1 Wavefield Calibration Using Regional Network Data R. B. Herrmann Saint Louis University

2 Questions What is a good Earth model for regional studies? –Location –Source Parameters –Structure What can be done with very good regional data in a simple region?

3 Answer It is difficult to obtain the Earth model

4 Presentation Tools to obtain earth models –Surface wave dispersion inversion –Receiver function inversion –Waveform inversion Adequacy of data set –Tests

5 Joint Inversion

6 Rayleigh Wave Sensitivity

7 RFTN Partials RFTN

8 Postulated Advantages of Joint Inversion Receiver function depends upon travel time and fine detail of structure related to conversions Surface wave is smoothly affected by velocities So Advantages of one overcome deficiencies of the other

9 Purpose of models Assist location by correctly predicting first arrivals Properly characterize dynamic wavefield to obtain quantitative estimates of source mechanism and strength

10 Receiver Function Sensitivity to Structure Perturb simple crust/mantle model Examine effect of gradient Design model to have same vertical travel time

11 Red = sharp / Blue = strong gradient

12 Gravity Anomaly Imagine sampling different structures within a region What would be seen in Bouguer anomaly

mGal variation among models

14 Love Rayleigh

15 Surface waves Subtle differences in dispersion for fundamental mode in second period range For surface waves to really contribute structure information, need dispersion for a fine grid of periods Need short periods to focus on upper crust

16 Receiver Functions Slides for different filter parameter - alpha =1.0 corresponds to a lowpass corner of about 1/3.14 Hz Focus on effect of Moho transition on nature of P-wave receiver function

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21 Comments 1st peak controlled by shallow structure Gradient indicated by absence of signal for high alpha, character by low alpha Sharp moho is indicated by distinct bounce arrivals for all alpha, especially higher Simultaneous fit to several alpha robust

22 KOREA Can events be located within 5 km? Can Earth model be defined that can –fit receiver function data –model regional waveforms or Is the joint inversion model of any value?

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25 21 NOV 2001

26 21 NOV 2001 WVFGRD

27 bp c np 2

28 bp c np 2

29 bp c np 2

30 Excellent fit, even at high frequencies but used Central US model - not model derived specifically or even valid for Korea so perform joint inversion surface - wave/receiver function for a Korea model

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32 Receiver functions Two filter parameters Stacked RFTN’s Arranged by similarity in shape Last 3 are from island stations Similarity in RFTN’s - > similarity in structure

33 Phase velocity (Herrmann, 2001) - Group velocity (Stevens, 1999) SNU (InvSNU)

34 Stacked receiver function for alpha 1 and stations Dispersion Stevens (1999) and a few phase velocity points joint96 same script (iterations controls ) for all AK135 (modified)

35 SNU (InvSNU)

36 All models Average of layer velocities, not slowness

37 P-wave 1st Arrival Surface focus

38 Good fits to both data sets Subtle differences in P-wave first arrival times Models do not fit 21 NOV 01 earthquake data - surface wave arrival too late - S-P times wrong Augment with CUS dispersion

39 SNU (nInvSNU)

40 Strong constraint on upper crust

41 Fits using CUS dispersion

42 P-wave 1st arrival surface focus

43 SNU Model comparison Red - first Blue - CUS

44 Discussion RFTN need very good dispersion Model requires independent test - waveform modeling?

45 New Dispersion Data Harvard group velocities Colorado group velocities Phase velocities from Korea –Treat BB network as array –Optionally apply match filter –Apply McMechan and Yedlin p-tau implemented as sacpom96

46 01 Jan 2001 Alaska Event - phase match output used from 10 stations

47 Blue - Colorado Green - Harvard Orange - Stevens Red - Korea phase velocity

48 Starting Model AK135 - depths > 50 km Upper 50 km is a halfspace with velocity of z=50 km Invert new dispersion Use stacked RFTN’s Use same script

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51 Test Compare data, CUS and Korea model Add modes to save time - so no P Bandpass Hz

54 Focal Mechanisms 21 NOV waveform Mw= NOV waveform Mw= DEC surface-wave radiation pattern Mw=4.0 –does not require precise location –does not require precise Earth model –does not require broadband signal –works with high noise

55 Rayleigh Wave

56 Love Wave

57 SEO spectra

58 New Mechanisms

59 KOREA Good data sets but little local activity Waveforms exhibit distinct P and sP –together these are good depth indicators Data are available to improve dispersion estimates Some events may have good azimuthal distribution and stations to 200 km

60 Future work Continued interaction with SNU and KMRI on –Ground motion scaling with distance –Location –Source mechanisms –Seismic Hazard maps

61 Organize and use waveform data set Get more data, especially events within 20 degrees away in NW and SE sector from Korea –Why? Get shorter period dispersion Digital data available from KMA website (if one reads Korean)

62 Other Data Teleseismic P-wave residuals –Effect of subduction zone –Usefulness for crustal wave propagation Detailed travel times - Pn, Pg, SmS 1 Hz dispersion for upper 1 km Calibration shots - not likely High frequency phase attenuation

63 Questions What degree of calibration is required? What can be done with a very good regional data set?