Applications of double-difference seismic attenuation tomography

地震产生的地震波穿过地球内部。当它们到达地表时，它们可以被用来揭示地球内部的结构，就像超声波在医学成像中的应用一样。地震波在传播过程中会损失能量，这种现象被称为地震衰减。衰减取决于波穿过的岩石，特别是它们的温度、裂缝的密度，以及裂缝的流体含量（例如部分熔体）。能量损失率与地震波的频率（振动率）有关。高频（快速振动）波比低频（缓慢振动）波损失能量更快。通过分析一组地震的地震图中的衰减，可以创建地球上地震衰减的三维图像。这个过程被称为衰减层析成像。它揭示了地球内部的运作过程。一个关键的方面是测量波幅作为频率的函数，即波谱。在这里，研究小组使用了在一组地震台站观察到的许多对附近地震的光谱之比。这种新方法被称为双差衰减层析成像。它允许在地震附近进行成像衰减，比传统方法具有更高的精度和空间分辨率。研究小组用它来成像加州间歇泉地热储层的剪切波（侧面振动）衰减结构。目标是确定由于地热储层的开采或增产而引起的压裂和流体含量的变化。研究人员还打算对日本两个地区的压缩波（推拉振动）衰减结构进行成像。在那里，正常的地震和异常的低频地震发生在大洋板块俯冲到地幔的深度。因此，这项研究有助于更好地理解深地震的起源和俯冲板块上边缘的作用过程。开发的软件，适用于不同的地质环境，提供给科学界。这个为期一年的项目还为一名博士后提供支持，为一名本科生提供培训，并向公众推广。在这里，该团队开发并应用了一种称为双差（DD）衰减层析成像的新方法。基本概念如下：使用差分全径衰减（dt*）的事件对测量，以及“绝对”t*测量，反演三维（3D）衰减结构，以q为参数化。使用事件对光谱比方法对dt*进行稳健估计。然后使用改进的地震层析成像代码对三维Q结构的绝对和微分t*测量进行反演。dt*测量允许在地震活跃区域对3D Q结构进行更高分辨率的成像，就像在DD速度层析成像的情况下一样。该团队用合成数据测试了该方法，并将其应用于2011年加利福尼亚Geysers地热区的数据，以获得P波的3D Q结构快照。他们通过仔细匹配的地震，确定了2005年至2011年间p波Q结构的变化。该项目支持将这种创新方法应用于间歇泉的s波结构成像，这为p波Q结果提供了补充信息。它还促进了日本两个俯冲带段纵波结构的成像。在The Geysers中，p波和s波Q模型的结合为地热储层的压裂和饱和状态提供了强有力的约束。对于本州北部的俯冲带段，该团队开发并分析了新的高分辨率3D p波和s波Q模型。这有助于限制双地震带的性质和两个地震带内地震活动的原因。对于四国的俯冲带段，先前的工作发现，在上低频地震和下常规地震的平面带之间夹着一个高纵波与横波比（Vp/Vs）的带。在这里，研究人员测试了关于在这两个俯冲带系统的不同部分Qp和q应该如何变化的预测。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Seismic waves, generated by earthquakes, travel through Earth’s interior. When they reach the surface, they can be used to reveal structures within the Earth, in a similar way than sonography is used in medical imaging. Seismic waves lose energy as they travel, a phenomenon known as seismic attenuation. Attenuation depends on the rocks the waves travel through, notably their temperature, how densely they are fractured, as well as on the fluid content of the fractures (e.g, partial melts). The rate of energy loss is related to the frequency (rate of vibration) of the seismic wave. High frequency (rapidly vibrating) waves lose energy faster than low frequency (slowly vibrating) waves. By analyzing the attenuation in seismograms of a set of earthquakes, it is possible to create a 3D image of seismic attenuation in the Earth. This procedure is known as attenuation tomography. It allows unveiling the processes at play within the Earth. A key aspect is measuring the wave amplitude as a function of frequency, known as the wave spectrum. Here the team uses the ratio of the spectra of many pairs of nearby earthquakes observed on a set of seismic stations. This new method is called double-difference attenuation tomography. It allows imaging attenuation in the vicinity of the earthquakes with greater accuracy and spatial resolution than the conventional method. The team uses it to image the shear-wave (side-to-side vibration) attenuation structure of the geothermal reservoir at The Geysers, California. The goal is to determine the change of fracturing and fluid content caused by the exploitation or stimulation of a geothermal reservoir. The researchers also intend to image the compressional-wave (push-pull vibration) attenuation structure of two areas in Japan. There, normal earthquakes and anomalous low-frequency earthquakes occur at great depth where oceanic plates are diving down (subducting) into the Earth’s mantle. The study, thus, contributes to a better understanding of the origin of deep earthquakes and the processes at work along the upper edge of subducting plates. The developed software, applicable in different geological contexts, is made available to the scientific community. The 1-year project also provides support to a postdoctoral associate, training for one undergraduate, and outreach to the public. Here, the team develops and applies a new method called double-difference (DD) attenuation tomography. The underlying concept is as follows: use event-pair measurements of differential whole-path attenuation (dt*), along with "absolute" t* measurements, to invert for three-dimensional (3D) attenuation structure, parameterized in terms of Q. An event-pair spectral ratio method is used to make robust estimates of dt*. A modified seismic tomography code is then used to invert the absolute and differential t* measurements for 3D Q structure. The dt* measurements allow for higher-resolution imaging of 3D Q structure in seismogenically active regions, just as in the case of DD velocity tomography. The team tested the approach with synthetic data and applied it to data from The Geysers geothermal area in California to obtain a snapshot of 3D Q structure for P waves in 2011. They determined the change in P-wave Q structure between 2005 and 2011 using carefully matched earthquakes. The project supports the application of this innovative method to image the S-wave structure at The Geysers, which provides complementary information to the P-wave Q results. It also fosters the imaging of the P-wave structure at two subduction zone segments in Japan. The combination of P-wave and S-wave Q models at The Geysers provides strong constraints on the fracturing and saturation state of the geothermal reservoir. For the subduction zone segment in Northern Honshu, the team develops and analyzes new high-resolution 3D P-wave and S-wave Q models. This helps constrain the nature of the double seismic zone and the cause of seismicity within the two zones. For the subduction zone segment in Shikoku, previous work finds a zone of high P-wave to S-wave ratio (Vp/Vs) sandwiched between planar zones of low-frequency earthquakes above and regular earthquakes below. Here the researchers test predictions regarding how Qp and Qs should vary in the different parts of these two subduction zone systems.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.