EARTHQUAKE SUMMARY MAPU.S. DEPARTMENT OF THE INTERIORU.S. GEOLOGICAL SURVEY

S O L O M O N I S L A N D S

PAPUA NEW GUINEA

San CristobalGuadalcanal

Santa Isabel

New Ireland

New Britain

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BougainvilleP.N.G.

S o l o m o n S e a

PA C I F I C O C E A N

C o r a l S e a

New Br it a in Trench

San Cris tob a l Trench

Choiseul

NEW HEBRIDES TRENCH

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NEW GUINEA

C A R O L I N E I S L A N D S

NORTHISLAND

CAROLINEPLATE NORTH

BISMARCKPLATE

SOUTHBISMARCK

PLATE

WOODLARKPLATE

SOLOMON SEAPLATE

NEWHEBRIDES

PLATE

BIRD'S HEADPLATE

MAOKEPLATE

KERMADECPLATE

BALMORALREEFPLATE

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FUTUNA PLATE

NIUAFO'OUPLATE

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P A C I F I CP L A T E

35-45 mm/yr

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AUSTRALIAN PLATE

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Brisbane

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M7.6, 7.8, and 7.3 Vanuatu Region Earthquakes of 7 October 2009

0 200 400100Kilometers

SOLOMON ISLANDSPAPUA

NEW GUINEAINDONESIA

A U S T R A L I A

San CristobalGuadalcanal

Santa Isabel

New IrelandNew Britain

Malaita

BougainvilleP.N.G.

New Caledonia

VANUATU

Choiseul

NEW ZEALAND

Brisbane

SydneyAdelaide

AucklandMelbourne

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Prepared in cooperation with the Global Seismographic Network

Tectonic Setting

Seismic Hazard

Epicentral Region

Torres Islands Earthquake 07 October 2009 23:13:49 UTC 13.145° S., 166.297° E.Depth 33.3 kmMw = 7.3 (USGS)

DISCUSSIONThe Vanuatu Region earthquakes of October 7, 2009: M7.6, Torres Islands, 22:03:15; M7.8 SantaCruz Islands, 22:18:26 UTC; and M7.3, Torres Islands, 23:13:49 occurred on or near the plateboundary between the Australia and Pacific plates. In the region of the earthquake, the Australiaplate moves to the east-northeast with respect to the Pacific plate at a velocity of about 91mm/year. The Australia plate thrusts under the Pacific plate at the New Hebrides trench and dipsto the east-northeast. The October 7 earthquake's location, depth, and focal mechanism areconsistent with the earthquake having occurred as thrust-faulting associated with subductionalong the Australia-Pacific plate boundary.The earthquake of October 7, 2009, 23:13 UTC occurred about 15 km southeast of, and 1 hourafter, a larger earthquake of the same date, 22:03 UTC. A second event of a similar size to the firstoccurred at 22:18 UTC, 60 km to the north of the first event. All three events are likely related;while the first two similarly sized events may be considered a earthquake doublet, preliminaryanalysis suggests the October 7, 2009, 23:13 UTC is likely a large aftershock of the previousevents.The Vanuatu region experiences a very high level of earthquake activity, with over a dozen eventsof magnitude 7 and larger having been recorded since the early decades of the twentieth century.Recent large earthquakes near the October 7 events include a M 7.2 earthquake in 2007 and a M7.3 earthquake in 1999. The subducting Australia plate is seismically active to depths of about350 km beneath the islands.

EXPLANATIONEarthquake Magnitude

5.50 - 5.996.00 - 6.997.00 - 7.99≥ 8.00

Earthquake Depth0 - 6970 - 299≥ 300

Aftershock Magnitude<5.05.0 - 5.96.0 - 6.97.0 - 7.9

Seismic hazard is expressed as peakground acceleration (PGA) on firmrock, in meters/sec², expected to beexceeded in a 50-yr period with aprobability of 10 percent.

EXPLANATIONMag ≥ 7.0

0 - 69 km70 - 299300 - 600

Plate BoundarySubductionTransformDivergentOthersVolcanoes

RELATIVE PLATE MOTIONSThe broad red vector represents the motion of the Australiaplate with respect to the Pacific plate in the epicentral region.

0 400 800 1,200 1,600200Kilometers

Significant Earthquakes Mag >= 7.5

Year Mon Day Time Lat Long Dep Mag1900 07 29 0659 -10.000 165.000 0 7.61901 08 09 1301 -22.000 170.000 0 7.91910 06 16 0630 -19.000 169.500 100 7.91910 11 09 0602 -16.000 166.000 70 7.51913 10 14 0808 -19.500 169.000 230 7.61920 09 20 1439 -19.919 168.530 35 7.81928 03 16 0501 -22.281 170.476 35 7.51931 10 03 1913 -10.932 161.016 35 7.81934 07 18 1940 -11.907 166.731 35 7.81939 01 30 0218 -7.080 155.386 35 7.71939 04 30 0255 -9.295 159.234 35 7.91953 11 04 0349 -13.189 166.516 35 7.51963 09 17 1920 -10.286 165.413 27.6 7.51965 05 20 0040 -14.642 167.504 9.7 7.61970 08 11 1022 -14.095 166.570 39.1 7.51980 07 08 2319 -12.487 166.482 56 7.51980 07 17 1942 -12.504 166.011 31.6 7.81980 10 25 1100 -21.941 170.056 39 7.51981 07 06 0308 -22.251 171.814 30 7.61984 02 07 2133 -9.957 160.522 17 7.61988 08 10 0438 -10.258 160.896 36.1 7.61990 03 03 1216 -21.956 175.171 35.5 7.61995 05 16 2012 -22.968 169.945 23.6 7.71997 04 21 1202 -12.560 166.738 29.8 7.71998 01 04 0611 -22.239 171.017 97.1 7.52007 04 01 2039 -8.466 157.043 24 8.12009 10 07 2203 -13.052 166.187 35 7.62009 10 07 2218 -12.554 166.320 35 7.8

Peak Ground Acceleration in m/sec**2

.2 .4 .8 1.6 2.4 3.2 4.0 4.8

Epicentral Region

Epicentral Region

DATA SOURCES and REFERENCESEARTHQUAKES AND SEISMIC HAZARD USGS, National Earthquake Information Center NOAA, National Geophysical Data Center IASPEI, Centennial Catalog (1900 - 1999) and extensions Engdahl, E.R. and Villaseñor, A., 2002, Global Seismicity: 1900 - 1999, chap. 41 of Lee, W.H.K., and others,eds., International Earthquake and Engineering Seismology, Part A: New York, N.Y., Elsevier Academeic Press, 932 p.EHB catalog (Engdahl et al., 1998)HDF (unpublished earthquake catalog, Engdahl, 2003)Global Seismic Hazard Assessment Program http://www.seismo.ethz.ch/GSHAP/PLATE TECTONICSBird, P., 2003, An updated digital model of plate boundaries: Geochem. Geophys. Geosyst., v. 4, no. 3, pp. 1027- 80.

DISCLAIMER

Base map data, such as place names and political boundaries, are the best available but may not be current or may contain inaccuracies and therefore should not be regraded as having official signifiance.Map prepared by U.S. Geological Survey National Earthquake Information Center8 October 2009http://earthquake.usgs.gov/Map not approved for release by Director USGS

BASE MAP NIMA and ESRI, Digital Chart of the World USGS, EROS Data Center NOAA GEBCO and GLOBE Elevation Models

Finite Fault Modelfor M7.6 Earthquake

FINITE FAULT MODELContributed byGavin Hayes, NEICDistribution of the amplitude anddirection of slip for subfault elements(small rectangles) of the fault rupturemodel are determined from the inversionof teleseismic body waveforms and longperiod surface waves. Arrows indicate theamplitude and direction of slip (of thehanging wall with respect to the footwall); the slip amount is also color-codedas shown. The view of the rupture planeis from above.The strike of the fault rupture plane isN14°W and the dip is 30.74° NE. Thedimensions of the subfault elements are15 km in the strike direction and 9 km inthe dip direction. The seismic momentreleased based on the plane is 4.28E+027dyne.cm.

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Distance (km)

Depth

(km)

Santa Cruz Islands Earthquake 07 October 2009 22:18:26 UTC 12.554° S., 166.320° E.Depth 35 kmMw = 7.8 (USGS)

Torres Islands Earthquake 07 October 2009 22:03:15 UTC 13.052° S., 166.187° E.Depth 35 kmMw = 7.6 (USGS)260 km (160 miles) S of Lata, Santa Cruz Isl., Solomon Isl.295 km (180 miles) NNW of Luganville, Espiritu Santo, Vanuatu785 km (490 miles) ESE of Honiara, Guadalcanal, Solomon Isl.2100 km (1310 miles) NE of Brisbane, Queensland, Australia

Slip (cm)0 - 5050 - 100100 - 150150 - 200200 - 250250 - 300300 - 350350 - 400400 - 450450 - 500500 - 550550 - 600

Cross-section of the New Hebrides subduction zonetaken perpendicular to the average strike of localcentroid moment tensor (CMT) solutions. YellowCMTs are mechanisms from the gCMT catalog plottedat their equivalent EHB catalog location (Engdahl etal., 1998), used to constrain trench strike and dip. Lightgray circles represent background seismicity from theEHB catalog. The trench location is marked with a redsquare. The red dashed line represents the best-fittingnon-planar geometry of the subduction zone. The initiallocations of the M7.6 2009/10/07 22:03 UTCearthquake are shown by black circles and marked witharrows corresponding to the gCMT centroid and NEICepicenter. Blue circles represent the locations ofsubsequent aftershocks.

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Dip 30.74° Northeast

(Cross-section source:http://earthquake.usgs.gov/eqcenter/eqarchives/subduction_zone/us2009mlcf/)

Cross-section defined by profile boundaries shown on the Epicenter Map