The structural architecture of the Whataroa Valley at the Alpine Fault (New Zealand) from first-arrival tomography and reflection imaging using an extended 3D VSP survey

2020 ◽  
Author(s):  
Vera Lay ◽  
Stefan Buske ◽  
Sascha Barbara Bodenburg ◽  
Franz Kleine ◽  
John Townend ◽  
...  
Keyword(s):  
New Zealand ◽  
Seismic Data ◽  
Average Velocity ◽  
Velocity Model ◽  
P Wave ◽  
3D Seismic ◽  
Data Set ◽  
Alpine Fault ◽  
P Wave Velocity ◽  
First Arrival

<p>The Alpine Fault along the West Coast of the South Island (New Zealand) is a major plate boundary that is expected to rupture in the next 50 years, likely as a magnitude 8 earthquake. The Deep Fault Drilling Project (DFDP) aims to deliver insight into the geological structure of this fault zone and its evolution by drilling and sampling the Alpine Fault at depth.  </p><p>Here we present results from a 3D seismic survey around the DFDP-2 drill site in the Whataroa Valley where the drillhole penetrated almost down to the fault surface. Within the glacial valley, we collected 3D seismic data to constrain valley structures that were obscured in previous 2D seismic data. The new data consist of a 3D extended vertical seismic profiling (VSP) survey using three-component receivers and a fibre optic cable in the DFDP-2B borehole as well as a variety of receivers at the surface.</p><p>The data set enables us to derive a reliable 3D P-wave velocity model by first-arrival travel time tomography. We identify a 100-460 m thick sediment layer (average velocity 2200±400 m/s) above the basement (average velocity 4200±500 m/s). Particularly on the western valley side, a region of high velocities steeply rises to the surface and mimics the topography. We interpret this to be the infilled flank of the glacial valley that has been eroded into the basement. In general, the 3D structures implied by the velocity model on the upthrown (Pacific Plate) side of the Alpine Fault correlate well with the surface topography and borehole findings.</p><p>A reliable velocity model is not only valuable by itself but it is also required as input for prestack depth migration (PSDM). We performed PSDM with a part of the 3D data set to derive a structural image of the subsurface within the Whataroa Valley. The top of the basement identified in the P-wave velocity model coincides well with reflectors in the migrated images so that we can analyse the geometry of the basement in detail.</p>

scholarly journals Seismic P  Wave Velocity Model From 3‐D Surface and Borehole Seismic Data at the Alpine Fault DFDP‐2 Drill Site (Whataroa, New Zealand)

2021 ◽  
Author(s):  
V Lay ◽  
S Buske ◽  
SB Bodenburg ◽  
John Townend ◽  
R Kellett ◽  
...  
Keyword(s):  
New Zealand ◽  
Wave Velocity ◽  
Seismic Data ◽  
Velocity Model ◽  
P Wave ◽  
Alpine Fault ◽  
P Wave Velocity ◽  
Drill Site ◽  
Borehole Seismic

No description supplied

scholarly journals Seismic P  Wave Velocity Model From 3‐D Surface and Borehole Seismic Data at the Alpine Fault DFDP‐2 Drill Site (Whataroa, New Zealand)

2021 ◽  
Author(s):  
V Lay ◽  
S Buske ◽  
SB Bodenburg ◽  
John Townend ◽  
R Kellett ◽  
...  
Keyword(s):  
New Zealand ◽  
Wave Velocity ◽  
Seismic Data ◽  
Velocity Model ◽  
P Wave ◽  
Alpine Fault ◽  
P Wave Velocity ◽  
Drill Site ◽  
Borehole Seismic

No description supplied

3D seismic imaging of the Alpine Fault and the glacial valley at Whataroa, New Zealand

2021 ◽  
Author(s):  
Vera Lay ◽  
Stefan Buske ◽  
Franz Kleine ◽  
John Townend ◽  
Richard Kellett ◽  
...  
Keyword(s):  
New Zealand ◽  
Fault Zone ◽  
Seismic Data ◽  
Seismic Imaging ◽  
P Wave ◽  
Seismic Survey ◽  
Fault System ◽  
3D Seismic ◽  
Alpine Fault ◽  
Drill Site

<p>The Alpine Fault at the West Coast of the South Island (New Zealand) is a major plate boundary that is expected to rupture in the next 50 years, likely as a magnitude 8 earthquake. The Deep Fault Drilling Project (DFDP) aimed to deliver insight into the geological structure of this fault zone and its evolution by drilling and sampling the Alpine Fault at depth. Here we present results from a seismic survey around the DFDP-2 drill site in the Whataroa Valley where the drillhole almost reached the fault plane. This unique 3D seismic survey includes several 2D lines and a 3D array at the surface as well as borehole recordings. Within the borehole, the unique option to compare two measurement systems is used: conventional three-component borehole geophones and a fibre optic cable (heterodyne Distributed Vibration Sensing system (hDVS)). Both systems show coherent signals but only the hDVS system allowed a recording along the complete length of the borehole.</p><p>Despite the challenging conditions for seismic imaging within a glacial valley filled with sediments and steeply dipping valley flanks, several structures related to the valley itself as well as the tectonic fault system are imaged. The pre-processing of the seismic data also includes wavefield separation for the zero-offset borehole data. Seismic images are obtained by prestack depth migration approaches.</p><p>Within the glacial valley, particularly steep valley flanks are imaged directly and correlate well with results from the P-wave velocity model obtained by first arrival travel-time tomography. Additionally, a glacially over-deepened trough with nearly horizontally layered sediments is identified about 0.5 km south of the DFDP-2B borehole.</p><p>With regard to the expected Alpine fault zone, a set of several reflectors dipping 40-56° to the southeast are identified in a ~600 m wide zone between depths of 0.2 and 1.2 km that is interpreted to be the minimum extent of the damage zone. Different approaches image one distinct reflector dipping at 40°, which is interpreted to be the main Alpine Fault reflector. This reflector is only ~100 m ahead from the lower end of the borehole. At shallower depths (z<0.5 km), additional reflectors are identified as fault segments and generally have steeper dips up to 56°. About 1 km south of the drill site, a major fault is identified at a depth of 0.1-0.5 km that might be caused by the regional tectonics interacting with local valley structures. A good correlation is observed among the separate seismic data sets and with geological results such as the borehole stratigraphy and the expected surface trace of the fault.</p><p>In conclusion, several structural details of the fault zone and its environment are seismically imaged and show the complexity of the Alpine Fault at the Whataroa Valley. Thus, a detailed seismic characterization clarifies the subsurface structures, which is crucial to understand the transpressive fault’s tectonic processes.</p>

scholarly journals Reference P-wave velocity in coal seams at great depths in Jastrzebie coal mine

2019 ◽  
Vol 133 ◽  
pp. 01011
Author(s):  
Jakub Kokowski ◽  
Zbigniew Szreder ◽  
Elżbieta Pilecka
Keyword(s):  
Wave Velocity ◽  
Coal Mine ◽  
Velocity Model ◽  
P Wave ◽  
Coal Seams ◽  
Data Set ◽  
Seismic Profiling ◽  
Reference Velocity ◽  
P Wave Velocity ◽  
Geological Conditions

In the study, the determining of the reference velocity of the P-wave in coal seams used in seismic profiling to assess increases and decreases in relative stresses at large depths has been presented. The seismic profiling method proposed by Dubinski in 1989 covers a range of depth up to 970 m. At present, coal seams exploitation in Polish coal mines is conducted at greater depths, even exceeding 1200 m, which creates the necessity for a new reference velocity model. The study presents an empirical mathematical model of the change of the P-wave velocity in coal seams in the geological conditions of the Jastrzebie coal mine. A power model analogous to the Dubinski’s one was elaborated with new constants. The calculations included the results from 35 measurements of seismic profiling carried out in various coal seams of the Jastrzebie mine at depths from 640 to 1200 m. The results obtained cause changes in the result of calculations of seismic anomalies. Future validation of the proposed model with larger data set will be required.

scholarly journals Semiglobal viscoacoustic full-waveform inversion

Geophysics ◽  
2019 ◽  
Vol 84 (2) ◽  
pp. R271-R293 ◽  
Author(s):  
Nuno V. da Silva ◽  
Gang Yao ◽  
Michael Warner
Keyword(s):  
Physical Properties ◽  
Wave Velocity ◽  
Seismic Data ◽  
Waveform Inversion ◽  
P Wave ◽  
Full Waveform Inversion ◽  
Data Set ◽  
Intrinsic Attenuation ◽  
Full Waveform ◽  
P Wave Velocity

Full-waveform inversion deals with estimating physical properties of the earth’s subsurface by matching simulated to recorded seismic data. Intrinsic attenuation in the medium leads to the dispersion of propagating waves and the absorption of energy — media with this type of rheology are not perfectly elastic. Accounting for that effect is necessary to simulate wave propagation in realistic geologic media, leading to the need to estimate intrinsic attenuation from the seismic data. That increases the complexity of the constitutive laws leading to additional issues related to the ill-posed nature of the inverse problem. In particular, the joint estimation of several physical properties increases the null space of the parameter space, leading to a larger domain of ambiguity and increasing the number of different models that can equally well explain the data. We have evaluated a method for the joint inversion of velocity and intrinsic attenuation using semiglobal inversion; this combines quantum particle-swarm optimization for the estimation of the intrinsic attenuation with nested gradient-descent iterations for the estimation of the P-wave velocity. This approach takes advantage of the fact that some physical properties, and in particular the intrinsic attenuation, can be represented using a reduced basis, substantially decreasing the dimension of the search space. We determine the feasibility of the method and its robustness to ambiguity with 2D synthetic examples. The 3D inversion of a field data set for a geologic medium with transversely isotropic anisotropy in velocity indicates the feasibility of the method for inverting large-scale real seismic data and improving the data fitting. The principal benefits of the semiglobal multiparameter inversion are the recovery of the intrinsic attenuation from the data and the recovery of the true undispersed infinite-frequency P-wave velocity, while mitigating ambiguity between the estimated parameters.

scholarly journals Microseismicity, tectonic stress, and exhumation rates near the central Alpine Fault, New Zealand

2021 ◽  
Author(s):  
◽  
Konstantinos Michailos
Keyword(s):  
New Zealand ◽  
Thermal Structure ◽  
Large Earthquake ◽  
Southern Alps ◽  
Focal Mechanisms ◽  
P Wave ◽  
Data Set ◽  
Alpine Fault ◽  
Brittle Ductile Transition ◽  
Average Maximum

<p>This thesis documents a detailed examination of the seismic activity and characteristics of crustal deformation along the central Alpine Fault, a major obliquely convergent plate-boundary fault. Paleoseismic evidence has established that the Alpine Fault produces large to great (M7−8) earthquakes every 250−300 years, in a quasi-periodic manner, with the last surface-rupturing earthquake occurring in 1717. This renders the fault late in its typical earthquake cycle, posing substantial seismic risk to southern and central New Zealand. Understanding the seismic and tectonic character of this fault may yield information of both societal and scientific significance regarding seismic hazard and late-interseismic processes leading up to a large earthquake. However, the central Alpine Fault is currently seismically quiescent when compared to adjacent regions, and therefore requires detailed, long-duration observations to study seismotectonic processes. The work in this thesis addresses the need for a greater understanding of along-strike variations in seismic character of the Alpine Fault ahead of an anticipated large earthquake.  To achieve observations with high spatial and temporal resolution across the length of the central Alpine Fault, I use 8.5 years of continuous seismic data from the Southern Alps Microearthquake Borehole Array (SAMBA), and data from four other temporary seismic networks and five local GeoNet permanent sites. Incorporating all of these temporary and permanent seismic sites provides us with a dense composite network of seismometers. Without such a dense network, homogeneous examination of the characteristics of low-magnitude seismicity near the Alpine Fault would be impossible.  Using this dataset, I have constructed the most extensive microearthquake catalog for the central Alpine Fault region to date, containing 9,111 earthquakes and covering the time between late 2008 and early 2017. To construct this catalog I created an objective workflow to ensure catalog uniformity. Overall, 7,719 earthquakes were successfully relocated with location uncertainties generally ≤ 0.5 km in both the horizontal and vertical directions. The majority of the earthquakes were found to occur southeast of the Alpine Fault (i.e. in the hanging-wall). I observed a lack of seismicity beneath Aoraki/Mount Cook that has previously been shown to be associated with locally high uplift rates (6–10 mm/yr) and high geothermal gradients (∼60◦C/km). Seismogenic cut-off depths were observed to significantly vary along the strike of the Alpine Fault, ranging from 8 km beneath the highest topography to 20 km in the adjacent areas.  To quantify the scale of the seismic deformation, a new local magnitude scale was also derived, corrected for geometric spreading, attenuation and site terms based on individually calculated GeoNet moment magnitude (Mw) values. Earthquake local magnitudes range between ML –1.2 and 4.6 and the catalog is complete above ML 1.1.  To examine the stress regime near the central Alpine Fault, I built a new data set of 845 focal mechanisms from earthquakes in our catalog. This was achieved by manually determining P wave arrival polarity picks from all earthquakes larger than ML 1.5. In order to determine the orientations and characteristics of the stress parameters, I grouped these focal mechanisms and performed stress inversion calculations that provided an average maximum horizontal compressive stress orientation, SHmax, of 121±11◦ , which is uniform within uncertainty along the length of the central Southern Alps. I observed an average angle of 65◦ between the SHmax and the strike of the Alpine Fault, which is consistent with results from similar previous studies in the northern and southern sections of the Alpine Fault. This implies that the Alpine Fault is misoriented for reactivation, in the prevailing stress field.  Using a 1-D steady-state thermal structure model constrained by seismicity and thermochronology data, I investigated the crustal thermal structure and vertical kinematics of the central Southern Alps orogen. The short-term seismicity data and longer-term thermochronology data impose complementary constraints on the model. I observed a large variation in exhumation rate estimates (1–8 mm/yr) along the length of the Alpine Fault, with maximum calculated values observed near Aoraki/Mount Cook. I calculated the temperature at the brittle-ductile transition zone, which ranges from 440 to 457◦C in the different models considered. This temperature is slightly hotter than expected for crust composed by quartz-rich rocks, but consistent with the presence of feldspar-rich mafic rocks in parts of the crust.</p>

3D P-wave velocity model of Ireland's crust from controlled source tomography

2020 ◽  
Author(s):  
Senad Subašić ◽  
Meysam Rezaeifar ◽  
Nicola Piana Agostinetti ◽  
Sergei Lebedev ◽  
Christopher Bean
Keyword(s):  
Wave Velocity ◽  
Velocity Model ◽  
P Wave ◽  
Uppermost Mantle ◽  
Data Set ◽  
Location Uncertainty ◽  
P Wave Velocity ◽  
1D Model ◽  
Minimum 1D Model ◽  
1D Velocity Model

&lt;p&gt;We present a 3D P-wave velocity model of the crust and uppermost mantle below Ireland. In the absence of local earthquakes, we used quarry and mining blasts recorded on permanent stations in the Irish National Seismic Network (INSN) and during various temporary deployments. We compiled a database of 1,100 events and around 20,000 P-wave arrivals, with each event associated with a known quarry. The source location uncertainty is therefore minimal. Both source and receiver locations are fixed in time and we used repeating events to estimate the travel time uncertainty for each source-receiver combination. We created a starting 1D velocity model from previously available data, and then used VELEST to calculate a preliminary minimum 1D velocity model. The 1D velocity model enabled us to remove outliers from the data set, and to calculate the final minimum 1D model used as the initial model in the 3D tomographic inversion. The resulting 3D P-wave velocity model will shed new light on the 3D crustal structure of Ireland.&lt;/p&gt;

Using 2D long-streamer seismic data waveform tomography to decipher sedimentary record of fault activity

2020 ◽  
Author(s):  
Amin Kahrizi ◽  
Matthias Delescluse ◽  
Mathieu Rodriguez ◽  
Pierre-Henri Roche ◽  
Anne Becel ◽  
...  
Keyword(s):  
Mass Transport ◽  
Seismic Data ◽  
Velocity Model ◽  
P Wave ◽  
Fault System ◽  
Fault Activity ◽  
Traveltime Tomography ◽  
P Wave Velocity ◽  
Waveform Tomography ◽  
Mass Transport Deposits

&lt;p&gt;Acoustic full-waveform inversion (FWI), or waveform tomography, involves use of both phase and amplitude of the recorded compressional waves to obtain a high-resolution P-wave velocity model of the propagation medium. Recent theoretical and computing advances now allow the application of this highly non-linear technique to field data. This led to common use of the FWI for industrial purposes related to reservoir imaging, physical properties of rocks, and fluid flow. Application of FWI in the academic domain has, so far, been limited, mostly because of the lack of adequate seismic data. While refraction seismic datasets include large source-receiver offsets that are useful to find a suitable starting velocity model through traveltime tomography, these acquisitions rarely reach the high density of receivers necessary for waveform tomography. On the other hand, multichannel seismic (MCS) reflection data acquisition has a dense receiver spacing but only modern long-streamer data have offsets that, in some cases, enable constraining subsurface velocities at a significant enough depth to be useful for structural or tectonic purposes.&lt;/p&gt;&lt;p&gt;In this study, we show how FWI can help decipher the record of a fault activity through time at the Shumagin Gap in Alaska. The MCS data were acquired on RV Marcus G. Langseth during the ALEUT cruise in the summer of 2011 using two 8-km-long seismic streamers and a 6600 cu. in. tuned airgun array. One of the most noticeable reflection features imaged on two profiles is a large, landward-dipping normal fault in the overriding plate; a structural configuration making the area prone to generating both transoceanic and local tsunamis, including from landslides. This fault dips ~40&amp;#176;- 45&amp;#176;, cuts the entire crust and connects to the plate boundary fault at ~35 km depth, near the intersection of the megathrust with the forearc mantle wedge. The fault system reaches the surface at the shelf edge 75 km from the trench, forming the Sanak basin where the record of the recent activity of the fault is not clear. Indeed, contouritic currents tend to be trapped by the topography created by faults, even after they are no longer active.&amp;#160; Erosion surfaces and onlaps from contouritic processes as well as gravity collapses and mass transport deposits results in complex structures that make it challenging to evaluate the fault activity. The long streamers used facilitated recording of refraction arrivals in the target continental slope area, which permitted running streamer traveltime tomography followed by FWI to produce coincident detailed velocity profiles to complement the reflection sections. FWI imaging of the Sanak basin reveals low velocities of mass transport deposits and velocity inversions indicate mechanically weak layers linking some faults to gravity sliding on a d&amp;#233;collement. These details question previous interpretation of a present-day active fault. Our goal is to further analyze the behavior of the fault system using the P-wave velocity models from FWI to quantitatively detect fluids and constrain sediment properties.&lt;/p&gt;

scholarly journals Microseismicity, tectonic stress, and exhumation rates near the central Alpine Fault, New Zealand

2021 ◽  
Author(s):  
◽  
Konstantinos Michailos
Keyword(s):  
New Zealand ◽  
Thermal Structure ◽  
Large Earthquake ◽  
Southern Alps ◽  
Focal Mechanisms ◽  
P Wave ◽  
Data Set ◽  
Alpine Fault ◽  
Brittle Ductile Transition ◽  
Average Maximum

<p>This thesis documents a detailed examination of the seismic activity and characteristics of crustal deformation along the central Alpine Fault, a major obliquely convergent plate-boundary fault. Paleoseismic evidence has established that the Alpine Fault produces large to great (M7−8) earthquakes every 250−300 years, in a quasi-periodic manner, with the last surface-rupturing earthquake occurring in 1717. This renders the fault late in its typical earthquake cycle, posing substantial seismic risk to southern and central New Zealand. Understanding the seismic and tectonic character of this fault may yield information of both societal and scientific significance regarding seismic hazard and late-interseismic processes leading up to a large earthquake. However, the central Alpine Fault is currently seismically quiescent when compared to adjacent regions, and therefore requires detailed, long-duration observations to study seismotectonic processes. The work in this thesis addresses the need for a greater understanding of along-strike variations in seismic character of the Alpine Fault ahead of an anticipated large earthquake.  To achieve observations with high spatial and temporal resolution across the length of the central Alpine Fault, I use 8.5 years of continuous seismic data from the Southern Alps Microearthquake Borehole Array (SAMBA), and data from four other temporary seismic networks and five local GeoNet permanent sites. Incorporating all of these temporary and permanent seismic sites provides us with a dense composite network of seismometers. Without such a dense network, homogeneous examination of the characteristics of low-magnitude seismicity near the Alpine Fault would be impossible.  Using this dataset, I have constructed the most extensive microearthquake catalog for the central Alpine Fault region to date, containing 9,111 earthquakes and covering the time between late 2008 and early 2017. To construct this catalog I created an objective workflow to ensure catalog uniformity. Overall, 7,719 earthquakes were successfully relocated with location uncertainties generally ≤ 0.5 km in both the horizontal and vertical directions. The majority of the earthquakes were found to occur southeast of the Alpine Fault (i.e. in the hanging-wall). I observed a lack of seismicity beneath Aoraki/Mount Cook that has previously been shown to be associated with locally high uplift rates (6–10 mm/yr) and high geothermal gradients (∼60◦C/km). Seismogenic cut-off depths were observed to significantly vary along the strike of the Alpine Fault, ranging from 8 km beneath the highest topography to 20 km in the adjacent areas.  To quantify the scale of the seismic deformation, a new local magnitude scale was also derived, corrected for geometric spreading, attenuation and site terms based on individually calculated GeoNet moment magnitude (Mw) values. Earthquake local magnitudes range between ML –1.2 and 4.6 and the catalog is complete above ML 1.1.  To examine the stress regime near the central Alpine Fault, I built a new data set of 845 focal mechanisms from earthquakes in our catalog. This was achieved by manually determining P wave arrival polarity picks from all earthquakes larger than ML 1.5. In order to determine the orientations and characteristics of the stress parameters, I grouped these focal mechanisms and performed stress inversion calculations that provided an average maximum horizontal compressive stress orientation, SHmax, of 121±11◦ , which is uniform within uncertainty along the length of the central Southern Alps. I observed an average angle of 65◦ between the SHmax and the strike of the Alpine Fault, which is consistent with results from similar previous studies in the northern and southern sections of the Alpine Fault. This implies that the Alpine Fault is misoriented for reactivation, in the prevailing stress field.  Using a 1-D steady-state thermal structure model constrained by seismicity and thermochronology data, I investigated the crustal thermal structure and vertical kinematics of the central Southern Alps orogen. The short-term seismicity data and longer-term thermochronology data impose complementary constraints on the model. I observed a large variation in exhumation rate estimates (1–8 mm/yr) along the length of the Alpine Fault, with maximum calculated values observed near Aoraki/Mount Cook. I calculated the temperature at the brittle-ductile transition zone, which ranges from 440 to 457◦C in the different models considered. This temperature is slightly hotter than expected for crust composed by quartz-rich rocks, but consistent with the presence of feldspar-rich mafic rocks in parts of the crust.</p>

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