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Towards Understanding Earthquakes

The analysis of earthquakes that occurred helps to understand why and how the earthquakes happened, and perhaps, even helps to predict where and when a new earthquake may occur. We present a comprehensive study on several recent earthquakes in China by a group of Chinese researchers using ADINA.

This study focused on China’s southwest region, shown in the map above. The region is comprised of several tectonic plates that are known to be very active. Figure 1 shows the plates and the faults.





Figure 1  Tectonic plates and faults in Southwest China



The objective of the study was to establish the effect of an earthquake on the stresses in the faults. Figure 2 is the flow chart of the simulation procedure in the study.






Figure 2  Flow chart of the simulation procedure


The researchers first mapped the plates and faults using a geological pre- and post-processor called GID (see Figure 3). Next, they created the 3D model and assigned the material properties based on the geological survey data.

Figure 4 shows the terrain of the region. Figure 5 gives the depth variation of the Moho surface (the Moho surface is the surface that separates the crust and the mantle). Shown in Figure 6 is a partial section of the 3D model in the vertical direction. Note that there are layers in the vertical direction. The thickness of the layers varies from 0.5 km to 100 km. Figure 7 shows the 3D model that was imported into ADINA for further pre-processing, such as applying loads, boundary conditions and initial conditions.






Figure 3  2D mapping of the tectonic plates and faults; plates shown in different colors






Figure 4  Relief map showing the terrain of the region studied






Figure 5  The Moho surface of the region studied






Figure 6  Partial section of the 3D model in vertical direction; the layers have different material properties






Figure 7  3D model of the region; different colors indicate different materials




The loading, initial conditions and boundary conditions were based on generally accepted assumptions and GPS survey data, and were applied through the ADINA User Interface. Figure 8 gives the GPS data of the velocity field of the region. Figure 9 shows the final ADINA finite element model.






Figure 8  GPS data for the velocity field of the region studied






Figure 9  The ADINA finite element model




Based on this model, the researchers studied the earthquakes with ML ≥ 7.0 on the Richter scale of the last two decades in this area, see Figure 10.






Figure 10  Earthquakes with ML ≥ 7.0 in the region since 1996




In particular, they studied the three major earthquakes of the last decade, the 2001 Kunlun Shan Earthquake (ML = 8.1), the 2008 Wenchuan Earthquake (ML = 8.0) and the 2010 Yushu Earthquake (ML = 7.1). The simulated results are shown in Figures 11 and 12. These band plots were created based on ADINA results using an in-house postprocessor.






Figure 11  Band plot of the increase of the Coulomb failure stress from the Kunlun Shan earthquake (in 2001). The Coulomb failure stress in the Wenchuan fault is increased 0.001~0.002 MPa due to this earthquake. Such increase is believed to be a factor for the Wenchuan earthquake in 2008.






Figure 12  Band plot of the increase of the Coulomb failure stress from the Wenchuan Earthquake (in 2008). The Coulomb failure stress in the Yushu fault is increased 0.001~0.002 MPa due to this earthquake. Such increase is believed to be a factor for the Yushu earthquake in 2010.




It is extremely interesting to see that the earlier earthquakes gave stress increases in regions where, indeed, a later earthquake occurred. This knowledge regarding stress variations is valuable in possibly predicting where — and perhaps even roughly when — future earthquakes may occur.

The use of ADINA in this study shows how the program can be used efficiently with third-party software in new application areas: from structures at the nano-scale (e.g., DNA-based nanostuctures) to structures at the kilometer-scale considered in this Tech Brief. In each case ADINA is providing reliable, accurate and efficient solutions.

 


Keywords:
Earthquake, tectonic plate, Coulomb failure stress, Moho surface, geological survey, Richter scale


Courtesy of Dr. Pingen Li, China Earthquake Administration

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