Implications of a composite source model and seismic-wave attenuation for the observed simplicity of small earthquakes and reported duration of earthquake initiation phase
Implications of a composite source model and seismic-wave attenuation for the observed simplicity of small earthquakes and reported duration of earthquake initiation phase
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复合震源模型和地震波衰减对观测到的小地震简单性和报告的地震起始阶段持续时间的影响
DOI：
10.1785/bssa0880051171
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发表时间：
1998
影响因子：
3
通讯作者：
P. Mandal
中科院分区：
文献类型：
--
作者：
S. K. Singh;M. Ordaz;T. Mikumo;J. Pacheco;C. Valdés;P. Mandal
对近源速度地震图上记录的P波的检验一般表明，大多数小地震（Mw < 2 ~ 3）是简单地震。另一方面，较大的地震（Mw4.4）通常是复杂的。Mw < 2至3的地震记录的简单性可能反映了震源的简单性（因此，可能意味着较小和较大的地震不是自相似的），或者可能是地震波衰减的结果。为了测试衰减是否是原因，我们从一个复合圆形震源模型中生成了合成P波地震图，在该模型中，假设震源下破裂区域遵循幂律分布。事件的破裂被假定为起始于故障上的随机点，并且以均匀的速度传播。当破裂前缘到达一个次火山口斑块（所有斑块都是圆形的）的中心时，辐射出一个P脉冲，这是根据Sato和Hirasawa（1973）的运动学震源模型计算的。合成P波地震图，这是所有复杂的，然后卷积与衰减算子的不同值的t*。结果表明，如果t* ≥ 0.02秒，则观察到的小事件（Mw < 2至3）的简单性可以完全由衰减来解释。
复合震源模型预测，破裂开始和最大斑块破裂之间的平均时间延迟τ为M0 1/3，log τ =（1/3）log M0 − 8.462。这一关系与Umeda等人（1996年）报告的M0与观察到的破裂开始和“亮点”破裂之间的时间差之间的关系非常相似。这与Ellsworth和Beroza（1995）以及Beroza和Ellsworth（1996）报道的M0与初始阶段持续时间之间的关系大致一致。该关系也令人惊讶地与Iio（1995）报告的关于缓慢初始阶段持续时间的数据tsip和M0吻合得很好。这种一致性的一个可能解释是，复合震源模型，这是本质上的“级联”模型，成功地捕捉到了震源过程的演变，以及破裂的启动和速度振幅的突然增加，由以前的研究人员观察到的地震图大致对应于复合震源模型中的第一个子事件的破裂和最大的子事件的破裂。
An examination of P waves recorded on near-source, velocity seismograms generally shows that most small earthquakes (Mw < 2 to 3) are simple. On the other hand, larger earthquakes (Mw ≧ 4) are most often complex. The simplicity of the seismograms of Mw < 2 to 3 events may reflect the simplicity of the source (and, hence, may imply that smaller and larger earthquakes are not self-similar) or may be a consequence of attenuation of seismic waves. To test whether the attenuation is the cause, we generated synthetic P-wave seismograms from a composite circular source model in which subevent rupture areas are assumed to follow a power-law distribution. The rupture of an event is assumed to initiate at a random point on the fault and to propagate with a uniform speed. As the rupture front reaches the center of a subevent patch (all of which are circular), a P pulse is radiated that is calculated from the kinematic source model of Sato and Hirasawa (1973). Synthetic P-wave seismograms, which are all complex, are then convolved with an attenuation operator for different values of t*. The results show that the observed simplicity of small events (Mw < 2 to 3) may be entirely explained by attenuation if t* ≧ 0.02 sec.
The composite source model predicts that the average time delay between the initiation of the rupture and the rupture of the largest patch, τ, scales as M01/3, such that log τ = (1/3) log M0 − 8.462. This relation is very similar to that reported by Umeda et al. (1996) between M0 and the observed time difference between the initiation of the rupture and the rupture of the “bright spot.” It roughly agrees with the relation between M0 and the duration of the initiation phase reported by Ellsworth and Beroza (1995) and Beroza and Ellsworth (1996). The relation also fits surprisingly well the data on duration of slow initial phase, tsip, and M0, reported by Iio (1995). One possible explanation of this agreement may be that the composite source model, which is essentially the “cascade” model, successfully captures the evolution of the earthquake source process and that the rupture initiation and the abrupt increase in the velocity amplitude observed on seismograms by previous researchers roughly corresponds to the rupture of the first subevent and the breaking of the largest subevent in the composite source model.
影响因子：
3
作者：
Y. Iio
通讯作者：
Y. Iio