Applications of double-difference seismic attenuation tomography

地震产生的地震波在地球内部传播。当它们到达地表时，它们可以用来揭示地球内部的结构，就像超声波在医学成像中使用的方式一样。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。衰减取决于波通过的岩石，特别是它们的温度、裂隙的密度以及裂隙的流体含量(例如，部分熔融)。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)波能量损失更快。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3D图像。这一过程称为衰减层析成像。它允许揭示地球内部正在进行的过程。一个关键的方面是测量波幅作为频率的函数，称为波谱。在这里，研究小组使用了在一组地震台上观测到的许多对附近地震的频谱比率。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。该研究小组使用它来成像加州间歇泉地热库的剪切波(侧向振动)衰减结构。其目的是确定地热气藏的开采或增产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纵波(推拉振动)衰减结构进行成像。在那里，正常地震和异常低频地震发生在海洋板块潜入(俯冲)地球地幔的很深的地方。因此，这项研究有助于更好地了解深部地震的起源和沿俯冲板块上缘的作用过程。开发的软件适用于不同的地质情况，可供科学界使用。这个为期一年的项目还为一名博士后助理提供支持，为一名本科生提供培训，并向公众宣传。在这里，该团队开发并应用了一种名为双差(DD)衰减断层扫描的新方法。其基本概念如下：使用差分全程衰减(DT*)的事件对测量与“绝对”t*测量一起，来反演三维(3D)衰减结构，该结构以Q为参数。事件对频谱比率方法被用来对DT*进行稳健估计。然后使用改进的地震层析成像程序来反演3DQ结构的绝对和差分t*测量。DT*测量允许在地震活动区对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这种方法，并将其应用于加利福尼亚州间歇泉地热区的数据，以获得2011年P波的3D Q结构快照。他们利用精心配对的地震，确定了2005至2011年间P波Q结构的变化。该项目支持应用这种创新的方法对间歇泉的S波结构进行成像，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的P波结构的成像。间歇泉的P波和S波Q波模型的结合，对地热储的破裂和饱和状态提供了很强的约束。对于本州北部的俯冲带段，该团队开发和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带内地震活动的原因。对于四国俯冲带段，前人的工作发现了一个高P波/S波比(Vp/Vs)的带，夹在上面的低频地震的平面带和下面的规则地震之间。在这里，研究人员测试了关于QP和QS在这两个俯冲带系统的不同部分应该如何变化的预测。这个奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.