REASONS TO GENERATE LIQUEFACTION / SAND BOIL AND ITS RELATIONSHIP WITH EARTHQUAKE AND FAULTS
Taiwan is located on the convergent boundary between the Philippine Sea Plate and the Eurasia Plate at the junction of the Manila and Ryukyu trenches (look at the fig. 1). From Taiwan to the northern Philippines, the rapid northwest motion of the Philippine Sea Plate, approximately 80 mm yr−1, is accommodated by the eastward subduction of the South China sea along the Manila trench. This subduction underthrusts the Chinese margin beneath Taiwan Island and leads to the collision with the Luzon volcanic arc, the suture zone being located along the longitudinal valley fault (LVF), east of Taiwan (Anne Loevenbruck, 2004).
Fig. 1 Geodynamic setting of Taiwan. LVF: longitudinal valley fault.
Therefore, Taiwan has many experiences of earthquake. And almost earthquakes are occurred in the faults trench or major faults which are located at the subduction (the edge of a plate of the earth’s crust), even they occur in the deep of ocean or in the depth of the earth which we’re on. The earthquake usually we don’t conscious the tremor when it is below than ~5 Mw, instead above it earthquake become dangerous, and indeed become devastating. Some faults have not shown these signs and we will not know they are there until they produce a large earthquake. When an earthquake occurs on one of these faults, the rock on one side of the fault slips with respect to the other. Faults can be centimeters to thousands of kilometers of length. The fault surface can be vertical, horizontal, or at some angle to the surface of the earth. Faults can extend deep into the earth and may or may not extend up to the earth’s surface. Surface features that have been broken and offset by the movement of faults are used to determine how fast the faults move and thus how often earthquakes are likely to occur (earthquakecountry, 2016).
A fault is a surface or narrow zone in the Earth’s crust along which one side has moved relative to the other in a direction parallel to the surface or zone (Robert J. Twiss, 2006). Therefore, we often find the earthquake caused by the meeting and shearing between the crusts, so called faults. Faults can often be recognized by the characteristic textures and structures that develop in rocks as a result of shearing (Robert J. Twiss, 2006).
Faults are structural features of importance think on the Earth’s surface and in its interior. They affect blocks of the Earth’s crust thousands or millions of square kilometers in area, and they include major plate boundaries hundreds or even thousands of kilometers long (Robert J. Twiss, 2006). We usually simulate that faults consist of two blocks land-crust which collision of each other with the various direction and shearing. The two blocks are called hanging wall and foot wall that each has difference position. For the hanging wall is above the fault and the foot wall is under the fault. In other word could be explained that for the hanging wall has a fault at the bottom surface, above the block and for the foot wall has a fault upper the surface, the lower block. But, for vertical fault we can’t use this theory to distinguish the hanging or foot wall. According to (Robert J. Twiss, 2006) we also divide faults into three categories depending on the orientation of the relative displacement, or slip, which is the net distance and direction that the hanging wall block has moved with respect to the footwall block (Figure 2). Dip-slip describes as the parallel movement of the fault surface, on the Strike-slip describes that the slip moves horizontally and parallelly, the last is Oblique-slip describes that the fault move obliquely or by simplify explanation is the vector or the horizontal and vertical movement. For the detailed information about the faults. I attached the image below about how does fault could be occurred.
Figure 2. Slip vector and slip component on fault surface
During the earthquakes the shaking of ground may cause a loss of strength of stiffness that results in the settlement of buildings, landslides, failure of earth dams or other hazards. As the result of large vibrations, the soil mixed with water-underground so behaves like a liquid. So, the sand experience under pressure and inability to keep its strength to hold back the building above, and the mud turn up through the gap. Why? Because in an earthquake, however, there is not enough time for the water-underneath to flow through the gap of pores (of soil). Instead, the water is “trapped” and prevents the soil particles from moving closer together. This is accompanied by an increase in water pressure which reduces the contact forces between the individual soil particles, thereby softening and weakening the soil deposit.
To understand how liquefaction happens, it is important to recognize the conditions that exist in a soil deposit before an earthquake. A soil deposit consists of an assemblage of individual soil particles. If we look closely at these particles, we can see that each particle is in contact with a number of neighboring particles (Fig 3 left side). The weight of the overlying soil particles produce contact forces between the particles, these forces hold individual particles in place and give the soil its strength.
Fig 3 (left). Loss of the resistance in a sediment due to liquefaction
Fig 4 (right). Diagram of the liquefaction phenomenon induced by earthquake
But, during or after earthquake, (Elena Candigliota, 2012) explained that a cyclic load applied to a saturated deposit can cause, for each cycle, an increase in pressure of water filling the pores between soil grains; if the water has not time to flow out before the next cycle, the hydraulic pressure can rapidly increase up to exceed the contact stress between grains (Fig. 4) with the consequent loss of shear strength. In this case, the layer couldn’t bear the weight on the surface and make it flow up or other direction like a liquid.
Here, liquefaction becomes the important think that has to be studied more, because it impacts to the urban area and this makes some building tilting or overbalance. Soil liquefaction always be the attention for researchers because it has caused many devastating of the urban areas.
- YUANLIN BY CHI-CHI EARTHQUAKE
Yuanlin is the one of many areas affected by the chi-chi earthquake, and Yuanlin is the worst damaged areas by liquefaction. It is a town which is located about 15 km from chelengpu fault rupture. Yuanlin is located on the alluvial deposit, which overlies older sedimentary deposits. According to (Hung, 2012) based on the survey, the ground water table is very swallow about 0.5 to 4 meters underneath to the surface, and there exists layers of very loose, sandy soils that are susceptible to liquefaction. Fortunately, these liquefiable sandy layers are often capped by a thick layer of clayey soils, which reduces the damages to buildings, said (Hung, 2012). You can see the figure 5 beside that describes simplified soil at Yuanlin.
Figure 5. Simplified Soil Profile at Yuanlin
In Yuanlin, many spot were found to have damages, including 17 were found to have suffered significant settlement and 8 were found to have evident liquefaction signs in the form of sand boils. In addition, (Hung, 2012) declares that there are ground motions were recorded in the Yuanlin area with horizontal peak ground acceleration (PGA) of approximately 0.19g. The effects of liquefaction caused by Chi-chi earthquake are surface sand boils, deformed roadways, and severe building settlement / tilting. Below a geologic data from some holes observed are indicated in Figure 6.
Fig. 6 Geological profil for Yualin area
Note that in Figure 6, the liquefiable layer is indicated by the dotted line drawn across the borehole logs.
Base on the image below: base on the research conducted by (Hung, 2012), they committed observation in Yuanlin area in order to zone were potentially vulnerable to liquefaction. Hung committed the survey with many results, but here I will pic up the one sample that indicates the probability of liquefaction. Using CPT data, we could know the liquefaction area with respect to the depth as the figure 7 below:
Figure 7. Profile of CPT data from an area west of downtown Yuanlin
Based on the graphic above, the vertical is indicating the depth (meter), and the horizontal is liquefaction value. So, we could know that, the soil vulnerable of liquefaction is from 2-4 meter, 5-8 meter depth and so on. And I will give the sample of the effect of liquefactions in Yuanlin area fig. 8 as follow:
Fig 8. Liquefaction-Induced Bearing Capacity Failure of buldings
- TAINAN BY MEINONG EARTHQUAKE 2016
Here in Tainan 6th February 2016, the same case of liquefactions are occurred, but different earthquake source. According to (USGS, 2016) earthquake east of Tainan in southern Taiwan occurred as the result of oblique thrust faulting at shallow-mid crustal depths (~ 20 km). We come at the sites with see several buildings tilting and many cracks, but the buildings keep stand and sturdiness. Below on figure 9 I took the pictures:
Figure 9. The tilting building caused by liquefaction. Crack at the path, and we also could see the puddle of mud here in the building.
The path move-up and the building move-down during/after earthquake, because the buildings have heavier than the path. Therefore, the ground couldn’t withstand the load of the building.
Figure 10. Here the liquefactions are happened, in the difference location at Tainan area. I saw the puddle of mud at the surface. It is looked more stable than the urban area, because perhaps the pressure on the surface almost the same at all. Therefore, we almost couldn’t see the vertical movement or deformation. For the accurate measurement we could also observe this location using GPS geodetic.
Then, from this study I infer some points which are:
- Before earthquake, the faults are the important thinks that have to be understood. Because the position or the direction of the fault could cause how large the scale of earthquake that would be happened.
- The liquefaction could be occurred depend on both the appearance of the soil structure and the dimension or scale of earthquake loadings usually greater than 5.5 Mw.
- The liquefaction could be also determined how the deep and thickness of potentially liquefiable deposit, about less than 15 – 20 meter from the ground surface, deep water table less than 5 meter, relative density, average diameter and contents in fine fraction of fill material.
- And to prevent the building from liquefaction become devastating or tilting, we should also consider that the building should have multiple basement levels or which have less than three stories above the ground.
I assume that the cause of the liquefaction in the part of Tainan area is the same sand/soil structural of the part yuanlin area. Both have groundwater several meter beneath the surface, which are vulnerable to experience liquefaction.
Brief explanation of Large M7.9 earthquake strikes offshore Sumatra
Based on the report from (earthobservatory, 2016) on the evening of March 2, 2016, a magnitude-7.9 earthquake struck off the west coast of Sumatra, Indonesia. According to the United States Geological Survey (USGS), the epicenter of the quake was approximately 800km from Padang, West Sumatra. At that time, Tsunami was issued but after several hours of the tremor, Tsunami noticing was canceled.
Based on the information, this earthquake is difference than the last decade on December 2014 Tsunami but very much like to April 2012 earthquake. Because on that earthquake, the two great tectonic plate was generated under the Indian Ocean. According to (earthobservatory, 2016) the earthquake was caused by a strike-slip rupture, during which the oceanic blocks moved horizontally with respect to one another. Since horizontal motions don’t cause large uplifts of the seafloor, no large tsunami was generated, according to Earth Observatory seismologist Assistant Professor Wei Shengji.
Figure 11. Map view of the historical earthquakes in the region and the location of recent earthquake. Inset: the source time function showing the time evolution of released moment rate (earthobservatory, 2016).
For the examples of liquefaction, I attached the link below :
- https://www.youtube.com/watch?v=qmVYbjiNWds the animation of liquefaction and explanation
- https://www.youtube.com/watch?v=2_0YvJsqw9w the real recorded of liquefaction.
- https://www.youtube.com/watch?v=2vCULflCPmw the simulation in the laboratory of liquefaction.
Anne Loevenbruck, R. C. (2004). Coseismic slip resolution and post-seismic relaxation time of the 1999 Chi-Chi, Taiwan, earthquake as constrained by geological observations, geodetic measurements and seismicity. Geophysics Journal International, 17.
Chih-Sheng Ku, D.-H. L.-H. (2004). Evaluation of soil liquefaction in the Chi-Chi, Taiwan earthquake using CPT. ELSEIVER (pp. 659-673). Taiwan: Soil Dynamics and Earthquake Engineering.
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earthquakecountry. (2016, 03 14). Retrieved from http://www.earthquakecountry.org/: http://www.earthquakecountry.org/roots/basics.html
Elena Candigliota, F. I. (2012). Sand liquefaction phenomena induced by the May 2012 Emilia Romagna Earthquake: geomorphological features and relations with the territory and building stability. Energia, Ambiente e Innovazione, 10.
Hung, V. (2012). A Study of the Effects of Liquefaction as a Result of the 1999 Chi-Chi Earthquake. Department of Civil Engineering, Massachusetts Institute of Technology, 19.
Iwasaki, T. (2003). Soil liquefaction studies in Japan: state-of-the-art. Soil Dynamics and Earthquake Engineering, 64.
Robert J. Twiss, E. M. (2006). Structural Geology (Second Edition). New York: W. H. Freeman and Company.
USGS. (2016, 02 6). Retrieved from http://earthquake.usgs.gov/: http://earthquake.usgs.gov/earthquakes/eventpage/us20004y6h#general_region
Yushan National Park. (2016, 03 12). Retrieved from http://www.ysnp.gov.tw/: http://www.ysnp.gov.tw/en/recourses/view_overview.aspx?cid=2