Abstract Title: Obtaining Focal Mechanisms for Small Earthquakes (M<3) from 
First-Motions and Amplitudes

Abstract Author(s): Preston, Leiph (UNR) - Vonseggern, David (UNR) - Smith, Ken 
(UNR)  

Abstract: Recordings from earthquakes with M<3 often are insufficient to 
adequately cover the focal sphere with first motions to obtain reliable focal 
mechanism solutions. We have developed a method for extracting double-couple 
focal mechanism solutions for earthquakes with M<3 from both P-wave 
first-motions and from low-pass filtered S-wave and/or P-wave amplitudes. The 
addition of amplitude information enhances our ability to select the 
appropriate solution whenever stations lie within one or two focal depths in 
distance from the epicenter. We have applied this method to a cluster of deep 
(30km) events occurring within the Tahoe Basin in 2003. Magnitudes of the best 
24 events we used in this study ranged from 1.5 to 2. Each of these events had 
typically 4-5 3-component, digital stations that were within two focal-depths, 
allowing us to utilize amplitude information from these stations, as well as 
first-motions out to about 100km offset. The inclusion of amplitude data 
allowed us to confine and verify focal mechanism solutions wherever several 
possible first-motion solutions existed. The use of amplitude information is 
limited to short epicentral distances due to the methodŐs inability to model 
complex S-waves or attenuation. This methodology could prove useful for 
determining focal mechanisms from small events especially with the advent of 
Earthscope. Increased station coverage, decreased inter-station distance, 
Bigfoot, PBO and an increased number of portable instruments available for 
smaller-scale arrays all increase the likelihood that reliable 3-component 
amplitude information will be able to play a larger role in determining focal 
mechanisms in the future.