Jökull - 01.01.2010, Blaðsíða 52

Martens et al.
the imaging method, final hypocentral locations are
not limited to individual grid points.
In an extension to CMM, we load waveforms
into an interactive phase-picking tool (PPICK), ini-
tially aligned by automatically determined CMM on-
set times. As illustrated in Figure 3, PPICK allows
the user to make fine adjustments to arrival times
prior to event relocation through a combination of
event gather and station gather modes. A variety of
zoom functions as well as an adjustable band-pass
filter improve the ease with which phase arrivals may
be identified. We estimate that PPICK facilitates the
selection of high-quality Upptyppingar phase onsets
to one tenth of the dominant period of the arrival (i.e.,
to ∼0.01 s).
Manually refined picks are generally slightly ear-
lier than the automated times produced by the CMM
algorithm because CMM locates events based on en-
ergy onsets as opposed to first arrivals. Weights
of integer values between 0 and 4 are manually as-
signed to each arrival within PPICK based on confi-
dence in the pick. Single-event locations are then de-
rived using HypoInverse-2000 (Klein, 2002) and sub-
sequently relocated relative to neighbouring events
using HypoDD, a double-difference algorithm (Wald-
hauser and Ellsworth, 2000; Waldhauser, 2001).
References to ASN in this manuscript imply the
processing of ASN data using the aforementioned
processing techniques.
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Figure 3. Panel (a) presents continuous waveform data as a PPICK event gather for an earthquake that occurred
at 18:34:40 GMT on 21 July 2007 (Ml 1.8). Panel (b) depicts a PPICK station gather from VADA (for location
see Figure 1) for a swarm of earthquakes that occurred between 18:15:52 GMT and 19:23:31 GMT on 21
July 2007. Red traces are Z-component and used to select P-wave arrivals. Pairs of blue traces are used to
select S-wave arrivals; the upper trace is E-W component and the bottom trace is N-S component. Signals
are aligned by manual time pick, band-pass filtered at 2–16 Hz, and normalised to the same peak-to-peak
amplitude. The green bars indicate CMM model time picks. – (a) Skjálftabylgjur frá mismunandi stöðvum
klukkan 18:34:40 þann 21. júlí 2007 (Ml 1,8). (b) Bylgjugögn frá stöðinni VADA sem sýnir nokkra skjálfta á
tímabilinu 18:15:52–19:23:31 þann 21. júlí 2007. Rauðar línur tákna Z-þáttinn, þar sem komutími P-bylgna
er lesinn, en bláu pörin sýna N-S og A-V þættina, þar sem S-bylgjan er lesin. Handvirkt ákveðinn komutími
bylgjunnar er settur sem núll, gögnin síuð á tíðnibilinu 2–16 Hz og stærsta útslag sett jafnt á öllum rásum.
Grænu merkin sýna komutíma samkvæmt CMM staðsetningu.
52 JÖKULL No. 60