EGU2012-12358 The Seismic Structure of the Mantle Wedge under , Northwestern U.S.A. Alan Levander1 ([email protected]), Kaijian Liu1,2, Rob Porritt3, Richard Allen3, Yingji Yang4 1: Rice University, Earth Science, 2: Rice University, Applied Physics Program, Houston, TX, USA 3: University of California at Berkeley, Earth & Planetary Sciences, Berkeley, CA, USA

XL-185 4: Macquarie University, Earth & Planetary Sciences, North Ryde, NSW, Australia

The Seismic Structure of the Mantle Wedge under Cascade Volcanoes Lassen and Shasta: FAME (Flexible Array Mendocino Experiment) C ascade Volcanoes (TA + FAME data) Right: Representative Ps receiver gathers as a function of Right: TA.A05A RcvrGthr Distance RcvrGthr Azimuth Depth Mapped Vs: Joint Inversion Under a number of Cascade volcanoes we have identi�ied distance and azimuth for a number of Cascade volanoes. 0 0 0 0 a characteristic seismic signature in individual station 126˚ 125˚ 124˚ 123˚ 122˚ 121˚ 120˚ 119˚ Map of FAME experiment stations Also shown are the depth mapped stacks of the gathers 50˚ 42˚ 50˚ aboutand location half of ofthe 3 TApro�iles spacing. shown below. Ps receiver functions and in Ps CCP image volumes made ambient noise and ballistic Rayleigh wave phase veloci- 5 5 FarAverage Right: instrument Thermal modelspacing of in Cascadia FAME was −50 50 from USArray Transportable Array and Flexible Array tiesand (Yang,the Vs etpro�iles al., 2008). from We joint thank inversions Y. Yang of for the providing RFs and subduction zone in central and stations. In the mantle wedge, the CCP images and the the phase velocities used in the TA station 49˚ interpretation of scattered wave image Depth (km) 49˚ inversions. Baker (Bostock et al., 2002). 10 10 RFs show a strong negative event just below the Moho, 40˚ Below: Map of Cascadia Volcanoes: Results −100 100 Below: Cross-sections of Vs determined paired with a weak to moderate positive event between Glacier from Baker, Rainier, St. Helens, and Lassen are shown. 48˚ Peak 48˚ 50-70 km, and a strong slab event. At most of these functions and ambient noise and ballistic Time (s) 15 15 −150 40 50 60 70 80 150 200 250 300 150 from joint inversion of FAME receiver 0. 3 4 5 volcanoes, a strong negative signal also appears Rayleigh wave phase velocity dispersion 237˚ 238˚ 239˚ 240˚ Distance (deg) Azimuth (deg) 38˚ between 15 and 25 km depth in the crust. The signature 47˚ 47˚ 50˚ 50˚ TA.B06A Rainier RcvrGthr Distance RcvrGthr Azimuth Depth Mapped Vs: Joint Inversion 0 0 is particularly clear under Mt. Lassen and Mt. Shasta (Liu et al., EPSL, 2012; Porritt et al., 2011, 0 0

in data from FAME (Flexible Array Mendocino Mount St Helens Adams EPSL) −126˚ −124˚ −122˚ −120˚ −118˚ 46˚ 46˚ Experiment), where instruments were close to the −6 −4 −2 0 2 4 v 5 5 −50 50 volcanic centers. Random averages using all stations Hood Lassen

W E Time (s) throughout the western U.S., and only stations in the 45˚ 45˚ Depth (km) Jefferson 49˚ 49˚ Right: joint inversion Vs −2. 10 −100 100 Cascadia backarc region show that this signature is 10 cross-section with depth mapped Ps Baker not common to the western U.S. as a whole, nor to the 3Sisters −3 44˚ Newberry 44˚ East-west −20 backarc region. Devils −3.2 15 15 −150 150 RFs through Mt. Lassen . 40 60 80 150 200 250 300 0 3 4 5 Cinnamon Butte Garden from Bostock et al. 2002. Distance (deg) Azimuth (deg) 43˚ Crater Lake 43˚ Temperature contours are modi�ied −40 600o −3.4 Glacier TA.E05A Joint inversion of the Ps receiver functions and ambient RcvrGthr Distance RcvrGthr Azimuth Depth Mapped Vs: Joint Inversion Below: Intepretation of cross- 48˚ 48˚ 0 0 0 −3.6 0 noise Rayleigh wave phase velocities (Porritt et al., section, shown with seismicity.

−60 800

42˚ 42˚ o −3.8 2011; Liu et al., 2012) for those volcanoes with v Medicine 1000 Shasta o the paired events provides 1D shear velocity pro�iles Lake −4 5 5 −50 0 1000 50 −80 1200 having common characteristics. A sub-Moho low 41˚ 41˚ Melt? o o −20 Moho −4.2 Time (s) velocity zone from 5 to 15 km thick gives rise to the Lassen Eagle Depth (km) 600o 47˚ 47˚ Lake −40 Cold Nose −100 −4.4 Rainier paired negative-positive signals in the RFs. The 40˚ 40˚ 1400 10 10 −100 100 −60 Gorda Plate800 o o 1000Melt? Rainier −4.6 St. sub-Moho low velocity zones, with velocities of o 1000 −80 o km/s LAB 1200 −120 Helens

3.7 < Vs < 4.0 km/s, are evident in 30 of the 39 stations 39˚ 39˚ o Adams −100 −124 −123 −122 −121 15 15 −150 150

1400 40 50 60 70 80 90 150 200 250 300 0 3 4 5 we examined. Stations not exhibiting this pattern also o Elev (km) Counter Distance (deg) Azimuth (deg) −120 Flow v show a characteristic seismic structure: There is no −8 −4 0 2 4 6 8 46˚ 46˚ -124 -123 -122 -121 Shasta TA.F04A Mount St Helens Imaging Experiment RcvrGthr Distance RcvrGthr Azimuth Depth Mapped Vs: Joint Inversion abrupt velocity increase at Moho depths, instead Vs 0 0 0 0 123º0 'W 122º30 'W 122º0 'W 121º30 'W W E increases gradually from the lower crust to as deep 46º3 N '

as ~70 km, forming a thick, relatively high velocity layer 0 0 Hood ' N

46º3 −3 −20 5 5 −50 50 (4.0 < Vs <4.5 km/s). Right: Vs cross-section with depth 400o −3.2 45˚ 45˚ 46º N 0 ' Time (s) ' 0 N Depth (km) 46º TemperatureEast-west contours are from Bostock et al. −40 Maps at right show the seismology and magnetotelluric 2000 lb. Shots 600 (2002).mapped Ps RFs through Mt. Shasta volcano. o −3.4 10 10 −100 100 1000 lb. Shots

Receiver Lines (1000 Texans)

experiment plans for the Mount Saint Helens Imaging Receiver Areas 45º3 0 4 8 16 24 32 N ' (1600 Texans) Kilometers 0 0 ' N

45º3 123º0 'W 122º30 'W 122º0 'W 121º30 'W −3.6 Experiment to be conducted 2013-2015. −60 1000 800 Broadband seismograph and magnetotelluric experiment o o 15 1200 −3.8 15 −150 150 plan (left) and active source transmission tomography o 1000 44˚ 44˚ 40 50 60 70 80 90 50 100 150 200 250 300 0 3 4 5 Distance (deg) Azimuth (deg) This project was initiated during the CIDER 2011 program. −80 o experiment plan (right) planned for Mt St Helens. −4 XQ.ME21 RcvrGthr Distance RcvrGthr Azimuth Depth Mapped Vs: Joint Inversion 0 0 0 0 −4.2 −100 1400 o Cascade Volcanoes −4.4 Baker St Helens Indian Heaven West Crater Hood Jefferson Blue Lake 5 5 −50 −50 0 0 0 0 0 0 0 −120 43˚ 43˚

48 −4.6 )

46 −50 −50 −50 −50 −50 −50 −50 km/s Time (s)

−124 −123 −122 −121 Depth (km 44 v −100 −100 −100 −100 −100 −100 −100 v 10 10 −100 −100 42 Lassen Shasta 40 −150 −150 −150 −150 −150 −150 −150 −124 −122 −120 −200 0 200 −200 0 200 −200 0 200 −200 0 200 −200 0 200 −200 0 200 −200 0 200 SE NW

Belknap 3Sisters Bachelor Medicine Lake Shasta Silver Lake Average 42˚ 42˚ Right: Vs cross-section with depth mapped Ps 15 15 −150 −150 0 0 0 0 0 0 0 40 50 60 70 80 100 150 200 250 300 0 3 3.5 4 4.5 5 Distance (deg) Azimuth (deg) SE-NW 3.2 −50 −50 −50 −50 −50 −50 −50 20 Shasta RFs through the Mt. Lassen and Mt. Shasta volcanoes. XQ.ME23 RcvrGthr Distance RcvrGthr Azimuth Depth Mapped Vs: Joint Inversion −100 −100 −100 −100 −100 −100 −100 3.4 0 0 0 0 −150 −150 −150 −150 −150 −150 −150 40 −200 0 200 −200 0 200 −200 0 200 −200 0 200 −200 0 200 −200 0 200 −50 0 50 100 3.6 41˚ 41˚

5 60 5 −50 −50 Back Arc: Random Samples of 27 RFs Western US Random Samples of 27 RFs References 3.8 Lassen 0 0 0 0 0 0 0 0 0 0 Bostock, M.G., Hyndman, R.D., Rondenay, S., Peacock, S.M., 2002. An inverted continental Moho and serpenti- nization of the forearc mantle. Nature 417, 536-538. Time (s) 45 45 Depth (km −50 −50 −50 −50 −50 −50 −50 −50 −50 −50 4.0 Levander, A. and M.S. Miller, Evolutionary aspects of lithospheric discontinuity structure in the western U.S., 80 40 40 10 10 −100 −100 −100 −100 −100 −100 −100 −100 −100 −100 submitted to Geochemistry, Geophysics, Geosystems, January 2012. 40˚ 40˚ −100 −100 35 35 Liu, K., A. Levander, Y. Zhai, R. Poritt, and R. Allen, 2012, Asthenospheric flow and lithospheric evolution near −150 −150 −150 −150 −150 −150 −150 −150 −150 −150 the Mendocino triple junction, Earth & Planetary Science Letters, 323-324, 60-71, doi:10.1016/j.epsl.2012.01.020. 4.2 −120−110−100 −200 0 200 −200 0 200 −200 0 200 −200 0 200 −200 0 200 −120−110−100 −200 0 200 −200 0 200 −200 0 200 −200 0 200 −200 0 200 Liu, K, and A. Levander, 2012, Three-dimensional Kirchhoff-approximate Generalized Radon Transform Imaging 100 using Teleseismic P-to-S Scattered Waves, submitted to Geophysical Journal International. 0 0 0 0 0 0 0 0 0 0 0 0 Porritt, R.W., Allen, R.M., Boyarko, D.C., Brudzinski, M.R., 2011. Investigation of Cascadia Segmentation with 4.4 15 15 −150 −150 50 60 70 80 100 150 200 250 300 0 3 3.5 4 4.5 5 −50 −50 −50 −50 −50 −50 −50 −50 −50 −50 −50 −50 Distance (deg) Azimuth (deg) Ambient Noise Tomography. Earth Planet Sc Lett 309, 67-76. - 120 most mantle beneath the western revealed by ambient noise and earthquake tomography, −100 −100 −100 −100 −100 −100 −100 −100 −100 −100 −100 −100 Yang,Journal Y., ofM.H. Geophysical Ritzwoller,1 Research, F.-C. Lin, 113, M. P.B12310. Moschetti, and N.M. Shapiro, 2008, Structure of the crust and upper 4.6 39.8 40 40.2 40.4 40.6 40.8 41 41.2 41.4 39˚ 39˚ −150 −150 −150 −150 −150 −150 −150 −150 −150 −150 −150 −150 km/s −200 0 200 −200 0 200 −200 0 200 −200 0 200 −200 0 200 −200 0 200 −200 0 200 −200 0 200 −200 0 200 −200 0 200 −200 0 200 −200 0 200