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Analysis procedures at the International Seismological Centre
Physics of the Earth and Planetary Interiors, 30 (1982) 85—93 Elsevier Scientific Publishing Company, Amsterdam — Printed in The Netherlands
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Analysis procedures at the International Seismological Centre R.D. Adams *, A.A. Hughes and D.M. McGregor International Seismological Centre, Newbury, Berks. RG13 JLZ (Gt. Britain) (Received October 29, 1981; revision accepted March 8, 1982)
Adams, R.D., Hughes, A.A. and McGregor, D.M., 1982. Analysis procedures at the International Seismological Centre. Phys. Earth Planet. Inter., 30: 85—93. Analysis at the International Seismological Centre (ISC) falls into three main categories—association, location and quantification. Difficulties of association on a global scale are not always appreciated. The ISC currently analyses 40—50 events a day, and as the first arrivals from any particular earthquake may span a time interval of up to 20 mm, readings from events occurring in different parts of the world may overlap considerably in time. The present association algorithm depends on time only, and results in many chance mis-associations, or even the synthesis of fictitious events. At present, these mis-associations are rectified by seismologists’ intervention, but full use of amplitude and period information could help to detect these errors automatically. Revision of locations follows standard least-squares procedures, based on Jeffrey’s method of uniform reduction and Jeffreys—Bullen travel times. Locations are made from P phases only (including crustal phases), but other first arrivals and secondary phases are identified and residuals calculated. If depth cannot be determined by geometric means a search is made for depth phases, or failing this, the depth is restrained to that given by another agency, or to a conventional value. No provision is made for local variations in travel time. Body-wave magnitudes are allocated within the distance range 21—100° from reported readings of A and T, or their ratio, using Gutenberg—Richter calibration curves. Surface-wave magnitudes are calculated from the “Prague” formula, using reports of long-period A and Tin the distance range 5—160°,but only values from stations at distances of 20°or more are used to determine an average M, for a particular event. There is no provision for the determination of local magnitude other than to reproduce values assigned by local agencies. Improvements in these procedures could be made through the automatic association of station readings, the introduction of local travel times, and better determination of earthquake size, particularly for local events.
1. Introduction The function of the International Seismological Centre (ISC) is the final collection, collation, analysis and publication of standard seismological information on a global scale. To ensure adequate time to achieve completeness in data collection, the start of processing is delayed by 23 months and publication usually follows within 3—5 months *
Also at Department of Geology, University of Reading, Reading, Gt. Britain.
0031-920l/82/0000—0000/$02.75
of this. The period being analysed at the time of writing is mid-1979, when some 1100 stations reported in a typical month. In each month 1500 events are listed, compared with 600 from the United States National Earthquake Information Service. Some 200 of these events have not been reported by other agencies. The Centre’s analysis procedures may be split into three phases—association, location and quantification. As the world reference for seismicity, the Centre is inclined to remain conservative rather than in-
© 1982 Elsevier Scientific Publishing Company
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novative. In particular, changes in procedures that affect the homogeneity of results must be kept to a minimum. The Centre has also always operated under severe financial restrictions, so that all procedures are oriented to producing its publications as efficiently as possible, and to keeping to an absolute minimum any extra facilities that might increase the flexibility of analysis but would increase costs. Nevertheless, the ISC has access to a unique collection of data on which new procedures can be tested. This paper describes only the analysis procedures used in producing the ISC’s final compilation of phases and events. A full description of other information included in ISC publications, such as felt effects and flagging of nuclear and chemical explosions, is given in English, French and Russian as an introduction to ISC Bulletins for the months January and July of each year, and also in the Regional Catalogue of Earthquakes.
2. Present procedures 2.1. Data handling Information is received in machine-readable form from the larger agencies, on magnetic tape. Smaller networks supply data on paper tape, by telex, on punched cards, or on forms from which direct keying is undertaken. In exceptional cases, information is keyed directly into the files from station bulletins, All information is converted into either an epicentre record, a phase record or a comment record, and stored in station-month packets on a disc file. When data for a particular month are to be analysed, all records for that month are put into a chronological list. Typically, this file contains 2200 epicentre records and a total of 75000 phase and comment records. Figure 1 shows the scheme of data input and analysis. 2.2. Association The association of records relating to the same event is significantly more complex on a global
40
1100
Agencies
Stations
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Comments
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T Fig. 1. Scheme of input, analysis and publication for typical monthly analysis at ISC.
scale than for regional or local analysis. These difficulties are compounded when, as at ISC, there is no direct access to seismograms to check the character of particular reported arrivals, or to confirm the accuracy of the time determinations. The first task is to associate origin estimates of the same event, submitted by different agencies. This is done using a gate of 1 mm in time and 2° in position. Separate origins within these limits given by the same agency are treated as separate events, and other estimates are associated with the closest. Special difficulties are encountered with array-determined origins, which are often misplaced by many degrees. These and other poorly determined origins result in an unreal multiplicity of events, which can cause a “split” of the phase readings that should belong to a single event. A particularly common form of this is when deep events in the Pacific are given shallow depths by European agencies, causing the origin time to be early by up to 2 mm. Once the program has grouped origin records, phase records are associated with the event they fit best. Initial association is made by considering P, S, PKP or SKS phases that have a residual of less than 70 s relative to an expected arrival time. This gate is kept wide to allow for possible minute
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errors. Many readings are submitted to ISC through several channels, for example from both preliminary and final bulletins, and also via NEIS. If an acceptable solution is found, such duplicates are automatically eliminated, together with readings having excessively large residuals. Decisions concerning which of several duplicate records is to be retained are made by a combination of such criteria as the time residual, the number of secondary phases, the presence of readings of direction of first motion, whether a record has been submitted directly from a station or via another agency, and in particular whether it contains information on amplitude and period. Much weight is now given to the reporting of the amplitude and period of surface waves, and it sometimes happens that the record retained is not that finally submitted by a station. The editor has the power to override the choice made by the program. Association by time alone causes many instances of mis-association. A mis-associated reading may be spurious, but is more likely to be a genuine reading from an unreported event. Given the fact that the Centre analyses 50 events a day, it is usual for several events to be occurring in different parts of the world within the time-window for association, and readings that do not fit closely with one origin (e.g., minute errors) are often erroneously associated with another origin. Exam-
QUE 0C125 14 34’27 36.5 N ISC~OC125 14 34’26.8 36.36 N 8 085 +0.47 +0.073 717:AFGHANISTAN—IJSSR BORDER CODE WRS PSH KBL NIL MNL QUE ND! MBC
ST~TI0NNAME WAPSAK ~M WAPSAK CAM PESHA WAR PESHAWAR KABUL NILORE NILORE MANGLA MANGLA QUETTA QUETTA NEW DELHI NE~DELHI MOULD 3M
70.5 E 71.23 E +0.053 REGION
ples of computer output showing the difficulties of association are given in Figs. 2—5. These examples, meant for illustrative purposes only, are taken directly from the computer output of various stages of analysis for the data month October 1978, and differ somewhat in format from the final Bulletin. All details of distance, azimuth, phase identification and residual relate to the last (starred) origin estimate, generally redetermined by ISC. Origin estimates of other agencies, where available, are given above. The column OP/ID shows the phase identification given by the station on the record as originally received. ISC/ID gives the ISC identification of all initial phases, and of secondary phases where the identification is changed. The residuals of all phases in the ISC/ID column with respect to the ISC solution are given in the column ISC/RESID. OP/RESID gives the residuals, also against the ISC solution, of secondary phases as originally identified by the operator. Crustal phases are assigned residuals only if they are initial phases, and no secondary phases are re-identified at distances less than 20°. Figure 2 gives an example of a reading from a sensitive station (Mould Bay) at teleseismic distance, improving the solution of an earthquake near Afghanistan and enabling a depth to be determined directly, rather than being assumed at a conventional value as was done by the local agency. In Fig. 3, however, it appears that the
160KM QUE 244KM 0.86 3.8B +6.6 18 /1
DELTA AZ. ICES OP 0E3 DEC NPIN ID 2.21 176 P/PKP 2 S 2.42 176 P/PKP 2 S 2.55 225 I IP 3.17 148 P/PKP 2 S 3.82 146 P/PKP 2 S 7.12 211 P/PKP 2 S 9.17 145 P/PKP 2 E ES 67.47 3 I IP
ISC ID P p p P P P p P
TIME C OP 1SC KR MN SEC 0 RESID RESID 14 35 10.0 —1.1 14 35 44.0 —1.7 14 35 13.2— 0.0 14 35 ‘.8.5 —1.1 14 35 14.5+ 0.0 14 35 22.3— +1.1 14 35 53.0 —10.8 14 35 29 +0.3 14 36 15.0 —2.0 14 36 10.0 +0.4 14 37 29.0 —1.2 14 36 35 —0.7 14 38 12 —4.9 14 44 57.4 0.0
Fig. 2. Example of good association of a teleseismic reading (MBC) with a locally determined event.
OTT OCT25 20 31’57
58.58 N
r33.32 W
OTT 3.3R
ISC*CCI2S 20 31’lR 56.0 N 136.0 14 4 tIES +2.9 +0.21 +0.55 20:OFF COAST OF SOUTH—EASTERN ALASKA COOE WI-IC 14110
KEY tNK DAG
STATION NAME WHITEHIRSE WHITEHOPSE
0KM 1.43 4.78 /4 /1
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20 33 10
20 33 02 20 33 51.5
+0.1
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20 34 17
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Fig. 3. Example of association of a teleseismic reading (DAG) causing mislocation of a locally determined event.
WEL 00127
3 42’lS.O
37.19 S
176.88 E
232KM
WEL 4.4
ISC*0C127 23 41’55 36.5 S 177.3 E 403KM 1.80 12 TIPS +1.4 +0.18 +0.23 +8.4 /9 160:OFF EAST COAST OF NORTH ISLAND, N.Z. CODE
STATION NAME
GBZ WTZ
GREAT BARRI~ WHAKATANE
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DELTA DES 1.48 1.54
2 3 2 1.59 2.02 141 223
WELLINGTON * * * WELLINGTON MAWSOM
2.25 165 2.34 183
3.02 205 3.11. 187
4.38 198
*
*
WEL OCT27 23 42’15.0 ISC*0C127 23 42’14.2 10 CBS +0.38 159:NORTH ISLAND, CODE
STATION NAME
WTZ WTZ KRP
WHAKATANE WHAKATANE WHAKATANE KARAPIRO KARAPIRO GREAT BARRIER EAST CAPE EAST CAPE TUAI TUAI GISBORNE GISBORNE NGAURUHOE NGAURUHOE TARADALE TARADALE MANGAHAO M&NGAHAO MANGAHA.O MANGAHAD WELLINGTON
GBZ ECZ TUA GNZ NCZ TRZ MNG MNG MNG WEL
1
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P
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37.19 5 176.88 8 37.05 S 176.91 E +0.050 +0.063 NEW ZEALAND
ES ES * * * 51(51(S) PIPKP P 232KM
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23 43 19 23 42 54.1+ 23 43 22 23 42 53.3 23 43 21 23 43 00.2
1 E *
TIME C OP HR MN SEC 0 RESID 23 42 50.3— 23 42 46.4 23 42 46.9 23.43 23 43 23 23 42 42
PP
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*
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23 43 34 23 43 30 23 43 37 23 43 13.5+ 23 43 14.0 23 43 50 23 43 58 23 44 15 *OIJPIICATE* 23 44 15 23 51 56
ISC -RESID +1,2 —3.0
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+1.3 —1.2
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DELTA AZ. ICES OP DEC DEC NPIN IC 0.93 176 P/PKP 2 1 I 3 S 1.40 231 1 PIPKP 2 S1(SKS) 1.42 305 I P/PKP 1.45 117 8 PIPKP 2 S/(SKS) 1.76 174 P/PKP 2 E ES 1.82 151 P/PKP 2 8 ES 2.36 205 I P1PKP 2 S/ISKSI 2.50 182 E 2 S1(SKS) 3.73 197 P/PKP 2 1 1 3 E E 4 E ES 4.55 201 S1(SKS)
/8 ZSC 10 P P P p P P P P
HR 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23
TIME C OP ISC MN SEC 0 RESID RESID 42 46.4 —0.? 42 46.9 43 10.0 —2.9 42 50.9— +0.5 43 19 0.0 42 50.3— -0.2 42 51 +0.2 43 21 +1.3 42 53.3 —0.1 43 21 —3.1 42 54.1+ +0.3 43 22 —2.8 43 00.2 +1.0 43 34 —0.3 43 30 43 37 40.2 43 13.5+ —1.1 43 14.0 43 50 43 58 —3.6 44 15 -4.1
Fig. 4. Example of removal of a mis-associated teleseismic reading (MAW) improving earthquake location.
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association of a reading from Danmarks Havn with an Alaskan earthquake gives a mathematically acceptable solution, but one which is displaced by several degrees from that given by the local agency, Ottawa. This reading, and the ISC origin, were removed from this earthquake, as the editor did not consider the association a likely one, Figure4 shows the improvement in solution that follows the removal of a reading from Mawson that had been associated with a New Zealand earthquake (a duplicate Wellington S reading is also removed). The second ISC solution agrees much more closely with that determined by Wellington, and gives much better residuals for the S phases. The examples given here are not difficult to interpret correctly, but there are. cases of real uncertainty, when the inclusion or exclusion of a lone reading from a sensitive station at teleseismic distance can alter a location substantially, and both solutions are equally acceptable. The availability of information on amplitude and period helps to resolve such ambiguities, and when an estimate of magnitude from closer stations is available it is possible to calculate a “likelihood” that a particular station of known position
ISC*OCT11 07 43’31.8
18.16
N
and sensitivity would or would not record a certam event. Information as to whether other nearby stations have or have not recorded a given event also helps in deciding the likelihood of a given association. When all credible associations between phase readings and known origins have been made, the remaining unassociated readings are subjected to a “search” procedure based on that of Engdahl and Gunst (1966) which examines each successive grouping of three consecutive phase arrivals. If these can be associated as from a single event, then further associated readings are sought, and a solution given if five consistent station readings are found. This preliminary origin is then subjected to the usual revision procedure applied to origins from other agencies. The difficulties of association become even more pronounced at this stage, and in extreme cases it is easy for local readings from two or more events in different parts of the world to combine to produce a fictitious event many tens of degrees from any real activity. About one half of these initial associations are rejected as spurious, but 200 such new events are found each month. Figure 5 gives an example of such a fictitious earthquake. Read-
153.93
15 CBS 612:HAWAIIAN ISLANDS REGION CODE STATION tiAME DELTA AZ. ICES DEC DEC NPIN RAB RABAUL 57.57 253 8 ULA ULAMONA 58.74 252 E 1MG LAMINGTON 63.17 250 1 * * * * * * * * * * * * * * 1MG LAMIPIGTCN 63.17 250 LAMINGTCN 2 E LAT LAE 63.30 253 8 *
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PORT MORESBY PORT MORESBY QUITO NASA COTOPAXI WARRAMUNGA AR WARRAMUNGA AR WARRAMUNGA AR
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TIME C OP ISC HR MN SEC C RESID RESID 07 53 02.0 +17,5 07 52 45.0 —7.2 07 53 13.0— —8.6 *DUPLICATE* * * * * 07 53 13.0— —8.6 07 53 52 —449.3 —3.5 C? 53 18.0 —4.4 *DUPLICATE* * * * * Q7 53 18 —4.4 ~ 53 25.5 +0.7 *
DUPLICATE *
07 01 07 07 07 07 C?
53 54 54 54 55 55 56
29.5 22 39.0 44.2 03.0 05.0 58.7
*DUPLICATE*
0? 55 C3.0 07 55 05.0 07 56 58.7 07 55 39.0 *DUPLICATE* 07 55 39 08 02 42.4—
Fig. 5. Example of a spurious earthquake found by mis-association of widely scattered readings.
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ings from the South Pacific, Ecuador, Australia and Africa have combined to produce a spurious event at a depth of 393 km near Hawaii. This event was removed from the file. Correct association can be aided by more complete availability of amplitude and period information, and by information on the character of the recording. Some algorithms have been devised to test these concepts, but at this stage the help of an experienced seismologist is still required to monitor such work. 2.3. Location Once station readings have been associated into events, a revision procedure is used to refine the origin parameters. This is a standard least-squares procedure, using weighting according to Jeffrey’s method of uniform reduction (Jeffreys, 1961). mitially a four-parameter solution, including depth, is sought. If this does not converge, the program seeks a solution with depth determined from depth phases, after scanning all secondary phases in an attempt to identify additional pP or sP phases. The next trial is to adopt a depth given by an external agency, and finally to adopt a conventional depth of 33 or 0 km. Once a solution has been found, duplicate readings are automatically eliminated and the process repeated. Seismologists have the facility to override the computer’s choice of option. Unsatisfactory events are rejected or modified, in batch mode, through five or six successively smaller runs until all remaining events have satisfactory solutions. Revision of location is done on P phases only, using the Jeffreys—Bullen (1940) travel-time tables. Since the data year 1978, improved use has been made of crustal phases P5, ~* and P~,according to the following scheme: (a) if at close distances and shallow depths, only one particular phase is geometrically allowable, phases are identified as such; (b) if the reporting agency identifies a particular phase, this identification is used provided that it is geometrically allowable; (c) if there are several possible identifications for an unspecified P phase, the program chooses the identification consistent with the earliest arrival time;
(d) the seismologist may override the program and specify any allowable phase identification. PKP, S and later phases are identified, and have residuals allocated, but are not used in the location process. Depth phases pP and sP are recognised if specified by the reporting station, and can be identified by the program for the determination of depth, either in addition to that derived geometrically, or as the definitive method if no geometrical determination is possible. No provision is made for the use of station or source corrections, regional travel-time tables, or joint methods of hypocentre location. The solution found by the ISC revision procedure is normally adopted as the “prime” estimate for cataloguing and reference purposes. When a local agency supplies an origin estimate with no supporting readings, Or not enough are available for a satisfactory revision, the local origin is adopted as the prime. Only in very exceptional circumstances is an event which is found by the ISC search procedure retained in the final listing if it fails to converge to a satisfactory solution. 2.4. Location errors Formal standard errors are given for each parameter determined by the least-squares procedure. These are calculated from the standard deviation of the residuals, and the number of degrees of freedom involved. The standard errors are generally small when the number of observations is large; for events determined from a hundred observations or more the errors in position are usually 2 or 3 km, and seldom more than 5—10 km. When the solution is based on data from only a few stations, the standard error may be unrealistically small, and represent only the degree of consistency of the observations. In such cases, the presence or omission of a single reading may result in solutions which each have a small error, but are separated by an amount greater than either. All errors determined are mathematical only, and assume that the velocity model used is physically correct. The use of inappropriate velocity models may result in systematic mislocations greater than the standard error of the solution.
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2.5. Quantification Present procedures determine magnitudes only at teleseismic distances. For P waves, magnitudes ~ are determined from submitted. readings of amplitude and period in the distance range 21— 1000, using the Gutenberg and Richter (1956) calibration functions, and for surface waves M~is determined from waves of period 5—60 s, recorded at distances of 5—160°, using the “Prague” formula of Vanek et al. (1962). For both kinds of magnitude, determinations at individual stations are averaged to be included in the origin estimate of each event, with the exception that determinations of M, in the distance range 5—20° are not included in the average, but appear only with the individual station reading. Magnitude determinations derived by external agencies are reproduced in their epicentral records, and if these remain as the “prime” solution they appear in the main historical file of events, ISC has no provision for the determination of local magnitudes, and it would be inappropriate for it to do so globally. The determination of local magnitudes can be carried out only by local agencies with knowledge of instrumentation and attenuation conditions. Events below a certain size cannot be recorded teleseismically, so some form of local magnitude determination is the only possible means of determining their size,
3. Possible ways of improving analysis The following suggestions are meant to be indicative of present weaknesses in procedures at ISC and other global data centres, and representalive of ways in which they could be improved. They are not necessarily all practicable at this time, and in some cases may not be desirable for a definitive agency such as ISC, until they have been tested as research options by other agencies. 3.1. Association Existing procedures perform well considering that they work in the time domain only. The addition of procedures to analyse amplitude and
period information would allow the likelihood of a given association to be assessed numerically. Improvement could also be achieved by using distance criteria. A single station associated with an event at much greater distance than all other stations is suspect, although such associations can occur with high-magnification stations and good transmission paths. Similarly, associations of an isolated group of stations at a large distance might suggest the occurrence of a separate event near those stations. For such associations to be analysed numerically full information would be needed on station operation, magnification, periods of non-operation, and completeness of reporting. 3.2. Location and velocity models At present each event is considered individually. Joint hypocentral determination or the use of master events could be useful in certain areas, particularly where some events are known to be well determined in position, either by local networks or by observations of controlled sources. The most serious deficiency is the use of mappropriate velocity models. The present Jeffreys— Bullen model has served seismology well, but is known to be deficient in several respects. When a new reference Earth model is adopted by IUGG (see for example, Dziewonski and Anderson, 1981), use of its derived travel-time functions should be considered by ISC. It is, however, quite unrealistic to expect highquality solutions from any laterally homogeneous Earth model, and it is surprising that such models have given results that have been acceptable for so long. The lack of differentiation between shallow continental and oceanic structure is a major deficiency: but note that NEIS now routinely restrains oceanic foci to a depth of 10 km, rather than the conventional value of 33 km. A much more serious deficiency is the neglect of provision for the major velocity inhomogeneity associated with zones of deep earthquakes, where velocities are typically 10% higher than in the surrounding mantle (Engdahl, 1973; Robinson, 1976). The deficiencies of the present models are not always evident, and not always appreciated by those not working on these problems. A feature of
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the least-squares procedure is that it will often find a good mathematical solution even if the physical model of velocity deviates significantly from reality. Examples of this are particularly noticeable in the South Pacific, where velocities higher than normal give residuals at close stations, such as Afiamalu and Rarotonga, that are often early by as much as 10 s or more for large events located by many teleseismic observations. For smaller events, on the other hand, with fewer or no teleseismic readings, the few regional stations can give apparently good solutions with small residuals, but which may be badly misplaced. Locations in such areas, that are based on small numbers of stations, must be treated with great caution, and the mathematical standard errors of the solutions must not be accorded any physical significance. It would be possible to use station and source corrections with the present model, preferably taking account of distance, azimuth and focal depth, and some models have been devised to approximate velocity inhomogeneities (see for example, Adams and Ware, 1977) and thus produce earthquake positions that are more realistic. The final desirable solution must be to use ray-tracing techniques in a realistic model, taking account of the three-dimensional velocity structure. Proper location of shallow local earthquakes also requires the use of correct velocity models, and for many restricted areas of local seismic activity detailed models of velocity structure are available that could be incorporated in the ISC programs to be invoked if both epicentre and station fall within a specified region. The present ISC model is stored mainly in the form of- traveltime tables for various phases, but it would be preferable if variations in shallow structure were stored in the form of velocity—depth distributions from which the desired local travel times would be calculated. This is already done for the upwardgoing direct P phase from the lower crust in the Jeffreys—Bullen model, 3.3. Weighting At present the only weighting of individual equations of condition depends on the residual of the arrival time. This is done by Jeffreys’ (1961)
method of uniform reduction which gives the residuals a normal distribution, to which a leastsquares procedure can more properly be applied. No weighting is applied to allow for variations in station distribution. In Bolt’s (1960) original leastsquares location program, station weighting was given inversely according to the number of stations in each quadrant of azimuth, and some such weighting according to the geographical density of stations is probably desirable; however, care must be taken lest a bad reading from an isolated station is given undue prominence. It is obviously bad for each individual station of a close network to be given equal weight in teleseismic determinations. The best solution may be to reduce progressively the weight of individual stations, once the station density in a given region exceeds a certain threshold. 3.4. Quantification The determination of earthquake size is probably the area in which the present ISC output is most deficient. Despite some improvement in procedures in the last few years, only 40% of ISC prime estimates contain any magnitude information at all. There are two main reasons for this. The first is the lack of information on amplitude and period submitted to the Centre. This arises in some cases from difficulty in deciphering these parameters from the record, and also from the need for revision of ISC reporting formats. The second factor is that it is proper for ISC to calculate magnitudes in the teleseismic range only, and not to determine local magnitudes for small events. Thus events small enough to be recorded only locally cannot be assigned a magnitude by present ISC procedures. In many of these cases, local agencies have determined origins, with local magnitudes, which are included in the ISC publications as non-prime origin estimates. Consideration is being given to the addition of locally determined magnitudes from other agencies to ISC origin parameters to form a “composite” prime estimate. One of the difficulties of systematic local magnitude determination is the diversity of recording instruments, on which different parameters may be read. For instance, in many conventional
93
short-period systems the recorders are too slow to enable the determination of ground period, and the number of Wood—Anderson-type seismographs existing for the determination of the classical magnitude ML from amplitude alone is not large. The parameter most easily determined for local earthquakes is the duration of recording, and this may be the best parameter to submit to global data centres, provided that instrumental effects are known. The routine determination of other source parameters, such as seismic moment, is again best carried out by the individual stations, or national centres, where all relevant instrumental and structural information should be known, but there is no reason why ISC should not reproduce values submitted to it. The present determinations of teleseismic magnitudes, mb and M~,could be improved by greater use of regional calibration curves and station and source corrections by some system such as the homogeneous magnitude system (Christoskov et al., 1979). Care must be taken, however, in applying such systems in tectonically active zones, in which three-dimensional variations in structure may cause anomalous attenuation as well as effects such as focusing and refraction of energy. 4. Conclusion Although deficient in various respects, the accumulated output of the ISC (since 1964) and of its predecessor, the ISS (1913—1963), forms a unique collection of earthquake data, which is continually being improved in quality. Care must be taken to reach a balance between the need to improve the analysis procedures used, and the desire to maintam a homogeneous collection of information. It is obvious that some provision must soon be made to adopt a more realistic global velocity model, to introduce methods to allow for the large velocity variations known to occur in the Earth’s
outer layers, and to improve earthquake quantification. May the results of this and similar Symposia help us to achieve this aim.
Acknowledgements The procedures described here have been developed over many years with the help of many people. Among those particularly involved are Drs. E.P. Arnold, S.J. Gibowicz, A.L. Levshin and P.L. Wilimore.
References Adams, R.D. and Ware, D.E., 1977. Subcrustal earthquakes beneath New Zealand: locations determined with a laterally inhomogeneous velocity model. N.Z. J. Geol. Geophys., 20: 5983. Bolt, B.A., 1960. The revision of earthquake epicentres, focal depths, and origin-time using a high-speed computer. Geophys. J., R. Astron. Soc., 3: 434—440. Christoskov, L., Kondorskaya, NV. and Vanek, J., 1979. Homogeneous Magnitude System of the Eui~asianContinent: P Waves. Rep. SE-18, World Data Center A for Solid Earth Geophysics, Boulder, CO. 57 pp. Dziewonski, A.M. and Anderson, D.L., 1981. Preliminary reference Earth model. Phys. Earth Planet. Inter., 25: 297—356. Engdahl, E.R., 1973. Relocation of intermediate depth earthquakes in the Central Aleutians by seismic ray tracing. Nature (Phys. Sci.), 245: 23—25. Engdahl, E.R. and Gunst, R.H., 1966. Use of a high speed computer for the preliminary determination of earthquake hypocentres. Bull. Seismol. Soc. Am., 56: 325—336. Gutenberg, B. and Richter, C.F., 1956. Magnitude and energy of earthquakes. Ann. Geofis., 9: 1—15. Jeffreys, H., 1961. The Theory of Probability. Oxford University Press, edn. Jeffreys, H. andOxford, Bullen, 3rd K.E., 1940. Seismological Tables. British Association for the Advancement of Science, London, 50 pp. Robinson, R., 1976. Relative teleseismic travel-time residuals, North Island, New Zealand, and their relation to uppermantle structure. Tectonophysics, 31: T41 —T48. Vanek, J., Zatopek, A., Karnik, V., Kondorskaya, N.y., Rizrnchenko, Yu. V., Savarensky, Y.F., Solovev, S.L. and Shebalin, N.Y., 1962. Standardization of magnitude scales. Izv. Acad. Sci. U.S.S.R., Geophys. Ser., 2: 153—158.
85
Analysis procedures at the International Seismological Centre R.D. Adams *, A.A. Hughes and D.M. McGregor International Seismological Centre, Newbury, Berks. RG13 JLZ (Gt. Britain) (Received October 29, 1981; revision accepted March 8, 1982)
Adams, R.D., Hughes, A.A. and McGregor, D.M., 1982. Analysis procedures at the International Seismological Centre. Phys. Earth Planet. Inter., 30: 85—93. Analysis at the International Seismological Centre (ISC) falls into three main categories—association, location and quantification. Difficulties of association on a global scale are not always appreciated. The ISC currently analyses 40—50 events a day, and as the first arrivals from any particular earthquake may span a time interval of up to 20 mm, readings from events occurring in different parts of the world may overlap considerably in time. The present association algorithm depends on time only, and results in many chance mis-associations, or even the synthesis of fictitious events. At present, these mis-associations are rectified by seismologists’ intervention, but full use of amplitude and period information could help to detect these errors automatically. Revision of locations follows standard least-squares procedures, based on Jeffrey’s method of uniform reduction and Jeffreys—Bullen travel times. Locations are made from P phases only (including crustal phases), but other first arrivals and secondary phases are identified and residuals calculated. If depth cannot be determined by geometric means a search is made for depth phases, or failing this, the depth is restrained to that given by another agency, or to a conventional value. No provision is made for local variations in travel time. Body-wave magnitudes are allocated within the distance range 21—100° from reported readings of A and T, or their ratio, using Gutenberg—Richter calibration curves. Surface-wave magnitudes are calculated from the “Prague” formula, using reports of long-period A and Tin the distance range 5—160°,but only values from stations at distances of 20°or more are used to determine an average M, for a particular event. There is no provision for the determination of local magnitude other than to reproduce values assigned by local agencies. Improvements in these procedures could be made through the automatic association of station readings, the introduction of local travel times, and better determination of earthquake size, particularly for local events.
1. Introduction The function of the International Seismological Centre (ISC) is the final collection, collation, analysis and publication of standard seismological information on a global scale. To ensure adequate time to achieve completeness in data collection, the start of processing is delayed by 23 months and publication usually follows within 3—5 months *
Also at Department of Geology, University of Reading, Reading, Gt. Britain.
0031-920l/82/0000—0000/$02.75
of this. The period being analysed at the time of writing is mid-1979, when some 1100 stations reported in a typical month. In each month 1500 events are listed, compared with 600 from the United States National Earthquake Information Service. Some 200 of these events have not been reported by other agencies. The Centre’s analysis procedures may be split into three phases—association, location and quantification. As the world reference for seismicity, the Centre is inclined to remain conservative rather than in-
© 1982 Elsevier Scientific Publishing Company
86
novative. In particular, changes in procedures that affect the homogeneity of results must be kept to a minimum. The Centre has also always operated under severe financial restrictions, so that all procedures are oriented to producing its publications as efficiently as possible, and to keeping to an absolute minimum any extra facilities that might increase the flexibility of analysis but would increase costs. Nevertheless, the ISC has access to a unique collection of data on which new procedures can be tested. This paper describes only the analysis procedures used in producing the ISC’s final compilation of phases and events. A full description of other information included in ISC publications, such as felt effects and flagging of nuclear and chemical explosions, is given in English, French and Russian as an introduction to ISC Bulletins for the months January and July of each year, and also in the Regional Catalogue of Earthquakes.
2. Present procedures 2.1. Data handling Information is received in machine-readable form from the larger agencies, on magnetic tape. Smaller networks supply data on paper tape, by telex, on punched cards, or on forms from which direct keying is undertaken. In exceptional cases, information is keyed directly into the files from station bulletins, All information is converted into either an epicentre record, a phase record or a comment record, and stored in station-month packets on a disc file. When data for a particular month are to be analysed, all records for that month are put into a chronological list. Typically, this file contains 2200 epicentre records and a total of 75000 phase and comment records. Figure 1 shows the scheme of data input and analysis. 2.2. Association The association of records relating to the same event is significantly more complex on a global
40
1100
Agencies
Stations
N
_______________________
u I
%%gr~inEs
1~m~tes and
Comments
ASSOCIATION A N A L
y
NO
>
.
with known events
Search YE~
“f’
LOCATION QUANTIFICATION
[,J 200,leL
fl~
e~s
‘NO
_________
unassociated Readings
0 Bulletin Publication ~00 events
)
Store Origins for 6-monthly Catalogue
T Fig. 1. Scheme of input, analysis and publication for typical monthly analysis at ISC.
scale than for regional or local analysis. These difficulties are compounded when, as at ISC, there is no direct access to seismograms to check the character of particular reported arrivals, or to confirm the accuracy of the time determinations. The first task is to associate origin estimates of the same event, submitted by different agencies. This is done using a gate of 1 mm in time and 2° in position. Separate origins within these limits given by the same agency are treated as separate events, and other estimates are associated with the closest. Special difficulties are encountered with array-determined origins, which are often misplaced by many degrees. These and other poorly determined origins result in an unreal multiplicity of events, which can cause a “split” of the phase readings that should belong to a single event. A particularly common form of this is when deep events in the Pacific are given shallow depths by European agencies, causing the origin time to be early by up to 2 mm. Once the program has grouped origin records, phase records are associated with the event they fit best. Initial association is made by considering P, S, PKP or SKS phases that have a residual of less than 70 s relative to an expected arrival time. This gate is kept wide to allow for possible minute
87
errors. Many readings are submitted to ISC through several channels, for example from both preliminary and final bulletins, and also via NEIS. If an acceptable solution is found, such duplicates are automatically eliminated, together with readings having excessively large residuals. Decisions concerning which of several duplicate records is to be retained are made by a combination of such criteria as the time residual, the number of secondary phases, the presence of readings of direction of first motion, whether a record has been submitted directly from a station or via another agency, and in particular whether it contains information on amplitude and period. Much weight is now given to the reporting of the amplitude and period of surface waves, and it sometimes happens that the record retained is not that finally submitted by a station. The editor has the power to override the choice made by the program. Association by time alone causes many instances of mis-association. A mis-associated reading may be spurious, but is more likely to be a genuine reading from an unreported event. Given the fact that the Centre analyses 50 events a day, it is usual for several events to be occurring in different parts of the world within the time-window for association, and readings that do not fit closely with one origin (e.g., minute errors) are often erroneously associated with another origin. Exam-
QUE 0C125 14 34’27 36.5 N ISC~OC125 14 34’26.8 36.36 N 8 085 +0.47 +0.073 717:AFGHANISTAN—IJSSR BORDER CODE WRS PSH KBL NIL MNL QUE ND! MBC
ST~TI0NNAME WAPSAK ~M WAPSAK CAM PESHA WAR PESHAWAR KABUL NILORE NILORE MANGLA MANGLA QUETTA QUETTA NEW DELHI NE~DELHI MOULD 3M
70.5 E 71.23 E +0.053 REGION
ples of computer output showing the difficulties of association are given in Figs. 2—5. These examples, meant for illustrative purposes only, are taken directly from the computer output of various stages of analysis for the data month October 1978, and differ somewhat in format from the final Bulletin. All details of distance, azimuth, phase identification and residual relate to the last (starred) origin estimate, generally redetermined by ISC. Origin estimates of other agencies, where available, are given above. The column OP/ID shows the phase identification given by the station on the record as originally received. ISC/ID gives the ISC identification of all initial phases, and of secondary phases where the identification is changed. The residuals of all phases in the ISC/ID column with respect to the ISC solution are given in the column ISC/RESID. OP/RESID gives the residuals, also against the ISC solution, of secondary phases as originally identified by the operator. Crustal phases are assigned residuals only if they are initial phases, and no secondary phases are re-identified at distances less than 20°. Figure 2 gives an example of a reading from a sensitive station (Mould Bay) at teleseismic distance, improving the solution of an earthquake near Afghanistan and enabling a depth to be determined directly, rather than being assumed at a conventional value as was done by the local agency. In Fig. 3, however, it appears that the
160KM QUE 244KM 0.86 3.8B +6.6 18 /1
DELTA AZ. ICES OP 0E3 DEC NPIN ID 2.21 176 P/PKP 2 S 2.42 176 P/PKP 2 S 2.55 225 I IP 3.17 148 P/PKP 2 S 3.82 146 P/PKP 2 S 7.12 211 P/PKP 2 S 9.17 145 P/PKP 2 E ES 67.47 3 I IP
ISC ID P p p P P P p P
TIME C OP 1SC KR MN SEC 0 RESID RESID 14 35 10.0 —1.1 14 35 44.0 —1.7 14 35 13.2— 0.0 14 35 ‘.8.5 —1.1 14 35 14.5+ 0.0 14 35 22.3— +1.1 14 35 53.0 —10.8 14 35 29 +0.3 14 36 15.0 —2.0 14 36 10.0 +0.4 14 37 29.0 —1.2 14 36 35 —0.7 14 38 12 —4.9 14 44 57.4 0.0
Fig. 2. Example of good association of a teleseismic reading (MBC) with a locally determined event.
OTT OCT25 20 31’57
58.58 N
r33.32 W
OTT 3.3R
ISC*CCI2S 20 31’lR 56.0 N 136.0 14 4 tIES +2.9 +0.21 +0.55 20:OFF COAST OF SOUTH—EASTERN ALASKA COOE WI-IC 14110
KEY tNK DAG
STATION NAME WHITEHIRSE WHITEHOPSE
0KM 1.43 4.78 /4 /1
DELTA AZ. ICES OP 9PKP D~G 750E~NPIN ~ 2 PG
WHITEIIIDRSE
3
KLUANE KIUANE INI~’~IK DANMARKS HAVN
IC
5.20 347 2 12.37 4 41.75 18
1
ISC ~
TIME ~ ~C
C OP 5D
RESID
1SC RES~D8
20 32 38
PG IG
PG
P/PKP
IP
20 33 10
20 33 02 20 33 51.5
+0.1
P
20 34 17
—1.1
P
20 39 09.9+
+0.2
Fig. 3. Example of association of a teleseismic reading (DAG) causing mislocation of a locally determined event.
WEL 00127
3 42’lS.O
37.19 S
176.88 E
232KM
WEL 4.4
ISC*0C127 23 41’55 36.5 S 177.3 E 403KM 1.80 12 TIPS +1.4 +0.18 +0.23 +8.4 /9 160:OFF EAST COAST OF NORTH ISLAND, N.Z. CODE
STATION NAME
GBZ WTZ
GREAT BARRI~ WHAKATANE
WTZ ECZ KRP
WHAKATANE WHAKATANE T CAPE EAST CAPE KARAPI’~O EAS KARAPj’~C
GNZ
GISBORNE GISEO3NE
TIJA
MNG
TUAT ThAI NGAURIJHOE NGAU~UHOE TAPACALE TAPAtIALE MANGAHAO
WIG TINS
MANGAHAC MANGAHATI
NGZ TRZ
WEL * I’ WEL MAW
DELTA DES 1.48 1.54
2 3 2 1.59 2.02 141 223
WELLINGTON * * * WELLINGTON MAWSOM
2.25 165 2.34 183
3.02 205 3.11. 187
4.38 198
*
*
WEL OCT27 23 42’15.0 ISC*0C127 23 42’14.2 10 CBS +0.38 159:NORTH ISLAND, CODE
STATION NAME
WTZ WTZ KRP
WHAKATANE WHAKATANE WHAKATANE KARAPIRO KARAPIRO GREAT BARRIER EAST CAPE EAST CAPE TUAI TUAI GISBORNE GISBORNE NGAURUHOE NGAURUHOE TARADALE TARADALE MANGAHAO M&NGAHAO MANGAHA.O MANGAHAD WELLINGTON
GBZ ECZ TUA GNZ NCZ TRZ MNG MNG MNG WEL
1
*
2
E
2
E
2
P/PKP ES
I E E
E
P
E
37.19 5 176.88 8 37.05 S 176.91 E +0.050 +0.063 NEW ZEALAND
ES ES * * * 51(51(S) PIPKP P 232KM
10.0 21 51 50.9—
23 43 19 23 42 54.1+ 23 43 22 23 42 53.3 23 43 21 23 43 00.2
1 E *
TIME C OP HR MN SEC 0 RESID 23 42 50.3— 23 42 46.4 23 42 46.9 23.43 23 43 23 23 42 42
PP
P/PKP P S/(SKS) E S/LSKS) P/PKP P
2
*
ISC ID P P
S/(SKS) P/PKP P ES
1
2 3 4
5.20 201 * * 5.20 201 65,24 203 *
I S S/(SKS) P/PKP P/PKP
EI
2
MA~GAHAr)
*
AZ. ICES OP DEC MPIN ID 279 I P/PKP 189 P/PKP
*
23 43 34 23 43 30 23 43 37 23 43 13.5+ 23 43 14.0 23 43 50 23 43 58 23 44 15 *OIJPIICATE* 23 44 15 23 51 56
ISC -RESID +1,2 —3.0
—22.4 —11.8
+1.3 —1.2
—17.7
+0.4 —1.0
—17.7 —19.9
40.9
—15.8 —14.1 —14.1 —11.8 * * —11.8
+1.6
*
*
—0.4
WEL 4.4
232KM 0.79
DELTA AZ. ICES OP DEC DEC NPIN IC 0.93 176 P/PKP 2 1 I 3 S 1.40 231 1 PIPKP 2 S1(SKS) 1.42 305 I P/PKP 1.45 117 8 PIPKP 2 S/(SKS) 1.76 174 P/PKP 2 E ES 1.82 151 P/PKP 2 8 ES 2.36 205 I P1PKP 2 S/ISKSI 2.50 182 E 2 S1(SKS) 3.73 197 P/PKP 2 1 1 3 E E 4 E ES 4.55 201 S1(SKS)
/8 ZSC 10 P P P p P P P P
HR 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23
TIME C OP ISC MN SEC 0 RESID RESID 42 46.4 —0.? 42 46.9 43 10.0 —2.9 42 50.9— +0.5 43 19 0.0 42 50.3— -0.2 42 51 +0.2 43 21 +1.3 42 53.3 —0.1 43 21 —3.1 42 54.1+ +0.3 43 22 —2.8 43 00.2 +1.0 43 34 —0.3 43 30 43 37 40.2 43 13.5+ —1.1 43 14.0 43 50 43 58 —3.6 44 15 -4.1
Fig. 4. Example of removal of a mis-associated teleseismic reading (MAW) improving earthquake location.
89
association of a reading from Danmarks Havn with an Alaskan earthquake gives a mathematically acceptable solution, but one which is displaced by several degrees from that given by the local agency, Ottawa. This reading, and the ISC origin, were removed from this earthquake, as the editor did not consider the association a likely one, Figure4 shows the improvement in solution that follows the removal of a reading from Mawson that had been associated with a New Zealand earthquake (a duplicate Wellington S reading is also removed). The second ISC solution agrees much more closely with that determined by Wellington, and gives much better residuals for the S phases. The examples given here are not difficult to interpret correctly, but there are. cases of real uncertainty, when the inclusion or exclusion of a lone reading from a sensitive station at teleseismic distance can alter a location substantially, and both solutions are equally acceptable. The availability of information on amplitude and period helps to resolve such ambiguities, and when an estimate of magnitude from closer stations is available it is possible to calculate a “likelihood” that a particular station of known position
ISC*OCT11 07 43’31.8
18.16
N
and sensitivity would or would not record a certam event. Information as to whether other nearby stations have or have not recorded a given event also helps in deciding the likelihood of a given association. When all credible associations between phase readings and known origins have been made, the remaining unassociated readings are subjected to a “search” procedure based on that of Engdahl and Gunst (1966) which examines each successive grouping of three consecutive phase arrivals. If these can be associated as from a single event, then further associated readings are sought, and a solution given if five consistent station readings are found. This preliminary origin is then subjected to the usual revision procedure applied to origins from other agencies. The difficulties of association become even more pronounced at this stage, and in extreme cases it is easy for local readings from two or more events in different parts of the world to combine to produce a fictitious event many tens of degrees from any real activity. About one half of these initial associations are rejected as spurious, but 200 such new events are found each month. Figure 5 gives an example of such a fictitious earthquake. Read-
153.93
15 CBS 612:HAWAIIAN ISLANDS REGION CODE STATION tiAME DELTA AZ. ICES DEC DEC NPIN RAB RABAUL 57.57 253 8 ULA ULAMONA 58.74 252 E 1MG LAMINGTON 63.17 250 1 * * * * * * * * * * * * * * 1MG LAMIPIGTCN 63.17 250 LAMINGTCN 2 E LAT LAE 63.30 253 8 *
*
*
*
*
*
*
*
*
*
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*
*
*
~
*
*
*
*
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*
*
*
*
+
WARRAMUNGA AR WARRAMUNGA AR WARRAMUNGA AR ALICE SPRINGS * * * * * ALICE SPRINGS BANGUI
*
*
*
63.30 253 64.2? 250 *
*
*
64.27 250
PORT MORESBY PORT MORESBY QUITO NASA COTOPAXI WARRAMUNGA AR WARRAMUNGA AR WARRAMUNGA AR
JAQ NCE WCB WCB *
*
LAE PORT MORESBY
76.23 95 76.37 95 79.88 245 *
*
*
*
*
*
*
*
2
81.77 242 * * * 81.77 242 156.35 19
*
*
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*
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3
*
393KM
ISC
OP IC EF EP IF * * P/PKP ES EP
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*
*
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* *
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P
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TIME C OP ISC HR MN SEC C RESID RESID 07 53 02.0 +17,5 07 52 45.0 —7.2 07 53 13.0— —8.6 *DUPLICATE* * * * * 07 53 13.0— —8.6 07 53 52 —449.3 —3.5 C? 53 18.0 —4.4 *DUPLICATE* * * * * Q7 53 18 —4.4 ~ 53 25.5 +0.7 *
DUPLICATE *
07 01 07 07 07 07 C?
53 54 54 54 55 55 56
29.5 22 39.0 44.2 03.0 05.0 58.7
*DUPLICATE*
0? 55 C3.0 07 55 05.0 07 56 58.7 07 55 39.0 *DUPLICATE* 07 55 39 08 02 42.4—
Fig. 5. Example of a spurious earthquake found by mis-association of widely scattered readings.
*
*
*
—432.8
*
*
*
+0.7 —1.2 +3.2 +3,3 —1.3 *
*
+3,3
—1.3 *
*
+29.6 * +29.6 +1.8
90
ings from the South Pacific, Ecuador, Australia and Africa have combined to produce a spurious event at a depth of 393 km near Hawaii. This event was removed from the file. Correct association can be aided by more complete availability of amplitude and period information, and by information on the character of the recording. Some algorithms have been devised to test these concepts, but at this stage the help of an experienced seismologist is still required to monitor such work. 2.3. Location Once station readings have been associated into events, a revision procedure is used to refine the origin parameters. This is a standard least-squares procedure, using weighting according to Jeffrey’s method of uniform reduction (Jeffreys, 1961). mitially a four-parameter solution, including depth, is sought. If this does not converge, the program seeks a solution with depth determined from depth phases, after scanning all secondary phases in an attempt to identify additional pP or sP phases. The next trial is to adopt a depth given by an external agency, and finally to adopt a conventional depth of 33 or 0 km. Once a solution has been found, duplicate readings are automatically eliminated and the process repeated. Seismologists have the facility to override the computer’s choice of option. Unsatisfactory events are rejected or modified, in batch mode, through five or six successively smaller runs until all remaining events have satisfactory solutions. Revision of location is done on P phases only, using the Jeffreys—Bullen (1940) travel-time tables. Since the data year 1978, improved use has been made of crustal phases P5, ~* and P~,according to the following scheme: (a) if at close distances and shallow depths, only one particular phase is geometrically allowable, phases are identified as such; (b) if the reporting agency identifies a particular phase, this identification is used provided that it is geometrically allowable; (c) if there are several possible identifications for an unspecified P phase, the program chooses the identification consistent with the earliest arrival time;
(d) the seismologist may override the program and specify any allowable phase identification. PKP, S and later phases are identified, and have residuals allocated, but are not used in the location process. Depth phases pP and sP are recognised if specified by the reporting station, and can be identified by the program for the determination of depth, either in addition to that derived geometrically, or as the definitive method if no geometrical determination is possible. No provision is made for the use of station or source corrections, regional travel-time tables, or joint methods of hypocentre location. The solution found by the ISC revision procedure is normally adopted as the “prime” estimate for cataloguing and reference purposes. When a local agency supplies an origin estimate with no supporting readings, Or not enough are available for a satisfactory revision, the local origin is adopted as the prime. Only in very exceptional circumstances is an event which is found by the ISC search procedure retained in the final listing if it fails to converge to a satisfactory solution. 2.4. Location errors Formal standard errors are given for each parameter determined by the least-squares procedure. These are calculated from the standard deviation of the residuals, and the number of degrees of freedom involved. The standard errors are generally small when the number of observations is large; for events determined from a hundred observations or more the errors in position are usually 2 or 3 km, and seldom more than 5—10 km. When the solution is based on data from only a few stations, the standard error may be unrealistically small, and represent only the degree of consistency of the observations. In such cases, the presence or omission of a single reading may result in solutions which each have a small error, but are separated by an amount greater than either. All errors determined are mathematical only, and assume that the velocity model used is physically correct. The use of inappropriate velocity models may result in systematic mislocations greater than the standard error of the solution.
91
2.5. Quantification Present procedures determine magnitudes only at teleseismic distances. For P waves, magnitudes ~ are determined from submitted. readings of amplitude and period in the distance range 21— 1000, using the Gutenberg and Richter (1956) calibration functions, and for surface waves M~is determined from waves of period 5—60 s, recorded at distances of 5—160°, using the “Prague” formula of Vanek et al. (1962). For both kinds of magnitude, determinations at individual stations are averaged to be included in the origin estimate of each event, with the exception that determinations of M, in the distance range 5—20° are not included in the average, but appear only with the individual station reading. Magnitude determinations derived by external agencies are reproduced in their epicentral records, and if these remain as the “prime” solution they appear in the main historical file of events, ISC has no provision for the determination of local magnitudes, and it would be inappropriate for it to do so globally. The determination of local magnitudes can be carried out only by local agencies with knowledge of instrumentation and attenuation conditions. Events below a certain size cannot be recorded teleseismically, so some form of local magnitude determination is the only possible means of determining their size,
3. Possible ways of improving analysis The following suggestions are meant to be indicative of present weaknesses in procedures at ISC and other global data centres, and representalive of ways in which they could be improved. They are not necessarily all practicable at this time, and in some cases may not be desirable for a definitive agency such as ISC, until they have been tested as research options by other agencies. 3.1. Association Existing procedures perform well considering that they work in the time domain only. The addition of procedures to analyse amplitude and
period information would allow the likelihood of a given association to be assessed numerically. Improvement could also be achieved by using distance criteria. A single station associated with an event at much greater distance than all other stations is suspect, although such associations can occur with high-magnification stations and good transmission paths. Similarly, associations of an isolated group of stations at a large distance might suggest the occurrence of a separate event near those stations. For such associations to be analysed numerically full information would be needed on station operation, magnification, periods of non-operation, and completeness of reporting. 3.2. Location and velocity models At present each event is considered individually. Joint hypocentral determination or the use of master events could be useful in certain areas, particularly where some events are known to be well determined in position, either by local networks or by observations of controlled sources. The most serious deficiency is the use of mappropriate velocity models. The present Jeffreys— Bullen model has served seismology well, but is known to be deficient in several respects. When a new reference Earth model is adopted by IUGG (see for example, Dziewonski and Anderson, 1981), use of its derived travel-time functions should be considered by ISC. It is, however, quite unrealistic to expect highquality solutions from any laterally homogeneous Earth model, and it is surprising that such models have given results that have been acceptable for so long. The lack of differentiation between shallow continental and oceanic structure is a major deficiency: but note that NEIS now routinely restrains oceanic foci to a depth of 10 km, rather than the conventional value of 33 km. A much more serious deficiency is the neglect of provision for the major velocity inhomogeneity associated with zones of deep earthquakes, where velocities are typically 10% higher than in the surrounding mantle (Engdahl, 1973; Robinson, 1976). The deficiencies of the present models are not always evident, and not always appreciated by those not working on these problems. A feature of
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the least-squares procedure is that it will often find a good mathematical solution even if the physical model of velocity deviates significantly from reality. Examples of this are particularly noticeable in the South Pacific, where velocities higher than normal give residuals at close stations, such as Afiamalu and Rarotonga, that are often early by as much as 10 s or more for large events located by many teleseismic observations. For smaller events, on the other hand, with fewer or no teleseismic readings, the few regional stations can give apparently good solutions with small residuals, but which may be badly misplaced. Locations in such areas, that are based on small numbers of stations, must be treated with great caution, and the mathematical standard errors of the solutions must not be accorded any physical significance. It would be possible to use station and source corrections with the present model, preferably taking account of distance, azimuth and focal depth, and some models have been devised to approximate velocity inhomogeneities (see for example, Adams and Ware, 1977) and thus produce earthquake positions that are more realistic. The final desirable solution must be to use ray-tracing techniques in a realistic model, taking account of the three-dimensional velocity structure. Proper location of shallow local earthquakes also requires the use of correct velocity models, and for many restricted areas of local seismic activity detailed models of velocity structure are available that could be incorporated in the ISC programs to be invoked if both epicentre and station fall within a specified region. The present ISC model is stored mainly in the form of- traveltime tables for various phases, but it would be preferable if variations in shallow structure were stored in the form of velocity—depth distributions from which the desired local travel times would be calculated. This is already done for the upwardgoing direct P phase from the lower crust in the Jeffreys—Bullen model, 3.3. Weighting At present the only weighting of individual equations of condition depends on the residual of the arrival time. This is done by Jeffreys’ (1961)
method of uniform reduction which gives the residuals a normal distribution, to which a leastsquares procedure can more properly be applied. No weighting is applied to allow for variations in station distribution. In Bolt’s (1960) original leastsquares location program, station weighting was given inversely according to the number of stations in each quadrant of azimuth, and some such weighting according to the geographical density of stations is probably desirable; however, care must be taken lest a bad reading from an isolated station is given undue prominence. It is obviously bad for each individual station of a close network to be given equal weight in teleseismic determinations. The best solution may be to reduce progressively the weight of individual stations, once the station density in a given region exceeds a certain threshold. 3.4. Quantification The determination of earthquake size is probably the area in which the present ISC output is most deficient. Despite some improvement in procedures in the last few years, only 40% of ISC prime estimates contain any magnitude information at all. There are two main reasons for this. The first is the lack of information on amplitude and period submitted to the Centre. This arises in some cases from difficulty in deciphering these parameters from the record, and also from the need for revision of ISC reporting formats. The second factor is that it is proper for ISC to calculate magnitudes in the teleseismic range only, and not to determine local magnitudes for small events. Thus events small enough to be recorded only locally cannot be assigned a magnitude by present ISC procedures. In many of these cases, local agencies have determined origins, with local magnitudes, which are included in the ISC publications as non-prime origin estimates. Consideration is being given to the addition of locally determined magnitudes from other agencies to ISC origin parameters to form a “composite” prime estimate. One of the difficulties of systematic local magnitude determination is the diversity of recording instruments, on which different parameters may be read. For instance, in many conventional
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short-period systems the recorders are too slow to enable the determination of ground period, and the number of Wood—Anderson-type seismographs existing for the determination of the classical magnitude ML from amplitude alone is not large. The parameter most easily determined for local earthquakes is the duration of recording, and this may be the best parameter to submit to global data centres, provided that instrumental effects are known. The routine determination of other source parameters, such as seismic moment, is again best carried out by the individual stations, or national centres, where all relevant instrumental and structural information should be known, but there is no reason why ISC should not reproduce values submitted to it. The present determinations of teleseismic magnitudes, mb and M~,could be improved by greater use of regional calibration curves and station and source corrections by some system such as the homogeneous magnitude system (Christoskov et al., 1979). Care must be taken, however, in applying such systems in tectonically active zones, in which three-dimensional variations in structure may cause anomalous attenuation as well as effects such as focusing and refraction of energy. 4. Conclusion Although deficient in various respects, the accumulated output of the ISC (since 1964) and of its predecessor, the ISS (1913—1963), forms a unique collection of earthquake data, which is continually being improved in quality. Care must be taken to reach a balance between the need to improve the analysis procedures used, and the desire to maintam a homogeneous collection of information. It is obvious that some provision must soon be made to adopt a more realistic global velocity model, to introduce methods to allow for the large velocity variations known to occur in the Earth’s
outer layers, and to improve earthquake quantification. May the results of this and similar Symposia help us to achieve this aim.
Acknowledgements The procedures described here have been developed over many years with the help of many people. Among those particularly involved are Drs. E.P. Arnold, S.J. Gibowicz, A.L. Levshin and P.L. Wilimore.
References Adams, R.D. and Ware, D.E., 1977. Subcrustal earthquakes beneath New Zealand: locations determined with a laterally inhomogeneous velocity model. N.Z. J. Geol. Geophys., 20: 5983. Bolt, B.A., 1960. The revision of earthquake epicentres, focal depths, and origin-time using a high-speed computer. Geophys. J., R. Astron. Soc., 3: 434—440. Christoskov, L., Kondorskaya, NV. and Vanek, J., 1979. Homogeneous Magnitude System of the Eui~asianContinent: P Waves. Rep. SE-18, World Data Center A for Solid Earth Geophysics, Boulder, CO. 57 pp. Dziewonski, A.M. and Anderson, D.L., 1981. Preliminary reference Earth model. Phys. Earth Planet. Inter., 25: 297—356. Engdahl, E.R., 1973. Relocation of intermediate depth earthquakes in the Central Aleutians by seismic ray tracing. Nature (Phys. Sci.), 245: 23—25. Engdahl, E.R. and Gunst, R.H., 1966. Use of a high speed computer for the preliminary determination of earthquake hypocentres. Bull. Seismol. Soc. Am., 56: 325—336. Gutenberg, B. and Richter, C.F., 1956. Magnitude and energy of earthquakes. Ann. Geofis., 9: 1—15. Jeffreys, H., 1961. The Theory of Probability. Oxford University Press, edn. Jeffreys, H. andOxford, Bullen, 3rd K.E., 1940. Seismological Tables. British Association for the Advancement of Science, London, 50 pp. Robinson, R., 1976. Relative teleseismic travel-time residuals, North Island, New Zealand, and their relation to uppermantle structure. Tectonophysics, 31: T41 —T48. Vanek, J., Zatopek, A., Karnik, V., Kondorskaya, N.y., Rizrnchenko, Yu. V., Savarensky, Y.F., Solovev, S.L. and Shebalin, N.Y., 1962. Standardization of magnitude scales. Izv. Acad. Sci. U.S.S.R., Geophys. Ser., 2: 153—158.