HR: 16:30h
AN: S24A-03 [Abstracts]
TI: Possible Links Between Ground Water Saturation and Rayleigh Wave Particle Motion in Microseisms: A
Comparison of Data and Simulation for Seismic Stations in Southern California
AU: * Eitzel, M
EM: melissaeitzel@cox.net
AF: Earth Sciences Department
University of California, Santa Barbara, 552 University Road - Bldg 526, Santa Barbara, CA 93106-9630
United States
AU: * Eitzel, M
EM: melissaeitzel@cox.net
AF: Institute for Crustal Studies, UC Santa Barbara
1140 Girvetz Hall, Santa Barbara, CA 93106-1100
United States
AU: Tanimoto, T
EM: toshiro@geol.ucsb.edu
AF: Earth Sciences Department
University of California, Santa Barbara, 552 University Road - Bldg 526, Santa Barbara, CA 93106-9630
United States
AU: Tanimoto, T
EM: toshiro@geol.ucsb.edu
AF: Institute for Crustal Studies, UC Santa Barbara
1140 Girvetz Hall, Santa Barbara, CA 93106-1100
United States
AB:
Seasonal variations in particle motion have been observed for microseisms in the 0.1 to 0.2 Hz range for many seismic
stations worldwide. The ratio of horizontal particle displacement to vertical particle displacement, called the HZ ratio,
has been adopted as a parameter with which to analyze this phenomenon. This study tests the hypothesis that the seasonal
change in the HZ ratio is driven by changes in ground water under the seismic station.
Selection criteria were applied to LH seismic data from southern California stations such that only stations and frequencies
with the 90-degree phase shift expected for Rayleigh waves would be considered in order to avoid contamination from other
seismic phases as much as possible. For these confirmed microseism datasets, normal modes were computed for the SCEC
Community Velocity Model (version 3.0) to generate synthetic HZ ratios. Simulation of seasonal ground water change was
incorporated into the model by perturbing velocity structures according to Budiansky and O'Connell (1974). Their equations
for seismic velocities in saturated and unsaturated cracked media were used to perturb the velocity structure of a ground
water layer in the model for both saturated and unsaturated cases. In addition, density perturbations were added to the model
to account for higher porosities found in soils, as compared to their study of cracks in bedrock.
A handful of seismic stations in southern California show similarities between simulation and data, particularly in the LA
basin. However, some stations do not match simulations, indicating that refinements of the hypothesis are necessary. We will
report on possible causes of seasonal variations in the HZ ratio, including (1) artificial effects from the remaining
contamination from other seismic phases (particularly in winter) and (2) potential causes other than groundwater.
DE: 0545 Modeling (4255)
DE: 1829 Groundwater hydrology
DE: 7200 SEISMOLOGY
DE: 7255 Surface waves and free oscillations
SC: Seismology [S]
MN: Fall Meeting 2005