Applications of double-difference seismic attenuation tomography

双差地震衰减层析成像的应用

基本信息

批准号：
2042919
负责人：
Clifford Thurber
金额：
$ 9.53万
依托单位：
University of Wisconsin-Madison
依托单位国家：
美国
项目类别：
Standard Grant
财政年份：
2021
资助国家：
美国
起止时间：
2021-03-15 至 2025-02-28
项目状态：
未结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=2042919&HistoricalAwards=false
关键词：
Applications double difference seismic attenuation

项目摘要

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.

地震产生的地震波穿过地球内部。当它们到达表面时，它们可用于在地球内露出与超声检查相似的方式在医学成像中使用的结构。地震波在旅行时会失去能量，这种现象称为地震衰减。衰减取决于海浪穿过的岩石，尤其是它们的温度，它们的压裂程度以及分数的流体含量（例如，部分熔体）。能量损失的速率与地震波的频率（振动速率）有关。高频（迅速振动）波比低频（缓慢振动）波更快地损失能量。通过分析一组地震的地震学的衰减，可以在地球中创建3D地震衰减的图像。此过程称为衰减断层扫描。它允许揭示地球内部发挥作用的过程。一个关键方面是测量波放大器作为频率的函数，称为波谱。在这里，小组使用了在一组地震站上观察到的许多近地震的光谱之比。这种新方法称为双差局部断层扫描。与常规方法相比，它允许在地震附近的成像衰减，其精度和空间分辨率更高。该团队使用它来对加利福尼亚州间歇泉地热储层的剪切波（左右振动）衰减结构进行成像。目的是确定由地热储层的开发或刺激引起的破裂和流体含量的变化。研究人员还打算对日本两个区域的压缩波（推力振动）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很大的深度，海洋板正在向下（俯冲）向地球的地幔潜水（俯冲）。因此，这项研究有助于更好地理解深层地震的起源以及沿俯冲板上边缘的工作过程。适用于不同地质环境的开发软件可供科学界提供。这项为期一年的项目还为博士后助理，一个本科生的培训以及向公众提供支持。在这里，团队开发并应用了一种称为双差（DD）衰减断层扫描的新方法。基本概念如下：使用差分全路径衰减（DT*）的事件对测量以及“绝对” T*测量值，以Q的三维（3D）衰减结构进行反转，以Q的参数为参数。使用事件对谱法进行参数化，以对DT*的鲁棒估计。然后，使用修改的地震层析成像代码将3D Q结构的绝对和差分测量倒置。与DD速度断层扫描一样，DT*测量值允许对地震活性区域的3D Q结构进行更高分辨率的成像。该团队用合成数据测试了该方法，并将其应用于加利福尼亚州的Geysers地热区域的数据，以获取2011年P波的3D Q结构的快照。他们确定了使用精心匹配的地震确定2005年至2011年之间P-Wave Q结构的变化。该项目支持这种创新方法在间歇泉中对S波结构进行映像的应用，该方法为P-Wave Q结果提供了完整的信息。它还促进了日本两个俯冲带段的P波结构的成像。间歇泉的P波和S波Q模型的组合对地热晶状体的破裂和饱和状态提供了强大的限制。对于北部北部的俯冲带，团队开发和分析了新的高分辨率3D P-WAVE和S-WAVE Q模型。这有助于限制双重地震带的性质和两个区域内的地震性原因。对于Shikoku的俯冲带段，以前的工作找到了一个高P波与S波比（VP/VS）的区域，该区域夹在上面的低频地震和下面的常规地震之间的平面区域之间。在这里，研究人员测试了有关QP和QS在这两个俯冲区系统的不同部分中如何有所不同的预测。该奖项反映了NSF的法定任务，并使用基金会的知识分子优点和更广泛的影响评估标准，被认为是通过评估而被视为珍贵的支持。