Saturday’s Swansea Valley earthquake / Torpantau tunnel (2 separate issues!)

**NB these two news items are unconnected in any way – it's mere coincidence that we're reporting them in a single Alert!!**
Torpantau Tunnel
We have received a report from Storey Arms Centre (thanks, Ryan Stamp) that a new bulge on the southern wall of the tunnel was noticed two weeks ago, along with some new blocks on the tunnel floor (see attachment). It is approximately 50 metres in from the northern entrance, in the first course of blockwork. Water can be seen flowing in the void made by the bulge, with cracks running through the blockwork. Members using the tunnel should be aware of this new development and monitor the area for any further changes.

Saturday's Earthquake
We have received the following report from Brecon Beacons National Park Authority, who will be carrying out appropriate checks – but Members should be alert to the possibility of rock movements affecting caves and slabs. Particularly vulnerable areas could include the the approaches and interior of Porth yr Ogof (including the roof slab of the Great Bedding Cave), the Gunpowder Works remains and some sections of roof within the silica mines behind Dinas Rock, as well as loose rock at Dinas Rock, Cribarth and Penwyllt and other locations further afield where individual rocks were 'near-critical'. Please make appropriate assessments and exercise due caution in these and other vulnerable areas.

Please let us know if any of you discover anything that we should pass on to Members.

From BBNPA: "The earthquake struck at 2.31pm on Saturday and was followed at 3.09 and 4.27 by two small aftershocks. The quake measured 4.4ML on the magnitude scale – the ML means ‘local magnitude’ often referred to as the Richter magnitude. The scale is logarithmic so a quake of magnitude 4 is ten times more powerful than one rated at 3. The original quake was felt over a wide area of Wales and England though the aftershocks are likely only to have been picked up by seismometers. The depth was calculated as 7.4km which is relatively shallow.  

The British Geological Survey (BGS) gave the epicentre as at 51.776 degrees north, 3.837 degrees west (Ordnance Survey grid reference SN 733102) beneath Bryn Melyn/Mynydd Uchaf which is near the northern edge of the South Wales Coalfield. That puts it about 2.8km southwest of the village of Cwmllynfell and 3.6km outside of the National Park and Geopark and in the NW corner of Neath Port Talbot unitary authority area. 

This is the largest earthquake to have been felt on the British mainland since the 5.2ML Market Rasen earthquake of 27 February 2008. There have been a cluster of relatively significant earthquakes (in British terms!) around the lower Swansea Valley over the centuries, the last major one being the 5.2ML Swansea Earthquake of 1906, an event which occurred just 3 months after the much larger, more well-known and more devastating quake in San Francisco. Others over 5ML were recorded near Swansea in 1727 and 1775. A 4.2ML quake occurred beneath the Bristol Channel in 2014 and was felt in our region. Other quakes emanating from within Wales and which hit the headlines include the Llŷn Peninsula quake of 19 July 1984 and the Sennybridge quake of October 27 1999.

An earthquake usually results from movement on a geological fault and represents the sudden relief of strain that has previously built up along the weakness. The BGS records numerous faults in the area – indeed the fact that this is a part of the (now largely industrially inactive) South Wales Coalfield means that the area has been geologically surveyed in considerable detail. Numerous N-S aligned faults are known and date from several hundred million years ago – they were active at that time as the coalfield was being stretched in an east-west direction. A notable one is the westerly-dipping CwmllynfelI Fault which runs north from Cilmaengwyn in the Swansea Valley beneath Bryn Melyn (epicentre) and extends to Herbert’s Quarry on the Black Mountain where it peters out to the north. It is not known however whether this event can be related to a known fault or else relates to a previously unknown one. 

The Swansea Valley itself follows the line of the Swansea Valley Disturbance, one of Britain’s major geological structures being a belt of faults and geological folds running NE from Swansea Bay towards Brecon and beyond and active during the Caledonian and Varsican orogenies (mountain building periods) of 450-400 million and around 300 million years ago. Dr Tony Ramsay, scientific director of Fforest Fawr Geopark has recently collaborated with other geoscientists into unravel more detail about earth movements associated with the Disturbance and an academic research paper is anticipated soon.

Britain has been a seismically quiet part of the world for a long while but our part of the Earth’s crust still experiences stresses related both to the continued opening of the Atlantic Ocean and to the ongoing Alpine Orogeny i.e mountain-building in southern Europe."

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