People | Randy Richardson

Geodynamics/Science Education

My past research efforts, summarized below, in conjunction with my students have concentrated on the dynamics of plate tectonic processes, including the driving mechanism for plate tectonics; intraplate deformation; the magnitude of tectonic stresses; strain accumulation and release at plate boundaries; the origin and support of mountains; and the role of gravitational potential energy in tectonics. I am also interested in earthquake prediction and numerical modeling techniques, including inverse modeling and finite element analysis of tectonic problems. Some of these projects are listed below.

1. Dynamics of the Indo-Australian and South American Plates
    
   In conjunction with a former student (David Coblentz) and a colleague in Australia (Mike Sandiford), we completed finite element modeling and analysis of stresses in the Indo- Australian and South American plates. For the Indo-Australian plate, we conclude that ridge torques are focused by collisional boundary conditions in the Himalaya and Papua New Guinea areas. An important conclusion of our work is that the time-history of ridge versus collisional boundary lengths may account for profound changes in collisional tectonics in the plate over the last 50 million years, including Miocene extensional faulting in the Tibetan Plateau. I am very satisfied that we were able to publish two papers on the subject.
    
   For the South American plate, we saw the publication (in JGR) of an important study limiting far-field intraplate stress magnitudes to about 25 MPa based on modeling of topographic effects in the Andes. We worked on a plate-wide finite element stress model for the South American plate that showed the important role of ridge torques and the modifying influence of other topographic effects. This work was published in JGR in 1996.
    
2. Modeling Failed Continental Rifts (Amazonas Rift in Brazil and the New Madrid Seismic Zone)
    
   In conjunction with Mary Lou Zoback, I completed work on a general study of failed rifts and the role that buried high-density material (called the rift pillow) associated with mantle injection into the lower crust plays in intraplate seismicity. An important element of this study was the recognition that we must consider not only the rotation of principal stresses, but the magnitude of resolved shear stress on pre-existing faults in these regions. With now former student Jeff Grana, we completed a time-dependent finite element study of the New Madrid Seismic Zone. This modeling represented a significant extension of our previous elastic modeling of failed rift basins, and indicates that the buried rift may be an important component of the source of stress in the region. These efforts with both Zoback and Grana resulted in companion papers published in JGR in 1996.
    
3. Testing Earthquake Prediction Models
    
   I have worked on testing the seismic gap hypothesis, and extended the effort into the general area of the statistical analysis of any earthquake prediction model, especially the fundamental role that the null hypothesis (against which any prediction is compared) plays in the testing. This research effort was recognized by an invited presentation at the fall 1995 AGU meeting.
    
4. Statistical Analysis of the Intraplate Stress Field for Trends
    
   The World Stress Map project resulted in the compilation of over 9000 intraplate stress indicators. Previous analysis of the data set has been primarily restricted to visual inspection in regions of interest. Together with former student David Coblentz, we applied a global statistical analysis to the entire data set and found that the data are dominantly strike-slip to compressional away from mid-ocean ridges and regions of high topography, which are dominantly normal to strike-slip. Also, over 50% of all data binned into 5 x 5 degree boxes have orientations for the maximum horizontal compressive stress consistent with a ridge-torque origin. This work resulted in a paper published in to JGR in 1995.
    
5. Global Potential Energy Studies
    
   Although gravitational potential energy differences play a fundamental role in plate tectonics, only recently has anyone attempted a global study based on topography and crustal structure. That effort, with former student David Coblentz and Australian colleague Mike Sandiford, resulted in a significant publication (at least judged by reprint requests!) in Tectonics in 1994. We demonstrated that the longest-recognized gravitational potential energy difference, the ridge-torque, is significant compared to other sources, such as elevated continental topography and continental margins.