Rocky Mountain Section - 64th Annual Meeting (9–11 May 2012)

Paper No. 22
Presentation Time: 8:00 AM-5:30 PM


CHEIFFETZ, Terry, Geology, Northern Arizona University, Box 4099, Flagstaff, AZ 86011 and BRUMBAUGH, David S., Flagstaff,

Preliminary analysis of small magnitude earthquakes (MD 1.47-2.83) that occur in the Northern Arizona Seismic Belt focused on the presence or absence of surface waves. We posed the question, is the presence or absence of surface waves in any way related to the depth of small magnitude earthquakes? The events included in the data set were limited to those earthquakes that were reasonably well located (rms <0.26). Another limiting factor was size. So the data set included only events of MD <3.0. Of the 25 seismic events included in the study 14 produced surface waves with periods ranging from 0.277 to 0.860 seconds for the initial recorded surface wave. The velocities ranged from 0.7962 to 2.910 kilometers/second. The phase velocities plotted against the depth of events shows a positive linear relationship as expected. A plot of wave periods against depths shows an increase of period with depth. The remaining 11 events of the data set showed no surface waves. A summary plot of depth against magnitude included both surface wave and non-surface wave propagating events. The plot suggests that below mid-crustal depths that the small magnitude events do not produce surface waves over the range of magnitudes included in the study.