Hannah S. Rabinowitz

Research

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My research focuses on the physics of earthquakes. This is accomplished through detailed studies of fault structure and the signatures of seismic slip left from frictional heating during an earthquake (paleoseismic indicators) as well as through laboratory experiments designed to measure the frictional behavior of geologic materials at a range of crustal deformation conditions.

Fault Heating in the Japan Trench

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Biomarker Thermal Maturity as a Paleoseismic Indicator

Hydrous pyrolysis reactor used to determine the kinetics of biomarker thermal maturity.

Hydrous pyrolysis reactor used to determine the kinetics of biomarker thermal maturity.

The first part of my thesis focuses on the shallow portion of the seismogenic zone at the Japan Trench where the 2011 Mw9.1 Tohoku-Oki earthquake slipped to the surface and caused a large tsunami, leading to the disaster at the Fukushima nuclear power plant. A year after the earthquake, an ocean drilling expedition sampled the fault zone that slipped during the Tohoku-Oki earthquake. I have used trace element geochemistry to develop a detailed stratigraphy of the plate boundary fault zone, which gives insight into the distribution of deformation in this region. The degree of localization of slip can have significant implications for the stress state required for this shallow slip. I have also developed a biomarker thermal maturity indicator (using alkenones and n-alkanes) to determine whether faults in the décollement region have experienced a significant temperature rise (inferred to be the result of frictional heating). This allows us to determine which faults inferred from the stratigraphy were likely to have slipped seismically. Through a series of hydrous pyrolysis experiments, I constrain the kinetics of the biomarker thermal maturation to estimate the temperature rise on these seismic faults. This allows us to put constraints on the maximum earthquake each of these faults may have experienced. 

Faulted material at the bottom of the JFAST core. This fault was thought to be a depositional unconformity. However, trace element stratigraphy which implies a significant age gap in this section of the core. In addition, biomarkers measured on surrounding samples indicate significant heating at this location. These two lines of evidence both suggest a significant fault here, thought to line up with the layer of crushed material between the green-ish mudstone (left) and the darker pelagic clay (right).

Faulted material at the bottom of the JFAST core. This fault was thought to be a depositional unconformity. However, trace element stratigraphy which implies a significant age gap in this section of the core. In addition, biomarkers measured on surrounding samples indicate significant heating at this location. These two lines of evidence both suggest a significant fault here, thought to line up with the layer of crushed material between the green-ish mudstone (left) and the darker pelagic clay (right).

Extracted organic material from core samples.

Extracted organic material from core samples.

Friction of Carbonate-Rich Sediments

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Map of the Ring of Fire with plate boundaries marked as thick black lines. In this map, symbols represent locations where ocean drill cores have recovered material penetrating through the entire thickness of the sediment overlying the oceanic crust. Diamonds show cores that have a significant amount of carbonate in the sedimentary sequence, with the color representing the concentration of carbonate. Cores used for this project are outlined in black and a schematic of the sedimentary stratigraphy at these sites is shown on either side of the map.

Map of the Ring of Fire with plate boundaries marked as thick black lines. In this map, symbols represent locations where ocean drill cores have recovered material penetrating through the entire thickness of the sediment overlying the oceanic crust. Diamonds show cores that have a significant amount of carbonate in the sedimentary sequence, with the color representing the concentration of carbonate. Cores used for this project are outlined in black and a schematic of the sedimentary stratigraphy at these sites is shown on either side of the map.

 
 

Frictional behavior of sedimentary inputs to subduction zones

Carbonate material tends to begin deforming plastically at shallower conditions (lower pressure and temperature) than silicate rock. However, carbonate-rich sediment has been observed to demonstrate unstable slip behavior over a wider range of conditions than more clay-rich sediment. I am investigating the deformation mechanisms that could be responsible for this enhanced seismic potential. 

To do this, I am measuring the frictional behavior of subducting sediments with varying amounts of carbonate, a significant input to the sedimentary budget of subduction zones. For example, the Pacific plate approaching the Hikurangi Trench (the subduction zone east of the North Island of New Zealand) has ~800 m of carbonate-rich sediment overlaying it. This subduction zone exhibits a type of seismic behavior known as slow slip, which normally is seen in the deeper parts of the seismogenic zone, occurring all the way to the surface. While many ideas have been proposed to explain this unusual behavior, one important aspect to consider is the mechanical behavior of the material in the faults here. I am also interested in how the variations in carbonate concentration could impact seismic behavior, and to investigate this question, I am also conducting experiments on the more carbonate-rich sediment from the Peru subduction zone. 

 
Spoleto fault in the Appenines, Italy. The white limestone hanging wall overlies a pink, marly footwall. 

Spoleto fault in the Appenines, Italy. The white limestone hanging wall overlies a pink, marly footwall. 

Zoom in of the Spoleto fault contact showing locations where we drilled into the fault contact to sample the material.

Zoom in of the Spoleto fault contact showing locations where we drilled into the fault contact to sample the material.

Monte Maggio fault in the Appenines. Heather and me for scale.

Monte Maggio fault in the Appenines. Heather and me for scale.

Long-term Strength of Pseudotachylyte

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During earthquakes, the strength of a fault can drop dramatically as a result of weakening mechanisms including the production of frictional melt. This melt is preserved in the rock record as pseudotachylyte and is one of the most widely used paleoseismic indicators. However, the long-term impact of psuedotachylyte on the strength of a fault zone is not well understood. Many natural faults show evidence that pseudotachylyte veins have served as localization features for future seismic slip. However, recent experimental work has also shown that pseudotachylytes weld, or restrengthen, faults at shallow crustal conditions. I have conducted deformation experiments on natural samples containing pseudotachylyte veins at high pressures and temperatures to determine the conditions under which pseudotachylytes may serve as weaknesses within a fault zone.

 
 
Griggs deformation apparatus at Washington University in St. Louis. This apparatus is used to deform samples at high pressures and temperatures.

Griggs deformation apparatus at Washington University in St. Louis. This apparatus is used to deform samples at high pressures and temperatures.

Samples from the Gole Larghe Fault in Adamello, Italy. These samples have been cored so that the pseudotachylyte vein (black planar feature) is oriented 45 degrees to the imposed maximum stress during the experiments (vertical in this picture). Ruler along the bottom shows a centimeter scale.

Samples from the Gole Larghe Fault in Adamello, Italy. These samples have been cored so that the pseudotachylyte vein (black planar feature) is oriented 45 degrees to the imposed maximum stress during the experiments (vertical in this picture). Ruler along the bottom shows a centimeter scale.