Evidence for Stability in Coda Q Associated with the Egion (Central Greece) Earthquake of 15 June 1995

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1 Bulletin f the Seismlgical Sciety f America, Vl. 87, N. 6, pp , December 1997 Evidence fr Stability in Cda Q Assciated with the Egin (Central Greece) Earthquake f 15 June 1995 by G-Akis Tselentis Abstract One hundred seventy-tw seismgrams, recrded at ne seismlgical statin lcated clse t the surce regin f the Egin (Ms = 6.2) 15 June 1995 earthquake, were used t measure values f Q frm the decay f the earthquake cda. The cllected data were cmpared between events that ccurred befre and after the main event. The analysis shwed n significant tempral variatin in the value f cda Q fr the regin. Intrductin Varius bservatins in different regins f the wrld have indicated a striking difference in the rate f amplitude decay between tectnically active and static regins. The decay rate f cda amplitude is in general a very stable parameter cmmn t all lcal earthquakes lcated near a given statin and is insensitive t surce and receiver lcatins within the regin, reflecting the average prperties f the crust. Many researchers have reprted tempral changes f the cda attenuatin parameter Q~- 1 befre and after large earthquakes. Table 1 depicts sme f these bservatins. Recently, Tselentis (1993) reprted a characteristic decrease in Q~-I after tw medium earthquakes in central Greece. This stimulated ur interest fr a mre detailed investigatin f the relatin between seismic attenuatin and seismicity in the tectnically active regin f central Greece. In this regin, the seismlgical labratry f Patras University has established a permanent micrearthquake netwrk, and in additin t the seismic activity, it als mnitrs cntinuusly a range f physical parameters, well knwn as earthquake precursrs (Tselenfis and Ifantis, 1996), such as the Earth's electric field, electrmagnetic anmalies, grundwater temperature, and chemistry and acustic emissin. A large earthquake f magnitude 6.2 ML ccurred n 15 June 1995 at :15 GMT in the western end f the Gulf f Crinth (Fig. 1) within the eastern regin cvered by the netwrk. This earthquake was fllwed 15 minutes later by a 5.4 M L aftershck and a rich aftershck sequence. This large bdy f data prvides a unique pprtunity t examine the tempral variatin f seismic attenuatin befre and after the mainsbck. Table 1 Observed Tempral Changes in Seismic Attenuatin (Q-t) Assciated with Earthquake Activity Authr Regin Variatin f Q 1 Wyss (1985) Gusev and Lemzikv (1985) Nvel et al (1985) Tsukuda (1985) Sat (1986) Jin and Aki (1986) Jin and Aki (1986) Wang et al. (1989) Lee et al (1986) Scherbaum and Kisslinger (1985) Rbinsn (1987) Peng et al. (1987) Jin and Aki (1989) Tselentis (1993) Aster et al. (1996) Antlik et al. (1996) Hawaii Kuril-Kamchatka Mexic Japan Central Japan China China E. Talwan and Nevada Aleutian New Zealand Central Greece 3% increase befre Sngpan 7.2 earthquake 2% increase befre three 8 earthquakes 3% increase befre ne 7.6 earthquake 15% increase befre ne 6.2 earthquake High Q- x befre ne 6. earthquake 3% increase befre Tangshan 7.8 earthquake 3% increase befre Halcheng 7.3 earthquake Increase after ne 6.1 earthquake Variatin in Qc befre large earthquakes N variatin Qc decreased when the number f all events relatively t the large increased Increase after ne 5.7 earthquake Qc increased when the number f small events relatively t the large increased Decrease after tw earthquakes N variatin N variatin 1679

168 Shrt Ntes [~ M=?. C) 11--6. O M=5. C) M=4. 2 krn IE]A (9 Depth(km) <5 5 <AND < 15 15 ~" 21~5 21.7 21.9 22.1 22:3 22.5 t 38,~ 38~3 S ~ 381 2!.5 2i.? ;~L9 22. i 2;L3-38.1 22.5 15/6/95-1/7/95 O- A At E~i ~ Egi qoit ~ ~ 2 ~ 4 4 l O 4~O 8~O N/21.

2 168 Shrt Ntes [~ M=?. C) O M=5. C) M=4. 2 krn IE]A (9 Depth(km) <5 5 <AND < ~" 21~ : t 38,~ 38~3 S ~ 381 2!.5 2i.? ;~L9 22. i 2;L /6/95-1/7/95 O- A At E~i ~ Egi qoit ~ ~ 2 ~ 4 4 l O 4~O 8~O N/21.7 E twards 38.l N/22.5 E ~W-ESE (K_m) I b " I I I " C 38.1 N/21.9 E twards N/22.3 E b SW-NE (Km) Figure 1. (a) Spatial distributin f the best lcated aftershcks during the first 17 days f the aftershck sequence and fcal mechanism f the main event, (b) AA' SW- NE and (c) BB' WNW-ESE crss sectin. The big star dentes the main event (Tselentis et al., 1996), Shadw area crrespnds t the uter circle f Figure 2. Data The present investigatin is based n data recrded by statin NAY (Fig. 1) f the Patras University seismlgical netwrk (PATNET). This statin is lcated within the western part f the aftershck regin and was perating at the time f the earthquake. It is equipped with a vertical-cmpnent 1-Hz seismmeter perating at 6-dB dynamic range in a lw-nise envirnment. The recrded signals are radi- linked using FM subcarriers t the central recrding site at Patras University, where they are antialias filtered with a 3- Hz Butterwrth lw-pass filter, sampled at 1 Hz and cnverted t digital frm with a reslutin f 32 bits. T minimize the effects f the surce functin and path prpagatin n the Q parameter, we have cnfined the events used in this study t be in the magnitude range f 2.5 < M L < 3. and within a cubic vlume apprximately 3 km n each side. The relative lcatin f the recrding statin and

3 Shrt Ntes 1681 a 38.7 I I I I I 38.7 Depth(kin) if) <5 5 s: AND < 2 2 if) M=3.O e M= * M=I.O M=O.O O 1 i i km 38.: I I I I I Events Scale 1: 6 b 38.7 I I I I 38.7 [] O D [] 2~ fi] [] m C~ []l~19 ff]~l,[ Depth(kin)< 5 5 K AND < 2 2 D M= [] ~t 38.3 M=2. * M=I.O M=O.O O 1 t J kin ( t 1 [ I [ 31].1 Figure Events Scale 1: 6 Epicentral distributin f the (a) aftershcks, (b) prir t the main event earthquakes used. Black symbls in (a) crrespnd t the events shwn in the insert f Figure 3. Star dentes the main event.

4 1682 Shrt Ntes a 4 ~ " ~ 2 -: 18 -: 1SO: 14 ; 12,- 1 : 8 : 6 : [ , i, i, ], i, i, i, O I~ 2 22 [ ' I ' I ' I ' I ' I ' I ' I ' I ' /2 \ / ~ O ~ \ ~8 8 / Q ~7, 5 I I ' I ' I ' I ' Mnths b s 4 3 O O " 2" 8 OO ' 1 I I I I I I Mnths Figure 3. Tempral variatin f cda Q befre and after the mainshck (a) fr 4 Hz and (b) 8 Hz central frequencies. Insert in (a) depicts the crrespnding cda Q variatin within the first week after the mainshck. Black triangles dente mnthly averages. all surce pints make the crrespnding ellipsidal sampling vlumes (Pulli, 1984) largely verlapping, and thus the btained Q-a values shuld be clsely related t attenuatin f the medium clse t the surce regin. The mainshck, as lcated by PATNET, is placed at 3818'54 " N, 22 8'46 " E and at a depth f 12.8 kin. A large number f aftershcks fllwed, and the principal spatial characteristics f the best lcated aftershcks (Tselentis et al., 1996) are illustrated in Figure 1. The distributin f hy- pcenters in crss sectin des nt immediately suggest a planar distributin but rather defines a vlume abut 15 km (depth) by 35 km (NW-SE) and by 2 km (NE-SW). The abve regin defines the sampling vlume frm which cda Q infrmatin is sught; the narrw magnitude range cnfines the surce excitatin t an apprximately unifrm frequency band (Wang et al., 1989). Sixty-five earthquakes in the magnitude range 2.5 < Mr < 3. (Fig. 2a) that ccurred after the main event have been

Shrt Ntes 1683 selected fr deriving the cda Q parameter. In additin t the abve data set, we have als cllected 16 events in the same regin fr a perid f 17 mnths preceding the mainshck (Fig. 2b).

5 Shrt Ntes 1683 selected fr deriving the cda Q parameter. In additin t the abve data set, we have als cllected 16 events in the same regin fr a perid f 17 mnths preceding the mainshck (Fig. 2b). Nrmally, waves in the interval 2 t 3 sec after the S arrival have been used t determine the decay rate. Methd The cda Q values were btained as fllws (Wdgld, 1994). Three-ple Butterwrth filters are applied with center frequencies at 2, 4, 8, and 12 Hz. The envelpes f the filtered traces are averaged ver 5-sec sliding windws with the beginning f the windws 2 see apart. The envelpe is calculated frm Ae(t) = ~/A2(t) + 112(, (1) where A is the seismic amplitude at time t and H(t) the Hilbert transfrm f A(t). We als averaged envelpes in a nise windw, and the signal amplitude is estimated as As = ~/~r - A 2, (2) where A r is the averaged envelpe fr the windw and A N is the averaged envelpe fr the nise. Fllwing Wdgld (1994), the functin F(t) = lg1 As (3) is calculated, and fr each event-statin-frequency cmbinatin, a regressin line is fit t F(t), where t is the lapse time at the center f the 5-sec windw, a = tits, t s is the lapse time f s-wave arrival, and Finally, Qc is calculated as K"(a) =,/1 - (1/aU). (4) Qc = l gl ezcf/s, (5) where s is the slpe frm the regressin andfis the frequency in Hertz. Results Tempral variatins f cda Q values fr earthquakes ccurring between January 1994 and December 1995 and fr frequencies 4 and 8 Hz are depicted in Figures 3a and 3b, respectively. In the same diagram, we present als the crrespnding mnthly averages. Judging frm these diagrams, we cannt assess any significant change in Qc. The verall means f the measured cda Q are similar befre (Of=4Hz = 183, QI=SHz = 266) and after (Of=4Hz = 184, Qf=sm = 273) the main event. In a similar research, Wang et al. (1989) bserved a significant drp f cda Q immediately after the Hualien 1986 earthquake in E. Taiwan. This drp lasted apprximately tw days befre returning t ambient level. T investigate fr a similar phenmenn in ur case, we calculated the cda Q crrespnding t 3 aftershcks ccurring within 1 week f the main event. The results are als presented as an insert in Figure 3a. N drp in the value f Qc can be seen. Acknwledgments The authr wishes t thank Mike Antlik and Rick Aster fr their mst useful cmments. This paper frms a cntributin under NATO Cllabrative Research Grant References Antlik, M., R. M. Nadeau, R. C. Aster, and T. McEvilly (1996). Differential analysis f cda Q using micrearthquakes in seismic Gaps. Part 2: Applicatin t seismgrams recrded by the Parkfield high reslutin seismic netwrk, Bull. Seism. Sc. Am. 86, Aster, R. C., G. Slad, J. Hentn, and A. Antlik (1996). Differential analysis f cda Q using micrearthquakes in seismic gaps. Part 1: Techniques and applicatin t seismgrams recrded in the Anza seismic gap, Bull. Seism. Sc. Am. 86, Gusev, A. A. and V. K. Lemzikv (1985). Prperties f scattered elastic waves in the lithsphere f Kamchatka: parameters and tempral variatin, Tectnphysics 112, Jin, A. and K. Aki (1986). Tempral changes in cda Q befre the Tangshan earthquake f 1976 and the Haicheng earthquake f 1975, J. Gephys. Res. 91, Jin, A. and K. Aki (1989). Spatial and tempral crrelatin between cda Q-1 and seismicity and its physical mechanism, J. Gephys. Res. 94, Lee, W. H. K., K. Aki, B. Chuet, E Jhnsn, S. Marks, J. T. Newberry, A. S. Ryall, S. W. Stewart, and D. M. Tttingham (1986). A preliminary study f cda Q in and Nevada, guk Seism. Sc. Am. 76, Nvel-Casanva, D. A., E. Berg, V. Hsu, and C. E. Helsley (1985). Time space variatin seismic S-wave cda attenuatin (Q) and magnitude distributin fr the Pelatan earthquake, Gephys. Res. Lett. 12, Peng, J. Y., K. Aki, B. Chuet, E Jhnsn, W. H. K. Lee, S. Marks, J. T. Newberry, A. S. Ryall, S. W. Stewart, and D. M. Tttingham (1987). Tempral change in cda Q assciated with the Rund Valley,, earthquake f Nvember 23, 1984, J. Gephys. Res. 92, Pulli, J. J. (1984). Attenuatin f cda waves in New England, Bull Seism. Sc. Am. 74, Rbinsn, R. (1987). Tempral variatin in cda duratin f lcal earthquakes in the Wellingtn regin, New Zealand, Pageph 125, Sat, H. (1986). Tempral change in attenuatin intensity befre and after the eastern Yamanashi earthquake f 1983 in central Japan, J. Gephys. Res. 91, Scherbaum, E and C. Kisslinger (1985). Cda Q in the Adak seismic zne, Bull Seism. Sc. Am. 75, Tselentis, G.-A. (1993). Tempral variatin f bdy wave attenuatin as related t tw earthquake dublets in W. Greece, Pageph 14,

6 1684 Shrt Ntes Tselentis, G.-A. and A. Ifantis (1996). Geelectric variatins related t earthquakes bserved during a 3-year independent investigatin, Gephys. Res. Lett. 23, n ~ Tselentis, G.-A., N. S. Melis, E. Sks, and K. Papatsimpa (1996). The Egin June 15, 1995 (6.2 ME) Earthquake, W. Greece; Pageph 147, Tsukuda, T. (1985). Cda Q befre and after a medium-scale earthquake, paper presented at the 23rd General Assembly, Internatinal Assciatin f Seismlgy and Physics f the Earth's Interir, Tky, Japan. Wang, J. H., T. L. Teng, and K. E Ma (1989). Tempral variatin f cda Q during Hualien earthquake f 1986 in Eastern Talwan, Pageph 13, n. 4, Wdgld, C. R. D. (1994). Cda Q in the Charlevix, Quebec, regin: lapse-time dependence and spatial and tempral cmparisns, Bull. Seism. Sc. Am. 84, Wyss, M. (1985). Precursrs t large earthquakes, Earthquake Predict. Res, 3, Seismlgical Labratry University f Patras Ri 261 1, Greece Tselenti@upatras.gr Manus pt received 14 April 1997.

5.4 Measurement Sampling Rates for Daily Maximum and Minimum Temperatures

5.4 Measurement Sampling Rates fr Daily Maximum and Minimum Temperatures 1 1 2 X. Lin, K. G. Hubbard, and C. B. Baker University f Nebraska, Lincln, Nebraska 2 Natinal Climatic Data Center 1 1. INTRODUCTION