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Plate-rate experimental deformation: Aseismic, transient or seismic fault slip

Periodic Reporting for period 3 - PREDATORS (Plate-rate experimental deformation: Aseismic, transient or seismic fault slip)

Reporting period: 2020-03-01 to 2021-08-31

The main focus of the project is understanding natural geologic disasters, primarily earthquakes. Large magnitude earthquakes are one of the world’s greatest risks to human life and damage to infrastructure and economy in densely populated regions. The hazard is compounded by the fact that these events can cause other natural disasters such as enormous tsunamis. Recently, earthquakes induced by humans, for example from wastewater injection associated with oil and gas extraction, have become a source of concern to the public.
Describing the slip behavior of major fault zones relies heavily on the results of laboratory shearing experiments simulating fault slip. The seismic behavior of major faults depends not only the absolute strength of a fault, but also how strength changes as a function of time and slip velocity. Such behaviors are measurable in laboratory friction experiments, which have provided the framework for assessing the in-situ fault conditions and fault rock properties that allow earthquakes nucleate and propagate.
Despite the many advances in knowledge over the years, the tendency for faults to host earthquake slip (or not, in the case of creeping faults) is far from well understood. I identify this missing fundamental piece of information as how faults slip when driven at plate-convergence slip rates. Furthermore, there is a need to conduct experiments on real samples from natural faults, in addition to analogue materials used as laboratory standards. To do this, we take advantage of a large suite of sample obtained by scientific drilling at plate-boundary fault zones recovered by the Integrated Ocean Drilling Program (IODP) and International Continental Scientific Drilling Program (ICDP).
To summarize, the main goals of this project are to: (1) Quantitatively describe the slip behaviour of geologic materials –both natural and analogue fault rocks – when driven at plate tectonic convergence rates as they are in nature, in order to explain seismologic and geodetic observations on real faults and predict fault slip behaviour in the future; and (2) Identify the important factors controlling the range of observed modes of fault slip, in other words, to identify the processes or material characteristics are important for causing aseismic creep, slow slip events, or locking and coseismic slip.
Much progress has been made in toward the project goals. Specifically, this includes achievement of several of the “Deliverables” named in the Grant Agreement Annex. Here only completed studies are listed; work in progress is discussed in the next section.

1. A comprehensive dataset of frictional behaviour for natural fault zone samples at cm/yr slip rates.

These tests have been tailored to answering site-specific questions. The measurements include basic strength, frictional velocity dependence, and parameters of slow or stick-slip events such as stress drop, peak slip velocity and duration (if observed).This has been completed and is published as two papers, with a third in progress.

2. First results of innovative measurements, for example velocity-cycling, power density or cohesion.

We have developed a new measurement that we call a “velocity-cycling” test, which is a hybrid of conventional slide-hold-slide and velocity-step tests that specifically simulates active loading from driving rates as low as 10-10 m/s during the interseismic period. We have also produced a study simulating lithification, or the creation of rock from sediment, and how it can enable earthquakes.

3. Synthesis of laboratory observations with geophysical observations (seismologic and geodetic) on natural fault zones. These studies are intended to: (1) explain current geophysical observations on fault zones which are specifically tested in this study, and (2) predict fault behaviour at locations where sampling and geologic characterization is limited.

One of these studies was published 2018, the first author is a former PhD student from Lamont-Doherty Earth Observatory at Columbia University. Two more were published in 2020, one by the PI focusing on the Hikurangi subduction zone offshore New Zealand, and one by Marum PhD student Alex Roesner focusing on the Nankai subduction zone offshore Japan. There are currently five studies in progress dedicated to this topic, these make use of scientific drilling samples from Hikurangi (New Zealang), Sumatra, and the Nankai Trough (Japan).
Scientific results beyond the state of the art expected by the end of the project include completing studies targeting the following deliverables, which includes ongoing studies:

1. A comprehensive dataset of frictional behaviour for rock and mineral standards at cm/yr slip rates, starting from room temperature and ~10 MPa up to potentially ~160 MPa and 240 °C simulating a depth range of ~1-16 km.

Several experiments for this dataset have been performed, and work is in progress.

2. An elevated temperature and pressure dataset at extremely slow rates designed to target the competition between interseismic strengthening and strength loss via shear failure. This will include testing of both standards and natural fault rocks.

Due to administrative issues, we have not been able to utilize the high-pressure high-temperature apparatus as planned. However, we have purchased a new higher-pressure shear device, and also hope to regain access to the high-temperature machine before the end of the project.

3. Results of innovative measurements, for example slide-creep-slide, power density or cohesion.

Direct measurements of cohesion are incorporated in two published studies: (1) Ikari and Hüpers; Frictional slip instability induced by simulated diagenetic cementation. This study uses analogue material (shale and salt) to simulate the transformation of sediment to rock, which we hypothesize is necessary for earthquake slip and quantified by the cohesion. (2) Rösner, Ikari, Saffer, Stanislowski, Eijsink, Kopf; Friction experiments under in-situ stress reveals unexpected velocity-weakening in Nankai accretionary prism samples. This is a study performed by Marum PhD student Rösner, who uses shearing tests using natural fault samples from the Nankai subduction zone offshore Japan show that when tested under accurately simulated conditions and in their originally recovered condition, the samples tend to be more frictionally unstable than previously thought.

4. Synthesis with existing laboratory data at intermediate and high slip velocities, providing a complete description of rock/sediment friction over the entire possible range of slip velocities.

The tests were performed on a sample from the Alpine Fault in New Zealand and is a continuation of an earlier project that has been further developed during the PREDATORS project. A manuscript is in preparation: Ikari, De Paola, Saffer, Nielsen, Smith, Marone, Kopf; Power density of near-surface faults.

5. Synthesis of laboratory observations with geophysical observations (seismologic and geodetic) on natural fault zones. These studies are intended to: (1) explain current geophysical observations on fault zones which are specifically tested in this study, and (2) predict fault behaviour at locations where sampling and geologic characterization is limited.

6. Microstructural images and roughness characterization of fault surfaces, for integration with field observations on exhumed faults by structural geologists.

This in-progress study is being conducted by. A. Eijsink and represents a first step at connecting laboratory-measured friction parameters, such as how strength changes during faster shearing, with topographic roughness measurements. The roughness, and roughness anisotropy of the shear surfaces are measured with a Zeiss Confocal Laser Scanning Microscope, which was purchased for the project.

7. Collaborative studies. A large number of studies are in progress, which are being conducted in cooperation with international scientists. The collaborations includes scientists in the USA (Univ California at Santa Cruz, Penn State University, University of Texas), Japan (University of Tokyo), New Zealang (GNS Science), the UK (Cardiff University), Canada (McGill University), France (University of Lorraine), and Italy (Sapienza University Rome, INGV Rome).

8. Improved boundary conditions and constraints for numerical simulations of fault slip.

Since the results from this project are quite new, there has not yet been sufficient time for follow-up modeling studies, but these are planned.