Focal Mechanisms of Weak Earthquakes from Amplitude Spectra and Polarities
Focal Mechanisms of Weak Earthquakes from Amplitude Spectra and Polarities
复制标题
从振幅谱和极性分析弱地震的震源机制
DOI：
10.1007/pl00001198
复制
发表时间：
2001
影响因子：
2
通讯作者：
K. Papatsimpa
中科院分区：
文献类型：
--
作者：
Jiří Zahradník;Jaromír Janský;K. Papatsimpa
摘要 — 用于从相对较弱的事件中检索震源机制的 ASPO 方法（振幅谱和极性）基于广泛可用的仪器设置：较密集的短周期网络内的几个宽带站。所有站点共同提供震中位置。完整的记录取自三分量宽带站，无需选择特定的波类型或选取振幅。它使该方法适合自动数据处理，并能够研究干扰地壳相。仅反转幅度谱。这是一个强大的功能，使得该方法对任何计时问题（例如由于不确定的起始时间或由于技术故障而导致的问题）不敏感。第一运动极性作为幅度谱反演的附加约束；只有少数（清晰的）极性是从最近的台站获取的，其中大部分属于直达 P 波。该方法寻求五个参数：焦深、标量矩、走向、倾角和前角。格林函数自动包含可能的近场效应和干扰（例如表面）波，通过离散波数方法计算。 ASPO 工作在转角频率以下，并且不检索时间函数。这一特性不仅最大限度地减少了反演参数的数量，而且还加快了计算速度，因为频率越低，离散波数运行得越快。反演不是采用极其缓慢的 5 参数网格搜索，而是分为两个步骤：(i) 通过走向、倾角和倾斜的粗网格搜索来确定深度和力矩，以及 (ii) 三个源角度的精细网格搜索。最佳拟合解的不确定性是根据最小误差值以及 min 和 min + 10% 之间节点线（和/或 P 和 T 轴）的散布来评估的。该方法在科林斯湾西部由三个CMG3-T宽带台站组成的临时网络记录的M ≈ 3.5级集群地震上进行了测试。一个根本问题是，如果所研究震级的地震发生在短距离（10-30公里）内，宽带站会系统地遭受事件引起的水平分量不稳定。因此，ASPO方法不能在0.1Hz以下应用。因此，结果对于未知的地壳结构细节很敏感，并且震源机制仍然相当不确定（最小误差高于0.34）。与扰动数据的综合测试相比，误差低于0.2，表明地壳模型需要进一步改进。
Abstract — The ASPO method (Amplitude Spectra and POlarities) for the focal-mechanism retrieval from relatively weak events is based on a widely available instrumental setup: A few broadband stations within a denser short-period network. Collectively all stations provide the epicenter location. Complete records are taken from three-component broadband stations, without selecting a particular wave type, or picking amplitudes. It makes the method suitable for automated data processing, and enables studies of the interference crustal phases. Only the amplitudespectra are inverted. This is a robust feature which makes the method insensitive to any timing problems (such as those due to uncertain origin time, or due to technical failures). The first-motion polarities serve as an additional constraint of the amplitude-spectra inversion; only few (clear) polarities are taken from the nearest stations, wher e they mostly belong to direct P waves. The method seeks five parameters: The focal depth, scalar moment, strike, dip, and rake. Green's function, automatically including possible near-field effects and interference (e.g., surface) waves, is calculated by the discrete wavenumber method. ASPO works below the corner frequency, and the time function is not being retrieved. This feature not only minimizes the number of the inverted parameters, but also speeds up the calculation, because the lower the frequency, the faster the discrete wavenumber run. Instead of an exceedingly slow 5-parameter grid search, the inversion is organized in two steps: (i) the depth and moment determination with a coarse grid search of the strike, dip and rake, and (ii) a fine grid search of the three source angles. Uncertainty of the best-fitting solution is assessed from the minimum error value and from the scatter of the nodal lines (and/or P and T axes) between min and min + 10%. The method was tested on the clustered M ≈ 3.5 earthquakes recorded by a temporary network of three CMG3-T broadband stations in western Corinth Gulf. A fundamental problem is that the broadband stations suffer systematically from event-induced instabilities at horizontal components if earthquakes of the studied magnitudes occur at short distances, 10–30 km. Therefore, the ASPO method could not be applied below 0.1 Hz. As such, the results are sensitive with respect to unknown crustal structure details, and the focal mechanisms remain rather uncertain (minimum error higher than 0.34). Compared to synthetic tests with perturbed data, in which the error is lower than 0.2, it is concluded that the crustal model needs further improvement.