Earthquakes are not in the Weather Forecast program, simply because they cannot be predicted. Although Vietnam is not in a strong earthquake zone, the data of more than 200 years of observations does not guarantee that a disaster will not occur. The knowledge of engineers helps construction works to be the place to preserve human life in earthquakes. This series of articles tries to explain this issue in the most popular way possible🆗
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📘Vocabulary
📖Earthquake:
is the shaking of the ground, caused by sudden movements of geological blocks along geological faults in the ground (called tectonic earthquakes), volcanic explosions (called volcanic earthquakes), cave collapses, landslides, meteorites and artificial explosions. (according to QCVN 02: 2009/BXD)
Location of earthquake origin
📖The epicenter is the place where the earthquake originates, where the earthquake energy is released and transmitted into the surrounding space in the form of elastic waves, causing ground vibrations. The epicenter is the vertical projection of the epicenter on the ground.
Due to geography, Vietnam is rarely affected by earthquakes (compared to areas where earthquakes occur frequently such as Japan, Taiwan, Indonesia, etc.). Based on the earthquake zoning map of Vietnam’s territory established by the Institute of Geophysics, most of Vietnam’s territory can experience weak and very weak earthquakes. Earthquakes occurring in Vietnam are usually of medium and weak medium intensity. The frequency of strong earthquakes is very low.
What is a Strong or Weak Earthquake?
In addition to human perception, only slight shaking or the earth and sky turning upside down, houses collapsing, roads collapsing, etc. In numerical quantification techniques. Previously, we were used to classifying earthquakes, level VII or VIII was considered very strong. The media is used to using Richter scale, for example, the earthquake in Kobe in 1995 was over 7 on the Richter scale, which is very strong. In construction according to TCVN 9386:2012, the concept of peak ground acceleration is used.
📖Earthquake level
is a quantity that represents the intensity of the shaking it causes on the ground and is assessed according to the scale of the impact of the earthquake on the types of houses, structures, objects, animals, humans and ground deformation. Earthquake levels are often assessed by the MSK-64 scale (Medvedev-Sponheuer-Karnik). The MSK-64 scale consists of 12 levels, approved by the European Seismological Council in 1964, specifically as follows:
Level I: The earthquake is not felt, only the machine can record it.
Level II: The earthquake is barely felt (very light). In individual cases, only people who are in a state of calm can feel it.
Level III: Weak earthquake. Few people can perceive the earthquake. The shaking is similar to that caused by a light truck passing by.
Level IV: The earthquake is clearly noticeable. Many people feel the earthquake, windows may rattle.
Level V: Awakening. Many people are awake, hanging objects swing.
Level VI: Most people feel the earthquake, houses shake slightly, plaster cracks.
Level VII: Damage to houses. Most people are scared, many people have difficulty standing, plaster cracks, walls crack.
Level VIII: House destruction; House walls have large cracks, eaves and chimneys fall.
Level IX: Total damage to houses; the ground may crack 10 cm wide.
Level X: Total damage to houses. Many houses collapse, the ground may crack 1 meter wide.
Level XI: Earthquake causes disaster. Houses, bridges, dams and railways are severely damaged, the ground is deformed, large cracks, large collapses in mountains.
Level XII: Change in terrain. Destroy all structures above and below ground, change the terrain over large areas, change the river, see the ground rise.
📖Richter scale:
is the logarithm to the base 10 of the maximum amplitude (microns) of the horizontal component of the seismic wave on the Wood Andersen standard short-period seismograph tape at a distance of 100 km from the epicenter
1–2 on the Richter scale: Not perceptible
2–4 on the Richter scale: Perceptible but no damage
4–5 on the Richter scale: Ground shaking, explosions heard, negligible damage
5–6 on the Richter scale: Houses shaking, some structures cracking
6–7 on the Richter scale, 7–8 on the Richter scale: Strong, destroying most conventional structures, with large cracks or subsidence on the ground.
8–9 on the Richter scale: Very strong, destroys most of the city or urban area, has large cracks, some buildings sink
>9 on the Richter scale: Very rare
>10 on the Richter scale: Extremely rare
📖Peak ground acceleration (PGA)
is the maximum acceleration of the horizontal ground motion, measured and recorded on seismic events. TCVN 9386:2012 describes the risk of large or small earthquakes through a number called the peak reference ground acceleration on type A ground (rock or rock-supported tectonic), $a_{gR}$, for a 475-year return period. This data is issued by the State in the standards for each administrative region of the construction site through the map.
The earthquake load on the structure is proportional to the ground acceleration. The vertical ground acceleration peak is often less concerned than the horizontal, because the vertical load bearing capacity of the structure is mostly safe enough to withstand additional vertical earthquakes.
📖Return period TR
is the average time period in which the earthquake data (such as ground acceleration) exceeds the reference value in the standard. This is a statistical concept, equivalent to saying: the data is exceeded with the probability PR, in the time TL.
📖Reference earthquake: is the “model” earthquake in the standard, with the probability of being exceeded being PR=10% in the time TL=50 years. According to 2.1.(1) of TC, the return period formula TR=-TL/ln(1-PR).
Thus, the return period of the reference earthquake is TR=475 years (or rounded up to 500 years), which is the basis of the earthquake zoning map in the standard and regulation QCVN 02:2009/BXD.
It is called “reference” because this ground acceleration data is a conventional value, applied to a certain type of construction, which is a construction of average importance, with a conventional return period of 500 years.
Real-life houses are subjected to “non-reference” earthquakes. The design ground acceleration needs to be multiplied by a factor of 1 to the reference ground acceleration $a_g=\gamma_Ia_{gR}$. This factor is called the 📖importance factor $\gamma_I$:
– In the case where the ground acceleration data is based on the probability of exceeding $P_{LR}$=10% in a period other than $T_{LR}$=50 years, as in foreign countries, it can be equivalently converted to the reference acceleration according to 2.1.(4) of the code:
+ same probability of exceeding, in a period of $T_L$ other than $T_{LR}$=50 years: $\gamma_I\approx(T_{LR}/T_L)^{(-1/k)}$
+ has a probability of exceeding $P_L$ other than $P_{LR}$=10%, in the same period of 50 years: $\gamma_I\approx(P_L/P_{LR})^{(-1/k)}$
usually k=3.
🎁attached is the file to calculate the ground acceleration conversion
– In addition to lengthening or shortening the earthquake recurrence period compared to the conventional recurrence period, the importance factor γI greater than 1.0 or less than 1.0 also has significance for structures with greater or lower importance than the standard.
TCVN divides 5 levels of importance for houses and structures, according to:
+ the consequences of collapse on human life,
+ their importance for public safety, for civil protection immediately after an earthquake
+ the socio-economic consequences caused by the collapse.
The importance levels corresponding to the importance factor and the equivalent recurrence period are as follows:
|
Level of importance |
Buildings |
Importance factor $\gamma_I$ |
Return period (years)
|
|
Special |
Buildings over 60 floors,
Nuclear power plants, dams >100m high, Towers >300m high …
|
calculated with the largest possible acceleration (*) |
|
|
I |
Buildings 20-60 stories high, Towers 200-300m high,
Public works with a high population of 20-29 stories high or total floor area of 10,000-15,000m2,
Large capacity hydroelectric and thermal power plants,
Warehouses, pipelines containing flammable and explosive substances,…
|
1,25 |
927 |
|
II |
9-19 storey buildings, 100-200m high towers,
Public works 9-19 storeys or TDTS 5,000-10,000m2,
Administrative headquarters, provincial agencies,
Train stations, bus stations, post offices with spans over 75m or TDTS over 10,000m2,
Factory, traffic works, tunnels, … large level I, II
National defense and security works, …
|
1,0 |
475 |
|
III |
4-8 storey buildings, 50-100m high towers,
Public buildings, smaller factories, level III,
Walls >10m high, …
|
0,75 |
200 |
|
IV |
Buildings ≤3 floors high, 1-storey livestock farms, warehouses, small factories, etc.,
buildings that are damaged by earthquakes cause little damage to people and property
|
no seismic calculation required |
|
See appendix E, F of the standard for construction description corresponding to the levels of importance.
📖Strong or weak earthquake according to ground acceleration?
According to 3.2.1 of TC, earthquake cases are classified according to the design ground acceleration $a_g$ on type A ground:
– Weak: $a_g\le0,08g$=0.78m/s2, design for reduced seismic resistance
– Very weak: $a_g\le0,04g$=0.39m/s2, no need for seismic design
– $a_g>0,08g$: common case in Vietnam, need to calculate and construct seismic resistance
📖Reference correlation between types of earthquake magnitude:
|
MSK – 64 scale |
Richter scale |
Ground peak acceleration $a_{gR}$ (g) |
|
I – II |
2,5 |
|
|
III – IV |
3,5 |
|
|
V |
4,9 |
0,012 – 0,03 |
|
VI |
5,0 |
> 0,03 – 0,06 |
|
VII |
5,9 |
> 0,06 – 0,12 |
|
VIII |
6,0 – 6,8 |
> 0,12 -0,24 |
|
IX |
6,9 – 7,6 |
> 0,24 – 0,48 |
|
X |
7,6 – 8,0 |
> 0,48 |
|
XI – XII |
> 8,0 |
|
g: gravitational acceleration g= 9.81m/s2
💎Meaning applied to some large cities, with many high-rise buildings, how large an earthquake must they withstand?
– Hanoi: peak ground acceleration 0.0747-0.1081g; level VII; 5 – 5.9 on the Richter scale
– Ho Chi Minh City: peak ground acceleration 0.0700-0.0856g; level VII; 5 – 5.9 on the Richter scale
Binh Chanh district is lighter, $a_{gR}$= 0.0589g; level VI; 5.0 on the Richter scale
– Hai Phong: the inner city and districts of An Duong, An Lao, Thuy Nguyen have a fairly large peak ground acceleration of 0.1276-0.1334g; up to level VIII; 6.0-6.8 Richter scale
some island districts have very weak earthquakes, such as Bach Long Vi, $a_{gR}$=0.0063g;
– Da Nang: peak ground acceleration 0.0674-0.1006g; level VII; 5 – 5.9 Richter scale
especially Hoang Sa island district, $a_{gR}$= 0.0544g, level VI, 5.0 Richter scale
– Phu Quoc: peak ground acceleration 0.004g; very weak earthquake, no need for anti-seismic design🥳
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The sequel is about the earthquake acting on the construction and its reaction.
References:
- TCVN 9386: 2012 “Siesmic design for Buildings”
- Theoretical basis for calculating earthquake resistant structures – Prof. Dr. Nguyen Le Ninh – Hanoi Science and Technology Publishing House – 2011
- Earthquake scales
- Earthquake Hazards 201 – Technical Q&A
