Fracturing and pore-fluid distribution in the Marlborough region, New Zealand from body-wave tomography: Implications for regional understanding of the Kaikōura area
Fracturing and pore-fluid distribution in the Marlborough region, New Zealand from body-wave tomography: Implications for regional understanding of the Kaikōura area
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体波断层扫描新西兰马尔堡地区的压裂和孔隙流体分布：对凯库拉地区区域了解的启示
DOI：
10.1016/j.epsl.2022.117666
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发表时间：
2022
影响因子：
5.3
通讯作者：
Bannister, Stephen
中科院分区：
文献类型：
--
作者：
Heath, Benjamin A.;Eberhart-Phillips, Donna;Lanza, Federica;Thurber, Clifford;Savage, Martha K.;Okada, Tomomi;Matsumoto, Satoshi;Iio, Yoshihisa;Bannister, Stephen
相对于较成熟的断裂带，累积总位移较小的不成熟断裂带具有应变局部化效率较低、地震破裂更加复杂的特点。成熟度的差异如何反映在区域尺度的上地壳破裂中尚不清楚。最近，与不成熟断层带相关的复杂地震破裂，例如2016年新西兰凯库拉地震，发生在较成熟断层区域附近（距离<100公里）的地区。在这里，我们研究了不太成熟的断层带中低效的应变局部化是否与数十公里距离上裂缝的更广泛分布和异常集中的裂缝有关。我们在凯库拉地震周围的大范围内使用区域地震到达时间层析成像来研究 Vp 和 Vp/Vs 的横向变化。重点关注发生地震的马尔堡地区断层广泛但成分均匀的 Torlesse-Pahau 地体，我们将 Vp 和 Vp/Vs 的横向变化归因于充满流体的裂缝浓度的差异。使用与地震速度和压裂相关的数值模型，我们解决了纵横比为 0.01 的充液裂缝浓度的横向变化。我们发现，与北部邻近区域相比，凯库拉断裂附近的区域裂缝孔隙度升高了 3% 以上。凯库拉地区的高位区域破裂被解释为由于区域均匀应变率场的应变定位效率低下，变形更加分散，地震分布更广泛，这凸显了上地壳断裂带的相对成熟度与区域上地壳性质的横向变化之间的关系。通俗易懂的语言摘要：当地震发生在不成熟断裂带（总位移积累较小的区域）时，它们往往会破裂多个方向不同的发育不良的断层。相比之下，更成熟的断层带与更发达、更平滑的断层和更局部的地震活动相关。这些成熟度的差异如何反映在裂缝的区域分布中尚不清楚。在这里，我们使用地震 P 波和 S 波的到达时间来约束地震速度（Vp 和 Vp/Vs），并使用数值模型将地震速度与裂缝集中度联系起来。我们重点关注新西兰南岛的马尔堡地区，该地区的不成熟断层带最近在 2016 年凯库拉地震中破裂，但在 100 公里之外也有更成熟的断层带。我们发现裂缝集中度升高（裂缝孔隙度高 3%），表明相对于更成熟的断层带，沿未成熟断层带的变形分布更均匀。在不成熟的环境中，压裂不能有效地沿着单个断层痕迹定位，从而导致广泛分布的裂缝网络。
Relative to more mature fault zones, immature fault zones that have accumulated smaller total displacement are characterized by less efficient strain localization and more complicated earthquake ruptures. How differences in maturation are reflected in regional-scale upper-crustal fracturing is not well known. Recently, complicated earthquake ruptures associated with immature fault zones, such as the 2016 Kaikōura earthquake in New Zealand, have occurred in areas that are in regional proximity (<100 km away) to more mature faults. Here we examine whether inefficient strain localization in less mature fault zones is associated with a broader distribution and anomalously elevated concentration of fractures over distances of tens of kilometers. We use regional seismic arrival-time tomography in a broad area around the Kaikōura earthquake to investigate lateral variations in Vp and Vp/Vs. Focusing on the extensively faulted but compositionally uniform Torlesse-Pahau terrane in the Marlborough region where the earthquake occurred, we attribute lateral variations in Vp and Vp/Vs to differences in concentration of fluid-filled fractures. Using numerical models relating seismic velocities and fracturing, we solve for the lateral variation in concentration of ∼0.01 aspect ratio fluid-filled fractures. We find that areas near the Kaikōura rupture have >3% elevated fracture porosity compared to the adjacent area to the north. The elevated regional fracturing in the Kaikōura area is interpreted to result from more distributed deformation, and broader distribution of earthquakes, due to inefficient localization of strain from a regionally uniform strain rate field, highlighting the relationship between relative maturity of upper-crustal fault zones and lateral variability of regional upper-crustal properties.Plain language summary:When earthquakes occur in immature fault zones (areas that have accumulated small total displacement), they tend to rupture multiple poorly developed faults of diverse orientations. In contrast, more mature fault zones are associated with more developed, smoother faults and more localized earthquake activity. How these differences in maturation are reflected in the regional distribution of fractures is not well known. Here we use the arrival times of P and S waves from earthquakes to constrain seismic velocities (Vp and Vp/Vs) and use numerical models to relate seismic velocities to fracture concentration. We focus on the Marlborough region of the South Island, New Zealand, where immature fault zones recently ruptured during the 2016 Kaikōura earthquake but which also has more mature fault zones <100 km away. We find elevated fracture concentrations (3% higher fracture porosity), indicative of more distributed deformation along immature fault zones relative to more mature fault zones. In immature settings, fracturing is not as effectively localized along individual fault traces, leading to a broadly distributed fracture network.