Seismicity and seismic tremor in the Hikurangi subduction zone

Abstract

Recent advances in geophysical monitoring have revealed a much broader spectrum of earth deformation processes than previously recognised, including several mechanisms operating on timescales between those of traditional seismology (in which earthquake rupture occurs in seconds) and geodesy (in which strain accumulates over years to decades).

Slow earthquakes are episodes of fault slip occurring over days or weeks, and were first identified using global positioning system (GPS) instruments. They have been detected in several subduction zones, where one tectonic plate is thrust beneath another, most notably in the western United States and Canada, Japan, and Mexico. Accompanying the slow earthquakes in western North America and Japan is a pronounced increase in coherent seismic noise, or “tremor”, which is manifest as episodic bursts of low-frequency noise.

Slow earthquakes have also been detected in the Hikurangi subduction zone, beneath the eastern North Island, using GPS instruments deployed as part of the national GeoNet monitoring system. This project focuses on detecting and interpreting tremor in the Hikurangi subduction zone and will provide a clearer understanding of the roles played by slow slip and associated phenomena in accommodating slip on major faults.

The key accomplishments and findings of this project are the following:

1. We have conducted a detailed visual inspection of continuous seismic data for a 7 week period spanning the 2004 Gisborne slow slip event;

2. A tremor signal similar to that observed in Cascadia and Japan and hypothesised to be present in Raukumara does not appear to have been generated during the 2004 Gisborne slow slip event;

3. We have distinguished three suites of earthquakes: nearby events detected by routine GeoNet analysis (“routine”); distant events (“teleseismic”); newly identified local events that had not triggered the automated processing system (“newly detected”);

4. The rate of earthquakes detected by routine GeoNet analysis does not appear to alter in the vicinity of the slow slip event during the period analysed;

5. In contrast, the slow slip event is accompanied by a pronounced increase in the number of newly detected small earthquakes;

6. These newly detected earthquakes appear to be preferentially concentrated near the Mahia Peninsula and to have occurred within the Pacific slab, beneath the principal plate boundary;

7. The newly detected earthquakes exhibit faulting characteristics compatible with reverse slip on a plane subparallel to the principal plate boundary interface;

8. The newly detected earthquakes, rather than seismic tremor, appear to be the seismological accompaniment to slow slip in this particular case;

9. Modelling results suggest that the newly detected events may constitute “coshocks” (Segall et al., 2006) occurring within the slab down-dip (deeper) than the area of the slow slip and triggered by the slow slip itself;

10. The reasons why slow slip near Gisborne triggers microearthquakes rather than tremor are not clear: however, this area differs from both Cascadia and southwest Japan in being generally cooler and in producing higher stress-drop slow slip. One or other of these two factors may have some influence on how the adjacent crust deforms in response to slow slip.

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Technical Abstract

Geodetically-detected episodes of slow slip appear in several subduction zones to be accompanied by bursts of low-frequency coherent noise known as seismic tremor, but whether a single physical process governs this association or even whether slow slip is invariably accompanied by tremor remains unresolved.

Detailed analysis of broadband seismic data spanning a slow slip episode in the Hikurangi subduction zone, New Zealand, reveals that slow slip was accompanied by distinct reverse-faulting microearthquakes, rather than tremor. The timing, location, and faulting style of these earthquakes are consistent with stress triggering down-dip of the slow slip patch, either on the subduction interface or just below it. These results indicate that tremor is not ubiquitous during subduction zone slow slip, and that slow slip in subduction zone environments is capable of triggering high-frequency earthquakes near the base of the locked subduction thrust. The stress drop during slow slip appears to control the nature of the triggered seismicity, with higher stress drops triggering high-frequency microearthquakes and lower stress drops triggering seismic tremor.