Nowack, Robert L.

Zeng, Qicheng

English

Purdue E-Pubs

Purdue University Purdue e-Pubs Geodata Science Presentations Department of Earth, Atmospheric, and Planetary Sciences 12-9-2019 Crustal Structure from Correlation Analysis of the Coda of Local Seismicity Beneath a Large-N Array Qicheng Zeng Robert L. Nowack Follow this and additional works at: https://docs.lib.purdue.edu/gdspres This document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu for additional information. ..CONCLUSIONSCrustal Structure from Correlation Analysis of the Coda of Local Seismicity Beneath a Large-N ArrayQicheng Zeng1 (zeng181@purdue.edu) and Robert L. Nowack11Dept of Earth, Atmos. and Planetary Sciences, Purdue University, West Lafayette, IN, USAACKNOWLEDGMENTSMap of IRIS Community Wavefield Experiment in Oklahoma ABSTRACTThe seismic coda from frequent local seismicity near the large-N IRIS Community Wavefield Experiment in Oklahoma was used to estimatecrustal structure beneath the array. Because of the number of these events, their coda make up a component of the local noise-field and alsohave predominantly body-wave components as recorded by the array. We first identified a subset of events located beneath the array using thefirst arrival times of the events. Several processing methods were then applied to analyze the P-wave and S-wave coda of the events recordedby the 247 short-period nodes on the east-west and north-south lines. These methods included correlation analysis, band-pass filtering,frequency and time-domain normalization, f-k filtering and stacking. Correlation analysis was used to eliminate the effects of the event depthand redatum the events to virtual source locations. The correlation results recovered seismic reflections from Moho and several crustalinterfaces with high reflectivity. Ray tracing was then applied to convert travel-times to depth based on the velocity models of OGS and Crust1.0 for the area. The recovery of crustal structure from the correlation analysis of local P-wave and S-wave coda provides a new aspect for theusage of local seismicity beneath large-N arrays.The first author thanks the Geodata Science Program at Purdue University. This work waspartially supported by NSF EAR-1839322 and a Purdue EAPS travel grant.a)Preliminary conclusions of this study include:• Near offset Moho reflections of PmP and SmS are enhanced on single eventstacks through NMO corrections, low pass filtering and gaining methods.• The stacking results of multiple near offset events show weak Moho reflectionsresulting from depth uncertainties and near source velocity variations.• Event static corrections are used to enhance the near offset Moho reflectionsignals. The NMO stacked times of PmP and SmS arrivals are used to estimatethe average Poisson’s ratio (0.258), with a Vp/Vs ratio of approximately 1.75.• The depth deviations obtained from the static corrections are within the depthuncertainties of the OGS catalog. However, the static corrections could resultfrom lateral changes in velocity as well.• The Moho reflections are strong on the composite record section for the post-critical SmS arrivals. This shows that the velocity model has the ability topredict the wide-angle Moho reflection signals correctly.REFERENCESDarold, A. P., et al. "Oklahoma earthquake summary report 2014." Okla. Geol. Surv. Open‐FileRept. OF1‐2015 (2015): 1-46.Sweet, J. R., et al. "A community experiment to record the full seismic wavefield inOklahoma." Seismological Research Letters 89.5 (2018): 1923-1930.The red triangles are the north-south and east-west linear nodal array stations, the magenta diamondsdenote stations from the nodal gradiometer array, and the cyan squares are the broadband stations(Sweet et al., 2018). The upper right inset map shows the location of the array by the triangle.a) Map of seismic events from OGS catalog over a period from 24 June to 20 July 2016. The eventsare denoted by gray dots. The dashed circles have radii of 50 km, 100 km, 150 km and 200 km fromthe central station. b) Map of near offset events within 50 km of the central station which are usedfor the pre-critical data processing. The stations are denoted by red triangles, magenta diamonds andcyan squares for linear array nodes, gradiometer array and broadband stations.Seismic Events in North Central Oklahoma Schematic Diagram of Normal Moveout for Events with Different Focal DepthsThe blue asterisks represent seismic events of differentoffsets and focal depths from the receiver array noted by redtriangle. Normal moveout is then performed on each seismicevent and receiver pair. The NMO time shift flattens thearrivals and corrects for the event depths to a depth of 5 km.Stacking for Single Event Receiver Gathers for the Vertical and Transeverse Componentsa) Vertical component event gather before event static corrections, where each trace is thestacked result for a single event on E-W linear array nodal stations. The cyan line predicts theNMO corrected PmP arrival. The rightmost trace is the event stack. b) Vertical componentevent gather after static corrections. Each trace on a) is cross correlated with the stacked traceto obtain the static corrections. The rightmost trace in b) is the stacked result after NMO andstatic corrections. c) and d) are similar results for the transverse component event gathers. Themagenta lines predict the NMO and NMO plus static corrected SmS arrivals.Near Offset Event Gathers for the PmP and SmS before and after Event Static Correctionsa) Single event receiver gather of the vertical component recorded by the East-West lineararray nodal stations for event 25. Each trace is low-pass filtered to 2 Hz and gained by apower of t to approximately equalize the amplitudes in time. The blue, red and cyan lines arethe predicted arrival times for the P, S and PmP from ray tracing, after an NMO correctionfor the PmP arrival. b) Single event receiver gather of the transverse component recorded byEast-West linear array stations for event 2. Each trace is low-pass filtered to 1.5 Hz andgained by a power of t to approximately equalize the amplitudes in time. The magenta line isthe predicted SmS arrivals after an NMO correction for the SmS. The rightmost trace in eachsingle event receiver gather is the NMO stack.The OGS Catalog Event Depths and Their Uncertainties, and Deviations from the Event Static Correctionsa) shows the OGS catalog event depths (blue asterisks) andtheir uncertainties (gray area). The red plus signs showdeviations obtained from the static corrections for the PmParrivals, and these deviations are within the depthuncertainties of the OGS catalog. However, the staticcorrections could also result from lateral changes invelocity. b) shows a similar plot for the SmS arrivals.Post-critical SmS Moho Reflectionsa) shows the event locations to the southof the receiver array for event distancesout to 200 km. b) displays a compositerecord section for the post-critical SmSarrival. This includes several eventgathers recorded by the left north-southlinear array stations for the transversecomponent. The direct S arrivals foreach event gather are denoted by redarrows. The magenta dashed lines andthe black arrows show the predictedSmS arrival on the composite recordsection.a)c)a)b)a)a )b )b)d)b)b)b)a)Velocity Models and Predicted Arrival Time of Seismic Phasesa) Velocity models of the study area. Thevelocity model from the OklahomaGeological Survey (Darold et al., 2015) isdenoted by solid blue and red lines. Thevelocity model from CRUST1.0 is shown bythe dashed blue and red lines for comparison.b) Travel time curves from the OGS velocitymodel for a 5 km focal depth source. The blueand red lines represent the direct P and Sarrivals. The solid and dashed cyan lines showthe downgoing and surface reflected PmP andpPmP arrivals from the Moho. The solid anddashed magenta similarly represent the SmSand sSmS arrivals. Surface wave arrivals aredenoted by gray area for velocities from 2.8km/s to 3.2 km/s.

Crustal Structure from Correlation Analysis of the Coda of Local Seismicity Beneath a Large-N Array

https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1000&amp;context=gdspres

Crustal Structure from Correlation Analysis of the Coda of Local Seismicity Beneath a Large-N Array

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