Pacific Array 2017
H. Kawakatsu, J. Gaherty & International Steering Committee*-a report intended for the ION business meeting @ AGU2017This short report updates the current status of the Pacific Array initiative1.
We are pleased to report that the first Pacific Array deployment will take place inspring of 2018 by the US team led by J. Gaherty (LDEO, Columbia University) via
funding provided by NSF for a proposal "Collaborative Research: Imaging small-scaleconvection and structure of the mantle in the south Pacific: a US contribution tointernational collaboration PacificArray"2. This is the first funded deployment of thePacific Array initiative that aims for two arrays (US1a and US1b in Figure 1). Proposals
submitted in 2016 by Japan (deployment) and Korea (cruise) were not funded. TheJapanese team led by H. Utada (ERI, U-Tokyo) has just submitted a renewed proposal
to JSPS as an international collaboration with Korea and Taiwan for two arrays "Oldest"and "WPH" in the figure. The Korean team led by S.-M. Lee (Seoul NationalUniversity) will submit a renewed proposal for cruises using the Korean ship. TheTaiwan team led by P. Lin (TORI: Taiwan Ocean Research Institute) plans to submit aproposal to conduct an BBOBS array deployment in the Philippine Sea (WPH) to which
the Japanese team will provide OBEM sensors.

Fig. 1. Planned arrays of Pacific Array. Two arrays US1a and US1b are
funded by NSF. Oldest and WPH arrays have been proposed by Japanese toJSPS. Small dots are past BBOBS deployments conducted either by Japaneseor US scientists. Other named and un-named spirals denote possible arraysthat are not more than just-an-idea.

Besides these planning activities, two international workshops for Pacific Array areheld in 2017 in Seoul and Tokyo. The first one is held in the Seoul National University(SNU) in April 13-14 entitled "Collaborative research in global ocean and subduction-toward Pacific Array-"3 (funding from NRF for YoungHee Kim (SNU) and from JSPS
for H Kawakatsu (ERI)) in which 10 Japanese scientists visited SNU to discuss withKorean scientists and students/postdocs on the related matters. Also P. Lin from Taiwan(TORI) participated the workshop to discuss future collaborations. The secondworkshop is held in Earthquake Research Institute (ERI) of The University of Tokyo on
May 23 after the JpGU-AGU joint meeting 2017. The title is "Ocean Mantle Dynamics
via Pacific Array"4 and various scientific problems related to the lithosphereasthenosphere system beneath the ocean were discussed.
In addition, a review paper (Kawakatsu and Utada5, 2017, Annual Review EPS) that
explicitly discusses about Pacific Array is also published in September, 2017 (written inJuly, 2016), in which the necessity and scientific merit are discussed. In the opening
plenary session of the IASPEI meeting in Kobe, B. Romanowicz, in her keynote
presentation, addressed the importance of the PA initiative. At the IRIS's 2017 OBS
Symposium6 held in Portland, Maine, Pacific Array was discussed in association withthe long-term seafloor seismographs (LTSS) strategy by Monica Kohler (chair of theworking group of LTSS).
So, overall, the Pacific Array initiative is moving forward and internationallywell-received. Further successful funding for individual proposals, as well as new array
proposals, are keenly awaited.

References:1

Pacific Array web: http://eri-ndc.eri.u-tokyo.ac.jp/PacificArray/
National Science Foundation web:https://www.nsf.gov/awardsearch/showAward?AWD_ID=1658491&HistoricalAw
ards=false
3
Workshop poster: http://eri-ndc.eri.u-tokyo.ac.jp/PacificArray/data/poster.pdf
4Workshop web: http://eri-ndc.eri.u-tokyo.ac.jp/PacificArray/postJpGUws2017.html
5
Kawakatsu, H., and H. Utada (2017), Seismic and Electrical Signatures of the
Lithosphere-Asthenosphere System of the Normal Oceanic Mantle, Annu. Rev.
Earth Planet. Sci., 45, 139-167.
6
Workshop web: http://www.obsip.org/about/2017-obs-symposium/
2* Pacific Array International Steering Committee:- JAPANH. Kawakatsu (ERI; chair), H. Utada (ERI)- USA
D. Forsyth (Brown), G. Ekstrom (Colombia),
R. Evans (WHOI), J Gaherty (LDEO)- EU
J.-P. Montagner (IPGP)- Australia B. Kennett (ANU)- S. Korea S.-M. Lee (SNU)- Taiwan
P-Y. P. Lin (TORI)US team funded project:

Collaborative Research: Imaging small-scale convection and structureof the mantle in the south Pacific: a US contribution to international
collaboration PacificArray (PI: Jim Gaherty)
All deformation on the surface of Earth, including faulting responsible for earthquakes,
is produced by the motion of tectonic plates. It is widely accepted that thermal
convection in the mantle drives plate motion, but details of that convection and how
exactly it moves the plates are poorly understood. Oceanic plates make up 70% of the
Earth's surface and offer important windows into mantle convection, yet they are largely
unexplored due to the lack of seismic data from the ocean basins. Questions aboundregarding the thermal structure of oceanic plates, the significance of volcanism in themiddle of oceanic plates, and how the convecting mantle beneath the plates controls
their movements. Waves in the gravity field and un-explained shallowing of the ocean
floors hint at small-scale convection beneath the oceanic plates. This project contributes
to an international effort to strategically place temporary arrays of instruments acrossthe Pacific Ocean basin that record the energy from earthquakes. Recent community
advances in ocean bottom seismographs will be used to record unique datasets inlocations where large gaps in coverage exist today. These data will allow us to infer
deformation and variations in mantle temperature related to small-scale convection. As
part of the international collaboration, all data will be openly available to scientists
worldwide. The project supports the training of graduate and undergraduate students.
This project will collect 12-15 months of broadband ocean bottom seismograph (OBS)
data in two 30-station arrays in the central and southern Pacific. These arrays, deployedat two distinct plate ages (~30 Ma and ~120 Ma), will address specific critical questions
on the dynamics of the oceanic asthenosphere, including its underlying state(temperature, presence of melt, water or other volatiles, and deformation mechanism).
The arrays are designed to image the anisotropic velocity signature of small-scaleconvection, which has been invoked to explain the flattening of the age versus depth
curve in old ocean plates, 140-200 km wavelength gravity lineations, and ubiquitousoff-axis, non-plume volcanism observed at a variety of scales. Anisotropic surface wave
and body wave tomographic models will be supplemented by shear wave splitting andattenuation measurements to obtain a multi-faceted understanding of the asthenosphereand base of the plates. Finally, the order-of-magnitude increases in path coverage forsurface and body waves in the south-central Pacific will enable new advances in global
tomography.