Description: Two short-period seismometers were permanently installed at depths of 26 and 383 m beneath the Eden Park stadium in central Auckland in October 2008 and incorporated into the Auckland Volcano Seismic Network in 2011. These borehole seismometers were temporarily augmented by a surface sensor to characterize the site response at this location. Despite the borehole installations, seismic monitoring is challenging in this urban environment due to high anthropogenic noise that superimposes the Earth’s signal. We analyze the power spectral density of continuous noise records over long and short time periods to quantify the reduction in noise with depth and the effect of temporal noise variations on the detection capabilities for earthquakes and volcanic tremor. We identify natural and anthropogenic noise sources that temporarily elevate noise levels by 10–15 dB using records from the Rugby World Cup matches held at the Eden Park stadium and by comparing records of daytime versus nighttime periods and windy versus calm days. Characterization of these noise sources shows that the frequency ranges of traffic and train noise at this site overlap (1–35 Hz and 8–35 Hz, respectively), however they exhibit distinct maxima at peak frequencies of 7 and 26 Hz. Modeling of the structure beneath the stadium shows that the noise spectrum generated by the nearby train excites frequencies within the topmost Waitemata sequences that get efficiently trapped in the low-velocity waveguide beneath the ~20 m thick basalt layer at the surface. Although the shallow borehole sensor generally shows a noise improvement of ≤5 dB in comparison to the surface sensor, it exhibits higher noise levels in the 8–35 Hz frequency range due to the trapped noise waves. Wind causes increased microseismic noise at 0.1–1 Hz that is coherent at all depth levels and shows the most pronounced time variation. Online Material: Figures of earthquake and quarry blast locations, spectrogram, and coherency.

Description: Fault rock assemblages reflect interaction between deformation, stress, temperature, fluid, and chemical regimes on distinct spatial and temporal scales at various positions in the crust. Here we interpret measurements made in the hanging-wall of the Alpine Fault during the second stage of the Deep Fault Drilling Project (DFDP-2). We present observational evidence for extensive fracturing and high hanging-wall hydraulic conductivity (∼10−9 to 10−7 m/s, corresponding to permeability of ∼10−16 to 10−14 m2) extending several hundred meters from the fault's principal slip zone. Mud losses, gas chemistry anomalies, and petrophysical data indicate that a subset of fractures intersected by the borehole are capable of transmitting fluid volumes of several cubic meters on time scales of hours. DFDP-2 observations and other data suggest that this hydrogeologically active portion of the fault zone in the hanging-wall is several kilometers wide in the uppermost crust. This finding is consistent with numerical models of earthquake rupture and off-fault damage. We conclude that the mechanically and hydrogeologically active part of the Alpine Fault is a more dynamic and extensive feature than commonly described in models based on exhumed faults. We propose that the hydrogeologically active damage zone of the Alpine Fault and other large active faults in areas of high topographic relief can be subdivided into an inner zone in which damage is controlled principally by earthquake rupture processes and an outer zone in which damage reflects coseismic shaking, strain accumulation and release on interseismic timescales, and inherited fracturing related to exhumation.

Description: During the second phase of the Alpine Fault, Deep Fault Drilling Project (DFDP) in the Whataroa River, South Westland, New Zealand, bedrock was encountered in the DFDP-2B borehole from 238.5–893.2 m Measured Depth (MD). Continuous sampling and meso- to microscale characterisation of whole rock cuttings established that, in sequence, the borehole sampled amphibolite facies, Torlesse Composite Terrane-derived schists, protomylonites and mylonites, terminating 200–400 m above an Alpine Fault Principal Slip Zone (PSZ) with a maximum dip of 62°. The most diagnostic structural features of increasing PSZ proximity were the occurrence of shear bands and reduction in mean quartz grain sizes. A change in composition to greater mica:quartz + feldspar, most markedly below c. 700 m MD, is inferred to result from either heterogeneous sampling or a change in lithology related to alteration. Major oxide variations suggest the fault-proximal Alpine Fault alteration zone, as previously defined in DFDP-1 core, was not sampled.

Description: Fault Zone Guided Waves (FZGWs) have been observed for the first time within New Zealand's transpressional continental plate boundary, the Alpine Fault, which is late in its typical seismic cycle. Ongoing study of these phases provides the opportunity to monitor interseismic conditions in the fault zone. Distinctive dispersive seismic codas (similar to 7-35Hz) have been recorded on shallow borehole seismometers installed within 20m of the principal slip zone. Near the central Alpine Fault, known for low background seismicity, FZGW-generating microseismic events are located beyond the catchment-scale partitioning of the fault indicating lateral connectivity of the low-velocity zone immediately below the near-surface segmentation. Initial modeling of the low-velocity zone indicates a waveguide width of 60-200m with a 10-40% reduction in S wave velocity, similar to that inferred for the fault core of other mature plate boundary faults such as the San Andreas and North Anatolian Faults.

Description: Using a newly compiled data set of 3424 focal mechanisms, we have estimated tectonic stress parameters at 100 locations throughout central New Zealand in the largest study to date of the tectonic stress field along the Australia-Pacific plate boundary. The results reveal pronounced changes in the azimuth of maximum horizontal compressive stress S-Hmax along the shallow portion (〈50 km depths) of the Hikurangi subduction margin, with a marked change from margin-parallel S-Hmax north of Hawke's Bay (latitude c. 40 degrees S) to a more oblique S-Hmax orientation further south. This change appears to coincide with the along-strike variations in subduction thrust coupling inferred from geodetic and seismological observations. In contrast, the orientation of S-Hmax is highly uniform across most of the South Island (averaging c. 115 degrees) and collinear with the axis of relative contractional strain rate. Analysis of focal mechanisms recorded before and after the damaging M(W)6.2 Christchurch earthquake of 22 February 2010 reveals no significant change in S-Hmax orientation or the overall stress regime: this suggests that even the high-stress drop Christchurch earthquake was incapable of substantially modifying the ambient stress field, at least on the scales at which focal mechanism stress estimation can be performed. (C) 2012 Elsevier B.V. All rights reserved.

Description: Temperature and fluid pressure conditions control rock deformation and mineralization on geological faults, and hence the distribution of earthquakes1. Typical intraplate continental crust has hydrostatic fluid pressure and a near-surface thermal gradient of 31 ± 15 degrees Celsius per kilometre2, 3. At temperatures above 300–450 degrees Celsius, usually found at depths greater than 10–15 kilometres, the intra-crystalline plasticity of quartz and feldspar relieves stress by aseismic creep and earthquakes are infrequent. Hydrothermal conditions control the stability of mineral phases and hence frictional–mechanical processes associated with earthquake rupture cycles, but there are few temperature and fluid pressure data from active plate-bounding faults. Here we report results from a borehole drilled into the upper part of the Alpine Fault, which is late in its cycle of stress accumulation and expected to rupture in a magnitude 8 earthquake in the coming decades4, 5. The borehole (depth 893 metres) revealed a pore fluid pressure gradient exceeding 9 ± 1 per cent above hydrostatic levels and an average geothermal gradient of 125 ± 55 degrees Celsius per kilometre within the hanging wall of the fault. These extreme hydrothermal conditions result from rapid fault movement, which transports rock and heat from depth, and topographically driven fluid movement that concentrates heat into valleys. Shear heating may occur within the fault but is not required to explain our observations. Our data and models show that highly anomalous fluid pressure and temperature gradients in the upper part of the seismogenic zone can be created by positive feedbacks between processes of fault slip, rock fracturing and alteration, and landscape development at plate-bounding faults.