Next Impending Earthquake in Northern Burmese Arc - Search for A Probable Precursor

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Next Impending Earthquake in Northern Burmese Arc- Search for A Probable Precursor

SUJIT DASGUPTA, BASAB MUKHOPADHYAY

AND MANOJ MUKHOPADHYAY1

Geological Survey of India, #27, J.L. Nehru Road, Kolkata-7000161Department of Geology and Geophysics,

King Saud University, P.O. Box 2455, Riyadh-11451, Kingdom of Saudi ArabiaE-mail: [email protected]

ABSTRACT

Subduction of the Indian plate below the Burmese Arc (BA) up to depths of about200 km in a continental environment presents a unique opportunity to focus onseismogenic behaviour of active tectonic blocks. Northernmost 450 km stretch of BA out-of-its 1100 km total length is seismically most active, where, the Benioff zone dips at 40°-45° degrees eastward and the Arc itself takes a sinuous bend following the Tripura FoldBelt and the Naga Belt of Schuppen. The continuing subduction is responsible foraccumulation of huge residual strain at lithospheric margins. Character of elastic strainthat is released from time to time from this rather short portion of the plate boundaryand the pattern of recurring major earthquakes provide the basis for the present study.Five major earthquakes occurring here since 1964 (when ISC data became available) are:[17.10.1969 (6.1), 6.8.1988 (6.6), 9.1.1990 (6.1), 5.1.1991 (6.1), and 6.5.1995 (6.3)]. Using these,the seismic quiescence and the corresponding b-values are estimated as precursors, wherean active seismic cycle distinguishes the alternate domains of quiescence (Q1, Q2 and Q3)and active seismicity. It is found that prior to the occurrence of 6.8.1988 earthquake (mag.6.6), there are many short and intermediate term precursors (Q1, Q2 and Q3), change inseismicity rate and b-value, that are either absent or camouflaged prior to the mainshocksof 9.1.1990 (6.1), 5.1.1991 (6.1) and 6.5.1995 (6.3). Seismicity pattern for last seven years(2002 to June 2008) starts with Q1 (2002, 13 events/yr, b = 1.45), increase in backgroundseismicity (2003-2004, 30 events/yr, b = 1.45), Q2 (2005, 19 events/yr, b = 0.86), increase inbackground seismicity (2006-May 21st 2008, 22.8 events/yr, b = 1.08), and followed by Q3for nearly one month up to the end of June 2008. Such Q3 quiescence should immediatelyfollow a fore - main - aftershock sequence. Typical tectonic set up for north part of BA andits seismic history suggest for the magnitude of the impending event as 6.0 M or greaterthat is likely to be associated with a strike slip fault or a thrust (with appreciable strikeslip). The exact size and timing of the event cannot however be deciphered from theavailable teleseismic data alone; this evidently requires data from local network onactive blocks.

Keywords: Subduction, Burmese Arc, b-values, Teleseismic data.

MEMOIR GEOLOGICAL SOCIETY OF INDIANo. 75, 2010, pp. 69 - 80

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INTRODUCTION

The BA is categorized as an intense seismic zone along the Indian platemargin where the ongoing subduction up to depths of about 200 km has producedseveral large earthquakes. A seismogenic area prior to a megathrust event usuallyexhibits precursors of various kinds like: hydrological, geochemical and geophysical.Geophysical precursors such as the changes in seismic source parameters, manifestedseismic quiescence, Accelerating Moment Release (AMR) and changes in the b-valueare taken to forecast an impending event. Further, these precursors can have short tolong time-durations depending on the expected time interval prior to an impendingevent.

A precursory phenomenon before the mainshock is regarded as a part ofphysical preparation for the main rupture to follow (Scholz, 2002). This physicalpreparation can exhibit seismic quiescence. At the same time changes in the seismicrate (Ogata, 2003) and high/ low b–values also act as meaningful precursors (Wyssand Martirosyan, 1998; Enescu and Ito, 2001, 2002; Huang et al., 2002; Monterrosoand Kulhánek, 2003; Schorlemmer et al., 2004; Nuannin et al., 2005; Wiemer et al.,2005).

Here we study the intermediate term geophysical precursors like: (i) Temporalquiescence, (ii) Seismicity pattern, (iii) Earthquake occurrence rate, (iv) Depthdistribution of the events prior to the main event and (v) the changes in temporal ‘b’values. We look for seismic cycles affecting BA for purposes of quantifying thephysical precursors, following the methodology by Scholz (2002), in order to searchfor a probable precursor having relevance to crustal mega-events in this region ofcontinental subduction.

TECTONIC SETTING AND SEISMICITY IN THE BURMESE ARC

Broad tectonic features of BA have resulted from the northeastward drift ofIndia landmass and its collision with the Shan-block belonging to the Asian landmassby early mid-Eocene. The Burmese ranges thus represent a continuous trench-arcsystem from north to Andaman Arc in the south where the northern part has emergedabove the sea level by the end of the Oligocene (Le Dain et al., 1984). The arc system(~ 1100 km length) has been represented by an east dipping Benioff zone penetratingto about 200 km depth sub-horizontally (Mukhopadhyay and Dasgupta, 1988;Dasgupta et al., 2003) in a continental environment and thus provide an uniqueopportunity to study the active arc system by surface investigations. It is nowcommonly accepted, as was initially proposed by Chhibber (1934), that the Burmesearc and Andaman arc further south together provide an important transitional linkbetween the Himalayan collision zone and the Indonesian arc; the latter is in directtectonic continuation of the Western Pacific arc systems. Northernmost 450 km stretchof BA is seismically most active, where the Benioff zone angle is 40°- 45° and arcitself takes a sinuous bend following Belt of Schuppen and Tripura Fold Belt innorth and south (Figs. 1a and 1b). This aerially exposed subduction related tectonicpacket (Mukhopadhyay and Dasgupta, 1988) represents a forearc basin (Tripurafold belt) in the west; folded and thrusted sedimentary arc with ophiolite mélange; MtPopa – Chindwin –Wuntho volcanic arc; and a back arc region represented by San-

DASGUPTA AND OTHERS: Next Impending Eartquake in Northern Burmese ... a Probable Precursor

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Fig. 1a: The seismotectonic map of Arakan Yoma sector in the Burmese Arc withearthquake data 1964-2008 [Data Source: 1964- 2004(ISC); 2004-2008 (NEIC)].Black stars are historical earthquakes (ABB source, 1897 – 1963) with magnitude(Ms e”7.0). CB – Central Basin, FMB – Fold Mountain Belt, Im – Imphal,Ti – Tiddim, Ga – Gangaw, Sa – Sagaing.

Indo-Myanmar Ranges in the Tectonic Framework of the Himalaya and Southeast Asia

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Fig. 1b: Seismic cross section along the line A – A’ of figure 1a showing the platemargins of Indian and Burma plate. Note the occurrences of four main-shocks,6.1(17.10.1969), 6.6 (6.8.1998), 6.1 (9.1.1990) and 6.3 (6.5.1995) along the platemargin.

Sagaing right lateral leaky transform towards east. Surface geology and physiographybroadly define three tectonic units here: Burmese Fold Mountain Belt (FMB), CentralBasin (CB) and Shan Plateau. A correlation between physiographic unit andmorphotectonic units will lead to a better understanding of tectonics of this region.The broad morphotectonic units recognized in this mobile belt from west to east are(Fig. 1a):

(i) The outer ridge (~ FMB) representing the Palaeogene Subduction –accretion complex where several dismembered ophiolite bodies andexotic blocks of metamorphics and Mesozoic rocks occur along theseaward flanks of the fore-arc trough.

(ii) Well-developed fore-arc basin (CB) in the Chindwin valley.(iii) The volcanic (magmatic) arc extends from Jade mines in the north –

Monywa – Mt. Popa towards south.(iv) A narrow linear faulted back arc basin (Shan Sagaing fault)

Details of subduction and accretion process in BA remain poorly studied:some workers, including us, have advocated for active subduction (Mukhopadhyayand Dasgupta 1988; Satyabala 1998; Dasgupta et al. 2003), still others argue forslow or no subduction at all (Le Dain et al., 1984; Ni et al., 1989; Chen and Molnar,1990; Rao and Kumar, 1999). Whatever may be the rate of subduction, the lithosphericplates in BA accumulate huge residual strain from the onset of subduction (earlymid-Eocene) which are released time to time by recurring shocks of variablemagnitudes extending in focal depths down to 200 km (Figs. 1a and 1b). The eventsare clearly more localized between the Eastern Boundary Thrust and the BurmeseVolcanic Arc and is also reflected as a linear tract along the segment of Shan-Sagaingfault north of 23°N latitude. The segments of Shan-Sagaing fault south of 23°Nlatitude are presently aseismic. The concentration of deeper focus earthquakesbetween 23° N and 25° N latitudes is particularly interesting as it calls for lithospheric


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flexure, where, we find moderate size earthquakes have been generated in associationwith thrust or strike-slip faults (Dasgupta et al., 2000).

SEISMICITY PATTERN AND SEISMIC QUIESCENCE

We utilize ISC bulletin data (period: 1964-2004) and NEIC USGS data (period:2005 – June 2008) for the present study. Considering the fact that monitoring capabilityfor smaller earthquakes (below magnitude 4.0) for this remote region for the datasampling period is rather inadequate, we consider data above this thresholdmagnitude for the analysis presented here. During the time period, the area produceda major shock of 6.6 magnitudes on 6.8.1988. The area had also spawned three majorshocks of comparable magnitude: 14.8.1932 (Ms 7.0), 16.8.1938 (Ms 7.2) and 12.9.1946(Ms 7.8) during last 100 years, thereby attesting to its seismic potentiality. Bar diagramof sequential magnitude occurrences over the years (Fig. 2) and number of earthquakesagainst years (Fig. 3) clearly reveal the high and low periods of earthquakeoccurrences. Prior to 1978 earthquake occurrence is less in number that suddenlyincreased subsequently, but part of it must be ascribed to an increase in stationcoverage in this time. Magnitude occurrence plot (Fig. 2) shows the existence of fivemainshocks above 6.0 magnitude and the corresponding seismicity pattern (Fig.1a). This is evaluated with reference to the five major earthquakes in BA [17.10.1969(6.1), 6.8.1988 (6.6), 9.1.1990 (6.1), 5.1.1991 (6.1), 6.5.1995 (6.3)].

Fig. 2: Plot of magnitude of earthquakes against year.

For any meaningful evaluation between seismic cycle and quiescence, it isconsidered relevant to have a quick reference to the theoretical background on seismiccycle and underlying quiescence model widely discussed in literature. Seismicquiescence is defined as periods where seismicity rate decreases to levels significantlybelow the normal seismicity rate. Scholz (1988) divided the seismic cycle of an activeseismic domain into alternate domains of seismic quiescence (Q1, Q2 and Q3) andactive seismicity. ‘Q1’ is defined as quiescence period between a majorshock –aftershock sequence and a further increase in background seismicity. ‘Q2’ is definedas a lull period between the previously defined increase in background seismicityand further renewed seismicity. ‘Q3’ is defined as a short time respite of seismicityprior to a main shock – aftershock sequence.

The seismicity pattern in BA is evaluated for the period 1969 to June 2008,with particular reference to four major earthquakes of respective focal depth > 100km: 17.10.1969 (6.1) depth 124 km, 6.8.1988 (6.6) depth 108 km, 9.1.1990 depth 118


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Table 1. Earthquake Statistics in the Study Area (22°- 26°N: 93°- 96°E).

Year No. of Mmax Seismic b-value Observation/ Analysisevents occurrencemb>= rate4.0 (events/yr)

1969 7 6.1 7 - MS 17.10.1969 (6.1) 1970 4 5.1 1971 7 5.6 1972 7 4.8 1973 8 5.8 7.13 0.63 Post Seismic quiescence: Q1 of Scholz 1974 12 4.9 (1988) 1975 9 5.3 1976 7 4.9 1977 3 5.2 1978 23 5.1 1979 20 5.2 1980 13 5.2 1981 19 5.7 1982 20 5.0 19.85 0.72 Increase in background seismicity 1983 20 5.7 (Scholz, 1988) 1984 24 5.7 1985 12 4.8 Intermediate term quiescence for 3 1986 13 5.2 14.33 0.86 years: Q2 of Scholz (19880 with 1987 18 5.7 comparative low seismicity 1988 31 6.6 31 0.73 Q3 of 17 days prior to MS {6.8.1988

(6.6)}. Increase in seismicity due toFS +MS +AS

1989 23 5.4 Unusual increase in seismicity due to 1990 27 6.1 presence of three Mainshock and 1991 30 6.1 corresponding aftershocks sequences 1992 30 5.8 which elevated the release in strain 1993 29 5.3 Q1, Q2 or Q3 not so prominent prior to 1994 20 5.9 the MS 1995 32 6.3 1996 33 4.9 The MS sequences are 1997 22 5.3 i) 9.1.1990 (6.1); Q3 8 days 1998 24 5.2 26.23 1.08 ii) 5.1.1991 (6.1); Q3 1 days 1999 27 5.3 iii) 6.5.1995 (6.30; Q3 1 days 2000 21 5.4 2001 23 5.2 2002 13 4.7 13 1.45 Post Seismic quiscence: Q1 of Scholz 2003 25 4.8 30 1.45 Increase in background seismicity 2004 35 4.8 2005 19 5.7 19 0.86 Intermediate term quiescence for 1 year:

Q2 of Scholz (1988) 2006 24 5.7 22.8 1.08 Increase in background seismicity 2007 23 5.1 Quiescence (Q3) of 1 month sets in Jan- 10 4.5 after 21.5.2008 (4.4) event-indicating June a big event ~M>6 is impending in near 2008 future.

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Fig. 3: Plot of number of earthquakes against year. Occurrence of maximummagnitude in a year placed as label.

km and 6.5.1995 (6.3) depth 120 km. All four earthquakes occurred at the coupledmargin of the Indian and Burma plates (Fig. 1b). The earthquake data from 1969 to margin of the Indian and Burma plates (Fig. 1b). The earthquake data from 1969 to2008 with number of earthquakes and Mmax for the respective year are listed (Table I).The mainshock of 17.10.1969 (6.1) with no identified foreshock and one aftershockat 29.10.1969 (4.7) is followed by a post seismic quiescence (Q1 of Scholz 1988) from1970 to 1977 with a low seismic occurrence rate of 7.13 events / year. This lowseismic occurrence rate indicates a stress relaxation after a large earthquake. Adistinct increase in background seismicity (18.85 events / year) is clearly noticedbetween 1978 – 1984, that was followed by intermediate term quiescence (Q2, 14.33events / year) during 1985-1987. Analyses of the data for the year 1988 indicate fiveforeshocks and a Q3 quiescence of 17 days prior to the megaevent of 6.6.1988 (6.6).The downward migration of seismic activity (Mogi, 1988) upto a depth of 129 kmprior to the main event at 108 km depth is also noticed. This event is followed by 17aftershocks in the ruptured area which increases the overall seismic occurrence rateto 31 events / year. The event of 6.8.1988 (6.6) has increased the seismic propensityto such an extent that the following years between 1989 and 2001, the area witnesseda huge increase in seismicity (26.23 events / year) and occurrence of three majorevents [9.1.1990 (6.1), 5.1.1991 (6.1), and 6.5.1995 (6.3)]. The 1990 and 1995 eventsare on the plate boundary and the 1991 event is located on Shan – Sagaing fault.Noticeably the precursory quiescence prior to these events is either absent orcamouflaged by the increase in background seismicity. Seismicity pattern in thefollowing years (2002 to June 2008) is quite interesting in the light of the precursorstudy. The precursory cycle starts with ‘Q1’ (2002, 13 events / year), increase inbackground seismicity (2003-2004, 30 events / year), ‘Q2’ (2005, 19 events / year)and increase in background seismicity (2006 – 21st May 2008, 22.8 events / year).The last event recorded is a 4.4 magnitude event in 21.05.2008 with a depthpenetration to 133 km. The non-occurrence of any event after 21.5.2008 up to the end


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of June 2008 may be considered as ‘Q3’ quiescence of Scholz (1988). We speculatethat this may follow an impending mainshock-aftershock sequence.

SEISMICITY RATE

Seismicity rate is defined as a ratio between the numbers of earthquakesoccurring against a specific time frame. High asperity along fault generally representshigh seismic rate (Wyss et al., 1984) and vice versa. Seismicity rate also consideredas precursor where low seismicity rate has been encountered by investigators priorto a large earthquake (Ogata, 2003) like Kalapana Hawaii earthquake of November1975 and the Irpinia earthquake in south Italy (see cross references in Wyss et al.,1998). The seismic rate given by the number of events / year is calculated for differenttime intervals between 1969 to 2008 (Table I) and is plotted on Fig. 4.

Fig. 4: Plot of seismic rate against year. Note the magnitude of mainshocks as label.

The highest seismic rates observed are in 1988, 1989-2001 and 2003-2004.Prior to 6.8.1988 (6.6) event the low seismicity rate 14.33 events/year in 1985-1987compared to 31 in 1988 indicates a precursor. The increase in seismicity rate during1988 is mainly due to the presence of aftershocks. The increase in seismic rate in2003-2004 is due to presence of many small events ~ magnitude 4.0. Significantly

Fig. 5: Time-dependence of b-value recognized for the Burmese Arc since 1970. Notethe magnitude of mainshocks as labeled.


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this seismic rate is rather low for the most recent past 2006-08 (only 22.8 events /year, Fig. 5), which may indicate a precursor prior to a major event.

b-VALUE ESTIMATES

The earthquake size distribution [frequency-magnitude distribution, FMD(Gutenberg and Richter, 1944)] follows the well-known power law, designated as b-value that is commonly used to designate the relative occurrences of large and smallearthquake events (Schorlemmer et al., 2005). The b-value in the present study hasbeen estimated by maximum-likelihood method using the equation (Aki, 1965; Ustu,1965; Bender, 1983), b = loge / (M – Mmin), where M denotes the mean magnitude andMmin the minimum magnitude of the given sample. The parameter ‘b’ depends on theeffective stress regime and tectonic character of the region (Hatzidimitriou et al.,1985; Tsapanos, 1990). Decreasing ‘b’ within the seismogenic volume correlateswith increasing effective stress levels prior to major shocks (Kanamori, 1981) or anincrease in applied shear stress / effective stress (Urbancic, et al., 1992). Severalstudies have been carried out to examine the potential of temporal changes in b-value as a short-term, medium-term and long-term earthquake precursor. Resultsshow that large earthquakes are often preceded by a medium-term increase in ‘b’,followed by a decrease in the weeks-months before the earthquake (Sammonds et al.,1992). Schorlemmer et al. (2005) demonstrated that b-value varies systematically fordifferent styles of faulting. Normal faulting is associated with the highest b-values(~1.2); strike-slip events show intermediate values (~1.0) and thrust events the lowestvalues (~0.90) from global catalogues. Thus ‘b’ acts as a ‘stressmeter’ in earth crustand depend inversely on the applied differential stress. Smith (1986) observed thatthe Cape Campbell earthquake of January 1977 was preceded by and located closeto a high b-value. In our previous study in search of precursor of great 26th December2004 Sumatra earthquake (9.3 Mw) in Sumatra – Great Nocober region, we haveestablished a temporal low in b-value (0.68-0.8) prior to the thrust related mega-events of 2.11.2002 (M 7.6) and 26.12.2004 (M 9.3) (Dasgupta et al., 2007) whichcorroborated well with the observed and experimental data of Schorlemmer et al.(2005).

The b-value calculated by equation 2 for different time period from 1970 to2008 (Table I) is plotted in a graph (Fig. 5). The b-value raises from 0.63-0.72-0.86prior to the 6.8.1988 (6.6) thrust type of earthquake (Table 2) corroborates well withthe values given by Schorlemmer et al. (2005) for thrust earthquakes. A slight increasein b-value to 1.08 during 1989 – 2001 takes place due to presence of three majorshock [9.1.1990 (6.1), 5.1.1991 (6.1), 6.5.1995 (6.3)] and aftershock sequences. Thesethree earthquakes either exhibit strike slip or a combination of strike slip and thrustmovement (Table 2). Furthermore, the b value of 1.08 corroborates well with theobserved and experimental data of Schorlemmer et al. (2005) for strike slipearthquakes. Major increases in b-value 1.45 during 2002 – 2004 indicates only arise in the background seismicity, with increase in smaller events (~ 4.0 Mb) andexhibit a form of intermediate time recovery precursor by strain softening (Meredithet al., 1990). This high b-value also indicates either or in combination of the followingphenomena - increase in stress intensity and crack density, creep along fault planes,decrease of effective stress due to shear slip on subsidiary fault and changes in porefluid pressure. Presently (2006-2008) the b value is 1.08. This drop in b-value


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

* Following Aki-Richards convention on slip

corresponds to a reduction in stress intensity, strain hardening prior to a criticalrupture by precursory slip behind an accelerating crack front and sudden stressdrop (Meredith et al., 1990). This intermediate b-value probably indicates animpending earthquake, with either strike slip or thrust slip, having appreciablestrike slip movement in north part of BA.

CONCLUSIONS

The seismic pattern in BA studied here using data from 1969 through June2008 brings out several salient characters of plate margin seismicity. Since theearthquake of magnitude 6.1 which occurred on 17.10.1969, a complete seismiccycle of 31 years with four mainshock events [6.8.1988 (6.6), 9.1.1990 (6.1), 5.1.1991(6.1), and 6.5.1995 (6.3)] with different phases of quiescence (Q1, Q2 and Q3) andactive seismicity has been identified. The latest Q3 quiescence from 21.5.2008 up tothe end of June 2008 may yield an impending event of 6.0 M or even greater. Theseismic rate changes and temporal b-value data indicate that the impending eventmay be with strike slip or thrust with appreciable strike slip movement along faultplane. The exact size and timing of the event cannot however be deciphered from theteleseismic data alone; this would require data from local seismic network as well.


Date 6.8.1988 9.1.1990 5.1.1991 6.5.1995Hr:Mn:Sec 00:36:38 18:51:36 18:51:36 01:59:14Latitude 25.2 24.4 23.6 24.8Longitude 94.89 94.95 96.18 95.02Mb 6.6 6.1 6.1 6.3Mw 7.3 6.3 7 6.4T axis Plunge 65 58 25 69T axis Azimuth 117 142 321 94P axis Plunge 5 20 6 9P axis Azimuth 217 16 228 209Focal Plane 1 strike 284 140 2 278Focal Plane 1 dip 45 32 68 39Focal Plane 1 Rake 55 139 166 60Focal Plane 2 strike 148 267 97 135Focal Plane 2 dip 54 69 77 57Focal Plane 2 rake 120 64 23 112Type of fault movement* Thrust Thrust with Strike slip Thrust

appreciablestrike slipcomponent

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Next Impending Earthquake in Northern Burmese Arc - Search for A Probable Precursor

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