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Margaret S. Boettcher (Back to profile »)

Mid-Ocean Ridge Transform Fault Projects

Mid-ocean ridge transform faults (RTFs) are exceptionally interesting and understudied tectonic regions. Their relatively simple geometric, compositional, and thermal structure lead to more predictable seismicity compared to continental faults. Below are a few examples of projects that are focused on understanding slip on ridge transform faults.

Scaling Relations:

Scaling Relations image

Tom Jordan (USC) and I have developed scaling relations between the physical fault parameters (fault length & tectonic slip rate). Our model ultimately shows that both large and small earthquakes on RTFs “know” the total fault area, and trade off in such a way as to maintain a constant coupling coefficient of ~15%, which is the percentage of plate motion accommodated by earthquakes. We are still investigating why there might be on average a constant coupling coefficient, independent of RTF length or tectonic slip rate.

Aftershocks and Foreshocks:

Aftershocks and Foreshocks image

Our work also showed that there are more than an order of magnitude fewer aftershocks on ridge transform faults (blue symbols) than on the better studied continental transform faults (green symbols). Jeff McGuire (WHOI), Tom Jordan (USC) and I also discovered that most magnitude 6.0 or greater earthquakes are preceded within 15 km and 1 hour by a foreshock. These faults cannot be described by standard models of earthquake seismicity (e.g. ETAS), which state that all earthquakes are triggered by the same mechanism. We are also still investigating the exciting topic of why seismicity on these faults is more predictable than that on continental faults.

Frictional Conditions:

Frictional Conditions image

Better constraints on the frictional properties of RTFs were needed to predict the depth extent of faulting. To address this topic Greg Hirth (Brown U.) and I conducted laboratory experiments on olivine powders at high temperatures and confining pressures. We found that the base of the seismogenic zone, as determined by the transition from velocity-weakening to strengthening conditions, corresponds to the 600°C isotherm. Our experiments exhibited localized sliding (see the offset enstatite layer, which is the light colored grains). The olivine grains in the photomicrograph were oxidized to highlight the dislocations near grain boundaries and provide us with insight into the details of the deformation mechanisms in our experiments.

Thermal Models:

Thermal Models image

Researchers have been using simple half-space cooling models to approximate the thermal structure of mid-ocean ridge transform faults for many decades. Mark Behn (WHOI), Greg Hirth (Brown U.) and I wanted to compare these models to models with more realistic parameters, including friction in the seismogenic zone and temperature-dependent viscosity in the asthenosphere below. Our models have produced a new predicted thermal structure that we will test with the improved seismic catalogs that are currently being collected by ocean bottom seismometer experiments (e.g. Jeff McGuire’s project on Quebrada, Gofar, & Discovery Transform Faults on the East Pacific Rise).


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