The project goals have been largely achieved. Specifically, this includes achievement of several of the “Deliverables” named in the Grant Agreement Annex.
A comprehensive dataset of frictional behavior for natural fault zone samples at cm/yr slip rates has been produced (Ikari and Kopf, 2017; Ikari, 2019). In addition, this dataset has been extended to five orders of magnitude faster velocities, for a total of 13 natural fault zone samples (Ikari, 2022 AGU Fall Meeting).
We have synthesized results of our plate-rate shearing velocity technique with geophysical observations (seismologic and geodetic) for site-specific studies of 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) help predict fault behavior at locations where sampling and geologic characterization is limited.
Several of these studies focused on the Hikurangi subduction zone offshore New Zealand, addressing the origin of slow slip events which are well-documented in the area (Rabinowitz et al., 2018; Ikari, Wallace et al., 2020; Eijsink and Ikari, 2022; Shreedharan et al., 2022). Another study focused on the lack of shallow seismic activity at the northern Cascadia subduction zone, discussing the possibility that the fault zone may not be locked (Stanislowski et al., 2022). Using slow, but not quite as low as plate-rate velocities, we have produced studies showing how natural samples from the Nankai subduction zone offshore Japan can produce shallow slow slip events (Roesner et al., 2020; Okuda et al., 2021; Roesner et al., 2022), which have been observed with borehole pressure sensors.
Results of innovative measurements have been produced and published. We have developed a new testing protocol 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 mm/yr during the interseismic period (Ikari, Carpenter et al., 2020). We have also produced successfully simulated lithification in the lab by adding salt to sediment and desciccated the sample, creating synthetic rocks from sediment powder. Using this technique combined with our unique capability of directly measuring cohesion, we show how lifthification process can enable earthquakes (Ikari and Hüpers, 2021). Another study produced a first quantitative relation between fault surface roughness and friction parameters, which can be scaled directly from the lab to the field (Eijsink et al., 2022). We have also developed a technique to modify apparatus stiffness in both the normal and shear directions, producing a wide range of slip events (Eijsink and Ikari, 2024).
For added value, we have carefully documented the stiffness of our apparatus over a wide range of conditions, which is neglected in the literature, and published this as a technical report (Ikari and Haberkorn, 2023).
Several studies are still in progress at the time of this report, and are expected to be completed and published in the future. These include a study examining the role of mineral surface chemistry in fault slip behavior (Ikari and Conin, 2024 EGU General Assembly), extending the conditions of the plate-rate experiments to higher pressures and temperatures (Zhang et al., 2023 AGU Fall meeting), and an intriguing study suggesting that SSEs can be used for earthquake early warning (Ikari, 2023 EGU General Assembly).
