D. Di Giacomo and D. Storchak A scheme to set preferred magnitudes in the ISC Bulletin D. Di Giacomo and D. Storchak www.isc.ac.uk IUGG 2015 Prague, Session S01 1

Background The ISC collects, integrates and processes seismic bulletins provided by agencies around the world in order to produce the ISC Bulletin. This is regarded as the most comprehensive bulletin of the Earth’s seismicity and its production is made possible thanks to a unique cooperation in the seismological community that allows the ISC to complement the work of seismological agencies operating at global and/or local-regional scale. Therefore, the ISC Bulletin contains the magnitude results of the reporting agencies as well as the ISC own solutions (currently only short-period body-wave mb and surface wave MS) based on amplitude data reported by contributing agencies…

Background and Motivation Example of magnitude data available in the ISC Bulletin for a great earthquake (Tohoku, 11 March 2011. The magnitudes listed here mimic the standard ISF output. Background and Motivation However, since a multitude of magnitude types is currently computed by seismological agencies, it is not always a straightforward task for users of the ISC Bulletin and related products the selection of a magnitude value (or multiple ones) for various research purposes or practical applications. Thus, the ISC has been routinely asked by its users if the magnitudes listed in the ISC Bulletin follow any order of preference or which one is the best-suited for the event… Type Magnitude Error N(sta) Agency Mw 8.7   NIED 9.0 JMA mb 7.3 9 BJI mB 7.7 54 Ms 98 Ms7 8.6 88 5.5 0.0 76 IDC mb1 82 mb1mx 0.1 83 mbtmp ML 4.6 0.2 7 MS 8.2 Ms1 ms1mx 8.0 73 86 MOS 8.4 93 MW 9.1 100 GCMT 7.2 298 NEIC ME 8.3 299 7.1 553 ISC 514 … To facilitate such task, we describe a scheme we intend to implement in routine ISC operations that will flag magnitude values as preferred ones so that ISC users will have a better tool to select magnitudes of their interest.

Overview of recent magnitude data in the ISC Bulletin Distribution of the number of magnitude entries for ISC relocated events in 2011-2012.

Overview of recent magnitude data in the ISC Bulletin Number of Mw (top left), MS, (top tight), mb (bottom left) and ML (bottom right) per event available for ISC relocated events in 2011-2012. The vast majority of the events does not have a Mw, whereas the other magnitude scales are much more recurrent and at times available by several agencies.

General components considered in our scheme Consistency of a magnitude type reported by an agency over time (e.g. GCMT) Documentation of magnitude procedures by a given agency (see IASPEI WG magnitude continuation plan) Network coverage (is the agency in a good position to compute a certain magnitude for a specific event?) Deviation from a reference magnitude value for the event (median magnitude value) Number of reported stations used to compute a magnitude Magnitude uncertainty

Setting preferred Mw: GCMT + agencies with known procedures Figure showing the events in 2011-2012 with Mw from GCMT Figure showing the events in 2011-2012 with Mw from other agencies

Setting preferred teleseismic magnitudes: MS (*) Maps showing the seismic stations contributing to ISC-recomputed magnitudes in 2011-2012 reported by NEIC, MOS, IDC and other agencies (*) In 2011-2012 only 14 agencies reported surface wave amplitudes to the ISC Before the ISC reviews a month (usually 24 to 30 months after real-time), preferred MS are associated to one of the global agencies (NEIC, MOS, IDC), or regional ones provided that amplitude-period data is available in the teleseismic range, Δ> 20° After a month is reviewed, only MS from ISC (if available) will be flagged as preferred MS

Setting preferred teleseismic magnitudes: mb (*) Maps showing the seismic stations contributing to ISC-recomputed magnitudes in 2011-2012 reported by NEIC, MOS, IDC and other agencies (*) In 2011-2012 49 agencies reported body-wave amplitudes to the ISC Before the ISC reviews a month (usually 24 to 30 months after real-time), preferred mb are associated to one of the global agencies (NEIC, MOS, IDC), or regional ones provided that amplitude-period data is available in the teleseismic range, Δ> 20° After a month is reviewed, only mb from ISC (if available) will be flagged as preferred mb

Local/regional magnitudes Figure showing ISC relocated events in 2011-2012 with at least one local/regional magnitude

Local/regional magnitudes ML available in the ISC Bulletin for a small seismic event (Austria/Germany border, 27 January 2011). The magnitudes listed here mimic the standard ISF output. Type Magnitude Error N(sta) Agency Ml 3.3 0.0   STR ML 3.7 0.5 20 CSEM 3.2 0.3 24 LDG 3.0 0.4 12 ROM 30 ZUR 3.1 23 BGR ml 3.5 13 VIE

Local/regional magnitudes First, we assign a score for each magnitude author depending on how many stations are reported to the ISC within a certain distance (300 km for ML) Station positions from the agencies reporting ML for an event in January 2011, Austria-Germany border

Local/regional magnitudes Another score is assigned for each author depending on the absolute difference between the agency magnitude and the median for the event. The median is only used as reference in order to lower the score of the magnitudes at the tails Type Magnitude Error N(sta) Agency Ml 3.3 0.0   STR ML 3.7 0.5 20 CSEM Md 3.0 1 LDG 3.2 0.3 24 2.9 0.1 12 ROM 0.4 30 ZUR 3.1 23 BGR mb 2.8 0.2 9 VIE ml 3.5 13 All available magnitudes in the ISC Bulletin for a small seismic event (Austria/Germany border, 27 January 2011). The last two scores are assigned for each author depending on N(sta) and magnitude uncertainty. The magnitude entry with the highest sum of all scores becomes the preferred

Preferred magnitudes: examples Example from the great Tohoku earthquake, 11 March 2011 Type Magnitude Error N(sta) Agency Comment MW 9.1   100 GCMT Preferred Mw MS 8.4 0.2 514 ISC Preferred MS mb 7.1 553 Preferred mb MJMA JMA Preferred local/regional magnitude Many users of the ISC Bulletin have been routinely asking which magnitude (one entry) should be used for an event? Should the ISC select, among the preferred magnitudes, a magnitude which will be the best-suited or prime for the event? Example from the small Austria-Germany border event, 27 January 2011 Type Magnitude Error N(sta) Agency Comment ML 3.1 0.3 23 BGR Prime magnitude for the event (also preferred ML) MD 2.9 0.1 12 ROM Preferred MD

Setting best-suited or prime magnitudes Once the preferred magnitudes are set, one becomes also the best-suited or prime magnitude for the event according to this magnitude type hierarchy: Mw Ms mb MJMA ML mbLG / MN MD Example from the great Tohoku earthquake, 11 March 2011 Type Magnitude Error N(sta) Agency Comment MW 9.1   100 GCMT Prime magnitude for the event (also preferred Mw) MS 8.4 0.2 514 ISC Preferred MS mb 7.1 553 Preferred mb MJMA JMA Preferred local/regional magnitude Example from the small Austria-Germany border event, 27 January 2011 Type Magnitude Error N(sta) Agency Comment ML 3.1 0.3 23 BGR Prime magnitude for the event (also preferred ML) MD 2.9 0.1 12 ROM Preferred MD

Summary The ISC Bulletin is probably the most comprehensive repository for magnitudes of seismic events. However, ISC users often face various difficulties in selecting the most appropriate magnitude for their particular need. To facilitate this task, we propose a scheme that: flags magnitude entries as preferred ones among the values available for a specific magnitude type; assigns, among the preferred magnitudes, a prime magnitude to the event according to a magnitude type hierarchy (first, if any, Mw, then MS, mb and finally local/regional magnitudes). We trust that once implemented in routine operations such a tool will help ISC users to fine tune their searches and also to simplify the use of the various datasets available at the ISC website (www.isc.ac.uk).