Jökull - 01.01.2010, Síða 54

Martens et al.
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Figure 5. CMM automated locations (red) with respect to SIL-system catalogue locations (blue) in easting, northing, and
depth. Hypocentre locations are plotted as reported directly from each system. For CMM, depths are reported below sea
level; for the SIL-system on-line catalogue locations, depths are reported below the average elevation of the nearest one or
two seismic stations to an event. Near Upptyppingar, the typical average offset for station elevation is 700 m. A correction in
depth of this amount, however, does not account for all of the offset in depth (Table 1). SIL-system locations (derived using
SIL data) appear deeper and northward of the CMM locations (derived using ASN data). – CMM sjálfvirkar staðsetningar
(rauðir punktar) í samanburði við yfirfarna SIL-skjálftalistann (bláir punktar). Gröfin sýna færslur til norðurs og austurs
miðað við strikstefnu besta flatar í gegnum skjálftaþyrpinguna, ásamt dýpi sem fall af tíma. CMM staðsetningar eru miðaðar
við sjávarmál, en SIL staðsetningar við meðalhæð næstu tveggja stöðva, eða um 700 m.y.s. Mismunandi hæðarviðmið skýrir
ekki að fullu muninn á dýpinu (sjá töflu 1). SIL staðsetningar lenda dýpra og norðar en CMM staðsetningar með ASN netinu.
II. Single-event locations
Secondly, we invert both SIL and ASN time-pick data
using HypoInverse-2000 for a direct comparison of
single-event locations. The SIL time-pick data is pro-
vided by the IMO in HypoInverse format, includ-
ing both P- and S-wave onset times as well as pick
weights for up to 18 SIL stations per event. Phase
onset times and pick weights for the ASN data are se-
lected in PPICK for up to 27 ASN stations (i.e., 21
Cambridge University and 6 SIL) per event. The re-
sults are presented in Figure 6, with each dataset plot-
ted as depth below sea level. An offset between the
SIL locations and the ASN locations of hundreds of
metres in both northing and depth is evident (Table 1,
second column). The minor difference in SIL cata-
logue locations derived by the IMO (Figure 5) rela-
tive to the SIL locations derived in HypoInverse-2000
(Figure 6) is attributed to different inversion programs
and weighting schemes.
In an effort to distinguish the effect of network
configuration from picking precision on absolute lo-
54 JÖKULL No. 60