HR: 1330h
AN: S22A-0421    [PDF]
TI: Investigation of the Low Velocity Zone Beneath the Southern Basin and Range
AU: * Savage, B
EM: savage13@gps.caltech.edu
AF: Caltech, 1200 E California Blvd MSC 252-21, Pasadena, CA 91125 United States
AU: Helmberger, D V
EM: helm@gps.caltech.edu
AF: Caltech, 1200 E California Blvd MSC 252-21, Pasadena, CA 91125 United States
AB: Following the work by Helmberger (1973), we use waveform recordings of P arrivals at distances from 6$^o$ to 20$^o$ to investigate the structure of the low velocity zone (LVZ) or asthenosphere. In contrast to the previous study, broadband data (TriNet and BDSN) is used at a much smaller station spacing providing higher along path and depth resolution. For this study, a well recorded earthquake in the central Gulf of California ($M_w$ 6.3) produces transitions from $P_{nL}$ to $P_{410}$ across all of California and western Nevada. The nature of these transitions indicates the thickness and gradients of the LVZ and the lithosphere. Initial findings show large variations of lithosphere and LVZ structure from east to west below California. By varying the lithosphere compressional velocity and depth of the LVZ in 1-D models, a database of synthetics waveforms is created to guide the development of realistic 2-D (along path) and 3-D (against azimuth) description of the lithosphere and asthenosphere. The character of the P arrivals changes dramatically near 9-11$^o$ with the emergence of a higher frequencies over-printing the longer-period P$_{nL}$ arrivals. Coastal California stations show these arrivals at the shortest distances, 9$^o$ indicating the lithosphere velocity and gradient below the LVZ are high. This is in opposition to those arrivals on the east which do not record the high frequency arrivals until 11$^o$. As the distances reach 13$^o$, a large amplitude, high frequency phase is present 10-15 seconds behind the initial P arrival. The emergence of the large secondary phase occurs at different distances across California with a pattern similar to before. At this distance, a change in the apparent velocity of the first arrival also occurs. Further in distance, the width of the initial P arrival and the energy following, or lack thereof, points to the shape of the underlying LVZ. Coastal stations and those in the central portion of California show larger amplitude arrivals following the initial P arrival than do those to the east. These large secondary arrivals may be due to larger than expected velocity jumps at the bottom of or just below the LVZ. Mapping the transition from the lithosphere to the asthenosphere, fine structure and lateral variation, should prove invaluable for tectonic reconstruction efforts, now in progress.
DE: 1236 Rheology of the lithosphere and mantle (8160)
DE: 7203 Body wave propagation
DE: 7205 Continental crust (1242)
DE: 7218 Lithosphere and upper mantle
DE: 8159 Rheology--crust and lithosphere
SC: Seismology [S]
MN: 2003 Fall Meeting