Local magnitudes at short distances Richard Luckett rrl@bgs.ac.uk Lars Ottemöller lars.ottemoller@uib.no Brian Baptie bbap@bgs.ac.uk Antony Butcher antony.butcher@bristol.ac.uk

Blackpool Earthquake Magnitude 2.3 ML earthquake at 02:34 on 1st April 2011. 4 temporary stations installed April/May. 58 more events detected, all smaller. Only 2 of these, on 26th and 27th of May, were also recorded at more distant stations. Similarity of waveforms suggests a highly repeatable source so locations assumed to be the same. 8 Waveforms on BHFF Now know depth ~2.5 km BHHF/BAVH 12/4/2011 to 25/11/2011 BPRH/BEVW 29/6/2011 to 23/11/2011 BKL1 7/4/2011 to 9/4/2011

Magnitude Problem Only BHHF was recording data suitable for amplitude measurement by end of May. 26th May 22:35 BHHF ML = 2.3 regional stations ML = 1.3 27th May 00:48 BHHF ML = 2.6 regional stations ML = 1.5 Don’t trust magnitudes only recorded on nearby stations

UK traffic light monitoring system. ‘Stop’ limit set at ML 0. 5 (www UK traffic light monitoring system. ‘Stop’ limit set at ML 0.5 (www.gov.uk).

Database Search 92 UK earthquakes where: Amplitudes measured on 5 or more stations With at least 1 station closer than 20km. 14 Manchester (2002) aftershocks. 9 from the Borders sequence (2004). 2 from Blackpool (2011). 10 New Ollerton mining events (2014). 57 others close to a station (1993-2014). Folkestone one is 2009 mag3 – 2007 saturated TFO 1253 Amplitudes from earthquakes with magnitude 0.6 ML to 3 ML

Station Magnitude Residuals

New Scale r is distance (km)   r is distance (km) A= 1.11, B=0.00185 (Hutton and Boore, 1987) D=-1.16, E=0.2 RMS residual 0.33 0.28 15% improvement

NEONOR2 deployment in Northern Norway 6664 amplitudes from 617 earthquakes with ML between -0.4 and 3.3 C=-0.74, D=0.09 RMS = 0.25 RMS = 0.22 Amatrice aftershocks 12045 observations from 287 earthquakes with ML between 0.4 and 3.3 C=-3.05, D=0.17 RMS = 0.32 RMS = 0.27

NOLA.HHE Sg NOLA.HHN NOLA.HHZ Pg Myers, et al (1999). measurements beyond 12 km will be altered by less than 0.1. For Amatrice the equivalent distance is 20km and for the NEONOR2 data, it is 22 km. Pg Myers, et al (1999).

Conclusions Local magnitude scales used in the UK and elsewhere are not suitable to calculate magnitudes from near source amplitude observations. Particularly problematic for small events only recorded by nearby stations. The addition of an extra, exponential, term results in near source magnitudes agreeing with those at other stations. The constants associated with the new term are different in different regions. Possibly a result of a different phase having the highest amplitude near source. This results in a single scale applicable at all distances that does not change the vast majority of existing catalogue magnitudes.