Ignored danger can cause damage weeks after an earthquake

In the aftermath of the 1999 Chi Chi earthquake in Taiwan, buildings were found to be precariously balanced on a slope. New modeling shows how to better plan for such damage.

Written by Judith Hubbard, Ph.D. (JudithGeology) and Reshav Malik, Earth Observatory in Singapore

Quote: Hubbard, J. and Mallick, R., 2021, Overlooked danger can cause damage weeks after earthquake, Temblor, http://doi.org/10.32858/temblor.212

When the 7.6-magnitude Chi Chi earthquake struck Taiwan on September 21, 1999, the tremors extensively destroyed homes, schools, and dams, among other buildings.

The earthquake also highlighted an additional danger – the tilting of the Earth’s surface. Houses about 1-2 miles (1.6-3.3 km) from the fault, which were once built on horizontal ground, were found precariously balancing on a slope in the days following the earthquake (Chen et al., 2007).

A building located on a retractable ramp constructed either during or shortly after the 1999 magnitude 7.6 earthquake in Chi Chi, Taiwan. Credit: Yu Gao Chen. Image reproduced from Chen et al. (2007).

This tendency was caused by slipping along the underlying fault. Some faults are vertical, but in areas where tectonic plates collide, faults typically descend at an angle into the Earth’s crust. When the rocks on either side of the fault move past each other during an earthquake, the rocks above the fault lift and tilt.

The tilt is just a small symptom of a much larger process: the growth of folds and eventually mountains, which takes place over millions of years.

Folding can occur near defects

Geologists know about folding and tilting from the rock record, but they still don’t know when it happens. Does the Earth suddenly flex during an earthquake, in the hours to days following as the crust re-adapts? Or do these processes occur during long periods between major earthquakes? We know that the Chi Chi earthquake caused the tilt, but the observations were made days after the event. The tilt could have occurred at any time in the days following the earthquake.

We and our colleagues recently tried to answer this question.

Ideally, we want to watch the folding happen in real time to learn more about the process. Unfortunately, earthquakes are intermittent, and having instruments recording in exactly the right place at the right time when an earthquake strikes is highly unlikely.

The next best option is to simulate the process using a model. We first model the long-term folding over the fault that builds the predicted terrain over hundreds of thousands of years. From this model, we calculate the rate of deformation or tilt along and around the fault. We use this information to create a second simulation focused on earthquakes.

The exposed sedimentary rocks in the Grand Canyon have been tilted over millions of years. Credit: James St. John (CC BY-2.0)

Folding occurs during, after, and between earthquakes

Our calculations show that folding does not only occur during one part of the earthquake cycle. Some folding occurs during the earthquake, but the tilt persists within hours to days after the earthquake and slows over time (Fig. 3).

The folding pattern is controlled by the fault size and direction, and the mechanical properties of the rock layers. Large earthquakes impart so much pressure on the surrounding rock that folding must occur quickly; On the other hand, smaller earthquakes may lead to a slower folding process that occurs over years.

Continuing danger after the earthquake

Our study highlights an important and unknown earthquake hazard – the tilt of the Earth’s surface within miles of the main fault impact, which can persist long after an earthquake.

Our results show that in the event of a major earthquake, the tilt can be as much as one degree within a plot hundreds of meters wide, paralleling the fault for up to 60 miles (100 km). Awareness of this danger is important, as re-entry into buildings while tilting is ongoing, either for reoccupation or for rescue efforts, could be dangerous, particularly given that these buildings may have already been weakened by the initial earthquake. Our work provides important insights into this process that can guide earthquake disaster mitigation plans.

Possible tilt areas should be highlighted as danger areas. Scientists have documented such folds in some densely populated areas – such as the Los Angeles Basin. These folds have been used to estimate long-term slip rates on underlying defects (eg Shaw et al., 2002; Leon et al., 2007; Bergen et al., 2017). People in these areas should be aware that they face not only the risk of shaking, but also the risk of subsidence that can persist for months to years after the earthquake.

Buildings on a foldable ramp after the Chi Chi earthquake Credit: Yue Zhao Chen. From Chen et al. (2007).

How long do you worry?

With our results, we can estimate the time ranges and expected tilt magnitudes. For a great earthquake (magnitude greater than 7.0) that explodes all the way to the surface, nearly all of the tilt (up to about 1 degree for a relatively narrow flexural zone) will occur during the first day, and by the end of the month the tendency may slow to imperceptible levels.

Earthquakes of the same size that don’t rupture all the way to the surface are likely to cause a similar amount of tilt, but can take months or even years to disintegrate at a tiny rate. The time scale will depend on the direction and amount of slip along the fault in the earthquake and the rock in the area, so direct observation during the post-quake period will be important to determine when the area is stable.

Engineering assessments of structural stability due to ground tilt indicate that a tilt of less than one degree can lead to foundation collapse. For specialized structures such as satellite towers, values ​​as low as 0.01° can cause problems (Charles and Skinner, 2004). In the aftermath of an earthquake, a tilt of less than 1 degree is likely to be dangerous since buildings may be damaged by vibration.

In our models, the amount of total tilt depends on the width at which the folding will occur. If the inclined region is a mile (1.6 kilometers) wide, the slope may be less than 0.2 degrees. In contrast, if the inclined area is 300 feet (90 m) wide, the tilt may be several degrees. This may explain how the building in Taiwan in the first photo curved so much in the Chi Chi earthquake. Our models are unable to reproduce such narrow folded regions due to computational cost, so these estimates are expected from simulations that considered wider fold regions.

The width of the tilt area and the amount of tilt depends on how wide the primary “fold joint” is in the rocks below. As the hinge width decreases, the maximum tilt increases (blue circles), while the tilt area width decreases (orange diamonds). Credit: Malik et al., 2021

Accurate tilt predictions will require observing actual earthquakes and accurately mapping their deformation patterns. However, before an earthquake occurs, scientists can identify potential tilt areas near the faults. Constructing buildings that are flexible to tilt and making sure that buildings are only reoccupied after tilting is slowed to minimal levels can save lives.

references

Bergen, KJ, Shaw, JH, Leon, L.A., Dolan, JF, Pratt, T.L., Ponti, DJ, Morrow, E., Barrera, W., Rhodes, EJ, Murari, MK, Owen, L.A. (2017). Accelerating slip rates on the Puente Hills blind thrust fault system below Los Angeles, California. Geology, https://doi.org/10.1130/G38520.1.

Charles, J.A. and Skinner, HD (2004), Leveling and tilting of low-rise buildings. Geotechnical Engineering, 157(2), p. 65-75, https://doig.org/10.1680/geng.2004.157.2.65.

Chen, Y. G.; , Lai, K.-Y. Lee, Y.-H. , Suppe, J., Chen, W.-S. , Lin, Y.-NN, Wang, Y., Hung, J.-H., Kuo, Y.-T. (2007). Regression of cosmic folds and their kinematic behavior in the 1999 Chi-Chi earthquake in Taiwan. Journal of Geophysical Research, 112, B03S02, https://doi.org/10.1029/2006JB004388.

Hubbard, c. Barbo, S. Hill, M., Taboneer, B. (2015). Cosmic slippage on shallow agglomerations: implications for earthquake and tsunami hazards. Earth Science Reviews 141, p. 45-55, https://doi.org/10.1016/j.earscirev.2014.11.003.

Leon, L.A., Christofferson, SA, Dolan, JF, Shaw, JH, and Pratt, T., Earthquake fold growth by seismicity over a blind thrust fault at Puente Hills, Los Angeles, CA: implications for fold kinematics and seismic hazards, J. Geophysics. Res, Vol. 112, B03S03, doi: 10.1029/2006JB004461, 2007

Malik, R, Burgman, R, Johnson, K, Hubbard, J. (2021). A unified framework for earthquake sequencing and geostructural growth in folded thrust belts. Journal of Geophysical Research: Solid Earth, https://doi.org/10.1029/2021JB022045

Shaw, J. H., Plesch, A., Dolan, J. F., Pratt, T. L. (2002). Blind Propulsion System Puente Hills, Los Angeles, California. Bulletin of the American Seismological Society, 92 (8), 2946-2960, https://doi.org/10.1785/0120010291.

van Zelst, I., Wollherr, S., Gabriel, A.‐A, Madden, EH, & van Dinther, Y. (2019). Modeling mega-earthquakes across scales: a unidirectional coupling from geodynamics and seismic cycles to dynamic rupture. Journal of Geophysical Research: Solid Earth, 124, 11414-11446. https://doi.org/10.1029/2019JB017539

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