Geophys. J. Int. (2004) 158, 915–921 doi: 10.1111/j.1365-246X.2004.01828.x

GJI

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Precursory changes in source parameters for the Koyna–Warna(India) earthquakes

Richa Jain, B. K. Rastogi and C. S. P. SarmaNational Geophysical Research Institute, Hyderabad 500 007, India

Accepted 2002 July 22. Received 2002 July 22; in original form 2001 March 7

S U M M A R YPrecursory changes of stress drop and corner frequency are found for five main earthquakesof M w 4.1 to 4.7 that occurred near the Koyna and Warna reservoirs during the period of ourclose monitoring from 1994 onwards. Earthquakes had started to occur in the area in 1962soon after the impoundment of the Koyna reservoir and are still continuing. The epicentresare located in a 30-km-long, N–S seismic zone extending southwards from the Koyna dam.A new Warna reservoir, situated 25 km south of Koyna, was filled to 60 m in 1992. Mostsubsequent epicentres are close to the new Warna reservoir. Most M ≥ 4 earthquakes in thisregion have been associated with foreshocks for 15–30 days and aftershocks for over a month.At the beginning of various foreshock sequences of M w 1.5 to 2.4, the maximum stress dropvalues ranged from 0.25 to 0.65 MPa; the corner frequency ranged from 7.0 to 10.0 Hz for allthe sequences. The stress drops decreased by over 50 per cent of their maximum value duringthe precursory period of 4–17 days prior to main shocks and remained at that low level for afew days after the main shocks. Following an initial increase, the corner frequency decreasedby 8 to 45 per cent. The decrease in corner frequency implies a 9–54 per cent increase in faultlength and is inferred to be caused by increased pore pressure as a result of dilatancy.

Key words: foreshocks, precursory changes, source parameters.

1 I N T RO D U C T I O N

Earthquake prediction research has become stagnant as a result ofexcessive criticism. However, in certain small, well-instrumented ar-eas with frequent shallow–moderate earthquakes (such as near largedams where reservoir filling influences the ambient stress regime),it may be possible to observe precursors. The area near the Koynaand Warna reservoirs in India is one such place. Rastogi & Mandal(1999) observed the nucleation process 100 hrs prior to moder-ate earthquakes in the Koyna region. Twenty-five boreholes weredrilled to approximately 200 m depth around the seismic zone tostudy pore pressure. These have shown precursory and coseismicchanges in pore pressure (Chadha et al. 1997, 2003; Gupta et al.1997). Here, we report a significant observation of changes in thespectral properties of foreshocks, implying changes in source pa-rameters. Various precursors have been studied by many investiga-tors for earthquake prediction, including Rhoades & Evison (1979),Kanamori (1981) and Wyss & Habermann (1988). Evison &Rhoades (1993, 1997, 1999) carried out studies of swarms and foundprecursors associated with major earthquakes in New Zealand andJapan. Ogata (1992) studied precursory relative quiescence time asa means of predicting earthquakes. Mogi (1990) studied the space–time distribution of seismicity. Precursory changes in source pa-rameters were reported by Urbancic & Young (1993) and Urbancic(1996) before and after large mine-induced earthquakes. Along the

same lines, we observed precursory changes in source parameters(corner frequency and stress drop) for Koyna main shocks of M w ≥4 that occurred during the 1994 to 1998 period of our close moni-toring. From 40 yr of observation of Koyna–Warna main shocks ofM w ≥ 4, it is seen that the main shocks in the region are preceded bya period of increased occurrence of small earthquakes for approx-imately 2–4 weeks (Rastogi et al. 1997; Rastogi & Mandal 1999).It is seen that, for most of the M ≥ 4 Koyna–Warna earthquakes,foreshocks cluster in the vicinity of the main shock. Source parame-ters such as corner frequency, stress drop and fault radius have beenstudied for five sequences in order to investigate any precursorychanges.

2 S E I S M O T E C T O N I C S O F T H EKOY N A – WA R N A R E G I O N

The Koyna and Warna dams are situated near the west coast ofIndia in the Deccan volcanic province that covers one-third of thePeninsular shield and is known as the Deccan traps terrain. Thevolcanics are 1 to 2 km thick and are probably underlain by Archeangranite and gneisses. These basalts are of Palaeocene age (65 Ma)with numerous fractures and tensional joints. The Deccan trap rockshave low permeability and the migration of water occurs throughfractures and faults.

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916 R. Jain, B. K. Rastogi and C. S. P. Sarma

Figure 1. Location of the Koyna–Warna region in the map of India (inset), reservoirs, continental divide and seismic stations.

The area has been seismically active since the start of the filling ofthe Koyna reservoir in 1962.Fig. 1 shows our study region markedin the map of India. No earthquake is known to have occurred within50 km of the Koyna dam prior to 1962. The Koyna earthquake of1967 December 10 (M 6.3) remains, globally, the largest recordedcase of a reservoir induced earthquake. The Koyna seismic zoneextends to approximately 30 km south from the Koyna dam. Im-pounding of the new Warna reservoir, 25 km south of Koyna, to alevel of over 60 m in 1992 led to enhanced seismic activity aroundthe new reservoir. To date, the region has witnessed 17 earthquakesof magnitude ≥5, more than 170 earthquakes of M ≥ 4 and manyearthquakes of smaller magnitude. The tectonic activity associatedwith these reservoirs, as revealed by the fault-plane solution of the1967 earthquake (M 6.3) and a composite fault-plane solution forthe 1995 events (Mandal et al. 1998) is mainly left-lateral strike-slipalong two parallel NNE-trending faults.

3 DATA A N D A N A LY S I S

With a dense network of 11 analogue and 5 digital stations in an areaof 50 × 50 km, the hypocentral locations and source parametersare well determined. Of these five digital stations, four wereequipped with three-component short-period seismometers (1 Hz)and one with a broad-band seismometer (0.01–20 Hz). All wereequipped with 16-bit recorders. Sampling rate was 100 samples persecond. Generally, the uncertainties in hypocentral locations esti-mated using the HYPO71PC program were less than 0.5 km hori-zontally and less than 1.0 km vertically.

During the study period from 1994 October through 1998 Febru-ary 7 earthquakes of magnitude M w ≥ 4.0 occurred. We excludedtwo earthquakes from our study because they are considered to beaftershocks (Rastogi & Mandal 1999). We considered foreshocksin the 2–4 week period. Foreshocks and aftershocks in the magni-tude range 1.5 to 2.4 only are considered, as the source parametersfor smaller shocks could not be reliably determined and some largershocks had a very high stress drop. Fig. 2 shows the epicentres of thefive selected main shocks and the epicentres of the entire sequences

(foreshock—main shock—aftershock) along with the locations ofthe digital stations. The main shocks of 1994 October 31 (M w 4.1),1995 March 12 (M w 4.7), 1996 April 26 (M w 4.4) and 1998 Febru-ary 14 (M w 4.2) lie in the Warna area while the 1997 April 25(M w 4.4) main shock lies in the Koyna area. Most foreshocks areclustered within 10 km or so of the respective main shock. Thefew foreshocks lying outside the cluster were excluded from thisprecursor study. For each main shock, between 13 and 40 foreshockswere selected during the 2–4 week period, as shown in Table 1.

The source parameters such as seismic moment, stress drop, cor-ner frequency and fault length are determined from the Fourier spec-trum of S-wave ground displacement, as described by Hanks (1977).The horizontal component seismograms are rotated to construct ra-dial and transverse components. A time window of 2.56 s is con-sidered on the S-wave of the transverse components. To minimizetruncation effects, a cosine taper of 10 per cent is applied to bothends of the window. The S-wave spectrum is corrected for attenua-tion, taking Qo = 169 as obtained by Mandal & Rastogi (1998). Thespectrum of the S-wave is corrected for instrument response so thatit represents displacement. On the S-wave spectra, a constant ampli-tude level �O (the long-period spectral level in m s) is determinedat low frequencies and the corner frequency, f c, is identified. Thesource parameters such as seismic moment, M o (in N m), sourceradius, r (in m) and static stress drop, �σ (in MPa), are estimatedwith the following relations:

Mo = 4πρV 3s R�O/F Rθϕ, (1)

r = 2.34Vs/2π fc, (2)

�σ = (7Mo/16r 3)10−6, (3)

where ρ is the density of the source material (2700 kg m−3), Ris the epicentral distance in m, V s is the S-wave velocity (3.8 ×103 m s−1), F is the free surface correction factor (F = 2) and R θϕ

is a mean radiation factor (0.63).Fig. 3 shows the temporal change in the distribution of corner

frequency and stress drop for each of the five selected main shocks.

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Figure 2. Epicentres of the five main shocks (stars) in the Koyna–Warna region. Locations of the digital seismic stations (triangles; maximum of five sitesoccupied at a time) and epicentres of foreshocks and aftershocks (circles) for the five sequences in the Koyna and Warna reservoir areas. Many epicentres thatwere touching or lying under the main shock epicentres are not plotted. Similarly, the overlapping epicentres in the cluster areas are not shown.

Table 1. Details of Mw ≥ 4.0 earthquakes in the Koyna–Warna region during 1994 October to 1998 February and precursorychanges in source parameters.

D M Y M w h NF NS f max f min Df σ max σ min Ds

311094 4.1 7.0 25 14 7.4 6.8 6 0.65 0.06 6120395 4.7 8.7 40 23 10.0 5.5 22 0.35 0.10 17260496 4.4 8.6 56 40 9.1 8.0 4 0.25 0.10 8250497 4.4 5.1 32 20 7.0 5.6 3 0.26 0.08 5140298 4.2 9.6 20 13 8.5 7.1 3 0.50 0.36 4

The location of the 1997 earthquake is near the Koyna reservoir. All others are near the Warna reservoir.Moment magnitude (M w); depth (h); number of foreshocks (NF); number of foreshocks selected for precursory study (NS;occurring within tight clusters); maximum corner frequency ( f max); minimum corner frequency ( f min); days prior to mainshock when decline in corner frequency is observed (Df); maximum stress drop (σ max in MPa); minimum stress drop (σ min inMPa); days prior to the main shock when decline in stress drop is observed (Ds).

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Figure 3. (a) and (b) Variation in corner frequency (Hz) and stress drop (MPa) with time (days) for the five sequences. The bold vertical arrow shows the timeof the main shock and the light vertical arrow shows the maximum value of the stress drop or corner frequency.

The bold arrow marks the date of occurrence of the main shock onthe horizontal axis that represents days. The thin arrow marks thepoint of inversion where the corner frequency or stress drop valuesstart to reduce. For the 1994 October 31 main shock, the corner fre-quency decreases from 7.4 to 6.8 Hz during the 6-day period priorto the main shock and remains at the same level for 3 days. After-wards, no distinct trend is seen for the aftershocks. The stress dropvalue drops to 0.06 MPa from a maximum value of 0.65 MPa (a 91per cent decrease) in 6 days. The main shock follows next day. Forthe next shock of 1995 March 12 of M w 4.7, the corner frequencyfirst increases to 10.0 Hz over 10 days then decreases to 6.5 Hz over22 days prior to the main earthquake and remains at the same levelfor 2 days after the main shock. For this earthquake, the foreshocksare considered over a 4-week period starting 1995 February 10. Dur-ing this period, the average stress drop is approximately 0.5 MPa.This average value decreases from 0.5 MPa at 25 days prior to mainshock, to 0.1 MPa 7 days prior to the main shock (an 80 per centdecrease). The stress drop envelope remains at a low level for 2days after the main shock and returns to the level of 0.5 MPa 1.5months later (not shown in the figure). However, in the present study

the single value of 0.5 MPa on the 2 day is treated as an outlier togive a more consistent result. Neglecting this outlier, the maximumvalue of 0.35 MPa (marked by the thin arrow) shows a substantialdrop of 71 per cent 17 days prior to the main shock of 1995 March12. The earthquake of 1996 April 26, M w 4.4, shows a decreasein the value of the corner frequency 4 days before the main shock.The corner frequency value rises from 8 to 9.1 Hz initially thendrops to 8 Hz. The stress drop starts decreasing 8 days prior to themain shock. From a value of 0.25 MPa, it goes down to 0.1 MPa (a60 per cent decrease) prior to the main shock. We again neglect theextreme point to give a consistent trend. For the main shock of 1997April 25, M w 4.4, the level of corner frequency drops from 7.0 to5.6 Hz during 3 days prior to the main shock. The stress drop valuedecreases by 73 per cent during the 5-day period just before themain shock. In this case, again one extreme point is treated as anoutlier. After the main earthquake, the stress drop value remains atthe level of approximately 0.1–0.05 MPa for 4 days. For the 1998February 14 main shock, the corner frequency value falls from 8.5to 7.1 Hz 3 days prior to the main shock. There is a 30 per centfall in stress drop value in 4 days. During the first few days, both

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Figure 3. (Continued.)

0.005.0010.0015.0020.0025.00DAYS

120.00

160.00

200.00

240.00

280.00

SO

UR

CE

RA

DIU

S (

m)

1994

1995

1996

1997

1998

Figure 4. Variation in source radius is plotted for all the five sequences (1994, 1995, 1996, 1997, 1998) as a function of time. The zero day marked by thearrow on the time axis indicates the day of occurrence of the main shock. Symbols plotted on each radius curve correspond to maximum, intermediate andminimum value on the corner frequency plot in Fig. 3.

corner frequency and stress drop fluctuate until they attain a max-imum value a few days before the main shock and then drop sub-stantially from the peak value just before the occurrence of the mainshock.

The temporal changes in source radii (half of the fault length) forthese earthquake sequences are shown in Fig. 4. For these source ra-dius curves, the corner frequencies in Fig. 3 were converted to radiiusing eq. (2). Zero on the time axis indicates the time of occurrence

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920 R. Jain, B. K. Rastogi and C. S. P. Sarma

of the main shock. The maximum value of the corner frequency inFig. 3 (shown by the vertical arrow) is taken as the first value foreach source radius curve (shown using different symbols for eachof the five sequences). The last point (close to the day of the mainshock) in Fig. 4 corresponds to the observed minimum frequencyvalue down to which corner frequency falls before the main shock inFig. 3. Intermediate points on the source radius curve correspond tothe intermediate values of the corner frequency between the max-imum and minimum values before the main shock in Fig. 3. Asexpected, Fig. 4 shows that there is a 9–54 per cent increase in faultlength.

4 D I S C U S S I O N

The Koyna main shocks of M w ≥ 4 are preceded by a number of fore-shocks (>15) for 2 to 4 weeks and a larger number of aftershocksfor approximately a month or more afterwards, corresponding toMogi’s type II pattern. Estimated values of M o, r and �σ are com-patible with similar observations from other regions of the world(Mandal et al. 1998). The source radii range from 140 to 253 m,while the seismic moment values range from 1012 to 1014 N m. Thestress drop values vary from 0.06 to 0.6 MPa for foreshocks andaftershocks of magnitude ≤2.4. For the five selected earthquakeshaving magnitude higher than 4, there is a 28–91 per cent drop inthe level of stress drop 4–17 days prior to the main shocks. The cor-ner frequency falls by 8–45 per cent. The largest precursory periodof 17 days is for the largest main shock of magnitude 4.7.

According to the dilatancy model, increased pore pressure willlubricate the fault and in turn fault length (2r) is increased as seenin Fig. 4. There is 9–54 per cent increase in fault length associatedwith a decrease of corner frequency over most of the anomalousperiod. Initially, some of the corner frequencies are observed to below with a corresponding lower source radius. If these earthquakesoccur in some weak zone with lower coefficient of friction, we mightanticipate that the fault length would be increased. The fall in thestress drop envelope is not the result of changes in foreshock loca-tions as the foreshocks occur in tight clusters. This study is basedon the variation in corner frequency and stress drop averaged overa number of foreshocks and aftershocks for each day, and hence isindependent of the number of events in foreshock and aftershockactivity. Also, the stress drop envelopes are seen to be independentof small depth variation from near surface to approximately 10 km(Fig. 5).

5 C O N C L U S I O N S

Koyna earthquakes of M ≥ 4 during 1994 to 1998 are preceded by2–4 weeks of foreshock activity. The maximum values of cornerfrequency and stress drop decrease for 3–22 days prior to the mainshocks. The corner frequency envelope drops by 8 to 45 per cent.The stress drop parameter decreases by 28 to 91 per cent. The mainshocks follow within a few days of at least 50 per cent decrease.The decrease in stress drop is associated with an apparent increasein fault lengths by 9 to 54 per cent. This may be the result of increasedpore pressure and dilatancy phenomena.

A C K N O W L E D G M E N T S

The Department of Science & Technology, New Delhi and Depart-ment of Space supported this study. Suggestions by the referees and

Figure 5. Depth of foreshocks for the five sequences located within 10 kmradius of the main shock. Big star represents the main shock.

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editorial corrections considerably improved the manuscript. The au-thors are thankful to these departments, the referees and the editorof this journal.

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