Only a small volume of Earth’s crust is comprised of fault zones, but the processes of strain, deformation, and rupture that occur there have an outsized impact in terms of landscape change, infrastructure damage, and human cost when earthquakes occur. The physical and chemical characteristics of fault rocks, as well as their textures, contain information about how faults behave on a variety of timescales. A recent article in Reviews of Geophysics describes what different kinds of magnetic analysis can reveal about faulting. Here, the authors give an overview of fault magnetism research.
What can the physical and chemical attributes of rocks in fault zones reveal?
During an earthquake, the energy of the accumulated stress in a fault zone is released into seismic waves (radiated energy), fragmentation of the rocks (fracture energy), and heat (frictional energy). Seismic waves can be detected by seismographs, but the other two types of energy are difficult to observe directly.
However, shear stress and high frictional temperatures induce physical and chemical changes in fault rocks, which in turn change their properties such as grain size, fabric, mineral composition, and magnetism. Fault rocks, therefore, may preserve a record of earthquake information as well as slip motion that does not come with earthquakes, so called (slow or) aseismic slip.
Analyzing the physical/chemical properties of fault rock thus helps scientists to understand the thermal history, fault weakening, kinematic mechanism, and related earthquake hazards. For example, under certain conditions frictional heating may even induce localized melting of rocks along principal slip zones (PSZs); the effects of this can be amazing.
What additional insights do the magnetic properties of rocks offer?
“Neoformation” or transformation of magnetic minerals through thermochemical reactions of iron-bearing minerals induced by seismic frictional heating provide a means for estimating the temperature experienced during seismic slip. Peak temperatures a fault rock has experienced are constrained by measuring the maximum temperature at which the magnetic response is reversible.
Note that this approach ignores time aspects: a short high temperature pulse is equivalent to longer period of elevated temperature at a lower temperature. Since earthquake durations (up to several minutes) are comparable to laboratory measurement time scales, this aspect is currently ignored but could be constrained by flash-temperature modeling.
Frictional heating typically generates magnetic minerals and makes PSZs stand out magnetically: even thin, mm-thick, difficult to track PSZs can be detected faithfully with magnetic means. Intricacies of deformation mechanisms operating in fault zones can be accessed by measuring the magnetic anisotropy.
What are the main techniques for analyzing rock magnetism?
Dedicated equipment can be used to measure the magnetic properties of samples. Temperature-dependent measurements indicate the magnetic mineralogy, applied magnetic field-dependent ones give information about magnetic grain size, and rock orientation-dependent ones reveal grain preferred orientation.
With modern equipment, the detection limits for each of these categories are low; particularly field-dependent measurements can have limits of detection as low as 1 in one million. Measurement of various types of laboratory-induced remanent magnetizations, that is the permanent magnetic moment of a sample after exposure to an applied magnetic field, as well as the natural remanent magnetization, also yields important information.
What strengths and weaknesses do magnetic methods have compared to other approaches of fault rock analysis?
A complete set of magnetic analyses requires less than approximately 500 milligrams of material of any shape (it can be chips, powder, or drilling/cutting residue): you can do a lot with little sample material.
A foremost asset of the magnetic approach is rapid information on grain size in the nanometer-micrometer range on bulk samples (albeit often small in fault rocks) so that a representative view of that sample is obtained. Such information is difficult to acquire with other techniques. Thus, in principle, magnetic changes at a small scale allow for resolving the faulting-related behavior. Magnetic measurements at room temperature and below are non-destructive so samples can be used afterwards for analysis with other techniques. Magnetic microscanning is gaining popularity which complements the bulk measurements.
A weakness of magnetic methods is that some of the outcome is non-unique in terms of interpretation, so those cases should be constrained by information from other approaches. However, this applies to other approaches as well.
What are some of the unresolved questions where additional research, data or modeling is needed?
Heating-induced reactions that affect magnetic minerals in faults may provide new and valuable constraints on the thermal history of faults. Yet, comprehensive deformation experiments are needed to calibrate these reactions under laboratory-controlled conditions.
Similarly, the paleomagnetic record of fault rocks cannot be compared to classic paleomagnetic poles because the latter are based on a time-averaged geomagnetic field (on short time scales – years to centuries – the field is quite variable). The duration of earthquakes (up to minutes) only provides a snapshot of the field. New analog experiments would advance our understanding of magnetization acquisition processes specific to fault rocks.
Meanwhile, laboratory simulation of fluid-rock interactions with natural and/or synthetic fault rocks under controlled physicochemical conditions are required to improve our understanding of alteration, removal, and/or neoformation of magnetic minerals as a result of fluid infiltration.
Fault rocks hold valuable kinematic information in their magnetic fabric. However, the minimum size required for magnetic analyses is still a significant limitation and there is a need for miniaturization of magnetic fabric techniques. Here, magnetic microscanning may be used advantageously.
Finally, most thermometers, including magnetic thermometers, measure the combined impact of heating temperature and duration. Flash-temperature modeling could enable separating these two factors.
—Mark J. Dekkers (M.J.Dekkers@uu.nl;
Dekkers, M. J.,Ferré, E. C.,Chou, Y.-M.,Yang, T.,Chen, J.,Yeh, E.-C., and Tanikawa, W. (2020), New insights from the magnetic properties of fault rocks, Eos, 101, https://doi.org/10.1029/2020EO151611. Published on 13 November 2020.
Text © 2020. The authors. CC BY-NC-ND 3.0
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