Constraints on the Timing and Distribution of Seismicity and Deformation on Oceanic Transform Faults: Implications for the Design and Deployment of Seafloor Monitoring Systems

Jian Lin, Geology & Geophysics
Greg Hirth, Geology & Geophysics

DOEI Project Funded: 2001

Proposed Research

Jian Lin and Greg Hirth are exploring how geodynamic models, incorporating experimental data on rheology, can constrain the resolution of seafloor measurements required to quantify the dynamics of deformation and earthquake processes at oceanic transform faults. The relatively simple tectonic setting of oceanic transforms make them an excellent natural laboratory for studying the processes that control faulting and the occurrence of earthquakes in the Earth’s lithosphere. This grant provided full or partial support for a number of research activities, including 1) discovery of evidence for earthquake triggering along Pacific transform faults and the Mid-Atlantic Ridge; and 2) geodynamic modeling showing evidence for low mechanical strength of oceanic transform faults; and 3) experimental determination of frictional properties of the oceanic lithosphere. Our collaboration, together with Jeff McGuire, also lead to a successful workshop on Seafloor Geodesy, sponsored by the DOEI, held at WHOI in October 2002. Furthermore, our collaboration resulted in a new MIT/WHOI Joint-Program class on Earthquakes and Faulting (taught for the first time in the Fall of 2002), formed the foundation of the DOEI–sponsored Geodynamics Seminar on Catastrophic Events held in the Spring of 2003, and a new seminar class of Oceanic Faulting and Earthquakes taught in the Fall of 2003.

Final Report

1. Evidence for earthquake triggering along Pacific transform faults and the Mid-Atlantic Ridge
We utilize hydroacoustic data collected by NOAA’s Equatorial Pacific autonomous hydrophone array to investigate the first order spatial and temporal variability of seismicity at the East Pacific Rise (EPR) and earthquake stress interaction and triggering at the Clipperton, Siqueirous, and Gofar transform faults. Our analyses reveal strong evidence of seismic clustering along the EPR. The majority of earthquakes (>90%) clustered in seismic swarms, in which individual events occurred within a few km and within minutes or hours of each other. Through correlating hydrophone data to teleseismically recorded events, we have located several moderate size earthquakes on the Siqueiros and Clipperton transforms. Some of these moderate size events occurred closely in space and time, suggesting the possibility of earthquake triggering. At 5.4°S we found evidence for possible interaction between two moderate size events, one of which occurred on the EPR axis and the other about 100 km northward on the Gofar transform fault. Stress calculations were carried out for several pairs of the moderate size events (Fig. 1). The close correlation of the calculated Coulomb stress changes with the observed spatial and temporal variations in seismicity patterns provides strong evidence for possible earthquake triggering along the transform faults. Results of this investigation were presented at the Fall 2003 AGU meeting (Gregg et al., 2003) and will be part of the generals project for MIT/WHOI Joint Program student Trish Gregg. The observed clustering of seismicity along the EPR ridge axis also have important implications on episodic diking events along fast spreading ridges and the associated rates of hydrothermal heat release and cooling of the oceanic crust (German and Lin, 2003). In addition, Jian Lin is currently working with colleagues on an investigation of potential triggering of ridge earthquakes along the slow-spreading Mid-Atlantic Ridge (Lin et al., in preparation).

German, C.R., and J. Lin, The thermal structure of the oceanic crust, ridge spreading, and hydrothermal circulation: How well do we understand their inter-connections? In Thermal Regime of Ocean Ridges and Dynamics of Hydrothermal Circulation, AGU Geophysical Monograph, in review, 2003.

German, C.R. and J. Lin, Reconciling geophysical and hydrothermal data for heat transfer from the lithosphere to the oceans at fast and slow ridges, Eos Trans. AGU, 84(46), Fall Meet. Suppl., 2003.

Gregg, P.M., D. K. Smith, and J. Lin, Spatial and temporal variability in seismicity of the East Pacific Rise: Constraints from hydroacoustic monitoring and evidence for triggering of transform earthquakes, Eos Trans. AGU, 84(46), Fall Meet. Suppl., 2003.

Lin, J., and others, Evidence for stress interaction between moderate size earthquakes along the Mid-Atlantic Ridge: Implications for the state of stress of slow-spreading ridges, in preparation.

2. Evidence for low mechanical strength of oceanic transform faults
We conducted a series of 3-D boundary element calculations to investigate the effects of oceanic transform faults on stress state and fault development at adjacent mid-ocean ridge spreading centers. We find that the time-averaged strength of transform faults is low, and that on time scales longer than a typical earthquake cycle transform faults behave as zones of significant weakness. Specifically, mechanical coupling of only ~5% best explains the observed patterns of strike-slip and oblique normal faulting near a ridge-transform intersection. On time scales shorter than a typical earthquake cycle, transient ‘‘locked’’ periods can produce anomalous reverse faulting similar to that observed at the inside corner of several slow-spreading ridge segments. Furthermore, we predict that extensional stresses will be suppressed at the inside corner crust due to the shear along the transform resisting ridge-normal extension. This implies that an alternative mechanism is necessary to explain the preferential normal fault growth and enhanced microseismicity observed at many inside corners of slow spreading ridges.

Behn, M. D., J. Lin, and M. T. Zuber, Evidence for weak oceanic transform faults, Geophys. Res. Lett., 29(24), 2207, doi:10.1029/2002GL015612, 2002.

3. Experimental Study of Frictional Properties of the Oceanic Lithosphere
After our initial discussion on the rheology of transform faults, it was clear that more experimental data on the frictional properties of oceanic faults would be aid in our modeling effort. Partly motivated by her generals project on earthquake aftershocks on oceanic transforms, MIT/WHOI Joint Program student Margaret Boettcher began a study on the frictional properties of olivine. The frictional properties at the base of the seismogenic zone in oceanic lithosphere are fundamental to our understanding of earthquake processes. While the composition of the oceanic lithosphere is probably the simplest and most well constrained of any seismogenic region on Earth, few data on its frictional properties exist. We are investigating the strength and sliding stability of olivine aggregates at temperature and effective pressure conditions close to those at the base of the seismogenic zone on a typical transform fault. For all experiments, strain became localized on faults and showed stick-slip events indicative of processes responsible for earthquakes in the Earth. Our new experiments are consistent with seismic data, which find that earthquake hypocenters regularly occur to temperatures well above 200oC, regularly to near the 600oC isotherm. Margaret presented her results at the 2003 SCEC (Southern California Earthquake Center) meeting and the Fall 2003 AGU.

The funds from the DOEI grant also provided support for Laurent Montesi’s modeling study of deformation mechanisms responsible for transient viscous deformation associated with earthquakes [Montesi and Hirth, 2003]. An understanding of these processes is critical for the interpretation of geodetic data measured during and after earthquake slip events [e.g., Freed and Lin, Nature, 411, 180-183, 2001]. We are looking forward to exploring how Laurent’s modeling studies can be incorporated into thermal models of oceanic transforms to establish predictions for geodetic motions associated with deformation along oceanic faults.

Boettcher, M., and G. Hirth, Olivine friction at the base of the seismogenic zone, SCEC, Fall 2003.

Boettcher, M, G. Hirth, and B. Evans, Olivine friction at the base of the seismogenic zone, Eos Trans. AGU, 84(46), Fall Meet. Suppl., 2003.

Montesi, L.G.J., and G. Hirth, Grain size evolution and the rheology of ductile shear zones: from laboratory experiments to postseismic creep, Earth Plant. Sci. Lett., 211, 97-110, 2003.

4. Seafloor Geodesy Workshop: Prospects and Challenges
This workshop, held October 10-11, 2002, examined how seafloor geodesy can be used to study deformation on the seafloor - an issue that has implications for the understanding of Earth's movements during earthquake cycles and volcanic eruptions, seafloor stability on the continental shelf, and the generation of tsunamis. The meeting was attended by approximately 60 scientists, engineers, and industry representatives from across the country, as well as international experts from Japan and France. The workshop objectives were

• To discuss the pressing marine geodynamics problem that can be addressed through advances in seafloor geodesy;
  
• To review the state-of-the-art technologies in space geodesy and discuss why their applications have revolutionized continental geodynamics; and
  
• To stimulate discussions on the promising directions of future advances in seafloor geodesy.