The Source-Scanning Algorithm: mapping the distribution of seismic sources in time and space

We introduce a new method, which we call the Source- Scanning Algorithm (SSA), for imaging the distribution of seismic sources in both time and space. Using trial locations and origin times, the method calculates the 'brightness' function by summing the absolute amplitudes observed at all stations at their respective predicted arrival times. The spatial and temporal distribution of sources is then identified by a systematic search throughout the model space and time for the maximum brightness. The greatest advantages of this method are that: (1) it exploits waveform information (both arrival times and relative amplitudes) without the need to calculate high-frequency synthetic seismograms; and (2) it requires neither pre- assembled phase- picking data nor any a priori assumptions about the source geometry. A series of tests using synthetic data have shown that this method is robust and can faithfully recover the input source configuration to within 1 grid interval. Finally, we demonstrate the value of the algorithm by locating a typical tremor event with emergent waveforms that occurred during the recent episodic tremor and slip (ETS) sequence in the northern Cascadia subduction zone.