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

Alpine Fault ◽

Horizontal Compressive Stress ◽

American Geophysical ◽

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.

Erosion rates of the New Zealand Southern Alps reflect long-term tectonics and transient climate

10.5194/egusphere-egu21-6571 ◽

2021 ◽

Author(s):

Duna Roda-Boluda ◽

Taylor Schildgen ◽

Hella Wittmann-Oelze ◽

Stefanie Tofelde ◽

Aaron Bufe ◽

...

Keyword(s):

New Zealand ◽

Strike Slip ◽

Southern Alps ◽

Fault System ◽

Tectonic Deformation ◽

Seismic Cycle ◽

Erosion Rates ◽

Alpine Fault ◽

Oblique Convergence

The Southern Alps of New Zealand are the expression of the oblique convergence between the Pacific and Australian plates, which move at a relative velocity of nearly 40 mm/yr. This convergence is accommodated by the range-bounding Alpine Fault, with a strike-slip component of ~30-40 mm/yr, and a shortening component normal to the fault of ~8-10 mm/yr. While strike-slip rates seem to be fairly constant along the Alpine Fault, throw rates appear to vary considerably, and whether the locus of maximum exhumation is located near the fault, at the main drainage divide, or part-way between, is still debated. These uncertainties stem from very limited data characterizing vertical deformation rates along and across the Southern Alps. Thermochronology has constrained the Southern Alps exhumation history since the Miocene, but Quaternary exhumation is hard to resolve precisely due to the very high exhumation rates. Likewise, GPS surveys estimate a vertical uplift of ~5 mm/yr, but integrate only over ~10 yr timescales and are restricted to one transect across the range.To obtain insights into the Quaternary distribution and rates of exhumation of the western Southern Alps, we use new 10Be catchment-averaged erosion rates from 20 catchments along the western side of the range. Catchment-averaged erosion rates span an order of magnitude, between ~0.8 and >10 mm/yr, but we find that erosion rates of >10 mm/yr, a value often quoted in the literature as representative for the entire range, are very localized. Moreover, erosion rates decrease sharply north of the intersection with the Marlborough Fault System, suggesting substantial slip partitioning. These 10Be catchment-averaged erosion rates integrate, on average, over the last ~300 yrs. Considering that the last earthquake on the Alpine Fault was in 1717, these rates are representative of inter-seismic erosion. Lake sedimentation rates and coseismic landslide modelling suggest that long-term (~103 yrs) erosion rates over a full seismic cycle could be ~40% greater than our inter-seismic erosion rates. If we assume steady state topography, such a scaling of our 10Be erosion rate estimates can be used to estimate rock uplift rates in the Southern Alps. Finally, we find that erosion, and hence potentially exhumation, does not seem to be localized at a particular distance from the fault, as some tectonic and provenance studies have suggested. Instead, we find that superimposed on the primary tectonic control, there is an elevation/temperature control on erosion rates, which is probably transient and related to frost-cracking and glacial retreat.Our results highlight the potential for 10Be catchment-averaged erosion rates to provide insights into the magnitude and distribution of tectonic deformation rates, and the limitations that arise from transient erosion controls related to the seismic cycle and climate-modulated surface processes.&#160;&#160;

Secondary faulting near the terminus of a seismogenic strike-slip fault: Aftershocks of the 1976 Guatemala earthquake

Bulletin of the Seismological Society of America ◽

10.1785/bssa0690020427 ◽

1979 ◽

Vol 69 (2) ◽

pp. 427-444

Author(s):

C. J. Langer ◽

G. A. Bollinger

Keyword(s):

Focal Mechanism ◽

Linear Trend ◽

Strike Slip ◽

Strike Slip Fault ◽

Aftershock Activity ◽

The North ◽

Composite Focal Mechanism ◽

Theoretical Pattern ◽

Main Fault

abstract Aftershocks of the February 4, 1976 Guatemalan earthquake (Ms = 7.5) were monitored by a network of portable seismographs from February 9 to February 27. Although seismic data were obtained all along the 230-km surface rupture of the causal Motagua fault, the field program was designed to concentrate on the aftershock activity at the western terminus of the fault. Data from that locale revealed several linear or near-linear trends of aftershock epicenters that splay to the southwest away from the western end of the main fault. These trends correlate spatially with mapped surface lineaments and, to some degree, with ground breakage patterns near Guatemala City. The observed splay pattern of aftershocks and the normal-faulting mode of the splay earthquakes determined from composite focal mechanism solutions, may be explained by a theoretical pattern of stress trajectories at the terminus of a strike-slip fault. Composite focal mechanism solutions for aftershocks located on or near the surface break of the Motagua fault, to the north and east of the linear trend splay area, agree with the mapped surface movements, i.e., left-lateral strike-slip.

Aftershocks and surface faulting associated with the intraplate Guinea, West Africa, earthquake of 22 December 1983

Bulletin of the Seismological Society of America ◽

10.1785/bssa0770051579 ◽

1987 ◽

Vol 77 (5) ◽

pp. 1579-1601

Author(s):

C. J. Langer ◽

M. G. Bonilla ◽

G. A. Bollinger

Keyword(s):

West Africa ◽

Focal Mechanism ◽

Main Shock ◽

Strike Slip ◽

Fault System ◽

Epicentral Area ◽

Lateral Strike Slip ◽

Short Period ◽

East West

Abstract This study reports on the results of geological and seismological field studies conducted following the rare occurrence of a moderate-sized West African earthquake (mb = 6.4) with associated ground breakage. The epicentral area of the northwestern Guinea earthquake of 22 December 1983 is a coastal margin, intraplate locale with a very low level of historical seismicity. The principal results include the observation that seismic faulting occurred on a preexisting fault system and that there is good agreement among the surface faulting, the spatial distribution of the aftershock hypocenters, and the composite focal mechanism solutions. We are not able, however, to shed any light on the reason(s) for the unexpected occurrence of this intraplate earthquake. Thus, the significance of this study is its contribution to the observational datum for such earthquakes and for the seismicity of West Africa. The main shock was associated with at least 9 km of surface fault-rupture. Trending east-southeast to east-west, measured fault displacements up to ∼13 cm were predominantly right-lateral strike slip and were accompanied by an additional component (5 to 7 cm) of vertical movement, southwest side down. The surface faulting occurred on a preexisting fault whose field characteristics suggest a low slip rate with very infrequent earthquakes. There were extensive rockfalls and minor liquefaction effects at distances less than 10 km from the surface faulting and main shock epicenter. Main shock focal mechanism solutions derived from teleseismic data by other workers show a strong component of normal faulting motion that was not observed in the ground ruptures. A 15-day period of aftershock monitoring, commencing 22 days after the main shock, was conducted. Eleven portable, analog short-period vertical seismographs were deployed in a network with an aperture of 25 km and an average station spacing of 7 km. Ninety-five aftershocks were located from the more than 200 recorded events with duration magnitudes of about 1.5 or greater. Analysis of a selected subset (91) of those events define a tabular aftershock volume (26 km long by 14 km wide by 4 km thick) trending east-southeast and dipping steeply (∼60°) to the south-southwest. Composite focal mechanisms for groups of events, distributed throughout the aftershock volume, exhibit right-lateral, strike-slip motion on subvertical planes that strike almost due east. Although the general agreement between the field geologic and seismologic results is good, our preferred interpretation is for three en-echelon faults striking almost due east-west.

PRESENT-DAY STATE-OF-STRESS OF SOUTHEAST AUSTRALIA

The APPEA Journal ◽

10.1071/aj05016 ◽

2006 ◽

Vol 46 (1) ◽

pp. 283 ◽

Cited By ~ 18

Author(s):

E. Nelson ◽

R. Hillis ◽

M. Sandiford ◽

S. Reynolds ◽

S. Mildren

Keyword(s):

Focal Mechanism ◽

Horizontal Stress ◽

Strike Slip ◽

Southeast Australia ◽

State Of Stress ◽

Earthquake Focal Mechanism ◽

Minimum Horizontal Stress ◽

Maximum Horizontal Stress ◽

Gippsland Basin

There have been several studies, both published and unpublished, of the present-day state-of-stress of southeast Australia that address a variety of geomechanical issues related to the petroleum industry. This paper combines present-day stress data from those studies with new data to provide an overview of the present-day state-of-stress from the Otway Basin to the Gippsland Basin. This overview provides valuable baseline data for further geomechanical studies in southeast Australia and helps explain the regional controls on the state-of-stress in the area.Analysis of existing and new data from petroleum wells reveals broadly northwest–southeast oriented, maximum horizontal stress with an anticlockwise rotation of about 15° from the Otway Basin to the Gippsland Basin. A general increase in minimum horizontal stress magnitude from the Otway Basin towards the Gippsland Basin is also observed. The present-day state-of-stress has been interpreted as strike-slip in the South Australian (SA) Otway Basin, strike-slip trending towards reverse in the Victorian Otway Basin and borderline strike-slip/reverse in the Gippsland Basin. The present-day stress states and the orientation of the maximum horizontal stress are consistent with previously published earthquake focal mechanism solutions and the neotectonic record for the region. The consistency between measured present-day stress in the basement (from focal mechanism solutions) and the sedimentary basin cover (from petroleum well data) suggests a dominantly tectonic far-field control on the present-day stress distribution of southeast Australia. The rotation of the maximum horizontal stress and the increase in magnitude of the minimum horizontal stress from west to east across southeast Australia may be due to the relative proximity of the New Zealand segment of the plate boundary.

Fluid Flux in Fractured Rock of the Alpine Fault Hanging-Wall Determined from Temperature Logs in the DFDP-2B Borehole, New Zealand

10.26686/wgtn.13876412 ◽

2021 ◽

Author(s):

L Janku-Capova ◽

Rupert Sutherland ◽

John Townend ◽

ML Doan ◽

C Massiot ◽

...

Keyword(s):

Fractured Rock ◽

Hanging Wall ◽

Surrounding Rock ◽

Fluid Flux ◽

Long Wavelength ◽

Alpine Fault ◽

Temperature Signal ◽

American Geophysical ◽

Diffusive Processes ◽

©2018. American Geophysical Union. All Rights Reserved. Sixteen temperature logs were acquired during breaks in drilling of the 893m-deep DFDP-2B borehole, which is in the Alpine Fault hanging-wall. The logs record various states of temperature recovery after thermal disturbances induced by mud circulation. The long-wavelength temperature signal in each log was estimated using a sixth-order polynomial, and residual (reduced) temperature logs were analyzed by fitting discrete template wavelets defined by depth, amplitude, and width parameters. Almost two hundred wavelets are correlated between multiple logs. Anomalies generally have amplitudes <1°C, and downhole widths <20m. The largest amplitudes are found in the first day after mud circulation stops, but many anomalies persist with similar amplitude for up to 15 days. Our models show that thermal and hydraulic diffusive processes are dominant during the first few days of re-equilibration after mud circulation stops, and fluid advection of heat in the surrounding rock produces temperature anomalies that may persist for several weeks. Models indicate that the fluid flux normal to the borehole within fractured zones is of order 10−7 to 10−6 m s−1, which is 2–3 orders of magnitude higher than the regional flux. Our approach could be applied more widely to boreholes, as it uses the thermal re-equilibration phase to derive useful information about the surrounding rock mass and its fluid flow regime.

The Alpine Fault hanging-wall viewed from within: Structural analysis of the ultrasonic image logs in the DFDP-2B borehole, New Zealand

10.26686/wgtn.13876364.v1 ◽

2021 ◽

Author(s):

C Massiot ◽

B Célérier ◽

ML Doan ◽

TA Little ◽

John Townend ◽

...

Keyword(s):

New Zealand ◽

Fault Plane ◽

Hanging Wall ◽

Anisotropic Permeability ◽

Ultrasonic Image ◽

Feature Sets ◽

Alpine Fault ◽

American Geophysical ◽

American Geophysical Union ◽

Topographic Relief

©2018. American Geophysical Union. All Rights Reserved. Ultrasonic image logs acquired in the DFDP-2B borehole yield the first continuous, subsurface description of the transition from schist to mylonite in the hangingwall of the Alpine Fault, New Zealand, to a depth of 818 m below surface. Three feature sets are delineated. One set, comprising foliation and foliation-parallel veins and fractures, has a constant orientation. The average dip direction of 145° is subparallel to the dip direction of the Alpine Fault, and the average dip magnitude of 60° is similar to nearby outcrop observations of foliation in the Alpine mylonites that occur immediately above the Alpine Fault. We suggest that this foliation orientation is similar to the Alpine Fault plane at ∼1 km depth in the Whataroa valley. The other two auxiliary feature sets are interpreted as joints based on their morphology and orientation. Subvertical joints with NW-SE (137°) strike occurring dominantly above ∼500 m are interpreted as being formed during the exhumation and unloading of the Alpine Fault's hangingwall. Gently dipping joints, predominantly observed below ∼500 m, are interpreted as inherited hydrofractures exhumed from their depth of formation. These three fracture sets, combined with subsidiary brecciated fault zones, define the fluid pathways and anisotropic permeability directions. In addition, high topographic relief, which perturbs the stress tensor, likely enhances the slip potential and thus permeability of subvertical fractures below the ridges, and of gently dipping fractures below the valleys. Thus, DFDP-2B borehole observations support the inference of a large zone of enhanced permeability in the hangingwall of the Alpine Fault.

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

10.26686/wgtn.13818725 ◽

2021 ◽

Author(s):

Calum Chamberlain ◽

John Townend

Keyword(s):

Focal Mechanism ◽

Matched Filter ◽

A Priori ◽

Low Frequency ◽

Aftershock Sequence ◽

Filter Method ◽

Detection Rates ◽

Earthquake Detection ◽

American Geophysical ◽

©2018. American Geophysical Union. All Rights Reserved. Matched-filters are an increasingly popular tool for earthquake detection, but their reliance on a priori knowledge of the targets of interest limits their application to regions with previously documented seismicity. We explore an extension to the matched-filter method to detect earthquakes and low-frequency earthquakes on local to regional scales. We show that it is possible to increase the number of detections compared with standard energy-based methods, with low false-detection rates, using suites of synthetic waveforms as templates. We apply this to a microearthquake swarm and an aftershock sequence, and to detect low-frequency earthquakes. We also explore the sensitivity of detections to the synthetic source's location and focal mechanism. Source-receiver geometry has a first-order control on how sensitive matched-filter detectors are to variations in source location and focal mechanism, and this likely applies to detections made using both synthetic and real templates.

Fluid Flux in Fractured Rock of the Alpine Fault Hanging-Wall Determined from Temperature Logs in the DFDP-2B Borehole, New Zealand

10.26686/wgtn.13876412.v1 ◽

2021 ◽

Author(s):

L Janku-Capova ◽

Rupert Sutherland ◽

John Townend ◽

ML Doan ◽

C Massiot ◽

...

Keyword(s):

Fractured Rock ◽

Hanging Wall ◽

Surrounding Rock ◽

Fluid Flux ◽

Long Wavelength ◽

Alpine Fault ◽

Temperature Signal ◽

American Geophysical ◽

Diffusive Processes ◽

©2018. American Geophysical Union. All Rights Reserved. Sixteen temperature logs were acquired during breaks in drilling of the 893m-deep DFDP-2B borehole, which is in the Alpine Fault hanging-wall. The logs record various states of temperature recovery after thermal disturbances induced by mud circulation. The long-wavelength temperature signal in each log was estimated using a sixth-order polynomial, and residual (reduced) temperature logs were analyzed by fitting discrete template wavelets defined by depth, amplitude, and width parameters. Almost two hundred wavelets are correlated between multiple logs. Anomalies generally have amplitudes <1°C, and downhole widths <20m. The largest amplitudes are found in the first day after mud circulation stops, but many anomalies persist with similar amplitude for up to 15 days. Our models show that thermal and hydraulic diffusive processes are dominant during the first few days of re-equilibration after mud circulation stops, and fluid advection of heat in the surrounding rock produces temperature anomalies that may persist for several weeks. Models indicate that the fluid flux normal to the borehole within fractured zones is of order 10−7 to 10−6 m s−1, which is 2–3 orders of magnitude higher than the regional flux. Our approach could be applied more widely to boreholes, as it uses the thermal re-equilibration phase to derive useful information about the surrounding rock mass and its fluid flow regime.

Focal mechanisms of Indian earthquakes

Bulletin of the Seismological Society of America ◽

10.1785/bssa0620020603 ◽

1972 ◽

Vol 62 (2) ◽

pp. 603-608 ◽

Cited By ~ 1

Author(s):

A. R. Banghar

Keyword(s):

Focal Mechanism ◽

Strike Slip ◽

Focal Mechanisms ◽

Peninsular India ◽

S Waves ◽

Focal Mechanism Solutions

abstract Focal mechanism solutions are presented for two earthquakes that occurred in peninsular India. The first motions of P, PKP and the polarization (or first motions) of S waves were used for this investigation. Both of these mechanisms are found to be associated with strike-slip faulting.