赤道大西洋中脊沿线地震的 Pn 震级表 mb(Pn)

IF 2.8 3区 地球科学 Q2 GEOCHEMISTRY & GEOPHYSICS
Won-Young Kim, Guilherme W S de Melo, Marcelo Assumpcao
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引用次数: 0

摘要

摘要 我们为沿赤道大西洋中脊的地震制定了短周期 Pn 震级表 mb(Pn)。由于信噪比较低,对于 mb < 4 的小地震,远震体波震级和长周期面波震级都无法确定。由于海洋环境不允许地壳相位传播,本地震级标度对这些地震也没有用处。然而,这些中小型(mb 3-6)地震产生的区域性高频 Pn 波在赤道大西洋地区有很好的记录,可用于确定震级。我们在巴西东北部约 20 个站点和非洲西部 11 个站点的垂直记录上测量了超过 2 041 个 Pn 峰值振幅,距离范围为 700-3 700 千米。我们分析了全球中心矩张量目录中 189 个事件的数据,将我们的 mb(Pn) 尺度与 MW 联系起来,从而可以方便地估算地震矩。Pn 波到达在短距离(∼1,000 公里)显示出介于 7.9 公里/秒和高达 9.1 公里/秒(3,500 公里)之间的表观群速。测量到的峰值振幅在 1 000-1 800 千米处的频率在 0.8 和 3 赫兹之间,但在更远的距离(1 800-3 700 千米)处,它们显示出非常一致的频率,约为 0.8 赫兹。峰值振幅在短距离衰减率较高(700-2,000 公里之间为 0.65 个等级单位),但在长距离衰减率较低(2,000-3,700 公里之间为 0.35 个等级单位)。振幅随距离衰减的速率较低,峰值振幅的频率含量几乎恒定,这表明 Pn 波在赤道大西洋盆地的上地幔下部传播效率很高。这些都是海洋 Pn 波的重要属性,可用于为赤道大西洋中部地区的中小型地震确定震级。估计的台站修正与上地幔低速异常有很好的相关性,特别是在巴西。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
A Pn magnitude scale mb(Pn) for earthquakes along the equatorial Mid-Atlantic Ridge
Summary We developed a short-period Pn magnitude scale mb(Pn) for earthquakes along the equatorial Mid-Atlantic Ridge. Due to low signal-to-noise ratios, teleseismic body wave magnitude and long-period surface wave magnitude cannot be confidently determined for small earthquakes of mb < 4. Local magnitude scales are also not useful for these events because the oceanic environment does not allow the propagation of crustal phases. However, regional high-frequency Pn waves from these small- to moderate-size (mb 3–6) earthquakes are well recorded in the equatorial Atlantic region and can be used to assign magnitudes. We measured over 2 041 Pn peak amplitudes on vertical records from about 20 stations in northeastern Brazil and 11 stations in western Africa in the distance range of 700–3,700 km. We analyzed data from 189 events from the global centroid moment tensor catalog to tie our mb(Pn) scale to MW so that seismic moments can be readily estimated. Pn arrivals show apparent group velocity between 7.9 km/s at short ranges (∼1,000 km) and up to 9.1 km/s at 3,500 km. The measured peak amplitudes have a frequency between 0.8 and 3 Hz at 1 000–1,800 km, but at greater distances, 1 800–3,700 km, they show a remarkably consistent frequency of about 0.8 Hz. The peak amplitude attenuates at a higher rate at short distances (∼0.65 magnitude units between 700–2,000 km) but attenuates at a lower rate at long distances (∼0.35 magnitude units between 2 000 and 3,700 km). The low rate of amplitude decay with distance and nearly constant frequency content of the peak amplitudes suggest that Pn waves propagate efficiently in the lower part of the upper mantle in the equatorial Atlantic Ocean basins. These are important attributes of oceanic Pn waves that can be used to assign magnitude for small- to moderate-size earthquakes in the equatorial mid-Atlantic region. The estimated station corrections correlate well with upper mantle low-velocity anomalies, especially in Brazil.
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来源期刊
Geophysical Journal International
Geophysical Journal International 地学-地球化学与地球物理
CiteScore
5.40
自引率
10.70%
发文量
436
审稿时长
3.3 months
期刊介绍: Geophysical Journal International publishes top quality research papers, express letters, invited review papers and book reviews on all aspects of theoretical, computational, applied and observational geophysics.
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