OR/18/029 Appendix 3 - Publication summaries

Baptie, B. 2018. Earthquake seismology 2017/2018. British Geological Survey Internal Report, OR/18/029.

The Moidart earthquakes of 4 August 2017
B Baptie, G Ford, D Galloway
The Moidart earthquake of 4 August 2017 (4.0 ML) was the largest earthquake in Scotland for 18 years. The earthquake was felt widely across the west of Scotland. Only five other earthquakes of this size or greater have been observed in the period of instrumental recording from 1970 to present. Historical observations and instrumental recordings have been used to estimate that an earthquake of 4.0 ML or greater occurs somewhere in Scotland roughly every 8-9 years on average. The earthquake hypocentre was calculated using an iterative linearized method. The results suggest that the earthquake occurred in the mid-Crust at a depth of approximately 12 km. This is largely consistent with observed focal depths for other earthquakes in the region, which are distributed throughout the upper 20 km of the Crust. The strong similarity between the recorded ground motions from the mainshock and the four recorded aftershocks suggests that they all occurred within a small source volume, of the order of a few hundred metres in extent and had similar source mechanisms. The modelled source displacement spectra provide a good fit for the observed displacement spectra and suggest a moment magnitude (Mw) of 3.6 ± 0.1. This is slightly less than that expected for an earthquake with a local magnitude of 4.0 ML using commonly used empirical relationships relating local and moment magnitude, which gives an expected moment magnitude of 3.7.

The calculated focal mechanism suggests that the earthquake resulted from strike-slip faulting on a fault plane that strikes either SW-NE or NW-SE and dips steeply, although the dip of both fault planes is rather poorly constrained. This is in good agreement with focal mechanisms calculated for other earthquakes across the region, which all show similar solutions. Seismicity in northwest Scotland is clustered around a number of large, steeply dipping major faults that strike either NE-SW or NW-SE suggesting that earthquake activity across the region is driven by reactivation of such fault systems by deformation associated with first-order plate motions rather than deformation associated with glacioisostatic recovery. Although there are no mapped major fault systems in the immediate vicinity of the Moidart earthquake, it seems likely that the earthquake also occurred on a steeply dipping fault that strikes either NE-SW or NW-SE but remains unmapped.

The South Wales earthquake of 17 February 2018
B Baptie, G Ford, D Galloway, 2018
The South Wales earthquake of 17 February 2018 (4.6 ML) was the largest earthquake on mainland Britain in almost 10 years, since a magnitude 5.2 ML earthquake near Market Rasen on 27 February 2008. The earthquake occurred in a part of South Wales that has experienced bursts of earthquakes with magnitudes of 5 ML or above at regular intervals in the last few hundred years, which may suggest that seismicity in this region is highly clustered in both space and time. However, there has been relatively little instrumentally recorded seismicity in the region in the last few decades. The epicentre of the earthquake on 17 February 2018 is close to the estimated epicentres of three earthquakes with magnitude greater than 5 ML in 1727, 1775 and 1906. We determined a hypocentre and source mechanism for the South Wales earthquake using P- and S-wave arrival times measured from instrumental recordings. The distribution of stations means that the error in the earthquake epicentre is less than 2.5 km. The focal depth of 7.5 km suggests that the earthquake may have nucleated at a relatively shallow depth. However, the error in the calculated depth is ±9.3 km, as the closest seismometer that recorded the earthquake was at a distance of 63 km. The calculated focal mechanisms show a near vertical, strike slip fault, with either left-lateral slip on a fault that strikes NE or right lateral slip on a fault that strikes NW. Neither of these is a good match for observed surface faulting near the epicentre. However, the NW plane is good match to the strike of the main Variscan Thrust, which cuts through the region. The NE plane is a reasonable match to the Acadian age faults that are observed at the surface to the north of the epicentre We suggest that the large difference between the moment magnitude (4.0 ± 0.2 Mw) and the local magnitude (4.6 ± 0.4 ML) is a result of the relatively high stress drop for the earthquake. This also results in higher recorded peak ground accelerations for the earthquake than those predicted by commonly used ground motion prediction equations used for seismic hazard assessments. Moment magnitudes measured at individual stations show considerably less scatter than local magnitude measurements, suggesting that the former are less dependent on site effects. We also observe that the stress drops calculated at each station increase with the local magnitude calculated at that station.

The Bulletin of British Earthquakes 2016
D Galloway
The British Geological Survey's (BGS) Seismic Monitoring and Information Service operate a nationwide network of seismograph stations in the United Kingdom (UK). Earthquakes in the UK and coastal waters are detected within limits dependent on the distribution of seismograph stations. Location accuracy is improved in offshore areas through data exchange with neighbouring countries. This bulletin contains locations, magnitudes and phase data for all earthquakes detected and located by the BGS during 2016, listed in Tables 1 and 2. Maps showing seismic activity in 2016 (Figure 1), and the larger magnitude events since 1979 (ML> 2.5) and since 1970 (ML> 3.5) are also included. The bulletin covers all of the UK land mass and its coastal waters including the North Sea (11°W to 6°E and 47°N to 65°N). All events believed to be of true tectonic origin are included. Coalfield events are also included. Acoustic disturbances, such as sonic booms from supersonic aircraft, are included when they are felt. The airborne waves are readily identified by their slow travel time across an array or by their signature on a microphone, but they are frequently mistaken as small earthquakes by the public. They are indicated by 'SONIC' in both the locality and comments column of Table 1. Significant non-natural events, such as explosions, which received media attention or were greater than magnitude 2.5 ML or felt by local residents, are also included in Table 1. Smaller events that are known, or suspected to be of explosive origin are excluded from the bulletin where possible. These include explosions due to quarrying, mining, weapon testing or disposal, naval exercises, geophysical prospecting and civil engineering. Unfortunately, identification by record character, location and time of occurrence is not always conclusive and some man-made events may be included in the bulletin or, more rarely, a small natural event may have been excluded.

Testing Earthquake Links in Mexico From 1978 to the 2017 M = 8.1 Chiapas and M = 7.1 Puebla Shocks
Margarita Segou, Tom Parsons
The M = 8.1 Chiapas and the M = 7.1 Puebla earthquakes occurred in the bending part of the subducting Cocos plate 11 days and ~600 km apart, a range that puts them well outside the typical aftershock zone. We find this to be a relatively common occurrence in Mexico, with 14% of M > 7.0 earthquakes since 1900 striking more than 300 km apart and within a 2 week interval, not different from a randomized catalog. We calculate the triggering potential caused by crustal stress redistribution from large subduction earthquakes over the last 40 years. There is no evidence that static stress transfer or dynamic triggering from the 8 September Chiapas earthquake promoted the 19 September earthquake. Both recent earthquakes were promoted by past thrust events instead, including delayed afterslip from the 2012 M = 7.5 Oaxaca earthquake. A repeated pattern of shallow thrust events promoting deep intraslab earthquakes is observed over the past 40 years.

Seismicity induced by longwall coal mining at the Thoresby Colliery, Nottinghamshire, UK
J P Verdon, J-M Kendall, A Butcher, R Luckett, and B Baptie
The United Kingdom has a long history of deep coal mining, and numerous cases of mining-induced seismicity have been recorded over the past 50 yr. In this study, we examine seismicity induced by longwall mining at one of the United Kingdom’s last deep coal mines, the Thoresby Colliery, Nottinghamshire. After public reports of felt seismicity in late 2013 a local seismic monitoring network was installed at this site, which provided monitoring from February to October 2014. This array recorded 305 seismic events, which form the basis of our analysis.

Event locations were found to closely track the position of the mining face within the Deep Soft Seam, with most events occurring up to 300 m ahead of the face position. This indicates that the seismicity is being directly induced by the mining, as opposed to being caused by activation of pre-existing tectonic features by stress transfer. However, we do not observe correlation between the rate of excavation and the rate of seismicity, and only a small portion of the overall deformation is being released as seismic energy.

Event magnitudes do not follow the expected Gutenberg–Richter distribution. Instead, the observed magnitude distributions can be reproduced if a truncated power-law distribution is used to simulate the rupture areas. The best-fitting maximum rupture areas correspond to the distances between the Deep Soft Seam and the seams that over- and underlie it, which have both previously been excavated. Our inference is that the presence of a rubble-filled void (or goaf) where these seams have been removed is preventing the growth of larger rupture areas. Source mechanism analysis reveals that most events consist of dip-slip motion along near-vertical planes that strike parallel to the orientation of the mining face. These mechanisms are consistent with the expected deformation that would occur as a longwall panel advances, with the under- and overburdens moving upwards and downwards respectively to fill the void created by mining. This further reinforces our conclusion that the events are directly induced by the mining process. Similar mechanisms have been observed during longwall mining at other sites.

Locating microseismic sources with a single seismometer channel using coda wave interferometry
Y Zhao, A Curtis and B Baptie
A novel source location method based on coda wave interferometry (CWI) was applied to a microseismic data set of mining-induced events recorded in Nottinghamshire, England. CWI uses scattered waves in the coda of seismograms to estimate the differences between two seismic states. We used CWI to estimate the distances between pairs of earthquake locations, which are then used jointly to determine the relative location of a cluster of events using a probabilistic framework. We evaluated two improvements to this location technique: These account for the impact of a large difference in the dominant wavelength of a recording made on different instruments, and they standardize the selection of parameters to be used when implementing the method. Although the method has been shown to produce reasonable estimates on larger earthquakes, we tested the method for microseismic events with shorter distinguishable codas in recorded waveforms, and hence, fewer recorded scattered waves. The earthquake location results are highly consistent when using different individual seismometer channels, showing that it is possible to locate event clusters with a single-channel seismometer. We thus extend the potential applications of this cost- effective method to seismic events over a wider range of magnitudes.