Dear Colleague, Standardizing amplitude measurements for magnitude determinations Standard magnitude determination is one of the old ongoing problems in observation seismology, especially for monitoring global seismic activity. In this letter I appeal for your support to enable ISC to provide you and the global seismological community, better magnitude assessments and more information about them. I would also like to draw your attention to the new recommendations made by IASPEI regarding magnitude determinations. The Working Group on Magnitudes (Magnitude WG) of the International Association of Seismology and Physics of the Earth’s Interior (IASPEI) Commission on Seismological Observation and Interpretation (CoSOI) was established to recommend standard procedures for taking measurements from digital data to be used in calculating several widely used types of earthquake magnitudes. The recommended procedures from the Working Group have been approved by the IASPEI Commission on Seismological Observation and Interpretation and are expected to be implemented by all seismological centres. The WG is planning to publish an article, in the open literature, that will explain the reasons and arguments that lead to those recommendations. ISC is preparing to implement the IASPEI recommendations and again your support in providing ISC standard amplitude measurements, as outlined in this document, will be highly appreciated. Information on current practices The ISC determinations of mb and Ms, published in the ISC Bulletin in the last several decades, are important and valuable references for quantifying earthquakes strength over a relatively long time span. It is known, however, that the current ISC published magnitudes involve the use of non-standard amplitude measurements. Moreover, relatively few agencies report amplitude measurements and therefore ISC magnitudes are strongly influenced by the measuring practice of few agencies. We at ISC and users of ISC Bulletin are not sure that we know what amplitudes were reported to us by all agencies or how they are measured. We therefore appeal to all agencies to provide ISC with the following information: 1) Do you report amplitudes for a) P waves? b) surface waves? c) others? (please specify) 2) What are the units of the reported amplitude? 3) Are the reported amplitudes "trace amplitudes" or "ground motions"? 4) On what part of the seismogram do you measure the required Amax or (A/T)max? e.g. within; a) the first few cycles after the phase onset? b) a fixed time window of a specified number of seconds after the phase onset? c) a flexible time window depending on the length of the P wave train/rupture duration? d) the whole surface wave maximum? 5) What is the frequency band pass of the instrument or the filtered seismogram? 6) Is the recording made from a displacement, velocity or acceleration transducer? 7) What type of instrument response is used (e.g. the actual response of the measuring instrument or a response imitating another seismograph)? 8) On what component (vertical or horizontal) is the measurement made? a) for P waves? b) for surface waves? c) other waves (please specify) 9) Do you measure maximum A and the corresponding T or do you measure the maximum (A/T)? 10) What equations do you use for determining the magnitudes that your institution reports in your bulletin? Please send us this information at your earliest convenience. ISC will compile the information and post it on the ISC web site. IASPEI standards of amplitude measurements for magnitude determinations The attached Appendix presents the magnitude formulae and the standards that the Magnitude WG has recommended and been approved by IASPEI. These standards are defined for determining the magnitudes ML, MS(20), MS(BB), mb, mB, MW and mb(Lg). ISC plans to provide determinations of the magnitude MS(20), MS(BB), mb, mB and MW. We shall not determine ML and mb(Lg) but rather provide the amplitude values reported to ISC to be used by the community for future developments. We realize, however, that many agencies will find it important to maintain the procedures they currently use for their magnitudes. Similarly and for the sake of continuity, ISC will continue to determine magnitudes according to the current non- standard practices, allowing different magnitude notation (for example; (mb) and (Ms)). Subject to the availability of data, ISC plans to compute standardized magnitudes MS(20), MS(BB), mb, mB and MW together with the magnitudes (mb) and (Ms). Please note that although MS(20) and (Ms) are expected to be, on average, close to each other, different practices in amplitude measurements will result in different magnitude values for individual earthquakes and specific distance ranges and travel paths. Reporting Amplitude Measurements The messages containing current amplitude measurements follow the notation (amplitude measurement codes): A (for any amplitude measure), AML (amplitude for agency's ML), AMB (Amplitude for estimating (mb)) and AMS (amplitudes for estimating (Ms)). ISC encourages its data contributors to report seismic moments and amplitudes and periods for determining MS(20), MS(BB), mb, mB and MW as specified by the IASPEI Magnitude WG, along with the amplitudes they currently measure. We ask you to report the standard amplitudes by using the notation: IAML, IAmb, IAmB, IAmbLg, IAMS20 and IAMSBB where "I" stands for IASPEI, "A" stands for amplitude and the rest stands for the corresponding magnitude convention proposed by the Magnitude WG of CoSOI. The Magnitude WG is currently monitoring an implementation of the IASPEI standard procedures, to be sure that the procedures as described in the Appendix are consistent with the intent of the WG.  Subsequently, the ISC will provide co-operating agencies with a more complete description of the IASPEI standard procedures, including filter parameters that will be necessary to replicate the responses of standard instruments that are specified in the standard procedures. The Magnitude WG recommendations are also presented on the USGS/NEIC site at: ftp://ftpext.cr.usgs.gov/pub/cr/co/golden/hazards/MagnitudeWG/. Updates will be provided on ISC and NEIC websites. Thank you very much for your continued support and co-operation. Avi Shapira Director International Seismological Centre Pipers Lane, Thatcham, RG19 4NS United Kingdom Tel: +44 1635 861022 Fax: +44 1635 872358 E-mail: admin@isc.ac.uk www.isc.ac.uk