1. Site effect characterization of the Ulaanbaatar basin
THÈSE DE DOCTORAT DE L’UNIVERSITÉ STRASBOURG
Site effect characterization of the Ulaanbaatar basin
Chimed Odonbaatar
Supervised by:
Luis Rivera & Antoine Schlupp

2. Plan of presentation Context of the study
Site effect characterization of the Ulaanbaatar basin:
Plan of presentation
Context of the study
Seismic activity
Weak motion and microtremor recordings
Amplification and amplified frequencies
Seismic wave modeling
Duration variation versus frequency
Conclusions

3. MONGOLIA: Population - 2.75 million Area - 1.56 million km2
Context of the study:
Population million
Area million km2 
MONGOLIA:

4. Ulaanbaatar Context of the study: Seismic activity 05/01/1967 Mw=7.2
D=260 km
4/12/1957
Mw=8.1
D=550 km
24/11/1998
ML=5.4
09/01/2010
ML=5.1
D=(190 km)

5. Context of Ulaanbaatar: Seismic activity
(after Ulziibat et al 2010)
Seismic crisis starts -2005

6. Context of the study: Seismic activity
Several earthquakes has been felt recently (1998 – 2010)
The number of buildings are rapidly increasing
Ulaanbaatar urban area
Ulaanbaatar is the capital of Mongolia where 1/2 (more than 1.2 million) of the total population of country is living
Ulaanbaatar is located on the Tuul river basin
(4 km x 25 km)

7. Mogod earthquake (05 Jan 1967, Mw=7.2), distance 260 km
Context of the study: Seismic activity
Mogod earthquake (05 Jan 1967, Mw=7.2), distance 260 km
(after Medvedeva C. V. 1971).

8. 104 sites Location of measured sites
Context of the study: Weak motion and microtremor recordings
Location of measured sites
:104 sites
:32 sites
8 short period 3 component station
104 sites

9. Amplified signal at sediment
Context of the study: Weak motion and microtremor recordings
Amplified signal at sediment

10. Plan of presentation Context of the study
Site effect characterization of the Ulaanbaatar basin:
Plan of presentation
Context of the study
Amplification and amplified frequencies
Spectral ratio methods and analysis
HV noise stability ?
Results
Seismic wave modeling
Duration variation versus frequency
Conclusions

11. Spectral ratio procedure
Amplification and amplified frequencies : Spectral ratio methods and analysis
SSR (Standard spectral ratio) is the spectral ratio between Reference and Sedimentary sites on S waves
Spectral ratio procedure
HVevent
Is the spectral ratio between Horizontal and Vertical components on S waves
HVnoise
Is the spectral ratio between Horizontal and Vertical components on microtremors

12. Reference site qualification
Amplification and amplified frequencies : Spectral ratio methods and analysis
Reference site qualification
Reference -> rock site without specific amplification
Site to Reference distance<< site to event distance
HV noise
on Reference
SSR between Reference and ALFM (permanent station at rock)

13. Comparison HVnoise, HVevent and SSR results
Amplification and amplified frequencies : Spectral ratio methods and analysis
Comparison HVnoise, HVevent and SSR results
- Same amplified frequency
- Different amplification factor
- HV noise gives minimum and SSR gives maximum amplification

14. Comparison HVnoise and HVevent with SSR results
Amplification and amplified frequencies : Spectral ratio methods and analysis
Comparison HVnoise and HVevent with SSR results
A-clear peak, B-wide peak, D-very wide or flat peak

15. Comparison HVnoise and HVevent with SSR results
Amplification and amplified frequencies : Spectral ratio methods and analysis
Comparison HVnoise and HVevent with SSR results

16. Influence of noise level on HVnoise ratio
Amplification and amplified frequencies : HV noise stability ?
Influence of noise level on HVnoise ratio

17. Influence of local noise sources on HVnoise ratio
Amplification and amplified frequencies : HV noise stability ?
Influence of local noise sources on HVnoise ratio

18. Influence of local noise sources on HVnoise ratio
Amplification and amplified frequencies : HV noise stability ?
Influence of local noise sources on HVnoise ratio
Day is higher noise level
Z noise level less increased than H

19. Polarization of the amplified peak
Amplification and amplified frequencies : HV noise stability ?
Polarization of the amplified peak
135

20. Azimuth dependence of H/V ratio
Amplification and amplified frequencies : HV noise stability ?
Azimuth dependence of H/V ratio

21. HVnoise ratio polarization / known noise sources
Amplification and amplified frequencies : HV noise stability ?
HVnoise ratio polarization / known noise sources
day time
Night time

22. Map of SSR amplification factor
Amplification and amplified frequencies: Results
Map of SSR amplification factor

23. Map of HVnoise (or SSR) amplified frequencies
Amplification and amplified frequencies : Results
Map of HVnoise (or SSR) amplified frequencies
Figure HV peak frequency distribution across Ulaanbaatar basin

24. Plan of presentation Context of the study
effect characterization of the Ulaanbaatar basin:
Plan of presentation
Context of the study
Amplification and amplified frequencies
Seismic wave modeling
Basin properties
1D and 2D modeling
Comparison of modeling with observations
Amplified frequency zoning
Duration variation versus frequency
Conclusions

25. Topography Geology Gravimetery Drillings Sediment thickness
Seismic wave modeling: Basin properties
Topography
GIS DATABASE
Geology
Gravimetery
Drillings
54 reached base rock
Sediment thickness

26. Available data => 3D basin model
Seismic wave modeling: Basin properties
Available data => 3D basin model

27. Microtremor array measurements to estimate S-wave velocity
Seismic wave modeling: Basin properties
Microtremor array measurements to estimate S-wave velocity

28. Results of the high resolution FK analysis
Seismic wave modeling: Basin properties
Results of the high resolution FK analysis
Vs~570 m/s
H=30m
Vs~520 m/s
H=28m
Vs~600 m/s
H=37m

29. Input of the 3D model for 1D and 2D wave modeling
Seismic wave modeling: 1D and 2D modeling
Input of the 3D model for 1D and 2D wave modeling
3D model + 2D horizontal grid => 1D vertical modeling at each node of the grid (4000 Nodes)
Shear wave velocity of each node is interpolated from nearest array velocity structure.
Use of Shake91, a program for equivalent linear seismic response analyses
3D model => 2D modeling (cross section)
Average shear wave velocity is used for all cross sections sediments
Use of Mka3D, a code using discrete element method, developed by Christian Mariotti,( CEA-DASE)

30. Seismic wave modeling: Comparison of modeling with observations
Map of amplified frequencies using 1D modeling: comparison with observed amplified frequencies
model accuracy (3D model , velocity)?

31. Location of the cross section used in the 2D simulation
Seismic wave modeling: 1D and 2D modeling
Location of the cross section used in the 2D simulation

32. ref sediment Base rock 2D simulation
Seismic wave modeling: 1D and 2D modeling
2D simulation
ref
Base rock
sediment

33. -1D modeling predicts well the amplified frequencies,
Seismic wave modeling: Comparison of modeling with observations
-1D modeling predicts well the amplified frequencies,
-2D modeling predicts better the shape of wide peaks

34. Data “layers” used to build amplified frequency zoning map
Seismic wave modeling: Amplified frequency zoning
Data “layers” used to build amplified
frequency zoning map
1D modeled amplified
frequencies +
Observed amplified frequencies +
Geological Map +
Topography and
Satellite image =
Amplified frequency
zoning map of the Ulaanbaatar basin

35. Amplified frequency zoning map of the Ulaanbaatar basin
Seismic wave modeling: Amplified frequency zoning
Amplified frequency zoning map of the Ulaanbaatar basin

36. Plan of presentation Context of the study
Site effect characterization of the Ulaanbaatar basin:
Plan of presentation
Context of the study
Amplification and amplified frequencies
Seismic wave modeling
Duration variation versus frequency
Duration based on the coda
Duration based on arias intensity
Conclusions

37. Plan of presentation Context of the study
Site effect characterization of the Ulaanbaatar basin:
Plan of presentation
Context of the study
Amplification and amplified frequencies
Seismic wave modeling
Duration variation versus frequency
Duration based on the coda
Duration based on arias intensity
Conclusions

38. Location of selected sites for the duration study
Duration variation : Data qualification
Location of selected sites for the duration study
Sites that recoded events with signal to noise ratio higher than 3.

39. Duration estimation based on the coda: measurement procedure
Duration variation : Duration based on the coda
Duration estimation based on the coda:
measurement procedure

40. Duration estimation based on the coda: Comparison with SSR
Duration variation : Duration based on the coda
Duration estimation based on the coda:
Comparison with SSR
* Lengthened frequency = amplified SSR frequency
related to amplification factor

41. Duration variation : Duration based on the coda
Impact of the analysis window length and the sediment thickness to the duration ratio

42. Duration variation : Duration based on the coda
Response of 1D modeling

43. Plan of presentation Context of the study
Site effect characterization of the Ulaanbaatar basin:
Plan of presentation
Context of the study
Amplification and amplified frequencies
Seismic wave modeling
Duration variation versus frequency
Duration based on the coda
Duration based on arias intensity
Conclusions

44. Duration variation : Duration arias

45. Response of 1D modeling
Arias duration does not include 1D amplification
=> Study of others lengthening sources

46. H<15m H~30 m UB9 Duration variation : Duration arias 5 7 6 8 3 5
6 8 2

47. Arias duration variation from 2D simulation
Duration variation : Duration arias
Arias duration variation from 2D simulation
SSR
The duration lengthening is at higher frequency than the amplified frequency

48. Site effect characterization of the Ulaanbaatar basin:
Conclusions 1/2
Amplified frequency is independent from the spectral ratio method (HVnoise, HVevent, SSR).
Amplification factor of HVnoise ratio is influenced by the noise level and local noise source azimuth.
The amplified frequency of the HVnoise ratio can be shifted in particular situations.
SSR gives higher amplification factor than HV ratio. We observe an amplification factor between 3 and 10 at the Ulaanbaatar basin with SSR on weak motions.
A 3D geometrical model was produced using available data. Velocity structure determined locally gives a shear wave velocity of ~ 550 m/s at sediment.
The amplified frequencies at Ulaanbaatar basin vary mainly between 2 and 5 Hz. An amplified frequencies zoning was deduced from observations and 1D modeling.

49. Site effect characterization of the Ulaanbaatar basin:
Conclusions 2/2
The lengthening duration, deduced from the coda wave, shows an absolute duration lengthening
It is the maximum at the same frequency that the amplification frequency.
The observed lengthening duration up to 100%.
The lengthening duration, deduced from normalized Arias intensity, is due to basin effects outside the 1D amplification factor.
We observe locally lengthening between 5 to 13 seconds for 40 to 80 seconds weak motion signals.
The most lengthened frequency is always higher than the amplified frequency.
The main duration lengthening, observed with weak motion at Ulaanbaatar basin, is due to 1D amplification.

50. Site effect characterization of the Ulaanbaatar basin:
Next …
The results of this study should be included in the Ulaanbaatar seismic hazard assessment, capital of Mongolia.
The strong motion site effect will be modeled, beyond the thesis, with a 3D modeling within the framework of the collaboration between the RCAG, the CEA-DASE and the EOST.
The GIS data base (model, measurements, modeling results) built during this PhD will be extended with additional data as geophysical exploration data, new drillings, new records, etc.

51. Thank you for your attention
Site effect characterization of the Ulaanbaatar basin:
I would like to thank
The CEA-DASE for its assistance and collaboration, specially for the instruments that have been provided and the 2D modeling.
The French Embassy for they financial support of my stays at Strasbourg.
Thank you for your attention