NEW_Accomplishments.indd - IRIS

More documents

Recommendations

Info

$Refraction - IRIS$

INNER CORE 2006 <strong>IRIS</strong> 5-YEAR PROPOSAL Hemispherical Transition of Seismic Attenuation at the Top of the Earth’s Inner Core Aimin Cao and Barbara Romanowicz • University of California, Berkeley In contrast to the liquid outer core, the earthʼs inner core is mostly solid, and its composition is more purely iron. Based on dynamic arguments related to the freezing process of the inner core, and the observation of much lower P-wave quality factor in the inner core (Qα 10,000), it has been suggested that a mushy layer with liquid inclusions may exist at the top of the inner core. On the other hand, seismic measurements indicate that Qα increases towards the center of the inner core. We here present estimates of Qα in the depth range 32-110 km beneath the Inner Core Boundary (ICB), based on the measurement of PKIKP/PKiKP amplitude ratios after a narrow band-pass filtering (0.7-2.0 Hz). Our measurements indicate that there are pronounced hemispherical differences in the values of Qα (~ 335 and ~ 160 in the western (180° W to 40° E) and eastern (40° E to 180° E) hemispheres, respectively), and in the depth of transition from decreasing to increasing Qα (< 32 km beneath the ICB in the eastern hemisphere and ~ 85 km in the western hemisphere). Below 85 km, the hemispherical pattern disappears. We also confirm the existence of a correlated hemispherical pattern in P velocity down to 85 km. The P velocity and Qα variations are compatible with an interpretation in terms of small hemispherical variations of temperature at the top of the inner core and their influence on the morphology of porosity and connectivity of liquid inclusions in the mushy zone. The disappearance of the differences in Qα beneath 85 km provide constraints on the likely depth extent of the mushy zone. (top) Differential travel time residuals (referring to PREM). The measurement error is less than ~ 0.1 second. (bottom) Q with respect to the epicentral distance and depth beneath the ICB. The average standard deviations are from ~24, ~43, to ~50. High-lighted green squares show the data sampling offshore northwest of Africa. The event epicentral distances were all calibrated with a reference focal depth of 100 km (Cao and Romanowicz, 2004) A. Cao, B. Romanowicz, Hemispherical Transition of Seismic Attenuation at the Top of the Earthʼs Inner Core, Earth Planet. Sci. Lett., 228, 243-253, 2004. NSF Grant No. EAR-0308750 192
2006 <strong>IRIS</strong> 5-YEAR PROPOSAL INNER CORE Differential Inner Core Superrotation From Earthquakes in Alaska Recorded at South Pole Station Xiaodong Song • University of Illinois at Urbana-Champaign Anyi Li • Lamont-Doherty Earth Observatory of Columbia University BC-DF residuals (s) 5 4 3 2 1 0 Alaskan Events to South Pole slope: 0.0230 +/- 0.0074 s/yr 1960 1970 1980 1990 Time (Year) Figure 1. Residuals of PKP (BC-DF) times for earthquakes in Alaska to the station at South Pole (SPA) as a function of earthquake time of occurrence. The residuals are calculated relative to the PREM using the Earthquake Data Report (EDR) location, normalized by the times that the rays travel through the inner core, and multiplied by the average (126.2 s) of the times in the inner core of all the rays. The solid line is the linear regression of the residuals on event occurrence times. One of the most convincing points of evidence for differential inner core rotation has been the temporal change of differential PKP (BC-DF) times from earthquakes in the South Sandwich Islands recorded at stations in Alaska (Song and Richards, 1996). Song and Li (2000) provided an independent support for a differential inner core rotation from a new ray path from earthquakes in southern Alaska to the South Pole station (SPA). The pathway has several characteristics that help us study the inner core rotation. (1) It is a north-south path with small ray angles from the spin axis, which was previously identified to have large BC-DF anomalies (Song and Helmberger, 1993). The largest anomalies and lateral variations from the inner core have all been identified with north-south paths; thus a northsouth path is a good start to detect travel-time changes. Furthermore, if the axis of the inner core rotation is the same or close to the spin axis, the effect of rotation on travel times is expected to be most easily observable along north-south ray paths. (2) The SPA station has a long history of continuous operation. (3) The great circle paths from anywhere in the globe to SPA are along the corresponding longitudes of the sources, -140 making it easy to sample a sweep of inner core longitudes needed to determine the lateral variation of the -145 patch sampled. We found that the differential PKP (BC-DF) travel times along the pathway have increased by about 0.85 s over 37 years with standard error of 0.27 s (Figure 1). Figure 2 plots the observed travel time residuals (normalized by the travel times in the inner core) as a function of longitude and time. Plotted also are equi-residual contours of the residuals and bilinear fits (dashed lines) on longitude and time to the residuals. The temporal trend and the longitudinal gradient are observable directly from both the observed residuals and the equi-residual contours. The temporal gradient (along horizontal direction) and the longitudinal gradient (along vertical direction) from the bilinear fits are 0.01849%/yr and -0.02192%/deg, respectively, giving a rough estimate of the rotation rate of 0.84 deg/yr, if we assume such travel time changes are caused by shifts of a laterally-varying inner core structure due to an inner core rotation around the spin axis (Creager, 1997). Using a technique to invert simultaneously for the inner core structure and the rotation rate (Song, 2000), the rotation rate determined from the new pathway is about 0.6 deg/yr faster than the mantle. Longitude ( o ) -150 -155 1.8 1960 1970 1980 1990 Time (Year) Figure 2. Travel time residuals from the Alaska-SPA paths as a function of event occurrence times and longitudes of the ray paths in the inner core. The residuals (circles) are expressed as percentages of the travel times that the rays travel through the inner core. The circle sizes indicate the relative sizes of the residuals. The equi-residual contours are plotted at every 0.2% interval. The dashed lines show bilinear fits on time and longitude to the normalized residuals at 0.2% intervals. 2 2.2 Song, X.D., Joint inversion for inner core rotation, inner core anisotropy, and mantle heterogeneity, J. Geophys. Res., 105, 7931-7943, 2000. Song, X.D. and A.Y. Li, Support for differential inner core superrotation from earthquakes in Alaska recorded at South Pole station , J. Geophys. Res., 105, 623-630, 2000. 193
Page 1 and 2:
Cornerstone Facilities for Seismolo
Page 3 and 4:
Cornerstone Facilities for Seismolo
Page 5 and 6:
2006 IRIS 5-YEAR PROPOSAL ACCOMPLIS
Page 7 and 8:
Page 9:
Page 12 and 13:
SCIENCE SUMMARIES 2006 IRIS 5-YEAR
Page 14 and 15:
Page 16 and 17:
Page 18 and 19:
Page 20 and 21:
Page 22 and 23:
Page 25 and 26:
2006 IRIS 5-YEAR PROPOSAL EARTHQUAK
Page 27 and 28:
Page 29 and 30:
Page 31 and 32:
Page 33 and 34:
Page 35 and 36:
Page 37 and 38:
Page 39 and 40:
Page 41 and 42:
Page 43 and 44:
Page 45 and 46:
2006 IRIS 5-YEAR PROPOSAL MONITORIN
Page 47 and 48:
Page 49 and 50:
Page 51 and 52:
Page 53 and 54:
Page 55 and 56:
Page 57 and 58:
Page 59 and 60:
Page 61 and 62:
Page 63 and 64:
Page 65 and 66:
2006 IRIS 5-YEAR PROPOSAL SIGNALS A
Page 67 and 68:
Page 69 and 70:
Page 71 and 72:
Page 73 and 74:
Page 75 and 76:
Page 77 and 78:
2006 IRIS 5-YEAR PROPOSAL SURFACE O
Page 79 and 80:
Page 81 and 82:
Page 83 and 84:
Page 85 and 86:
Page 87 and 88:
Page 89 and 90:
Page 91 and 92:
Page 93 and 94:
Cedar Mtn State Line Estes Park 200
Page 95 and 96:
Page 97 and 98:
Page 99 and 100:
Page 101 and 102:
Page 103 and 104:
Page 105 and 106:
Page 107 and 108:
Page 109 and 110:
Page 111 and 112:
Page 113 and 114:
Page 115 and 116:
Page 117 and 118:
Page 119 and 120:
Page 121 and 122:
Page 123 and 124:
Page 125 and 126:
Page 127 and 128:
Page 129 and 130:
Page 131 and 132:
Page 133 and 134:
2006 IRIS 5-YEAR PROPOSAL UPWELLING
Page 135 and 136:
Page 137 and 138:
Page 139 and 140:
Page 141 and 142:
Page 143 and 144:
Page 145 and 146:
Page 147 and 148:
Page 149 and 150:
Page 151 and 152: 2006 IRIS 5-YEAR PROPOSAL UPWELLING
Page 153 and 154: 2006 IRIS 5-YEAR PROPOSAL UPWELLING
Page 155 and 156: 2006 IRIS 5-YEAR PROPOSAL GLOBAL MA
Page 173 and 174: 2006 IRIS 5-YEAR PROPOSAL CORE-MANT
Page 191 and 192: 2006 IRIS 5-YEAR PROPOSAL INNER COR
Page 201: 2006 IRIS 5-YEAR PROPOSAL INNER COR
Page 207 and 208: 2006 IRIS 5-YEAR PROPOSAL EDUCATION
Page 227: 2006 IRIS 5-YEAR PROPOSAL EDUCATION
show all

NEW_Accomplishments.indd - IRIS

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?