North Dakota's Current Seismic

Fred Anderson

Priorto2008,thestateofNorthDakotadidnothaveanyseismicmonitoring instrumentation in place capable of recordingearthquakes that could originate either locally, regionally, orglobally.Currently,NorthDakotahasthreeoperatingbroadbandseismic monitoring stations in the south-central and easternpartsofthestate(fig.1).InOctoberof2008NorthDakota’sfirstbroadband seismic monitoring instrument was installed near Maddock,ND(Anderson,2009)asapartoftheUSArrayReferenceNetwork. TheMaddock station was installed ahead of, and inanticipationof,thearrivaloftheEarthScopeTransportableArray(TA)thatwasslowlytraversingthelower48statesfromwesttoeast. From 2008 to 2012 the EarthScope project deployed 36temporary broadband seismicmonitoring stations across NorthDakotaasitcontinueditswayacrossthecontinentalUnitedStates(Anderson,2010).TheEarthScopeTAcompleteditstraverseofthelower conterminous 48 states in the fall of 2013 and is currently deployedacrossthestateofAlaska(IRIS,2016).

WhentheTAofficiallyleftNorthDakotainthesummerof2012,twoseismicmonitoringstationswereselectedtobeleftinplaceto eventually become a part of the Central and EasternUnitedStatesNetwork (CEUSN). Oneof thesestations isnearHuff,15milessouthofBismarck-Mandan,andtheotherstationis inthesouthwesternRedRiverValleynearGardner(fig.2).Thisbroughtthe total number of broadband seismic monitoring stations inNorthDakotatothree.

OurclosestseismicmonitoringstationtothewestisnearDagmar,Montana and is operated by the Montana Bureau of Mines and Geology’s Earthquake Studies Office. A second station, whichwasonce(ca.1965)apartoftheformerLargeApertureSeismicArray (LASA), is located near Angela in easternMontana. TheregionalstationnetworkcanprovidecompleteseismicmonitoringcoverageacrossNorthDakotaandincludesthefollowingstations:Rapid City and Miller, South Dakota; two stations in easternMinnesota (5 Mile Ranch and at the Agassiz National WildlifeRefuge);onestationinsoutheasternSaskatchewanatAquistore,andoneinsouthwesternManitobaatLacDuBonnet.CollectivelythesestationscouldbeconsideredtobesomewhatofaninformalregionalvirtualseismicnetworkforthestateofNorthDakota.

Seismometers in Use in North DakotaThe instruments currently deployed inNorthDakota are three-componentbroadbandseismometerscapableofdetectingground

Figure 1. Seismic monitoring stations located in and around NorthDakota. Stations (shownasblack triangles) include the LASAArrayStationnearAngela,Montana(LAO),Dagmar,Montana(DGMT),RapidCity, South Dakota (RSSD), Aquistore, Saskatchewan, Canada (SV3S),HuffHills,NorthDakota(E28B),Maddock,NorthDakota(MDND),Mill-er,SouthDakota(SUSD),Gardner,NorthDakota(D32B),5MileRanch,Herman,Minnesota(F33B),AgassizNationalWildlifeRefuge,Minneso-ta(AGMN),andLacduBonnet,Manitoba,Canada(ULM).ThelocationsofallhistoricalearthquakesusedbytheUSGSinthegenerationofthelatestUSseismichazardmaps,non-inclusiveofrecentinducedseismic-ityinthecentralUS,areshownasredcircles(USGS,2014).

Monitoring Capabilities

Figure 2. Seismicmonitoring station located northwest ofFargonearGardner,NorthDakotashortlyafter installation.Theseismometerishousedinasealedcircularvaultjustbe-lowgroundlevel(buriedbeneaththeexcavatedsoils). Thesolar panel powering the instrument and supporting elec-tronicsissurroundedbyprotectivefencing.

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motionsoverawiderangeoffrequencies(i.e.abroadbandsensor).These seismometersare capableofdetectingearthquakes fromaroundtheworld(fig.3)inadditiontoweakmotionsfromregionalteleseismicearthquakesandambientnoise(PASSCAL,2016).Theinstrumentscontainthreesensorsarranged inauniquemannerto detect motions in the north-south, east-west, and verticaldirections and resolve the signals captured simultaneously andonthesamescale (HavskovandAlguacil,2016). ThesestationsarethenorthwesternmostintheIRIS-operatedCEUSNsub-array,whichcoversthecentralportionoftheUSfromNorthDakotatoTexas and east to the Atlantic seaboard (IRIS, 2015). Inclusionof North Dakota’s stations into the CEUSN rather than, say, awesternnetworkwasbasedonmanyfactors,butmainlybecausethegeologicalandseismologicalcharacteristicsinEarth’scrustintheeasternUSaredifferentthaninthewest.IntheeasternUSthecrustisolderwithlessattenuation(meaninglessdampeningoftravelingseismicwaves).Asaresult,seismicwaveswillcarryfurther intheeastthan inthegeologicallyyoungerwesternUS.Stations in the CEUSN are anticipated to operate through fiscalyear 2017 under the Incorporated Research Institutions forSeismologyprogram.

Accessing Seismograms and Earthquake DataEach station continuously records the ground movementsmeasuredbytheseismometersandcanbeviewedanumberofdifferentwaysontheweb. AnexceptionallyconvenientwaytoviewdatafromanystationintheCEUSNisthroughtheIRISCEUSNNetwork Station Monitor at http://ds.iris.edu/gmap/_CEUSN.Also, thepast 24hours of stationdata canbequickly accessedby linking to the USArray Station Monitor website, hosted bythe University of South Carolina, at http://usarray.seis.sc.edu/.Simplyenterthecodeforyourstationofinterest(e.g.,MDNDforMaddock,E28BforHuff,orD32BfortheGardnerstation)andthelatest 24 hours of recordeddata, in the formof a seismogram,will bedisplayed. As anexample,oneof themoredevastatingearthquakes that occurred recently was the M 6.2 ItalianEarthquakenearNorcia,ItalyonWednesday,August24th(USGS,2016). This earthquake,whichoccurrednearly halfway aroundtheworld,wasreadilyrecordedontheseismometers locatedinNorthDakota(fig.4).

Inadditiontoearthquakes,otherformsofnaturalphenomenathatrelease energy in short but intense fashion can also be recorded by the instruments inNorthDakota. Anexampleof thiswouldbe the severe summer storms that most North Dakotans arefamiliar with and the associated thunder and lightning commonly producedduringtheseevents(fig.5).Miningactivities,suchastheinfrequentblastingeventsthatareconductedat thecoalminesincentralNorthDakota,canalsoberecordedonNorthDakota’scurrently operating seismic stations. Rock blasting produces asomewhatcharacteristiclookingseismogram(fig.6).

Comparison of Earthquake Energy ReleaseInordertogainabetterperspectiveontheearthquakesthathaveoccurredinthestateandtheircomparisontootherearthquakes,we can compare them on the basis of the energy that radiates awayfromtheearthquakeepicenterasseismicwaves.Thereisatremendousdifferenceintheamountofenergyreleasedbetweenearthquakesofdifferentmagnitudes.Generallyspeaking,eachfull

Figure 3. The Streck-eisenSTS-2isthetypeof three-componentbroadband seismome-ter currently in opera-tionatNorthDakota’sthree active seismicmonitoringstations.

Figure 4. Vertical component seismogram of the August 24th,M 6.2ItalianEarthquakerecordedby theGardnerstation inNorthDakotaasextractedusingtheIRISWilber3seismicwaveformdataservicesextrac-tionapplication.FirstarrivalofP-Wave(compressional)energy,about11minutesaftertheearthquakeoccurredinItaly,ismarkedbytheredverti-callineontheseismogram.ThearrivalsofthereflectedPwave,denotedasPPandreflectedSwave,denotedasSSaremarkedwiththeyellowandblueverticallines,respectively.

Figure 5. Storm seismogram from the severe summer storms thattrackedthroughsouth-centralNorthDakotathispastAugust.Thisseis-mogram,fromthestationlocatednearHuff,ND,coversthe24-hourpe-riodfrom3:00pmCDTTuesdayafternoonofAugust9thto3:00pmthefollowingday.Individuallightningstrikes,thunderrolls,andlargethun-derandlightningsequencesareobservableontheseismogram.

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numericvalueincreaseinearthquakemagnitudeisequaltoabout30timesmoreenergyreleasedthanthepreviousmagnitude(Bolt,1988).

Themost recent earthquake in North Dakota, theM 3.3 eventjust southeastofWilliston thatoccurred in2012,wassimilar incharactertothe“typical”minorearthquakesthathaveoccurredinNorthDakotaoverthepastcentury,whichwereroughlyequivalenttotheenergyreleaseofalargelightningbolt,orabouthalfatonofTNT(Anderson,2015).Interestingly,NorthDakota’slargestandperhapsmostwell-known earthquake,which also happened tobe thefirst instrumentally verifiedearthquake in the state,wastheM4.4eventnearHuff(CoffmanandCloud,1970).This1968earthquake had an energy release of approximately 50 timesthe2012Willistoneventandwasfeltacrosssouth-centralNorthDakotaandwithintheStateCapitolbuilding.Moreimportantly,thelargestearthquakeknownhistoricallyfortheregionwastheM5.3NorthernPlainsEarthquakeof1909(Bakunandothers,2011),whichisthoughttohaveoccurredinnortheasternMontananearthe Montana, North Dakota, and Saskatchewan border. Thisearthquakehadanenergyreleasethatwas22timesgreaterthantheM4.4Huffearthquake.

Comparatively,theamountofenergyreleasedduringtherecentdevastating earthquake in central Italy, an M 6.2 event, wasequivalenttotwicethatoftheatomicbombusedattheendoftheWorldWarII(fig.7).

Looking aheadThrough the vision of IRIS and the continued success of theEarthScopeproject,alongwiththecontinuedoperationofseismic

monitoring stations in and around the state, we are betterpreparedandpositionedtomonitorseismicevents.

ReferencesAnderson,F.J.,2009,USArrayReferenceSeismicStationInstalledinNorth

Dakota:GeoNews,v.36,no.1,p.10-13.Anderson,F.J.,2010,EarthScopeTransportableSeismicArrayOperational

inNorthDakota:GeoNews,v.37,no.1,p.23-25.Anderson, F.J., 2015, Earthquakes in North Dakota: North Dakota

Geological Survey Geologic Investigation No. 187, 1:1,000,000scalemap.

Bakun,W.H., Stickney,M.C., andRogers,G.C., 2011, The16May1909Northern Great Plains Earthquake: Bulletin of the SeismologicalSocietyofAmerica,ShortNote,v.101,no.6,p.3065-3071.

Bolt,B.A.,1988,Earthquakes:W.H.FreemanandCompany,116p.Coffman, J.L., andCloud,W.K., 1970,UnitedStatesEarthquakes, 1968:

U.S.EnvironmentalScienceServicesAdministration,111p.Havskov, J., and Alguacil, G., 2016, Instrumentation in Earthquake

Seismology(2nded.):Springer,p.87-88.IRIS,2015,USArray,CentralandEasternUnitedStatesNetwork,http://

www.usarray.org/ceusn,(retrievedSeptember16,2016).IRIS, 2016, USArray, Transportable Array Deployment to Alaska and

Western Canada, http://www.usarray.org/Alaska, (retrievedSeptember16,2016).

PASSCAL, 2016, Portable Array Seismic Studies of the ContinentalLithosphere Instrument Center, NewMexico Tech, https://www.passcal.nmt.edu/content/instrumentation/sensors/broadband-sensors/sts-2-bb-sensor,(retrievedAugust26,2016).

USGS, 2014,Documentation for the 2014Updateof theUnited StatesNational Seismic Hazard Maps: US Geological Survey Open FileReport 2014-1091, 243 p., http://pubs.usgs.gov/of/2014/1091/pdf/ofr2014-1091.pdf,(retrievedJuly8,2016).

USGS,2016,M6.2–Norcia, Italy EarthquakeOverview:USGeologicalSurvey Earthquake Hazards Program, http://earthquake.usgs.gov/earthquakes/eventpage/us10006g7d#executive, (retrievedOctober24,2016).

Figure 6. Seismogram from recent rock blast-ing at the Falkirk Mine in north-central North Da-kota showing the charac-teristic seismic signatureof a typical mining or quarryblast.

Figure 7. Relative amountsof earthquake energy radi-ated as seismic waves fromearthquakes of interest inNorth Dakota and the sur-roundingregionandthedev-astatingAugust2016, Italianearthquake.

The July 8, 1968 M 4.4 earthquake near Huff is, to date, the largest earthquake to have occurred in North Dakota, and the first earthquake in the state with an instrumentally verified epicenter.

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