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The initiation of intraslab earthquakes and their location depend on multiple factors such as slab heterogeneity, pre-subduction anisotropy, thermal parameters and the metamorphic history of the lithosphere slab. As a result, these failures occur over a range of temperature-pressure conditions through different mechanisms. Therefore, to unravel the intricacy of intraslab seismicity, it is crucial to investigate the details of failure mechanisms using multiple datasets, at different scales. Details on rupture mechanisms are now accessible through advanced, high-resolution seismologic approaches. However, seismologic archives are limited to events of the past few decades. Further, quasi-synchronous multiple-source mechanisms can be challenging to distinguish from seismic data only. Alternatively, the geologic records of intraslab seismicity, preserved in pseudotachylytes, provide a higher spatiotemporal resolution than seismic data, resulting in direct insight into the rupture triggering mechanisms.. These fault rocks also inform, over the geologic timescale, on the integrated deformation history of the slab. Despite this fact, only a handful of natural exposures have been investigated, and these include localities in Corsica, Italy, Zambia, and Japan. In addition, despite some strain rate limitations, deformation experiments, conducted using a wide range of starting materials and over broad pressure-temperature conditions, may further constrain the causes of intraslab rupture. These experiments, being intrinsically performed on relatively small rock volumes cannot capture large scale aspects of intraslab deformation. Finally, numerical modelling may overcome some of the above limitations as long as accurate and realistic parameters are chosen. Ultimately, our review shows that to fill the gap in our knowledge of intraslab seismicity, we need a multiscale-multimethod approach that integrates seismologic, geologic, experimental and numerical perspectives.

The pseudotachylyte of the Markagunt slide in Utah is the best example of gravity slide frictional melt. A key exposure near Panguitch provides opportunities to investigate the kinematics of frictional melts. Here we analyze the anisotropy of magnetic susceptibility (AMS) in eight oriented samples and show that magnetite dominates the magnetic susceptibility (97.7% in generation veins and 87.6% in injection veins) and most likely also the AMS. These rocks have magnetic susceptibility one order of magnitude greater (∼46,106 • 10−6 [SI]) than their host rock (∼1850 • 10−6 [SI]), which shows that magnetite must have formed during the gravity slide. The AMS of these rocks records the laminar flow kinematics of the frictional melt, with a North-South direction and a top-to-South sense of displacement consistent with regional observations. The dominantly planar symmetry of the AMS points to a flow regime dominated by pure shear, towards the end of the gravity slide movement. Our results, obtained on a landslide pseudotachylyte where the emplacement kinematics was already known, fundamentally validate the use and interpretation of AMS in other pseudotachylytes including those formed along seismogenic faults.