IRIS NEWSLETTER12

When most of us associated with theIRIS Consortium speak of “seismicdata,” we usually mean seismograms.But for much wider communities ofEarth scientists and engineers, forpolicymakers—and for people injournalism, emergency management,nuclear test-ban monitoring, insurance,and business generally—the basic“seismic data” are not seismograms, butproducts derived from seismograms. Ofthese, the most important are lists ofearthquake and explosion locations, withtheir origin times and magnitudes (orsome other measure of ground motion).Such hypocenter lists, or seismicitycatalogs, are often backed up by apublished seismicity bulletin givingmeasured arrival times at the detectingstations—which may be regional orteleseismic.

Bulletins of seismicity, whether theyare produced on a local, regional,national, or global basis, are nowundergoing profound changes. Betteraccuracy, and/or better coverage tolower magnitude, has often been the keyto new insight into earthquake processesand Earth structure, and has enablednew levels of confidence in the ability tomonitor a region of interest. Of course,new insights and new monitoringcapabilities are the very rationales uponwhich much work in seismology isfunded.

Different Types of BulletinFor some users, prompt reporting on

all types of seismicity is essential. Forothers, the most complete catalog ofearthquakes or explosions is needed,even if locations are not worked upuntil a few years after the events occur.For some users, accuracy of theestimated event parameters(hypocenter, magnitudes, moment) isparamount and it is acceptable ifsmaller events with poorly estimatedparameters are ignored. For other users,it is more important to be sure that all

Future Directions for Global BulletinsPaul G. Richards, Columbia University

possible seismic events are reported,even if special studies later show thatsome of these events are the result offalse associations, and/or have poorlydetermined locations.

Since these different users cannot allbe satisfied with the same type of globalbulletin, it is helpful to consider theproduction of a range of products.

Such a range of products would helpsummarize seismic activity for manyinterested users outside seismology —in geophysical research, in quantitativeestimation of seismic hazard, and inmonitoring arms control treaties. Forusers inside the research community ofseismologists these products couldprovide feedback to the question ofwhere new stations should be sited.Improved reporting on seismicity would

renew debate on the merits of boreholeinstrumentation, and increase theimportance of quiet sites and stationreliability.

The commitment to produce anaccurate global bulletin that is completedown to magnitude 4 (about 20 eventsper day) is surely now a realistic goalwith openly available data. Since 70%

of seismicity occurs beneath the oceans,and each decrease by one magnitudeunit corresponds approximately to aneightfold increase in numbers of events,it follows that there are approximately50 events per day on continents withmagnitude 3. The goal of monitoringdown to magnitude 3 on continentsappears attainable on a time scale ofabout a decade. The number of eventshere, about 50/day, is comparable to the

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7577 earthquake locations estimated by the Northern California Earthquake Data Center, forthe Calaveras Fault from 1984 to the present. (a) Map view of events rotated along the 146degree strike of the Calaveras Fault. (b) Depth section displaying earthquakes on the faultwith estimated source sizes based on a circular rupture model using a 3 MPa stress drop.

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COMMENTARY

13IRIS NEWSLETTER

number of events handled routinely byregional data centers in seismicallyactive areas such as California. (Ofcourse, such data centers achievecomplete regional coverage well belowmagnitude 3.)

New Procedures for Event LocationAt present, all three of the global

bulletins described in this newsletterrely heavily upon standard one-dimensional Earth models for purposesof interpreting arrival times, in theprocess of iteration to find the best-

fitting location. Resulting locationestimates can still be quite accurateprovided there are enough reportingstations, with no large gap in azimuthalcoverage. But for a sparse network, suchas the IMS associated with the CTBT, anew approach must be adopted. It is

Improved location estimates using the correlation method for measuring relativearrival times at each station and a double-difference technique (Waldhauser andEllsworth, Bull. Seismol. Soc. Am., in press). The same 7577 events are shown, andon the same scale. (a) Note the fine structure (seismicity lineaments) as well asseveral off-fault structures. (b) The great reduction of vertical errors shows thatseismicity is largely concentrated into several discrete bands that contain events ofwidely varying magnitudes. [Figures are courtesy of David Schaff, Goetz Bokelmann,Greg Beroza, Felix Waldhauser, and Bill Ellsworth.]

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desirable to calibrate each IMS stationso that in effect the location of a newevent can be located with reference toanother event, whose location is knownaccurately and which, preferably, is notfar from the new event. By using asufficiently large number of calibrationevents, whose location is accuratelyknown and whose signals are detectedreliably at IMS stations, it is possible togenerate a station-based travel timesurface (a function of distance andazimuth), for each seismic phase.Different surfaces are needed for

different event depths. For CTBTmonitoring, the most important surfaceis that for zero depth. The IDC hasbegun using station-based empirically-determined travel times for stations inNorth America and northwesternEurasia; and plans are in place to obtain

and use such travel times for stations inNorth Africa, the Middle East, and EastAsia. At present, errors in event locationare caused by pick errors and modelerrors, with model errors being far thelarger (at least for events above aboutmagnitude 4.5). The use of station-specific travel times can be expected toachieve a significant reduction in themodel errors.

Looking further to the future, it willbe important to apply to globalbulletin production some of the newmethods of event location recentlyfound useful in regional studies. Thefirst method that has clearly been veryimportant, is the use of cross-correlation techniques to measurerelative arrival times accurately fortwo or more events observed at thesame station. Such an approachreduces pick errors. The figures hereshow catalog locations for about 8000events on the Calaveras Fault,California, and their relocations basedupon inversion of relative arrival timesdetermined by cross-correlation (workreported by Schaff and others, at theDecember 1999 AGU meeting: seealso http://pangea.stanford.edu/~beroza/location.html). A key to suchfuture work, needing millions of cross-correlations, is very fast access todigital waveform data. The underlyinglocation method, developed byWaldhauser and others (in press withthe Bulletin of the SeismologicalSociety of America), can useconventional phase picks or cross-correlations. It is based on a doubledifference scheme that analyzes allpossible pairs of events, and theirrelative arrival times at detectingstations.

Bulletins of global seismicity are theproduct of a vast community, rather thanof a few smart hard-working people. Butthey reflect what the larger communitywants. Given what is achievable overthe next several years, this is probablythe time to think how archives ofseismic signals should be established, toachieve the kinds of improvement inseismic event location that now appearpossible.

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