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Journal of Earthquake and TsunamiVol. 7, No. 4 (2013) 1350030 (8 pages)c© World Scientific Publishing CompanyDOI: 10.1142/S1793431113500309

b-VALUE ANOMALIES ALONG THE NORTHERN SEGMENTOF THE SUMATRA–ANDAMAN SUBDUCTION ZONE:

IMPLICATIONS FOR UPCOMING EARTHQUAKES

SANTI PAILOPLEE∗, PEERASIT SURAKIATCHAIand PUNYA CHARUSIRI

Earthquake and Tectonic Geology Research Unit (EATGRU )Department of Geology, Faculty of Science, Chulalongkorn University

Bangkok 10330, Thailand∗Pailoplee.S@gmail.com

Received 21 October 2012Revised 27 February 2013Accepted 29 April 2013Published 20 June 2013

The potential areas of upcoming earthquakes were investigated along the Northernsegment of the Sumatra–Andaman Subduction Zone according to the b-value of thefrequency-magnitude distribution. After enhancing the completeness of the earthquakecatalogue, two datasets, those recorded during (i) 1980–1994 and (ii) 1980–2003, weretested in order to verify the effective correlation between precursory b-values and thelocation of subsequent earthquakes. The results confirmed that areas with low b-valuesagreed well with the locations of the subsequent earthquakes in that region. Accord-ingly, the present-day dataset from 1980–2010 was carefully evaluated to determine theb-values across the region. Within this spatial investigation, three areas of low b-valuesand so potential hazards were found. These consisted of the (i) West coast of Myan-mar, and (ii) North and (iii) South of the Nicobar Islands. From 2010–2012, a majorearthquake with magnitude 7.5 mb was recorded as being generated in the region Southof the Nicobar Islands. Thus, attention should be paid to the remaining two until nowquiescent areas, and mitigation plans should be raised for both seismic and tsunamihazards.

Keywords: Sumatra–Andaman subduction zone; seismicity; frequency-magnitude distri-bution; b-value; earthquake prediction.

1. Introduction

After the devastation caused by the Mw-9.0 earthquake on December 26th, 2004and the subsequent public reaction to the earthquakes with Mw 8.2 and 8.6 onApril 11th, 2012 (Fig. 1), coastal communities around the Indian Ocean realizedthat the Sumatra–Andaman Subduction Zone (SASZ) is a significant hazard source,in particular for earthquakes and tsunamis, and so might pose a threat again in thefuture.

∗Corresponding author.

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90oE 95oE 100oE

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8.6(2012)

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Fig. 1. Map of the SASZ showing the epicenters of the alert earthquakes mentioned in the text.The rupture area of Mw-9.0 earthquake is bounded by the dashed line. The grey circles withinthe study area (black square) are the main shocks after declustering with the algorithm accordingto Gardner and Knopoff [1974].

Among the various techniques that have been used to predict earthquakes, the b-value defined from the frequency-magnitude distribution (FMD) of seismicity data,is one of the statistical methods that appears to predict fairly reliably upcomingearthquakes [e.g. Schorlemmer et al., 2003; Nuannin et al., 2005]. For instance,Nuannin et al. [2005] investigated the b-values along the subduction zone fromSumatra to Nicobar Islands corresponding to the Mw-9.0 earthquake’s rupture area(Fig. 1). They concluded that the variations in the b-value evaluated from thepreceding seismicity data strongly related to the subsequent earthquake of Mw-9.0in both temporal and spatial domains.

However, the Northern segment of SASZ, i.e. northwards from the NicobarIslands, was excluded from the survey of Nuannin et al. [2005] and so is stillunclear. Geographically, if this segment ruptures, in particular for vertical move-ment, the earthquake will generate the potential hazards of both ground shaking andlocal tsunami for Myanmar and India, including Thailand. This study, therefore,

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b-Value Anomalies along the Northern Segment of the Sumatra–Andaman Subduction Zone

attempted to clarify the hazardous situation of the Northern segment of SASZ byinvestigating the anomalies in the b-values. The obtained results should be usefulfor preparing long-term mitigation plans for both seismic and tsunami hazards.

2. Dataset and Completeness

The datasets used in this b-value investigation were the earthquake cataloguesderived from the Incorporated Research Institutions for Seismology (IRIS), theU.S. National Earthquake Information Center (NEIC) and the Thai MeteorologicalDepartment (TMD), which together recorded a total of 35,900 events during 1964–2012. The recorded earthquakes have magnitudes in the range of 1.0–7.5 Richter,and are mostly reported in the body-wave magnitude (mb) scale. It is noted thateach magnitude scale is derived by a specific assumption and analytical methodthat have a valid but different value and unique meaning. Thus, the other minorreported magnitude scales, i.e. the moment magnitude (Mw), surface-wave magni-tude (MS), and local magnitude (ML), were converted systematically to the singlestandard mb according to the empirical relationships of Pailoplee et al. [2009]. Theevents with a depth of more than 35km [Rao et al., 2011], defined as earthquakesgenerated by the subduction slab, were excluded from this analysis in order torepresent the interplate crustal earthquake activities. After the declustering proce-dure [Gardner and Knopoff, 1974], 1,100 clusters of earthquakes were found anda total of 1,920 events are identified as the main shocks representing exactly theseismotectonic activities (Fig. 1).

From the plot of the cumulative number of events against time (Fig. 2), threeprominent periods were found that could identify the different seismicity rates;namely between the years of (i) 1964–1980, (ii) 1980–2010, and (iii) 2010–2012(Fig. 2). The time spans of 1964–1980 and 2010–2012 were inferred to relate to thenetwork improvements that have been subjected to many adjustments, as reportedby Zuniga and Wiemer [1999]. Thus, the seismicity data reported during those timespans were excluded in this b-value investigation.

Regarding the consistency of the dataset reported during 1980–2010, theGENAS algorithm [Habermann, 1983, 1987] implemented in the ZMAP program[Wiemer, 2001] was applied to scan the detailed rate changes at an individual mag-nitude level. No obvious change in the seismicity rate was identified in the reportingof earthquakes with mb ≥ 4.0, which implied no man-made changes significantlyaffected the mb ≥ 4.0 dataset. Thus, the seismicity data recorded during 1980–2010with a magnitude of ≥4.0mb were finally selected to investigate the b-value alongthe Northern segment of the SASZ.

3. b-Value Investigation

According to the FMD, as shown in Eq. (1) [Ishimoto and Iida, 1939; Gutenbergand Richter, 1942], the values of b including a were positive, real constants with

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Fig. 2. Cumulative number of earthquakes with mb ≥ 4.0 (grey line) showing the three differentrates of seismicity reported during 1964–2012. The symbols are the earthquake with magnitude≥6.0 mb reported in the time spans of 1964–1994 (triangles), 1994–2003 (squares), 2003–2010(diamonds), and after 2010 (circle).

b representing the ratio of the occurrence of small to large earthquakes and a theentire seismicity rate. Both values were obtained from plotting the number (N) ofearthquakes with a magnitude equal to or larger (cumulative distribution) than M

which is in the mb scale in this study.

log(N) = a − bM. (1)

For a given certain area and time interval, variations in the b-value reflect the tec-tonic stress in a specific seismogenic volume. A number of successful applications ofthis approach have been reported, such as finding out the magma chamber beneathvolcanoes [Wiemer and McNutt, 1997; Sanchez et al., 2004] and the characteristicsof rock fractures [Rao and Lakshmi, 2005], including precursory investigation ofearthquakes [Schorlemmer et al., 2003; Nuannin et al., 2005], which is the mainaim of this study.

In b-value investigations, initially the entire selected data (i.e. in this studythat for 1980–2010 and mb ≥ 4.0) are observed and the FMD is plotted in orderto estimate the magnitude of completeness (Mc) that represents the level of thecomplete report. Figure 3 illustrates the FMD plot for this study, from which a = 5.4and b = 0.9. By the entire-magnitude-range method [Woessner and Wiemer, 2005],the estimated Mc was found to be approximately 5.0mb.

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Fig. 3. FMD plot of the seismicity data recorded mb ≥ 4.0 during 1980–2010. Triangles indicatethe number of earthquakes of each magnitude; squares represent the cumulative number of earth-quakes equal to or larger than each magnitude. The solid line is the line of best fit according toWoessner and Wiemer [2005]. Mc is defined as the magnitude of completeness.

With respect to the spatial investigation of the SASZ region evaluated here, theearthquake data were divided chronologically into the three groups of (a) 1980–1994, (b) 1980–2003, and (c) 1980–2010 (Fig. 2) and analyzed separately in orderto (i) verify the assumptions of Nuannin et al. [2005], and then (ii) predict theprospective sources of any upcoming earthquakes. The b-values were calculated for0.25◦ × 0.25◦ grid cells along the Northern segment of the SASZ. In any individualgrid cell, 50 earthquake events closest to the grid node [Nuannin et al., 2005] wereselected to evaluate the b-value using the ZMAP program [Wiemer, 2001]. Theobtained b-values were then contoured to construct the b-value maps, as shown inFig. 4.

The distribution of b-values, as analyzed from the seismicity data recorded dur-ing 1980–1994, reveals two regions that showed a low level of b (i.e. b ∼ 0.5–0.8)compared with the others (i.e. b ∼ 0.9–1.8), namely South and North of the NicobarIslands (Fig. 4(a)). By overlaying this with the recorded earthquakes with mb ≥ 6.0generated during 1994–2003 (squares in Fig. 2), at least four events were found tobe located in areas of low b-values in the region South of the Nicobar Islands. How-ever, note that there were also two events located nearby the inland region of theNicobar Islands with higher b-values of around 1.0–1.3.

Rechecking with the extended dataset recorded during 1980–2003 revealed threemore regions with low b-values similar to that in Fig. 4(a), including the West coastof Myanmar (Fig. 4(b)). The subsequent earthquake events with mb ≥ 6.0 (i.e.diamonds in Fig. 2) were found to be mostly located within those three regions of lowb-values. This two-case study, therefore, ensured that the hypothesis of precursory

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92o

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Eo

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(b)

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low b-values with the analyzing condition proposed by Nuannin et al. [2005] waseffective in locating the source of potentially likely upcoming earthquakes.

In addition, for evaluating the hazard situation, the present-day dataset from1980–2010 was investigated carefully. The result (Fig. 4(c)) revealed that there wasa progressive drop in the b-value compared with the previous b mapping (Fig. 4(b)).However, it is noticeable that subsequently there has only been an earthquake withmagnitude 7.5 mb (i.e. circle in Fig. 2) recorded in the southward offshore region ofthe Nicobar Islands on June 12th, 2010. The other two regions of low b-values, (i)the offshore area North of the Nicobar Islands and (ii) the West coast of Myanmar,are still quiescent up to the present day (2013).

4. Conclusions and Recommendations

Based on the seismicity data recorded during 1964–2012, the consistency of thedataset were checked and adjusted in order to get a complete seismicity data setthat represented the actual natural tectonic activities. Over the entire investigatedregion, the value of b obtained in this study was around 0.9, which is higher thanthe b = 0.7 proposed by Nuannin et al. [2005]. This implies the regional tectonicactivities of the Southern segment of SASZ investigated by Nuannin et al. [2005]are higher than the Northern segment of this study.

With respect to the spatial investigation, the 50 events closest to each individual0.25◦ × 0.25◦ grid nodes were used to calculate the respective grid b-value. Atfirst, different chronological periods were used to test the consistency of the b-value evaluation with respect to the location of subsequent earthquakes. The resultssupported that most subsequent earthquakes were located within areas showing lowb anomalies (Figs. 4(a) and 4(b)). Using the present-day dataset (i.e. 1980–2010),three obvious areas with a low b-value were revealed. Within these three regionsin 2010–2012 an earthquake with 7.5mb was generated in the region South of theNicobar Islands whereas the other two regions are still quiescent. One may surmisethat they may pose a potential risk (Fig. 4(c)).

It is difficult to determine correctly whether the fault movement is vertical orhorizontal. However, based on the low b-value anomalies at the offshore area of theNorthern part of the Nicobar Islands, the coastal communities around the AndamanSea, including the Indian Ocean, should be aware of the prospective tsunami hazard.Meanwhile, the existence of the low b-value area nearby to Sittwe City, on the Westcoast of Myanmar (Fig. 4(c)) should not be ignored. The contribution of effectivemitigation plans is urgently needed for reducing the hazardous effects in the future.

Acknowledgments

The Faculty of Science, Chulalongkorn University provided the financial supportto S. Pailoplee through the Thai Government Stimulus Package 2 (TKK2555:PERFECTA), the National Research University Project of CHE and the

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Ratchadaphiseksomphot Endowment Fund (Project Code CC508B-56). Thanks arealso given to T. Pailoplee for the preparation of the draft manuscript. We thank thePublication Counseling Unit (PCU), the Faculty of Science, Chulalongkorn Univer-sity for a critical review and improvement of the English. We acknowledge thought-ful comments and suggestions by Prof. Dr. Fook-Hou Lee, the Managing Editors andanonymous reviewers which enhanced the quality of this manuscript significantly.

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