Crustal Imaging Using Earthquake Waveforms From Precision-located Earthquake Sequences.
Abstract
Our ability to accurately locate crustal earthquakes has markedly improved with recent improvements in the density of temporary and permanent arrays, and with new approaches for relative earthquake location, using cross-correlation analysis and double-difference location techniques. If these approaches are applied to earthquake swarm sequences, such as commonly observed in the active Taupo Volcanic Zone of New Zealand, waveforms from closely-related earthquakes in a swarm sequence can be subsequently used for crustal imaging, once errors in relative locations are much less than the shortest wavelength. Here we use waveforms from more than 700 earthquakes (2 < ML < 4) in the 2005-2007 Matata earthquake swarm to image crustal structure in the Taupo Volcanic Zone. We first relocate the sequence using both absolute and waveform-based differential time measurements, simultaneously inverting for the local velocity structure using double-difference tomography. The relocated events are clearly aligned c.NE-SW (strike 35 degrees) on several sub-parallel planes, with events clustered at 2-6 km depth, just northwest of the rupture area of the 1987 M6.2 Edgecumbe earthquake and southwest of deep 'bright spot' seismic reflections observed at 10-11 km depth. Super-receiver gathers of the earthquake waveforms, processed using coherency and frequency filtering, show excellent consistency out to 110 km+ from some of the event clusters. Coherent phases with strong amplitude are observed in the S-wave coda, 2-4 seconds after direct S, especially at offsets between 23 and 52 km. These may relate to lower crustal changes in S-wave reflectivity associated with intrusions. This interpretation is being examined using elastic wave finite difference synthetics and the observed 3-component data.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFM.S33D..06B
- Keywords:
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- 3285 Wave propagation (0689;
- 2487;
- 4275;
- 4455;
- 6934);
- 7203 Body waves;
- 7230 Seismicity and tectonics (1207;
- 1217;
- 1240;
- 1242)