Since the observations of the first instrumentally recordedearthquake in the late 1800s, the discovery and documen-tation of the mysteries of the earth’s deep interior and earth-quake rupture processes have proceeded hand-in-hand withimprovements in instrumentation and data exchange. Thepioneering discoverers in the first half-century of instru-mental seismology were those scientists fortunate enoughto have a seismograph at their institute. The excitement ofobserving a distant earthquake as its energy was recordedwas a powerful catalyst for curiosity-driven studies of thedeep earth. The results were dramatic. Within about a decadeof recording the first seismogram, the gross internal struc-ture of the earth (crust, mantle, core) was established andgeoscientists began to ponder the evolution of the earthfrom the perspective of these new discoveries. Forty yearsago, the Cold War and the need to discriminate under-ground nuclear explosions from the background of naturalearthquakes spawned the second revolution in whole earthseismology with the installation of the World WideStandardized Seismograph Network and a more centralizedarchiving approach that facilitated effective data exchange.The WWSSN immediately contributed to another revolu-tion in geoscientific thought: plate tectonics. Its 20-plus yeararchive of seismograms continues to contribute to new dis-coveries about the earth, such as the differential rotation ofthe inner core.

One hundred years after the first wiggly line hit paper,we are in the midst of discovery spawned by a third revo-lution in seismic instrumentation. It began in the mid-1980swhen IRIS (The Incorporated Research Institutions forSeismology; www.iris.edu) was established by the UnitedStates’ university community and the U.S. National ScienceFoundation to modernize seismic instrumentation to addressoutstanding problems on a wide spectrum of scales. In addi-tion to installation and operation of new instrumentation,a key component of the present revolution is in data deliv-ery capabilities as current and emerging technologies allowus to deliver seismological data to an individual’s desktopfrom almost anywhere on the planet in near-real time. CoreIRIS facilities for instrumentation and data delivery includeThe Global Seismic Network (GSN), The Program for ArraySeismic Studies of the Continental Lithosphere (PASSCAL),and the Data Management System (DMS).

DMS operates the Data Management Center (DMC) atthe University of Washington in Seattle. This is the centralarchive for both GSN and PASSCAL data and for data frommany other sources around the world. DMC provides avariety of mechanisms to request and receive seismic datafrom its archive, from real-time data feeds to email requestsfor data to be mailed on tape. DMC has a 50 terabyte roboticmass storage device and a number of smaller disk farms toaccommodate the several terabyte per year influx of world-wide data. DMC data can be requested by anyone in theworld; it is not subject to any distribution restrictions.

The IRIS GSN maintains and operates a network of ~128permanent seismological observatories around the globe(Figure 1). These stations and similar installations operatedby the international community (loosely organized as theFederation of Digital Broadband Seismograph Networks orFDSN) are high fidelity digital instruments with broadband

frequency sensitivity. Digitizers have 24 bits of resolutionwith sampling rates of at least 20 samples per second (sps)for continuous recording with 80-100 sps becoming morecommon. Timing is provided through GPS systems. A majordesign goal for GSN is to deploy instruments with a mini-mum station spacing of 2000 km. Perhaps the largest receiverspacing deployment ever discussed in TLE, the GSN designwas considered the absolute minimum necessary to improvetomographic imaging of the whole earth using surface wavesand long-period body waves. The longer-period sensors ofGSN have responses similar to the blue line in Figure 2 andare those used for whole earth tomographic studies. Siteswith higher sampling rates are often also equipped with asensor of response similar to the yellow line. The higher fre-quency recording is motivated by the need to monitor theglobe for clandestine underground nuclear tests, a role thatGSN inherited from its analog predecessor, WWSSN. ManyGSN and FDSN stations contribute data to the InternationalMonitoring System under development in anticipation of aComprehensive Nuclear Test Ban Treaty (www.ctbto.org).

The second element of the IRIS instrumentation facilityis PASSCAL (Program for Array Seismic Studies of theContinental Lithosphere). As its name implies, PASSCAL’soriginal focus was on detailed studies of the continentallithosphere. With the ultimate goal of collecting unaliasedsampling of the seismic wavefield from both active (planned

220 THE LEADING EDGE MARCH 2003

New instrumentation drives discovery of the earth’s deep interiorTHOMAS J. OWENS, University of South Carolina, Columbia, U.S.JAMES FOWLER, Incorporated Research Institutions for Seismology, Socorro, New Mexico, U.S.

Figure 1. Permanent broadband seismograph stations operated by IRISand a variety of international organizations around the globe. Open starsare planned new IRIS stations.

Figure 2. The relative frequency response of sensors of the IRIS facility.

explosions and vibrators) and passive (earthquakes andother unexpected seismic signals) sources, PASSCAL hasdeveloped a very versatile instrument pool (Figure 3). Withsensors available with a frequency sensitivity range of overfive orders of magnitude (Figure 2), university researchershave deployed PASSCAL instrumentation at receiver spac-ings ranging from a few meters to a couple of hundred kilo-meters to address an equally diverse range of scientificproblems. In addition to more familiar temporary instrumentdeployments such as active source experiments using indus-try-style equipment available in the PASSCAL pool and

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Figure 5. Instrumentcheck-out during arefraction experimentin Antarctica.

Figure 6. Broadbandstation deployment inAlaska.

Figure 3. Members ofthe PASSCAL familyof sensors. Clockwisefrom the top: Silversensor = three-compo-nent L-22 with anatural period of 2 Hz;green sensor =Strekheisen STS-2broadband sensor witha frequency responseshown by the yellowline in Figure 2;orange single compo-nent 40-Hz geophone; and three-component 4.5-Hz geophone similar tothose used in the exploration industry. Responses are shown in Figure 2.

Figure 4. Past, present, and future PASSCAL broadband experimentdeployments.

earthquake aftershock arrays, the PASSCAL instrument poolhas allowed for rapid growth of an entirely new type ofexperiment, the passive-source broadband experiment. Priorto PASSCAL, broadband instruments were strictly thedomain of permanent observatories. Through acquisition ofan instrument pool based on new, rugged, low-power 24-bit digitizers and portable broadband seismometers, PASS-CAL allows collection of essentially observatory qualitydata anywhere in the world. As a result, the deep structureof the continental lithosphere is but one of many targets thatare the focus of PASSCAL deployments that span the globe(Figure 4).

A successful PASSCAL broadband experiment requiresa good design and a lot of hard work. Broadband PASSCALexperiments depend on the rate and distribution of globalseismicity. Companion articles by James, and Aster andWilson in this issue outline the ways in which the academiccommunity is using distant, or teleseismic, earthquakes toimage the earth’s lithosphere. To design an experiment toexploit global seismicity requires consideration of the loca-tion of the major source regions, primarily the Pacific Rim,relative to the subsurface targets. Teleseismic P- and S-wavessample the crust-mantle boundary offset roughly 6-10 kmlaterally from the recording station in the direction of theearthquake. However, to examine the structure of the man-tle transition zone at a depth of 660 km, an experimentdesigner must account for the ~200 km lateral offset betweenthe station and the subsurface sampling point. By using therate of seismicity from a previous “typical” year, the prob-able distribution of sampling points in the vicinity of the tar-

get of interest can be calculated and an effective experimentdesign developed. Although the earth will never produce theexact number and magnitude of earthquakes used in theexperiment design, the overall rate and distribution rarelychange drastically from year to year, so experiments can bedesigned with some confidence ... and with a little bit of luck,global seismicity can be very cooperative!

Deploying a PASSCAL experiment in a remote region ofthe world is no small feat. Roughly 250 lbs of equipment mustbe shipped per site from the PASSCALInstrument Center (PIC)in Socorro, New Mexico (www.passcal.nmt.edu). So, a modestdeployment of 20 instruments generates an outgoing shipmentof more than 5000 lbs (Figure 5). Construction materials andbatteries are normally bought locally. Funding realities restrictexperiments to a bare-bones installation crew; most investi-gators are thrilled to be able to afford a crew of four people,including themselves and often a technician from the PIC.These folks handle all of the custom clearances, site permit-ting, site design and construction, and instrument installation(Figure 6). Depending on the terrain and local regulations, eachPASSCAL site takes 3-4 days to get up and running.

A broadband PASSCAL station normally consists of two“vaults.” One houses the sensor itself and is hopefully locatedwhere bedrock is covered by a meter or two of soil. Buryingthe vault in a thin veneer of soil provides both security andthermal stability for the sensor. Sensors themselves are sealed,triaxial units that must be carefully aligned with geographiccoordinates (Figure 7). The second vault houses the digitizer,batteries, and related gear, leaving only the solar panels andthe GPS antenna visible at the surface (Figure 8). Sites typi-cally record to a hard disk and thus must be visited for ser-vicing every 3-5 months although recent developments intelemetry capabilities are allowing more experiments to useFreeWave radio technology to transmit their data to anInternet-capable site. In these cases, the data then flow directlyto the investigator’s desktop for immediate analysis.

What do PASSCALinvestigators get for their efforts? Data.Lots of data. Never as much as they hoped, of course, butalways enough to make significant headway toward their sci-entific objectives. Atypical experiment will record earthquakesfrom a wide range of distances, producing in a year or twoenough data to address their problem and a wide variety ofother problems in earth structure (Figure 9). Through com-munity consensus, investigators are allowed proprietary accessto the data for a period of two years. After that point, it isreleased to the community through the IRIS DMS for use byany interested investigator in the world.

While most PASSCAL broadband experiments are under-taken to address a key problem in earth structure, they areoften located in seismically active regions as well. And, some-times, they get very lucky. In March 1994, a PASSCAL exper-iment in Fiji/Tonga was ideally located above a majorearthquake, a moment magnitude (Mw) ~7.6 event at about536 km depth. This event was the largest deep earthquake inabout a quarter of a century and the first major deep earth-quake since the advent of high-resolution digital instrumen-tation. A bonanza unlikely to be repeated in our careers?Obviously; yet three months later, Bolivia was hit by anMw=~8.3 event at 645 km depth. And, another PASSCALexperiment was sitting right on top of the epicentral area! Thesetwo remarkable strokes of serendipity supplemented by anew digital GSN re-energized the debate on the origin andrupture processes of deep earthquakes, revitalizing a fieldthat had stagnated due to the lack of appropriate data.

While this third revolution in seismic instrumentation willcontinue to result in new discoveries about the earth throughdeployments around the globe, we can look forward to

222 THE LEADING EDGE MARCH 2003

Figure 7. An STS-2sensor deployed in a55-gallon drum, acommon “vault” usedby PASSCAL investi-gators. The hole for thevault is normally dugdown to bedrock in alocation with a meteror so of soil cover. Thisprovides thermalstability of the sensorand some security. Thesensor pad may be apaving stone or a concrete pad secured to the underlying bedrock. In thissite, river stones are used to enhance drainage in the vault; many sitesalso use a French drain to keep water away from the equipment. The steelbar extending from the sensor in this photo is used to align it with geo-graphic coordinates. Sensor alignment is both crucial and difficult. ABrunton compass, care, and patience are still the primary tools for sensoralignment and can result in an accuracy of a degree or two. A gyroscope-based tool, whose use is increasing, may improve the accuracy by an orderof magnitude.

Figure 8. A completed PASSCAL-style installation. This site is actually aprototype USArray installation near the PASSCAL Instrument Center inSocorro, New Mexico.

USArray, described in a companion article by Meltzer, as per-haps the fourth revolution in seismic instrumentation. Thatrevolution is driven primarily by numbers of instruments, inte-gration with other geologic and geophysical observations,

and a focus on a “local” problem—the evolution of the NorthAmerican continent! TLE

Corresponding author: owens@seis.sc.edu

MARCH 2003 THE LEADING EDGE 223

Figure 9. A composite global record section from the 1991-92 Tibetan Plateau Seismic Experiment. The section was assembled by selection of the high-est signal-to-noise ratio seismogram from each 100-km distance bin. Some bins had no seismograms, others had more than 100. The seismograms werethen adjusted to a common depth and plotted with a reducing velocity of 13.8 km/s. Reduced time equals arrival time—distance/Vr, where Vr is thereducing velocity. 1° = about 111.19 km. Energy from a number of seismic phases is used by PASSCAL researchers to investigate earth structure.