Local magnitudes at short distances Richard Luckett

Local magnitudes at short distances Richard Luckett Lars Ottemöller Brian Baptie Antony Butcher Murchison House West Mains Road Edinburgh EH 9 3 LA Tel 0131 667 1000 © NERC All rights reserved rrl@bgs. ac. uk lars. ottemoller@uib. no bbap@bgs. ac. uk antony. butcher@bristol. ac. uk

Blackpool Earthquake • Magnitude 2. 3 ML earthquake at 02: 34 on 1 st April 2011. • 4 temporary stations installed April/May. • 58 more events detected, all smaller. • Only 2 of these, on 26 th and 27 th of May, were also recorded at more distant stations. • Similarity of waveforms suggests a highly repeatable source so locations assumed to be the same.

Magnitude Problem • Only BHHF was recording data suitable for amplitude measurement by end of May. • 26 th May 22: 35 BHHF ML = 2. 3 regional stations ML = 1. 3 • 27 th 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. gov. uk).

Database Search 92 UK earthquakes where: Amplitudes measured on 5 or more stations With at least 1 station closer than 20 km. 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). 1253 Amplitudes from earthquakes with magnitude 0. 6 ML to 3 ML

Station Magnitude Residuals

New Scale 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

NEONOR 2 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).

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.