Analysis of Earthquake Hazardin Papua New Guinea

Lawrence AntonPort Moresby Geophysical Observatory

OutlineIntroductionTectonicsSeismologyEarthquake hazardDiscussionsConcluding remarksRecommendations

•Top right: aerial view of CBD Port Moresby, National Capital

•Bottom: National Parliament

The aims of the study are to

evaluate (and map) earthquake hazard in PNG at different scales, utilizing data and improved methods now available

focusing on specific sites of population density and sites of important national industrial activity

provide the basis for a much-needed revision of the PNG Earthquake Loading (Building) Code

update results in previous studies

Global tectonic plate configuration & kinematics

Regional tectonics and related elements

PNG region is situatedWithin the collision zone of the India – Australia, Pacific and Eurasian Plates

Left-lateral shearing across New Guinea resulting from highly obliquecollision (Gochioco et al., 2002)

world’s fastest continental shear zone (75-80 mm/yr), relative to northern Australia (McCaffrey, 1996)but accounts for only minimum release of seismic moment in the region

Ontong Java Plateau dominating the Pacific Plate frontthick massive oceanic crustcaused break up of Melanesian Arc and reversal of subduction polarity

Convergence is accommodated by thrusts and strikeslip faulting along frontal Highlands thrust belt; involved in mountain building (crustal shortening/thickening) spanning the central axis of New Guinea Island

the central collisional belt; 300 km wide, 1300 km long & peaks of over 3 km

Existence of minor plates within the collision zone (Wallace et al., 2004; this study)

Many more being recognised/confirmed; eg. North Bismarck, New Guinea Highlands and Woodlark, amongst many others

TECTONICSTwo mountain ranges dominate the region:

Terrains of the Melanesian arc – subjected to oblique collision in early Pliocene; still involved in active subduction in the Solomon IslandsHighlands (Irian Jaya Fold Belt & Papuan Fold Belt) –formed by two tectonic events

Obduction of Papuan Ophiolites during Oligocene resulting in metamorphism of continental margin sedimentsContinental-arc collision during Pliocene causing intra-continental deformation

Tectonic structure of New Guinea (Abers and McCaffrey, 1988; Abers, 1994)

Geological provinces of New Guinea (Davies, 1990)

Geological provinces of SW Pacific region (Audrey-Charles, 1991)

Free-air gravity anomalies of OJP and New Guinea (Mann and Taira, 2004)

GPS data from Tregoning et al. (1998)

Tectonic plates configuration of PNG (Ripper and Letz, 1991; 1993; PMGO)

Seismicity of PNG based on PMGO earthquake catalogue

SEISMOLOGYEarthquake activity is a manifestation of and delineate

Plate boundaries – all possible types Subduction zones, continental and oceanic convergenceSeafloor spreading centres and continental riftingContinental and oceanic transcurrent faulting

Zones of crustal deformation, e.g. due to convergenceCrustal fracture zonesVolcanic centres

Significant earthquakes are frequent, including six having significant tectonic effects; 1907, 1935, 1941, 1993, 2000, 2002, 2007

There may have been numerous others

Seismicity of the PNG region based on ISC earthquake catalogue – showing regional earthquake distribution

Red circles denote shallow events (0-34 km)Yellow diamonds and blue triangles denote intermediate depth events (35-299 km)Green stars denote deep events (>300 km)

Significant earthquakes of PNGGreat Earthquakes (Pacheco and Sykes, 1992; Engdahl et al., 1998 and ISC)Year Date Time Lat Long Depth Ms Moment Mw

hr:min °S °E Km 1020Nm .1906 14 Sep 16:04 7.00 149.00 S 7.4 12.70 8.01935 20 Sep 01:46 3.50 141.75 S 7.9 14.50 8.01971 14 Jul 06:11 5.50 153.90 53 7.8 12.00 8.01971 26 Jul 01:23 4.90 153.20 48 7.7 18.00 8.12000 16 Nov 04:54 3.98 152.17 33 8.2 12.40 8.02007 01 April 20:39 8.466 157.043 24 7.9 8.1

Significant earthquakes with tectonic effects (PMGO)Year Date Place Mw Effect .1907 15 Dec North coast New Guinea 7.3 subsidence1935 20 Sept Torricelli Mtn/N New Guinea 8.0 uplift1941 13 Jan New Britain 7.0 horizontal displacement1970 31 Oct Adelbert Range 7.0 Slumping - tsunami1993 25 Oct Eastern New Guinea 7.1 vertical displacement1998 17 Jul Northern New Guinea 7.1 submarine slumping -

tsunami2000 16 Nov Southern New Ireland 8.0 5.5m horizontal

displacement2002 08 Sept North coast New Guinea 7.8 30-40cm uplift – tsunami2007 01 April New Georgia Group, SI 8.1 1 metre uplift

Coastal uplift during Eq of 1 April 07

Earthquake activityAmongst the most intense in the worldFrequent magnitude 7 and above earthquakes

about 2 per yearHigh stress release;

Low stress accumulationHowever, locations are poor;

poor seismic network coverageEarthquakes smaller than magnitude 4 not locatedLocations have huge errors

Active plate interactionIndicative of rapidly evolving of Plate boundariesInter-slab and intra-slab activityDiverse earthquake wave propagation pathPoor local seismograph coverage

So most earthquakes are located using global seismograph data

Locations and depths include very large errors

Future great eqs are imminent & of concern

Seismic instrumentationProper monitoring started in the 1962 by USGSLocal networks of soft and strong motion began about the same time;

but have currently ceased due to lack of funding commitmentReplacement networks an urgent need;

For public information and early warningtsunami warning including

For hazards mitigation and awareness – risk managementFor proper hazard assessment – for engineering useFor proper identification of source zones as a result of improved location and magnitude determinationsTo improve earthquake catalogs which will benefit local and regional seismicity and tectonic studies; including hazard studies

EARTHQUAKE HAZARDAfter an understanding of the regional seismicity and tectonics having gained, the second stage for an earthquake hazard study is largely concerned with calculation of ground motion recurrence,

representing the hazard due to shaking by seismic waves. other earthquake hazards such as liquefaction, surface rupture, landslides, and tsunami, are treated separately.

Two main models required to compute ground motion recurrence are a seismotectonic model that specifies the assumed distribution of earthquake magnitude recurrence of earthquakes, and a ground motion model that specifies the expected ground motion from the earthquakes.

Available dataCompiled using databases established based on ISC, USGS, and sourced from many workers; of

Hypocentres – this studyIntensities – futureStrong motion – future component of the project

Seismicity maps are now possible; therefore,Identification of seismic source zones made easier

Hazard mapping was attempted in previous yearsProper data and methods not available then

Revise existing seismic hazard maps – building code seismic zone

The current PNG Earthquake (Building) Code does not reflect current knowledge of the tectonic structure of PNGWhen the review is completed, proper legislation is called for to include the revisions.Rapid national growth requires urgent legislation to ensure compliance by town planners and engineers.

The Code was developed in the 1970s using data from abroad, as PNG didn’t have the data then. PNG Statutory Instrument No.44 of 1971 (1971), Regulations made under the “Building Ordinance 1971”, documented by Papua New Guinea Government Printer, November, 1971 (plus Amendments).First revised in 1982 and documented by PNG National Standards Council (1983), Code of practice for general structural design and design loadings for buildings, Part 4, Earthquake loadings.

Revisions were not taken onboard

PNG Earthquake Loading Code

The hazard is represented by uniform probability response spectra computed using the 4-staged Cornell method

1. Develop seismotectonic map2. Quantification of seismicity3. Determine attenuation function4. Earthquake hazard, and hazard mapping

Based on the comprehensive PNG earthquake catalogueThe seismotectonic model was developed, and ground motion recurrence for selected sites computed

After several iterations of the model, earthquake hazard maps are produced

Method

1. Develop seismotectonic mapDivided the region in to seismotectonic zones based on:

Existing earthquake catalogue; the reformat of which is based on ISC, PMGO, USGS databases

Checked against regional geology, and geophysics, especially gravity and magnetics, Quaternary faults, topographic and geographic features

There must be prior knowledge of the tectonicsModels used influence hazard assessments

Tectonics

Collision zone

Seismotectonics – uniform earthquake distribution

Six layers of the model – PNG1

A total of 120 source zones

PNG region is classed as low to moderate resolution; Australia as low

2. Quantification of seismicityFor each source zone (as well as faults – future work)

Based on available data – distribution patterns defining activity in zones, including sources at depth

Determine per zone:Rate of recurrence of earthquakes varying with magnitude (magnitude recurrence)Relative proportion of small to large earthquakes (b-value) Maximum earthquake magnitude (Maximum Credible Earthquake); tectonic settings considered too!

Example of source zone quantification: New Britain Arc

A typical source zone, the New Britain Arc, is presented as anexample of quantification of a source zone

The earthquakes within the zone were extracted from the catalogue (using the MapInfo GIS system)

The catalogue had previously been declustered

independent mainshocks distinguished from dependent foreshocks and aftershocks

the declustered listing was used for earthquake magnitude recurrence estimates

Figure shows magnitude-time plot for earthquakes in the New Britain Arc zone, and the catalogue completeness is estimated (considering seismograph coverage, and linearity of the earthquake magnitude recurrence plot) and represented by the blue line. The plot shows magnitude against time for the known declustered earthquakes, with dependent foreshocks and aftershocks removed.

The earthquake magnitude recurrence plot for the New Britain Arc. This is a plot of Nx, the number of earthquakes per year equal to or larger than magnitude x, against magnitude x. Nx values indicating the number of earthquakes in entire source region per year.An alternative measure of earthquake activity is Ax, which is the number of earthquakes per year per 100 x 100 km. The Nx values depend on source region size so can't be compared (large zones have more earthquakes than small zones), but Ax values can be compared between zones.The gradient of this plot gives the b-value (a measure of the proportion of small to large earthquakes) for the zone, which in this case is a very high value of 1.386.

Table lists earthquake magnitude recurrence for the New Britain Arc zone.These estimate of earthquake magnitude recurrence for the New Britain Arc zone of model PNG1 used the earthquake catalogue to 2008-09-30. The zone covers 65,730 km2. The gradient is represented by beta = 3.19, which corresponds to b-value = 1.39.

Events/year Events/year Ret Period Ret Period

Whole zone /100x100km /100x100km (yr) for zone (years)

No = 7952015.4 Ao = 1209800 0.00 0.000

N1 = 326796 A1 = 49718 0.00 0.000

N2 = 13430 A2 = 2043.21 0.00 0.000

N3 = 551.921 A3 = 83.968 0.01 0.002

N4 = 22.6818 A4 = 3.45075 0.29 0.044

N5 = 0.932132 A5 = 0.141812 7.05 1.073

N6 = 0.0383069 A6 = 0.00582792 171.59 26.105

N7 = 0.00157426 A7 = 0.0002395 4175.30 635.219

This process was repeated for every source zone for the seismotectonic model PNG1. At this stage the model PNG1 has a total of 120 zones in six depth ranges.

3. Determine attenuation functionGives earthquake ground motion as a function of magnitude, distance and other parametersNo one attenuation relationship is possible for the entire region due to:

Complex geology – resulting in varying seismic energy propagation pathNumerous crustal tectonic blocksDiverse tectonic structureTerrains and hilly topography

As local data is not sufficient, relationship of similar tectonic environments was used

Atkinson and Boore (2003) – for subduction zonesChiou and Youngs (2008) – for crustal, intra-plate zones

Attenuation relationships to be developed in next stage of the project – future work

4. Earthquake hazardComputed spectral ground motion recurrence (SGMR), integrating probabilities of motion from all earthquakes in space, magnitude and freq of motion

using commercial software EZ-FRISK (McGuire, 1993)Computed SGMR at specific sites/points depending on complexity of seismotectonic and attenuation models; on bedrock – as strong motion data lackingAfter several iterations of the model, earthquake hazard maps will be produced

Repeating the process at many points on a grid covering the region, for better resolutionSites of Port Moresby, Lae, Kokopo, Kimbe, Buka, Madang, Wewak and Honiara have been attempted

Source zone contributions for Port Moresby ground motion.This plot gives contributions for peak ground acceleration.

1. Port Moresby

Figure shows that the high-frequency peak ground acceleration at Port Moresby is dominated by hazard contributions from shallow or crustal seismic zones using the attenuation of Chiou-Youngs (2008), but higher accelerations may be originating from the intra-slab zones, using the attenuation function of Atkinson-Boore (2003).

PGA recurrence for Port Moresby.This is for bedrock motion, considering magnitudes 5.0 and higher.

The peak ground acceleration is numerically equal to the response spectral acceleration at near zero period, for all the left-most points on each of the plots. The spectra for other return periods can be found by re-calculation, or approximated by interpolation.

Figure shows the uniform probability response spectra at Port Moresby for return periods of 475 years, 3,000 years and 10,000 years, for bedrock, and using magnitudes from 5.0.

Figure shows the deaggregation plot for 1.0 second period motion at a return period of 975 years for Port Moresby (corresponding to a relatively low amplitude of 0.07 g spectral acceleration).

It shows three main source of hazard, including moderate magnitude nearby events, larger events at distances of 200 to 500 km, and some contribution from great earthquakes at distances of 500 km and beyond (cumulatively plotted at 500 km).

Magnitude 6 earthquakes near Port Moresby occur infrequently, while the maximum credible magnitude of 7.3 has been determined, with a near-zero recurrence rate.

Zones contributing most to the hazard are those closest to Kokopo and the most active are the zones of the New Britain Arc and New Britain Trench. These zones consequently contribute highest to the total hazard, or otherwise is spread amongst many other zones.

Figure shows source zone contributions for Kokopoground motion. This plot gives contributions for motion with spectral acceleration at a period of 1.0 seconds.

2. Kokopo

Figure shows the strength of seismic zones of the subduction zone in the total hazard using the attenuation function by Atkinson and Boore (2003), especially in the lower and upper PGA. There was a reasonable contribution from zones non-subducting lithosphere using the attenuation function by Chiou and Youngs (2008), especially at the mid-range PGAs.

Note that for the total motion at the site, the Atkinson-Booreattenuation functions for subduction events are each weighted by 0.5, while the plot shows the ground motion recurrence for full weighting for each.

Figure shows response curves for 475, 3,000 and 10,000 year return periods. Spectra for other return periods can be derived from re-calculation, or approximated by interpolation.

Figure shows the response spectra for Kokopo. These uniform probability response spectra are for bedrock, using magnitudes from 5.0.

Figure shows the deaggregation for all source zones within 500 km, with associated hazard parameters. Two parts are shown to contribute earthquakes, from very local distance (0-25 km) and that from distance up to 150 km. No seismic zones is observed to contribute hazard at Kokopo from farther distances, even in adjoining subduction zones are observed.Maximum magnitude earthquake in Kokopo is 7.6 and maximum acceleration of 0.5g

Figure magnitude-distance deaggregation for motion at Kokopo. Results for bedrock motion of period 1.0 seconds, considering magnitudes 5.0 and higher.

Figure shows source zone contributions for Lae ground motion. This plot gives contributions for peak ground acceleration. It shows that the source contributions in Lae are dominated by the Huon seismotectonic zone within which the city is located, and the neighbouring Schrader and Adelbertzones, as well as Huon Peninsula at depth.

3. Lae

Figure shows that the high-frequency peak ground acceleration at Lae is totally dominated by contributions from nearby local earthquakes, determined using the Atkinson-Boore (2003) attenuation function.

Figure shows PGA recurrence for Lae. Results for bedrock motion and peak ground acceleration, considering magnitudes 5.0 and higher.

The peak ground acceleration is numerically equal to the response spectral acceleration at near zero period, for all the left most points on each of the plots.The uniform probability response spectra for return periods of 475 years, 3000 years and 10,000 years are shown in the Figure. Spectra for other return periods can be found by re-calculation.

Figure shows response spectra for Lae. These uniform probability response spectra are for bedrock, using magnitudes from 5.0.

For Lae, the deaggregation plot of 1.0 second for a return period of 975 years shows that most of the contributing earthquakes are originating locally, but the situation extends gradually and tapers off to a distance of about 210 km.

Maximum acceleration of 0.35g and MCE of 7.1, at a mean distance of 71 km

Outcomes The earthquake source zones are quantified using historical and recent seismicity data, and checked against geology, geophysics and geodesy (GPS deformation) during the current tectonic regimeAttenuation functions that were derived using data from comparable tectonic environments were selected

No check for consistency with the few existing PNG strong motion data, and isoseismal dataA component of the next stage of the study

Ground motion recurrence computations were then performed for selected locations

PGAs for 475 yr Return Period

64

91

83

41

66

60

71

254

Mean dist (km)Sites PGA Max Cred Eq

Port Moresby 0.07 7.3

Lae* 0.35 7.1

Kokopo 0.50 7.6

Madang 0.28 7.4

Wewak 0.24 6.9

Kimbe 0.32 7.6

Buka 0.36 7.7

Honiara 0.31 7.5

CONCLUDING REMARKSPNG a geologically, seismically and geographically complex region

Tectonic structure (debated) redefined and updated continuously

Data and methods for earthquake hazard determination are now available; could have better dataIt was determined that earthquake hazard is significantAs local strong motion data is not sufficient, that of similar tectonic setting could have been used – data required to determine attenuation function There is a need for modern seismic equipment

Replace the almost non-existing local networkMonitoring, parameter determination and hazard

assessmentBuild earthquake catalog for future hazard updates

Expected outcomes – in the near future

Following completion of the earthquake catalogue, computation of earthquake hazard was undertaken for representative locations within two years, and is anticipated that these will lead to revise hazard maps in the following two yearsThis work will lead to the development of a set of modern earthquake hazard maps of the PNG region and the determination of earthquake response spectra at other selected sitesThe earthquake hazard maps and response spectra would form the basis for a revision of the PNG Earthquake (Building) Code

RECOMMENDATIONSThings proposed to be achieved for the purpose of realising in

full the value of the study. These include: (1) Determine earthquake hazard at other additional sites, to

improve the resolution and therefore better hazard mapping(2) Improve on the seismotectonic model developed and hazard

maps covering the whole geographic region (3) Immediate use of the hazard maps to facilitate the

replacement of the existing earthquake building code(4) Replace seismic station network to improve data acquisition

required which will in turn improve earthquake hazard analysis, and be able to sustain maintenance of the network

(5) To improved hazard analysis; delineate active faults(6) Develop plans for the future updates of the hazard map(7) Acquire EZ-FRISK or similar tools for immediate use, and for

future earthquake hazard updates

FUTURE WORKWill include:

Determination of PNG earthquakes (epicenters, depths, magnitudes and mechanisms) using a local seismograph network to reduce the uncertainty and current scatter in epicentre and depth estimates;

and hence allow delineation of active faults. Update and increase resolution of the seismotectonic model, particularly further re-iteration of the analysis process to include more sites, and by computing the hazard contributions by specific active faults rather than assuming uniform area source zones.

THANKS…