Tsunameter network design in Pacific and Atlantic 89

7. Array Summary

Now that all elements of U.S. array, as envisioned at the DART® Siting

Workshop, have been deployed and have been returning data for some time,

it is appropriate to report some measures of the performance of the array

as a whole. This is likely to be an ongoing process as factors such as data

return rate, ambient current patterns, commercial and fishing vessel traffic

patterns, become evident.

7.1 The Final Array

The distribution of the 39 elements of the U.S. array, and of the five cooperatively deployed instruments as of October 2008 is provided in Fig. 5

and Table 11 below. The U.S. sites were chosen in light of available knowledge so as to optimize the detection of seismically generated tsunamis that

pose a threat to U.S. interests. The data used to compile the data availability statistics presented in Table 11 are based on 15-min data readily

obtained from the NDBC web site www.ndbc.noaa.gov/dart.shtm; other

historic BPR data beginning in the 1980s, including those with higher sampling rates, can be found at the NGDC (National Geophysical Data Center)

site www.ngdc.noaa.gov/hazard/DARTData.shtml. It should be noted that

the percent completeness values in Table 11 are, in many instances, related

more to the logistics of reestablishment cruises which can result in major data

gaps. Data return statistics, more relevant to instrumental performance and

environmental impact, are provided in Tables 2–10 of Section 6.

Some statistics relating to the data return are presented in Section 7.2

and have led to consideration of the impact of individual instrument failure

on the array. This is done in Section 7.3, though an interim report on the

impact of multiple failures in the Gulf of Alaska has been provided to the

Warning Centers. The other factors, such as the impact of ambient currents

or ship track and fishing activity on mooring survivability, may suggest revisions to the array layout as data acquisition from the array progresses.

Two sites, 42408 and 44402, have already been adjusted from their initially

recommended locations due to their exposure to the Loop Current and Gulf

Stream, respectively, and as noted earlier 42408 was disestablished in December 2008 in favor of a new site 42409. The isolation of the Gulf of Mexico

from the Atlantic and Caribbean limits the impact of this change on the joint

array statistics reported below, and insufficient data have been acquired at

42409 to warrant reporting its return statistics. Section 7.3 includes some

preliminary results from recently acquired datasets on ship tracks and fishing

pressure.
90 Spillane et al.

Table 11: Array layout and data availability as of 30 October 2008. Asterisk denotes a non-U.S.

site.

Region DART® Latitude Longitude Depth (m) Data Availability

Aleutians-AK 21414 48.958◦N 178.266◦E 5431 11 Aug 2006 (90.8%)

46413 48.868◦N 175.586◦W 5504 11 Aug 2006 (81.4%)

46408 49.623◦N 169.848◦W 5372 14 Aug 2006 (98.0%)

46402 51.062◦N 164.002◦W 4712 1 Jan 2003 (79.8%)

46403 52.637◦N 156.932◦W 4508 1 Jan 2003 (89.5%)

46409 55.301◦N 148.495◦W 4187 29 Sep 2005 (98.2%)

46410 57.503◦N 143.989◦W 3742 28 Jul 2006 (72.5%)

U.S./Canada West Coast 46419 48.798◦N 129.584◦W 2773 25 Jul 2006 (75.0%)

46404 45.863◦N 128.776◦W 2740 11 Jan 2003 (63.3%)

46407 42.595◦N 128.894◦W 3266 10 Dec 2007 (97.7%)

46411 39.329◦N 127.011◦W 4266 27 Sep 2005 (89.0%)

46412 32.246◦N 120.699◦W 3777 29 Sep 2005 (98.8%)

Central and South America 43412 16.031◦N 107.001◦W 3239 21 Mar 2007 (87.3%)

43413 10.841◦N 100.085◦W 3468 24 Mar 2007 (97.5%)

32411 4.924◦N 90.685◦W 3247 27 Mar 2007 (98.2%)

32412 17.970◦S 86.392◦W 4326 2 Nov 2007 (99.9%)

32401* 19.577◦S 74.814◦W 4881 23 Mar 2005 (98.2%)

Northwest Pacific 21415 50.166◦N 171.836◦E 4709 28 Jul 2007 (87.3%)

21416 48.046◦N 163.490◦E 5782 26 Jul 2007 (99.5%)

21417 43.187◦N 157.140◦E 5483 24 Jul 2007 (32.3%)

21418 38.707◦N 148.694◦E 5665 23 Jul 2007 (77.6%)

West Pacific 21413 30.550◦N 152.093◦E 5822 27 Nov 2006 (90.0%)

52401 19.256◦N 155.759◦E 5569 12 Dec 2006 (69.1%)

52402 11.564◦N 154.585◦E 5799 15 Dec 2006 (99.6%)

52403 4.023◦N 145.580◦E 4436 19 Dec 2006 (93.7%)

52404 20.942◦N 132.307◦E 5925 1 Dec 2006 (98.6%)

52405 12.884◦N 132.317◦E 5967 4 Dec 2006 (99.2%)

Southwest Pacific 52406 5.340◦S 165.078◦E 1849 4 Mar 2008 (94.1%)

51425 9.499◦S 176.246◦W 4962 28 Feb 2008 (98.7%)

51426 22.982◦S 168.100◦W 5637 11 Feb 2008 (96.2%)

54401 33.005◦S 172.968◦W 5837 17 Feb 2008 (96.2%)

Coral Sea 55012* 15.800◦S 158.500◦E 3284 27 Mar 2008 (99.1%)

Tasman Sea 55015* 46.932◦S 160.461◦E 4944 27 Apr 2007 (91.0%)

Equatorial Pacific 51406 8.489◦S 125.017◦W 4473 1 Jan 2003 (95.6%)

Hawaii 51407 19.627◦N 156.526◦W 4718 28 Jun 2005 (97.5%)

Atlantic 41420 23.302◦N 67.662◦W 5659 27 Apr 2006 (80.6%)

41421 23.401◦N 63.912◦W 5802 11 Apr 2006 (89.2%)

41424 32.928◦N 72.468◦W 5252 7 Apr 2006 (69.4%)

44401 37.551◦N 49.985◦W 5391 31 Aug 2007 (96.0%)

44402 39.487◦N 70.595◦W 2434 28 Aug 2007 (65.0%)

Caribbean 42407 15.255◦N 68.236◦W 4486 11 Apr 2006 (86.8%)

Gulf of Mexico 42408 25.410◦N 86.797◦W 3259 17 Apr 2006 (58.7%)

Indian Ocean 23401* 8.907◦N 88.540◦E 3546 4 Dec 2006 (93.7%)

53401* 0.051◦N 91.900◦E 4481 27 Sep 2007 (97.9%)
Tsunameter network design in Pacific and Atlantic 91

7.2 Array Detection Capability

To summarize the capabilities of the array for the detection of seismically

generated tsunamis we rely on statistics derived from the propagation

database (Gica et al., 2008). This contains MOST model simulations for

the propagation of waves generated by “unit sources” each consisting of an

Mw = 7.5 event whose slip is distributed over a rectangular region, measuring

100 km along the strike and 50 km along the descending plate. When complete the database will span all potential sources (subduction zones and segments of oceanic convergent plate boundaries) worldwide. Currently those

sources of greatest threat have been computed, and are available to the SIFT

tool for use in tsunami forecasting. What remains is to expand the width

of the unit source tiling in some regions and add coverage of more remote

sources in the eastern Atlantic and South Shetland Islands.

To quantify the utility of a DART® site in tsunami monitoring, two

parameters are employed. The first is the arrival time, Tf , of the first peak

of the tsunami wave train. This can be computed from the propagation

database and is usually taken as the earliest time at which a peak exceeding

20% of the overall maximum for wave train occurs at a location (see Fig. 51.)

The 20% (or other) requirement is to exclude spurious low-level peaks that

might exist in the numerical solution. The other parameter needed quantifies

the amplitude of the signal expected at the site. Tsunami waves of low

amplitude may be difficult to detect in the presence of background pressure

fluctuations and residual seismic noise. As noted earlier, the unit source

model solutions, while computed for a magnitude 7.5 event, may be linearly

scaled and combined in the open ocean. The scale factor is related to source

magnitude through the relationship

Scale = 101.5(Mw−7.5) (4)

This relationship (see also Fig. 51) is used to define a “detectable magnitude” for each combination of unit source and DART® location. Based on

past experience, a tsunami signal is visible in a BPR record if its amplitude exceeds a detectability threshold At of about 0.5 cm. If the predicted

amplitude for a unit source (Mw = 7.5) at a DART® location is Au then,

using the relationship (4) above with a threshold of 0.5 cm, the source is

detectable at a magnitude of

Mwd = 7.3 − 0.667 log(Au) (5)

Small values for Mwd are desirable since they imply that weaker tsunami

events are detectable by a DART® site, thereby facilitating early warning

cancelation. Large values, on the other hand, indicate that only strong

tsunamis emanating from that source region can be detected.

For the purpose of evaluating detection time and detectable magnitude

the unit sources are treated individually; this is realistic for lower magnitude

events, but less so for event magnitudes of 8.0 and greater where more than

a single unit source is involved. Nonetheless Tf and Mwd together allow

a straightforward means of representing the ability of the array to detect
92 Spillane et al.

Figure 51: Definition of first wave arrival time and amplitude. The latter, based on a unit source

magnitude Mw = 7.5 and an assumed threshold of 0.5 cm at a DART®, can be used to determine

the lowest magnitude event at the source location that is detectable.

tsunamis generated in the far-flung distribution of potential sources. The

results are tabulated in Table 12 and illustrated graphically in Figs. 52–56.

In Table 12, the Pacific and Atlantic Oceans are partitioned into regions

for purposes of discussion, though the statistics presented consider all pairings of DART® site and B-row unit source. For each instrument the number

of (B-row) unit sources for which it provides the primary and secondary detection are given. The importance of a site combines its ability to provide an

early warning and to serve as backup in the event of failure of a neighboring

site. Use of the word backup is not intended to imply redundancy; a SIFT

inversion can proceed once the first wave peak is resolved but the quality of

the forecast is improved when tsunami detection results from two or more

locations are assimilated. For those unit sources for which it provides the

primary (earliest) detection, the mean detection time (in minutes) is given.

This is followed by a degraded detection time, for those unit source locations,

in the event the primary instrument fails. The quality of the detection and

the impact of loss of the instrument are given by the mean detectable magnitudes, with and without the DART® site in question. For the Aleutians and

Alaska, both the primary and secondary DART®s are capable of providing
Tsunameter network design in Pacific and Atlantic 93

Table 12: Source detection contributions for elements of the U.S. DART® array. Three nonU.S. sites, indicated with an asterisk, are included, since their presence influences Pacific array

performance.

# Sources Mean Detection Mean Detectable

Detected As Time (min) Magnitude (Mw)

Region DART® Primary Backup With Without With Without

Aleutians and Alaska 46413 3 5 17 26 6.87 7.15

46408 4 4 17 25 6.87 7.10

46402 5 4 17 28 6.91 7.15

46403 5 4 17 33 6.92 7.20

46409 4 15 18 30 6.85 7.15

46410 13 4 34 57 7.03 7.19

West Coast 46419 9 3 34 57 7.10 7.18

46404 4 9 29 36 6.90 7.11

46407 2 4 32 37 6.97 7.15

46411 2 1 25 35 7.02 7.11

Central and South America 43412 9 5 45 89 7.11 7.41

43413 10 30 61 89 7.14 7.39

32411 29 8 109 174 7.44 7.76

32412 11 67 109 124 7.46 7.64

32401* 59 8 154 209 7.70 7.57

Northwest Pacific 21418 19 15 49 78 7.19 7.28

21417 7 11 34 48 6.97 7.18

21416 11 7 37 57 7.02 7.11

21415 9 8 26 42 7.06 7.12

21414 4 6 17 26 6.89 7.15

West Pacific 21413 11 16 70 89 7.27 7.43

52401 8 7 68 77 7.43 7.38

52402 8 19 67 81 7.33 7.50

52403 38 21 87 145 7.60 7.73

52404 21 24 72 118 7.39 7.59

52405 32 28 70 107 7.25 7.35

Southwest Pacific 52406 5 22 67 76 7.32 7.25

51425 5 5 54 86 7.27 7.63

51426 12 26 49 81 7.21 7.64

54401 22 14 60 126 7.26 7.58

Coral Sea 55012* 32 8 94 127 7.47 7.84

Tasman Sea 55015* 7 1 32 309 7.00 8.32

Equatorial 51406 0 0 n/a n/a n/a n/a

Hawaii 51407 0 0 n/a n/a n/a n/a

Atlantic 41420 13 58 55 69 7.53 7.56

41421 20 13 79 95 7.51 7.65

41424 0 0 n/a n/a n/a n/a

44401 0 0 n/a n/a n/a n/a

44402 0 0 n/a n/a n/a n/a

Caribbean 42407 43 21 75 136 7.53 8.12

Gulf Mexico 42408 16 0 104 155 7.87 8.20
94 Spillane et al.

early detection of low-magnitude events. Loss of 46410, at the end of the

line (see Fig. 52), would have a noticeable impact on array performance for

sources off the Alaskan panhandle and Canada.

Note that the southern California DART® provides neither primary

nor secondary coverage for any of the (seismic) sources in the propagation

database. This is true of other sites in the table, in the mid-Pacific and in

the Atlantic. These sites were chosen to provide warning of non-seismic or

remote tsunami sources (or other less-well characterized threats).

Off Central and South America, some of the statistics in Table 12 (and

Fig. 55) indicate weaknesses in the array. Sites 43413 and those further

south must each cover long stretches of the source line. The mean detection times for 32411, 32412 and the Chilean tsunameter 32401 are long,

reflecting poorer coverage of sources off Panama, Colombia and Ecuador in

the north and the extensive tsunami-prone central and southern coastline of

Chile. The hazard is more local than remote due to the long travel time to

points of direct U.S. impact, but the degraded detectable magnitudes will

increase the basin-wide uncertainty for events in these regions. It is hoped

that these weaknesses will be addressed through the addition of Ecuadorean

and further Chilean tsunameters. There has been some discussion (see the

site recommendation reports in the Appendices) of the possibility of reallocating one of the U.S. array elements (likely 51406) to southern Chile.

There appears to be some reluctance to do so, given the long history of the

site and its potential to provide validation of SIFT predictions for a wide

range of sources. Preferable would be a modest expansion of the number of

instruments in the U.S. array, should the anticipated Chilean sites not be

realized.

The close spacing along the Aleutian/Alaskan and West Coast source

regions is evident in the early portion of the table. With the exception of

46410, which provides primary cover from the northern Gulf of Alaska to

the Queen Charlotte Islands, each DART® need only cover 300–500 km of

the source line.

Proceeding to the northwestern Pacific, the statistics provided in Table 12, and illustrated graphically in Fig. 52, show that this region is comprehensively covered, too. The orientation of the Kamchatka and Kuril

trenches, while directing tsunami energy toward the Hawaiian Islands and

the U.S. West Coast, does permit them to be detected by western elements

of the Aleutian/Alaskan DART® line. The situation is somewhat poorer

in the west and southwest, where increased detection intervals and poorer

detectable magnitudes are encountered. This may be of greater concern for

local than for remote impacts. The region with its many island chains may

limit the basin-wide broadcast of tsunami energy. For example, both ends

of the New Guinea to Vanuatu source line (see Fig. 53) are poorly detected

due to the screening effect of New Guinea itself and New Caledonia. The

Australian site 55012 in the Coral Sea is better placed to detect sources in

the center section of the line than is 52406 to the north of the island chain.

The New Zealand to Tonga source line is well served by 51425, 51426, and

54401. Australia’s Tasman Sea tsunameter 55015, in addition to its primary

role of monitoring Puysegur Trench off New Zealand’s South Island, is better
Tsunameter network design in Pacific and Atlantic 95

Figure 52: Detectability and detection time for seismic sources in the North Pacific.

Figure 53: Detection capability in the Western Pacific.
96 Spillane et al.

Figure 54: Detection capability in the Southwest Pacific.

Figure 55: Detection capability in the Eastern Pacific.
Tsunameter network design in Pacific and Atlantic 97

Figure 56: Detection capability for Atlantic and Caribbean seismic sources.

placed to detect waves generated near Macquarie Island, or other segments

of the plate boundary further south, than is DART® 54401 to the northeast

of New Zealand. These sources are of major concern to Tasmania and New

South Wales but may direct some energy into the Pacific.

In the Atlantic basin the primary threat to the U.S. East Coast for seismically generated events comes from sources between Antigua and Hispaniola.

The DART® pair 41420/42421 north of Puerto Rico primarily covers these.

The timing and quality of the detection degrades somewhat in moving away

from Puerto Rico, though this is somewhat compensated for by the presence

of shielding shallows (Turks and Caicos and Bahamas) to the west and the
98 Spillane et al.

orientation of the main tsunami beam in the east. Puerto Rico, in addition

to being threatened (and monitored) to the north, is exposed to a range of

sources to its south. The Muertos Trough is perhaps of greatest concern

to Puerto Rico and the U.S. Virgin Islands, but there is the potential for

tsunami generation off Venezuela, with relatively short travel times. The

sole DART® site in the Caribbean, 42407, provides some cover for many of

these sources but with late detection and less than desirable quality in many

instances (see Fig. 55.) There are further sources in the western Caribbean

basin that are not well served. Given the almost total EEZ ownership, siting in the western Caribbean would be impossible except in a collaborative

mode. Further north in the Atlantic, sites 41424 and 44402 can provide

delayed observations of remote events impinging on the East Coast as well

as local landslide-generated events not fully treated in this report. The easternmost site, 44401, was selected to give approximately 3 hours warning of

waves that might originate in the eastern Atlantic: Lisbon or possibly Las

Palmas.

7.3 Data Return Statistics

Overall data return statistics for each element of the array are summarized in

Fig. 57. Although the data return rate has been substantial, there are occasions when severe intermittency or complete loss of data occurs. The reasons

for such occurrences are not always completely known, though weekly status reports from NDBC attempt to classify them. Apart from instrument

failure or damage during deployment, intermittency may be due to communication interruptions between the surface unit and the BPR, or the satellite

link. Sea state or strong ambient currents are external factors that may be

responsible for such intermittency. Total loss on the other hand, though it

may be due to a hostile natural environment, can arise through vandalism

or collision with merchant or fishing vessels.

The issue of intermittency, as possibly the result of ambient currents, has

been addressed to some extent in the individual site descriptions of Section

6. This analysis is not complete to the extent that contemporaneous, in situ,

current observations are not available and the NLOM hindcast currents do

not substantially overlap the DART® records. In some locations NDBC

meteorological and wave buoys lie in the vicinity of a DART® and will in the

future be used in an attempt to associate intermittency with environmental

factors.

7.4 Commercial and Fishing Vessel Considerations

Total loss of the data stream is occasionally the result of a collision with a

surface vessel. Two sources of relevant data in this regard have recently been

considered. The first is an analysis of VOS (Voluntary Observing Ship) Program data conducted by Halpern et al. (2008) as part of a study of human

impact on the marine environment. VOS vessels are estimated to comprise

some 11% of traffic for vessels in excess of 1,000 tons, and perhaps a greater
Tsunameter network design in Pacific and Atlantic 99

Figure 57a: DART® data return statistics.

Figure 57b: DART® data return statistics (cont.).
100 Spillane et al.

Figure 57c: DART® data return statistics (cont.).

Figure 58: Global commercial vessel activity, derived from VOS ship reports by Halpern et al.

(2008).

proportion in such heavily traversed regions as the North Pacific and Atlantic. By interpolating between successive reports from individual vessels,

Halpern et al. (2008) achieve resolutions of about 1 km in their estimates

of vessel transits per square kilometer per year, and so finely delineate the

shipping lanes, as seen in Fig. 58.

Fishing boat activity is not included in the VOS traffic analysis. However, another product, “Lights At Night,” available from NGDC and the Defense Mapping Satellite Program, identifies transient lights over water and
Tsunameter network design in Pacific and Atlantic 101

Figure 59a: Sea-lanes and fishing activity near DART® Site 21417.

discriminates between those due to gas flares from drilling platforms and

those associated with fishing boats. Among the regions than stand out in

the fishing activity results is the area east of the Kuril Islands where DART®

21417 has twice been lost in circumstances that suggest a collision. Another

“hot spot” of fishing pressure appears south of the Galapagos. While these

products have not yet been systematically included in the DART® siting

process at NCTR, a case study for DART® 21417 has been performed and

the instrument will be relocated when the site is next visited. Figure 59 is

a composite of sea-lane and fishing activity generated during that study.
102 Spillane et al.

Figure 59b: Sea-lanes and fishing activity near DART® Site 21417 (cont.).
Tsunameter network design in Pacific and Atlantic 103

8. Conclusions

8.1 Major Considerations in DART® Siting

Some of the major issues involved in site selection for an array of DART®

instruments are:

1. Water Depth and Strong Current Regimes: the DART®s currently in use can only be deployed in water depths between 1500 and

6000 m. Because of the need for a surface buoy, strong current regions

where it is difficult to maintain the tethered surface buoy are to be

avoided.

2. Bottom Roughness: A DART® needs to communicate acoustically

with its surface unit. For this to work it must lie on a reasonably flat

seabed. This is a local bathymetric constraint, as is the need to avoid

areas of strong depth gradients, or where submarine landslides may

occur.

3. Seismic waves: If a DART® is located too close to the seismic event

that generates the tsunami, the shaking of the sea floor can cause spurious BPR fluctuations unrelated to the passage of tsunami water waves.

This seismic noise can be largely avoided by locating the instruments

no closer than 30-min of tsunami travel time from the closest source.

4. Tsunami Scattering: While consideration (2) above relates to the

local bathymetry, the presence of seamounts or other major seafloor

features between a DART® and likely tsunami sources is also to be

avoided. The complete detection/forecast system works by “inverting”

the waveform of a passing tsunami wave train to extract the characteristics of the tsunami source. This process involves comparison with

modeled waves that in complex regions may not fully represent scattering by rugged bathymetry.

5. Timing issues: Consideration (3) above suggests that DART®s not

be placed too close to potential tsunami sources. Conversely, if sited

too far from the action, too much time is lost between the seismic event,

which is detected almost instantly and the arrival of an unambiguous

sea surface disturbance at a DART® site. Suitable sites must therefore

be close, but not too close, to potential sources.

6. Detectability: Until it reaches the coast, a tsunami wave in the open

ocean may have only an amplitude of a few centimeters. While the

influence of the larger (but much shorter period and wavelength) wind

waves is largely filtered out in a BPR record, there is a noise threshold of a few millimeters that limits the detectability of weak tsunami

signals. Given the lobed or beamlike pattern of tsunami energy, the

best location to observe the tsunami is somewhat “in the beam” rather

than too far to the side. Actually, though, observations directly off-
104 Spillane et al.

shore of a source can complicate the discrimination of the unit sources

it represents.

7. Number of Systems: For the U.S. DART® system that will monitor

the Pacific and Atlantic it was decided that 39 instruments would be

deployed. Potential tsunami sources span virtually the entire perimeter

of the Pacific and the Atlantic has sources in the Caribbean as well as

to the east (between Portugal and the Azores, and perhaps landslide

or volcanic sources elsewhere).

8. EEZ Issues: For source regions along the Aleutians, and the Alaskan

and U.S. west coast mainland, likely DART® sites are either in international waters or within U.S. EEZ (Exclusive Economic Zone) limits.

Elsewhere, notably in the Caribbean and western Pacific, the seafloor

is a patchwork of differing ownerships. Other related issues include

shipping routes, seafloor infrastructure, piracy, or a history of damage

to unattended buoys that make some areas less desirable for DART®

siting.

9. Logistics: Although DART®s are typically deployed for 2-year stints,

there is considerable expense associated with deploying and maintaining them in remote regions. For some potential sites in the equatorial Pacific, where the TAO (www.pmel.noaa.gov/tao/) buoy array or

elsewhere where meteorological buoys exist, there may be efficiencies in

collocation if there is not conflict with special DART® requirements.

10. Redundancy: Either the bottom unit or the surface buoy of a DART®

may fail and, in remote locations, repair/replacement may not be an

immediate option. Of course tsunamis do not occur every day but

nonetheless it would be desirable to have some redundancy in the array. Not in the sense of having DART®s virtually side-by-side (the

bottom units have dual systems to partially achieve this), but more

in having a sufficient density of DART®s that failures near source regions of particular risk are partially compensated for by having more

than one DART® capable of providing a timely, high-quality signal.

Other nations, Chile and Australia, have deployed DART® in the Pacific,

which extend the coverage, and some Indonesian plans in the Western Pacific

may assist the detection of sources in the vicinity of the Philippines and

Bird’s Head. Nevertheless some gaps remain, most notably off central and

southern Chile where some assumptions of the DART® Siting Workshop

have not as yet been met. Now that the full U.S. array is in place, as seen in

Fig. 5, it is probably time to revisit these assumptions and decide whether

additional buoys may be forthcoming or whether an existing instrument,

such as 51406, whose role in the expanded array is altered, should be reallocated.

8.2 Future Directions

With the U.S. DART® array in place and, together with similar instruments

put in service by other nations, providing input data to the Tsunami Warning Centers, it is appropriate to identify the ongoing efforts and additional
Tsunameter network design in Pacific and Atlantic 105

products or analyses that might improve performance and reliability into

the future. Chief among these would be the identification of regions that

are poorly covered, for example the southeast Pacific, where an ongoing discussion may lead to a relocation of DART® 51406 to a site south of Chile’s

Juan Fernandez Island.

The design of the U.S. array described in this report is mainly aimed at

detecting seismically generated tsunami waves, and providing adequate forewarning of their far-field impact. Seismic noise and geometric/bathymetric

constraints to siting make local impacts more difficult to cover, though the

duration of tsunami impacts give value to even a “late” forecast. A recent

analysis of the detection in the Western Mediterranean of tsunami waves

generated off North Africa (Schindel ́e et al., 2008) highlights the difficulty.

The NCTR propagation database employed, while well suited to estimating

DART® arrival times and amplitudes, is less satisfactory to quantify local

coastal impact; a database of higher-resolution propagation solutions, appropriate to Cascadia, the Caribbean, and other regions with local threats,

is needed to refine the analysis.

Considerable effort within the tsunami research community is being directed toward landslide-generated waves whose dynamics are far more difficult to model. The distribution of the landslide threat has been considered

by McAdoo et al. (2000) and others, and a case study of the Currituck

landslide off North Carolina (geologic age ∼25–50 ka) by Geist et al. (2008)

suggests that the local impact of such events need to be considered in ongoing

refinements and expansion of the array.

Technological improvements in tsunami detection will likely need to be

considered in the future. An effort underway at NOAA/PMEL involves the

development and intercomparison of the DART®-ETD (Easy To Deploy)

buoy configuration with the standard DART®-II. The ETD version is considerably more compact, allowing it to be deployed from smaller vessels,

reducing costs, and providing options for expanding the array and speeding replacement when a failure occurs. Part of the intercomparison effort

involves attempts to quantify and explain differences between neighboring

instruments. The impact of environmental factors is part of this effort and

should lead to insights that might routinely be applied at other sites.

While it is hoped that the material in this report may assist site selection

processes by other nations to serve their own needs, the global nature of the

threat suggests that NCTR staff should stand ready to cooperate in foreign

decision-making, since the U.S. will benefit from array synergies. Likewise

the audience for this report may be aware of other information sources that

could be usefully employed in site selection.

9. Acknowledgments

This publication is contribution 3192 from NOAA/Pacific Marine Environmental Laboratory and was [partially] funded by the Joint Institute for the

Study of the Atmosphere and Ocean (JISAO) at the University of Wash-
106 Spillane et al.

ington under NOAA Cooperative Agreement No. NA17RJ1232, JISAO contribution 1607. The authors would like to acknowledge the contributions of

the USGS and NDBC for information pertinent to the siting choices, and to

online data sources used in the analysis and graphics.

The authors benefited from discussions with Diana Greenslade of the

Australian Bureau of Meteorology in regard to their methodology for site

selection and forecast procedures. Dr. Benjamin Halpern of the UCSB

National Center for Ecological Analysis and Synthesis, and coworkers, are

gratefully acknowledged for permission to reproduce their graphic of VOS

shipping lanes and for the data it represents. Technical assistance and editing suggestions from Ryan Layne Whitney of PMEL were invaluable in the

production of this report.
Tsunameter network design in Pacific and Atlantic 107

10. References

Bird, P. (2003): An updated digital model of plate boundaries. Geochem. Geophys.

Geosyst., 4(3), 1027, doi:10.1029/2001GC000252.

Blewitt, G., C. Kreemer, W.C. Hammond, H.-P. Plag, S. Stein, and E. Oka (2006):

Rapid determination of earthquake magnitude using GPS for tsunami warning

systems. Geophys. Res. Lett., 33, L11309, doi: 10.1029/2006GL026145.

Chaytor, J.D., D.C. Twichell, U.S. Ten Brink, B.J. Buczkowski, and B.D. Andrews

(2007): Revisiting submarine mass movements along the U.S. Atlantic continental margin: Implications for tsunami hazards. In Advances in Natural and

Technological Hazards Research, Vol. 27, Springer, The Netherlands.

Driscoll, N.W., J.K. Weissel, and J.A. Goff (2000): Potential for large-scale submarine slope failure and tsunami generation along the U.S. mid-Atlantic coast.

Geology, 28(5), 407–410.

Eble, M.C., and F.I. Gonz ́alez (1991): Deep-ocean bottom pressure measurements

in the Northeast Pacific. J. Atmos. Oceanic Technol., 8(2), 221–233.

Geist, E., F. Gonz ́alez, and U. ten Brink (2005): Workshop on Optimizing the

DART Network for Tsunami Forecasting. Sound Waves Monthly Newsletter,

October 2005 (http://soundwaves.usgs.gov/2005/10/meetings.html).

Geist, E., V. Titov, A. Kelly, and H. Gibbons (2007): Tsunami-forecasting system

tested by recent subduction-zone earthquakes. Sound Waves Monthly Newsletter, April 2007 (http://soundwaves.usgs.gov/2007/04/research2.html).

Geist, E.L., P.J. Lynett, and J.D. Chaytor (2008): Hydrodynamic modeling

of tsunamis from the Currituck landslide. Mar. Geol., doi: 10.1016/jmargeo.2008.09.005, in press.

Gica, E., M. Spillane, V.V. Titov, C.D. Chamberlin, and J.C. Newman (2008):

Development of the forecast propagation database for NOAA’s Short-term Inundation Forecast for Tsunamis (SIFT). NOAA Tech. Memo. OAR PMEL-139,

NOAA/Pacific Marine Environmental Laboratory, Seattle, WA, 89 pp.

Gonz ́alez, F.I. (1999): Tsunami! Sci. Am., 280(5), 56–65.

Halpern, B.S., S. Walbridge, K.A. Selkoe, C.V. Kappel, F. Micheli, C. D’Agrosa,

J.F. Bruno, K.S. Casey, C. Ebert, H.E. Fox, R. Fujita, D. Heinemann, H.S.

Lenihan, E.M.P. Madin, M.T. Perry, E.R. Selig, M. Spalding, R. Steneck, and

R. Watson (2008): A global map of human impact on marine ecosystems. Science, 319(5865), 948–952, doi: 10.1126/science.1149345.

McAdoo, B.G., L.F. Pratson, and D.L. Orange (2000): Submarine landslide geomorphology, U.S. continental slope. Mar. Geol., 169, 103–136.

Mofjeld, H.O., V.V. Titov, F.I. Gonz ́alez, and J.C. Newman (2001): Tsunami scattering provinces in the Pacific Ocean. Geophys. Res. Lett., 28(2), 335–337.

Okada, Y. (1985): Surface deformation due to shear and tensile faults in a halfspace. Bull. Seismol. Soc. Am., 75, 1135–1154.
108 Spillane et al.

Schindel ́e, F., A. Loevenbruck, and H. H ́ebert (2008): Strategy to design the sealevel monitoring networks for small tsunamigenic oceanic basins: the Western

Mediterranean case. Nat. Hazards Earth Syst. Sci., 8(5), 1019–1027.

Shriver, J.F., H.E. Hurlburt, O.M. Smedstad, A.J. Wallcraft, and R.C. Rhodes

(2007): 1/32◦ real-time global ocean prediction and value-added over 1/16◦

resolution. J. Mar. Syst., 65, 3–26.

Smith, W.H.F., and D.T. Sandwell (1997): Global sea floor topography from satellite altimetry and ship depth soundings. Science, 277, 1956–1962.

Smith, W.H.F., R. Scharroo, V.V. Titov, D. Arcas, and B.K. Arbic (2005): Satellite

altimeters measure tsunami—Early model estimates confirmed. Oceanography,

18(2), 10–12.

Synolakis, C.E., J.P. Bardet, J. Borrero, H. Davis, E. Okal, E. Sylver, J. Sweet, and

D. Tappin (2002): Slump origin of the 1998 Papua New Guinea tsunami. Proc.

Roy. Soc. Lond. A, 458, 763–789.

Tang, L., C. Chamberlin, and V.V. Titov (2008): Developing tsunami forecast

inundation models for Hawaii: Procedures and testing. NOAA Tech. Memo.

OAR PMEL-141, NOAA/Pacific Marine Environmental Laboratory, Seattle,

WA, 46 pp.

Titov, V., and F.I. Gonz ́alez (1997): Implementation and testing of the Method

of Splitting Tsunami (MOST) model. NOAA Tech. Memo. ERL PMEL-112

(PB98-122773), NOAA/Pacific Marine Environmental Laboratory, Seattle, WA,

11 pp.

Titov, V.V., F. Gonz ́alez, E.N. Bernard, M.C. Eble, H.O. Mofjeld, J.C. Newman,

and A.J. Venturato (2005): Real-time tsunami forecasting: Challenges and

solutions. Nat. Hazards, 35(1), Special Issue, U.S. National Tsunami Hazard

Mitigation Program, 41–58.

USGS (2006): JAMSTEC multibeam surveys and submersible dives around the

Hawaiian Islands: A collaborative Japan-USA exploration of Hawaii’s deep

seafloor. U.S. Geological Survey Data Series 171 (pubs.usgs.gov/ds/2006/

171/home.html).

VLIZ (2005): Maritime Boundaries Geodatabase available from the Flanders Marine Institute. www.vliz.be/vmdcdata/marbound.

Wei, Y., E. Bernard, L. Tang, R. Weiss, V. Titov, C. Moore, M. Spillane, M. Hopkins, and U. Kˆano ̆glu (2008a): Real-time experimental forecast of the Peruvian

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J. Geophys. Res., in press.
Tsunameter network design in Pacific and Atlantic 109

Appendix A. The DART® Siting

Workshop

This two-day meeting, 6–7 July 2005, was conducted jointly by NOAA and

USGS at NOAA’s Pacific Marine Environmental Laboratory, and was attended by staff from the Tsunami Warning Centers, members of the tsunami,

seismological, and oceanographic research communities, and engineers from

PMEL and NDBC knowledgeable in DART® design and operational matters. Speakers with their affiliations were as follows:

Frank Gonz ́alez, NOAA Center for Tsunami Research, PMEL

Eddie Bernard, NOAA Center for Tsunami Research and PMEL Director

Sam Johnson, USGS Western Coastal and Marine Geology Team

Paul Whitmore, NWS West Coast and Alaska Tsunami Warning Center

Shannon MacArthur, NOAA/National Data Buoy Center

Hal Mofjeld, NOAA Center for Tsunami Research, PMEL

John Dennis, Rice University (Consultant on optimization methods)

Mick Spillane, NOAA Center for Tsunami Research, PMEL

Eric Geist, USGS

Ken Hudnut, USGS Earthquake Hazards Program

Brian Atwater, USGS

Uri ten Brink, USGS

Aurelio Mercado, University of Puerto Rico

Chris Newhall, USGS Volcano Hazards Program

Vasily Titov, NOAA Center for Tsunami Research, PMEL

Doug Luther, University of Hawai‘i

Other experts, including Peter Bird, David Jackson, and Yan Kagan of

UCLA, though unable to attend, communicated their views through the

participants.

The discussion was very fruitful, beginning with the practical needs of

an operational warning system, then focusing on the distribution and associated threat levels associated with various tsunami sources with a goal of

narrowing and refining the initial network concept. Logistical considerations

bearing on maintenance of a virtually global array were described, together

with options for incorporating the needs and resources of other oceanic observational programs. The potential application of optimization techniques

to array design were described. A concept of a fully-featured warning system,

melding data from seismometers and DART® buoys with pre-computed numerical models of tsunami propagation and tuned site-specific inundation

models (now available in the SIFT system), was presented.

The workshop concluded with a consensus-building discussion in which

Frank Gonz ́alez (then Program Leader of PMEL’s Tsunami Research group)

sought an assignment of the 39 U.S. DART® buoys in the Pacific and Atlantic/Caribbean basins which would achieve the best coverage of the perceived threat. The resulting allocation, encapsulated in Fig. 4 of the body
110 Spillane et al.

of this report, assigned 7 buoys to the Atlantic region (including one each to

the Caribbean and Gulf of Mexico), with the remaining 32, including several

existing instruments, distributed in several groupings throughout the Pacific. The Workshop recommendations included priorities for the sequence

of deployments at the new sites, and agreement on the distribution of effort

among those involved in order to make the expanded array a reality. The

completion of the array in March 2008 marks the success of the Workshop

deliberations.

More specific detail on the content of the Workshop discussions, summarized by Geist et al. (2005), are available online from the USGS at

soundwaves.usgs.gov/2005/10/meetings.html.

Reference

Geist, E., F. Gonz ́alez, and U. ten Brink (2005): Workshop on Optimizing the

DART Network for Tsunami Forecasting. Sound Waves Monthly Newsletter,

October 2005 (soundwaves.usgs.gov/2005/10/meetings.html).
Tsunameter network design in Pacific and Atlantic 111

Appendix B. West Pacific DART®

Location Assessment (Stations 2-1 to 2-6)

Hal Mofjeld, Mick Spillane, Frank Gonz ́alez, Vasily Titov

NOAA/PMEL National Center for Tsunami Research

(Originally circulated on 21 October 2005)

Introduction

NDBC has scheduled the deployment of six new DART® stations in the

West Pacific, with the deployment cruise set to begin February–March, 2006.

At the request of Shannon McArthur (NDBC), PMEL conducted a highly accelerated evaluation of a set of candidate locations developed by the Tsunami

Warning Centers (TWCs). The rapidity of this evaluation was driven by the

lead time needed to secure advance permission from other nations to deploy

DART® systems in their territorial waters, if such deployments prove necessary in order to meet DART® Network operational requirements. The

purpose of this effort is to recommend refinements in the DART® locations

to the TWCs and NDBC, as well as to identify choices that will need to be

made before the final locations can be established.

Analysis

The assessment and subsequent recommendations were guided by the following set of criteria:

(a) Tsunami travel times from potential tsunami sources,

(b) Positions relative to tsunami propagation paths to U.S. impact sites,

(c) Suitability of bottom conditions for hardware deployment,

(d) Avoidance of wave scattering islands, seamounts, and ridges,

(e) Location relative to political boundaries.

Overall, only minor changes in location appear to be necessary in order to

meet these criteria. The original TWC candidate sites, the recommended

locations, and relevant comments and issues are listed in Table B1, and a

graphical representation is provided in Fig. B1.

To better understand the role of these recommended DART® stations

in observing tsunamis from the full suite of possible subduction zone sources

in the West Pacific Region, it is helpful to view the tsunami beam patterns

from the sources. These patterns are available at the website: nctr-people.

pmel.noaa.gov/spillane/WestPacAnim.html.
112 Spillane et al.

Table B1: Original and recommended DART® positions in the West Pacific Ocean.

Original Sites Recommended Sites

Latitude Longitude Depth Latitude Longitude Depth

DART® Station (◦N) (◦E) (m) (◦N) (◦E) (m) Issue/Comment

2-1 18.30 152.10 5873 18.30 152.10 5873 Unchanged

2-2 18.28 155.57 5678 18.00 157.00 5722 Scatterers nearby

2-3 11.20 154.60 5740 11.20 154.60 5740 Unchanged

2-4 20.84 134.97 5849 21.00 134.00 5934 Ridge nearby

2-5 02.38 145.65 4570 04.00 145.50 4414 Beam coverage

2-6 13.00 132.00 5695 13.00 132.00 5695 Unchanged

Summary and Recommendations

Each individual DART® Station is addressed here in turn, together with

a recommendation to either leave the candidate location unchanged or to

move it, based on the analysis criteria listed above.

2-1. Placed on the 1.0-hr travel time curve from South Honshu Ridge

and Japanese sources, the original location appears to be fully adequate. It

is located in International Waters.

2-2. Suggest moving to the east to avoid scattering centers near the original location. This does increase the minimum travel time slightly from 1.0

hour to 1.25 hour. Location within the Northern Marshall Islands. Moving

it eastward beyond 158◦E would place it in the Marshall Islands.

2-3. Also on the 1.0-hour travel time curve, the original location appears

to be fully adequate. Location is within the Northern Marshall Islands.

Moving it eastward beyond 158◦E would place it in the Marshall Islands and

moving it southward below 10.5◦N would place it in the Federated States of

Micronesia.

2-4. This station is in a difficult location because it is between the

Kyushu Palau Ridge and water to the west with depths >6000 m; the latter

are beyond the depth limit of the DART® bottom units. The suggested location is farther away from the ridge but in deeper water. A local bathymetric

survey during the deployment cruise will be necessary to find a suitable location in terms of water depth. The station is within waters claimed by

Japan.

2-5. Suggest moving the station northward to get within the main beam

of more sources to the south. Location is within the Federated States of

Micronesia. Moving it westward a relatively short distance will place it

within waters claimed by the Palau Islands.

2-6. Well-located. The station is in international waters. Moving it

farther west will eventually place it within 200 nm of The Philippines.
Tsunameter network design in Pacific and Atlantic 113

Figure B1: Recommended DART® locations in the western Pacific Ocean. Also shown are tsunami

travel times to nearest tsunami source (white lines, in hours), potential tsunami sources (black

squares), TWC warning points (pink squares), and bathymetry (indicating islands, seamounts, and

mid-ocean ridges).

Note on EEZ Issues

Even modest changes in some of the candidate locations can move these

locations from the territorial waters claimed by one nation (Fig. B2) into

those of another. In addition, some of the borders separating these waters

are in dispute.

It may be that the NOAA/NWS already has existing relationship with

some of the Island Nations that will facilitate getting permission for the

DART® deployments in their waters. It may be that the NWS Pacific

Region HQ in Honolulu has experience in dealing with instrumentation deployments and maintenance in these nations that would be useful in this
114 Spillane et al.

Figure B2: Recommended DART® station locations and territorial waters. Minimum tsunami

travel times are shown in black, while political boundaries are shown as magenta lines. Color code

refers to the earliest DART® network detection of a tsunami generated by the associated source.

The EEZ boundaries for Japan are from the online source www.gdrc.org/oceans/un-seahorse/

maps.html; the island nation boundaries are extracted from The New International Atlas (Rand

McNally, 1991).

regard. This possibility needs to be explored further, since delays associated in getting permission from other nations can be substantial, potentially

delaying the DART® deployments in the west Pacific or leading to a lessthan-optimal DART® array there1.

1Further discussion following this recommendation, coupled with a more accurate representation of the EEZ boundaries, led to minor adjustments that placed all of the sites

in International waters.
Tsunameter network design in Pacific and Atlantic 115

Appendix C. Atlantic/Caribbean/Gulf of

Mexico DART® Location Assessment

(Stations 3-1, 3-2, 3-3, 7-1, 7-2, 7-3, 8-1)

Hal Mofjeld, Mick Spillane, Frank Gonz ́alez, Vasily Titov

NOAA/PMEL National Center for Tsunami Research

(Originally circulated on 2 February 2006)

Introduction

NDBC has scheduled the deployment of five new DART® stations in the Atlantic/Caribbean/Gulf of Mexico Region, with the deployment cruise scheduled to begin on 24 March. At the request of Shannon McArthur (NDBC),

PMEL has conducted an accelerated evaluation of candidate locations of all

seven DART® stations that have been allocated for the region. Alternate

locations are suggested for some stations as well, based on various technical

considerations. This evaluation is meant to serve as a guide to the Tsunami

Warning Centers (TWCs) who will make the final decision of the locations

or who may request additional analyses for some locations. Having the seven

locations, Shannon can then plan the deployment cruise, taking into account

the relative costs of ship time for the various choices of the five stations.

The original DART® locations were established at the DART-NOW

Workshop. The priorities for each group of stations were also established relative to the overall U.S. DART® Network. In the Atlantic/Caribbean/Gulf

of Mexico Region, the stations in the Caribbean/Puerto Rico area were assigned third priority in the overall DART® network based on the frequency

and damage potential of regional tsunamis. The stations off the East Coast

have seventh priority, and the Gulf of Mexico station eighth priority. However, it should be noted that NOAA is committed to deploying all seven

stations by the end of FY07 and that logistical contributions must largely

determine the order in which the stations are deployed.

Analysis

The DART® location assessment and resulting recommendations were guided

by the following set of criteria:

(a) Tsunami travel times from potential tsunami sources,

(b) Positions relative to tsunami propagation paths to U.S. impact sites,

(c) Suitability of bottom conditions for hardware deployment,

(d) Avoidance of wave scattering islands, seamounts and ridges,

(e) Avoidance of strong current regimes,
116 Spillane et al.

Table C1: Original and recommended DART® positions in the Atlantic/Caribbean/Gulf of Mexico

Region.

Original Sites Recommended Sites

Latitude Longitude Depth Latitude Longitude Depth

DART® Station (◦N) (◦W) (m) (◦N) (◦W) (m) Comment

7-1 28.46 56.69 5413 37.55 50.00 5454 Moved NNE

7-2 38.57 67.95 4112 38.21 67.93 4317 Moved South

7-3 31.72 73.55 5158 32.93 72.47 5283 Moved NE

3-1 21.50 66.81 5374 23.33 67.64 5750 Moved North

3-2 15.00 73.00 3144 15.26 68.23 4462 Moved East

3-3 21.28 62.79 5498 23.40 63.90 5840 Moved North

8-1 25.41 86.80 3312 25.41 86.80 3312 Unchanged

(f) Location relative to political boundaries.

The database of tsunami propagation model runs was expanded to cover

potential earthquake sources in the Atlantic/Caribbean/Gulf of Mexico Region, with the guidance of Uri Ten Brink of the USGS. Unit sources with

100 km spacing were sited along the major known faults. Off the East

Coast and in the Gulf of Mexico, submarine landslides are the most likely

sources of tsunamis. Travel times appropriate to such sources were computed

based on the TTT program (Geoware), but amplitude information for the

landslide-generated tsunamis is as yet unavailable. Preliminary results from

an analysis of the submarine landslide hazard, being prepared by the USGS,

were employed in the revisions to the East Coast DART® locations (7-2 and

7-3).

The original locations were established during the DART-NOW Workshop. They, and the recommended locations, are listed in Table C1; a graphical representation is provided in Fig. C1. Further graphical information relevant to site selection in the Caribbean Sea (3-2) and Gulf of Mexico (8-1)

are provided in Fig. C2.

Note added during the writing of this Technical Memo

The surface buoy of the Gulf of Mexico DART® 8-1 (46408), which was

deployed as recommended here, broke loose in November 2007. The loss

was likely due to strong currents associated with the Loop Current sweeping

across the site. The mooring for DART® 7-2 (44402) has been similarly

impacted by strong currents of the Gulf Stream. Clearly the treatment

above, based on a climatological representation of these strong currents, was

inadequate to appropriately represent the major excursions they exhibit. A

full treatment of siting should include actual current observations, preferably

moored but perhaps supplemented with drifter data; Such data are, however,

sparse.
Tsunameter network design in Pacific and Atlantic 117

Figure C1: DART® site selection in the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico. Also

shown are the 30-min tsunami travel times to nearest tsunami source, potential tsunami sources,

bathymetry, and boundaries of international waters.

Figure C2: Detection times as a factor in Caribbean site selection. The position recommended for

a single DART® in the Caribbean lies in the southwest of the Puerto Rico EEZ. Color-contoured is

the advance arrival (in hours) at potential buoy locations when compared to Ponce for the tsunami

sources indicated by red stars. Sources shown as blue stars are excluded from the computation

because their detection relies on other DART®s (north of Puerto Rico), or other methods (for the

closest Muertos Trough sources and those off the chart to the east of the Lesser Antilles.) The

original 3-2 site, while providing greater advance warning of tsunamis originating near Panama,

provides no lead time for events to the south and near Hispaniola.
118 Spillane et al.

In attempting to address this issue and quickly generate revised recommendations for the 46408 and 44402 sites, hindcasts from the Navy LayeredOcean Model (NLOM, Shriver et al. (2007)) have been employed (see Appendix D of this report). For data and visualizations see www7320.nrlssc.

navy.mil/global_nlom32/skill.html.

Reference

J.F. Shriver, H.E. Hurlburt, O.M. Smedstad, A.J. Wallcraft, and R.C. Rhodes.

2007, 1/32◦ real-time global ocean prediction and value-added over 1/16◦ resolution. J. Marine Systems, 65, 3–26.
Tsunameter network design in Pacific and Atlantic 119

Appendix D. Revised Siting

Recommendation for the New England

DART® (44402)

Mick Spillane

NOAA Center for Tsunami Research, Seattle WA

(Originally circulated on 6 March 2008)

Introduction

The initially recommended site for the northernmost of the two DART®s

along the U.S. East Coast has proved to be too often in the path of the Gulf

Stream and is, in consequence, difficult to maintain. A revised recommendation, based on the surface current statistics of the Navy Layered-Ocean

Model (NLOM, Shriver et al., 2007), is proposed that should substantially

reduce the risk of mooring loss. The revised site, 39.30◦N, 70.65◦W, is

closer to shore, which may improve the warning time somewhat in the case

of locally generated tsunami waves caused by submarine landslides, but possibly degrade the detection of slides at intermediate distances. However,

it should still permit an updated forecast, two hours or so in advance of

coastal impact, for waves from remote seismic sources off Puerto Rico or in

the Eastern Atlantic that have already been detected by other elements of

the DART® array.

Analysis

An initial report (Mofjeld et al., 2006; Spillane et al., 2008) that led to the

deployment of DART® 44402 at 38.196◦N, 67.851◦W employed a representation of the climatological mean path of the Gulf Stream that has proved

inadequate. The surface buoy broke loose in November 2007, severing communications with the sea floor pressure recorder, and a replacement was

lost shortly after deployment. It is believed that strong currents from the

meandering Gulf Stream were responsible for the losses.

While a thorough mooring design and selection of a new site should

include an analysis of all available current meter data for the region, this

is precluded by the urgency of restoring tsunami coverage. Instead, surface

current hindcasts from an operational U.S. Navy circulation model NLOM,

available online for the period January 2005 through August 2007, were

analyzed. The overall maximum and median speed statistics, displayed in

Fig. D1, confirm that the original 44402 site is frequently in the path of the

Gulf Stream with excessive current speeds. The meandering nature of the
120 Spillane et al.

Stream can be readily visualized at the NLOM website and exposes a wide

area off New England to its influence. Sorting the daily model speeds over

the 965 days available allows the computation of a the 99th percentile speed

for each grid point. This is the speed (denoted by S1%) that is likely to be

exceeded during 1% of the days of a long-term deployment. In Fig. D2, S1% is

plotted for the Gulf Stream region, south and east of Cape Cod. The inshore

bathymetry and 1,500–3,000-m isobaths are based on Smith and Sandwell

(1997). The latter indicate the presence of the New England seamount chain

to the east that should be avoided as potential scatterers of tsunami wave

energy. Color contours show the 1% exceedance level for speed in units of

meters/second, south of the stepped boundary which represents the inshore

boundary of the NLOM domain. The initial site of DART® 44402 is shown,

as is the site of a NDBC wave and meteorological buoy 44004 at which some

knowledge of environmental conditions should be available.

Highlighted in red in Fig. D2 is the 1 m/s S1% contour. This encloses

a region that has a considerable extent to the south of the Stream but is

more constrained to the north. The 0.75 m/s contour is shown in blue.

Based on these, two tentative sites, marked in red, were circulated to the

DART® user group with an estimate of their impact on warning times.

Based on the positive response, via e-mail and the DART® teleconference

of 5 March 2008, a more detailed study of this sub-region enclosed in black

was performed. The results are shown in Fig. D3. Here the underlying

bathymetry is from the NGDC (2005) and has 9-arcsecond resolution; the

color contour interval is 100 m. Submarine canyons are clearly evident,

cutting through the continental slope and in places incised into the more

gradually sloping plane on which the DART® will be placed. Cross-hatching

is used to display the S1% statistic. Regions where speeds of 2 m/s and

1 m/s might be exceeded during 1% of deployment days are hatched in

red and orange. These are clearly unsuited as survivable DART® sites.

Further inshore, and hatched in yellow, are the regions of minimum S1%,

with levels between 0.4 and 0.5 m/s. Considering these and the isobath

spacing, highlighted in red, the location 39.30◦N, 70.65◦W with water

depth ∼2644 m is recommended as the new DART® 44402 site.

In addition to reduced exposure to strong currents, the warning characteristics of the site must be considered. Potential sources for tsunami hazard

for the U.S. East Coast include local submarine landslides and remote seismic events. For the latter, the subduction zone of the Puerto Rico Trench is

probably of greatest concern. Considering travel times, to the initial and revised 44402 site from unit sources of the propagation database near Puerto

Rico, it is estimated that a reduction of approximately 12 min in arrival

time will result from the move. However, detection at the revised 44402 site

still comes 2 hours in advance of coastal impact along the northern portions

of the East Coast, and the primary warning of such events will have been

produced hours earlier by DART® 41420 or 41421. Being 270 km westward

of the initial site, a delay of 20–30 min might be expected in waves originating in the eastern Atlantic (Portugal or the Azores), but again the role of

DART® 44402 in such a situation is to refine the estimate of waves initially

detected by DART® 44401.
Tsunameter network design in Pacific and Atlantic 121

Figure D1: Maximum and median surface current speeds from the Navy Layered-Ocean Model

(based on 965 days of hindcasts: 1 January 2005 to 23 August 2007.)
122 Spillane et al.

Figure D2: 99th percentile of NLOM surface speed distribution (m/s) expected to be exceeded on

1% of days during a long-term deployment. Details of the region within the box are presented in

Fig. D3.

In the absence of a complete characterization of the local submarine

landslide hazard, both in terms of location and the waves that might be

generated, it is not possible to do more than comment generally on the

result of the 44402 re-siting. For events in the immediate vicinity the move

inshore might improve matters, but slower alongshore propagation and the

reduction in the length of shoreline effectively monitored might degrade the

detection of other landslide events.

Conclusion

A revised location for DART® 44402 deployment in the vicinity of 39.30◦N,

70.65◦W is recommended. The precise location may be subject to revision by NDBC staff on consideration of seafloor infrastructure, sea lanes,

or seafloor surveys during deployment. The new site is expected to greatly

reduce the currents to be expected, though no site in sufficiently deep water can be expected to completely escape the meandering path of the Gulf

Stream.
Tsunameter network design in Pacific and Atlantic 123

Figure D3: Bathymetry in the vicinity of the revised site recommendation for DART® 44402.

The statistic S1% estimates the surface current speed that may be exceeded on 1% of days during a

long-term deployment and is computed from NLOM hindcasts. At the recommended site, 39.30◦N,

70.65◦W, S1% is less than 0.5 m/s.

References

Shriver, J.F., H.E. Hurlburt, O.M. Smedstad, A.J. Wallcraft, and R.C. Rhodes

(2007): 1/32◦ real-time global ocean prediction and value-added over 1/16◦

resolution. J. Marine Systems, 65, 3–26. For NLOM data and visualizations

see: www7320.nrlssc.navy.mil/global_nlom32/skill.html.

Mofjeld, H., M. Spillane, F. Gonz ́alez, V. Titov (2006): Atlantic/Caribbean/Gulf

of Mexico DART® Location Assessment. Unpublished document circulated to

Tsunami Warning Centers and NDBC and reproduced in Appendix C.

Spillane, M.C., E. Gica, V.V. Titov, and H.O. Mofjeld (2008): Tsunameter site selection for the U.S. DART® arrays in the Pacific and Atlantic Oceans. NOAA

Tech. Memo. OAR PMEL-143 (this document).

Smith, W.H.F., and D.T. Sandwell (1997): Global seafloor topography from satellite

altimetry and ship depth soundings. Science, 277, 1957–1962.

National Geophysical Data Center (2005): East Coast and Gulf Coast and Caribbean

Nine Second Tsunami Propagation Grids Compilation Report, prepared for the

NOAA Center for Tsunami Inundation Mapping Efforts (TIME).

Tsunameter network design in Pacific and Atlantic 125

Appendix E. Aleutian/Alaska/West Coast

DART® Location Assessment (Stations

4-a, 46401, 1-2, 1-1, 5-3, and 46405)

Hal Mofjeld, Mick Spillane, Vasily Titov

NOAA/PMEL National Center for Tsunami Research

(Originally circulated on 31 March 2006)

Introduction

The NDBC is scheduling a cruise to deploy four new DART® stations in the

Aleutian/Alaska/West Coast region and to service, and possibly redeploy in

new locations, some others. This report is intended to provide guidance as

to likely sites, which best meet the goals of the Tsunami Warning Centers

with regard to coverage of potential tsunami sources and satisfy operational

constraints of the instruments. Precise siting is generally determined by

NDBC, taking into account detailed bathymetry, the presence of seafloor

infrastructure, national boundaries, etc. In this case all but one of the

locations lie within U.S. waters. The exception, P26/5-3, lies within the

Canadian EEZ.

The DART® network design combines the legacy of the originally instrumented sites, the conceptual array that emerged from the DART-NOW

Workshop, and the results of tsunami propagation studies that are ongoing at PMEL. The Workshop assigned priorities to the recommended sites

but, since all of the sites are to be instrumented in the next year or two,

the order of deployment is in large part dictated by instrument production

and ship scheduling. One location (P1/4-b) discussed below will not be instrumented at this time. However it seemed appropriate to include it, and

existing DART®s unaffected by the cruise, in this report in order to fully

reflect the current concept for the regional array.

Note on DART® Site Naming: Unfortunately there is more than

one name associated with each site. The initial array concept for the Pacific

was described with labels P1–P32. The workshop employed names, such as

5-3, to provide regional and local index information for the planning process.

Finally, when a DART® is deployed it gets a NDBC buoy name, such as

46401. Where more than one name is appropriate to the likely users of this

report, all the relevant ones are given.
126 Spillane et al.

Analysis

The DART® location assessment and resulting recommendations in this

report were guided by the following set of criteria:

(a) Tsunami travel times and amplitudes for potential tsunami sources,

(b) Positions relative to tsunami propagation paths to U.S. impact sites,

(c) Suitability of bottom conditions for hardware deployment,

(d) Avoidance of wave scattering features such as seamounts and ridges,

(e) Provide timely data for real-time forecasting of tsunami amplitudes,

as well as early warning cancelation capability.

The database of tsunami propagation model runs for the Pacific Ocean basin

is essentially complete and allows the estimation of the travel time and amplitude of the first wave crest arriving offshore at any location from all potential

sources. It should be noted that these model results do not extend to the

beach but, in addition to serving as primary information for DART® siting

studies, provide input to more detailed real-time models of inundation. The

15- and 30-min minimum travel time envelopes that result from the propagation database are shown as light blue lines in Fig. E1. These lines show the

arrival time at a location from the closest source in the database. DART®

locations are generally sited close to one or other of these lines. In the case of

the Aleutian DART®s, the 15-min envelope is preferred since the detection

and computation time needed to estimate impacts on the Hawaiian Islands

make the 3-hour notice desired for evacuation difficult to achieve. Elsewhere the 30-min envelope is generally chosen, except where frequent but

rarely damaging events are expected. Here placement of instruments closer

to the sources is important for rapid cancelation of local tsunami warnings

as needed.

Summary and Recommendations

The Tsunami Warning Centers have indicated that an even distribution of

DART® sites best provides coverage of a largely unpredictable hazard. This

has been a major consideration in the choice of Aleutian and Alaskan sites,

as has their positioning along the 15-min line as dictated by travel times to

the Hawaiian Islands. In particular, the Aleutian sites P1 through P14 will,

after adjustments, have a nominal 6◦ zonal spacing. Details of the rationale

for sites involved in the cruise being planned are given below.

P26/5-3. The area to the west of Vancouver Island, and northward

toward Queen Charlotte Island, has frequent earthquake activity. While

the tsunami amplitude associated with such events is generally low, it is

desirable to have a DART® site in the vicinity to verify that the warning

issued on the basis of the seismic signal can be canceled. For this reason

it is recommended that the original DART® site be moved inshore to the

location shown in Fig. E2. The sea floor is relatively flat and scattering

features to the west and southwest do not rise much closer than 2000 m

to the surface. Earlier work by Hal Mofjeld suggests that features need to
Tsunameter network design in Pacific and Atlantic 127

Figure E1: The current design for the DART® array in the Aleutian/Alaska/West Coast region.

Red squares indicate instruments to be installed or repositioned in upcoming cruises; green marks

those already in place, and yellow future deployments or repositioning actions. Arrows indicate

recommended moves. Instruments at P8/46405 and P9/46404 will be serviced in the April 2006

cruise; the repositioning of P8/46405 may await the following servicing cruise.

rise to within 1200 m of the surface to cause significant scattering. The

tsunami beam patterns of nearby sources in the propagation database were

examined. They tend to be quite directional but, given their proximity to

the recommended P26 site, their amplitude should be adequate for detection.

The overall distribution of DART®s (Fig. E1), and the presence of faults for

the entire shelf northward to Yakutat, suggests that an additional DART®

to the north, perhaps in the vicinity of 55◦N, 138◦W, would be desirable to

provide some redundancy to P10/1-1. P26/5-3 does add redundancy for P9

and P8, lying in the path of waves propagating from the northern Gulf of

Alaska toward the West Coast. Its recommended location is however within

the Canadian EEZ; all others discussed below are in U.S. waters.

P8/46405. This should be moved from its current location near the

45-min line to a point on the 30-min line, approximately midway between

P9/46404 and P13/46411. In Fig. E3, the Smith-Sandwell bathymetry shows

a relatively flat valley, separated from the open ocean by a low ridge (which

might provide an alternate site, in the vicinity of 42.8◦N, 129.5◦W). To the

east, where the plate boundaries are seen in the wider view, there are some

minor seamounts that should not cause significant scattering of Cascadia

events. Waves originating off California are likely to be distorted by the

Mendocino escarpment and other features to the south and southeast, as
128 Spillane et al.

Table E1: Original and recommended DART® positions in the Aleutian/Alaska/West Coast region. Bold text highlights positions to be visited in the upcoming cruise. Other stations, existing or

planned, are included, here and in Fig. E1, to show the overall array design for this region. Several

naming conventions are in use for DART® positions; those most appropriate to likely readers are

included here.

DART® Original Position Recommended Position

Station Latitude Longitude Depth (m) Latitude Longitude Depth (m) Comment

4-b P1 50.667◦N 171.961◦E 5000 50.175◦N 171.850◦E 4681 Future deployment

(Fig. E6)

4-a P17 47.273◦N 179.709◦E 4899 48.920◦E 178.300◦E 5714 New DART®

(Fig. E3)

46401* P16 46.638◦N 170.790◦W 5658 48.870◦E 175.580W 5502 Substantial move

(Fig. E4)

1-2 P15 48.727◦N 168.814◦W 3686 49.610◦N 169.900◦W 5184 New DART®

(Fig. E8)

46402 P14 50.442◦N 165.028◦W 4900 51.060◦N 164.000◦W 4687 Future adjustment

(Fig. E9)

46403 P12 52.649◦N 156.940◦W 4517 52.649◦N 156.940◦W 4517 No action

46409 P11 55.300◦N 148.500◦W 4175 55.300◦N 148.500◦W 4175 No action

1-1 P10 56.455◦N 143.971◦W 3890 57.500◦N 144.000◦W 3771 New DART®

(Fig. E7)

5-3 P26 48.787◦N 130.896◦W 3289 48.800◦N 129.600◦W 2838 New DART®

(Fig. E2)

46404 P9 45.849◦N 128.778◦W 2765 45.849◦N 128.778◦W 2765 Service visit

46405* P8 42.903◦N 130.909◦W 3500 42.600◦N 128.900◦W 3243 Possible move

(Fig. E3)

46411 P13 39.349◦N 127.008◦W 4320 39.349◦N 127.010◦W 4320 No action

46412 P7 32.246◦N 120.699◦W 3782 32.246◦N 120.700◦W 3782 No action

*Note that, subsequent to their resiting to the recommended locations, 46401 and 46405 were given the

WMO buoy numbers 46413 and 46407, respectively.

are trans-Pacific tsunamis, but this is unavoidable for any site in the region.

A more precise characterization of the sea floor is desirable to avoid the risk

of submarine landslides. The recommended location is within the U.S. EEZ.

P16/46401. The current and recommended locations for P16/46401 are

shown in Fig. E4. The original site was south of the 30-min minimum travel

time envelope, a location chosen perhaps to monitor sources near Kamchatka

and the West Aleutians as well as those to the north. This need is reduced

by the placement of other DART®s to the west, including others, not seen

in Fig. E1, off Kamchatka and east of Japan. By moving P16/46401 north,

close to the 15-min envelope, earlier forecasts and/or warning cancelation

are achieved, particularly for Hawaii. The recommended site lies in relatively

flat topography within the U.S. EEZ boundary; the feature to the NNW lies

some 4000 m below the surface and ought not cause significant tsunami wave

scattering.

P17/4-a. This site was chosen to extend westward the picket line of

DART® instruments monitoring Aleutian tsunami sources to the north. In

addition it will provide some backup and another reading on events whose

origin lies further west for which the DART® at P1/4-b (see Fig. E1) and

other instruments (not shown) are the primary detectors. The recommended
Tsunameter network design in Pacific and Atlantic 129

Figure E2: Smith-Sandwell bathymetry in the vicinity of the recommended location for P26/5-3.

This site is in Canadian waters and is intended to enhance early warning cancelation capability

for the Pacific Northwest and British Columbia for local events. Light blue lines show the 15- and

30-min minimum travel time envelopes; black, red, and green contour lines characterize the beam of

three unit sources in the region (AASZ-B44, CCSZ-B1, and CCSZ-B2).

site (see Fig. E5) is on the 15-min envelope and lies within the U.S. EEZ

boundary. When placed, P17/4-a will be for a while the westernmost of

the DART® array. It lies near the dateline in broken, but not too rugged

topography, within the U.S. EEZ.

P1/4-b. DART® site P1/4-b, when deployed in a future cruise, will be

the westernmost of the array in U.S. EEZ waters. Its location (see Fig. E6),

near the 15-min envelope, maintains the nominal 6◦ spacing of the DART®s

to the east. Farther west, DART®s 4-c, 4-d, and 4-e of the group (not

shown in Fig. E1) will be aligned parallel to the Kuril-Kamchatka subduction

zone. However, P1/4-b is well placed to monitor the northern portion of this

subduction zone and its junction with the Aleutian subduction zone. The

recommended location is within the U.S. EEZ.

P10/1-1. This station is intended to monitor potential tsunami sources

at the eastern end of the Aleutian/Alaska subduction zone, as well as the

Yakutaga Zone and a range of other faults extending southward toward

Queen Charlotte Island. Because of the vee-shaped distribution of these

sources, reflected in the shape of the 15- and 30-min envelopes, P10/1-1

provides early detection for quite a few sources. In addition to providing

the primary detection capability for waves propagating toward Hawaii, it

should provide coverage for West Coast impact sites, too. However, while

other DART®s to the west provide some redundancy for Hawaii, this is less
130 Spillane et al.

Figure E3: Current and recommended locations for P8/46405. The bathymetry of the region in

the local (a), and wider view (b) is based on the Smith-Sandwell topography.

true for the West Coast, and an additional DART® between P10/1-1 and

P26/5-3 (off Vancouver Island) would be desirable.

P15/1-2. This is a new DART® site whose location was chosen to continue the nominal 6◦ spacing of the sites to the west, and placement near

the 15-min minimum travel time envelope. The Smith-Sandwell bathymetry

shows a relatively smooth sea floor in the region, without significant scattering features.
Tsunameter network design in Pacific and Atlantic 131

Figure E4: Repositioning of DART® P16/46401 from its current position outside the 30-min line

(panel a) to the position recommended in this report will improve early detection and hence the

forecast and warning cancelation capability. The bathymetry shown is from Smith-Sandwell.
132 Spillane et al.

Figure E5: Siting recommendation for DART® P17/4-a. The bathymetry data is from Smith and

Sandwell (1997).

Figure E6: Siting recommendation for P1/4-b, which will be deployed in a future cruise, with

bathymetry from Smith-Sandwell.
Tsunameter network design in Pacific and Atlantic 133

Figure E7: Siting recommendation for P10/1-1 in the northern Gulf of Alaska, with bathymetry

from Smith and Sandwell (1997).

Figure E8: Siting recommendation for P15/1-2 with bathymetry from Smith-Sandwell.
134 Spillane et al.

Figure E9: Potential future adjustment of the P14/46402 DART® with bathymetry from SmithSandwell.

P14/46402. The current cruise does not entail servicing this DART®

location. The current position is generally quite suitable and only slightly

removed from the alternate location, which was picked to achieve 6◦ spacing.

There may be merit in maintaining the continuity of observations at the

current P14/46402 site.
Tsunameter network design in Pacific and Atlantic 135

Appendix F. Central/South America

DART® Location Assessment (Stations

6-1, 6-2, 6-3, 6-4, and relocated 51406)

Mick Spillane and Vasily Titov

NOAA Center for Tsunami Research

(Originally circulated on 29 December 2006)

Introduction

NDBC is working toward the completion of the DART® array in the Pacific

during 2007. This report recommends locations suitable for DART® siting

in the eastern Pacific, between the now complete U.S. West Coast array and

the existing Chilean tsunameter 32401. The sites discussed include the four

designated as Group 6 in previous planning documents, and a relocation

of the existing DART® 51406 from its current position near 8.5◦S, 125◦W.

Site recommendations for DART®s in the northwest Pacific and southwest

Pacific will be provided in separate reports.

The “Equatorial” DART® 51406 (previously named 46406) was intended

to warn Hawaii of events either in the Chile or the Tonga region. The

Southwest Pacific DART®s of Group 9, to be sited north and east of Tonga,

will greatly improve the coverage of events in that area, allowing 51406 to

be better employed in the eastern Pacific where it essentially becomes an

extra element of Group 6.

Analysis

Site recommendations are based on the following considerations:

• Water depth and sea floor slope and roughness: Instruments must be

deployed in water depths of 1,500–6,000 m and the seafloor unit must

be level to communicate with the surface satellite link.

• Proximity to tsunami sources: Early warning cancelation or accurate

forecasts of expected impacts require each DART® to lie close to the

source region it is intended to monitor. The finite number of DART®

systems available, the extent of potential source regions, and the need

to provide backup in the event of instrument failure, limit this.

• Signal quality: In the open ocean tsunami amplitudes are quite small

and their beam patterns complex. Scattering features such as seamounts between the source and DART® are to be avoided; extreme
136 Spillane et al.

proximity to a potential source is to be avoided to prevent contamination of the true tsunami signal by seismic seafloor shaking. Highquality time series from one or more DART®s are needed to accurately

forecast impacts.

• Deployment/servicing efficiencies: To facilitate routine servicing or repair in the event of failure, DART®s should, where possible, be placed

within U.S. or International waters. Coordination with the cruise

plans of other U.S. research programs or NDBC’s other buoy activities may lead to efficiencies. Areas with a history of vandalism should

be avoided.

To quantify the above considerations, Smith-Sandwell seafloor bathymetry,

the database of precomputed tsunami propagation solutions, a database of

EEZ boundaries, and known buoy locations are employed in producing the

following recommendations.

The recommended locations are given in Fig. F1 and Table F1. Shown as

blue triangles are the closest DART®s to the north and south; blue squares

and lines are the TAO array sites with typical servicing cruise tracks (dark

blue for the Ron Brown, Chile-Panama via the WHOI Stratus buoy; light

blue for the Ka’imimoana, San Diego–Manzanillo.)

Tsunami Source Distribution in the Region

Virtually the entire west coast of Central and South America is a potential

source of tsunamis with several notable events in the historical record. The

propagation database contains unit sources representing Mw = 7.5 events

in 100 km segments from Manzanillo, Mexico to Cape Horn and provides

useful information to aid in site planning for DART® instruments to monitor the region. Figure F2 shows, for each unit source, its potential risk for

the Hawaiian Islands; squares at each unit source have areas proportional

to the relative amplitude to be expected offshore from Hilo. Contour lines,

for selected sources posing the greatest risk, show the extent to which the

first wave from a unit source has a readily DART®-detectable 1-cm amplitude. These patterns have lobes, which illustrate the complexity of tsunami

detectability and impact at remote locations.

An alternate presentation, in Fig. F3, contrasts timing and offshore

amplitude at Hilo and San Diego. Note that the amplitude scales refer to

offshore conditions and that SIM results are needed to reflect conditions

at the beach. While the amplitudes (indicated by bar length) at the two

locations are not directly comparable, first wave arrival times are; arrival

times at San Diego for Central American sources are considerably earlier

so that DART® placement needs to be closer to those sources than those

further south to enable early warning cancelation and forecasts for the west

coast.
Tsunameter network design in Pacific and Atlantic 137

Table F1: Locations recommended for Group-6 DART® deployment and 51406 relocation.

DART® Latitude Longitude Water Depth Comments

6-1 16.033◦N 107.000◦W 3468 m Rugged bathymetry; avoid seamounts

6-2 4.924◦N 90.685◦W 3216 m Close but outside Peru/Ecuador EEZ

6-3 10.840◦N 100.085◦W 3304 m Near TAO 95◦W–110◦W transit track

6-4 17.980◦S 86.350◦W 4359 m Relatively smooth bathymetry

51406 7.000◦S 93.943◦W 4013 m Plan in relation to TAO schedule

Figure F1: Recommended locations for the four Group-6 DART®s and the relocated 51406.

Summary and Recommendations

The suggested layout and locations of the Central and South American

DART®s were given in Fig. F1 and Table F1 above. Further detail and

site-specific graphics are provided below.
138 Spillane et al.

Figure F2: Relative impact offshore at Hilo for East Pacific Sources.
Tsunameter network design in Pacific and Atlantic 139 Figure F3: Offshore amplitude (indicated by bar length) and arrival time (color coded.)
140 Spillane et al.

Figure F4: Recommended location for DART® 6-1.

Site 6-1. Figure F4 shows the recommended location for DART® 6-1.

The site is in International waters with a travel time of about 36 min from

the nearest source. The bathymetry is quite rugged in the area though only

a few features rise close enough to the surface to scatter tsunami waves.

Tsunami waves from the sources that this site is intended to monitor could

reach San Diego within 3 hours, and the resort areas of western Mexico

even earlier. Were the array to be augmented in the future, sites within the

Mexican EEZ should be considered.
Tsunameter network design in Pacific and Atlantic 141

Figure F5: Recommended location for DART® 6-3.

Site 6-3. The next DART® to the south of 6-1 has been designated

6-3 in planning to date. The bathymetry in its vicinity, shown in Fig. F5,

is quite smooth and free of seamounts so that there is some flexibility to

choose the site in relation to the TAO servicing ship tracks. When the TAO

servicing of lines 95◦W and 110◦W is done on a cruise from Chile to Panama,

the transit between lines is east to west at the north (the blue line). Visiting

DART® 6-3 involves no added steaming; its location is about 300 nm west

of the 12◦N, 95◦W buoy. The other mode of servicing is on a Manzanillo

to San Diego cruise where the transit between 95◦W and 110◦W is done at

8◦S. In this case a departure from the cruise track from Manzanillo (shown

in yellow) would be required, adding about 170 nm (25%) to that leg. Site

6-3 is approximately 53 min from the closest unit source.
142 Spillane et al.

Figure F6: Recommended location for DART® 6-2.

Site 6-2. Proceeding south, the next DART® site of Group-6 is 6-2,

which serves the dual role of monitoring sources in Central America and

those off Colombia, Ecuador, and northern Peru. The recommended site

(see Fig. F6) is in International waters close to the EEZ boundaries of

Ecuador and Costa Rica, about one degree east of the “placeholder” site

(5◦N, 92◦W). This position reduces somewhat the travel time (68 min) to

the closest sources, gives a clearer view to the southeast, and places it closer

to the Ron Brown’s route from TAO buoy 8◦S, 110◦W to Panama. A position closer to the 5◦N, 95◦W TAO buoy would place 6-2 more in the lee

of the Galapagos Islands, resulting in more complex time series for likely

tsunami sources.
Tsunameter network design in Pacific and Atlantic 143

Figure F7: Recommended relocation of DART® 51406.

Relocated 51406. As noted earlier, the Group-9 DART®s will detect

tsunami waves from the Tonga or southern Chilean sources covered by the

current 51406 location (8.5◦S, 125◦W). By moving it eastward, 51406 could

improve the coverage of South American sources that historically have impacted U.S. interests. A likely location SSE of the Galapagos Islands is

detailed in Fig. F7. In the absence of strong scattering features, the suggested site was chosen approximately 120 nm from the TAO buoys at 8◦S,

95◦W and 5◦S, 95◦W to facilitate overnight steaming between site visits

during servicing cruises of either the Ron Brown or the Ka’imimoana.
144 Spillane et al.

Figure F8: Suggested location for DART® B-4.

Site B-4. The southernmost site of the Central/South American group

of U.S. DART®s is designated B-4. Its location, well removed from any EEZ

boundaries and in a region of relatively smooth sea floor, is less constrained

than most of the other sites. The suggested location was chosen to be close

to the servicing cruise track from the WHOI Stratus buoy to the southernmost TAO buoy in the 95◦W line and on the 90-min minimum travel time

envelope for sources south of Callao. It acts as a backup to 32401 and benefits from the presence of the Nazca Ridge, which appears to focus the energy

from sources far to the south, thereby improving their detectability. It also

provides backup coverage to the relocated 51406 for sources between Callao

and Buenaventura. The bathymetry allows some flexibility in the precise

location; the suggested location is some 140 nm northeast of the WHOI

Stratus buoy.
Tsunameter network design in Pacific and Atlantic 145

Appendix G. Northwest Pacific DART®

Location Assessment (Stations P1/21415,

P18/21416, P3/21417, P19/21418)

Mick Spillane and Vasily Titov

NOAA Center for Tsunami Research

(Originally circulated on 19 June 2007)

Introduction

NDBC plans to complete the tsunami-warning array in the north Pacific

with the deployment of four new DART® systems during July 2007. The

positions, as recommended in this report, will extend the Aleutian line of

DART® sites, which currently ends at 21414 near the dateline, and bridge

the gap along Kamchatka, the Kuril Islands, and northern Japan to link with

the western Pacific group of DART® sites whose northernmost member is

21413. The current Aleutian line has successfully detected some moderate

tsunamis emanating from the Kuril region in November 2006 and January

2007. The signals received, however, were not in the main beam of these

events; the additional sites to be instrumented will reduce the detection time

and, by improving the spatial sampling of future events, improve the quality

of the source characterization and forecasts derived from it.

Analysis

Site recommendations are based on the following considerations:

• Water depth and sea floor slope and roughness: Instruments must be

deployed in water depths of 1,500–6,000 m and the seafloor unit must

be level to communicate with the surface satellite link.

• Proximity to tsunami sources: Early warning cancellation or accurate

forecasts of expected impacts require each DART® to lie close to the

source region it is intended to monitor, and to provide backup in the

event of instrument failure.

• Signal quality: In the open ocean tsunami amplitudes are quite small

and their beam patterns complex. Scattering features such as seamounts between the source and DART® are to be avoided; extreme

proximity to a potential source is to be avoided to prevent contamination of the true tsunami signal by seismic seafloor shaking. Highquality time series from one or more DART®s are needed to accurately

forecast impacts.
146 Spillane et al.

Figure G1: The recommended sites for the Northwest Pacific are shown as red triangles, with the closest existing DART® sites to the south (21413) and east (21414)

marked in green. Green lines indicate EEZ boundaries, red the plate boundaries.

Minimum travel time envelopes for the first wave are drawn at 15-min intervals

and are based on the B row of unit sources in the propagation database for the

northwest Pacific.

• Deployment/servicing efficiencies: To facilitate routine servicing or repair in the event of failure, DART®s should, where possible, be placed

within U.S. or International waters.

To quantify the above considerations, Smith-Sandwell seafloor bathymetry,

the database of pre-computed tsunami propagation solutions, a database of

EEZ boundaries, and existing DART® locations are considered. The recommendations are summarized in Fig. G1 and Table G1 below, and supported

by the text and graphics that follow.

Analysis

P1/21415. This site will be the westernmost element of the DART® array

in U.S. waters, south of the Near Islands. It was described in and accepted
Tsunameter network design in Pacific and Atlantic 147

Table G1: Characteristics and comments for the four recommended northwest Pacific DART®

sites.

DART® Latitude Longitude Water Depth Comments

P1/21415 50.175◦N 171.850◦E 4681 m Extend Aleutian Trench coverage

P18/21416 48.011◦N 163.503◦E 5858 m Assist 21415; Kamchatka-Kuril cover

P3/21417 43.188◦N 157.125◦E 5510 m Kamchatka-Kuril cover

P19/21418 38.710◦N 148.670◦E 5712 m Kamchatka-Kuril cover; assist 21413

following an earlier report (Mofjeld et al., 2006), but some of the content

is repeated here for convenience, and illustrated in Fig. G2. The site maintains the nominal 6◦ spacing of its neighbors to the east and lies, as they do,

along the 15-min minimum travel time envelope for the closest sources. The

latter choice, in the case of 21415, is dictated less by the need to minimize

the warning time for Hawaii than to avoid the more serious wave scattering

features of the Emperor Seamount Chain. Mofjeld et al. (2001) have shown

that the potential for scattering by submarine features increases as the summit approaches the surface; the threshold for major scattering in the water

depths of the NW Pacific is about 1500 m. The Detroit Seamount (∼1500 m,

some 340 km to the WNW of 21415) and the Meiji Guyot off Kamchatka

(∼3000 m, visible in Fig. G1) should not cause major scattering. However,

younger members of the chain: Suiko (45◦N), Nintoku (41◦N), Koko (36◦N),

and Kimmei (34◦N), could induce substantial scattering that increases the

uncertainty in modeled tsunami signals. The intent is then that 21415 monitor potential local tsunami sources in the Aleutian subduction zone as well

as those to the west in the corner region where that subduction zone joins

those off Kamchatka and the Kuril Islands. DART® 21415 can provide

backup and a second look at events in the Kuril region but, for these, the

primary detection will come from the remaining three sites that are not in

the lee of the seamount chain.

P18/21416 to P19/21418. The role of 21416 (and its neighbors to

the south: 21417 and 21418) is to cover the active tsunamigenic region off

Kamchatka, the Kuril Islands, and northern Japan. DART® 21413, the

northernmost of the western Pacific DART® sub-array, provides further

coverage for events off Japan, as do Japanese pressure gages and seabed

seismometers connected to shore via cable in an alternate methodology for

tsunami detection (see KPG1 and KPG2 in Fig. G1). The bathymetry

is relatively smooth in this region, allowing considerable flexibility in the

choice of sites. Those chosen are essentially evenly spaced along the 30-min

minimum travel-time envelope for local potential sources in the interests of

permitting early warning cancellation for the relatively frequent events that

originate in the Kuril region.

The bathymetry in the vicinity of the recommended sites is shown in

Figs. G3–5. In the case of 21416, the strong bathymetry of the Emperor

Seamount Chain, evident in the upper right of Fig. G3, but extending south
148 Spillane et al.

Figure G2: Topography and minimum travel-time envelopes near the recommended 21415 site.

Figure G3: Recommended site for DART® 21416 which, together with 21417

and 21418, provide an essentially uninterrupted view of the Kamchatka, Kuril, and

north Japan subduction zone sources.
Tsunameter network design in Pacific and Atlantic 149

Figure G4: Recommended site for DART® 21417.

Figure G5: Recommended site for DART® 21418. Three topographic features

have been labeled, but none rise sufficiently close to the surface to induce major

scattering of tsunami waves.
150 Spillane et al.

to near Midway then east to the Hawaiian Islands, is “downstream” of the

Kamchatka-Kuril-Japan sources this DART® and its neighbors to the south

are intended to monitor. In the case of 21418, three topographic features

have been labeled, none of which should cause significant scattering. All

three DART®s are located on the 30-min minimum travel-time envelope.

There is a possibility that seismic noise may overlap the actual tsunami signal

and need to be removed during data processing, particularly in the case of

the DART® closest to the source, but the relatively frequent occurrence of

medium-sized events in this region suggests a need for early detection and,

hopefully, early warning cancelation.

References

Mofjeld, H.O., V.V Titov, F.I. Gonz ́alez, and J.C. Newman (2001): Tsunami scattering provinces in the Pacific Ocean. Geophys. Res. Lett., 28(2), 335–337.

Mofjeld, H.O., M. Spillane, and V.V. Titov (2006): Aleutian/Alaska/West Coast

DART® Location Assessment (Stations 4-a, 46401, 1-2, 1-1, 5-3, and 46405)

(Appendix E of this report).

Note added in the production of this report:

Following deployment at the recommended location, DART® 21417 has

apparently been involved in two collisions with vessels. As described in

Section 7.4, sea lanes and fishing activity ought to be a greater consideration

in site selection.
Tsunameter network design in Pacific and Atlantic 151

Appendix H. The U.S. DART® Array:

Recommendations for the Remaining

Stages of the Initial Deployment

M.C. Spillane and V.V. Titov

NOAA Center for Tsunami Research

(Originally circulated on 5 November 2007)

Introduction

The initial deployment of the U.S. array of DART® tsunameters in the Pacific Ocean is nearing completion; only 5 of the 39 instruments allocated to

the array remain. By and large the deployment has followed the arrangement

shown in Fig. H1, which is an annotated version of an image produced in

September 2005 to encompass the recommendations of the DART® Siting

Workshop held that year. Instruments yet to be deployed are the Southwest Pacific group (designated 9-1 to 9-4) and one DART® (6-4) of the

Central/South American group. The allocation of DART® buoys in the

Pacific was predicated on the expectation of further Chilean tsunameters

south of that designated 24 in Fig. H1. To add coverage of this historically

active region, there has been some discussion of relocating the long-deployed

“Equatorial” DART®.

Two further additions to the original figure reflect the actions of Australia:

• a DART® has been deployed in the Tasman Sea in recognition of the

threat posed to Tasmania and the southeastern mainland of Australia

by a subduction zone to the south of New Zealand.

• another DART® in the Coral Sea is believed to be planned to provide

warning to northeastern Australia.

While the first of these has little bearing on the U.S. array, the second

duplicates much of the role intended for the DART® designated 9-1 in the

figure. This issue is discussed below and leads to a suggestion that this

instrument might be more effectively employed off South America.

Discussion

As in previous reports the task of DART® site selection is to find locations

that satisfy instrument constraints:

• depths in the range 1500–6000 m to seafloor-to-surface acoustic communication,
152 Spillane et al.

Figure H1: Finalizing the initial deployment of the U.S. DART® Array.

• relatively smooth and level local bathymetry both for the bottom unit

and for the anchoring of the surface unit,

• absence of strong topographic features between the DART® and the

tsunami sources it is intended to monitor, to avoid scattering,

• lie within international or U.S. EEZ waters to facilitate deployment

and service cruise planning without lengthy advance notification requirements,

• avoid areas with seafloor infrastructure, strong currents, intense fishing

activity, or a history of vandalism.

In addition a suitable location should be:

• far enough from likely sources to reduce the contamination of the

tsunami signal with seismic noise,

• close enough to provide adequate warning time to threatened shores,

• in a location that, in cooperation with other array elements, provides comprehensive coverage and perhaps some duplication/backup

for likely tsunami sources.

Constraints in the first set can be addressed through bathymetric, EEZ,

and marine navigation datasets. The second set is investigated using the

NCTR propagation database which, outside the near-shore zone, allows the

prediction of the timing and amplitude of tsunami wave trains from likely

sources.
Tsunameter network design in Pacific and Atlantic 153

Figure H2: SW Pacific DART® array planning.

Southwestern Pacific—DART® 52406

In Fig. H1 four DART®s were shown to be allocated to this region, which is

shown in greater detail in Fig. H2. The three DART®s to the east (51425,

51426, and 54401) are intended to cover the line of potential tsunami sources

stretching from Tonga in the north, via the Kermadec region, and terminating at New Zealand’s North Island. The remaining DART® (52406) of

the group was intended to cover a further source line extending from New

Guinea via the Solomon and New Hebrides Islands to Vanuatu. Figure H2

illustrates these sources, as represented in NCTR’s propagation database,

the island and mainland Warning Points considered by the TWC’s, existing

DART®s (red) and those subject to planning (green). Among the latter is

the Coral Sea DART® 55012 (originally designated 52407), to be deployed

by Australia in the vicinity of 14◦20

S, 161◦E.

A fact not recognized earlier in the planning was the extent to which

the island chains between New Guinea and Vanuatu attenuate the passage

of tsunami energy northward into the Pacific. In Fig. H3 the maximum

tsunami amplitude, over the set of unit sources shown, is contoured. The
154 Spillane et al.

Figure H3: Maximum amplitude from any unit source (Mw = 7.5) of the set

shown.

source line lies south and west of the islands and only through a few passages can significant energy penetrate into the Pacific. As a consequence,

a Coral Sea DART® is far better at detecting waves emanating from the

New Guinea to Vanuatu source subset. For a limited number of these sources

52406 would receive the first wave earlier than at 55012 but, except for major

events, the amplitude may not be easily detectable. There are severe limits

on the location of 52406 due to the merging EEZ territories (see Fig. H4)

of the island nations of the region. However, even if this constraint were

removed, it is not likely that a single DART® north of the islands could do

more than provide a poor backup to a Coral Sea DART®. However, should

it be decided to deploy 52406, the location indicated does lie in international waters in proximity to a TAO buoy (see below), and has reasonable

bathymetry.

Note that neither 52406 nor 55012 is well positioned to monitor the

ends of the source set shown in Fig. H3. However, while the New Guinea

end poses mainly a local threat, those near Vanuatu could, in the case of

a substantial event, direct energy toward Hawaii. In the discussion below,

there is the possibility that DART® 51425 might be able to detect such

energy with sufficient lead time to be of value to Hawaii.
Tsunameter network design in Pacific and Atlantic 155

Southwestern Pacific—DART®s 51425, 51426, and 54401

Of the eastern DART®s in the SW Pacific group, only 51425 is substantially

constrained by EEZ considerations. In Fig. H4, the EEZ limits of American

Samoa and other U.S. interests are shown in red; those of other nations are

shown in blue. International waters are un-hatched and are preferred in

order to facilitate deployment and service cruises. Buoys of the TAO array

are drawn and there are logistical efficiencies in placing a DART® about

120 miles from a TAO buoy, thus allowing overnight steaming between sites.

For DART®s 51426 and 54401 the transit to Tahiti, which lies to the east

and is a likely staging area for cruises, is a consideration.

The 30- and 60-min minimum travel time envelopes are also drawn in

Fig. H4. A location close to the 30-min envelope is generally a good choice

where the DART® array is dense. In other circumstances a location closer to

the 60-min envelope allows a DART® to cover a greater length of subduction

zone. The initial choices for the three sites, indicated in Fig. H4, were based

on such considerations, as well as bathymetry and tsunami beam geometry.

Unlike 51425, likely sites for 51426 and 54401 lie in international waters

permitting greater flexibility.

The original site suggested for 51425/9-2 in the area of international

water north of American Samoa (Fig. H4) arose at a time when a closer

location for 9-1/52406 in the sliver of international water closer to the dateline was envisioned. With 9-1 much further to the west, or dropped entirely,

a site such as 9.5◦S, 176◦15

W deserves reconsideration. As seen in Fig. H5,

such a site is in fact preferable. To the north of American Samoa a DART®

would be in the lee of those islands and subject to greater attenuation and

scattering than at 9.5◦S, 176◦15

W. In addition, the latter site is in the

path of waves transiting from Vanuatu toward Hawaii. Vanuatu sources are

those for which a Coral Sea DART® performs poorest and, while the amplitude and timing of waves from smaller events may not help much with

early warning cancellation, the lead time of over 4.5 hours to Hawaii possible

with detection at 9.5◦S, 176◦15

W is for larger events a valuable bonus. The

bathymetry at this recommended location 51425 site is acceptable.

Returning to the sites east of the Tonga Trench, we require a pair of

locations that monitor those sources whose beam patterns are directed primarily in the eastward direction. In Fig. H4, we see that between 22◦S and

40◦S and east of 174◦W, the choice is only constrained by bathymetry and

timing. By locating 51426 at 22.95◦S, 168.1◦W and 54401 at 33◦S, 173◦W

then together with 51425 all of the sources in Fig. H5, with the exception

of those adjacent to New Zealand’s North Island, should be detected within

65 min. Waves generated at the southernmost sources have travel times to

54401 that are as much as 2 hours but, given their proximity to land, initial estimates of their threat will likely come from shore locations such as

Gisborne or Napier.
156 Spillane et al.

Figure H4: Some factors to consider in Southwest Pacific

DART® siting.

Assessing the Array

Assuming that three U.S. DART®s are deployed in the Southwest Pacific,

as described above, and that the Coral Sea DART® becomes a reality, it

is instructive to view the overall effectiveness of the array to date. This

is done in Fig. H6, where at each source location a color-coded symbol is

drawn representing the earliest time at which it would be detected if all

of the DART®s shown are operational. The undeployed South American

DART® and the Equatorial DART® are excluded from the calculation since

they, and potentially the fourth southwest Pacific DART®, are candidates

for relocation.
Tsunameter network design in Pacific and Atlantic 157

Figure H5: Moving 51425 toward the West.

The majority of sources, with the exception of those off Colombia-Ecuador

and southern Chile, are detected within 2 hours. Excluding the South American deficiencies, where detection times (first wave arrival at the closest

DART®) can be as high as 6 hours, the source regions with detection times

of 2 hours or more lie off New Zealand’s South Island, southern Japan, Vanuatu, and New Guinea’s Bird’s Head. For New Zealand and Japan these

near-shore sources are likely to be detected by shore stations. The Vanuatu

sources should be seen at 51425 while close to 5 hours remain before impact

at Hawaii. Perhaps at greater risk, from late-detected Vanuatu sources, is

the North Cape of New Zealand, to which sea floor structures may channel

and focus energy. It would, however, likely take a dedicated tsunameter in

the northern Tasman Sea to cover this threat.

Absent from the propagation database at this time is a comprehensive set

of sources in the East Indies. There is a source line, not shown here, west of

the Philippines and energy from the most northerly of these could leak into
158 Spillane et al. Figure H6: Detection times for the Pacific array with three DART®s unallocated.
Tsunameter network design in Pacific and Atlantic 159

Figure H7: South American DART®s.

the Pacific south of Taiwan. A similar situation may exist for the northern

Molucca Sea, though this region has not been modeled. There are, however,

ongoing discussions with the Indonesians, who wish to place a DART®-ETD

there. If these plans came to fruition, it is possible that earlier detection of

Bird’s Head and East Philippines events might be a by-product.
160 Spillane et al.

Figure H8: Siting considerations for a DART® off southern

Chile.

Expanding the South American Array

Clearly South America, with its tsunamigenic history, is a region most deserving of extra instrumentation. Figure H7 illustrates the situation, with

the existing DART®s, the previously proposed addition 32412, and two

other sites (6.6◦S, 93.9◦W and 35◦S, 80◦W) that have arisen in earlier discussion. The EEZ boundaries are indicated together with South American

events since 1900 for which tsunamis were reported.

The southern site falls within the EEZ region surrounding the Juan Fernandez Islands and will need to be nudged either south or north. The site

at 6.6◦S was mooted earlier as a potential place to relocate the Equatorial
Tsunameter network design in Pacific and Atlantic 161

DART®. Considerations of vandalism, and logistical efficiencies associated

with the TAO line at 95◦W, lead to a suggestion that, in light of the Peruvian event of August 2007, it is too far west. The recommended site for

32412 will also be re-examined in the discussion that follows.

Although the historic events indicated in Fig. H7 are concentrated mainly

to the north of 35◦S, the propagation database considers sources as far as

55◦S.

Figure H8 was generated in an attempt to quantify the joint arrival time

and amplitudes of the first wave peak from the 92 sources shown in red. A

source is said to be “detected” by either 32401 or a location in the color-coded

region southwest of Juan Fernandez if its amplitude exceeds 0.5 cm. Color

contours indicate the number of sources detected as a function of DART®

placement. The contour lines represent the worst-case detection time (in

minutes). The results seem to indicate that a location near 42◦S, 85◦W

might be a reasonable choice, but the seafloor in the vicinity is remarkably

rugged, with multiple fracture zones. A compromise might be to locate

the DART® somewhat to the north, perhaps near 38◦S, 80◦W, though this

would leave several sources to the south inadequately detected.

Conclusion

In the Southwest Pacific the three sites north and east of Tonga are recommended. A site is also shown for the final DART® initially allocated to

this region. However, in light of the discussion above based on the superior

performance of a Coral Sea site in achieving the purpose of 9-1/52406, it is

recommended that this fourth DART® be reallocated to South America.

Off South America there seems to be a consensus that the “Equatorial DART®,” 51406, be moved once the southwestern DART®s are in

place. If this is the only extra DART® available it should be placed off

Central/Southern Chile near 38◦S, 80◦W. If an extra DART® from the

southwest Pacific becomes available, a DART® near 3.5◦S, 85◦W would

provide improved coverage of the Columbia-Ecuador-Northern Peru region.

Note added during the preparation of this report:

The issue of relocation of DART®s 42406 and 51406 is discussed further

in Appendix I. DART® was in fact deployed, and the present plan is to

relocate the Equatorial DART® to a site south of Juan Fernandez Island

where it can improve coverage of Chilean sources.

Tsunameter network design in Pacific and Atlantic 163

Appendix I. Addendum To “The U.S.

DART® Array: Recommendations for the

Remaining Stages”

M.C. Spillane and V.V. Titov

NOAA Center for Tsunami Research

(Originally circulated on 9 November 2007)

Discussion

During the DART-TWC Teleconference of 7 November 2007 the siting of the

SW Pacific DART®s was discussed. While the desirability of reallocating

one of these instruments to the eastern Pacific was agreed, the existence

of a milestone to have the full array deployed in the current fiscal year

and the upcoming deployment cruise to the south west Pacific in January

2007 pushed the decision toward the retention of the DART® previously

designated 9-1. The group agreed with the siting recommendations for the

remaining three instruments in the region in the Draft Report (see Appendix

H).

In light of this decision a tentative site in the vicinity of 5◦S, 166◦E was

agreed, with NCTR tasked to optimize a location in that vicinity. A factor

influencing the siting decision is the deployment by the Australians of a

tsunameter in the Coral Sea. Diana Greenslade, visiting NCTR from the

Australian Bureau of Meteorology, confirmed that this will occur but at a

slightly less optimal location than that initially selected. The new location,

selected to lie within the Australian EEZ, was incorporated in the analysis

that follows. It has essentially the same properties as the former site.

The salient features of the recommendation for the 9-1 site are summarized in Fig. I1. Color-coded is the bathymetry in the depth range (1500–

6000 m) suited to the DART® system. The overlaid cross-hatching delineates the Exclusive Economic Zones of other nations; un-hatched regions are

those best suited logistically to deployment and servicing. Unit sources from

the NCTR propagation database are shown as red rectangles and the first

wave amplitude and timing from these is a prime factor in site selection. The

inset in the upper left contrasts the amplitude (red, in centimeters) expected

for a DART® deployed at the optimum U.S. DART® site, north of the island chain, and the Australian Coral Sea site (blue). The (revised) Coral

Sea site does far better at detecting sources near Vanuatu (C) and provides

lower but quite uniform response to sources to the north and west (A). The

U.S. DART® site, though less uniform in its response, does receive a better

signal for sources in the north and west of the New Guinea-Vanuatu line that
164 Spillane et al.

Figure I1: Revised site recommendations for the Southwest Pacific DART®

Group.

direct substantial energy into the Pacific. To that extent the U.S. DART®

does complement the Coral Sea site, though for the overall capability of a 32-

element Pacific array, an eastern Pacific site for the final instrument would

be more effective. Note that near A and C, and at B (near Guadalcanal),

neither DART® site is very effective.

The recommended site is indicated in the upper right inset panel. While

the originally suggested site at 5◦S, 166◦E has only a slightly less favorable

response, the recommended site at 5.34◦S, 165.08◦E is on the source side of

the topographic feature shown. The feature does not rise close enough to

the surface to cause major scattering in light of Hal Mofjeld’s study of this

effect (Mofjeld et al., 2001). A 25-nm circle is drawn around the site and,

within that, the response is generally similar; it is hoped that a sufficiently

flat bottom can be found there. The recommended site is at about 30 min

travel time from the closest source and close to the TAO line at 165◦E.

Though the range of available sites to the south is constrained by the EEZ,
Tsunameter network design in Pacific and Atlantic 165

Table I1: Revised site recommendations for the Southwest Pacific DART® Group.

DART® Designation Latitude Longitude Approximate Depth (m)*

9-1 5.34◦S 165.08◦E 1904

9-2 9.50◦S 176.25◦W 4843

9-3 22.96◦S 168.10◦W 5637

9-4 33.00◦S 173.00◦W 5677

*From Smith and Sandwell (1997)

consideration of seismic noise would suggest that sites in that direction would

be less favorable in any case.

Recommendation

As a result of this analysis the siting recommendation for the Southwest

Pacific has been revised as detailed in Table I1 and Fig. I1.

Reference

Mofjeld, H.O., V.V Titov, F.I. Gonz ́alez, and J.C. Newman (2001): Tsunami scattering provinces in the Pacific Ocean. Geophys. Res. Lett., 28(2), 335–337.

Smith, W.H.F., and D.T. Sandwell (1997): Global seafloor topography from satellite

altimetry and ship depth soundings. Science, 277, 1957–1962.