Joint Seismic and Magnetotelluric Imaging of the San Andreas Fault Zone

圣安德烈亚斯断层带的联合地震和大地电磁成像

• 批准号: 0838249
• 负责人: Clifford Thurber
• 金额: $ 9.84万
• 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: 2009
• 资助国家: 美国
• 起止时间: 2009-02-01 至 2012-01-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0838249&HistoricalAwards=false
• 关键词: Joint; Seismic; Magnetotelluric; Imaging; San

Geophysical methods are key tools for exploring the deep structure of fault zones. Different geophysical techniques are sensitive to different physical properties. Commonly, multiple techniques are applied to the same study area with structural results for each technique obtained separately, and the resulting structures compared for interpretation. Joint inversions provide an alternative in which a single structural model is used to try to fit multiple datasets, most commonly with empirical relationships linking the different physical properties (for example, seismic velocity and density). Our work will involve the joint inversion of seismic wave travel time and magnetotelluric profiling data to determine seismic wave velocity and electrical resistivity structure across two segments of the San Andreas Fault in California. One profile passes through the San Andreas Fault Observatory at Depth (SAFOD) where in-situ information is available from the SAFOD borehole, and the other is along the creeping section of the San Andreas near Hollister. This work will improve our understanding of the structure of active transform faults and the complex inherited framework in which they are emplaced. The constrained inversion methods we will develop should have broad applicability to the study of fault zones. The spatial relationship between subsurface structure, seismicity, and hydrologic constraints will result in a better understanding of the primary factors (lithology, fluids, fault geometry) controlling fault behavior.We will apply a novel variation of the joint inversion approach that involves a constraint where spatial variations in model properties are encouraged to occur at the same places in the model. Specifically, the gradient fields of the physical properties (in our case seismic velocity and electrical resistivity) are computed and the cross product of the two gradients at each cell in the model space is evaluated. Models are "penalized" for having a non-zero cross product, which has the effect of producing models for which a spatial change in one parameter is accompanied by a change in the other that shares the same geometry but can have the same or opposite sign (parallel or anti-parallel gradient), or has no change (zero gradient).

地球物理方法是探索断裂带深层结构的关键工具。 不同的地球物理技术对不同的物理特性敏感。 通常，将多种技术应用于同一研究区域，并分别获得每种技术的结构结果，并对所得结构进行比较以进行解释。 联合反演提供了一种替代方案，其中使用单个结构模型来尝试拟合多个数据集，最常见的是连接不同物理特性（例如，地震速度和密度）的经验关系。 我们的工作将涉及地震波传播时间和大地电磁剖面数据的联合反演，以确定加利福尼亚州圣安德烈亚斯断层两段的地震波速度和电阻率结构。 一条剖面穿过圣安地列斯断层深度观测站 (SAFOD)，可从 SAFOD 钻孔获取现场信息，另一条剖面则沿着霍利斯特附近的圣安地列斯断层蠕变部分。 这项工作将增进我们对主动变换断层结构以及它们所处的复杂继承框架的理解。 我们将开发的约束反演方法应该对断裂带的研究具有广泛的适用性。 地下结构、地震活动和水文约束之间的空间关系将有助于更好地理解控制断层行为的主要因素（岩性、流体、断层几何形状）。我们将应用联合反演方法的一种新颖变体，该方法涉及一个约束，其中鼓励模型属性的空间变化发生在模型中的相同位置。 具体来说，计算物理属性（在我们的例子中是地震速度和电阻率）的梯度场，并评估模型空间中每个单元的两个梯度的叉积。 模型因具有非零叉积而受到“惩罚”，这会产生这样的效果：生成的模型中一个参数的空间变化伴随着另一个共享相同几何形状但可以具有相同或相反符号（平行或反平行梯度）或没有变化（零梯度）的模型。