Tsunameter network design in Pacific and Atlantic 25

6. Site Parameters for the U.S. DART®

Array

In this section, the details of each of the 39 sites comprising the current

U.S DART® array are provided. As noted earlier, some sites predated the

decision to expand the array in the wake of the 2004 tragedy and were retained. In particular, the primary role of the Hawaiian DART® 51407 is

the detection of a locally generated tsunami. As such its siting is outside

the scope of this study, as are the sites of the “Equatorial” DART® 51406,

and the non-U.S. Chilean tsunameter 32401. Although the two Australian

sites 55012 and 55015, in the Coral Sea and Tasman Sea, respectively, were

chosen by the Australian Bureau of Meteorology (Greenslade, personal communication) and those in the Indian Ocean are the responsibility of Thailand

(23401) and Indonesia (53401), similar graphical products for these sites are

included for completeness.

With the exception of the mid-ocean DART® sites, 51406 and 51407,

each graphic has a common format. To the left a broad region in the vicinity

of the finally chosen site is shown with EEZ boundaries in red (based on the

freely available dataset from VLIZ at www.vliz.be/vmdcdata/marbound),

and land in black. Water depths exceeding the engineering limit of 6000 m

are crosshatched in blue. Following the analysis of Mofjeld et al. (2001)

the locations of scattering features in the bathymetry are indicated by color

contours in shades of red of the scattering index S:

S = 1 − 2ε/(1 + ε2) (2)

where

ε = H1/H0 (3)

represents the extent to which the actual depth of a feature H1 deviates

from the average depth H0 in the vicinity. For this study the Smith-Sandwell

bathymetry is used to represent H1 while ETOPO60, regridded linearly from

one degree to 2 resolution, is employed as H0. Substantial scattering of

tsunami waves is to be expected where S exceeds a value of 0.2 (H1 < 1,500 m

in regional depths of 4,000–6,000 m) while major scattering results when S is

greater than 0.5 (H1 < 400 m). Ideally, to avoid excessive complexity in the

detected tsunami signal that may not be fully represented in the comparison

model series of the propagation database, a DART® should not be placed

where there are significant scattering features between it and the sources it

is intended to monitor. In these graphics sources are represented with the

green rectangles of the propagation database unit sources.

The propagation database allows the computation of first wave arrival

time, for a given unit source, at each point in the basin. Combining this
26 Spillane et al.

information it is possible to determine the earliest arrival from any of the

sources in the database and contour that field. The contour lines are referred

to as “minimum travel time envelopes” and, in the accompanying graphics,

the 15-, 30-, 45-, and 60-min envelopes are drawn. Although there have been

to date too few sizable seismic events near DART® sites to fully explore the

relationship between seismic noise amplitude and duration, the conventional

wisdom is that at least 30 min of tsunami travel time is needed for the

faster propagating (2–4 km/s) seismic waves to sufficiently outrun the true

tsunami signal. The travel time envelopes thus aid in site selection, and

most sites have been chosen at or beyond the 30-min envelope. Because

the travel times between Alaskan and Aleutian sources and the Hawaiian

Islands are in many cases close to the 3-hour warning period desired by

emergency managers, sites closer to the 15-min envelope have been selected.

Local bathymetric considerations have in some instances supported such

recommendations. Near-source placement will result in earlier detection,

though possibly at the expense of forecast quality. As noted earlier, to

compensate for this possible degradation, the spacing of the Alaskan and

Aleutian DART® sites has been reduced in order to provide cleaner though

somewhat later additional signals to possibly refine the early forecast based

on the first-reporting DART®.

Travel time envelopes are based on all unit sources within a basin. Generally the propagation database holds two rows of sources: the B-row adjacent

to the plate boundary, and an A-row representing earthquakes deeper on the

descending slab. In places, additional source rows are necessary to represent

submerged areas of shelf inshore of the A-row. To avoid excessive complexity in the graphics and tables that follow, attention is generally focused on

B-row statistics. Because this row is typically closer to the DART® sites,

and because the other rows are generally beneath shallower water depths

with slower speeds of propagation, the B-row detection times reported are

less than those of neighboring inshore unit sources,

Details of the siting environment for each DART® location are provided

in a set of graphics in the following sections. Common features of these

images are as follows: EEZ boundaries are drawn in red, land mass in black

(Japan’s Honshu in Fig. 7, for example), and waters exceeding the maximum deployment depth of 6,000 m shaded in blue. The primary instrument

location is the black square within a 5◦ × 3◦ rectangle whose topography

and speed characteristics are shown in the panels to the right; neighboring

sites are marked with smaller blue squares. Color-contoured in shades of red

is the scattering index, described at the beginning of this section. Strong

scattering of a wave between origination and detection can increase the uncertainty in the source identification (“inversion”) process. Minimum travel

time envelopes (in black), are drawn at 15-min intervals. Unit sources from

the propagation database are drawn in green with some B-row detection

times (in minutes) shown for the prime site. The detection time of concern

in the forecast setting is the earliest one available when instruments of the

array act in concert. Graphics and a table providing such overall statistics

are available in Section 7. Note that the figures in the PDF version of this
Tsunameter network design in Pacific and Atlantic 27

report www.pmel.noaa.gov/pubs/PDF/spil3192/spil3192.pdf are better

suited to close examination.

Bathymetry is based on Smith-Sandwell and is color-contoured in the

uppermost panel to the right. Line contours at 50-m intervals are drawn

within a 400-m band near the DART® depth. This aspect of the graphic,

intended to reflect depth variability near the site, is not fully satisfactory.

A larger scale image would be better and can, in many cases, be found in

the graphics of the Site Recommendation Reports of Appendices B through

I. In practice, an on-site survey during deployment is needed to adequately

determine the bathymetry.

Surface current speed statistics, based on a 600-day sample of NLOM

hindcast results near and at the site are shown in the remaining panels.

The 5% exceedance statistic (or 95th percentile) of the NLOM surface current speed distribution is color-contoured. Based on daily speed estimates

between 1 January 2006 and 23 August 2007 from this numerical hindcast

model (which assimilates satellite altimetry), the value contoured is exceeded

on 30 (5%) of the 600 readily available days. The extent to which such conditions might persist at the chosen DART® site is displayed in the final two

panels as a time series and a histogram of the speed distribution. Note that

for the speed histograms, the same speed scale (0–2 m/s) is employed for

all sites to facilitate identification of the moorings most at risk of damage

by excessive speeds. The NLOM model has been applied for multiple years

and provides hindcasts over the whole water column. The 600-day subset

of surface speeds employed here was acquired via anonymous FTP access.

The process was somewhat time consuming and, as noted earlier, occurred

after the site selection in response to the difficulties experienced with 42408

and 44402. Access to a more complete set of NLOM results would permit a

more thorough exploration of mooring design and deployment issues, but is

beyond the scope of this memo.

6.1 The Western Pacific

Six sites forming the West Pacific Group were the subject of the Site Recommendation Report reproduced in Appendix B. They are intended to provide

warning to the island nations of the western Pacific and, more remotely, to

Hawaii, of events in the complex of sources extending from Japan southward via the Marianas Arc to Yap, and from the Nankai Trough to New

Guinea via the Ryukyus and Philippines. Site details and data return details are provided in Table 2. The Data Return statistic provided in this

and later tables is intended to reflect intermittency in the data stream; periods of extended outage are excluded. As such the statistic provided in

these tables relates to the performance criteria for tsunameter reliability

(see www.ndbc.noaa.gov/dart/dart2_pc_1.shtml) where a return rate in

excess of 80% is mandated. As will be seen, this standard is significantly

exceeded for all but one of the DART® sites (77.6% at 21418 in the Northwest Pacific. A graphical representation of intermittency and major gaps
28 Spillane et al.

Table 2: Location and performance of the Western Pacific DART® Group (to 30 October 2008).

WMO Identifier Location Data Begins Major Disruptions Data Return (%)

21413 30.55◦N, 152.11◦E, 5827 m 27 Nov 2006 One 97.0

52401 19.26◦N, 155.76◦E, 5569 m 12 Dec 2006 One 98.3

52402 11.56◦N, 154.58◦E, 5799 m 15 Dec 2006 None 99.6

52403 4.02◦N, 145.58◦E, 4436 m 19 Dec 2006 One 95.4

52404 20.95◦N, 132.31◦E, 5926 m 1 Dec 2006 None 98.6

53405 12.88◦N, 132.32◦E, 5957 m 4 Dec 2006 None 99.2

in the data stream is presented in Section 7.3, where an overall data return

statistic is provided.

The northernmost site, 21413 lies near the 60-min envelope, essentially

midway between sites 21418 and 52401, to the east of an area of excessive

depths. As seen in Fig. 7 it provides detection times between 77 and 87 min

for the unit source segment shown but has successfully detected the minor

Kuril Island event of January 2007 further to the north. For sources directly

to the west the path is open; in other directions, especially the SW, are

numerous scatterers. DART® 21413 is located on a stretch of flat seafloor

but is rather close to the path of the Kuroshio with sustained surface currents

in excess of 0.6 m/s not uncommon.

Some 1,300 km to the SSE, and on the 60-min envelope for sources in the

Marianas, lies 52401 whose characteristics are displayed in Fig. 8. Numerous

scattering centers are present in this region; the dominant ones along the

southern edge are the Caroline Islands, which act to limit the penetration of

tsunamis generated off New Guinea into the wider Pacific. Current speeds

are somewhat weaker, though steadier, than at 23413, with a median of

0.3 m/s.

Further south, just outside the EEZ boundary of the Federated States of

Micronesia, is DART® 52402. The site (see Fig. 9) provides good detection

possibilities for events on the southern portion of the Marianas Arc. Sources

of the Manus oceanic convergent boundary, north of New Guinea, are about

2 hours to the south. The site should normally be beyond the influence of

the North Equatorial Current and has a median speed of about 0.3 m/s and

only rare current spikes exceeding 0.5 m/s.

DART® 52403 is the southernmost of the West Pacific group and is located in International waters just south of the Federated States of Micronesia

EEZ. The location (see Fig. 10) is well placed to intercept waves from Manus,

northern New Guinea, the Marianas, and the East Philippines. The current

speed regime may be somewhat severe and, based on NLOM statistics, a

move to the southwest may be indicated. Notice two sites (marked with

flag icons) in the southwest that Indonesia plans to instrument in November

2008 (one with a DART®-ETD) which will supplement the U.S. array.

Coverage of Ryukyu-Nankai and the northern portion of the East Philippines source region is provided by DART® 52404, which was placed in In-
Tsunameter network design in Pacific and Atlantic 29 95th Percentile NLOM Speeds 21418 52401 30min 45min 60min 15 min mm/s 30 min 45 min 60 min 21413 Figure 7: Siting environment for DART® 21413 in the Western Pacific.
30 Spillane et al. 52401 52402 21413 15 min 30 min 45 min 60 min 95th Percentile NLOM Speeds mm/s

Marianas

Figure 8: Siting environment for DART® 52401 in the Western Pacific.
Tsunameter network design in Pacific and Atlantic 31 52401 52402 52403

Marianas

Manus

15 min 30 min

45 min

60 min

95th PercentileNLOM Speeds

m

m/s

Figure 9: Siting environment for DART® 52402 in the Western Pacific.
32 Spillane et al. 15 min 15 min 30 min 30 min 45 min 60 min 52403 52405 52402 95th Percentile NLOM Speeds mm/s Manus Marianas Biak Halmahera Yap PalauFigure 10: Siting environment for DART® 52403 in the Western Pacific.
Tsunameter network design in Pacific and Atlantic 33

Marianas

52404

52405

15 min

30 min

45 min

60 min

95th PercentileNLOM Speeds

m

m/s

Luzon

Ryukyus

Figure 11: Siting environment for DART

® 52404 in the Western Pacific.
34 Spillane et al. 52405 52404 52403 Yap

Marianas

Palau

East Philippines

15 min

30 min

45 min

45 min

30 min

15 min

95th PercentileNLOM Speeds

m

m/s

Ryukyus

Figure 12: Siting environment for DART® 52405 in the Western Pacific.
Tsunameter network design in Pacific and Atlantic 35

Table 3: Location and performance of the Northwest Pacific DART® Group (to 30 October 2008).

WMO Identifier Location Data Begins Major Disruptions Data Return (%)

21414 48.94◦N, 178.27◦E, 5420 m 11 Aug 2006 One 96.0

21415 50.17◦N, 171.84◦E, 4709 m 28 Jul 2006 One 92.6

21416 48.05◦N, 163.49◦E, 5782 m 26 Jul 2007 None 99.5

21417 43.19◦N, 157.17◦E, 5482 m 24 Jul 2007 Two 87.0

21418 38.71◦N, 148.69◦E, 5665 m 23 Jul 2007 Several Minor 77.6

ternational waters just outside the Japanese EEZ (Fig. 11). It may detect

waves propagating westward from the Marianas, perhaps with reversed polarity (time order of leading peak/trough) to that seen at 52401 and 52402,

but the wave trains may be attenuated by passage through the island arc.

Site selection was constrained by the EEZ boundary and areas of deep water to the south and west. While surface speeds are generally acceptable,

Kuroshio meanders may occasionally impinge on the site.

Site 52405 (Fig. 12) provides coverage of the entire East Philippines

source region and the southern portion of the Marianas arc near Yap. For

the latter a reversed polarity signal to that seen by 52403 may be expected.

As for 52404, the water depth at 52405 is close to the instrument limit of

6,000 m. The site is on the northern edge of a zone of high current speed

extremes.

6.2 The Northwest Pacific

Five sites form the Northwest Pacific Group and were the subject of the

initial Site Recommendation Report reproduced in Appendix G. The objective of this group is to extend westward the coverage of the Aleutian Trench

sources provided by the legacy sites 46401 (now relocated to 46413), 46402,

and 46403 that predated array expansion. Site details and data return details are provided in Table 3.

The group begins with 21414 near the dateline, south of Amchitka, and

extends to 21418 east of Honshu, providing earlier and superior detection

for sources in the western Aleutians, and the Kamchatka and Kuril trenches.

DART® 21414 is located on the 15-min envelope (see Fig. 13) in order to

provide adequate (3 hour) warning to Hawaii of events associated with the

Aleutian Trench. Its scope is much wider and produced useful information,

though at delayed times, of the Kuril Island events of 2006 and 2007. These

wave trains traversed the Emperor Seamount Chain, which appears as a

series of scattering features along 170◦E in Fig. 13. Local bathymetry is

quite benign but high NLOM current speeds, not far to the east, are possibly

associated with eddies shed by the Alaskan Stream and might have caused

data loss had 21414 been deployed at the time.

Moving westward, site 21415 is located south of Attu within the U.S.

EEZ. The location, as seen in Fig. 14, provides a clear view of sources near
36 Spillane et al. 21414 21415 46413 46408 mm/s 95th Percentile NLOM Speeds 21414 15 min 30 min 45 min 60 min

Date Line

Figure 13: Siting environment for DART® 21414 in the Northwest Pacific.
Tsunameter network design in Pacific and Atlantic 37

m

m/s

95th PercentileNLOM Speeds

21415

21414

15 min

30 min

45 min

60 min

21415

Emperor Seamounts

21416

Date Line

Figure 14: Siting environment for DART

® 21415 in the Northwest Pacific.
38 Spillane et al. 15 min 30 min 30 min 60 min 21415 21416 21417 mm/s 95th Percentile NLOM Speeds 21416 Emperor Seamounts Figure 15: Siting environment for DART® 21416 in the Northwest Pacific.
Tsunameter network design in Pacific and Atlantic 39 21417 21416 21418 15 min30 min 45 min 60 min mm/s 95th Percentile NLOM Speeds 21417 Kuril Islands Figure 16: Siting environment for DART® 21417 in the Northwest Pacific.
40 Spillane et al. 21418 21417 21413 15 min 30 min45 min mm/s 95th Percentile NLOM Speeds 21418 60 min Figure 17: Siting environment for DART® 21418 in the Northwest Pacific.
Tsunameter network design in Pacific and Atlantic 41

the junction of the Aleutian and Kamchatka trenches, and those further

south, with a reduced influence of the Emperor Seamount Chain compared to

21414. Surface current speeds are higher than desirable, perhaps accounting

for the increased intermittency in the data return (90%). To the extent to

which this issue persists, and NLOM adequately represents the speed regime,

a move to the southwest of its current location may be indicated.

To the southwest lies the next site, 21416, of the Northwest Pacific

group. The terrain in the region (see Fig. 15) and the greater travel times

to U.S. interests permit considerable flexibility in siting. The chosen site

is on the 30-min envelope, outside the Russian EEZ, and provides coverage

of sources from the Western Aleutians to Hokkaido. In an area of weak

surface currents, data return has been good (99.49%) in the 15 months of

deployment analyzed, compensating for the early loss of 21417 off the active

Kuril Island region to the south.

Site 21417 lies directly offshore from the epicenters of the November 2006

and January 2007 Kuril Island events (Fig. 16). As for 21416, the terrain

made this an easy site to select. Fortuitously, the site lies in a local minimum

of the NLOM speed field. Whether the structure is persistent or an artifact

of the 600-day 2006–2007 sample of hindcasts available is not known. The

circumstances of the early loss of 21417: the dragging of the surface unit

eastward, out of range of acoustic communication with the seafloor unit,

following 5 weeks of 100% data return, suggests contact with a vessel. The

instrument was redeployed in May 2008 and the data return exceeded 99%

from then until another loss occurred. Later in this report, data on shipping

lanes and fishing activity are considered as a possible explanation of these

losses. Such data will be incorporated in future siting studies.

The southernmost element of the Northwest Pacific group of DART®s

is 21418. The site, seen in Fig. 17, was chosen based on terrain, EEZ, and

tsunami travel time considerations, but without due regard for the ambient

current regime. Based on the NLOM statistics it would appear that the

mooring is exposed to the northern edge of the Kuroshio, perhaps explaining

the severe intermittency of the data return in the present deployment, which

has reduced the data return rate to 77.6%. During the first deployment (July

2007–May 2008), the return rate of 84.7% was lower than that at any other

site, though exceeding the 80% standard. Should the intermittency persist,

and an association with speed distribution can be confirmed, a move to a

site to the northeast along the 30-min envelope may be indicated.

6.3 The Aleutians and Alaska

Six sites comprise the Aleutian/Alaska Group and were the subject of the

Site Recommendation Report reproduced in Appendix E. The sites, ordered

from the west near Adak to the Gulf of Alaska southeast of Anchorage,

and their data return details, are provided in Table 4. The region has a long

history of the use of BPR technology to detect tsunamis (Eble and Gonz ́alez,

1991). In particular, 46413 represents a re-siting of a previous instrument

(46401) from a location 46.64◦N, 170.79◦W further offshore to the southeast;
42 Spillane et al.

Table 4: Location and performance of the Aleutian and Alaska DART® Group (to 30 October

2008).

WMO Identifier Location Data Begins Major Disruptions Data Return (%)

46413 48.87◦N, 175.59◦W, 5568 m 10 Aug 2006 Two 95.7

46408 49.63◦N, 169.86◦W, 5387 m 14 Aug 2006 None 98.0

46402 51.06◦N, 164.00◦W, 4712 m 1 Jan 2003 Two 98.7

46403 52.64◦N, 156.93◦W, 4508 m 1 Jan 2003 Three 97.5

46409 55.30◦N, 148.50◦W, 4184 m 29 Sep 2005 None 98.4

46410 57.50◦N, 143.99◦W, 3740 m 28 Jul 2006 One 89.6

46402 is a slightly adjusted continuation of the site previously occupied since

2003 near 50.44◦N, 165.03◦W. Taken together with sites 21415 and 21414 of

the Northwest Pacific group, a picket line with nominal zonal spacing of 6◦,

close to the 15-min envelope results that should provide adequate warning

to the Hawaiian Islands for Aleutian and Alaskan events.

Site 46413 is the westernmost of the Aleutian/Alaska group and is described by Fig. 18. There are no scattering centers of note between 46413 and

the source line inshore of the Aleutian Trench. The bathymetry and current

speed regime (based on NLOM) are also benign and result in little intermittency in the data stream. The site registered the Kuril Island tsunami events

of November 2006 and January 2007 which, before the Northwest group were

fully deployed, provided input to a useful assessment of these mild events,

and an unplanned test for SIFT system components under development.

Further to the east along the 15-min minimum travel time envelope is site

46408, whose characteristics are displayed in Fig. 19. As was the case for

46413, scattering features are confined to the island chain of the Aleutians,

and the Bering Sea basin and shelf to their north, and should not affect

tsunami signal reception. In the vicinity of 46408 the currents are weak,

though there is evidence of the Alaskan Stream to the north. With a 97.91%

data return rate since its establishment in August 2006, 46408 appears to be

a reliable element for the detection of tsunami signals in the North Pacific.

Proceeding eastward, the next site of the Aleutian/Alaska group is 46402.

As noted earlier, though slightly moved from its original location, this site

has a long history of tsunami monitoring in the region. Its current location

appears to be well removed from the Alaskan Stream, though there is an

episode in the time series and a noticeable tail to the speed distribution

(lower right panel in Fig. 20) that suggests that the site may not be immune

to eddies shed by the Stream. Note however that, as yet, no direct link

has been established between high-speed episodes in the NLOM record and

intermittency in the DART® data stream.

The next site in the group is 46403, offshore from the Shumagin Islands

as portrayed in Fig. 21. Although there are no scattering features of concern,

for the sources between Unimak and Kodiak that are detected early by 46403,

the beginnings of such features are evident near 150◦W. These may influence
Tsunameter network design in Pacific and Atlantic 43 46413 21414 21415 46408 46402 15 min30 min 45 min 60 min 95th Percentile NLOM Speeds mm/s Aleutian Trench

Date Line

Figure 18: Siting environment for DART® 46413 in the Aleutian/Alaska group.
44 Spillane et al. 46408 46402 46413 21414 15 min 30 min45 min 60 min 95th Percentile NLOM Speeds mm/s

Date Line

Figure 19: Siting environment for DART® 46408 in the Aleutian/Alaska group.
Tsunameter network design in Pacific and Atlantic 45 46402 46403 46408 46413 95th Percentile NLOM Speeds mm/s 15 min 30 min 45 min 60 min Figure 20: Siting environment for DART® 46402 in the Aleutian/Alaska group.
46 Spillane et al. 46403 46402 46409 46410 46408 15 min 30 min 45 min 60 min mm/s 95th Percentile NLOM Speeds Figure 21: Siting environment for DART® 46403 in the Aleutian/Alaska group.
Tsunameter network design in Pacific and Atlantic 47 46419 46410 46409 46403 15 min30 min 45 min 60 min 95th Percentile NLOM Speeds mm/s Yakataga Figure 22: Siting environment for DART® 46409 in the Aleutian/Alaska group.
48 Spillane et al. 46403 46409 46410 46419 46404 46407

Cascadia

Yakataga

95th PercentileNLOM Speeds

m

m/s

15 min

30 min

45 min

60 min

 QueenCharlotte Islands

Figure 23: Siting environment for DART® 46410 in the Aleutian/Alaska group.
Tsunameter network design in Pacific and Atlantic 49

the reception of signals originating further east for which the Gulf of Alaska

sites, 46409 and 46410, are the primary detectors. Surface current speed

would appear to be of little concern at 46403, with no values exceeding

0.45 m/s in the 600-day record of NLOM hindcasts studied.

Site 46409 lies the northern Gulf of Alaska, south of the Kenai Peninsula.

Only the B and A rows of unit sources are displayed in Fig. 22, but the

propagation database includes source rows further inshore. Though not

identified as subducting plate boundaries in the Bird (2003) database, the

Yakataga Zone and the plate boundary that extends southward past the

Queen Charlotte Islands are a potential source of tsunami waves and have

been incorporated into the propagation database. Surface speeds near 46409,

based on NLOM, would not appear to be of concern but the presence of

numerous scattering features were a major constraint in site selection. The

chosen site lies midway between two major groups of such features.

Site 46410 has the distinction of being the most northerly site in the

U.S. array. Given the distance to site 46419, the closest of the West Coast

group, it might have been preferable to place 46410 somewhat to the south

near the 30-min envelope. However, the multiple scattering features in that

direction might increase the complexity of the tsunami signals, whereas the

selected site should provide clean detection of the source regions that have

historically posed a greater threat. The surface speed pattern exhibits a lobe

of higher values in the vicinity of the chosen site and another further north.

The frequency of such events, beyond the 2006–2007 NLOM period analyzed,

is not known and, while the intermittency of the data return (91.9%) from

46410 is the poorest in the group, other possible causes, such as sea state,

have not been ruled out.

6.4 The U.S. West Coast

Five sites (one within the Canadian EEZ) form the West Coast Group, and

were the subject of the initial Site Recommendation Report reproduced in

Appendix E. Site details and data return details are provided in Table 5.

As was true for the Aleutian/Alaska group, the West Coast has a long history of DART® data collection predating the Indian Ocean disaster of 2004

which led to the expansion in tsunami detection capability in the Pacific

and Atlantic Oceans. For the West Coast itself the threat is multi-faceted,

including the severe local threat associated with Cascadia, the potential for

local landslide-associated wave generation off southern California, and the

exposure of the entire coast to tele-tsunamis. Cascadia, of course, poses a

severe threat to the entire basin; West Coast elements of the U.S. DART®

array must serve the need of forecasts to remote areas. As discussed earlier,

in the event of a major event in Cascadia, ground motion, recognized by an

informed public, may be the only advance warning to coastal communities in

the region. High-quality DART® data, processed through the SIFT system,

can provide estimates of inundation and the expected duration of significant

wave activity to guide the emergency response. The reality that “warning”

and “forecast” may differ in meaning from their usual interpretation in the
50 Spillane et al.

Table 5: Location and performance of the U.S. West Coast DART® Group (to 30 October 2008).

WMO Identifier Location Data Begins Major Disruptions Data Return (%)

46419 48.74◦N, 129.61◦W, 2775 m 25 Jul 2006 One 89.1

46404 45.86◦N, 128.78◦W, 2740 m 11 Jan 2003 Three 90.8

46407 42.60◦N, 128.89◦W, 3266 m 10 Dec 2007 None 97.7

46411 39.33◦N, 127.01◦W, 4266 m 27 Sep 2005 One 95.2

46412 32.25◦N, 120.70◦W, 3777 m 29 Sep 2005 None 98.8

context of a locally generated tsunami is not unique to Cascadia, but can

complicate siting decisions in such circumstances.

Site 46419, plotted in Fig. 24, lies off Vancouver Island, slightly outside

the 15-min envelope and within the Canadian EEZ. The main rationale for

its choice is that, in combination with its three regularly spaced neighbors

(46404, 46407, and 46411) to the south, it provides early detection of tsunami

waves generated in the vicinity. The detection times 46419 provides are

about an hour for sources near the Queen Charlotte Islands and 90 min for

those at the southern end of Cascadia. Further detail on its siting, together

with a more useful chart of the bathymetry, can be found in Appendix E. In

hindsight, the choice from the point of view of current speeds is reasonable.

There may be episodes of speeds in excess of 0.5 m/s but the median is

about 0.2 m/s in the NLOM analysis. Between its deployment in July 2006

and its malfunction in January 2008, the instrument provided a data return

in excess of 98%.

Among U.S. West Coast DART®s, 46419 plays another role: to provide

a “second look” at waves generated and detected off Alaska, but propagating

toward the Pacific Northwest. Though not computed with that in mind, the

proximity of the Cascadia source line to the open coastlines of Vancouver

Island and the U.S. between Neah Bay and Cape Mendocino permit another

interpretation for 46419’s “detection times” of Fig. 24. Detection by 46419

can provide “warning times” of at least 30–90 min for waves arriving from

the north. Warning times have not been emphasized in this document since

the coarse spatial resolution of the propagation database, and possible nonlinear behavior close to shore, limits the ability to determine coastal arrival

times without better representation of inshore bathymetry.

Sites 46404 and 46407, portrayed in Figs. 25 and 26, respectively, provide the primary coverage for Cascadia subduction zone events. Like 46419

they have the potential to provide an assessment of impacts to the Northwest Pacific Coast for remote events, though quantifying their utility in this

regard is beyond the scope of this memo. In particular 46407 is well placed

to detect waves directed toward Crescent City, California, whose harbor,

perhaps due to resonance and the action of the Mendocino escarpment in

directing energy toward it, is often impacted by remote events. Site 46407,

as described in Appendix E, is a relocation of the long-term site 46405 that

lay beyond the 60-min envelope; both 46404 and 46407 now lie close to the
Tsunameter network design in Pacific and Atlantic 51 46410 46419 46404 46407 46411 15 min 30 min 45 min 60 min

Cascadia

 QueenCharlotte Islands

95th PercentileNLOM Speeds

m

m/s

Yakataga

Mendocino Escarpment

Figure 24: Siting environment for DART

® 46419 in the U.S. West Coast group.
52 Spillane et al. 4641946404 46411 46407 15 min 30 min 45 min 60 min

Cascadia

95th PercentileNLOM Speeds

m

m/s

Mendocino Escarpment

Figure 25: Siting environment for DART® 46404 in the U.S. West Coast group.
Tsunameter network design in Pacific and Atlantic 53 46419 46412 46411 46407 4640415 min 30 min 45 min 60 min 95th Percentile NLOM Speeds mm/s

Cascadia

Mendocino Escarpment

Figure 26: Siting environment for DART® 46407 in the U.S. West Coast group.
54 Spillane et al. 46419 46404 46407 46411 46412

Cascadia

Mendocino Escarpment

Mendocino Escarpment

95th PercentileNLOM Speeds

m

m/s

15 min 30 min

45 min

60 min

Figure 27: Siting environment for DART® 46411 in the U.S. West Coast group.
Tsunameter network design in Pacific and Atlantic 55 60 min 60 min 46411 46412 43412 95th Percentile NLOM Speeds mm/s Channel Islands Channel Islands Figure 28: Siting environment for DART® 46412 in the U.S. West Coast group.
56 Spillane et al.

Table 6: Location and performance of the Central/South American DART® Group (to 30 October

2008).

WMO Identifier Location Data Begins Major Disruptions Data Return (%)

43412 16.03◦N, 107.00◦W, 3239 m 21 Mar 2007 None 87.3

43413 10.84◦N, 100.08◦W, 3468 m 24 Mar 2007 None 97.5

32411 4.92◦N, 90.69◦W, 3247 m 27 Mar 2007 None 98.2

32412 17.97◦S, 86.39◦W, 4326 m 2 Nov 2007 None 99.9

32401 (Chile) 19.58◦S, 74.81◦W, 4881 m 23 Mar 2005 None 98.2

30-min envelope. There are some minor scattering centers to the northwest

of 46404 but, while rugged, the bathymetry is quite amenable to DART®

operations as is the current regime as portrayed in NLOM hindcasts.

Proceeding southward, DART® 46411 lies offshore between Cape Mendocino and Point Arena. Prominent in the bathymetry panel of Fig. 27 is the

Mendocino Escarpment. The surface current speed distribution represented

by NLOM has features oriented east-west that are aligned with strong topographic gradients, but the site of 46411 lies in a local minimum with few

spikes in speed exceeding 0.4 m/s in the time period examined.

Completing the U.S. West Coast group is DART® 46412, west-southwest

of San Diego, California. While there is not a direct threat to this region

from seismically generated tsunamis (the earthquake mechanisms being more

strike-slip than normal or reverse-thrust in nature and, in consequence, involving less vertical motion of the sea floor), there is the risk that the frequent

earthquakes may trigger a submarine landslide. The site chosen for 46412,

as shown in Fig. 28, reflects that hazard, being located at the closest point to

the Channel Islands at which the water depth is adequate. With a detection

time in excess of 3 hours, 46412 is the first DART®, other than 43412 and

43413 off Central America, to detect waves generated south of Baja California that may impact the U.S. mainland. The paths for such waves are likely

to be quite oblique to the main beam and would likely only be detectable

for larger source magnitudes (above Mw = 7.8 using the methods discussed

in the Section 7.2).

6.5 Central and South America

Four U.S. sites form the Central and South American Group and were the

subject of the initial Site Recommendation Reports reproduced in Appendix

F. Site details and data return details are provided in Table 6. Note that

details of the Chilean tsunameter (32401) have been added to Table 6, and

the discussion that follows, for completeness.

Sites 43412 and 43413 have primary detection responsibilities for sources

associated with the Central American Trench, of which they have a clear

view, unobstructed by significant scattering features. As seen in Fig. 29,

the northern element of the pair is placed close to the 30-min envelope to
Tsunameter network design in Pacific and Atlantic 57

intercept waves en route to the U.S. West Coast as well as providing some

warning and relevant information to local emergency managers of the popular beach resorts from Cabo San Lucas to Acapulco. While episodes of

higher current speed are not uncommon, the median is less than 0.2 m/s.

Site 43413 (Fig. 30) is somewhat further offshore in order to monitor sources

from southern Mexico to Costa Rico. It lies midway between 43412 and the

site of the northernmost of the South American DART®s, 43413. While to

the south of some strong current features, the extended tail of the NLOM

speed histogram suggests that currents may on occasion be an issue.

The South American subduction zone, offshore from Columbia and Peru,

is oriented so as to pose a greater threat to U.S. interests than sources off

Central America. The presence of multiple scattering features, including

the Galapagos, Cocos Ridge, some small island groups, and their associated

EEZs, make the choice of a location for 32411 difficult. The selected site,

fortunately, is near a minimum of the 5% exceedance current speed distribution, but the possibility of short bursts of excessive speeds is greater than at

most recommended sites. As seen in Fig. 31, there is a strip of international

water between the Galapagos and mainland Colombia that is closer to the

South American sources, but the recommended 32411 location in a notch of

the EEZ boundary will, it is hoped, provide superior wide coverage. There

is the possibility that a DART® might be deployed by Ecuador, most likely

within their EEZ limits because of the local threat. This would be a valuable

addition to the overall array, which is rather sparse off South America.

Continuing southward, that sparseness is evident in Fig. 32, which shows

the site chosen for the U.S. DART® 32412. Initially a site further to the

north was considered but reports of a greater likelihood of vandalism, south

of the Galapagos, were provided by NDBC. The selected site is well offshore

(at about the 80-min envelope) in order to capture events from a wider range

of sources, including those further south than the long-standing Chilean

tsunameter 32401. Numerous scattering features, and the lengthy travel

times, may diminish the utility of 32412 to detect sources far to its south.

The current speed environment near 32412 is quite benign, with a median

less than 0.2 m/s and few daily NLOM hindcasts exceeding 0.35 m/s. The

poor coverage of southern Chilean sources, in the absence of tsunameters

south of 32401, has been recognized as a deficiency in the array. After much

discussion, it is planned to move the instrument currently named 51406 to

a site south of the Juan Fernandez Islands as indicated in Fig. 32

Though the siting of the Chilean tsunameter predated and was not part

of the array expansion effort, Fig. 33 summarizes its characteristics in the

same format employed for the others. The Peruvian earthquake of August

2007, in addition to causing casualties and damage inland, generated a minor

tsunami, which was detected by 32401. This provided an unscheduled test

for SIFT components under development (Wei et al., 2008a). The near

shore location of the source demonstrated the need to add extra rows to the

propagation database; this has since been done.
58 Spillane et al. 43412 43413 15 min 30 min 45 min 60 min mm/s 95th Percentile NLOM Speeds Figure 29: Siting environment for DART® 43412 in the Central and South American group.
Tsunameter network design in Pacific and Atlantic 59 43412 43413 32411 95th Percentile NLOM Speeds m/s m 15 min 30 min 45 min 60 min Figure 30: Siting environment for DART® 43413 in the Central and South American group.
60 Spillane et al. 32411 15 min 30 min 45 min 60 min mm/s 95th Percentile NLOM Speeds Figure 31: Siting environment for DART® 32411 in the Central and South American group.
Tsunameter network design in Pacific and Atlantic 61 95th Percentile NLOM Speeds 43412 32401 mm/s Recommended Site for 51406 Relocation 15 min 30 min 45 min 60 min Juan Fernandez Islands Figure 32: Siting environment for DART® 32412 in the Central and South American group.
62 Spillane et al. 60 min 45 min 30 min15 min 32401 32412 mm/s 95th Percentile NLOM Speeds Figure 33: Siting environment for the Chilean Tsunameter 32401.
Tsunameter network design in Pacific and Atlantic 63

Table 7: Location and performance of the Southwest Pacific DART® Group (to 30 October 2008).

WMO Identifier Location Data Begins Major Disruptions Data Return (%)

52406 5.34◦S, 165.08◦E, 1849 m 4 Mar 2008 One 97.4

51425 9.50◦S, 176.25◦W, 4962 m 28 Feb 2008 None 98.7

51426 23.01◦S, 168.11◦W, 5675 m 11 Feb 2008 One 96.5

54401 33.00◦S, 172.99◦W, 5837 m 17 Feb 2008 None 96.2

55012 (Aust.) 15.80◦S, 158.50◦E, 3284 m 27 Mar 2008 None 99.1

55015 (Aust.) 46.93◦S, 160.47◦E, 4944 m 16 Apr 2007 Two 96.5

6.6 The Southwest Pacific

Four U.S. sites form the Southwestern Pacific Group and were the subject of

the initial Site Recommendation Reports reproduced in Appendices H and

I. Site details and data return details are provided in Table 7. Note that

details of two Australian DART®s (55012 in the Coral Sea and 55015 in

the Tasman Sea) are included in Table 8 and the discussion that follows.

The latter sites were selected by the Australian Bureau of Meteorology to

provide warning to their East and South coasts, but extend the capability

and source coverage of the Pacific array.

This portion of the array extends the coverage provided by the Western

Pacific group, discussed in subsection 6.1, for source segments in the vicinity

of New Guinea and extending eastward via the Solomon Islands to Vanuatu,

then from Tonga via the Kermadec Islands to New Zealand. One Australian

tsunameter, 55012 in the Coral Sea, supplements the coverage provided by

the four U.S. DART®s. Another Australian instrument, 55015 in the Tasman Sea, has primary responsibility to monitor sources near the Puysegur

Trench, which extends southward from New Zealand’s southern island and

poses a significant hazard for Australia’s southeastern seaboard in addition

to the local threat. Occasional earthquakes occur along the plate boundary

south of Puysegur which 55015 is better placed to detect than is 54401, the

southernmost of the U.S. Southwest Pacific group. Some graphics are provided for these Australian sites since, together with the Chilean tsunameter,

their capabilities need to be considered in assessing the performance of the

array as a whole.

DART® 52406, as its WMO designation suggests (the first two digits

representing the sea area), is closest (2,300 km) to 52403 of the West Pacific

group. As shown in Fig. 34, the extensive EEZ ownership resulting from the

numerous island groups, which act as scattering features, makes site selection

quite difficult in this region. The New Guinea-Vanuatu source line is to a

large extent screened from the open Pacific by the island chains, though

the Solomon Islands tsunami of April 2007 was detectable, though mild, off

Hawaii. The gap between San Cristobal and the Santa Cruz Islands is one

of the wider openings and 52406 was sited to monitor the source line in this

area (between the Solomons and Vanuatu) with detection times of about
64 Spillane et al.

1 hour. It could also detect tsunami waves that might propagate eastward

from Manus and northern New Guinea. The topographic (upper right) panel

of Fig. 34 shows that the BPR lies on the south-facing slope of a bank. This

increases the signal somewhat since the low amplitude of the waves from most

unit sources in the region is an issue. Normally the caveat that imperfect

representation of the bathymetry in the propagation runs reduces the fidelity

of the model results might preclude the site. However, the combination of

weak signal, complex bathymetry, and EEZ boundaries limited the selection.

In hindsight the NLOM speed statistics is another count against the site.

While it would appear to avoid the strongest NLOM speed features, the

frequency and strength of high-speed events, if valid, would be of concern.

The data return statistics (discussed in Section 7.3 using Fig. 57b) do not

show much intermittency. NLOM statistics are not available for the period

of 52406’s deployment to shed light on this inconsistency.

Siting of DART® 51425 was also difficult, as a result of the almost complete EEZ ownership in the target area, the numerous scattering features,

and the sharp turn the source line takes between Tonga and the Samoa Islands. The selected site is in a narrow strip of international water where

it monitors the east-west portion of the source line (with detection times

of about 50 min), and is in the energy beam of waves originating in the

Vanuatu region but directed toward Hawaii. Though the latter have detection times of about 3 hours, the warning time for Hawaii is still adequate.

Another area available for siting is American Samoa, but the selected site,

in combination with the two other elements of the group, provided a more

comprehensive coverage. Of concern are the somewhat elevated statistics of

the NLOM speeds, though, as for 52406, the data returns to date have been

highly reliable.

The main section of the Tonga and Kermadec trench line, running from

NNE to SSE between Samoa and the east coast of New Zealand, is monitored

by two DART®s, 51426 and 54401 as shown in Figs. 36 and 37. International waters extend southward from the Niue EEZ. This island, together

with the Louisville Ridge (see Fig. 37), are the most notable scattering features in the region. The Tonga Trench portion of the source line is well

covered by 51426 near the 45-min envelope, though the instrument is in the

lee of Niue for the most northerly sources. The site chosen for 54401 lies

on the 30-min envelope with an uninterrupted view of Kermadec Trench

sources. For both sites the NLOM speed statistics suggest that ambient

currents should not be of concern, particularly at 54401.

As noted earlier, the Australian sites in the Coral and Tasman Sea are

a valuable complement to the U.S. array. The sites were chosen by staff of

the Australian BOM (Bureau of Meteorology) (Greenslade, personal communication) using methods, and a propagation database, similar to those

described here. The calculations were shadowed at NCTR, providing a useful comparison of the underlying databases that were separately generated

by the two programs. Because the source line between New Guinea and

Vanuatu lies between the island arc and the Australian mainland, the Coral

Sea DART® 55012 is better positioned than is 52406 to directly detect

most unit sources in the line. The exception to this is the southern portion
Tsunameter network design in Pacific and Atlantic 65 52406 51425 55012 15 min30 min 45 min 60 min Coral Sea Vanuatu Solomon Islands 95th Percentile NLOM Speeds mm/s

Date Line

Figure 34: Siting environment for DART® 52406 in the Southwest Pacific group.
66 Spillane et al. 51425 51426 52406 15 min 30 min45 min 60 min 95th Percentile NLOM Speeds mm/s Tonga Fiji Samoa Vanuatu

Date Line

Figure 35: Siting environment for DART® 51425 in the Southwest Pacific group.
Tsunameter network design in Pacific and Atlantic 67 51426 51425 54401

Louisville Ridge

Tonga

Samoa

Kermadec Islands

15 min 30 min

45 min

60 min Niue 95th Percentile NLOM Speeds

m

m/s

Figure 36: Siting environment for DART

® 51426 in the Southwest Pacific group.
68 Spillane et al. Louisville Ridge

Kermadec Islands

15 min 30 min 45 min

60 min

54401

51426

95th PercentileNLOM Speeds

m

m/s

Date Line

Figure 37: Siting environment for DART® 54401 in the Southwest Pacific group.
Tsunameter network design in Pacific and Atlantic 69 55012Solomon Islands Coral Sea New Caledonia 95th Percentile NLOM Speeds 52406

m

m/s

Figure 38: The Australian Coral Sea Site 55012.
70 Spillane et al. 15 min 30 min 45 min 60 min55015 Puysegur Ridge Auckland Islands 95th Percentile NLOM Speeds mm/s Non-subducting Plate Boundary Macquarie Island Event April 12, 2008, Mw=7.1 254 Figure 39: The Australian Tasman Sea Site 55015.
Tsunameter network design in Pacific and Atlantic 71

of the source line where New Caledonia interrupts the direct path. DART®

52406 is slightly better positioned to detect waves originating south of New

Britain. However, because of the numerous islands near the “tail” of New

Guinea, as well as the Great Barrier Reef itself, these sources are unlikely

to significantly impact Queensland.

Of greater concern to Australia is the Puysegur source line, which extends southward from New Zealand’s southwest coast. The DART® 55015

site in the Tasman Sea (originally named 55401), selected by BOM scientists

(Greenslade, personal communication), endeavors to provide 90 min of warning to the threatened eastern coastlines of Tasmania and New South Wales.

The unit sources and timing information in Fig. 39 are based on the NCTR

propagation database. The “regular” Pacific grid, employed for most sources

currently in the propagation database, cuts off at 50◦S. Since this cannot

adequately represent trans-basin propagation of southern sources such as

Puysegur (and the southernmost South American sources described elsewhere in this memo), these were derived from MOST runs on an “extended”

Pacific grid whose southern limit is Antarctica. Merging of “regular” and

“extended” first wave results in the Pacific resulted in the apparent cut off

at 50◦S in Fig. 39. The current version of the database consistently employs

the extended grid; future versions may use a global grid. Representations of

the plate boundary and the occurrence of a “thrust”-type earthquake near

Macquarie Island in April 2008, reveal the presence of some short segments

of subduction further south than Puysegur Ridge. DART® 55015 is well

placed to detect such sources, which are far removed from the next instrument 54401. For both of the Australian sites discussed here, current speeds

(based on NLOM) should not be of concern for mooring integrity.

6.7 The Hawaiian and Equatorial Sites

Two long-standing U.S. sites (see Table 8), pre-date the expansion of the

U.S. array in the Pacific and the rationale for their selection is not included

in the Appendices. The Hawaiian site 51407, 140 nm southeast of Honolulu,

was selected to provide warning of locally generated tsunami waves. The

role of 51406 in the equatorial Pacific was originally to provide wide area

coverage for the extended source regions of South America and the Southwest

Pacific. Data from 51606 or 51407 can also serve to validate the SIFT

forecast methodology when they register tsunami signals from remote events.

The DART® allocation decided at the Siting Workshop was to some

extent predicated on the addition of further Chilean tsunameters south of

32401. In their absence, given the history of significant events off central

and southern Chile, this stretch of subduction zone has the poorest coverage.

There has been some discussion (some of it included in Appendices F and

H) of how this issue might be resolved. Options include:

• The addition of one or more DART®s (beyond the initial 39)

• Redeployment of 51406 to a point south of 32401

• Reassignment of the westernmost instrument of the SW Pacific group
72 Spillane et al.

Table 8: Location and performance of the Equatorial and Hawaiian DART®s (to 30 October 2008).

WMO Identifier Location Data Begins Major Disruptions Data Return (%)

51406 8.49◦S, 125.02◦W, 4480 m 1 Jan 2003 One 97.4

51407 19.63◦N, 156.53◦W, 4718 m 28 Jun 2005 None 97.8

The latter option is based on the premise that the Australian Coral Sea

DART® 55012, which was not envisaged during the Siting Workshop, is

somewhat better placed to detect many of the sources between New Guinea

and Vanuatu, than is 52406. This is because many of these sources lie to

the south of the Solomon Islands, thus distorting and possibly limiting the

penetration of tsunami energy into the Pacific. For sources east and south

of New Caledonia, however, 55012 is less effective. Redeployment of 51406

can be argued against, based on its long history and the secondary coverage

it provides for several source regions that potentially impact Hawaii. Ideally

the addition of one or more instruments off Chile, either by the U.S. alone

or in collaboration, would seem the most desirable choice.

The site occupied by DART® 51406 has a long history of occupation

and has a near continuous record since 2003; prior to 2006 it was designated

46406. As discussed in Appendix F and in Section 7, 51406 is not the

primary detector of any sections of the source line. It does, however, provide

a later evaluation for waves from major events that might be in transit from

South America to Hawaii, or from Tonga-Kermadec toward the West Coast.

Despite this role and the history of the site, there is a valid argument that,

if additional instruments are not forthcoming for the central and southern

sections of the Chilean trench, the overall performance of the array would be

better served by redeploying this DART® to one of the locations suggested

in Appendix F. The bathymetry and current speed regime in the vicinity

of the present site are portrayed in Fig. 40, where alternate sites that have

been discussed are also indicated.

The major purpose for the Hawaiian DART® 51407, is to monitor waves

generated locally by landslides and subsidence triggered by earthquakes.

The Halina fault along the south coast of Hawaii Island is associated with

active slumping. In 1975 and 1868 earthquakes (Mw = 7.9 and 7.2, respectively) generated tsunamis that caused loss of life and damage near Hilo

and in California. Detailed bathymetric and seafloor data accumulated during the JAMSTEC-Hawaii project (USGS, 2006) is summarized in a chart

(geopubs.wr.usgs.gov/i-map/i2809/bathy.pdf) that identifies numerous

past slumps and slides along the Halina and Kona coasts. DART® 51407

is located west of the North Kona Slump and is well placed to intercept

slump-generated waves propagating toward Honolulu and the other islands.

In October 2006 the instrument detected the disturbance associated with

near-shore submarine earthquakes (Mw = 6.7 and 6.0) that caused a 10-cm

wave in Kawaihae. The USGS-JAMSTEC chart illustrates, in greater detail
Tsunameter network design in Pacific and Atlantic 73 43412 43413 324113241232401 51406 51407 51425 51426 54401 Present Site of Equatorial DART Recommended Site for 51406 Relocation 95th Percentile NLOM Speeds

m

m/s

Tonga

Southern Chile

Figure 40: The Equatorial DART

® 51406.
74 Spillane et al.Figure 41: The Hawaiian DART® 51407.
Tsunameter network design in Pacific and Atlantic 75

Table 9: Location and performance of the Atlantic DART® Group (to 30 October 2008).

WMO Identifier Location Data Begins Major Disruptions Data Return (%)

41420 23.31◦N, 67.65◦W, 5667 m 7 Apr 2006 Two 98.0

41421 23.40◦N, 63.90◦W, 5800 m 11 Apr 2006 One 97.7

41424 32.93◦N, 72.47◦W, 5254 m 7 Apr 2006 Three 91.5

42407 15.26◦N, 68.24◦W, 4486 m 11 Apr 2006 Two 97.4

42408 25.21◦N, 87.00◦W, 3293 m 17 Apr 2006 Three 88.5

44402 39.49◦N, 70.59◦W, 2434 m 31 Aug 2007 None 96.0

44401 37.56◦N, 50.00◦W, 5391 m 28 Aug 2007 Two 90.6

than available for the other sites of this section, the presence of numerous

seamounts southwest of the 51407 site that might scatter teletsunami waves

from the Southwest Pacific. Nonetheless the April 2007 Solomon Islands

tsunami was detected at DART® 51407 and used in the validation of the

SIFT forecast scheme (Wei et al., 2008b). In Fig. 41 the current speed statistics in the vicinity (derived from NLOM hindcasts) are inset to an extract

from the USGS-JAMSTEC chart. While the instrument is sited where the

speeds are on occasion among the strongest in the region, these have not in

the past significantly impacted the data return.

6.8 The Atlantic, Caribbean, and Gulf of Mexico Sites

Though addressed and prioritized separately during the Siting Workshop,

all seven sites of the Atlantic and its marginal seas are discussed together.

They were the subject of a single Site Recommendation Report, reproduced

in Appendix C, and some follow-up discussion when two sites were found to

be exposed to excessive ocean currents. Site details and data return details

are provided in Table 9.

The propagation database for the Atlantic is primarily based on potential seismic sources within the Caribbean and an arc outside the Caribbean

extending from Trinidad to Hispaniola. Of these sources, the subset of greatest concern to the U.S. East Coast lie near Puerto Rico. They also threaten

Puerto Rico itself and the Virgin Islands, though these are further threatened by sources along the Muertos Trough and offshore of South America.

Other coastlines within the Caribbean are threatened by sources further

west, including those off Panama. However, propagation database statistics

reveal that the narrow passages of the Yucatan and south of Florida reduce

the direct impact of seismically generated waves entering the Gulf of Mexico.

For the Gulf, as well as the East Coast, landslide sources, perhaps triggered

by seismic activity, are an additional threat.

The more remote seismic sources of the South Sandwich Island arc have

recently been added to the propagation database and are of little concern.

Various representations of the less well-known Eastern Atlantic source region

that devastated Lisbon in 1755 have been explored. As noted earlier, the
76 Spillane et al.

general layout of the Atlantic portion of the array, decided at the Siting

Workshop, reflects the main threats with two DART®s north of Puerto

Rico, one in the Caribbean, two close to the East Coast, and one in the Gulf

of Mexico, to validate earlier detected waves of remote origin or potentially

landslide-generated waves. The final site 44401 lies well to the east and

should act to provide adequate warning for waves emanating from the eastern

Atlantic.

The rationale for the specific sites chosen to the north of Puerto Rico is

provide in Figs. 42 and 43, as well as in the original site recommendation

report in Appendix C. Both lie in international waters along the 30-min

envelope. Acting alone, DART® 41420 provides detection times of between

30 and 60 min for the sources of greatest concern along the Puerto Rico

Trench and Hispaniola. DART® 41421 provides a similar coverage, though

favoring the more eastern sources. Their combined capability, and similar

results for the array as a whole, are provided in Section 7. Scattering features

are confined mainly to the region north of the Dominican Republic and in

the vicinity of the Turks and Caicos Islands. These, and the even greater

impediment to tsunami passage posed by the Bahamas, provide protection

to southern Florida. Survivability of the moorings would not appear to be a

concern, based on the NLOM speed statistics. However, as seen in Fig. 57c

of Section 7.3, both sites failed within a month of each other in March

2008. Other failures and intermittency were negligible and the reason for

the failures may lie elsewhere.

Proceeding northward DART® 41424 lies near the U.S. EEZ boundary,

east of Charleston, South Carolina. The site lies well to the east of the Gulf

Stream path, which appears in the upper left corner of the 5% exceedence

panel in Fig. 44. While occasional higher speed episodes are not unexpected,

due to eddies associated with Gulf Stream recirculation, the extended loss

of data at 41424 beginning in August 2006 does not seem related to the

NLOM speed record. The record was interrupted again in November 2007

and in April 2008, resulting in a poor overall data return rate of 65%. In

between breakages the site had a good record with few and brief interruptions. The NDBC Status Reports indicate a BPR failure though the mooring

subsequently parted and the surface buoy drifted until it was recovered.

Serving as it does as the sole DART® in the Caribbean Sea with the

role of providing warning to Puerto Rico, the Virgin Islands, and other

island locations from widespread seismic and volcanic sources, 42407 had

an excellent early record of data return (see Fig. 57c in Section 7.3). This

has been marred of late by two interruptions, in December 2007 and April

2008. Both incidents were cited in NDBC Status Reports as possible BPR

failures and may not be associated with environmental conditions. The

passage nearby of Hurricane Dean in August 2007 (with winds in excess of

130 mph) caused a brief loss of data (4 hours) but otherwise registered the

low atmospheric pressures associated with the storm. As seen in Fig. 45, the

warning that can be provided for events on the Muertos Trough south and

west of Puerto Rico, or off the Netherlands Antilles and western Venezuela,

are likely to be short. Surprisingly, 42407 (as well as 41421 and 41424, though

not 41420) were triggered seismically by the far distant Peru earthquake of
Tsunameter network design in Pacific and Atlantic 77 95th Percentile NLOM Speeds m/s m 41420 41421 41424 42407 60 min 45 min 30 min15 min Bahamas Figure 42: Siting environment for DART® 41420 in the Atlantic group.
78 Spillane et al. 41421 41420 41424 42407 15 min 30 min 45 min 60 min 95th Percentile NLOM Speeds mm/s Figure 43: Siting environment for DART® 41421 in the Atlantic group.
Tsunameter network design in Pacific and Atlantic 79 41420 41421 41424 44402 15min30min 45min 60min mm/s 95th Percentile NLOM Speeds Bermuda Figure 44: Siting environment for DART® 41424 in the Atlantic group.
80 Spillane et al. 95th Percentile NLOM Speeds mm/s 41420 41421 42407 15 min 15 min 30 min 15 min 30 min 30 min Muertos Trough Figure 45: Siting environment for DART® 42407 in the Caribbean Sea.
Tsunameter network design in Pacific and Atlantic 81

15 August 2007. The nearby event near Martinique (29 November 2007)

also caused triggering, though the tsunami wave, if any, was very weak.

The Gulf of Mexico DART® 42408 has had poor overall data return

statistics with the surface buoy breaking free on a number of occasions and

an episode of high intermittency (see Fig. 57c, Section 7.3) that may be

associated with excessive Loop Current activity. A slightly revised site,

based on the NLOM speed statistics, was occupied when the instrument

was redeployed in August 2008 but another failure followed soon afterward.

In December 2008, the site was abandoned in favor of a more protected site

designated 42409 at 26.655◦N, 85.797◦W, some 200 km NE of 42408. Since

the decision and redeployment came when this report was near completion,

details of 42408 are retained and the new site is not discussed further. It is

likely that as experience with 42409 accumulates, and the threats associated

with submarine landslides are characterized, the issue of Gulf of Mexico

DART® placement will require further study.

The role of DART® 44402, in addition to perhaps monitoring local

landslide-associated tsunami activity, is in the assessment of seismically generated waves, propagating toward the northeastern U.S. states and Canada,

but which were likely detected earlier, close to their source. The initially recommended and occupied site for this instrument did not properly account

for the variability of the Gulf Stream axis (whose climatological axis was

drawn in Fig. C1 of Appendix C), or its strength. The survivability of the

mooring has hopefully been improved by a move inshore in March 2008, as

detailed in Appendix D and the NLOM speed statistics in Fig. 47.

Sources north of Puerto Rico are those of most concern and arrive at

44402 some 3 hours after the time of the earthquake. From Figs. 42 and 43,

such waves would have first been detected by DART® 41420 or 41421 within

30–45 min of generation and perhaps been further assessed in transit using

data from 41424. The closest inshore cells of the propagation database for

the New England region suggest that coastal impact will occur some 90-180

min after the waves transit the 44402 site (the lesser lead times between

Atlantic City to Nantucket, the longer for Boston to Portland, ME).

Waves directed toward the U.S. coast from the eastern Atlantic should,

it is hoped, be first seen at DART® 44401. As seen in Fig. 48, and also

in Fig. C1 of Appendix C, the location is southeast of Newfoundland with

a view to the east that is largely uninterrupted by scattering features. The

current speed regime near 44401, based on NLOM hindcasts, would not seem

too extreme and apart from some periods of intermittency (see Fig. 57c,

Section 7.3) the record is complete since deployment in December 2007.

The site recommendation took into consideration some sample beam paths

and travel times of potential sources in the vicinity of Lisbon and La Palma.

Research is ongoing to characterize the eastern Atlantic source distribution,

which is not, as yet, incorporated in the propagation database.
82 Spillane et al. 30 min 45 min 60 min 15 min 42408 mm/s 95th Percentile NLOM Speeds Figure 46: Siting environment for DART® 42408 in the Gulf of Mexico.
Tsunameter network design in Pacific and Atlantic 83 44402 44401 41424 41421 41420 Bermuda Original Location15 min 30 min 45 min 60 min 95th Percentile NLOM Speeds mm/s Figure 47: Siting environment for DART® 44402 in the Atlantic group.
84 Spillane et al. 44401 44402 41424 41421 41420 30 min 15 min 45 minBermuda 60 min 95th Percentile NLOM Speeds mm/s Figure 48: Siting environment for DART® 44401 in the Atlantic group.
Tsunameter network design in Pacific and Atlantic 85

Table 10: Location and performance of the Indian Ocean DART® instruments (to 30 October

2008).

WMO Identifier Location Data Begins Major Disruptions Data Return (%)

23401 8.91◦N, 88.54◦E, 3546 m 4 Dec 2006 None 93.7

53401 0.05◦N, 91.90◦E, 4481 m 27 Sep 2007 None 97.9 (before loss)

6.9 DART® Sites in the Indian Ocean

Though perhaps beyond the scope of a report on the U.S. DART® Array,

it nonetheless seems appropriate to include a brief discussion and graphics

relating to the siting choices for the two DART® instruments deployed in the

Indian Ocean. These were established, by the governments of Thailand and

Indonesia in cooperation with USAID, as part of an evolving international

effort to provide detection and warning capabilities there. NCTR staff were

involved in site selection and DART® data are available to the international

community via the same NDBC websites that serve the U.S. arrays in the

Pacific and Atlantic Oceans. Two further sites have been established by

Australia using DART® technology (Greenslade, personal communication)

and numerous other sites are in various stages of operation or planning.

During 2005, before a propagation database had been computed, some

preliminary steps were taken to envision an Indian Ocean array. The tentative number and layout of array nodes were based on detection and warning

time estimates drawn from travel-time computations. The TTT software

(from Geoware) was employed for this purpose with point sources spaced

at 100 km intervals along the plate boundaries off Indonesia, the AndamanNicobar region, and the Makran region of the northern Arabian Sea. The

intent of this initial USAID effort was to select two sites whose joint coverage

would optimize a warning capability for those nations most impacted by the

2004 disaster. The pair of sites that best satisfied that mandate among the

set of locations identified in the preliminary array design are described in

Table 10. Other relevant site parameters, including those derived from the

now-available Indian Ocean propagation database, are provided in Figs. 49

and 50.
86 Spillane et al. 95th Percentile NLOM Speeds mm/s 23401 53401 45 min 60 min 15 min 30 min Sri Lanka Andaman Sea Figure 49: Siting environment for the Thailand DART® 23401 in the Indian Ocean.
Tsunameter network design in Pacific and Atlantic 87 15 min 30 min 45 min 60 min 95th Percentile NLOM Speeds mm/s 23401 53401 Andaman Sea Figure 50: Siting environment for the Indonesia DART® 53401 in the Indian Ocean.