I looked at the focal mechanism. There were two versions, and they were very different: one was strike-slip and the other was normal. But neither fit the first arrivals all that well. Maybe it was a complex event, even though only a M4. Or, if really that shallow, maybe you (Brian) may want to Google earth whether there is a quarry there....although the first arrival pattern should be different for an explosion. Explosions are usually removed by seismologists.

Chris