The 2010 Mw 6.9 Yushu earthquake produced a ~33-km-long co-seismic surface rupture zone along the pre-existing active Yushu Fault on China’s central Tibetan Plateau. Sand boils occurred along the tension cracks of the co-seismic surface rupture zone, and locally spouted up above the ground to coat the top of limestone blocks that had slid down from an adjacent ~300-m-high mountain slope. Based on our observations, the relations between the arrival times of P- and S-waves at the sand-boil location and the seismic rupture velocity, we conclude that 1) the sand boils occurred at least 18.24 s after the main shock; 2) it took at least 4.09 - 9.79 s after the formation of co-seismic surface rupture to generate liquefaction at the sand-boil location; 3) the spouting height of sand boils was at least 65 cm. Our findings help to clarify the relationships between the timing of lique-faction and the spouting height of sand boils during a large-magnitude earthquake.
The Mw 6.9 Yushu earthquake occurred on 14 April 2010 in Qinghai Province of the central Tibetan Plateau,
China (
[
Liquefaction caused by large-magnitude earthquakes has been responsible for a tremendous amount of damage to buildings and infrastructure, including roads [
The study area is located in a high mountain region on the central Tibetan Plateau, with an average elevation of ~4000 m, along the strike-slip Ganzi-Yushu Fault Zone (
Straight valleys trending WNW-ESE mark the position of the Yushu Fault, and the topography is characterized by sags and saddles, fault depressions, systematically offset gullies, river channels, and ridges (
The 2010 Mw 6.9 Yushu earthquake produced a 33-km-long co-seismic surface rupture zone characterized by tension cracks, mole track structures, and discrete shear faults, most of which developed in unconsolidated alluvial and fluvial deposits along the pre-existing left-lateral strike-slip Yushu Fault [
Sand boils caused by the 2010 Yushu earthquake occurred mainly in the lowland areas of the Changu Temple
and Buqionggei segments, where fluvial deposits are developed (
Grain sizes of the boiled sands at Loc. 2 were analyzed by a Master Sizer 2000 instrument, and the analytical results are shown in
As stated above, the sand boils observed at Locs. 1 - 3 are distributed within and along both sides of the tension cracks that form part of the co-seismic surface rupture zone, and they sometimes form circular sand volcanoes
(
The seismic inversion results reveal an S-wave velocity (Vs) of 3.4 km/s for the Yushu earthquake [
The rupture velocity is generally less than that of the P- and S-waves [
where L in the length of the slope (274 m), g is acceleration due to gravity (9.8 m/s2), a is the sliding velocity of stone block along the slope (sin31˚ g), and t is the time taken for the stone block to fall down the mountain to the sand-boil location (10.42 s), assuming the friction resistance of the slope is zero. If we consider the collapse of the stone blocks to have been caused by S-wave shaking, the time taken for the stones to arrive would be 23.94 s after the main shock (which equals the arrival time of the S-wave, plus the time taken for the stone block to drop down to the sand-boil location), 9.79 s after the formation of co-seismic surface rupture in this location.
If the collapse of stone blocks had been caused by P-wave shaking, then the arrival time of stone blocks at the sand-boil location would have been 18.24 s after the main shock (which equals the arrival time of the P-wave, plus the time taken for the stone block to drop down to the sand-boil location), 4.09 s after the formation of co- seismic surface rupture in this location.
Liquefaction is a physical process that occurs during large-magnitude earthquakes, and it may lead to a reduction in strength and stiffness of a saturated granular sand layer. Our findings indicate that the time required to generate liquefaction, subsequent to the formation of co-seismic surface rupture during the Yushu earthquake, was at least 4.07 - 9.77 s, at which time the strength of the sediment was reduced and ground failure started to develop.
Boiled sands observed on the ground surface were extruded from water-saturated soil-sand layers in which the
shear strength approached zero. The extrusion of the boiling soils and sands indicates that high hydro-pressures must have existed in these water-saturated soil-sand layers. If the high pressure of the water-saturated sand had been released rapidly as a result of fluid extrusion, the boiling sand deposits would have been squirted upwards to a certain height above the ground, similar to a volcanic eruption. In fact, water-sand mixtures were reported to have reached heights in excess of 0.9 m above the ground during the 2000 Ms 7.2 Tottori earthquake, western Japan [
Based on our observations of liquefaction caused by the 2010 Mw 6.9 Yushu earthquake, and using the data presented above, we arrive at the following conclusions:
1) During the earthquake, the time at which ground strength was reduced to the point of failure, causing liquefaction, was at least 4.09 - 9.79 s after the formation of co-seismic surface ruptures at the sand-boil location. The time scale is in consideration of possibility of either P-wave shaking or S-wave shaking induced liquefaction.
2) During liquefaction, the boiled sands were spouted up by high hydro-pressures to heights in excess of 65 cm. The height of boiled sand provides referable evidence for study of pure pressure status during liquefaction caused by earthquakes.
Our findings help to clarify the relationships between the timing of liquefaction and the spouting height of sand boils during a large-magnitude earthquake.
We thank Drs. T. Wan and X. Wu for assistance in the field. This work was supported by a Grant-in-Aid for Scientific Research (A) (No. 23253002 awarded to A. Lin) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.