JOURNAL GEOLOGICAL SOCIETY OF INDIAVol.80, September 2012, pp.393-402

0016-7622/2012-80-3-393/$ 1.00 © GEOL. SOC. INDIA

Earthquake Forerunner as Probable Precursor – An Examplefr om North Burma Subduction Zone

SUJIT DASGUPTA1, BASAB MUKHOPADHYAY

2 and MANOJ MUKHOPADHYAY3

1Ex Geological Survey of India, Kolkata, India2Geological Survey of India, Central Headquarters, 27 J.L. Nehru Road, Kolkata-700016, India

3Department of Geology & Geophysics, King Saud University, P.O. Box 2455,Riyadh 11451, Kingdom of Saudi ArabiaEmail: basabmukhopadhyay@yahoo.com

Abstract: The Burmese Arc seismic activity is not uniform for its ~ 1100 km length; only the Northern Burmese Arc(NBA) is intensely active. Six large earthquakes in the magnitude range 6.1 – 7.4 have originated from the NBA Benioffzone between 1954 -2011, within an area of 200 x 300 km2 where the Indian plate subducts eastward to depths beyond200 km below the Burma plate. An analysis on seismogenesis of this interplate region suggests that while the subductinglithosphere is characterized by profuse seismicity, seismicity in the overriding plate is rather few. Large earthquakesoccurring in the overriding plate are associated with the backarc Shan-Sagaing Fault (SSF) further east. The forecastingperformance of the Benioff zone earthquakes in NBA as forerunner is analysed here by: (i) spatial earthquake clustering,(ii) seismic cycles and their temporal quiescence and (iii) the characteristic temporal b-value changes. Three such clusters(C1- C3) are identified from NBA Benioff Zones I & II that are capable of generating earthquakes in the magnituderanges of 7.38 to 7.93. Seismic cycles evidenced for the Zone I displayed distinct quiescence (Q1, Q2 and Q3) prior to the6th August 1988 (M 6.6) earthquake. Similar cycles were used to forecast an earthquake (Dasgupta et al. 2010) to comefrom the Zone I (cluster C1); which, actually struck on 4 February 2011 (M 6.3). The preparatory activity for an eventhas already been set in the Zone II and we speculate its occurrence as a large event (M > 6.0) possibly within the year2012, somewhere close to cluster C3. Temporal analysis of b-value indicates a rise before an ensuing large earthquake.

Keywords: Northern Burmese Arc (NBA), Precursor, Seismic clusters, Seismic cycles, Temporal quiescence, Temporalb-value.

INTRODUCTION

Short-time earthquake precursors within a reasonabletime-frame are often looked for as an indicator or warningof an approaching event and prove useful for forecasting.Seismic parameters like the enhanced foreshocks, seismicitypattern and seismic quiescence that occur before a mainshock are linked to the ground preparation of the main shock(IASPEI Sub-Commission on Precursors); these areconsidered as physical preparation for the principal rupturingto follow (Scholz, 2002). The IASPEI lists of significantprecursors include foreshocks, pre-shocks, seismicquiescence before major aftershocks, radon decrease inground water, and increase in ground water level (Wyss,1997). Historically, the precursory evidences have beenclassified into A1, A2 and B types (Rikitake, 1975). Thetype B precursors observed in terms of foreshocks have nomagnitude-dependent precursor time. The A2-type precursorsobserved by means of geodetic work, changes in seismic-

wave velocities have a precursor time which is closelycorrelated to the magnitude of coming earthquakes. TheA1-type precursor, mostly observed several hours prior tothe main shock, is caused by a creep-like failure before themain rupture (Rikitake, 1975). The changes in the seismicrate (Ogata, 2005) and its later version that combinesAccelerated Seismic Rate (ASR) power-law and geometry(Mignan, 2011) are important precursors. Both high/low b–values in and around the epicentral zone reflectingthe nature of ensuing movement (thrust/strike-slip/normal) along causative fault planes, material heterogeneity,high and low stress condition are also treated as precursors(Wyss and Martirosyan, 1998; Enescu and Ito, 2001, 2002;Huang et al. 2001; Monterroso and Kulhánek, 2003;Schorlemmer et al. 2004; Nuannin et al. 2005; Wiemer etal. 2005).

Prediction of an earthquake has both success and failure.There are some controversial successful stories in the USA

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and China in 1970’s whilst there are cases where thepredicted earthquake never took place. Even several largeearthquakes in recent times (like 26th December 2004Sumatra earthquake (Mw 9.3); 11th March 2011 Sendaiearthquake (Mw 8.9) in Japan) do not produce appreciablerecognizable precursors. Nevertheless, the advancedresearch efforts are on to look for different types ofprecursors in the earthquake forecasting; refer to the recentreviews by Cicerone et al. (2009), Murru et al. (2009) andUyeda et al. (2009).

The NBA is in direct continuation of the Sunda–Andaman arc system within a land environment thatultimately connects to the Himalayas and thereby forms animportant seismic region where studies of seismicityprecursor could contribute to a better understanding ofthe underlying physical processes and seismic hazardassessment (Fig.1). More so, since, the NBA subduction zonehas the added advantage of utilizing the data from surfacegeological mapping. Studies on earthquake precursors for

its immediate western region in the Eastern Himalayanforedeep have earlier been attempted by various researchers(Khattri and Weiss, 1978; Gupta and Singh, 1982, 1986,1989; Guha and Bhattacharya, 1984; Gupta 1985, 1987,1993; and Khattri, 1987, 1993, 1999). Gupta and Singh(1986, 1989), Gupta (1987) through an analysis ofprecursory earthquake swarms, made an earthquake forecastvalid up to 1990 in the North Burma that eventually struckin August 1988.

Our objective here is to look for the earthquakeforerunners in NBA by using: (i) Short-term spatialearthquake clustering, (ii) Seismicity pattern, Seismic cyclesand their temporal quiescence and (iii) Temporal b-values.This exercise will hopefully contribute to assess the seismicpotentiality of NBA in near future.

SEISMOLOGICAL SETTING OF NBA

The 450 km stretch of NBA (Fig. 1) is seismically most

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Fig.1. Seismotectonic Map of Northern Burmese Arc (NBA); earthquake data period: 1954 through 2010, data source cited in text;precursor studies carried with earthquake events that locate within the rectangular areas marked Zones I and II. Black stars 1 to6 are large Benioff zone earthquakes for which precursor analysis undertaken. A-A' and B-B' are the section lines across thetwo zones illustrated on Fig.3. BS – Belt of Schuppen, CB – Central Basin, DF – Dauki Fault, EBT – Eastern Boundary Thrust,SSF – Shan Sagaing Fault, VA – Volcanic Arc.

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active in the entire region, where the Benioff zone plungesbelow the Burma plate at an angle of 40° - 45° (Figs. 2 and3). For constructing the seismic sections (lower panel ofFig.3), the data from the earthquake catalogue (describedin the next section) and available EHB relocated data of theevents are used. The geological sections on the top panel ofFig.3 are after Mukhopadhyay and Dasgupta (1988). Detailson seismicity and geometry of the interplate region arealready discussed in our earlier studies (Mukhopadhyayand Dasgupta, 1988; Dasgupta et al. 2003). Most notablefeature of NBA is that while seismicity is profuse todefine the subducting lithosphere, earthquake incidence isrelatively sparse within the overriding plate. It is intriguingthat NBA has not produced large shallow foci interplateearthquakes within the recorded history and neither thereare significant events originating from the seismogenicforearc crust of the overriding plate. Only some moderateto large shallow foci earthquakes are found with SSF system.Though their location error could be quite large (CentennialEarthquake Catalog, Engdahl and Villaseñor, 2002), somesignificant events occurring close to SSF are: 27.01.1931(M 7.6; h 35 km); 12.09.1946 (M 7.7; h 35 km); 16.07.1956(M 7.1; h 25 km). Here, the most recent relocated EHBevent being of 05.01.1991(M 7.0; h 18 km) that wasassociated with the SSF and it shows dextral strike slipmovement.

DATA AND METHODS

Earthquake Data Catalogue

In order to decipher the seismic clusters and cyclesaffecting the NBA, we utilize ISS/CGS data for the period1954 through 1963; ISC bulletin data from 1964 through2004 and NEIC data for the period 2005 through February2011. The Centennial Catalogue and EHB event data(courtesy: Prof. E. R. Engdahl, pers. comm.) is also consultedfor cross checking of earthquake parameters. Theseearthquake data are plotted on topographic DEM showingmajor tectonic features (Fig. 1), where, the configuration ofthe subducting slab, Benioff Zone and other associatedfeatures are adopted from Guzman-Speciale and Ni (1996)and Dasgupta et al. (2003). For an analysis of b-values, it isvery important to inspect the magnitude completeness asthe b-value is particularly affected by the missing events.This is discussed next.

Magnitude Thr esholds

The data catalogue from the year 1954 through February2011 contains 1444 events with magnitude ranging from2.9 to 7.4 and focal depth down to 285 km. To workout themagnitude completeness, we adopt the methodology basedon the assumption of self-similarity (Wiemer and Wyss,2000). A fast and reliable estimate of the magnitudecompleteness (Mc) by this method is to define the point ofthe maximum curvature (MAXC) as ‘magnitude ofcompleteness’. This is done by computing the maximumvalue of the first derivative of the frequency-magnitudecurve. In practice, this matches the magnitude bin with thehighest frequency of events in the non-cumulative frequency-magnitude distribution. Highest frequency of events in thenon-cumulative curve with the NBA data is having a flatpeak with magnitude 3.9 as the highest frequency class whichby definition may be taken as Mc (Fig.4). Here, the G-Rrelation: Log (cumulative frequency) – magnitude illustratesthat for magnitude 4.1 and above, the curve is smooth andfollows a straight-line, thereby implying that all earthquakesof magnitude ≥ 4.1 in the region were detected and thecatalogue is by and large complete above this cut-offmagnitude. To maintain a rational between over- and under-estimations which may take place within statisticalprocedures, an intermediate magnitude 4.0 as Mc is adoptedfor further calculations.

PRECURSOR STUDIES

Earthquake Clustering Epidemic Model for NBA

A seismic cluster is suspected in a region if it consists of

�Fig.2. Schematic interplate configuration below NBA (approach

adopted after Sibson, 2009); BP: Burmese Plate, CSZ –Crustal Seismic Zone; within CSZ of Burma plateseismicity is sparse in the forearc domain below CB –Chindwin Basin while large earthquakes occur in thebackarc domain associated with SSF – Shan Sagaing fault;SBB – Shwebo backarc basin; SP – Shan Plateau; VF –Volcanic front; note that no great shallow interplateearthquakes have occurred within the recorded history alongLI – locked interface between the subducting Indian andoverthrust Burmese plate. Benioff zone below the lockedinterface is highly seismogenic. Overpressure fluidgenerated from plate interface, metamorphism anddiagenesis of sediments in the basins (CB and SBB) alongBurmese plate generates small-intermediate shocks.

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multiple events with magnitude greater than a threshold valueoriginating within an acceptable time period. ‘Point density’estimate (after Silverman, 1986), a spatial statistical function,is applied to constrain the extent of such seismic clusters asit successfully identifies the areas of concentration of points.To calculate the point density, ‘distance’ between theadjacent earthquakes (mb ≥ 4.0; time frame 1954-2010) ismeasured on the earthquake distribution map (Fig. 1) by‘near analysis’. From the ‘distance data’, a mean distance(~10 km) between the earthquake points is calculated. The‘mean distance’ is taken as the radius of the circular influencefor point density calculation. Point density is calculated asthe total number of earthquake epicentral points that fallwithin the chosen circular area (here: 10 km radius) dividedby the area of the circle. A factor resulting from the size ofearthquake is also considered for deriving the point densityvalue, i.e. the size of the earthquake is taken as its count.This is done to offer more weight to larger earthquakes.The measurement is then carried out on an overlapping gridpattern where the centre of the circle has moved across themap, both along latitude and longitude, by a sliding distanceequivalent to the search radius (10 km). Resulting pointvalues obtained are interpolated to generate a grid. This gridhas a mean (m) 308.14 and standard deviation (sd) of 427.50.

The areas with anomalous point density (value > (m + 2 sd)= 1163.14) are identified as zones of spatial clusters, shownby closed polygons (Fig.5).

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Fig.3. Section view through A- A' and B- B' across NBA (Fig. 1 for location) illustrate geology (top panel; after Mukhopadhyay andDasgupta, 1988) and underlying seismic zones; note the locations of five large earthquakes within the inclined seismic slabunder Zone I and one earthquake under Zone II. Earthquakes defined by the grey band constitutes cluster C1 in Zone I whileclusters C2-C3 in Zone II (refer Fig.5). AB – Aseismic Belt; EBT – Eastern Boundary Thrust; and VA – Volcanic Arc.

�Fig.4. Frequency-magnitude distribution of the data (1954 –February 2011) in the catalogue used. The result of theMAXC approach is indicated with the arrow (Mc = 3.9)and for Gutenberg-Richter relationship with Mc = 4.1. Thegray coloured line on cumulative frequency curve indicatesthe best fit line by G-R relationship.

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The process identifies four such “spatial seismic clusters”of variable sizes within NBA (designated as C1 through C3)representing mainly the zone between EBT and volcanicarc whereas the fourth cluster (C4) locates along the SSF inthe back-arc. Notice that six moderate to large earthquakeshave originated within the clusters or from their periphery(Fig. 5). Four earthquakes of: 06.08.1988; 09.01.1990;06.05.1995 and 04.02.2011 (all are thrust type events; seeTable 2) locate within cluster C1 while one along itsperiphery (21.03.1954). Similarly, the earthquake of17.10.1969 locates within cluster C3. Seismo-geologicsections drawn across clusters C1 (Fig. 3a) and C2 and C3(Fig. 3b), show that the earthquakes which cluster on mapview (Fig. 5) is largely supported by events originating fromthe Benioff zone, with a negligible contribution from theupper plate fore-arc seismicity.

We have suggested elsewhere (Mukhopadhyay et al.2010; Mukhopadhyay et al. 2011) that in regions of profuseseismicity, identifying earthquake cluster zones mightactually serve as the first step to statistically segregatesegments of recurrent seismicity and then to proceed withother probes in and around the cluster. It is noteworthy thatseismic clusters in other active belts also delineate zones ofstrain accumulation and destined for periodic release. Toestimate this release through earthquakes in the time domain

one has to fall back on other precursors, be its statistical,geophysical or mixed approach.

Seismic clusters C1 - C3 in the NBA originating fromthe Benioff zone are sub-parallel to the trend of the Burmeseorogen; assuming that the individual cluster ruptures in asingle earthquake, we employed the empirical relationbetween rupture length (RL) and size of earthquake (M)as a function of type of faulting (reverse/strike slip). Thestrike lengths of clusters C1 to C4 were measured on themap (Fig.5). These strike lengths are indicative of themaximum rupture length (RL) that can be generated by anearthquake. On measuring RL, the expected size ofearthquake is empirically calculated using the equation:log RL = -2.86 + 0.63M (for reverse faults) or log RL =-3.55 + 0.74M (for strike-slip fault) (Wells and Coppersmith,1994). We adopt the equation for reverse fault for C1 to C3since earthquakes within the clusters show predominantthrust movement (CMT data in Table 2 and also refer toFigure 6 in Mukhopadhyay et al. 2010). For cluster C4,which is generated by a predominant strike slip motionalong SSF, we used the second equation. These estimatesgives a first approximation on the Maximum CredibleEarthquake (MCE) that C1 (7.93 M), C2 (7.56 M), C3(7.38 M) and C4 (6.91 M) can generate. Notice that therecorded earthquake magnitudes from the NBA Benioff

Fig.5. Seismic clusters, C1 through C4 delineated by point density analysis. The clusters are designated as source zones of recurrentseismicity including large earthquakes. For other abbreviations refer to Fig.1.

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zone are lower than these estimated values. Larger rupturelength cutting across the clusters is not consideredimprobable, although unlikely.

Seismicity Pattern, Cycles and Temporal Quiescence

Seismic quiescence is defined as the period where theseismicity rate decreases to levels significantly below thenormal seismicity. Seismic cycle of an active seismic domainis divided into alternate temporal segments of the seismicquiescence (Q1, Q2 and Q3) and an increase in backgroundseismicity (Scholz, 1988). ‘Q1’ is defined as a quiescenceperiod between a mainshock – aftershock sequence and afurther increase in background seismicity; ‘Q2’ is a lullperiod between the previously defined increase inbackground seismicity and further renewed seismicity, while‘Q3’ is defined as a short time-respite of seismicity prior toanother main shock – aftershock sequence. Though statisticaltests for significance of quiescences are available in literature(see Zoller et al. 2002 for details), Scholz (1988; 2002) hasused quiescence as an integral part of seismic cycle toforecast the earthquake in an area without focussing on itsstatistical significance. It is therefore emphasized that

occurrence of a major shock immediately after the ‘Q3’quiescence is of significance by itself.

For evaluation of the seismic cycles precisely, the NBAsubduction zone is partitioned into two broad zones havingidentifiable seismic characteristics (Zones I and II; Fig.1)that are amenable to produce larger shocks. Shallow fociback-arc seismicity, located east of the volcanic line anddistinctly associated with the SSF is not taken into accounthere for further study. Thus, the basic seismicity pattern inNBA with cycle and quiescence is evaluated for the period1954 through February 2011. In Zone I five largeearthquakes (magnitude > 6.0) occurred during the periodwhile in Zone II there is only one event. Tectonically, ZoneI and Zone II are different. Zone I is with NNE-SSW trendingsubduction interface oriented sub-parallel/parallel to Indianplate movement vector with well developed volcanic arc,whereas, Zone II has N-S trending subduction front,tangential to the Indian plate movement vector with welldeveloped subduction zone, back-arc basin and volcanicfront (Figs. 1 and 3).

In Zone I, a large earthquake occurred on 21.03.1954(M 7.4; h 180 km). This is followed by a post seismic

Table 1. Earthquake cycles and quiescence model for North Burma subduction zone

Cycle in Zone – I

Year No of events Mmax b-value Observation / Analysis on earthquake cycle asper Scholz (1988)

1954 1 7.4 - Occurrence of 21.03.1954 (M 7.4)

1955 - 1968 17 (Mb ≥ 4.6) 5.7 - Post Seismic quiescence: Q1

1969 - 1974 20 (Mb > 4) 5.6 0.87 Increase in background seismicity

1975 - 1977 6 (Mb ≥ 4) 4.7 Intermediate term quiescence for 3 years:0.96 Q2 with comparatively low seismicity

1978 - 1988 78 (Mb ≥ 4) 5.7 Increase in background seismicityQ3 of 25 days Occurrence of 06.08.1988 (M 6.6)

1988 - 1989 28 (Mb ≥ 4) 5.3 1.08 Increase in background seismicityQ3 (?) of 10 daysOccurrence of 09.01.1990 (M 6.1)

1990 - 1995 66 (Mb ≥ 4) 5.9 0.86 Increase in background seismicityQ3 (?) of 2 days Occurrence of 06.05.1995 (M 6.3)

1995 - 2000 83 (Mb ≥ 4) 5.2 0.85 Post Seismic quiescence: Q1

2001 - 2004 100 (Mb ≥ 4) 4.9 1.08 Increase in background seismicity

2005 11(Mb ≥ 4) 5.7 Intermediate term quiescence for 1 year:1.08 Q2 with comparatively low seismicity

2006 - 2010 52 (Mb ≥ 4) 5.9 Increase in background seismicityQ3 of 7 days Occurrence of 04.02.2011 (M 6.3)

Cycle in Zone – II

1956 - 1968 20 (Mb ≥ 4.6) 5.9 - Background SeismicityQ3 of 8 months Occurrence of 17.10.1969 (M 6.1)

1969 - 1995 160 (Mb ≥ 4) 5.4 0.86 Post Seismic quiescence: Q1

1996 - 2004 185 (Mb ≥ 4) 5.1 0.80 Increase in background seismicity

2005 - 2006 23 (Mb ≥ 4) 5.7 Intermediate term quiescence for 2 years:1.08 Q2 with comparatively low seismicity

2007 - 2010 45 (Mb ≥ 4) 5.5 Increase in background seismicityQ3 has not set in; last event is recorded on25.06.2011 (4.7)

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quiescence (Q1) from 1955 to 1968 with occurrence of only17 earthquake events in 14 years (Table 1; temporal b-valuesreferred in the table are discussed in the next section). Anincrease in the background seismicity (20 events) is clearlynoticed over the next 6 years pertaining to the time interval1969 – 1974; followed by intermediate term quiescence (Q2,only 6 events; Mmax 4.7)) during 1975-1977. The nextincrease in background seismicity with 78 events between1978 and 1988, though considered significant could partlybe due to the increase in earthquake detection threshold inthe region. In the year 1988, five foreshocks could distinctlybe linked with the ensuing large event along with quiescence(Q3) of 25 days prior to the large earthquake of 6.8.1988[M 6.6; h 100 km; (Mw 7.3 EHB)]. The downward migrationof seismic activity (Mogi, 1988) up to a depth of 139 kmprior to the main event of 100 km depth is also noticed.Thus there were signals for a possible medium termforecasting of the 1988 earthquake. In fact Gupta and Singh(1986, 1989), Gupta (1987) through an analysis ofprecursory earthquake swarms made a forecast valid up to1990 that eventually struck in August 1988. This event isfollowed by 17 aftershocks all along the ruptured area thatincreases the overall seismicity of the region in the followingtwo years 1988 and 1989 that spawn 28 earthquake events.The large earthquake of 6.8.1988 has increased the seismicpropensity within the Benioff zone to such an extent thatthe following years (between 1989 and 1995), the areawitnessed a huge increase in seismicity with an occurrenceof two large events [9.1.1990 (M 6.1; h120 km) and 6.5.1995(M 6.3; h 117)] with doubtful Q3 quiescence of 10 and 2days respectively. Noticeable precursory quiescence (Q1 orQ2) prior to these events is either absent or camouflaged byan increase in the background seismic activity. Seismicitypattern in the following years (1995 through February 2011)is quite interesting in the light of the precursor study. Theprecursory cycle starts with ‘Q1’ (1995-2000, 83 events);increase in background seismicity (2001-2004, 100 events),

followed by ‘Q2’ (2005, 11 events) and increase inbackground seismicity (2006 – 2010, 52 events). Thisincrease in background seismicity is followed by a Q3

quiescence of 7 days with the occurrence of a main shockof 04.02.2011 (M 6.3; h 88 km). In fact, after studying theseismic cycle-quiescence pattern utilizing earthquake databetween 1969 through June 2008, we speculated animpending event of M 6.0 or greater from this part of NBA(Dasgupta et al. 2010); the event actually occurred on04.02.2011.

The analysis is next extended further south to Zone II,following the same approach adopted for Zone I. In zone II,there is a background seismicity of 20 events during theperiod 1956 – 1968, followed by a Q3 quiescence of 8 monthsbefore a large shock on 17.10.1969 (M 6.1; h 124 km). Thisearthquake is again from the deeper level of the subductingIndian plate (Fig. 3b). The precursory cycle starts with ‘Q1’(1969-1995, 160 events in 25 years) followed by increasein background seismicity (1996-2004, 185 events in 9 years);this is succeeded by intermediate term quiescence‘Q2’ (2005-2006, 23 events) and increase in background seismicity(2007 – 2010, 45 events) (Table 1). This period of increasein background seismicity is still ongoing (as evidenced byan earthquake of M 4.7 that occurred on 25.06.2011) andQ3 quiescence has not set in. It is probable that Zone- II iswaiting for a large earthquake (estimated M 6.0 - 6.5) tooccur possibly within 2012, if it follows the trend of theZone I.

Temporal b-value Characterization in Zone I and Zone II

The temporal distribution of b-value within the zones iscalculated by maximum-likelihood method (Aki, 1965; Ustu,1965; Bender, 1983), b = loge / (M – Mmin), where M denotesthe mean magnitude and Mmin the minimum magnitude ofcompleteness in the given sample]. As b-value is dependenton data, the earthquake data has been treated separately asper the techniques described by Kulhanek (2005) to make

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Fig.6. Temporal variation in b-value with corresponding shocks in the Zone I (leftpanel) and Zone II (right panel) of NBA

the calculated b-values statistically robustand tectonically significant.

The b-value for different time periodsfrom 1969 through 2010 (Table 1; Fig.6) inZone I indicates that there is an increase inb-value prior to a mainshock. The b-valueraise from 0.87 to 0.96 prior to the 6.8.1988(6.6) thrust type of earthquake (Table 2)corroborates well with the calculated valuesgiven by Schorlemmer et al. (2005) forthrust earthquakes. Immediate increase inthe b-value to 1.08 is due to a possible largeheterogeneities in the area. While there is

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no significant temporal b-value precursor registered priorto the earthquakes of 9.1.1990 (M 6.1) and 6.5.1995 (M6.3) which occurred within a b-value of ~ 0.85, an increaseof b–value from 0.85 to 1.08 before the 04.02.2011 (M 6.3)earthquake is worth noting (Figure 6).

Similarly, an increase in b-value 0.86-0.80-1.08 in ZoneII (Table 1; Fig. 6) exhibits a form of intermediate timerecovery precursor. This high b-value also indicates eitheror in combination of the following phenomena - increase instress intensity and crack density, creep along fault planes,decrease in effective stress due to shear slip on subsidiaryfault and changes in the pore fluid pressure. Presently (2005– 2010), the b-value is 1.08. This increase in b-valuecorresponds to a reduction in the stress intensity, strainhardening prior to a critical rupture by precursory slip behindan accelerating crack front and sudden stress drop (Meredithet al., 1990). This intermediate b-value also indicates animpending earthquake, with thrust slip, having appreciablestrike slip movement in Zone II of NBA. If Zone II followsthe trend of Zone I, a large earthquake (~ M 6.5) may occurwithin 2012.

CONCLUSIONS

The seismic pattern in the NBA studied here usingdata from 1954 through February 2011 brings out severalsalient characters of the plate margin seismicity andprecursory evidences. Spatial clusters of seismic events inregions of high seismic propensity are segregated throughstatistical analysis and tagged as source zones of recurrentearthquakes. Identification, mapping and validation throughforthcoming events should constrain the geometry of

these clusters and designated as precursor zones in theparlance of medium to long- term earthquake forecasting.Within the constraints of present dataset, we postulatethat the future earthquake source zone in NBA are theclusters C1 to C3 with projected MCE to the order of 7.38M to 7.93 M. The cluster analysis thus provides possiblelocation and size window of the future earthquake in aseismically active domain; other precursor studies shouldfollow surrounding the clusters to shortlist temporalprecursors.

Since the earthquake of magnitude 7.4 in Zone–I thatoccurred on 21.03.1954, a complete seismic cycle of 37years with four main shock events [6.8.1988 (6.6), 9.1.1990(6.1), 6.5.1995 (6.3) and 04.02.2011 (6.3)] with differentphases of quiescence (Q1, Q2 and Q3) and active seismicityhas been identified. A forecast was made previously(Dasgupta et al. 2010) for the earthquake of 4 February 2011.Similarly, in Zone–II, a seismic cycle has been initiated byan earthquake dated 17.10.1969 (6.1) which is followed by‘Q1’ (1969-1995, 160 events), increase in backgroundseismicity (1996-2004, 185 events), ‘Q2’ (2005-2006, 23events) and again an increase in background seismicity(2007 – 2010, 45 events). The increase in back groundseismicity is still continuing and fortunately Q3 quiescencedoes not engrave. We postulate that NBA Zone-II is waitingfor a large earthquake (M 6.0- 6.5) to occur possibly within2012, if it follows the trend of Zone I.

The temporal ‘b value’ analysis in the source zonesindicates an increase in ‘b value’ prior to main shocks inthis region that may be regarded as precursor. Themoderate b–value (0.8 to 1) in the source zones is indicativeof a future thrust type of earthquake.

Table 2. CMT solution for major earthquakes of the Burmese Arc in Zone - I (source HRVD)

Date Hr:Mn:Sec Lat. Long. Mb Mw T T P P Focal Focal Focal Focal Focal Focal Typeaxis axis axis axis Plane_1 Plane_1 Plane_1 Plane_2 Plane_2 Plane_2of fault

Plunge Azimuth Plunge Azimuth strike dip Rake strike dip rake movement*

6.8.1988 00:36:38 25.2 94.89 6.6 7.3 65 117 5 217 284 45 55 148 54 120 Thrust

9.1.1990 18:51:36 24.4 94.95 6.1 6.3 58 142 20 16 140 32 139 267 69 64 Thrust with strikeslip component

6.5.1995 01:59:14 24.8 95.02 6.3 6.4 69 94 9 209 278 39 60 135 57 112 Thrust

4.2.2011 13:54:6.7 25.14 94.80 6.3 6.3 58 105 1 196 133 54 131 257 53 48 Thrust with strikeslip component

* Following Aki –Richards convention on slip

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(Received: 10 October 2011; Revised form accepted: 14 November 2011)