— V11B-01 —Trapdoor faulting at Kita-Ioto Caldera, Japan:

Five-min. Summaryis available on YouTube

Quantification of magma overpressure beneath a submarine caldera
Feel free to contact mevia osm3@bosai.go.jp(1) National Research Institute for Earth Science and Disaster Resilience (NIED), Japan, (2) JSPS Research Fellow (PD), Japan

1. Introduction

2. Methods: Mechanical model of trapdoor faulting

A volcanic earthquake at Kita-Ioto caldera
generated a tsunami wave. By using the tsunami
data, we here attempt to estimate the magma
overpressure that caused the volcanic tsunami.

Hypothesizing the trapdoor faulting mechanism for the earthquake,
we newly develop a mechanical model of trapdoor faulting to relate
the magma overpressure as a driving force to the resultant tsunami.

Study target: Kata-Ioto caldera, south of Japan
Kita-Ioto caldera has been known to be active, but recent
volcanic activity is unknown.
Mw 5.2–5.3 non-double-couple earthquakes, often calledvertical-CLVD earthquakes, occurred every several years.

••aac 06/12/2008

c 06/12/2008

03/11/2019

Funka
Asane

Funka
FunkaAsaneAsane

Mw 5.308/20/1992

Topography (km)b
DART52404b

12/25/2010

Kita-Ioto
Island

11/15/2017

Water depth (m)c08/20/1992

Topography (km)Kita-Ioto
IslandIotoIslandbc

Kita-Ioto
Kita-Ioto
Islandcalderad

Minami-Ioto
IslandWater depth (m)Topography (km)Kita-Ioto
IslandDART 52404

Estimated tsunami
arrival time

Ioto
IslanddFig. 1: A vertical-CLVD
earthquake near Kita-Ioto caldera, asubmarine
calderawitha
size of 12 km x 8 km near Kita-Ioto
Minami-Ioto
IslandIsland,in the Izu-Bonin arc. Each dot represents locations ofTopography (km)Estimated tsunami
repeating vertical-CLVDearthquakes.
The 2008 event is plotted
DART 52404
arrival timewith its focal mechanism reported in the GCMT catalog.

Milli-meter tsunami due to the 2008 earthquake•

Following an Mw 5.3 event in 2008, a tsunami signal wasrecorded by an ocean-bottom-pressure gauge.
Tsunami signal

Δp = –1.97 (MPa)ΔV = 0.0030 (km3)(a)

Reverse slip of RF of max. ~ 9 m (c)

(e)

(f)

b) Best-fit model (Crack depth, Arc length, Dip angle) = (3.0 km, 2/3-ring, 85.0°)

Stress/dislocation interactions between(1) a ring fault (RF) and (2) a horizontal
crack (HC) filled with magma are solvedin a 3-D half-space elastic medium by
the boundary element method (BEM).

(b)

Opening & closure
Reverse slip ofof the horizontal
crackopeningthe
fault
Verticalorring
closureReverse slipof a ring fault
Bof a magma-filled horizontal crack(d)Vertical opening/closure of HC

What equations to solve?
Water
Assumptions:depth(m)I. TF is caused by magma overpressure (p0)II. TF occurs with (1) reverse slip of RF, s,(2) opening and closure of HC, δ,and (3) magma pressure change, Δp.

cdAB

Fig. 4. Source structure of a mechanical model.

Fig. 6. Mechanical source model of the 2008 Kita-Ioto(c)(e)
caldera earthquake constrained by the tsunami data.

Boundary conditions (BCs): Pre-assuming p0 , BEM determines RF/HC motions (s , δ) and magma pressure change (Δp) during TF.
BC I. On RF, shear stress reduces to zero.

! "!

+ $% + &' = )Shear stressbefore TF

Change ofshear stressduring TF

Shear stressafter TF

BC II. On HC, normal stress = magma pressure.

*% + +'
Change ofnormal stressduring TR= Δ" - where ∆" = − ! ∆%"! #"Change of!
= − " # ∑$ '$ ($magma pressure! "during TF

BEFORE trapdoor faulting

P, Q, R, S: Interaction matrices that map dislocations of
RF/HC into normal/shear stresses on RF/HC,
computed by the triangular dislocation method(Nikkhoo & Walter, 2015).
ΔV: HC volume change during TF
V0: Initial crack volume(here V0βm = 1.50 m3/Pa)
βm: Magma compressibility(d)A: Area of each HC mesh

AFTER trapdoor faulting
Trapdoorfaulting
occurs!

Shear stress on RF reduces zero

Estimated tsunami
arrival time

Fig. 2: Tsunami signal from the 2008 Kita-Ioto caldera earthquake,recorded by an ocean-bottom-pressure gauge of the DART 52404
station (orange triangle in Figure 1a).

Input parameters(g) depth = 2.0 (km)Crack
V0βm = 1.50 (m3/Pa)
Ring-fault dip angle = 83°
Elastic moduli λ,μ = 5.0 (GPa)Output parameters
pcrit = 11.79 (MPa)Δp = –1.97 (MPa)ΔV = 0.0030 (km3)

AHigh shear stress on RFDART 52404

V0βm = 1.50 (m3/Pa)

1) TF with RF slip of ~9 m explains the tsunami data, as well as seismic data
(notdipshownhere).
Ring-fault
angle = 83°
Elastic moduli λ,μ = 5.0 (GPa)2) Magma overpressure of p0~12 MPa is required to cause the TF.
Output parameters
3) Magma pressure drop is Δp0~2 MPa, only ~16 % of the overpressure before
TF.
pcrit = 11.79(MPa)Fig. 3: Trapdoor faulting in Sumisu submarine
caldera (Sandanbata et al., 2022).

•

12/15/2015

12/25/2010
11/15/2017

By comparing a mechanical-model-predicted tsunami waveform with the tsunami data,
Input parameters
we determine the TF motions/size and the magma overpressure thatcaused the TF.
Crack depth = 2.0 (km)Recently found at Sumisu caldera.
1) Mw~5 vertical-CVLD earthquakes
2) Efficient tsunami generation
3) Recurrence at a caldera

Model setting

12/15/2015

Mw 5.3b•---(b)

aHypothesis: “Trapdoor faulting (TF)”

CLICK HERE ▶

3. Results: Source model of the 2008 Kita-Iotocaldera earthquake
Discussion

03/11/2019

Figure 2

DART
52404

(a)

Osamu SANDANBATA1,2, and Tatsuhiko SAITO1

HC opens due tomagma overpressure p0

Normal stress on HC
is equilibrium with magma pressure

Fig. 5: SE–NW profile of displacement fields in the caldera (left) before and (right) after TF.

Tsunami data helps us to estimate remotely
the magma overpressure in a submarine caldera

Tsunami!

How much p0?

ModelData

Fig. 7. Tsunami of the model and data: (top)waveforms, and (middle & bottom) spectrograms.

4. Discussion and Conclusions• Our results suggest that the 2008 earthquake at Kita-Ioto
caldera was caused by a trapdoor faulting under water.
• The estimated magma overpressure of >10 MPa showsthat magma beneath the caldera was highly pressurized;
Modelthis overpressure value is comparable
to those estimated at(g)Axial Seamount and Sierra Negra when the eruptions
initiated (Cabaniss et al., 2020; Gregg et al., 2022).
• A trapdoor faulting reduces the magma overpressure by
only 10–20%, suggesting that the potential for volcanic
Data
unrest remains high even after a trapdoor faulting.
• Although the estimated values vary depending on assumed
source geometries, fault friction laws, and/or magma
properties, trapdoor faulting data can be utilized toinvestigate the physical status of a submarine volcano.
(f)

Trapdoor
faulting!