Microseismicity in Southern South Island, New Zealand: Implications for the Mechanism of Crustal Deformation Adjacent to a Major Continental Transform

Abstract

Shallow (<25 km), diffuse crustal seismicity occurs in a zone up to 150 km wide adjacent to the southern Alpine Fault, New Zealand, as a consequence of distributed shear and thickening in the obliquely convergent Australian‐Pacific plate boundary zone. It has recently been proposed that continental convergence here is accommodated by oblique slip on a low‐angle detachment that underlies the region, and as such, forms a previously unrecognized mode of oblique continental convergence. We test this model using microseismicity, presenting a new, 15 month high‐resolution microearthquake catalog for the Southern Lakes and northern Fiordland regions adjacent to the Alpine Fault. We determine the spatial distribution, moment release, and style of microearthquakes and show that seismicity in the continental lithosphere is predominantly shallower than ~20 km, in a zone up to 150 km wide, but less frequent deeper microseismicity extending into the mantle, at depths of up to 100 km is also observed. The geometry of the subducted oceanic Australian plate is well imaged, with a well‐defined Benioff zone to depths of ~150 km. In detail, the depth of continental microseismicity shows considerable variation, with no clear link with major active surface faults, but rather represents diffuse cracking in response to the ambient stress release. The moment release rate is ~0.1% of that required to accommodate relative plate convergence, and the azimuth of the principal horizontal axis of contraction accommodated by microseismicity is 120°, 15–20° clockwise of the horizontal axis of contractional strain rate observed geodetically. Thus, short‐term microseismicity, independent of knowledge of intermittent large‐magnitude earthquakes, may not be a good guide to the rate and orientation of long‐term deformation but is an indicator of the instantaneous state of stress and potential distribution of finite deformation. We show that both the horizontal and vertical spatial distribution of microseismicity can be explained in terms of a low‐angle detachment model.

Detecting Real Earthquakes Using Artificial Earthquakes: On the Use of Synthetic Waveforms in Matched‐Filter Earthquake Detection

“…This example uses data from the COSA ((Warren‐Smith, Lamb, et al, ) and GeoNet (Petersen et al, ) networks, and the manual picks and initial locations generated by Warren‐Smith, Chamberlain, et al () for 31 events recorded on the first day of the sequence. We use the same process to generate templates as described in section , except that we adopt the velocity model used by Warren‐Smith, Chamberlain, et al () and the mainshock focal mechanism (252/58/170, strike/dip/rake in degrees) to synthesize Green's functions.…”

Variations in Seismogenic Thickness Along the Central Alpine Fault, New Zealand, Revealed by a Decade's Relocated Microseismicity

“…The most recent large Alpine Fault earthquake occurred in 1717 CE, and the southern section of the fault has a recurrence interval for ground‐rupturing earthquakes of 291 ± 23 years (Cochran et al, ). The central section of the Alpine Fault exhibits high rates of geodetic deformation (Beavan et al, ), low levels of contemporary (past 50 years) earthquake activity (Boese et al, ; Bourguignon et al, ; Chamberlain, Boese, & Townend, ; Eiby, ; Evison, ; Feenstra et al, ; Leitner et al, ; Scholz et al, ; Warren‐Smith et al, ), little on‐fault seismicity, and no measurable creep (Evison, ; Sutherland et al, ). The oblique motion of the Alpine Fault has exposed a thin zone of metamorphic rocks within the hanging wall (Norris & Cooper, ).…”

Structural Controls on Shallow Cenozoic Fluid Flow in the Otago Schist, New Zealand

“…Analysis of borehole breakouts and earthquake focal mechanisms indicates that the contemporary crustal stress field in the South Island is relatively homogenous and is characterized in most areas (including Otago) by a regional strike-slip stress regime (i.e., σ 2 is subvertical) and a maximum horizontal compressive stress axis (σ 1 ) between c. 110°and 120° [36][37][38][39][40][41][42]. This σ 1 orientation is broadly compatible with active reverse faulting along NE-NNE striking structures, although this would require σ 3 to be subvertical in a typical "Andersonian" faulting regime.…”

“…Our interpretation is that the vein-and breccia-bearing strike-slip fault networks exposed at Akatore Creek and Bruce Rocks (Figures 2-4) represent shallowly formed, post-Early Miocene structures that formed in a stress field similar to the contemporary stress field. If this is correct, the fault networks broadly overlap in age with reverse movements on the nearby Akatore Fault, which has accumulated several hundreds of metres of reverse displacement at the surface since the Miocene [35][36][37][38][39][40][41][42][44][45][46][47][48][49][50][51][52][53][54][55][56][57]. The fault networks exposed along the coast may therefore represent the 21 Geofluids manifestation of broadly distributed, upper-crustal deformation within the "damage zones" of regional-scale reverse faults such as the Akatore Fault.…”