Chen Ji, Ph.D - US grants

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)

The 12 May 2008 Mw 7.9 Wenchuan, China earthquake is the largest intraplate earthquake recorded by modern geophysical observations. It ruptured unilaterally about 250 km beneath the predominantly northeast-trending Longmen Shan, a steep high mountain range that bounds the Tibetan Plateau to the northwest and the Sichuan Basin to the southeast. The strong ground shaking caused over 5 million buildings to collapse and nearly 70,000 fatalities. The occurrence of this major earthquake surprised most of the geoscience community because the source region historically lacks large earthquakes and has a very low slip rate based on geological and geodetic data. This catastrophic earthquake and its aftershock sequence will be investigated with seismic, geodetic and geological observations. The rupture process will be constrained to match the tectonic background and surface observations, and at the same time be consistent with rupture dynamics that includes a constitutive friction law. Quantitative understanding of such giant intraplate earthquakes and their aftershocks can contribute to hazard analysis that will ultimately be useful for guiding societal response for the future events. There are many areas in United States with very low strain rates, e.g., New Madrid, Charleston, where large magnitude events have occurred. This research provides an excellent opportunity to take a large step forward in understanding the character and processes of intraplate earthquakes in areas of low strain rates.


This is an unusual complex earthquake sequence involving multiple faults beneath steep topography, but with abnormally few large aftershocks. The approach is to use the newest nonlinear methods for inverting the seismic and geodetic data to determine a kinematic model. Critical to the analysis will be an all out effort to assess the uncertainties in the kinematic model and their relation to uncertainties in earth structure, data distribution and the type of data being inverted. This kinematic model will serve as the baseline for full dynamic modeling in 3D that will involve multiple fault planes, topography and 3D structure. This research addresses the following questions: i) Are the rupture characteristics of this event different from other well studied thrust earthquakes, such as 1999 Chi-Chi earthquake, perhaps reflecting its abnormal tectonic loading process? ii) How did the thrust rupture initiate? What effect did the nucleation have on the subsequent rupture? iii) How did the ruptures on two faults dynamically interact? iv) What are the characteristics of the ground motion excited by this earthquake? Are the ground motions unusual compared with other large thrust events?

Because the process of an earthquake cannot be directly observed, we must infer the process by determining source parameters that describe the relative motion between the two sides of the fault, i.e., the kinematics of the rupture. At each point on the fault the source parameters are: 1) the slip-rate time function, characterized by a functional form and generally specified with a few parameters, 2) the rupture time?a time relative to the origin time of the earthquake at which the slip rate function starts and 3) the final slip, a vector with one component along strike and the other up-dip (i.e., perpendicular to the strike component), that gives the relative displacement of one side of the fault with respect to the other. The investigators assume a priori the velocity and attenuation structure of the medium in which the fault is embedded as well as the geometry of the fault and the hypocenter of the earthquake. With this information we can compute the ground motion at any point in space, in particular, at the locations of the seismic and geodetic instruments that recorded the earthquake. To find the source parameters they will solve an inversion problem in which they continuously adjust the source parameters. With each adjustment they will compute ground motion time histories (synthetics) at the recording sites and compare the synthetic time histories with the recorded time histories. At the same time they compute the static displacements for comparison with the geodetic measurements, GPS or InSAR. Using a predetermined cost function that measures the misfit between data and synthetics, we continue the procedure of adjusting the source parameters until we have an acceptable level of misfit.

Until relatively recently the emphasis has been on reducing the misfit between the data and the synthetics. However, that is now being recognized as not the issue because any number of different kinematic descriptions can fit the data. The number of free parameters (i.e., source parameters) is often far greater than the number of independent data making it possible to find nearly perfect agreement between synthetics and data. This raises two questions to be addressed in the proposal: 1) Is there additional data, not yet used, to constrain the source parameters? 2) Rather than measure the misfit between synthetics and data, how can we determine the difference between models? That is, how can we decide if one source description is better than another?



An earthquake is one of the most fundamental phenomena in the natural world. How two massive blocks of earth move past one another in a matter of seconds, tens of seconds, perhaps a few hundred seconds for the very largest earthquakes (e.g., 2004 Sumatra; 2010 Maule, Chile; 2011 Tohoku, Japan) releasing strain energy accumulated over hundreds or thousands of years is a basic scientific question. It is also a societally relevant question. The shaking and subsidiary effects (e.g., tsunamis, landslides) created by an earthquake critically affect the built environment. In order to prepare for future earthquakes it is essential to understand the process of earthquake ruptures. By using the data from an earthquake we can infer some of its behavior. By studying many different earthquakes we can understand what features are common among earthquakes. We can also understand what features of an earthquake contribute most to the strongest shaking and to the subsidiary effects.

Because we cannot directly observe an earthquake, we invert the seismic, geodetic and geologic data created by the earthquake to infer the relative motion between the two blocks of earth. The kinematic source parameters describe how the rupture evolves in time and space, i.e., the time when a point on the fault starts to slip (controlled by the rupture velocity) and the growth of the slip at each point on the fault (controlled by the stress relaxation of the medium). Conceptually the inverting the data is similar to finding the parameters (slope and intercept) of the best fitting line to a group of points. The difficulty with inverting for the source parameters of an earthquake is that the number of parameters being sought exceeds the number of independent data. Thus multiple, best-fitting earthquake models may be found. The problem is assessing how different one model is with respect to other models that fit the data equally well and what features different models have in common. In both respects the information can guide simulations of earthquakes for estimating the range of ground motions from future earthquakes.