SEISMIC LOCATION CALIBRATION FOR 30 INTERNATIONAL MONITORING SYSTEM STATIONS IN EASTERN ASIA: FINAL RESULTS Pul G. Richrds, 1 John Armruster, 1 Vleriu Burlcu, 2 Vernon F. Cormier, 4 Mrk D. Fisk, 2 Vitly I. Khlturin, 1 Won-Young Kim, 1 Igor B. Morozov, 3 Elen A. Morozov, 3 Chndn K. Siki, 5 Dvid Schff, 1 Anstsi Stroujkov, 4 nd Felix Wldhuser 1 Lmont-Doherty Erth Oservtory of Columi University, 1 Mission Reserch Corportion, 2 University of Wyoming, 3 University of Connecticut, 4 URS Group 5 Sponsored y Defense Thret Reduction Agency Contrct No. DTRA01-00-C-0029, 4 DTRA01-00-C-0031, 1,2,3 DTRA01-00-C-0033 5 ABSTRACT We hve completed three-yer consortium effort to improve the cpility to locte seismic events sed on dt cquired y 30 Interntionl Monitoring System (IMS) sttions in Est Asi. We hve developed nd tested Source Specific Sttion Corrections (SSSCs) for Pn nd Sn trvel times t these 30 IMS sttions (or suitle surrogtes), nd for 127 other sttions used for vlidtion testing. The SSSCs were initilly computed y the method of Bondár (1999), using regionlized 1-D trvel-time curves estlished fter extensive review of pulished studies including mny from the Russin literture. Susequently we developed 3-D model of the P-wve velocity for Est Asi (using set of 36 different regions in ech of which we otined velocity s function of depth), nd used 3-D ry trcing in the ltter model to compute SSSCs. These model-sed SSSCs were refined empiriclly y pplying kriging lgorithm to trvel-time residuls for ground-truth (GT) events. Off-line vlidtion tests were performed y evluting trvel-time residuls nd y relocting GT events, with nd without using SSSCs. To test the vlidity of the model directly, reloction tests were first performed using model-sed SSSCs without kriging. Tests were then performed to evlute the kriged SSSCs, using leve-one-out pproch so tht events were not simultneously used to oth compute nd test the SSSCs. Nucler explosions dominted our ground-truth dtsets in the first two yers of this project. In prticulr we used source prmeters for Soviet-er Peceful Nucler Explosions (PNEs). But this pproch, while quite stisfctory for clirting sttions in much of Russi nd Centrl Asi (which mde up pproximtely hlf the IMS sttions we studied) could not e extended to the remining sttions, for which it ws necessry to develop GT informtion on significnt numers of erthqukes. By use of the doule-difference method nd detiled fult mps, we otined 64 GT5 erthqukes y re-nlyzing the Annul Bulletin of Chinese Erthqukes (ABCE) for 15-yer period (1985 to 1999). It contins phse picks for pproximtely 1000 erthqukes in nd ner Chin, ech yer. A preliminry exmintion of digitl wveforms for out 14,000 events, in nd ner Chin, shows tht pproximtely 9% of them (1301 events) hve the property tht ny one event hs lmost the sme Lg wveform s t lest one other event. These events re grouped into 494 sets of events, ech of which hs essentilly the sme short-period wveform nd thus the events of ech set must e within out 1 km of ech other. These event sets provide good method for ssessing the qulity of stndrd event ctlogs. When comined with other informtion, they cn provide high-qulity solute loctions. Using Pn nd Sn rrivl times for our GT dt sets, we relocted 525 events recorded y vrious comintions of 140 regionl sttions. Misloctions were reduced for 66% of the events using the model-sed SSSCs, nd for 85% of the events using model-sed SSSCs refined y kriging. Medin misloction improved from 16.9 km to 11.4 km nd 6.5 km, respectively. Medin error ellipse re ws reduced from 2,616 km 2 to 1,633 km 2 nd 722 km 2, respectively. Error ellipse coverge (percentge of GT loctions within 90% error ellipses) ws 89% without using SSSCs, 91% using model-sed SSSCs, nd 92% using kriged SSSCs. These results were otined for source loctions, sttions, nd pths tht smple very extensive nd diverse geologicl provinces throughout much of Asi. The SSSCs re expected to perform, on verge, s well s the test results using the model-sed SSSCs, nd sustntilly etter for res where GT clirtion dt were utilized to refine the SSSCs.
OBJECTIVE The gol of this project hs een to improve the ccurcy of estimtes of the loction of seismic events nd to reduce the uncertinty of such estimtes on the sis of n interprettion of the rrivl times of regionl seismic wves oserved t 30 sttions of the Interntionl Monitoring System (IMS) locted in Estern Asi. RESEARCH ACCOMPLISHED Introduction Our project egn s three-yer effort in Mrch 2000 nd hs now een completed. It hs een collortive cdemic-industry reserch project led y Lmont nd involving consortium of five institutions. We developed structurl model of Est Asi uilt up from 36 su-regions, shown in Figure 1, nd we lso developed the cpility to compute trvel times in this model y 3D ry trcing. We developed set of 525 ground-truth events (lmost ll them of GT5 qulity or etter), nd their ssocited phse picks. In ddition to clirting 30 IMS sttions we clirted nother 110 sttions in Est Asi. The totl set of 140 sttions nd GT loctions, nd the Pn pths etween them, re shown in Figure 2. We derived set of model-sed Source-Specific Sttion Corrections (SSSCs) for Pn nd Sn from 3D ry trcing in our regionlized model, nd we then used empiricl rrivl times nd GT informtion to refine the model-sed SSSCs y kriging. In this pper, we emphsize the end-to-end vlidtion of our clims of significnt loction improvement when our SSSCs re pplied to relocte seismic events in Est Asi. 0 20 40 60 80 100 120 140 160 180 80 80 60 40 20 1 2 18 3 4 5d 5 6 5c 7 8 9 10 f g c h 19 20 e d 20 15 12 11 c 13 14 16 17 60 40 20 0 20 40 60 80 100 120 140 160 180 Figure 1. The region oundries nd numering system used for 36 1D regions in Est Asi nd surrounding res, which together mke up the 3D velocity model in which 3D ry trcing ws used to give our model-sed SSSCs. Further detils of our work, including much informtion on how we otined our regionlized model nd numerous new GT events in Est Asi (mostly, in nd ner Chin), re contined in Fisk (2002), Wldhuser nd Richrds (2003), Schff nd Richrds (2003), Yng et l. (2003), nd Burlcu et l. (2003). A CD hs een prepred with our min finl report (Burlcu et l., 2003) nd 1871 dditionl files giving our dt, including detils of our 3D model, our GT events (including phse picks in CSS 3.0 formt), nd our model-sed nd kriged SSSCs. Our work hs lso een descried in 11 ppers presented t four Oslo workshops on loction clirtion orgnized y NORSAR from 1999 to 2003 (nd ppering in the proceedings of those workshops), eight presenttions t meetings
of the Americn Geophysicl Union, the Seismologicl Society of Americ, nd the Interntionl Assocition of Seismology nd Physics of the Erth's Interior, nd in Seismic Reserch Review meetings of 2000, 2001, nd 2002. 80 N 70 NRIS TIXI BIL 60 50 40 30 20 10 BRVK ZAL AKTO KURK MKAR AAK PRPK COC EVN KMI CHT CMAR PDYAR YAK TLY URG JAVM HIA YSS USK BJT LZH KSAR XAN SSE km SEY MAG 0 1000 2000 20 40 60 80 100 120 140 160 E Figure 2. Mp of events (red strs) nd recording seismic sttions (lue tringles) of the dt set used for model vlidtion. The green tringles represent the 30 IMS sttions tht our consortium ws tsked to clirte. Also shown re gret circle Pn pths etween events nd sttions. Tle 1. Men nd stndrd devition of Pn nd Sn trvel-time residuls, in seconds, for ll the sttions tht recorded t lest 3 GT events. IASP91 Model-Bsed SSSCs Model + Kriged SSSCs Cse µ Τ (s) σ Τ (s) µ Τ (s) σ Τ (s) µ Τ (s) σ Τ (s) Pn 1.89 1.77 1.33 1.48 0.22 1.01 Sn 6.08 4.24 3.08 3.76 1.34 3.76
The tles nd figures in this pper contin descriptions of our clims of loction improvement, which hve een the suject of successful integrtion test conducted in My 2003 y the Reserch nd Development Support System stff of the group then known s the Center for Monitoring Reserch. As n overll indiction of how well our SSSCs reduce the misfit etween oserved nd clculted rrivl times, Tle 1 shows RMS vlues for the men nd stndrd devition of the Pn nd Sn trvel-time residuls for ll the sttions tht recorded t lest 3 GT events. The kriged results were otined vi leve-one-out pproch in the genertion of SSSCs, so tht the rrivl times from ny one event were not used to provide the loction estimte in tht cse. From this Tle, we see tht very significnt reduction of residuls is otined y kriging. In sections tht follow, we descrie discovery concerning rod re seismicity of Chin which proved useful in generting ground truth reference events nd which points the wy to future in which seismicity will e locted y mking use of wveforms rther thn eing sed on phse picks. We then descrie our model vlidtion, nd endto-end vlidtion of our kriged SSSCs, efore giving our conclusions nd recommendtions. A discovery concerning rod-re seismicity As prt of our work to generte ground-truth reference events in Est Asi, we found tht whole wveforms of regionl signls, from few seconds prior to the P rrivl nd running to severl tens of seconds fter the Lg rrivl, were very similr for certin clusters of events. This result ws first otined for events (foreshocks, ftershocks) ssocited with mgnitude 5.9 erthquke in 1999 in the Xiuyn region of Lioning Province, Chin (Schff nd Richrds, 2003). The correltion coefficient ws ove 0.9, for time window of few hundred seconds (recorded in the nd from 0.5 to 5 Hz for sttions severl hundred km distnt from the events). Becuse it ws possile to mesure reltive rrivl times with precision of few milliseconds (out three orders of mgnitude etter thn trditionl Lg phse picks), reltive loctions ccurte to out 150 m could e otined. 45 N 30 N 15 N 75 E 90 E 105 E 120 E 26-plet 1 11-plet 2 10-plet 1 8-plet 4 7-plet 3 6-plet 7 5-plet 16 4-plet 31 3-plet 86 2-plet 343 Figure 3. The loction of 494 multiplets, totling 1301 seismic events (9% of the ABCE). For ech multiplet, the cross-correltion is greter thn or equl to 0.8 for window running from 4 s efore the P rrivl, to 40 s fter the Lg rrivl, nd pssed in the nd from 0.5 to 5 Hz. nos 135 E
Our studies of Chinese seismicity for the yers 1985 to 1999 included nlysis of out 15,000 events reported in the Annul Bulletin of Chinese Erthqukes (ABCE), nd llowed us to derive 64 GT5 qulity events s documented y Wldhuser nd Richrds (2003). Knowing the pproximte loction of these 15,000 events, we mde mjor dt request to the IRIS (Incorported Reserch Institutes for Seismology) Dt Mngement Center for the regionl wveforms of out 14,000 events in nd ner Chin, s recorded y ll digitl sttions within 20 for ech event ( totl of 115 sttions). When this dtset (out 12 Gytes) ws serched for repeting wveforms, it ws found tht set of 1301 events hd the property tht ech event hd lmost exctly the sme seismogrms s t lest one other event. These 1301 events, out 9% of the ABCE, were composed of 494 susets of repeting signls (Schff nd Richrds, 2003). Ech suset (in most cses composed of just pir of events, i.e. doulet) must consist of events tht were within 1 km of ech other, given the time window nd ndwidth of the signls tht were cross-correlted. Numerous different uses my e mde of our discovery tht significnt frction of seismicity in rod region is mde up of repet events. Excellent reltive loctions my e otined for the events within cluster. The clusters my in some cses e locted in n solute sense (enling their use s GT events). And the fct tht ech cluster is so smll in size (less thn 1 km cross) enles simple evlution of the precision of event ctlogs tht locte the events one-t--time (Schff nd Richrds, 2003). Misloction Distnce (without SSSCs) [km] 10 2 10 1 10 0 Better Worse 10 0 10 1 10 2 Misloction Distnce (with SSSCs) [km] Figure 4. Misloction distnces with nd without using model-sed SSSCs with respect to corresponding GT loctions. The green symols show the events for which the misloction error is smller using SSSCs thn without. Red symols show the events for which the misloction errors re smller without using SSSCs. The isecting line corresponds to equivlent misloction errors for the two solutions (with nd without SSSCs).
Evlution of our trvel time model For model vlidtion we use 525 events recorded t 140 sttions. The test consists of relocting these events using Pn (5677 picks) nd Sn (1586 picks) rrivls. All the reloctions re performed with depth fixed t the surfce. Figure 2 shows the distriutions of events nd seismogrphic sttions used in this nlysis. Also shown in Figure 2 re gret circle Pn pths etween events nd sttions. The reloction procedure is first pplied using the IASP91 trvel-time tles, without ny SSSCs. This is followed y relocting the sme events using the SSSCs. Executing EvLoc with nd without SSSCs resulted in 525 events with loction estimtes tht converged. First we look t the misloctions, then t the trvel time residuls, nd then t the size of the error ellipses nd their coverge, when using our model-sed SSSCs. Misloction is expressed s the difference in distnce etween the GT loction nd the loction otined y EvLoc. Of the 525 events, the loctions using SSSCs improved for 348 events (66%) nd deteriorted for 177 events (34%). The medin misloction ws reduced from 16.9 km to 11.4km. For 276 events (53%) the solutions improved y more thn 20%, while for 140 events (27%) the deteriortion is more thn 20%. Figure 4 shows the misloction results. The green symols represent the events for which the reloction with SSSCs is closer to the GT loction thn without SSSCs. The red symols show the events for which the misloction without SSSCs is smller thn with SSSCs. The stndrd error of oservtions, mesure of the fit tht depends on the root-men-squred (rms) trvel-time residuls, shows improvement for 304 solutions (58%) nd deteriortion for 221 solutions (42%). 186 solutions (35%) re improved y more thn 20% nd 147 solutions (28%) deteriorted y more thn 20%. 10 6 Are (without SSSCs) (km**2) 10 5 10 4 10 3 Better Worse 10 2 10 2 10 3 10 4 10 5 10 6 Are (with SSSCs) (km**2) Figure 5. Sctter plot of error ellipse res computed with (x-xis) nd without (y-xis) using model-sed SSSCs. Green symols represent error ellipse res tht re smller when using the SSSCs thn without.
Our SSSCs led in generl to smller error ellipses. Thus, using model-sed SSSCs, error ellipse re is reduced for 522 of 525 solutions (99%), 498 of which (95%) re improved y more thn 20%. Only 3 solutions (1%) do not hve smller error ellipses. The decrese in the medin error ellipse re is 953 km 2 (from 2,616 km 2 to 1,633 km 2 ). Figure 5 shows the sctter plot of error ellipse res computed with nd without SSSCs. Error ellipse coverge is defined s the percentge of GT event loctions tht fll within the corresponding 90%- confidence error ellipse. For reloction solutions without using SSSCs, 466 events (89%) hve 90%-confidence ellipses tht contin the GT loctions. Using SSSCs, 476 events (91%) hve 90%-confidence ellipses tht contin the GT loctions. In oth cses the coverge is ner the trget of 90%, while the medin re of the error ellipses is reduced significntly when relocting with SSSCs. 46 03' km 0 10 20 45 54' 45 45' Ground-Truth IASP91 (20.8 km) Model SSSC (8.5 km) Kriged SSSC (6.6 km) 45 36' 67 30' 67 39' 67 48' 67 57' Figure 6. Reloction results, with nd without using SSSCs, for PNE (Meridin-2) in the Former Soviet Union on 19 Septemer 1973. Misloction errors reltive to the ground-truth loction re 20.8 km without using SSSCs, 8.5 km using model-sed SSSCs, nd 6.6 km using kriged SSSCs. The error ellipse res re 710 km 2 without using SSSCs, 425 km 2 using model-sed SSSCs, nd 357 km 2 using kriged SSSCs. Evlution of our kriged SSSCs We evluted loction performnce using the kriged SSSCs. At loctions ner clirtion dt, the kriged corrections converge to the men of the nery dt vlues nd the uncertinty converges to the residul (i.e., locl) vrince. For grid points fr from clirtion dt, the correction surfce pproches the model-sed SSSC, with lrger uncertinty tht is the sum of the clirtion nd residul vrinces. Thus, the kriged SSSCs should perform t lest s well s the model-sed SSSCs, nd much etter for loctions close to clirtion points. In this nlysis, we used computtionlly intensive ut necessry leve-one out procedure in which the event to e relocted ws
excluded from the kriging clcultion of the SSSCs. We then relocted ech of the 525 events with kriged SSSCs tht re re-computed for ech event so tht we do not use the sme dt to oth compute nd test the SSSCs. As n exmple, Figure 6 shows reloction results without SSSCs, with model-sed SSSCs, nd with kriged SSSCs for PNE, Meridin-2, which ws detonted on 19 Septemer 1973 in the Former Soviet Union. The kriged SSSCs reduce the misloction error from 20.8 km to 6.6 km nd reduce the error ellipse re from 710 km 2 to 357 km 2. For this event, the reloction results do not differ significntly when using the model-sed or kriged SSSCs. Note tht 48 09' km 0 10 20 48 00' 47 51' 47 42' Ground-Truth IASP91 (19.6 km) Model SSSC (6.8 km) Kriged SSSC (1.0 km) 47 51' 48 00' 48 09' 48 18' 48 27' Figure 7. Reloction results, with nd without using SSSCs, for PNE (Azgir-10) in the Former Soviet Union on 24 Octoer 1979. Misloction errors reltive to the ground-truth loction re 19.6 km without using SSSCs, 6.8 km using model-sed SSSCs, nd 1.0 km using kriged SSSCs. Mgent squre mrkers represent clirtion points. The error ellipse res re 455 km 2 without using SSSCs, 264 km 2 using model-sed SSSCs, nd 79 km 2 using kriged SSSCs. the error ellipses re smller when using either version of the SSSCs, nd contin the GT loction, unlike the error ellipse sed on IASP91 without SSSCs. Another cse, depicted in Figure 7, shows tht kriging, when clirtion points re ner the event to e locted, cn hve significnt impct. In this cse gin, the error ellipse sed on IASP91 without SSSCs does not include the GT loction. Wht is different in this cse is tht there re 23 GT events inside 4 rdius round the Azgir-10 PNE nd only one in the previous cse (Meridin-2). With mgent squres we disply in Figure 7 the nery GT reference events. The residuls of the 23 GT events re eing used in the kriging process. The kriged SSSCs, used in reloction, contriute to solution tht is 1 km distnce from the GT loction compred to 6.8 km when model-sed SSSCs re used in the reloction process. The improvement of
results shown in Figure 7, compred to the results in Figure 6, is n indiction of the effectiveness of empiricl dt, in sitution where such dt re ville. Next we look t the misloctions in generl, then t the size of the error ellipses nd their coverge, when using our model-sed SSSCs refined y kriging. Of the 525 GT events, 445 solutions (85%) hve smller misloction errors using kriged SSSCs thn those otined using just the IASP91 trvel-time tles. Of these, 410 events (78%) hve misloction errors tht re reduced y more thn 20%. Only 80 solutions (15%) deteriorted nd 53 solutions (10%) deteriorted y more thn 20%. The medin misloction is reduced from 16.9km to 6.5 km when kriged SSSCs re used. Figure 8 shows sctter plot of the misloction distnces, reltive to the GT loctions, otined with (x-xis) nd without (y-xis) using the SSSCs. Misloction Distnce (without SSSCs) [km] 10 2 10 1 10 0 Better Worse 10 0 10 1 10 2 Misloction Distnce (with kriged SSSCs) [km] Figure 8. Misloction distnces with nd without using kriged SSSCs with respect to corresponding GT loctions. Mrkers nd the line re defined s in Figure 4. As in Figure 4, the green symols represent events for which loction estimtes using the kriged SSSCs re closer to the GT loctions, while the red symols show solutions tht re etter without using SSSCs. Using kriged SSSCs, error ellipse re is reduced for ll the 525 solutions (100%), 523 of which (99.6%) re improved y more thn 20%. The medin ellipse re is reduced from 2,616 km 2 to 722 km 2. The results re shown in Figure 9. Error ellipse coverge, computed s the percentge of GT event loctions contined within the 90%-confidence error ellipses, is 92% (483 GT events) when using the kriged SSSCs, s compred to 89% (466 GT events) without using SSSCs (i.e., using IASP91 only).
10 6 Are (without SSSCs) (km**2) 10 5 10 4 10 3 Better 10 2 10 2 10 3 10 4 10 5 10 6 Are (with kriged SSSCs) (km**2) Figure 9. Sctter plot of error ellipse res computed with (x-xis) nd without (y-xis) using kriged SSSCs. Mrkers re defined s in Figure 5. CONCLUSIONS AND RECOMMENDATIONS In this pper we hve riefly presented results vlidting our regionlized trvel-time model of Est Asi nd evluting the effectiveness of the regionl Pn nd Sn SSSCs developed y the Lmont Consortium for Group 1 IMS sttions. In Tle 2 we summrize the min loction performnce metrics when Pn nd Sn rrivls were used with nd without SSSCs. More extensive documenttion is ville. We elieve the results indicte the generl vlidity of the model nd the resulting SSSCs for this region. In ll cses, the results demonstrte tht the regionliztion nd trvel-times curves, developed y the Lmont Consortium for Group 1 sttions, long with the computtionl methods of 3D ry trcing nd kriging, developed y the Consortium, hve produced Pn nd Sn SSSCs nd modeling errors tht improve the performnce of loction nd uncertinty estimtes in Est Asi. We expect tht these SSSCs will perform, on verge, s well s indicted y the vlidtion test results for the model-sed SSSCs, nd sustntilly etter for regions surrounding the Lop Nor, Semipltinsk, Indin nd Pkistni nucler test sites, where high-qulity clirtion dt hve een utilized. We note tht the degree of loction improvement using our SSSCs significntly exceeds the criteri developed t Loction Workshops held in 1999, 2000, 2001, 2002, nd 2003 in Oslo, Norwy (see for exmple CTBT/WGB/TL- 2/18 for 1999). In prt, this is reflection of the fct tht the IASP91 trvel times re poor representtion of regionl trvel times in much of the region we hve studied. We therefore recommend tht ny users of regionl signls recorded y the 140 sttions tht we hve clirted consider our SSSCs for seismic event loction in Est Asi. In prticulr, we recommend tht our SSSCs e considered for use in opertionl systems tht interpret seismic rrivl-time dt from these sttions. Finlly, we note tht our overll method (of first otining model trvel times nd then kriging with empiricl dt) is well suited to clirtion of ny sttions in Est Asi tht hve significnt rchive of relily mesured rrivl times.
Tle 2. Loction performnce metrics for Pn nd Sn. Cse IASP91 Model-Bsed SSSCs Model + Kriged SSSCs Medin misloction (km) 16.9 11.4 6.5 Events with reduced misloction 66% 85% Medin error ellipse re (km 2 ) 2,616 1,663 722 Events with smller ellipses 99% 100% 90% coverge 89% 91% 92% REFERENCES Bondár, I (1999), Comining 1-D models for Regionl Clirtion, in Proceedings of Workshop on IMS Loction Clirtion, Oslo, Jnury 1999. Burlcu, V., M. Fisk, J. Armruster, V. Khlturin, W.-Y. Kim, P. Richrds, D. Schff, F. Wldhuser, M. West, I. Morozov, E. Morozov, V. Cormier, A. Stroujkov, nd C. Siki (2003), Development nd Vlidtion Testing of Regionlized Trvel-Time Model, nd Source-Specific Sttion Corrections for Thirty IMS Sttions nd Other Sttions in Est Asi, 300 pge drft Technicl Report to DTRA. Fisk, M. (2002), Accurte loctions of nucler explosions t the Lop Nor Test Site using lignment of seismogrms nd IKONOS stellite imgery, Bull. Seism. Soc. Amer., 92, 2911-2925. Schff, D., nd P. Richrds (2003), Lg-wve cross correltion nd doule-difference loction: ppliction to the 1999 Xiuyn, Chin, sequence, sumitted for puliction June 25, Bull. Seism. Soc. Amer. Schff, D., nd P. Richrds (2003), High precision loction of seismic ctivity cross Chin, ms in preprtion for SCIENCE. Wldhuser, F., nd P. Richrds (2003), Reference events nd empiricl Source Specific Sttion Corrections for regionl phses t IMS sttions in Chin, sumitted for puliction June 9, Bull. Seism. Soc. Amer. Wldhuser, F., nd P.. Richrds (2003), Loction of underground nucler explosions t the Lop Nor Test Site, 1969 to 1996, in preprtion for sumission to Bull. Seism. Soc. Amer. Yng, Z.-Y., F. Wldhuser, Y.-T. Chen, nd P. Richrds (2003), Reloction of erthqukes in centrl-western Chin using the doule-difference erthquke loction lgorithm, sumitted for puliction, Journl of Seismology.