Clarification Needed on Surface Level Hazard Computations Using Vs30 Data

[Ramesh Pudi]

Dear all,

I need a small clarification regarding surface-level hazard computations using Vs30 data. When computing hazard on base rock, Vs30 is assumed to be hard rock (760 m/s) for the entire study area. However, when conducting hazard computations using actual Vs30 values, if any site has a Vs30 value greater than 760 m/s, it results in a lower PGA than the base rock.

Generally, base rock PGA values should not be higher than surface-level hazard values. Is there any solution to avoid this issue?

I would appreciate any clarifications and suggestions on this matter.

Thank you,

Ramesh

[Peter Pažák]

Hi, can you plese clarify what you mean by surface-level hazard values?

Yes, PGA and SA values are usually bigger for softer soil and lower for stiffer so not sure I understand what is the problem.

Peter

Dátum: streda 5. júna 2024, čas: 6:51:59 UTC+2, odosielateľ: pudira...@gmail.com

[Ramesh Pudi]

Hi Peter,

Here I'm comparing PGA estimation using Vs30 (surface level) and without Vs30 ( hard rock).

Best regards,

Ramesh 

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[Peter Pažák]

Ok, so can you describe in a bit more clearer way what you would like to achieve?

Of course if you put into the site model higher Vs30 values than rock, you would get lower hazard

and if lower Vs30, higher hazard...

Peter

Dátum: štvrtok 6. júna 2024, čas: 4:18:23 UTC+2, odosielateľ: pudira...@gmail.com

[Ramesh Pudi]

Dear Peter,

I have a question regarding the computation of hazard for bedrock where I assumed Vs30 to be 760 m/s. In the case of surface level computations, several locations may have Vs30 values greater than 760 m/s (indicating hard rock). For these sites, I would expect to get higher PGA values at the surface level compared to the bedrock level.

From my understanding, the hazard at the surface level is generally higher than at the bedrock level. Please correct me if I'm wrong. Additionally, when conducting risk assessments for these two cases, it seems that I would obtain higher loss figures for bedrock compared to when I use Vs30 for certain locations.

How can I address this issue during my surface level hazard computations? I am interested in learning how others approach their computations in similar scenarios.

I hope my question is clear. Please let me know if you need any further clarification.

Best regards,
Ramesh

[Peter Pažák]

Hi, so my understanding is sometimes the bedrock (having certain S wave velocity, say 760 m/s) is in depth of say 500m and vs30 may be say 200 m/s as there are sediments on top of the bedrock, on other locations the bedrock may reach surface and Vs30 reaches 760 m/s. Or more if some crystalline rock is reaching surface, there the hazard would be lower than on the bedrock.

To avoid having lower hazard at these places than on reference rock (760 m/s) you would need to cap the Vs30 to 760 m/s.

But I am still unsure why this is a problem - this "default rock" calculations for Vs30 = 760 m/s are done as a standard but it is not a problem

if with real site model you reach lower hazard than on this "default rock" on some places and higher at others - maybe someone else can also comment...

Peter

Dátum: pondelok 10. júna 2024, čas: 8:30:45 UTC+2, odosielateľ: pudira...@gmail.com

[Ramesh Pudi]

Hi Peter,

Thank you for your response and suggestions.

Yes, I have implemented capping of Vs30 to 760 m/s in one of my computations. I had also considered that in default rock, we assume a uniform Vs30. However, in real scenarios, local site effects can increase or decrease the hazard value. I wanted to confirm this approach with other experts.

Best regards,
Ramesh

[Peter Pažák]

OK, in my opinion the approach will depend on what is your use-case, still struggling a bit what you would like to achieve and what is your exact problem :)

Peter

Dátum: utorok 11. júna 2024, čas: 7:57:24 UTC+2, odosielateľ: pudira...@gmail.com

[Aulia Khalqillah]

Hi Ramesh.

I would like to elaborate more about the discussion based on my opinion.

To calculate the PGA, we use the attenuation equation or GMPE. The GMPE input requires some input, two of which are distance and site condition (Vs30). The distance describes how far the seismic source (fault source, area source, etc.) is from the target site (spatially). Then, the Vs30 describes the site conditions (soft soil, medium soil, and hard soil).

Either the Vs30 is assumed to equal or vary for the entire region of interest, as a function of distance, the PGA will decrease with increasing distance, because of the attenuation effect.

Of course, the PGA will be smaller on the hard soil (high Vs30) than on the soft soil (low Vs30). But, the distance must be considered as well.

I hope this helps

Best regards

Aulia Khalqillah. S.Si., M.Si
Tsunami and Disaster Mitigation Research Center
Universitas Syiah Kuala, Banda Aceh, Indonesia
auliakh...@usk.ac.id

To view this discussion on the web visit https://groups.google.com/d/msgid/openquake-users/195144b1-5aa9-4ba6-8464-679e8f9a6fedn%40googlegroups.com.

[Ramesh Pudi]

Dear Peter and Aulia,

Thank you for your response.

Peter: From a risk assessment perspective, bedrock losses are generally lower than those at the surface level, correct? However, if I obtain higher PGA values for bedrock compared to Vs30, the loss values would be reversed. This is why I wanted to seek expert opinions on the matter.

Anyway, I understand now that I do not need to use bedrock PGA for risk assessment, so there should not be any issue.

Aulia: Thank you for your explanation.

Best regards,

Ramesh

[Peter Pažák]

Generally lower - depends on your assumptions, this 760 m/s is a theoretical "bedrock" and thus it is no problem, if you are getting lower or higher values in the realistic calculations...

Peter

Dátum: streda 12. júna 2024, čas: 9:15:31 UTC+2, odosielateľ: pudira...@gmail.com

[Ramesh Pudi]

ok, thank you very much for your time and suggestions.