Analyzing data from multiple measurement stations, scientists at the Georgia Institute of Technology found that the quake weakened subsurface materials by as much as 70 percent. That nonlinear response from the top layer of the Earth’s crust affected how the movement of faults deep beneath the surface was delivered to buildings, bridges and other structures.
Understanding how the soil responds to powerful earthquakes could be important to engineers and architects designing future buildings to withstand the level of acceleration measured in this quake. The information will also help seismologists develop new models to predict the effects of these rare and extremely powerful events.
“The degree of nonlinearity in the soil strength was among the largest ever observed,” said Zhigang Peng, an associate professor in Georgia Tech’s School of Earth and Atmospheric Sciences. “This is perhaps not too surprising because the ground shaking generated by this earthquake – acceleration as much as three times the Earth’s gravity – is also among the highest ever observed.”
The findings were reported in a special issue of the journal Earth, Planets and Space (EPS). The research was sponsored the National Science Foundation (NSF) and by the Southern California Earthquake Center (SCEC).
Peng and graduate student Chunquan Wu were among the first scientists to examine data recorded by the high-quality seismometers that are part of the Japanese Strong Motion Network KIK-Net. The stations have accelerometers both on the surface and in boreholes located on bedrock far beneath it. The researchers chose to study data from six stations that have strong velocity contrasts between the surface soil layers and the underlying bedrock.
“In this study, we were trying to understand the relationship between soil nonlinearity and peak ground acceleration (PGA), which is a measure the ground shaking,” said Wu. “We want to understand what parameters control this kind of response.”
By comparing data on the acceleration of motion from sensors on the bedrock to comparable information from surface sensors, they were able to study how the properties of the soil changed in response to the shaking. The researchers computed the spectral ratios of each pair of station measurements, and then used the ratios to track the temporal changes in the soil response at various sites at different levels of peak ground acceleration.
“The shear modulus of the soil was reduced as much as 70 percent during the strongest shaking,” Wu explained. “Typically, near the surface you have soil and several layers of sedimentary rock. Below that, you have bedrock, which is much harder than the surface material. When seismic waves propagate, the top layers of soil can amplify them.”
Nonlinear response from soils is not unusual, though it varies depending on their composition. Similar but smaller effects have been seen in other earthquake-prone areas such as California and Turkey, Wu said. The shallow layers of the Earth’s upper crust can be complex, composed of varying types of soil, clay particles, gravel and larger rock layered in sediments.
Because the March 11 quake lasted an unusually long time and generated a wide range of ground motions of greatly varying strengths, it provided an unprecedented data set to scientists interested in studying nonlinear soil behavior.
Beyond the immediate effect of the strongest shock, the researchers were interested in how the soils recover their strength after the shaking stops. That recovery time can vary from fractions of a second to several years, Wu said.
“It is still not clear whether there could be longer recovery times at certain sites,” Wu noted. “This is a function of soil type and other factors.”
If the soils are very porous, water can lengthen the recovery. “For porous media, the ground shaking could cause water to go into the pores, which will also reduce the shear modulus of the soil. If water is involved, the recovery time will be much longer.”
Soil response to aftershocks, which ranged up to magnitude 7.9 after the main Tohoku earthquake, was also studied.
Information developed by the Georgia Tech researchers will be provided to seismologists developing new hazard models of very powerful earthquakes. Knowing how soils respond to strong shaking is also important to predicting how motion deep within the Earth will be translated to structures built on the surface.
“Understanding how soil loses and regains its strength during and after large earthquakes is crucial for better understanding and predicting strong ground motions,” Peng noted. “This, in turn, would help earthquake engineers to improve the design of buildings and foundations, and could ultimately help to protect people in future earthquakes.”Research News & Publications Office
Writer: John Toon
John Toon | Newswise Science News
Multi-year submarine-canyon study challenges textbook theories about turbidity currents
12.12.2017 | Monterey Bay Aquarium Research Institute
How do megacities impact coastal seas? Searching for evidence in Chinese marginal seas
11.12.2017 | Leibniz-Institut für Ostseeforschung Warnemünde
DNA molecules that follow specific instructions could offer more precise molecular control of synthetic chemical systems, a discovery that opens the door for engineers to create molecular machines with new and complex behaviors.
Researchers have created chemical amplifiers and a chemical oscillator using a systematic method that has the potential to embed sophisticated circuit...
MPQ scientists achieve long storage times for photonic quantum bits which break the lower bound for direct teleportation in a global quantum network.
Concerning the development of quantum memories for the realization of global quantum networks, scientists of the Quantum Dynamics Division led by Professor...
Researchers have developed a water cloaking concept based on electromagnetic forces that could eliminate an object's wake, greatly reducing its drag while...
Tiny pores at a cell's entryway act as miniature bouncers, letting in some electrically charged atoms--ions--but blocking others. Operating as exquisitely sensitive filters, these "ion channels" play a critical role in biological functions such as muscle contraction and the firing of brain cells.
To rapidly transport the right ions through the cell membrane, the tiny channels rely on a complex interplay between the ions and surrounding molecules,...
The miniaturization of the current technology of storage media is hindered by fundamental limits of quantum mechanics. A new approach consists in using so-called spin-crossover molecules as the smallest possible storage unit. Similar to normal hard drives, these special molecules can save information via their magnetic state. A research team from Kiel University has now managed to successfully place a new class of spin-crossover molecules onto a surface and to improve the molecule’s storage capacity. The storage density of conventional hard drives could therefore theoretically be increased by more than one hundred fold. The study has been published in the scientific journal Nano Letters.
Over the past few years, the building blocks of storage media have gotten ever smaller. But further miniaturization of the current technology is hindered by...
11.12.2017 | Event News
08.12.2017 | Event News
07.12.2017 | Event News
15.12.2017 | Life Sciences
15.12.2017 | Life Sciences
15.12.2017 | Physics and Astronomy