We develop a 3D parallel visco-elasto-plastic finite element model, which is aimed at simulating long-term slip on faults with arbitrary geometry. The main characteristic of the model is that it incorporates plasticity to simulate both slip on faults and plastic deformation in the out-of-fault crust. Other characteristics of the model include three dimension and parallel computation. All of these characteristics make the model a new powerful too to simulate long-term fault slip in a complex fault system.

Finite element model development

Mantle convection and Magma Production on Mars

Mars has remained volcanically active into the geologically recent past, as demonstrated both by low impact crater densities on some lava flows and by radiometric ages of the shergottite meteorites. This evidence for recent volcanism is an important constraint on the thermal structure of Mars. Adiabatic decompression melting in hot, upwelling mantle plumes rising from a thermal boundary layer at the base of the mantle provides a logical explanation for the roughly point-source nature of individual large volcanoes such as Olympus Mons or Ascraeus Mons.

 

We simulate mantle plumes on Mars using spherical axisymmetric geometry and a temperature-dependent, Arrhenius viscosity law. We test our models with geologic observations of the present-day volcanism rate, geochemical estimates of the melt fraction in the shergottite meteorites, and geophysical constraints on the surface and core-mantle boundary heat fluxes. We test the sensitivity of our models to the activation energy, thermal Rayleigh number, and the partitioning of radioactive heating between crust and mantle.

 

In the ongoing work, we’ll explore effects of water content in martian mantle and chemical convection. Furthermore, we’ll explore the nature of 3D mantle convection and plumes on Mars.

Interactions between the SAF and SJF

The initiation of the SJF and other secondary faults in the SAF system and their interactions with the SAF are of fundamental importance for understanding the plate boundary zone dynamics and the associated earthquake hazards. The SJF initiated between 1.5 and 1.0 Ma, based on geological and stratigraphic evidence. The timing roughly coincided with the formation of a major restraining bend in southern SAF, suggesting a causative relationship between the SAF and the SJF. Here we test this relationship and explore the dynamic interaction between the SAF and the SJF in a three-dimensional visco-elasto-plastic finite element model.

Lithosphere thickness variation and seismicity in the CEUS

Earthquakes in the CEUS appear to concentrate along the margins of the seismologically inferred North American craton, or the “tectosphere” defined by the abnormally thick lithosphere. Could the lithosphere-tectosphere transition zone concentrate stresses and thus contribute to seismicity in the CEUS? To address this question, we developed a finite element model for the CEUS. To simulate the long-term stress pattern, we treat the lithosphere as a power-law fluid continuum with a relative high viscosity (10²⁴ Pa s), underlain by a viscous asthenosphere with a lower viscosity of 10²¹ Pa s. The thickness of the model lithosphere is based on seismologically derived thermal lithosphere thickness (Goes and van der Lee, 2002).

Geometric impacts of the SAF

In northern and central California, up to ~34 mm/yr of the plate motion is accommodated by the SAF and some of the closely subparallel faults; most large earthquakes occurred on or clustered to the main trace of the SAF. However, in southern California the relative plate motion is distributed among a complex system of faults. Slip rate on the main-trace of the SAF drops to 24-25 mm/yr. Seismicity in southern California is much diffuse, with many of the large earthquakes occurred off the main-trace of SAF. Although along-strike variations of seismicity and slip rate may have numerous causes, such as stressing rate [Parsons, 2006] and distribution and properties of active secondary faults [Bird and Kong, 1994], a particularly important cause may be the geometry of the SAF, especially the Big Bend, a ~25° counterclockwise bending in southern California. In this study, we apply a three-dimensional dynamic finite element model to investigate how the particular geometry of the SAF may have impacted on long-term fault slip, stress pattern, and seismicity in California.

New Madrid seismic zone

During the winter between 1811 and 1812, at least three large earthquakes (Mw 7.0-7.5) [Hough et al., 2000] occurred in the New Madrid Seismic zone (NMSZ), which is delineated by instrumental seismicity. The cause of seismicity in the NMSZ is not well understood [Johnston and Schweig, 1996]. Among the uncertainties of the NMSZ seismicity one fact stands solid – some large earthquakes occurred here in 1811-1812. In this study we choose to focus on the potential impact of the 1811-1812 large events on seismicity within the NMSZ and surrounding regions in the following two centuries. Although thousands of micro-earthquakes (M<4) have been recorded within the NMSZ in the past decades, most of the major earthquakes (M>5) in this region since 1812 occurred not within the NMSZ fault zone but in the surrounding areas. This raises the question of where the next large earthquake in the central United States would most likely occur.

The 2001 Bhuj, India, earthquake

The M_w= 7.7 earthquake near Bhuj in western India, which occurred on 1/26/2001, has stimulated considerable interests and debate. Some workers regard the Bhuj earthquake as a new example of intraplate earthquakes that may provide a rare chance for understanding intraplate earthquakes in general and the large earthquakes in the NMSZ in particular [Bendick et al., 2001; Ellis et al., 2001]. Others, however, recognize the diffuse plate boundary zone deformation in western India and many other places in the world and suggest that the Bhuj earthquake resulted directly from the plate boundary processes and thus may provide more insight into the dynamics of diffuse plate boundaries than intraplate deformation [Stein et al., 2001]. To get to the heart of this debate, we need to address the question of what caused the Bhuj earthquake. The specific questions we attempt to address in this work are: 1) Why did the Bhuj event and many historic earthquakes concentrate in this part of the Indian plate? 2) Were these earthquakes mainly controlled by plate boundary processes or by the rift complex? We explore the answers to these questions by numerically simulating stress evolution in the lithosphere of western India in a three-dimensional finite element model.