UNITED STATES

DEPARTMENT OF COMMERCE

Carlos M. Gutierrez

Secretary

NATIONAL OCEANIC AND

ATMOSPHERIC ADMINISTRATION

William J. Brennan

Acting Undersecretary for Oceans

and Atmosphere/Administrator

Office of Oceanic and 

Atmospheric Research

Richard W. Spinrad

Assistant Administrator

NOAA Technical Memorandum OAR PMEL-143

TSUNAMETER NETWORK DESIGN FOR THE U.S. DART®

ARRAYS IN THE PACIFIC AND ATLANTIC OCEANS

Michael C. Spillane1,2

Edison Gica1,2

Vasily V. Titov1,2

Harold O. Mofjeld2,3

1

Joint Institute for the Study of the Atmosphere and Ocean (JISAO)

University of Washington, Seattle, WA

2

Pacific Marine Environmental Laboratory

Seattle, WA

3

(retired)

Pacific Marine Environmental Laboratory

Seattle, WA

December 2008
NOTICE from NOAA

Mention of a commercial company or product does not constitute an endorsement by

NOAA/OAR. Use of information from this publication concerning proprietary products

or the tests of such products for publicity or advertising purposes is not authorized. Any

opinions, findings, and conclusions or recommendations expressed in this material are those

of the authors and do not necessarily reflect the views of the National Oceanic and Atmospheric Administration.

Contribution No. 3192 from NOAA/Pacific Marine Environmental Laboratory

Also available from the National Technical Information Service (NTIS)

(http://www.ntis.gov)

ii
Contents

List of Figures iv

List of Tables vii

Abstract 1

1 Introduction 1

2 Tsunami Detection and the DART® Technology 5

2.1 Tsunami Detection in Deep Water . . . . . . . . . . . . . . . 5

2.2 Noise Reduction . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.3 Bathymetric Considerations . . . . . . . . . . . . . . . . . . . 8

2.4 Territorial Issues . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.5 A Note on DART® Site Identification Numbers . . . . . . . . 9

3 From Initial Concept to the Final Design 11

3.1 Initial Array Concept . . . . . . . . . . . . . . . . . . . . . . 11

3.2 The DART® Siting Workshop . . . . . . . . . . . . . . . . . 11

3.3 The Completed Array . . . . . . . . . . . . . . . . . . . . . . 13

3.4 A Note on Optimization . . . . . . . . . . . . . . . . . . . . . 14

4 The Propagation Database: A Tool in the Array Design 17

4.1 Unit Sources and the SIFT System . . . . . . . . . . . . . . . 17

4.2 Using the Propagation Database in Array Design . . . . . . . 20

4.3 Site Selection Using Travel Time Only . . . . . . . . . . . . . 21

5 Strong Current Regimes and Mooring Feasibility 23

5.1 Availability of Current Data . . . . . . . . . . . . . . . . . . . 23

5.2 Landslide Avoidance in Site Selection . . . . . . . . . . . . . 24

6 Site Parameters for the U.S. DART® Array 25

6.1 The Western Pacific . . . . . . . . . . . . . . . . . . . . . . . 27

6.2 The Northwest Pacific . . . . . . . . . . . . . . . . . . . . . . 35

6.3 The Aleutians and Alaska . . . . . . . . . . . . . . . . . . . . 41

6.4 The U.S. West Coast . . . . . . . . . . . . . . . . . . . . . . . 49

6.5 Central and South America . . . . . . . . . . . . . . . . . . . 56

6.6 The Southwest Pacific . . . . . . . . . . . . . . . . . . . . . . 63

6.7 The Hawaiian and Equatorial Sites . . . . . . . . . . . . . . . 71

6.8 The Atlantic, Caribbean, and Gulf of Mexico Sites . . . . . . 75

6.9 DART® Sites in the Indian Ocean . . . . . . . . . . . . . . . 85

7 Array Summary 89

7.1 The Final Array . . . . . . . . . . . . . . . . . . . . . . . . . 89

7.2 Array Detection Capability . . . . . . . . . . . . . . . . . . . 91

7.3 Data Return Statistics . . . . . . . . . . . . . . . . . . . . . . 98

7.4 Commercial and Fishing Vessel Considerations . . . . . . . . 98

iii
8 Conclusions 103

8.1 Major Considerations in DART® Siting . . . . . . . . . . . . 103

8.2 Future Directions . . . . . . . . . . . . . . . . . . . . . . . . . 104

9 Acknowledgments 105

10 References 107

Appendix A The DART® Siting Workshop 109

Appendix B West Pacific DART® Location Assessment 111

Appendix C Atlantic/Caribbean/Gulf of Mexico DART® Location Assessment 115

Appendix D Revised Siting Recommendation for the New

England DART® (44402) 119

Appendix E Aleutian/Alaska/West Coast DART® Location

Assessment 125

Appendix F Central/South America DART® Location Assessment 135

Appendix G Northwest Pacific DART® Location Assessment 145

Appendix H The U.S. DART® Array: Recommendations

for the Remaining Stages of the Initial Deployment 151

Appendix I Addendum To “The U.S. DART® Array: Recommendations for the Remaining Stages” 163

List of Figures

1 Schematic of a DART® site. . . . . . . . . . . . . . . . . . . . . 3

2 The mild Kuril Island tsunami of 13 January 2007 as seen by

DART® 46408. . . . . . . ...................... 7

3 Initial concept for the expanded U.S. DART® network . . . . . . 12

4 Refined concept of the DART® network resulting from the Siting

Workshop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5 The completed U.S. DART® array in March 2008 . . . . . . . . 14

6 The maximum amplitude pattern for the 15 November 2006 Kuril

Island tsunami. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

7 Siting of DART® 21413 in the Western Pacific . . . . . . . . . . 29

8 Siting of DART® 52401 in the Western Pacific . . . . . . . . . . 30

9 Siting of DART® 52402 in the Western Pacific . . . . . . . . . . 31

10 Siting of DART® 52403 in the Western Pacific . . . . . . . . . . 32

11 Siting of DART® 52404 in the Western Pacific . . . . . . . . . . 33

iv
Contents

v

12 Siting of DART

® 52405 in the Western Pacific . . . . . . . . . . 34

13 Siting of DART

® 21414 in the Northwest Pacific . . . . . . . . 36

14 Siting of DART

® 21415 in the Northwest Pacific . . . . . . . . 37

15 Siting of DART

® 21416 in the Northwest Pacific . . . . . . . . 38

16 Siting of DART

® 21417 in the Northwest Pacific . . . . . . . . 39

17 Siting of DART

® 21418 in the Northwest Pacific . . . . . . . . 40

18 Siting of DART

® 46413 in the Aleutian/Alaska group . . . . . 43

19 Siting of DART

® 46408 in the Aleutian/Alaska group . . . . . 44

20 Siting of DART

® 46402 in the Aleutian/Alaska group . . . . . 45

21 Siting of DART

® 46403 in the Aleutian/Alaska group . . . . . 46

22 Siting of DART

® 46409 in the Aleutian/Alaska group . . . . . 47

23 Siting of DART

® 46410 in the Aleutian/Alaska group . . . . . 48

24 Siting of DART

® 46419 in the U.S. West Coast group . . . . . 51

25 Siting of DART

® 46404 in the U.S. West Coast group . . . . . 52

26 Siting of DART

® 46407 in the U.S. West Coast group . . . . . 53

27 Siting of DART

® 46411 in the U.S. West Coast group . . . . . 54

28 Siting of DART

® 46412 in the U.S. West Coast group . . . . . 55

29 Siting of DART

® 43412 in the Central and South American group 58

30 Siting of DART

® 43413 in the Central and South American group 59

31 Siting of DART

® 32411 in the Central and South American group 60

32 Siting of DART

® 32412 in the Central and South American group 61

33 Siting of the Chilean Tsunameter 32401 . . . . . . . . . . . . . . 62

34 Siting of DART

® 52406 in the Southwest Pacific group . . . . . 65

35 Siting of DART

® 51425 in the Southwest Pacific group . . . . . 66

36 Siting of DART

® 51426 in the Southwest Pacific group . . . . . 67

37 Siting of DART

® 54401 in the Southwest Pacific group . . . . . 68

38 The Australian Coral Sea DART

® 55012 . . . . . . . . . . . . . 69

39 The Australian Tasman Sea DART

® 55015 . . . . . . . . . . . 70

40 The Equatorial DART

® 51406. . . . . . . . . . . . . . . . . . . 73

41 The Hawaiian DART

® 51407. . . . . . . . . . . . . . . . . . . . 74

42 Siting of DART

® 41420 in the Atlantic group . . . . . . . . . . 77

43 Siting of DART

® 41421 in the Atlantic group . . . . . . . . . . 78

44 Siting of DART

® 41424 in the Atlantic group . . . . . . . . . . 79

45 Siting of DART

® 42407 in the Caribbean Sea . . . . . . . . . . 80

46 Siting of DART

® 42408 in the Gulf of Mexico . . . . . . . . . . 82

47 Siting of DART

® 44402 in the Atlantic group . . . . . . . . . . 83

48 Siting of DART

® 44401 in the Atlantic group . . . . . . . . . . 84

49 The Thailand DART

® 23401 in the Indian Ocean . . . . . . . . 86

50 The Indonesia DART

® 53401 in the Indian Ocean . . . . . . . 87

51 Definition of first wave arrival time and amplitude . . . . . . . . 92

52 Detectability and detection time for seismic sources in the North

Pacific. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

53 Detection capability in the Western Pacific. . . . . . . . . . . . . 95

54 Detection capability in the Southwest Pacific. . . . . . . . . . . . 96

55 Detection capability in the Eastern Pacific. . . . . . . . . . . . . 96

56 Detection capability for Atlantic and Caribbean seismic sources. 97

57a DART

® data return statistics. . . . . . . . . . . . . . . . . . . . 99

57b DART

® data return statistics (cont.). . . . . . . . . . . . . . . . 99
vi Contents

57c DART® data return statistics (cont.). . . . . . . . . . . . . . . . 100

58 Global commercial vessel activity . . . . . . . . . . . . . . . . . . 100

59a Sea-lanes and fishing activity near DART® Site 21417. . . . . . 101

59b Sea-lanes and fishing activity near DART® Site 21417 (cont.). . 102

B1 Recommended DART® locations in the western Pacific Ocean . 113

B2 Recommended western Pacific DART® locations and territorial

waters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

C1 DART® site selection in the Atlantic Ocean, Caribbean Sea, and

Gulf of Mexico . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

C2 Detection times as a factor in Caribbean site selection . . . . . . 117

D1 Maximum and median surface current speeds from the Navy

Layered-Ocean Model . . . . . . . . . . . . . . . . . . . . . . . . 121

D2 99th percentile of NLOM surface speed distribution . . . . . . . 122

D3 Bathymetry in the vicinity of the revised site recommendation

for DART® 44402 . . . . . . . . . . . . . . . . . . . . . . . . . . 123

E1 The current design for the DART® array in the Aleutian/Alaska/West

Coast region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

E2 Smith-Sandwell bathymetry in the vicinity of the recommended

location for P26/5-3 . . . . . . . . . . . . . . . . . . . . . . . . . 129

E3 Current and recommended locations for P8/46405 . . . . . . . . 130

E4 Repositioning of DART® P16/46401 . . . . . . . . . . . . . . . . 131

E5 Siting recommendation for DART® P17/4-a . . . . . . . . . . . 132

E6 Siting recommendation for P1/4-b . . . . . . . . . . . . . . . . . 132

E7 Siting recommendation for P10/1-1 . . . . . . . . . . . . . . . . . 133

E8 Siting recommendation for P15/1-2 . . . . . . . . . . . . . . . . . 133

E9 Potential future adjustment of the P14/46402 DART® . . . . . . 134

F0 Recommended locations for the four Group-6 DART®s and the

relocated 51406. . . . . . . . . . . . . . . . . . . . . . . . . . . . 137

F2 Relative impact offshore at Hilo for East Pacific Sources. . . . . . 138

F3 Offshore amplitude and arrival time . . . . . . . . . . . . . . . . 139

F4 Recommended location for DART® 6-1. . . . . . . . . . . . . . . 140

F5 Recommended location for DART® 6-3. . . . . . . . . . . . . . . 141

F6 Recommended location for DART® 6-2. . . . . . . . . . . . . . . 142

F7 Recommended relocation of DART® 51406. . . . . . . . . . . . . 143

F8 Suggested location for DART® B-4. . . . . . . . . . . . . . . . . 144

G1 Recommended sites for the Northwest Pacific . . . . . . . . . . . 146

G2 Topography and minimum travel-time envelopes near the recommended 21415 site. . . . . . . . . . . . . . . . . . . . . . . . . . . 148

G3 Recommended site for DART® 21416 . . . . . . . . . . . . . . . 148

G4 Recommended site for DART® 21417. . . . . . . . . . . . . . . . 149

G5 Recommended site for DART® 21418 . . . . . . . . . . . . . . . 149

H1 Finalizing the initial deployment of the U.S. DART® Array. . . 152

H2 SW Pacific DART® array planning. . . . . . . . . . . . . . . . . 153

H3 Maximum amplitude from any unit source (Mw = 7.5) of the set

shown. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154

H4 Some factors to consider in Southwest Pacific DART® siting. . . 156

H5 Moving 51425 toward the West. . . . . . . . . . . . . . . . . . . . 157
Contents vii

H6 Detection times for the Pacific array with three DART®s unallocated. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158

H7 South American DART®s. . . . . . . . . . . . . . . . . . . . . . 159

H8 Siting considerations for a DART® off southern Chile. . . . . . . 160

I1 Revised site recommendations for the Southwest Pacific DART®

Group. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164

List of Tables

1 Sub-region allocations and priorities within the overall U.S. array. 15

2 Location and performance of the Western Pacific DART® Group 28

3 Location and performance of the Northwest Pacific DART® Group 35

4 Location and performance of the Aleutian and Alaska DART®

Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

5 Location and performance of the U.S. West Coast DART® Group 50

6 Location and performance of the Central/South American DART®

Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

7 Location and performance of the Southwest Pacific DART® Group 63

8 Location and performance of the Equatorial and Hawaiian DART®s 72

9 Location and performance of the Atlantic DART® Array . . . . 75

10 Location and performance of the Indian Ocean DART® instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

11 Array layout and data availability as of 30 October 2008 . . . . . 90

12 Source detection contributions for elements of the U.S. DART®

Array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

B1 Original and recommended DART® positions in the West Pacific

Ocean. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

C1 Original and recommended DART® positions in the Atlantic/

Caribbean/Gulf of Mexico Region . . . . . . . . . . . . . . . . . 116

E1 Original and recommended DART® positions in the Aleutian/

Alaska/West Coast region . . . . . . . . . . . . . . . . . . . . . . 128

F1 Locations recommended for Group-6 DART® deployment and

51406 relocation. . . . . . . . . . . . . . . . . . . . . . . . . . . . 137

G1 Characteristics and comments for the four recommended northwest Pacific DART® sites. . . . . . . . . . . . . . . . . . . . . . 147

I1 Revised site recommendations for the Southwest Pacific DART®

Group. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165

Tsunameter network design for the U.S. DART® arrays in the

Pacific and Atlantic oceans

M.C. Spillane1,2, E. Gica1,2, V.V. Titov1,2, and H.O. Mofjeld2,3

Abstract. In March 2008 a significant milestone in the quest to provide adequate warning of propagating tsunami waves in the Pacific and Atlantic Oceans was accomplished with the completion of

the U.S. array of 39 tsunameters (www.noaanews.noaa.gov/stories2008/20080310_buoy.html).

In conjunction with five additional instruments, deployed in cooperation with other nations (including two sites in the Indian Ocean), this network is monitored continuously by the U.S. Tsunami

Warning Centers in Hawaii and Alaska and the data stream provided freely in real-time to the international community. While the occurrence of a significant submarine earthquake can be detected

rapidly to provide a basis for an early warning, not all such events generate significant waves. Unambiguous quantitative observation of the passage of tsunami waves in the deep ocean can permit

either early warning cancelation or, in conjunction with numerical models, forecasts of the hours-long

trains of potentially damaging waves at threatened sites. This report documents the considerations

and methodology used in site selection.

1. Introduction

Following the Indian Ocean tsunami disaster of December 2004, which demonstrated the catastrophic and widespread potential for death and destruction

inherent in tsunami waves, an early decision was made to greatly expand

the then-existing capability for warning U.S. coastal communities. Tsunami

Warning Centers (TWCs) were established in Hawaii and Alaska in the wake

of destructive tsunamis, caused by seismic activity in the Aleutian Islands

(1946), Chile (1960), and Alaska (1964). By monitoring submarine earthquakes the staff at these centers have been able to quickly establish a potential for tsunami generation, and promulgate warnings to the regions most

threatened.

However, in addition to there being non-seismic mechanisms for tsunami

generation, there is not as yet the ability to relate tsunami amplitude to

earthquake magnitude and associated parameters, at least within a time

frame that might provide timely warnings. Perhaps due to the triggering of

submarine landslides, as appears to have occurred in the case of the PapuaNew Guinea event of 1998 (Synolakis et al., 2002), an earthquake of lesser

intensity can be highly dangerous. Conversely, a sizeable earthquake may

generate only minor tsunami waves. The significance of the former eventuality is self-evident, but the costs associated with “false alarms” are not

negligible. An unwarranted evacuation can cost millions of dollars and,

while the costs associated with the loss in public confidence are less easy

to quantify, the effectiveness of a warning system is ultimately grounded in

credibility.

1Joint Institute for the Study of the Atmosphere and Ocean (JISAO), University of

Washington, Seattle, WA, 98195 2NOAA, Pacific Marine Environmental Laboratory, Seattle, WA, 98115

3(retired)
2 Spillane et al.

A proven technology for the unambiguous detection and assessment of

tsunami waves as they propagate had been available since the 1980s, though

it has been significantly improved since then. A bottom pressure recorder

(BPR), installed on the seafloor and communicating to shore in real time

via a surface buoy and satellite link (Fig. 1), can register the amplitude and

timing of a tsunami wave train. Combined with a pre-computed database of

propagation models, the time series from one or more such DART® (Deepocean Assessment and Reporting of Tsunami) instruments can be “inverted”

to produce basin-wide estimates of the tsunami’s threat and the input needed

to drive site-specific forecast models of inundation for those communities

most at risk. While the signature of the Indian Ocean tsunami was discernable in hindsight from satellite altimetry (Smith et al., 2005), and schemes

relying on GPS receivers on surface buoys have been proposed, their capabilities in actual real-time emergency situations have yet to be demonstrated.

[GPS-derived ground motion estimates have the potential to improve early

estimates of seismic moment (Blewitt et al., 2006), and the quality of the

seismic forecast.] Another technology that has been successfully proven in

the field (www.gloria-symposium.com/pdf/22-7Kaneda.pdf) has been deployed off Japan and incorporates BPR units and seafloor seismometers,

with communication to shore via submarine cable. The latter, with a capacity for high-frequency data transfer, is well suited to the locally generated tsunamis that pose the greatest hazard to Japan. Cabled systems in

the Northeast Pacific too are under development, or operating in research

mode, with the potential to provide valuable input for local tsunami detection. NSF’s Ocean Observatories Initiative and the NEPTUNE Canada

project envisage (www.ooi.washington.edu/) regional scale nodes extending offshore with a variety of sensors providing real time data via the internet.

Bottom pressure sensors off Monterey Bay (www.mbari.org/mars/), and in

the Strait of Georgia east of Vancouver Island (www.neptunecanada.ca),

currently deliver such data and the promise of things to come. Such systems

are, however, expensive and DART® systems provide a more cost-effective,

readily available and scalable solution given the global extent of potential

tsunami sources. The “Ring of Fire” spans thousands of kilometers along

the peripheries of the Pacific and Indian Oceans and other extended source

regions occur in the Atlantic, Caribbean, and Mediterranean.

The utility of the early, limited array of DART® buoys off Alaska and

the U.S. west coast, and the forecast methodology for generating a usable

forecast was demonstrated during an actual tsunami event (Titov et al.,

2005). A full-featured system with the acronym SIFT (Short-term Inundation Forecasting for Tsunamis) for processing seismic and DART® data, and

generating a suite of forecast products, is in the process of being installed at

the Pacific (PTWC) and West Coast/Alaska (WCATWC) Tsunami Warning Centers. SIFT has been described elsewhere (Gica et al., 2008) and has

performed well during several mild tsunamis since 2004 (Wei et al., 2008a).

Installation and servicing of elements of the U.S. DART® array (and other

sites instrumented in cooperation with Australia, Chile, Indonesia, and Thailand) by the National Data Buoy Center (NDBC) has been proceeding apace.

The initial deployment of the full U.S. array of 39 DART® systems was com-
Tsunameter network design in Pacific and Atlantic 3

DART II System DART II System

Optional

met sensors

Tsunameter

1000-6000 m

Optional

sensor mast

Lifting 

handle Electronic systems

and batteries

Iridium &

GPS antennas

Bi-directional ectional communication communication & contr & control

Tsunami 

Warning 

Center

O

Signal flag

Glass ball 

flotation

13 mm 

polyester

Acoustic release

CPU

Sensor

Anchor 325 kg

1.8 m

19 mm nylon

22 mm nylon

25 mm nylon

Swivel

25 mm chain (3.5 m) 

13 mm chain (5 m) 

Anchor 3100 kg

Acoustic transducers

(2 each)

Batteries

Acoustic 

transducer

Surface Buoy 2.5 m diameter

4000 kg displacement

Iridium 

satellite

NATIONAL OCEA NIC AND ATMOSPHERIC AD MINISTRAT OI N

U.S. DEPARTM ENT OF COMMERCE

NOAA

2.0 m

~ 75 m

Figure 1: Schematic of a DART® site.
4 Spillane et al.

pleted in early 2008. The rationale and methodology for the siting decisions

is the subject of this Technical Memorandum. Based on Site Recommendation Reports produced at the NOAA Center for Tsunami Research (NCTR)

in consultation with the TWC’s, and with input from the NDBC and the

U.S. Geological Service (USGS), and other interested parties, the goal is

to provide information of use in refining and extending the U.S. array, and

to aid efforts by the international community to extend the coverage. As

the 2004 event demonstrated, a tsunami may wreak destruction over many

nations, and can only be effectively combated through education and cooperative detection efforts.
Tsunameter network design in Pacific and Atlantic 5

2. Tsunami Detection and the DART®

Technology

Before proceeding to array considerations it is well to summarize in brief

those characteristics of tsunami waves and DART® instrumentation that

influence site selection. While they may be generated by several phenomena, the most frequent source of tsunami waves is seismic activity beneath

the sea floor (Gonz ́alez, 1999). A submarine earthquake, particularly one

associated with a subduction fault, can displace upward and/or downward

a large area of seafloor (Wei et al., 2008b). The resulting displacement of

the overlying water column then propagates outward from the source region in all directions as a wave train of peaks and troughs. The waves have

a long wavelength, with the distance between successive peaks often being

several hundred kilometers in the deep ocean. As a consequence, though

it seems counterintuitive, tsunamis behave as “shallow-water” waves since

their wavelengths significantly exceed even the greatest ocean depths.

2.1 Tsunami Detection in Deep Water

Shallow-water waves travel with a speed (c) that depends on water depth

through the relationship

c = gH (1)

where H is the water depth and g the gravitational acceleration (∼9.8 m/s).

In the deep ocean, where the mean depth is ∼3,800 m, a tsunami may

travel at a speed comparable to an airliner! As the waves spread out their

amplitude decreases from an initial value, typically of the order of a meter,

to a few centimeters or less. However, as the wave approaches land and the

water depth decreases, the wave train slows, becomes compressed along the

direction of travel, and grows in amplitude again. Seafloor features can act to

focus the tsunami in particular locales, resulting in a mass onshore movement

of water, better described as a “bore” rather than a wave. Sometimes the

leading crest may be preceded by a noticeable drawdown of water level,

which, to a knowledgeable person on the beach, should be a signal to seek

high ground. There may be several waves in succession over a period of

minutes to hours; the leading wave may not be the greatest and indeed,

due to reflection by seafloor features away from the direct path, the most

dangerous waves may arrive many hours after the first. Extended monitoring

of the DART® network in the hours following the generation of a significant

tsunami can provide valuable guidance to the emergency response.

Tsunami wave properties, such as those described above, have several

implications for the instruments that might detect them. Because of their

greater speed in the deep ocean, the waves can arrive earlier at detectors
6 Spillane et al.

placed offshore, than at a site at a lesser distance from the source but in

shallower water. Measurements at coastal sea level gages, or the direct

observation of tsunami waves by persons in impacted areas, may provide

further indication of the presence of a destructive tsunami. However, due

to topographic focusing, such observations are generally more complex and

less suited to analysis and basin-wide prediction than a “clean” time series

in deep water.

The reduced amplitude of tsunami waves in deep water may seem to mitigate against detection, particularly as their amplitudes of a few centimeters

are much less than the several-meter heights of ambient wind waves. This

would be true for a surface-based detector, but a BPR in deep water senses

the pressure signal associated with changes in the water column height and

essentially filters out high-frequency wind wave fluctuations or swell. A typical DART® signal (see Fig. 2) appears, at first glance, as a smooth tidal

variation with an episode of high-frequency activity of much lesser amplitude

highlighted in red. The data in units of pressure (PSIA) have been converted

to units of water depth in meters using a conversion factor (670 mm/PSIA).

Once the tides are removed, by suitable prediction and filtering algorithms,

the high-frequency variability can be examined in detail.

In the early hours of 13 January 2007 an earthquake of moment magnitude (Mw) 8.1 occurred beneath the seafloor in the vicinity of the Kuril

Islands. Seismic waves from the event epicenter, radiating outward at 2–

3 km/s, triggered DART® 46408 (among others) from its regular 15-min

reporting mode into event mode. One-minute data stored aboard the instrument were transmitted for a period preceding the trigger and the transmissions of 1-min samples continued for several hours. The seismic noise

signal, aliased since its frequency is much higher than the 0.0333 Hz Nyquist

frequency of the event mode reports, died away to a level not much greater

than the ambient noise level before the event, allowing the true tsunami signal to stand out clearly though its peak-to-trough amplitude is only 4 cm.

DART® 46408 lies 2,650 km east of the epicenter. Like the November 2006

Kuril event that preceded it, the tsunami waves caused some damage in

Crescent City, CA (6,220 km away) where waves of 37 cm peak-to-trough

were reported. Such mild events are providing useful unscheduled tests of

the DART® and SIFT systems in addition to assisting in TWC operations

(Geist et al., 2007; soundwaves.usgs.gov/2007/04/research2.html).

2.2 Noise Reduction

While communication with a seafloor BPR via a seafloor cable would be

the “gold standard” in terms of the sampling rate with which the signal

could be monitored, the distances of tens to hundreds of kilometers to suitable shore stations make the costs of such installations prohibitive. With a

limited number of BPRs to employ (the current U.S. array is based on 39

instruments) individual instruments need to be placed well offshore in order

to monitor a significant stretch of potential source and derive the benefits
Tsunameter network design in Pacific and Atlantic 7

Figure 2: The mild Kuril Island tsunami of 13 January 2007 as seen by DART® 46408.

of deep-water detection. The earth’s subduction zones, which are the major

source of tsunamigenic earthquakes, extend for thousands of kilometers.

Offshore placement has other benefits for DART® operations. In addition to generating a tsunami, a submarine earthquake is the source for

a variety of seismic waves that propagate through the surface layers and

body of the earth. Indeed, these waves, which travel through the solid earth

considerably faster than the tsunami’s speed through water, are detected

at land-based seismometers around the globe providing an early indication

of the epicenter and magnitude of the event. Even though no changes to

water level are caused by these fast-moving seismic waves, their effect is

high-frequency “seismic” noise in the BPR (see Fig. 2). Frequently it is this

seismic signal that trips the BPR from its stand-by reporting interval of

15 min to the 1-min interval needed to “see” a tsunami. The amplitude of

this noise signal may exceed the true water column signal associated with

the tsunami and, to avoid contamination of the latter, the BPR should be

sufficiently removed from the tsunami source region so that the fast-moving

seismic noise signal will have died away before the desired tsunami signal

arrives. There is clearly a trade-off here between the need for avoidance of

seismic noise contamination (and suitable spacing of BPRs) and the desire

to provide an early assessment of tsunami amplitude to the SIFT forecast
8 Spillane et al.

system upon whose predictions early warning cancelations or inundation

forecasts depend.

2.3 Bathymetric Considerations

Next to consider in DART® siting is water depth and the character of

the bathymetry, both local and in the direction of a tsunami source. As

illustrated in Fig. 1, the BPR signal, measured at the seafloor, is sent to

an adjacent surface buoy via an acoustic link. From there, the signal is

transmitted to shore by a satellite link. In early versions of the DART®

this communication was one-way, with the BPR operating autonomously

when a “trigger” condition in the BPR signal was detected. Current versions

of the DART® permit two-way communication, allowing an instrument to

be controlled by an operator on shore. This may be desirable during a

tsunami event to initiate or extend the pre-programmed interval of 1-min

sampling, or to circumvent the premature seismic triggering of a remote

DART® that would drain battery power without returning relevant data.

(In addition, two-way communication facilitates testing, system resets, and

software update.) The acoustic link requires separations of between 1,500

and 6,000 m between the seafloor and surface units. This, and the need

for a reasonably flat seabed on which the BPR will sit, are constraints to

DART® site selection. Relatively uniform depth is also needed in order to

pre-cut the cables needed to anchor the surface unit with a suitable watch

circle. While sites that are generally suitable can be selected in advance

using bathymetry datasets (such as that of Smith and Sandwell, 1997), the

precise positioning is usually chosen during deployment, based on an on-site

shipboard survey.

Another bathymetric consideration, not unique to the DART® technology, is the avoidance of tsunami scattering. Scattering can result when the

seafloor terrain between the tsunami source and the detection site is overly

rugged. Bathymetric features, such as seamounts, cause zones of shadowing

or of reinforcement in their lee. To the extent that these effects are imperfectly represented in the tsunami propagation model database on which

the SIFT inversion scheme relies, forecast quality will be adversely affected.

Confirmation of general site suitability is performed in advance by NDBC

personnel, who also provide guidance for the avoidance of seabed infrastructure, sea lanes and excessive fishing pressure, high probability of vandalism,

and issues such as strong currents that might impact mooring design.

2.4 Territorial Issues

While tsunami detection can produce benefits to all, which would ideally

suggest that sites for DART® systems be selected strictly on scientific and

engineering grounds, in reality elements of the U.S. DART® array are placed

either in U.S. or international waters. This choice is primarily for logistical

reasons. Access to a foreign EEZ (Exclusive Economic Zone) generally requires several months of advance notice. With deployment cycles of two or
Tsunameter network design in Pacific and Atlantic 9

more years this might not be a problem, but unplanned visits at irregular

intervals may be required due to instrument failure or to avail of ship-ofopportunity cruises. Generally this has not been a major inconvenience

since the need to avoid the immediate vicinity of earthquake source regions

usually dictates sites outside the 200 nm limit. However in some locations,

such as the Caribbean and the southwest Pacific, most of the seabed falls

within some nation’s EEZ. A database of EEZ boundaries is available online

from the Flanders Marine Institute (VLIZ, 2005).

There is one exception to the siting of the U.S. DART® array in international or U.S. waters. The site of 46419 off Vancouver Island is within the

Canadian EEZ, reflecting the joint U.S.-Canada interest in tsunami warnings

for the Pacific Northwest coast.

2.5 A Note on DART® Site Identification Numbers

DART® sites and other marine buoy locations are assigned 5-digit World

Meteorological Organization (WMO) identification numbers. The first two

digits are based on WMO association areas and sub-areas (see www.wmo.

int/pages/prog/amp/mmop/Images/wmo_area_sub_area.gif for a chart of

these areas). In the case of U.S. DART® sites the third digit is consistently

“4,” providing a convenient means of distinguishing them from buoys with

other purposes. The final two digits are generally sequentially assigned.

A consequence of this naming procedure is that sites with adjacent WMO

numbers may be widely separated or vice versa, as in the case of 46413 (midAleutians) and 46412 (off Los Angeles) whose closest neighbor to the south

is 43412!

Tsunameter network design in Pacific and Atlantic 11

3. From Initial Concept to the Final

Design

So far, tsunami sources in relation to suitable DART® sites have only been

discussed in general terms, based on considerations of signals and noise,

engineering issues, and a loose statement that an array, with a finite number

of elements, cannot be very closely spaced. To proceed further one needs to

consider the sources of tsunami risk and the assets, both of human life and

property, in need of protection.

3.1 Initial Array Concept

In the early days following the Indian Ocean tsunami of 2004, when there

were seven U.S. and one Chilean DART® systems in place, it was estimated

that to protect U.S. interests in the Pacific and Atlantic Oceans the U.S.

array or network should be increased to 39 sites. This number, based on

experience with the existing instruments off the Aleutians, the West Coast,

and Chile and, no doubt, some “back of the envelope” estimates, was in

retrospect quite a reasonable starting point. Figure 3 illustrates the initial

concept for the network with 32 U.S. sites in the Pacific (P1–P32) and 7

in the Atlantic (A1–A7). Five of the occupied locations (01–05) were to be

adjusted; two others (06 in the Equatorial Pacific and 07 off Hawaii, whose

main role is to provide warning of a local source) were to remain unchanged.

3.2 The DART® Siting Workshop

The first step in refining the conceptual array was to convene a meeting

of experts and interested parties to identify the major threats, and impact sites at greatest risk, in order to make a general allocation of the

resources. This was done in July 2005 at a joint NOAA-USGS DART®

Siting Workshop held at NOAA’s Pacific Marine Environmental Laboratory

in Seattle, Washington. A brief report of the discussion is available online (Geist et al., 2005; soundwaves.usgs.gov/2005/10/meetings.html).

Some salient points from the discussion are repeated here and in Appendix

A.

Attendees identified the primary source types, which, in addition to the

historically dominant submarine earthquake mode, include volcanic activity

(both submarine and sub-aerial), landslides (again both submarine and subaerial with the latter including potential massive slope failure in the Azores

as well as the notable event in Lituya Bay, Alaska), and extraterrestrial

impact. In light of the historical record in the Pacific, earthquake sources

would appear to pose the dominant threat. The paucity of long-term records

outside of Japan pointed to the difficulty in assigning recurrence probabili-
12 Spillane et al.

Figure 3: Initial concept for the expanded U.S. DART® network. Sites marked in red

were in place at the time of the Indian Ocean disaster; included is the Chilean tsunameter

not counted in the U.S. allocation of 39 instruments (32 Pacific, 7 Atlantic).

ties with any reliability. Though the likelihood of an event might be greater

where there is a “seismic gap,” and there may be grounds to make spatial

estimates of the greatest event magnitude likely, the consensus seemed to

be that it would be unwise to discount any portion of the circum-Pacific

stretches of subduction zone as a potential source. Likewise, though the

Hawaiian Islands are perhaps most exposed to tele-tsunamis from widespread

sources and clearly warrant extra consideration, the West Coast has areas

under high threat (associated with the Cascadia subduction zone) or with

severe risk exposure both of population and infrastructure in the case of California. There are, in addition, widespread centers of U.S. interest spread

throughout the Pacific and Atlantic basins. Based on the consensus of opinion at the workshop (which assumed that additional Chilean DART®s to the

south of the existing one were likely), the approximate allocation indicated

in Fig. 4 was decided.

The placement of DART® buoys in the Atlantic region was considerably

revised as a result of the Siting Workshop discussion. With the exception

of the Caribbean, where tsunamis are not infrequent, the source distribution is less well defined. In addition to the subduction zone that arcs from

Trinidad to Hispaniola on the Atlantic side of the island chain, there are

the incompletely documented sources that caused earthquake and tsunami

activity near Lisbon in 1755. In addition to the volcano/landslide threat

from the Azores there was a landslide-generated event off Newfoundland in

1929 and the potential exists for slope failure along the U.S. mid-Atlantic

coast (Driscoll et al., 2000; Chaytor et al., 2007). In the Gulf of Mexico there
Tsunameter network design in Pacific and Atlantic 13

Figure 4: Refined concept of the DART® network resulting from the Siting Workshop.

Sub-regions were identified (outlined in green) and deployment priorities assigned to each.

The Aleutian-Alaska sub-region had highest priority (1), the Southwest Pacific lowest (9).

Allocations presumed expansion of the Chilean network to the south of their existing

tsunameter (labeled 24 in the figure).

is some threat of seismically generated tsunamis from subduction zones off

Panama and Venezuela, though waves would be greatly attenuated by the

Yucatan Strait (or the Florida Strait in the case of seismic activity to the

east). There are, however, potential landslide sources (McAdoo et al., 2000).

The south coast of Puerto Rico and U.S Virgin Islands are at some risk due to

the Muertos Trough subduction zone immediately to its south (see Fig. 45),

and from other seismic sources off South America and along the island chain

to the east as well as the submarine volcano Kick’em Jenny and other island

volcanoes. Bearing all these factors in mind, the consensus of the workshop

attendees was to allocate one DART® each to the Caribbean and the Gulf

of Mexico, two north of Puerto Rico, two inshore along the East Coast and

the final DART® well to the east to provide a long lead time for ill-defined

threats in the eastern Atlantic.

3.3 The Completed Array

Figure 4 and Table 1 summarize the recommendations of the workshop in

terms of asset allocation and the priorities for deployment. Sub-groups

within the array were established which, to a large extent, were followed in

the site recommendation reports that are included as appendices. The status

of the array, and both data and metadata, are available online (www.ndbc.

noaa.gov/dart.shtml); the situation in March 2008, following the completion of the array, is shown in Fig. 5. This image, adapted from the NDBC
14 Spillane et al.

Figure 5: The completed U.S. DART® array in March 2008 (shown in red) and other

tsunameter stations that report via NDBC (adapted from the locator image at: www.ndbc.

noaa.gov/dart.shtml).

website to include the 5-digit WMO buoy identifiers, can be compared with

the initial array concept (Fig. 3) and the revised design (Fig. 4) that emerged

from the Workshop. Figure 5 includes non-U.S. sites that employ DART®

instruments and report freely through NDBC. NCTR staff members have

cooperated in international siting decisions with Chile and Australia for the

Pacific, and Thailand and Indonesia in the case of the Indian Ocean. USAID

contributed to these Indian Ocean sites and it clearly benefits both the U.S.

and the global community to collaborate in the positioning of tsunameters

and in sharing the data they provide. India and Germany have deployed

tsunameters in the Indian Ocean and plans from other nations under the

IOTHWS umbrella will be monitored to ensure cooperative efficiencies. [In

October 2008 two new DART® sites were established by Australia, in the

North Australian Basin, south of the Java Trench.]

3.4 A Note on Optimization

Following the DART® Siting Workshop the utility of optimization was explored. To the extent that the constraints on siting can be quantified,

and the benefits expressed in functional form, array design might be approached as a problem in optimization. This avenue was explored with

the assistance of John Dennis (of Rice University) and his colleagues, using their tool NOMAD (www.gerad.ca/nomad/). Routines were added at

NCTR to communicate to NOMAD (implemented in Matlab) via Ferret

(ferret.pmel.noaa.gov/Ferret/, a PMEL-designed analysis tool) data
Tsunameter network design in Pacific and Atlantic 15

Table 1: Sub-region allocations and priorities within the overall U.S. array.

Array Sub-Group Instruments Assigned Pre-existing Sites Priority

Alaska/Aleutians 6 3 1

Western Pacific 6 0 2

Puerto Rico/Caribbean 3 0 3

West Coast 5 2 4

Southwest Pacific 4 0 5

Central/South America 4 0 6

Atlantic 3 0 7

Gulf of Mexico 1 0 8

Northwest Pacific 5 0 9

Hawaii/Mid-Pacific 2 2 N/A

extracted from the propagation database (see Section 4) and the SmithSandwell bathymetry. Constraints on water depth and bottom roughness

were easily incorporated as a mask; NOMAD would then position a selected

number of DART® sites within a sub-region of the ocean so as to optimize

the detection time for a set of unit sources.

The scheme was tested for a relatively simple case: siting six DART® off

the Aleutians and Alaska. Here the linear source arrangement, and the lack

of EEZ constraints, resulted in a solution that conformed to expectation.

Another case explored was that of the western Pacific, where the source distribution is considerably more complex. The array suggested by NOMAD

had some similarity to that finally adopted, but required considerable adjustments to accommodate the limitations imposed by territorial issues. Other

issues, such as the representation of scattering and the expression of the

optimization function, could not be addressed within the time constraints of

the site selection process. Some of the difficulty lay no doubt in the fact that

the NOMAD expertise resided offsite while the voluminous datasets and Ferret expertise were local to NCTR. Thus, though in principle NOMAD and

the optimization approach may yet be employed in fine-tuning the existing

array, the siting recommendations were based on calculations performed at

NCTR, implemented using the analysis and graphic capabilities of Ferret

alone.

Tsunameter network design in Pacific and Atlantic 17

4. The Propagation Database: A Tool in

the Array Design

The refraction due to the depth-dependence of tsunami wave speed, together

with scattering and reflection from bathymetric features, can result in complex patterns for the spread of tsunami waves even in the deep ocean. In

consequence, numerical modeling is required to make basin-wide predictions.

This is illustrated in Fig. 6, where the pattern of maximum amplitude (occurring during a 24-hour model hindcast) is color contoured for the relatively

weak Kuril Island tsunami of 15 November 2006. The pattern shows lobes or

fingers of enhanced energy pointed in various directions. Although there was

little overall damage associated with this event, some communities such as

Crescent City, California experienced unusually strong waves and currents.

4.1 Unit Sources and the SIFT System

The numerical models that can produce charts such as that shown in Fig. 6,

require detailed bathymetry and can be quite demanding of computer resources. The overall task is generally partitioned into two components:

Figure 6: The maximum amplitude pattern for the 15 November 2006 Kuril Island tsunami.
18 Spillane et al.

open ocean propagation and coastal inundation. While the latter process

is strongly nonlinear, and in consequence requires real-time, event-specific

computer runs, the propagation of tsunami waves in deep water can be

treated linearly and stored as a database. The seismic source regions are

partitioned into blocks, or “unit sources” (see the inset panel in Fig. 6) each

representing an earthquake of moment magnitude 7.5 in a 100 × 50 km

rectangle whose long side is aligned with the subduction zone axis. This

“strike” direction and the associated dip and source depth are chosen based

on seismological knowledge with the assistance of the USGS, resulting in a

tiled layout covering the source regions. The “rake angle” of each unit source

is set to 90◦, since normal or reverse faulting generates the greatest vertical

seafloor deformation. The distribution of seafloor deformation, as computed

using the algorithms of Okada (1985), provides the initial condition for sea

surface elevation.

The propagation and evolution of the initial elevation can be computed

using the MOST (Method Of Splitting Tsunamis) hydrodynamic code (Titov

and Gonz ́alez, 1997). Usually 24 hours of propagation is modeled and the

results for velocity and surface elevation stored at 1-min intervals. For situations where 24 hours is not adequate for wave propagation to extend to all

potential impact points, or where major reflections can cause later waves to

be of concern, the model run is extended appropriately. For these propagation runs a coastal “wall” is placed, typically at the 20-m isobath, to exclude

the non-linear processes that can occur in shallow water. When the results

have been quality-controlled, to ensure that no numerical instabilities have

occurred, they are loaded to the propagation database. This database now

contains in excess of 1,450 unit sources covering all likely seismic sources in

the Pacific, Atlantic, and Indian Oceans.

Because of their underlying linearity, the unit source propagation solutions can be scaled up or down to represent event magnitudes other than

Mw = 7.5, or combined with those of neighboring unit sources to form composite sources of greater spatial extent than the 100 × 50 km appropriate to

each. In operational use, within the SIFT system, the propagation database

is employed as follows:

1. Detection of an earthquake with potential tsunami risk prompts the

TWCs to issue a preliminary warning. The epicenter coordinates and

magnitude estimate identify which unit sources are likely involved.

2. The composite solution for these unit sources, with appropriate scaling,

generates a chart (similar to that shown in Fig. 6), timing information,

and other products such as a coastal estimate. These quickly identify

the communities most threatened should a significant tsunami in fact

have been generated.

3. While historical information concerning the source region may indicate the likelihood that a damaging tsunami has been generated, the

expense of evacuation and the loss of public confidence that can result

from a false alarm requires that unambiguous evidence of the tsunami

be obtained to refine the initial warning. This validation is provided
Tsunameter network design in Pacific and Atlantic 19

by the DART® array, whose BPR sensors measure and transmit via

satellite the height of the water column above them. A DART® may

be triggered into event-reporting mode (one sample per minute; the

standby mode has a 15-min interval) by the seismic signal or, based on

the predicted arrival time at the DART® predicted by the propagation

database, may be triggered by an operator at a TWC.

4. The time series reported by one or more DART® buoys are ingested

by the SIFT system and used to establish which linear combination of

unit sources best represents the observations. Now, an actual forecast

of the size and distribution of tsunami energy is possible and perhaps

a warning cancelation or revised warning can be issued.

5. In the event that significant impacts are forecast for one or more communities, the propagation database performs its second function. As

noted earlier, the propagation model results exclude the coastal zone

itself and do not have the spatial resolution to represent, for example,

a harbor and its infrastructure. However, the composite propagation

solution in deep water is adequate to produce the boundary conditions

for a set of nested model runs of increasing spatial resolution that culminate in inundation predictions of use to local emergency managers.

The SIFT system launches this phase of detailed model runs under the

direction of TWC operators. Referred to as SIMs (Standby Inundation

Models), these are optimized to run quickly in real time so that the

detailed forecasts are available while the tsunami is still far from land.

Some graphical examples of nested SIM models and the finely resolved

topographical data needed to forecast inundation are given by (Tang

et al., 2008).

6. If a severe tsunami has occurred, extended monitoring over many hours

of the DART® array can lead to refined predictions, detection of reflected waves, damage estimates in the face of possible communications

difficulties, and other information of value to emergency responders

until tsunami energy in the basin has dissipated.

Further detail on the SIFT system and its development can be obtained

elsewhere (Gica et al., 2008). However, the issues raised, and the availability

of the propagation database, do have a direct bearing on the design of the

DART® array as discussed below.

The goal of the array is to provide timely observations, on which an accurate forecast can be based, while adequate time remains for emergency

procedures to be initiated. TWC guidelines indicate that a 3-hour lead-time

is desirable. This requirement is most appropriate in the case of remote

sources and may not be practicable in situations where an impact site lies

in close proximity to a potential source. For example, tsunami waves from a

rupture of the Cascadia Subduction Zone would impact the coasts of Washington, Oregon, and California in less than an hour. It is however to be

noted that, even where a forecast cannot be provided with sufficient or indeed any lead time, it is not without value. An accurate though delayed
20 Spillane et al.

“forecast” may be late in terms of the first waves, but may establish the

likelihood of later waves and the extent of the damage: information of value

to emergency responders.

4.2 Using the Propagation Database in Array Design

With regard to array design, the availability of the propagation database

allows the prediction of what a DART® would “see” under various source

scenarios. The timing of the expected signal allows an estimate of whether

the seismic noise is likely to have dissipated or whether an adequate warning time can be produced by a DART® in a given location. While the

number of actually observed seismic and tsunami wave trains is limited,

the typical speeds with which each propagates suggests that to reduce seismic noise contamination a DART® not be placed closer than 30 min of

tsunami travel time from the closest source. The locus of points meeting

this criterion, referred to as the 30-min envelope, can be computed from the

propagation database and is frequently quoted in the site recommendation

reports provided in Appendices B–I. Sites are generally recommended at or

outside this line. In the case of Aleutian and Alaskan sources, the placement of a DART® at or outside the 30-min envelope would not provide

a 3-hour warning for the Hawaiian Islands. A number of sites in this region have been chosen closer to the 15-min envelope. While the quality of

the signal may be expected to be somewhat degraded by seismic noise at

the closest DART®, the observations should be adequate to establish the

presence/absence of a significant tsunami and an initial forecast of its impact. The alongshore spacing of the array off the Aleutians and Alaska has

been reduced to permit improved new forecasts to be generated as less contaminated peripheral DART® time series are added to the SIFT inversion

process. Mofjeld (personal communication) has noted that, due to similar

travel times from adjacent unit sources, data obtained directly offshore may

be ill-suited to discriminate between potential source scenarios.

Simulated time series at potential DART® sites can be used to pick those

that should provide the largest and least complex tsunami signals. The lobelike patterns seen in a tsunami “beam plot” such as Fig. 6 identifies locations

that are in the lee of some bathymetric feature such as a seamount and

therefore less suited as a DART® site. Such shadow regions where excessive

scattering might be expected are to be avoided. The full suite of beam plots

available from the propagation database were not examined due to the time

constraints imposed by the need to deploy the array as soon as was practical.

Instead, an analytic study by Mofjeld et al. (2001) was often consulted to

identify regions to be avoided. The study showed that scattering becomes

significant when a feature extends to within 1,200 m of the surface. Thus, in

selecting DART® sites to provide warning to threatened coasts by a given

source region, locations were chosen that had as “clear” a view of the sources

as possible. The choices made may be subject to revision in light of further

analysis and the accumulation of experience from deployed instruments.
Tsunameter network design in Pacific and Atlantic 21

4.3 Site Selection Using Travel Time Only

Since they provide information on both the amplitude and timing of tsunami

waves emanating from a source region, numerical results, such as those produced by MOST and available in a propagation database, are highly valuable

in array design. Such a comprehensive database may however not be available when decisions regarding tsunameter placement are being made. Since

the wave speed for shallow-water wave speed is related to depth, various

computer programs exist to derive travel time contours for waves produced

by a point source. With a set of such solutions, originating from a set of

points spaced along a plate boundary, or within a region subject to submarine landslide, minimum travel time envelopes, detection times, and estimates of impact times can be derived for a study region. This approach

was taken in site selection for DART® sites 23401 and 53401 in the Indian Ocean. The sites were selected to provide the best possible warning

times to surrounding nations by the placement of two tsunameters. Clearly,

given the extent of the source region, two instruments are only the starting

point for a more extensive array that can be designed with the aid of the

subsequently available Indian Ocean propagation database. A similar approach, also employing the TTT (Tsunami Travel Time) program by Paul

Wessel/Geoware (www.geoware-online.com/tsunami.html) was taken for

the U.S. East Coast and Gulf of Mexico where some of the threat is from

landslides, as noted in Section 5.2 below, and illustrated in Appendix C.

Tsunameter network design in Pacific and Atlantic 23

5. Strong Current Regimes and Mooring

Feasibility

One aspect in which the initially recommended sites could have been better

thought out is that of mooring feasibility. While the BPR on the seabed is

not impacted directly by ambient currents, the surface unit that serves as

a link between the BPR and shore via acoustic and satellite transmission

must be capable of surviving in the marine environment. In choosing the

locations for the Gulf of Mexico and off New England, representations of

the Loop Current and Gulf Stream were employed that failed to adequately

represent the position and strength of these currents. A likely result of this

failure was a loss of data from these sites as the surface unit dragged beyond

the range of acoustic contact or even broke free. The early loss of DART®

21417 off the Kuril Islands may have a similar origin though the movement

of the surface unit suggests contact with a vessel.

5.1 Availability of Current Data

A comprehensive siting study should include the accumulation of in situ current data and involve the expertise of a mooring designer. Again, due to time

constraints and the sparseness of such data in the global ocean, this was not

possible and, in proposing alternate sites, numerical model results have been

employed. Available online (www7320.nrlssc.navy.mil/global_nlom32/

skill.html) are hindcast currents for the global ocean from the operational Navy Layered-Ocean Model (NLOM). This model, which assimilates

altimetry data, is described by Shriver et al. (2007) and provides daily current estimates on a 1

32nd degree grid. Some 32 months of model results were

acquired and analyzed to extract relevant statistics for the Loop Current

and Gulf Stream regions to produce revised site recommendations. Parameters derived include the spatial distribution of overall maximum and median

speeds in the siting areas, and the likelihood of recurrence of excessive speed

episodes. The results, presented in Appendix D for the New England case,

employ a 1% exceedance criterion. Based on the available modeled surface

current speed data, the colored contours of Fig. D3 represent the speed that

is expected to be exceeded on only 1% of deployment days. Since a DART®

deployment cycle may be 2 years or more, decisions based on this criterion

may be somewhat optimistic and the overall maximum from the full set of

model days more appropriate.

A more thorough study of the vertical structure of model currents could

provide relevant parameters for future mooring design and siting recommendations. With the time constraints of the initial array deployment, and the

production of this technical memorandum, a limited surface current study

has been performed at each of the chosen DART® sites. At each, 600 days
24 Spillane et al.

of surface speed hindcasts were obtained in a 5◦ × 3◦ region adjacent to

the site. Statistical results in the form of a 5% exceedance contour in the

vicinity of the recommended (and occupied) sites and the estimated NLOM

surface speed history and histogram are included in the graphics presented

in the next section.

5.2 Landslide Avoidance in Site Selection

Some other factors relevant to site selection are also incompletely assessed.

For example, the potential for submarine landslides is relevant, both in terms

of possible tsunami generation and potential destruction of a BPR. The

speed and extent of the 1929 landslide off Newfoundland were estimated

from the times at which a series of seafloor telegraphic cables were ruptured.

Such observations are rare; typically the sites of ancient submarine slides are

inferred from seafloor surveys. Preliminary results from a survey of submarine features associated with offshore sediment transfer, for the eastern U.S.,

were provided by the USGS (Geist, personal communication) and employed

to determine locales to be avoided during the selection of a site for DART®

41424.

Of more direct concern of course is the potential for tsunami generation by mass movements like submarine landslides. The implications of the

survey mentioned above, spanning the continental slope and rise between

Georges Bank and the Blake Spur, were assessed by Chaytor et al. (2007)

in regard to tsunami hazard. Identifying past landslides requires detailed

seafloor mapping, and dating such events is difficult. Numerical modeling of

landslide-generated tsunamis, more complex than seismic generation, is an

active area of current research. The rarity of such events limits the observational base, but laboratory models can provide measurements with which to

test numerical predictions. In the coming years, it is hoped, a propagation

database of landslide-generated tsunami waves will be available to complement the seismic source database and, in particular, serve to guide future

siting studies. As an interim step in East Coast site selection, an analysis of

travel times from potential landslide sources was conducted (see Appendix

C). The travel time calculation program TTT (available from Paul Wessel/Geoware, www.geoware-online.com/products.html) was employed to

generate propagation times for point “sources” evenly spaced along the shelf

break. As noted earlier, though it does not provide amplitude estimates,

TTT may also play a useful role in studies where a propagation database

of seismic sources is not available; such was the case in the early days of

selecting DART® sites for the Indian Ocean.