Focal mechanisms were routinely determined on the basis of first-motion picks; however, few solutions exist at Mammoth Mountain for earthquakes below the 10-km depth due to the deterioration of focal sphere coverage with increasing depth and the small magnitudes of the earthquakes. P wave arrivals were commonly clear, and first motions were easily picked at three stations at the edge of the local network: MRD, MDC, and MCV (Fig. 4). First-order trends in possible focal mechanisms can be determined by looking at the different combinations of first-motion picks at these three stations (table S2). The two most common combinations by far have MDC with opposite polarity of MRD and MCV, consistent with a focal plane of strike and dip close to that of the trend of the relocated seismicity. We were able to determine representative double-couple focal mechanisms on a small number of relatively well-recorded earthquakes (Fig. 6). We used HASH (32) without P/S amplitudes, as most of our stations are a single component, and assumed relocated hypocenters.

Below each mechanism is a subset of associated waveforms. Most solutions have a steeply dipping nodal plane close to the strike of the trend of seismicity but a wide variety in slip vectors. The bottom right mechanism is an example that does not follow this pattern.

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