LEG 186
WEST PACIFIC SEISMIC NETWORK

Modified by K. Suyehiro from Proposal 431 Submitted by:

K. Suyehiro, H. Fujimoto, T. Kanazawa, J. Kasahara, Y. Fukao, H. Momma, K. Fujioka,
T. Matsumoto, J. Kinoshita, S. Sacks, A. Linde, R. Hino, A. Hasegawa, M. Shinohara, and
I. Kawasaki

Staff Scientist:  Gary Acton
Co-Chief Scientists: Kiyoshi Suyehiro and Selwyn Sacks


ABSTRACT

During Leg 186, we will drill two sites from the deep-sea terrace on the landward side of the Japan
Trench off northeast Japan to monitor crustal response to stable (aseismic) or unstable (seismic)
sliding of the subducting Pacific Plate. This will be the first attempt to establish such a long-term
seafloor borehole observatory in one of the world's most active and most studied subduction
zones. Near-field data from an active plate boundary are crucial in quantifying the plate's elastic and
anelastic behaviors in relation to earthquakes. Borehole strainmeters and broadband seismometers
will be installed at the bottom of the two holes to continuously monitor strain changes and seismic
activities associated with episodic plate motions. Secondary objectives include (1) determining the
forearc subsidence history and placing constraints on the subduction mass balance at this tectonic
erosional environment and (2) obtaining arc volcanism reference since 3 Ma.


INTRODUCTION

During Leg 186, we will drill two sites from the deep-sea terrace on the landward side of the Japan
Trench off northeast Japan to monitor crustal response to stable (aseismic) or unstable (seismic)
sliding of the subducting Pacific Plate. The western Pacific area, where the plate-consuming
boundaries are concentrated, is the best suited region on Earth to address the dynamics of the
subducting plates, formation and evolution of island arcs and marginal seas, and their relation to
mantle convection (e.g., Fukao, 1992; Fig. 1). In particular, the Japan Trench region has both high
seismicity and plate convergence rates compared to other trenches. Dense regional geophysical
networks have been expanded in the land area across Japan over the years. Such data accumulation
has made it possible to precisely locate the two proposed sites in seismically active (JT-1) and
inactive (JT-2) areas about 10 km immediately above the interplate seismogenic zone (Fig. 2).

The Japan Trench subduction zone is the world's best studied area, but it lacks near-field data that
are crucial in quantifying the plate's elastic and anelastic behaviors at an active plate boundary in
relation to earthquakes. Borehole strainmeters and broadband seismometers will be installed at the
bottom of the holes to continuously monitor strain changes and seismic activities associated with
episodic plate motions.

Secondary, but important, geological objectives include (1) determining the forearc subsidence
history and placing constraints on the subduction mass balance at this tectonic erosional
environment and (2) obtaining arc volcanism reference since 3 Ma.


BACKGROUND

The scientific importance of establishing long-term geophysical stations in deep oceans has been
acknowledged by earth sciences and Ocean Drilling Program (ODP) communities and is
expressed in various articles (COSOD II, 1987; Purdy and Dziewonski, 1988; BOREHOLE,
1995; Montagner and Lancelot, 1995; Ocean Drilling Program-Long Range Plan, 1996). In
essence, we want to understand active processes driving Earth's dynamics from a global to a
regional scale, but 71% of Earth's surface is covered by oceans that can only be probed by utilizing
state-of-the-art digital sensors linked with land-based stations. Many sensors, whose locations will
be carefully selected to maximize results, are needed around the world to obtain the goals of the
international geoscience programs. We have selected the western Pacific area for installation of
ocean-bottom sensors because it is ideal for addressing problems related to plate subduction. 

In the Japan Trench area, seven large (magnitude [M] > 7) interplate events occurred in the last 30
yr between 38 and 41 deg N. Recent large events are the 1968 Tokachi-Oki earthquake (~41 deg N
moment magnitude [Mw] 7.9) and the 28 December 1994 Far-off Sanriku earthquake (~40 deg N
Mw 7.7). These events, however, are not sufficient to account for the subducting rate of about 10
cm/yr. Thus, the seismic coupling seems much smaller along the Japan Trench (35-41 deg N) as
compared with the Kurile Trench or Nankai Trough regions, which have a higher seismic energy
release rate. Subduction at the Japan Trench may be proceeding by stable sliding either only with
relatively small (surface-wave magnitude [Ms] < 8) events or with occasional large events.

There is a third important category whereby the subduction rate is accommodated by episodic
aseismic events of time constants on the order of 10 min to several days (slow earthquakes). Such
events, if they exist, are presently extremely difficult to detect. Kawasaki et al. (1995) reported that
an ultra-slow earthquake estimated to be Mw 7.3-7.7 accompanied the 1992 Off-Sanriku
(39.42 deg N, 143.33 deg E, Mw 6.9) earthquake based on strain records observed ~120-170 km away
from the source. A postseismic strain change of 10-7 to 10-8 of a time constant of about a day
were
observed by quartz-tube extensometers (devices that measure absolute strain). Historically, in the
same area, the 1896 Sanriku tsunami earthquake (Mw ~8.5 but body-wave magnitude [Mb] ~7)
killed about 22,000 people. Tsunami earthquakes rupture in a much longer time constant of
minutes compared to normal type (Tanioka and Satake, 1996).

More recently, the Japanese global positioning system (GPS) network has revealed a postseismic
motion of northern Japan after the 1995 Far-off Sanriku earthquake (M 7.2) that can be explained
by a stress diffusion model assuming slow slip on the earthquake fault (Heki et al., 1997). A
different, but previously more prevailing interpretation, is that the postseismic deformation is due
to aseismic slip at a depth extended deeper down from the seismogenic zone. If such a slow slip
really occurred in the vicinity of the normal seismogenic zone, then strainmeters in the proximity
would have recorded signals not only much larger in magnitude but also would have resolved how
and where the slip initiated relative to a normal earthquake and how it proceeded. Furthermore, one
can test if aseismic and episodic slips occur irrespective of normal earthquakes.

The strain waveforms of slow earthquakes are of a ramp type. The amplitudes of strain steps
decay reversely proportional to the distance cubed, much more rapidly than seismic waves. It is
essential, therefore, to measure the strain signatures as near to these events as possible (within 20
km for an event equivalent to Mw 7.0; e.g., Johnston et al., 1990) to estimate how the regional
tectonic stress affects earthquake occurrences.

Because the primary objective is the establishment of long-term borehole observatories, once the
instruments are installed, they must be serviced for data analyses, distribution, and archiving.
There is an ongoing national program within Japan to achieve this (Ocean Hemisphere Project). A
new fiber optic cable owned by the University of Tokyo already exists and currently terminates
near Site JT-1. Once Site JT-1 proves to be useful, connections will be made to supply power,
send commands, and retrieve data in real-time on land. Furthermore, a 50-km cable extension is
planned to connect Site JT-2 as well. These stations will make invaluable additions to the existing
geophysical network over the western Pacific. The data will eventually become accessible
worldwide through the Internet.


SCIENTIFIC OBJECTIVES

Dynamic Sliding of the Subducting Plate and Earthquake Process
The seismic coupling efficiency of the subduction zone off Tohoku appears to be as low as 25%.
This means that of the total Pacific plate motion expected, only one-quarter is seen as strike-slip
motion leading to thrust-type earthquakes. One possibility is that three-quarters of the motion is
released as slow earthquakes, which are not recorded on normal seismographs. In the past, sparse
observations suggest that the slow strain release may consist of multiple episodes in which each
event is rather small. For this reason, installation of an instrument of the highest achievable
sensitivity is required. Any data leading to better understanding of the partitioning of strain release
into damaging "fast" events and slower events will be extremely valuable and may lend further
insight into the whole earthquake process.

The plate boundary off northeast Japan fulfills three important conditions for a long-term
geophysical observatory:

1.Dense geophysical networks to which our proposed observatories can be optimally linked
already exist on land.
2.Moderately large (M ~7) seismic events occur frequently, and aseismic slips (slow
earthquakes) with comparable or larger magnitude are expected to occur even more frequently.
3.Crustal and uppermost mantle structures have been well studied by reflection-refraction
seismic surveys and tomographic inversions (Suyehiro et al., 1985a, 1985b, 1990; Suyehiro
and Nishizawa, 1994; Ito, 1996).

Earthquake Source Studies
Stations at proposed Sites JT-1 and JT-2 will greatly improve source location (particularly depth),
focal mechanism, and rupture process determinations of the earthquakes near the Japan Trench
(Nishizawa et al., 1990, 1992; Suyehiro and Nishizawa, 1994; Hino et al., 1996). Near-field data
obtained from the stations at Sites JT-1 and JT-2 will particularly improve the resolution of the
source mechanisms of very slow rupture events such as tsunami earthquakes.

High-Resolution Geometry of the Plate Boundary
The two stations at proposed Sites JT-1 and JT-2 will be linked to the network of the broadband
and/or very broadband seismometers on the main Japanese islands and will make a dense seismic
network of 50-km in scale. The observations of various phases of body waves from many shallow
to deep earthquakes within the network will provide sufficient data to improve the structural image
of the plate boundary, particularly, the changes in physical properties associated with tectonic
erosion and seismogenesis.

Miocene and Younger Volcanic Ash Stratigraphy in the Western Pacific (Site JT-1)
The cores will represent an important reference section near Japan to compare to the remote ash
deposits already cored to the east. They will also provide important information about eruptive
processes, volcanic hazards, and aspects of climate such as response to wind, sand, and
volcanogenic input of greenhouse and related gases (J. Natland, pers. comm., 1997).

During Leg 132, a number of rhyolitic to dacitic volcanic ash beds on Shatsky Rise, east of Japan
were recovered (Fig. 1). Comparison with ash stratigraphy at Deep Sea Drilling Project (DSDP)
Sites 578-580, about halfway between Shatsky Rise and Japan, indicates that the Shatsky ash beds
were derived either from Japan or the Kurile-Kamchatka arc systems and that they were carried far
to the east on the high-speed polar and sub-tropical jet streams (Natland, 1993). A summary
appraisal is that 25-40 eruptions produced ash that reached one or more of those sites in each of the
past 3 m.y., with ~10% of these reaching Shatsky Rise in the form of discrete ash beds or pumice
drops. Some of the eruptions were extremely large, resulting in deposits 5 to 15 cm thick, even on
Shatsky Rise. The last drilling in this region was during DSDP Legs 56 and 57, before the advent
of hydraulic piston coring. An important, but seriously incomplete and at times highly disturbed,
ash record was recovered in Holes 438A and 440B (e.g., Cadet and Fujioka, 1980). Fluctuations in
accumulated ash thickness through time over the 15¯ of latitude represented by the proposed Leg
186 sites indicates that both the position and velocity of the jets have changed during the past 3
m.y., during the period of pronounced climatic deterioration since the early Pliocene. 

Age and Nature of the Cretaceous Basement
Only one hole (Hole 439) touched the Cretaceous basement during Legs 56 and 57 (Shipboard
Scientific Party, 1980). This Cretaceous unit is unconformably underlain at the drill site by a 48-m-
thick breccia conglomerate, which contains 24-m.y.-old hypabyssal dacitic to rhyolitic boulders
(von Huene et al., 1994).

Subsidence History Across the Continental Slope to Constrain the Processes 
of Tectonic Erosion
Quantitative estimates of the tectonic erosion process were made for the Neogene history of the
Japan Trench region based on drilling and seismic records (Fig. 3; von Huene and Lallemand,
1990; von Huene et al., 1994). Key evidence came from Site 439. Evidence collected from
additional drilling will further constrain the timing and erosion volumes in relation to backarc
opening and the style of convergence.

Subduction Rate
It is planned that proposed Sites JT-1 and JT-2 will be used as a benchmark for geodetic
measurements in the future. Repetitive measurements of the positions of Sites JT-1 and JT-2 using
an acoustic technique and differential GPS positioning will provide information on the subducting
rate of the western Pacific Plate at the Japan Trench.


DRILLING STRATEGY

The two proposed sites are to become legacy holes, thus requiring them to be equipped with
reentry cones and casing through unstable sections. Because the instrument must match the rigidity
of the surrounding rock, we need to drill into the Cretaceous basement. Installation of borehole
sensors are to be made by the drillship. The sensor package is about 8" in diameter and is to be
attached at the end of the drill string and to be disconnected at the seafloor level after cementing the
package section about 30 m from the bottom (Fig. 4).

To obtain volcanic ash records, piston coring to refusal is required, ideally at least twice, to ensure
complete recovery in the Pliocene and younger section, and single extended core barrel (XCB)
coring into the Miocene section below that (from ~250 to 450 mbsf).

LOGGING PLAN

The logging program for the two holes is designed to measure physical properties, anisotropy, and
hole shape. These objectives are quite similar to the objectives at the pilot site OSN-1 during Leg
136. The anazimuthal resistivity tool (ARI) could be used in place of the laterolog to measure
electrical anisotropy at ~1-m resolution, complementing the high-resolution formation
microscanner (FMS) images. Standard geophysical logs can be used to measure physical
properties, and  hole volume can be estimated with high accuracy using a borehole televiewer
(BHTV) log in the basement intervals. This will significantly improve grouting procedures for the
strain sensors and emplacement for the seismometers. High-resolution temperature logs should be
emphasized to identify permeable zones and inflow/outflow from both drilling-induced and natural
fractures in the holes.

All sites should be logged with (1) the standard triple-combo tool with ARI or dual laterolog
(DLL); (2) the FMS/Sonic/Temperature tool; and (3) the BHTV (logging speed is ~1 m/min).


PROPOSED SITES/OBSERVATORIES

Site JT-1B
Proposed Site JT-1B is located at the deep-sea terrace for observatory installation within a
seismically active zone (Fig. 2). This zone is known to be capable of generating from micro- to
large (M 7) earthquakes.

Site JT-2B
Proposed Site JT-2B is located at the deep-sea terrace south of Site JT-1B for observatory
installation within a seismically inactive zone (Fig. 2). Slip within this zone has not been
accompanied by detectable earthquakes for more than a decade. No clear historical record is
available that indicate seismic slips in this zone except possibly in 1678 or 1915.

Observatory Design
All the instruments will be third-party tools. Both sites are to be equipped with (1) a high-
resolution volumetric strainmeter (Carnegie Institution of Washington/University of Tokyo joint
development), (2) broadband seismometers (Guralp CMG-1) and back-up sensors (PMD2023),
(3) a temperature sensor, (4) a pressure gauge, and (5) a tilt sensor. Any heat generating
instrument will be separated from the strainmeter.

(1) Strainmeter
The Sacks-Evertson borehole volumetric strainmeter has proven to have resolution of better than
10-11 at various locations on land, including San Andreas, California, Iceland, and Japan (Sacks et
al., 1978; Linde et al., 1988, 1993, 1996). The instrument must be buried and cemented in solid
contact within a competent rock section. The deepest installation so far has been at ~500 m. The
high dynamic range and very broad frequency response (up to some tens of Hertz) of the borehole
strainmeter will result in an ocean-bottom installation providing valuable data for subduction zone
earthquakes. For example, on-land borehole strainmeters have proven to be effective in detecting
the slow initial stage of large earthquakes.

(2) Seismic Sensor
In September 1989, a feed-back type accelerometer capsule was installed in Hole 794D in the
Japan Sea during Leg 128 (Ingle et al., 1990; Suyehiro et al., 1992, 1995). The instrument
recorded a teleseismic event (Mb 5.4 at ~4000 km epicentral distance) clearly showing a surface
wave dispersion train (Kanazawa et al., 1992). In May 1992, a comparison of seafloor and
borehole (Hole 396B) sensors was made using a deep-sea submersible for installation and
recovery (Montagner et al., 1994). Although at this stage, there is no clear conclusion as to how we
should establish seafloor seismic observatories, it is becoming clear that oceans can provide low-
noise environments. For this particular case, where seismic sensors are to be installed as near to
the source as possible, borehole installation should give better constraints on hypocenter depths. It
is imperative that no fluid motion occur around the sensor; therefore, the seismometer and the
strainmeter must be cemented at the same location in the same operation.

(3) Pressure
Two sensors will be equipped to measure pressure at the sea bottom and at the bottom of the hole.

(4) Tilt
Tiltmeters (two component) will be included to measure crustal deformation.

(5) Temperature
The vertical profile of temperature in the hole will be measured by placing temperature sensors at
the top of the hole and at the top and the bottom of the bottom housing. Temperature inside the
bottom housing will be measured to compensate for temperature variations and the sensitivity of
the sensor.


REFERENCES

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Ocean-Bottom and Land-Based Observation. J. Phys. Earth, 38:347-360.
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redistribution. Nature, 275:599-602.
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the inner trench slope of the Japan Trench. Tectonophysics, 112:155-191.
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Crust of the Japan Trench Inner Slope. Tohoku Geophys. Jour. (Sci. Rep. Tohoku Univ.,
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downhole digital seismometer experiment at Site 794: a technical paper. In Tamaki, K.,
Suyehiro, K., Allan, J., McWilliams, M., et al., Proc. ODP, Sci. Results, 127/128 (Pt. 2):
College Station, TX (Ocean Drilling Program), 1061-1073.
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FIGURE CAPTIONS

Figure 1. Map of western Pacific area where at least five major plates with consuming boundaries
interact. Gray dots are DSDP sites. Bathmetry is in meters. 

Figure 2. Map of Japan Trench area with seismicity. The locations of proposed Sites JT-1 and JT-
2 are shown. Focal depth symbols: open circle = 0-10 km, open square = 10-20 km, open triangle
= 20-30 km, closed circle = 30-40 km, closed square = 40-50 km, closed triangle = 50+ km.

Figure 3. Tectonic subsidence history (from von Huene and Lallemand, 1990).

Figure 4. Schematic configuration of the instrument package with multisensors for crustal strain
and broadband seismometry.


TABLE 1 (revised)

PROPOSED SITE INFORMATION AND DRILLING STRATEGY


SITE:	  JT-1B		PRIORITY:  1		POSITION:  39¯10.878'N, 143¯20.095'W
WATER DEPTH: 2700 m	SED. THICKNESS:  ~1300 m	TOTAL PENETRATION:
1400 m
SEISMIC COVERAGE:  Intersection of KH96-3, Lines Leg 2-1 and Leg 2-4

Objectives:  (1) Install long-term geophysical borehole observatory to monitor changes in strain,
seismic
signals, tilt, pressure, and temperature. Quantify episodic plate motions within seismically active
part of the
interplate seismogenic zone. (2) Recover past 3 m.y. volcanic ash stratigraphy for comparison with
other
western Pacific ash deposits in relation to eruptive processes and transport mechanisms. (3)
Constrain
subsidence history of the Japan Trench forearc. (4) Determine nature of Cretaceous basement.  

Drilling Program:  Double APC to 250 m, XCB to 450 m, RCB to 1400 m, reentry cone. Case
through unstable section 

Logging and Downhole Operations:  Triple combo, FMS/Sonic/Temp, BHTV; install long-term
sensor
package and cement at the bottom 

Nature of Rock Anticipated:  Sandy, silty, pebbly clay with ash layers (50 m); clayey diatom ooze,
diatomaceous clay, ash layers (~350 m); diatomaceous claystone, clayey diatomite (~250 m);
claystone,
diatomaceous claystone, calcareous claystone (~200 m); sandy claystone, diatomaceous claystone
(~200 m);
turbidite sand silty claystone (~100 m), sandstone and siltstone (~100 m), boulder to pebble
conglomerate and
breccia, dacite and mudstone clasts (~50 m); silicified claystone (~100 m)


SITE:  JT-2C		PRIORITY:  1		POSITION:  38¯42.151'N, 143¯20.506'E
WATER DEPTH:  2143 m	SED. THICKNESS:  ~1300 m	TOTAL PENETRATION: 
1600 m 
SEISMIC COVERAGE:  Near intersection of KH96-3,Lines Leg 2-2 and KH90-1 Line JT90

Objectives: (1) Install long-term geophysical borehole observatory to monitor changes in strain,
seismic
signals, tilt, pressure, and temperature. Quantify episodic plate motions within seismically inactive
part of the
interplate seismogenic zone. (2) Constrain subsidence history of the Japan Trench forearc. (3)
Determine nature of Cretaceous basement.

Drilling Program: APC, RCB to 1400 m; reentry cone; case through unstable section

Logging and Downhole Operations: Triple combo, FMS/Sonic/Temp, BHTV; install long-term
sensor
package and cement at the bottom

Nature of Rock Anticipated:   Sandy, silty, pebbly clay with ash layers (50 m); clayey diatom ooze,
diatomaceous clay, ash layers (~350 m); diatomaceous claystone, clayey diatomite (~250 m);
claystone,
diatomaceous claystone, calcareous claystone (~200 m); sandy claystone, diatomaceous claystone
(~200 m);
turbidites and silty claystone (~100 m), sandstone and siltstone (~100 m), boulder to pebble
conglomerate and
breccia, dacite and mudstone clasts (~50 m); silicified claystone (~100 m)