Joint Structures Determined by Clustering Microearthquakes
Using Waveform Amplitude Ratios
Adam Roff , W. Scott Phillips and Donald W. Brown
International Journal of Rock Mechanics and Mining Sciences
and Geomechanics Abstracts
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A new method of clustering seismic events has been developed to identify features
within a large cloud of microearthquakes induced by the pressurization of a 3.5-km deep,
jointed region of granitic rock. The relative amplitudes of shear- and compressional-
waves were taken as an expression of the source mechanism for each microearthquake.
Then, the events were clustered according to their similarity in S/P ratio and a second
criterion, their spatial proximity to each other. Thus, each cluster contains events with
closely spaced hypocenters and similar S/P ratios, and therefore a high probability of the
same (or very similar) focal mechanism. This method was applied to a data set of 8,000
microearthquakes recorded during hydraulic stimulation of the Hot Dry Rock geothermal
reservoir at Fenton Hill, New Mexico, located on the western flank of the Valles Caldera.
Over 50 clusters of more than 10 events each were identified as planar features and over
20 such clusters were identified as linear features, together containing 15% of the usable
events. Two predominant sets of planar features strike north to N45*W, roughly tangent
to the ring fault system circling the caldera and dip 60* in either direction. These planes
represent joints that may be associated with stress fields accompanying caldera formation.
Most observed planes contain significant resolved shear within the current stress field.
However, wide ranges of equally probable orientations are not observed, indicating a
relatively unbiased sampling of the actual range of joint orientations. Most linear features
are oriented parallel to one or both sets of planar features and may be associated with joint
intersections or terminations. These results support a reservoir flow model composed of
water storage within aseismic joints oriented perpendicular to the minimum stress
direction and of limited dimension, connected by high impedance paths along joints
striking north to N45*W as defined by the cluster results.
Figure 9. Lower hemisphere, equal area projections of (a) poles to planar clusters and (b)
axes of linear clusters identified by the S/P ratio method.