中小地震的破裂过程研究

研究断层上地震的破裂过程以及发震机制，有助于人们认识发震断层的性质和状态，进而更好地评估断层潜在的地震危险性。相比于大地震，5、6级中等地震有较高的发生频率，它能够给断层上的破裂研究带来丰富的地震观测资料。基于近源宽频带台站的视震源时间函数（AMRFs）数据，我们研究了2021年云南漾濞M_W 6.0地震的破裂过程。该地震发生在青藏高原东南缘川滇块体西南边界的未知右旋走滑断层上。破裂沿断层走向向东南方向单向传播了~8 km，破裂速度为2.4 km/s，峰值滑动量为~2.1 m。由于应力被分散，主震破裂在三个分叉断层的位置处停止。余震和前震主要发生在主要滑动区域的外围，这表明在主要滑动区域的应力基本被释放了。漾濞地震有相对低的E_R/M₀（1.5 × 10^-5）以及丰富的余震和前震，这表明该未知断层可能是一个被重新激活的不成熟断层。我们也使用远场体波和GNSS数据反演了2022年四川泸定M_W 6.6地震的破裂过程。该地震发生在川滇块体东北边界的左旋走滑鲜水河断层南端的磨西段。磨西段是一个地震空区，其位于与龙门山断层和安宁河断层交汇处。主震破裂主要向东南方向传播，峰值滑动量为~2.8 m，其破裂了磨西段南部~20 km，基本释放了上次地震以来积累的滑动亏欠。但是，磨西段北部并没有大的破裂并且泸定地震在其上产生了高达0.1 MPa的正静态库伦破裂应力变化（∆CFS），这使得北段存在较大的潜在地震危险性。位于主震西侧、贡嘎山东侧的余震活动性强且为正断层震源机制。我们认为贡嘎山东侧陡峭的地势有利于下方北西-南东走向的正断层形成，且地下发达的结构裂隙有利于水渗入到深部从而弱化断层使其有相对较高的地震活动性。

研究小地震的震源过程对于认识震源物理机理是非常重要的。基于简单圆盘震源模型使用震源谱估计小地震的震源参数是一种常用方法。但是，对于复杂破裂小地震, 使用震源谱估计的震源参数可能会存在较大误差, 因此我们直接反演了它们的滑动分布。我们使用近源宽频带台站的AMRFs数据估计了四川威远两个2级地震的破裂过程。两个地震都表现出复杂的滑动分布，一个为双向破裂，一个为单向破裂。它们的震源参数值有明显区别，其应力降分别为~1.4 MPa和~0.4 MPa，E_R/M₀分别为~1.6 × 10^-5和~2.5 × 10^-6。同时，我们使用经验格林函数方法获得了57个3-5级地震的AMRFs和视震源谱并通过震源谱分析估计了它们的震源参数。一些地震的AMRFs简单且视震源谱符合简单震源模型的震源谱，称为简单自停止破裂地震；一些地震的视震源谱存在“凹陷”且AMRFs较为复杂，称为复杂破裂地震。复杂破裂地震的拐角频率通常会被错误低估，从而导致估计的应力降偏小。应力降估计公式中k_s/k_p在1.0 - 1.5之间，平均值为~1.26，这可能意味着更小的破裂速度或者更小的P波和S波速度比。特定的震源模型会给使用震源谱估计的震源参数带来不确定性。基于破裂相图理论，我们使用破裂动力学模拟方法研究了不同动力学参数状态的自停止破裂地震的震源性质。自停止破裂地震呈现不同特征，较小临界滑移距离(Dc)地震的震源时间函数在开始阶段上升快而呈现右偏态，随着Dc增加，逐渐变为左偏态。Dc越小，E_R/M₀、地震辐射效率以及估计应力降公式中的k系数越大。

Investigating the causes and rupture process of earthquakes along faults is beneficial to understanding the properties and state of faults, which will enable a better assessment of the potential seismic hazard on the faults. Compared to larger earthquakes, M5-6 moderate earthquakes have a higher frequency, providing abundant seismic observations to investigate the rupture on faults. Based on the apparent moment rate functions (AMRFs) data extracted from near-source broadband seismic stations, we study the rupture process of the 2021 M_W 6.0 Yangbi, Yunnan earthquake. The earthquake occurred on an unmapped right-lateral strike-slip fault as the southwest boundary of the Sichuan-Yunnan block along the southeastern margin of the Tibetan Platea. The rupture propagated unilaterally toward the southeast along the strike with a rupture length of ~8 km, a rupture velocity of 2.4 km/s, and a peak slip of ~2.1 m. The rupture propagation stopped near the three bifurcating faults due to the stress dispersed by these faults. Foreshocks and aftershocks are distributed near the periphery of the main slip area, suggesting the substantial release of stress in the main slip area. The low moment scaled radiated energy E_R/M₀ of 1.5 × 10^-5 and high foreshock and aftershock productivity may be the result of reactivation of an immature fault. We also invert the rupture process of the 2022 M_W 6.6 Luding, Sichuan earthquake using teleseismic and GNSS data. The earthquake occurred on the Moxi segment in the southernmost part of the left-lateral strike-slip Xianshuihe fault as the northeast boundary of the Sichuan-Yunnan block. The Moxi segment is a seismic gap, located at the intersection of the Longmenshan fault and the Anninghe fault. The rupture mainly propagated toward the southeast along the strike with a peak slip of ~2.8 m. It ruptured in the southern part of the Moxi segment with a spatial extent of ~20 km, which approximately balanced the accumulated slip deficit on it since the last earthquake. However, no large ruptures have occurred in the northern part of the Moxi segment, and the positive ∆CFS  is up to 0.1 MPa on the northern Moxi segment induced by the Luding mainshock, indicating a relatively high seismic potential in the northern Moxi segment in the future. The aftershocks that occurred in the eastern flank of Mt. Gongga west of the mainshock have high activity and normal-faulting mechanisms. We speculate that the steep Mt. Gongga tends to favor the NW-SE trending normal faults underneath it, and the well-developed structural fissures benefit the water to penetrate into deep faults to weaken the faults, causing high seismicity.

Investigating the source process of small earthquakes is important for understanding the physics of the source process. It is a common method that estimating the source parameters of small earthquakes using the source spectra based on a simple circular source model. However, for small earthquakes with complex ruptures, the source parameters estimated using the source spectra can have significant uncertainties, so we can invert the slip distribution to estimate their source parameters. We estimated the rupture processes of two o M ~2.0 earthquakes in the Weiyuan region, Sichuan, using the AMRFs extracted from near-source broadband stations. Two earthquakes exhibited complex slip distributions, with one showing bilateral rupture and another showing unilateral rupture. They have significantly different source parameters with the stress drop of ~1.4 MPa and ~0.4 MPa, and the E_R/M₀  of ~1.6 × 10^-5 and ~2.5 × 10^-6, respectively. Meanwhile, we utilize the empirical Green's function (EGF) method to obtain AMRFs and apparent source spectra for 57 M 3-5 earthquakes and then estimate the source parameters for these earthquakes by analysis of source spectra. Some earthquakes with simple AMRFs and the apparent source spectra fitted well by those of a simple source model are referred to as simple self-arresting ruptures. Others with complex AMRFs and the apparent source spectra having "holes", are referred to as complex ruptures. The corner frequencies of complex rupture earthquakes usually be mistakenly underestimated, causing the underestimation of stress drop. The ratio of k_s/k_p in the stress drop estimation formula are between 1.0 and 1.5 with a mean of ~1.26, implying a low rupture velocity or ratio of P wave to S wave velocity. Specific source models can introduce uncertainties for the source parameters estimated using source spectra. Based on the rupture phase diagram, we investigate the source properties of self-arresting rupture earthquakes with varied dynamic parameters by dynamic simulation. The source time functions of the earthquakes with less critical slip distance (Dc) exhibit a right-skewed shape with a rapid rise at the initiation, and turn a left-skewed shape with a large Dc. As Dc decreases, the E_R/M₀, seismic radiation efficiency, and the k coefficient in stress drop estimation increase.

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