wave velocity surface,
which correlates with basin depth. And the use and abuse of Vs30.
So what exactly is Vs30 we keep waving our arms about?
It’s the time-averaged shear wave velocity in the upper 30 meters
of a site. I emphasized “time” so that we don’t average over depth.
In our one-dimensional Vs profile shown here, the depth –
maximum depth is Z-sub-p. And we can take that and divide it
by the total travel time to go through all these layers,
which is the time-averaged velocity in the whole profile.
That travel time is the sum of these velocities divided by –
layer thicknesses divided by velocities – excuse me.
Plug that in. Get that travel time. And replace the profile depth
with 30 meters, and you have Vs30. It’s very simple.
Over time, in the past, we had – originally had qualitative and
subjective site descriptions, such as soil and rock, stiff and soft, etc.
But Roger Borcherdt of the USGS in 1970 identified the relation of
local geology, including its shear wave velocity of the materials to
the amplification of ground motion. And the USGS commenced a
Vs measurement campaign, starting with downhole profiles,
in the early to mid-1970s and released many Open-File reports to date,
including the starting views of non-invasive methods as well –
besides invasive ones in boreholes.
Dave Boore and others first adopted Vs30 for their ground motion
attenuation relations in 1993, and there’s a screenshot of their
model here, and the Vs30 values for different qualitative site classes
that they used in their effort.
The National Earthquake Hazards Reduction Program, or NEHRP, started
adopted Vs30 for building codes after Roger Borcherdt’s paper in 1994.
And preliminary – it’s – I’m sorry – it’s the primary term that is used
in ergodic site response models implemented in ground motion
models – GMMs – or called GMPEs – to date.
So why are we using Vs30? It’s a single continuous variable
instead of those discrete classes we talked about earlier.
As Dave Boore showed in his 2004 study, there is a reduction
in uncertainty when you use Vs30 compared to discrete site
classifications or using nothing to classify your site at all.
It effectively captured first-order linear and nonlinear site effects.
The nonlinear is shown at the top of the page,
and the linear correlates – linear ground – linear scaling
of Vs30 with ground motion in the bottom set of plots.
Vs30 correlates with shallower and deeper sedimentary structure,
as Dave Boore and his colleagues showed in 2011 in this global
data set of Vs30, which correlates to deeper time-average values –
say, Vs100 or even up to Vs600 in Japan.
And Vs30 has been shown to correlate with basin sediment thickness terms
like that Z2.5 parameter or Z1.0 that we discussed earlier.
Ned Field of the USGS showed that – in 2000 that ground motion residuals
correlate at long periods with these basin depth terms.
And Ebuka Nweke and others at UCLA in recent years have updated
correlation terms for Z1.0, which is the 1-kilometer isosurface depth –
1 kilometer per second shear wave velocity correlated to Vs30.
Continuing on, other parameters may not be obtainable at every site,
such as fundamental or dominant site frequency,
or those basin depth terms – Z-sub-X.
This plots shows horizontal-to-vertical spectral ratio – HVSR – curves that
are collected at a granite outcrop site in southern – near Ridgecrest.
And we cannot identify a predominant site frequency at this site.
Vs30, as I mentioned, as been started –
has been adopted by building codes, and we continue to use them.
It correlates to high-frequency attenuation parameter, kappa,
and Fourier amplitude spectral space.
And it’s much more readily available than other site parameters.
This is a screenshot of the USGS Vs30 data compilation website,
which is active and available, has over 4,000 Vs30 sites and counting.
We are ongoingly updating this database.
So why would we not use Vs30? Arguments against its use.
Well, in our classic schematic of interpreting how ground motions
occur at a site, we know that it’s a combination of source,
wave propagation path, and site effects. And one Vs30 value does not indicate
our complexity here. And our one Vs profile for our site
can actually give us a Vs30 value which can be represented by
many Vs profiles. It’s not really a unique problem.
Site amplification is directed also by the directivity of the ground motion
and topographic effects. And the complex geology we see
in this plot in active tectonic regions, especially with basin edges,
cannot be captured by one parameter as well.
H-over-V peak frequency better predicts the site amplification
at frequencies near the fundamental period of the site,
or frequency of the site.
And, just in summary, it’s not a fundamental site parameter.
It’s a proxy for site response, as has been stated in the literature
and discussed over the years.
Just a quick argument against the use of fundamental frequency in ground
motion models instead of Vs30. As Grace Parker, our colleague
in the ESC, has shown, a consensus has not yet emerged on the
appropriateness of estimating site parameters from attributes of
ground motion recordings when the performance of the resulting ground
motion models is then judged against those same recordings.
So this is a circular argument that we are doing ongoing research
to ascertain whether or not this is going to be figured out.
I’m going to put a plug in for my co-authors for two abstracts that
will be presented at SSA to understand the consistency of using H-over-V
from earthquakes and from other measurements to then
assign them to station characteristics for recording stations
in ground motion model development.
I’m going to make a quick argument for slowness, which is the inverse
of velocity. You see two figures here. One is shear wave velocity and
one is shear wave slowness at a site in southern California.
Travel time is linearly proportional to S-S, instead of inversely proportional
to Vs, which makes it less complicated. And thus, it can be linearly averaged
to get the depth average values, which would alleviate the concern
I mentioned earlier when we defined Vs30, that you have to
do a time-averaged velocity. And some people have been
making this mistake in industry applications.
Statistical properties fitting travel time data apply to S-S rather than Vs.
And S-S emphasizes the material properties that amplify ground motions
rather than diminishing them in the velocity profile or emphasizing
stiffer materials that are less important for site response.
Another plug for slowness is recent work we’ve done with Utkarsh Mital
and others at the USGS and Japanese colleagues that we’re looking at
S-S30 distributions that can be fit with a normal distribution.
But then the inverse of that, like the inverse of S-S is Vs,
would be the reciprocal normal distribution.
And we’re trying to investigate the efficacy of this in describing Vs30
data sets as compared to the longstanding belief that Vs30 data sets
have been fit with log normal distributions.
That might just be statistical and not physical.
Quick plug for V-R40, which is the 40-meter wavelength Rayleigh wave
phase velocity that can be easily obtained from surface wave
measurements, such as the SASW surface – spectral analysis
of surface waves. And Alan Yong and others have shown that
there’s almost a 1-to-1 correlation between Vs30 and V-R40.
So, while we’re still – would be using Vs30 if we implemented
V-R40 correlation, it gives us another way to quickly obtain the parameter.
So concluding remarks for why we’re using these terms.
Most major ground motion models and building codes use Vs30 because
of widespread data accessibility that continues to grow, and the
momentum that is built, it just – we’re just kind of taking advantage
of that and moving along with it.
Admittedly, it has been used for a long time.
It’s a stable measurement across multiple measurement methods
so that we can now start to ascertain epistemic uncertainties at our sites
across methods and across soil types and geologic regimes.
The arguments against Vs30 are still value – it is just a proxy.
It’s not a physical parameter.
And it’s an index parameter. It cannot capture the effects of
basins and other complicated three-dimensional geologic
constraints at our site. And so that means there’s
definitely room for more work, and this is an active area of research.
I will – I will conclude this part of the talk with John Anderson from
University of Nevada-Reno reminding us that the upper 30 meters only
represents less than 1% of the path from the source to the site, but this
Vs30, the observed value of kappa, attenuation, and resonant frequencies
that we mentioned might be sufficient to characterize a site
where 1D models are appropriate. So there’s still use in it.
Shotgun efforts showing you what Vs data is being used and the
data sources that we have so far. Ongoing work to update the National
Seismic Hazard Model in Hawaii where we have compared Vs30 and
site amplification models from existing ground motion models
with Hawaiian data. This is unpublished work, and it’s ongoing.
CEUS – central and eastern U.S., Atlantic and Gulf Coastal plain
amplification models that have been published, and we are collecting
a new database of Vs30 and Vs profiles to determine the efficacy of those
amplification models and ongoing updates to the National Seismic Hazard
Model that will be published, hopefully, in 2023.
The USGS hosts a Vs30 Map Viewer, which has an underpin of the
topographic slope-based Vs30 that was initially proposed by Dave Wald
and Trevor Allen and has since been updated by Eric Thompson and others
and put into a mosaic that the USGS – mosaic map that the USGS hosts that
includes effects of surface geology and topography and those Vs30
measurements from the data compilation that I mentioned earlier.
We are hoping to expand these mosaic inlays for specific
geographic regions in other parts of the U.S.
The data compilation that I mentioned earlier is online, and we encourage
people to access it and collect the data that they need.
And the U.S. – the UCLA organized project – the Vs Community
Profile Database, which now has H-over-V, or HVSR, measurements
implemented in it – these are – this is the map of sites to date.
And a smattering of Lakers colors across the Bay Area
for you Warriors fans out there.
And this is the online web interface for the UCLA Vs Profile Database
and now has – that now has the HVSR measurements as well.
We are – we have thousands of sites, and we continue to add to them.
And I thank you for your time. If, for no other reason that
I mentioned herein, we should continue to collect Vs30 and
other site data so that we can get out in the field and enjoy field work.
Thank you.
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