Microseismicity and stress in the vicinity of the Alpine Fault, central Southern Alps, New Zealand

We investigate present-day microseismicity associated with the central Alpine Fault and the zone of active deformation and uplift in the central Southern Alps. Using 14 months of data, robust hypocenter locations have been obtained for ∼1800 earthquakes of magnitudes between -0.3 and 4.2. We derived a magnitude scale with a frequency-dependent attenuation factor, γ(f) = γ0f, where γ0 = 1.89 ± 0.02 × 10-3 s/km, that enables magnitudes to be calculated consistently for earthquakes of different sizes and frequency contents. The maximum depth of the seismicity varies systematically with distance from the Alpine Fault, from 10 ± 2 km near the fault to 8 ± 2 km within 20 km and 15 ± 2 km further southeast. This distribution correlates with lateral variations in crustal resistivity: earthquake hypocenters are concentrated in areas of strong resistivity gradients and restricted to depths of resistivities >100 Ωm. Rocks at greater depth are too hot, too fluid-saturated, or too weak to produce detectable earthquakes. Focal mechanism solutions computed for 211 earthquakes (ML > 0.44) exhibit predominantly strike-slip mechanisms. We obtain a maximum horizontal compressive stress direction of 115 ± 10° from focal mechanism inversion. This azimuth is consistent with findings from elsewhere in the central and northern South Island, and indicates a uniform crustal stress field despite pronounced variations in crustal structure and topographic relief. Our stress estimates suggest that the Alpine Fault (with a mean strike of 055°) is poorly oriented in an Andersonian sense but that individual thrust and strike-slip segments of the fault's surface trace have close to optimal orientations. Copyright 2012 by the American Geophysical Union

Tectonic tremor and deep slow slip on the Alpine Fault

Tectonic tremor is characterized by persistent, low-frequency seismic energy seen at major plate boundaries. Although predominantly associated with subduction zones, tremor also occurs along the deep extension of the strike-slip San Andreas Fault. Here we present the first observations of tectonic tremor along New Zealand's Alpine Fault, a major transform boundary that is late in its earthquake cycle. We report tectonic tremor that occurred on the central section of the Alpine Fault on 12days between March 2009 and October 2011. Tremor hypocenters concentrate in the lower crust at the downdip projection of the Alpine Fault; coincide with a zone of high P-wave attenuation (low Q p) and bright seismic reflections; occur in the 25-45km depth range, below the seismogenic zone; and may define the deep plate boundary structure extending through the lower crust and into the upper mantle. We infer this tremor to represent slow slip on the deep extent of the Alpine Fault in a fluid-rich region marked by high attenuation and reflectivity. These observations provide the first indication of present-day displacement on the lower crustal portion of the Australia-Pacific transform plate boundary. © Copyright 2012 by the American Geophysical Union

Microseismicity and P–wave tomography of the central Alpine Fault, New Zealand

<p>We utilise seismic data from the central section of the Alpine Fault to locate earthquakes and image crustal structure in three dimensions. Tomography results from c. 6500 sources reveal the fault as either a southeast-dipping low-velocity zone or a marked velocity contrast in different parts of the study region. Where our model is best resolved, we interpret the Alpine Fault to be listric in nature, dipping steeply in the upper crust (50–60°) and flattening to 25–30° in the lower crust. The base of the seismogenic zone shallows from c. 15 km beneath the footwall and Alpine Fault to c. 6 km beneath the Southern Alps Main Divide, and then deepens to c. 15 km by c. 10 km further southeast. The shallow brittle–ductile transition overlies a broad low-velocity zone, which together likely result from the presence of fluids and elevated temperatures brought about by enhanced exhumation rate in this section of the Alpine Fault.</p