Improved location procedures at the International Seismological Centre

Abstract

S U M M A R YThe International Seismological Centre (ISC) is a non-governmental, non-profit organization with the primary mission of producing the definitive account of the Earth's seismicity. The ISC Bulletin covers some 50 yr of seismicity. The recent years have seen a dramatic increase both in the number of reported events and especially in the number of reported phases, owing to the ever-increasing number of stations worldwide. Similar ray paths will produce correlated traveltime prediction errors due to unmodelled heterogeneities in the Earth, resulting in underestimated location uncertainties, and for unfavourable network geometries, location bias. Hence, the denser and more unbalanced the global seismic station coverage becomes, the less defensible is the assumption (that is the observations are independent), which is made by most location algorithms.To address this challenge we have developed a new location algorithm for the ISC that accounts for correlated error structure, and uses all IASPEI standard phases with a valid ak135 traveltime prediction to obtain more accurate event locations. In this paper we describe the new ISC locator, and present validation tests by relocating the ground truth events in the IASPEI Reference Event List, as well as by relocating the entire ISC Bulletin.We show that the new ISC location algorithm provides small, but consistent location improvements, considerable improvements in depth determination and significantly more accurate formal uncertainty estimates. We demonstrate that the new algorithm, through the use of later phases and testing for depth resolution, considerably clusters event locations more tightly, thus providing an improved view of the seismicity of the Earth.

The Profound Reach of the 11 April 2012 M 8.6 Indian Ocean Earthquake: Short-Term Global Triggering Followed by a Longer-Term Global Shadow

“…A million phase records were digitized, and all earthquakes were relocated using Bondár and Storchak (2011). Approximate completeness is M ≥ 7:50 since 1900, M ≥ 6:25 since 1918, and M ≥ 5:50 since 1965.…”

Rebuild of the Bulletin of the International Seismological Centre (ISC), part 1: 1964–1979

“…It is thus expected that usage of additional phases by the new locator generally improves the location. Further to this argument, it was proved during the new ISC locator testing stage (Bondár and Storchak 2011) that the new locator does not introduce additional bias when its locations are tested against the locations in the IASPEI Reference Event List (GT).…”

“…As previously explained, relocation of selected events has been made using the new standard earthquake locator (Bondár and Storchak 2011), ak135 velocity model (Kennett et al 1995) and the uniform set of seismic phases (Storchak et al 2003). Location error ellipse parameters have been computed for all ISC hypocentres.…”

“…A new seismic event location program (locator) based on different approaches and techniques was initially introduced into the ISC operations with the beginning of data year 2009 (Bondár and Storchak 2011). The locator was altered and fine-tuned during the processing of data years 2009 and 2010. Throughout the ISC history different sets of seismic phases were used: P to start with and then also S from data month of January 2001 (Storchak et al 2000).…”

“…Also, computation of ISC magnitudes was regularised and improved with the introduction of the new locator (shown as ISCloc II in Fig. 1) (Bondár and Storchak 2011).…”