Report on the 2023 Slow-to-Fast Workshop held in Tokyo
The workshop portion of this meeting was held at the Ito International Center of TokyoUniversity, which provided a good room for the meeting’s main session, coupled with space
just outside where posters could be discussed during coffee breaks. The workshop format ofa single suite of talks, with a fair amount of time to discuss talks and interact with thespeakers worked very well, as did the broad range of topics that were scheduled for theplenary sessions. The posters were able to be placed in a good viewing space.
The meeting was a very effective way for an interested scientist to become aware of thecurrent state-of-the-art in research on seismic slip associated with a range of slip styles interrestrial shear zones (ranging from tremor activity to regional and megathrustearthquakes), and the geophysical, geological, and geodetic tools used to study them. It also
proved very useful for stimulating connections between researchers investigating different
aspects of these phenomena, and in introducing the novice practitioner to the wide range ofapproaches and field areas that are useful for studying these phenomena. It was a pleasure
to learn so much in a concentrated meeting, with the area near the venue also providing anice range of relatively inexpensive meals and evening activities.

Report on 1-day Field Trip to see elevated Boso Penisula (Tokyo) beach terraces that werecreated by the 1923 Taisho (M7.8) and the 1703 Genroku (M8.2) Megathrust Earthquakes
The field trip was led by Junki Komori (NTU, Singapore) and Ryosuke Ando (U. Tokyo) withthe help of Asuka Yamaguchi (U. Tokyo). We travelled in a group bus that left around 8am on
Saturday 16 September from the University of Tokyo Campus where the meeting had been
held, and were dropped at the JR Soga Train station near Chiba (roughly 1 hour metro ridefrom downtown Tokyo) around 18:00 the same day. Approximately 40 people participated inthis field excursion.
The first stop was along the Kenbutsu coastline (Tokyo Bay) where marine terraces resulting
from the 1923 Taisho EQ along the underlying megathrust are well exposed. A short (200 m)
walk was needed to reach the beach. The gentle (bay exposure) of this outcrop meant thatthe ~2 m beach uplift associated with this event was well-observed as a discrete beachterrace. (Fig. 1 and 2)

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Figure 1. This figure shows the typical geometry of the former coastline (beach terrace) thatrises above the present-day beach level in this part of the coast.

Figure 2. Paronamic view of Kenbutsu coastline at stop 1.
The exposed rocks were also interesting, in that they showed material making up thisportion of the Japan Trench/Nankai Trough (and the Nankai Trough’s northward extension,
the Segami Trough that currently subducts offshore the southern Boso Peninsula)
subduction system. In particular, we could see frequent clasts within a volcanoclastic matrix(Fig. 3).

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Figure 3. Blocks of fine grained (silty clay material) within a volcanoclastic matrix. (The smallumbrella is ~20 cm, for scale.)Stop 2 - Shirahama area along the coast between Kenbutsu and Chikura.

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At this stop we also had the chance to appreciate both sets of beach terraces, with the lower
one (1923) being almost at sea level. At this stop we also saw beautiful examples of block-inmatrix complexes where the clasts had deformed after they became less strong than thematrix in which they were embedded.

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Figure 4. Embedded clast.

Figure 5. Flame structures within the volcanoclastic sandstone.

5Figure 6. Embedded clast behaving like a weak block within its deforming matrix.
In this outcrop it was possible to see small-scale rheological behaviour in the accretionary
prism. This was as interesting as the beautiful examples of beach terraces that we saw on all
3 field stops during this trip.

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During the second stop (the Shirahama area) the larger, earlier up to ~6m high terraces of
the 1703 Genroku event were particularly well exposed, and at the 3rd stop near Chikura wecould see examples of even earlier beach terraces preserved as terraces in agricultural andresidential areas.
Stop 3 - Chikura
Because the 3rd stop was on a Pacific-ocean-facing area of the coastline, the smaller uplifts
associated with the 1923 event were harder to see, implying that beach erosion processes
are a key factor in the preservation or non-preservation of these features. However atChikura we were able to appreciate a longer time frame, with terraces dating back to 6000
years ago (Figure 7).

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Figure 7. Here the train line appears to be built on a pre-existing terrace (this terrace wasseen at the end of the small hike associated with the 3rd stop).

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