REPORT ON SLOW-TO-FAST EARTHQUAKES WORKSHOP AND FIELD TRIP 2022
Author: Simone Puel
Affiliation: The Univ. of Texas at AustinWorkshop
The International Joint Workshop on Slow-to-Fast Earthquakes 2022 held in Nara, Japan,
focused on the difference between “slow” and “fast” earthquakes. While for “fast” we mean
regular earthquakes which cause worldwide destructions and trigger early warning systems,“slow” earthquakes include low-frequency earthquakes (LFEs), tectonic tremors, very lowfrequency earthquakes (VLFEs), and slow slip events (SSEs).
A thorough overview of the two phenomena was given by the first keynote speaker, Dr. Ide. He
suggested considering the slow earthquakes as manifestations of a single slow deformation
process. For instance, a cluster of tremor/LFEs can be seen as an SSE. The rupture process ofboth fast and slow events may be self-similar with scale-invariant stress drop and energy, making
the distinction of these events very challenging at the early stage of rupture propagation. A
difference between the two processes seems to be the different scaling of maximum seismic
moment magnitude with respect to duration. For example, the moment magnitude scales withearthquake duration for slow earthquakes, while the cube of duration for fast earthquakes.
However, the most fundamental difference between fast and slow earthquakes may be thegoverning equation coupled with the friction/fracture equation: wave equation and diffusion
equation for fast and slow earthquakes, respectively. Diffusional mechanisms are various anddependent on regional environmental conditions. Hence, slow earthquakes appear diverse.
A concept that has been discussed extensively during the workshop is the importance and role ofheterogeneities. These heterogeneities may be lateral and vertical variations in Earth properties
both in laboratory experiments (Dr. Faulkner), numerical modeling (Dr. Elbanna), and in nature(Dr. Townend). Another type of heterogeneity may be related to variations in fault frictions (Dr.
Kaneko) or the presence of asperities with different sizes and geometry along the fault interface(Dr. Aochi).
This wide range of heterogeneities seems to play a crucial role in influencing the type of slip
behavior. While the first day of the workshop was focused more on the terminology of the fastand slow earthquake (features and scaling), the importance of heterogeneity, and the role of slow
phenomena (Dra. Mouchon, Dr. Ohyanagi, and Dr. Cruz-Atienza), the second day started off
with a focus on new techniques to better monitor these seismic events.
These new techniques include distributed acoustic sensing (DAS) and fiber optic sensing (Dr.
Zhan and Dr. Li, Dr. Araki, and Dr. Chen), which can resolve higher frequency signals than thestandard seismometers, and the potential use of machine learning for more accurate seismic
detection of slow phenomena (Dr. Rouet-Leduc, Dr. Chiu, Dr. Constantino). The afternoon
session concentrated on the use of numerical modeling to infer the current state of the Nankai
subduction zone (Dra. Sherrill, and Dr. Noda and Saito), and the use of seismic reflection forinferring fault geometry and 3D heterogeneous structure (Dr. Tsuji, and Dr. Bassett), which may
control different slip behavior along the trench.
Lastly, the third day focused on the geological constraints of slow and fast earthquakes. Thegeologic record of exhumed rocks seems to indicate the presence of a fluid-rich source inproximity to the area of slow events. Both geological and geophysical observations suggest theimportance of the fluid-rich and high-pressure region for the slow slip and the tremor source
region (Dra. Behr, Dr. Condit, and Dr. Kaneki). Besides the geologic record, Dr. Tanaka
suggested using temporal variations in gravity anomaly to quantify the presence of water in theregion of slow slip events in Nankai and Cascadia.

Field trip
The goal of the two-day geological field trip was to observe several outcrops of the Median
Tectonic Line (MTL), which may have recorded a multi-stage faulting history of brittle-ductile
deformation due to island-arc collision. Unfortunately, the entire field trip was affected by thetyphoon Nanmadol bringing heavy rains and strong winds in the southwest of Japan (Figure 1).

Figure 1: Heavy rainfall during the field work in Matsusaka due to typhoon Nanmadol.

On the first day we observed outcrops of MTL in Matsusaka that experienced shallow brittle
deformation (geologic fault contact, Figure 2) and deeper plastic deformation.

Figure 2: Example of brittle deformation along the MTL represented by the fault contact.
On the second day, we moved to the east to the Ise, where outcrops of high-pressure
metamorphic rocks of the Sanbagawa Belt and the tectonic mélange of the Kurosegawa Belt
formed at the subduction plate boundary (Figure 3).

Figure 3: Outcrop of high-pressure metamorphic rocks with quartz veins of the Sanbagawa Belt
at Arashima Beach.