ISC Focal Mechanisms




Introduction

Unlike hypocentre solutions and phase arrival picks, earthquake source mechanisms are systematically reported to the ISC by only a few global agencies such as HRVD/GCMT and NEIS/NEIC/USGS and other contributors, notably the Japan Meteorological Agency (JMA), the National Research Institute for Earth Science and Disaster Prevention (NIED) for Japan, the Pacific Northwest Seismic Network (PNSN), the Swiss Seismological Service (ZUR-RMT), and the Med-Net Regional Centroid Moment Tensor project (MED-RCMT) for Europe.

The majority of these source mechanisms are derived by using techniques which are based on waveform modeling, and as a consequence, the associated seismic events are usually of magnitude mb 5.0 and above. Seismic events with lower magnitude show a proportional decrease in the number of available source mechanisms in the ISC Bulletin.

An effort to bridge this gap and lower the magnitude threshold to mb ~ 4.5 is attempted by either introducing data from temporary deployments (see Rebuild of the ISC Bulletin, www.isc.ac.uk/projects/rebuild) or by computing our own ISC focal mechanisms based on first motion P-wave polarities.





Data and Method

The data being used come from two main sources. These are: (i) reported first motion P-wave polarities from the ISC Bulletin, and (ii) automatic picks from waveform data of broadband stations up to 90 degrees, and waveform data from short period instruments up to 10 degrees.

The ISC keeps its own inventory of waveform and response files as part of the CTBTO link project (www.isc.ac.uk/projects/#ctbto). The FDSN station codes of the waveform data (International Federation of Digital Seismograph Networks, www.fdsn.org) are matched with the seismic station codes of the reported phase arrivals in the ISC Bulletin (IR, www.isc.ac.uk/registries), based on the station location information.

Details on the computations of the focal mechanisms are given in Lentas, 2018. The Earth's topography and ellipticity corrections are now implemented in the theoretical calculations.


Global distribution of seismic stations kept in the ISC inventory. The map shows the locations of seismic stations, and the availability of several vertical component instrument types, which are successfully matched with stations registered in the ISC International Registry.

Results

ISC focal mechanism solutions are available for the ISC reviewed and relocated earthquakes with mbISC >= 4.5 starting from data month January 2011, and will be routinely added for every data month added in the Reviewed ISC Bulletin.

Additionaly we have added focal mechanism solutions obtained from reported polarities for the ISC relocated earthquakes for the time period 1964 - 1979 as part of the Rebuild project (www.isc.ac.uk/projects/rebuild), and focal mechanism solutions obtained from reported polarities in the ISS Bulletin for the time period 1938 - 1963.

If a well-trusted ISC focal mechanism is available it will be shown in the online ISC Bulletin using the IMS1.0 format below the ISC hypocentre solution. A link at the top of the page will redirect the user to a dedicated page showing the distribution of the polarities being used in the computations and any possible solutions (well-trusted and deprecated) derived with respect to the a priori hypocentre, velocity and first-motion polarity errors. Moreover, a list of the polarity data (reported to the ISC and auto-picked) is also available. Please note that in some cases the focal mechanism algorithm may discard some of the reported polarities if they do not correspond to first arrivals, but they will still show in the list.


References

Lentas, K. (2018). Towards routine determination of focal mechanisms obtained from first motion P-wave arrivals. Geophys. J. Int.. 212(3). 1665-1686.
doi: 10.1093/gji/ggx503



Global map showing the distribution of ISC computed focal mechanism solutions and ternary plot presenting the distribution of the obtained source mechanism types.