Where these events were noted, they were referred to as floods, not major earthquakes. As the Mersey was still a maritime route to Warrington, utilised less than the Dee but still used, you’d also think that a passing sailor might have noted the sudden appearance of a new natural harbour suddenly appearing as he floated upriver.
The Mersey doesn’t get a mention in these records, but there are probably reasons for that. First, the Mersey is a more powerful river, with its own tidal bore, and depending on the season and tide, the effects could have been far less catastrophic than the open and placid lower Dee, and second that the coastal population was far less ( and less exposed), with no significant settlements until you reach Warrington, by which time the effects could have been much subsided.
It is worth pondering if there was ‘some’ effect however, even if it is purely speculative. The riverside of Liverpool sits on boulder clay, flat and floodable before rising to slightly higher ground where the castle was later to sit, with sandstone forming the higher ground above. If there ever was a wooden fishing settlement in pre-Christian Liverpool, it could have been totally washed away by a high tide tsunami like flood. Highly unlikely but not beyond the realms of possibility. Second, a similar high tide flood in a tidal opening like ‘the pool’ could have washed away a portion of the alluvium that surrounds the boulder clay of the higher ground, ‘cleaning out’ the natural harbour for easier use.
We’ll never know of course, but one of the mysteries that does remain is what such a practical inlet, identified by King John as suitable for a port, has no recorded use by the mariners and settlers of Roman and Celtic times. Unless something changed after the Roman settlements and ports were established…