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

2. 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…

3. Background and Motivation
Example of magnitude data available in the ISC Bulletin for a great earthquake (Tohoku, 11 March 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.

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

5. 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 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.

6. 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

7. Setting preferred Mw: GCMT + agencies with known procedures
Figure showing the events in with Mw from GCMT
Figure showing the events in with Mw from other agencies

8. Setting preferred teleseismic magnitudes: MS
(*)
Maps showing the seismic stations contributing to ISC-recomputed magnitudes in reported by NEIC, MOS, IDC and other agencies
(*) In 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

9. Setting preferred teleseismic magnitudes: mb
(*)
Maps showing the seismic stations contributing to ISC-recomputed magnitudes in reported by NEIC, MOS, IDC and other agencies
(*) In 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

10. Local/regional magnitudes
Figure showing ISC relocated events in with at least one local/regional magnitude

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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 (