Osaka, Japan — Slow earthquakes have been discovered to exhibit anomalously slow, long-lasting and small slips, adjacent to regular earthquakes where we sometimes feel catastrophic vibration (Fig. 1). However, no one knows the reason why slow earthquakes show such strange characteristics. In a study published in a scientific journal Nature Communications, researchers at The University of Osaka succeeded in experimentally reproducing the multiple features of slow earthquakes in the lab (Fig. 2) and suggested the grain-scale origin of them based on their direct observations.
More than 20 years after the discovery of slow earthquakes, which cause weak or no perceptible vibration but associate with slow slip for about a year at maximum, the unified explanation for the characteristics of slow earthquakes does not exist. Up to now, most experimental studies that reproduced slow earthquakes have focused only on the slowness of fault slip. However, what makes slow earthquakes most distinct from regular earthquakes are rather their statistical features revealed through seismic observations (Fig. 2 upper panels). These statistical characteristics of slow earthquakes, however, had rarely been reproduced or explained in laboratory experiments.
"Slow earthquakes have traditionally attracted attention mainly for their slow slip rate, but the statistical properties of their duration and recurrence relative to earthquake magnitude have so far been addressed only in a few limited experiments," says a study author Yuto Sasaki. "Needless to say, experiments using actual rocks and natural fault grains are of great importance; however, we realized a necessity of simpler physical equivalent, which enables us to directly observe internal structures during deformation. This is a major advantage of our experimental system, as such direct internal observation is usually very difficult in rock-deformation experiments."
The researchers have prepared very simple "gel jelly beads raft" (Fig. 3). As the fault region of slow earthquakes is expected rich in fluid and soft grains, they conceived its physical analog of soft gel jelly beads in a liquid solution. "This table-top experimental system is available even in your home, but it shows a surprising variety of behaviors and offers us a wealth of fascinating clues about slow earthquakes underground," says Sasaki. "If you put rigid glass beads into a dry cup and mix it slowly, you can feel intermittent and fast scratches. Actually, these scratch events show the statistics similar to regular earthquakes." However, the mixture of gel jelly beads and a liquid solution shows significantly different features with longer and smaller events, as with slow earthquakes (Fig. 2 lower panels). "You can feel sluggish, intermittent slide by mixing bubble tea, but the two phenomena seem to be fundamentally different."
In contrast to dry rigid beads, soft wet beads are inefficient in transmitting force and deformation. This property potentially induces longer and isolated small slips. "In retrospect, this experimental system seems to have been well suited for studying fault systems of slow earthquakes, while our prior target was deeper part of tectonic plate," says Sasaki. "We expect the similar experimental report from high temperature and pressure experiments using rock and fault material."
Slow earthquakes often occur adjacent to the source regions of regular, destructive earthquakes (Fig. 1). "Based on the results, observed slow earthquake statistics could be interpreted as fault conditions," says Sasaki. This would contribute to probabilistic assessments of earthquakes. Moreover, the longstanding mystery of the mechanisms underlying slow earthquake occurrence is expected to be further addressed not only through laboratory experiments but also through observational studies and geological analyses. "This experimental result will serve as a starting point for further contributions from a wider range of fields, ultimately advancing our understanding of slow earthquakes and enabling better assessment of their influence on conventional, destructive earthquakes," says Sasaki.
Since the experimental setup used in this study is simple, the observed results can also represent general characteristics of the mixture of soft beads and liquid. "By analyzing the detailed relationship between microscopic bead rearrangements and macroscopic slips, fundamental aspects of sheared soft-matter systems can be revealed, as well as the origin of characteristic features of slow earthquakes. Nothing excites me more than realizing that tabletop experiments in soft matter can unlock the mysteries of both fundamental soft-matter physics and geological-scale phenomena," says an author Hiroaki Katsuragi.
