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Attenuation Characteristics of High Frequency Seismic Waves in Southern India

机译：印度南部高频地震波的衰减特征

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摘要

We present a systematic study of seismic attenuation and its related Q structure derived from the spectral analysis of P-, S-waves in the southern India. The study region is separated into parts of EDC (Eastern Dharwar Craton), Western Dharwar Craton (WDC) and Southern Granulite Terrain (SGT). The study is carried out in the frequency range 1-20 Hz, using a single-station spectral ratio technique. We make use of about 45 earthquakes, recorded in a network of about 32 broadband 3-component seismograph-stations, having magnitudes (M (L)) varying from 1.6 to 4.5, to estimate the average seismic body wave attenuation quality factors; Q (P) and Q (S). Their estimated average values are observed to be fitting to the power law form of Q = Q (0) f (n) . The averaged power law relations for Southern Indian region (as a whole) are obtained as Q (P) = (95 +/- A 1.12)f ((1.32 +/- 0.01)); Q (S) = (128 +/- A 1.84)f ((1.49 +/- 0.01)). Based on the stations and recorded local earthquakes, for parts of EDC, WDC and SGT, the average power law estimates are obtained as: Q (P) = (97 +/- A 5)f ((1.40 +/- 0.03)), Q (S) = (116 +/- A 1.5)f ((1.48 +/- 0.01)) for EDC region; Q (P) = (130 +/- A 7)f ((1.20 +/- 0.03)), Q (S) = (103 +/- A 3)f ((1.49 +/- 0.02)) for WDC region; Q (P) = (68 +/- A 2)f ((1.4 +/- 0.02)), Q (S) = (152 +/- A 6)f ((1.48 +/- 0.02)) for SGT region. These estimates are weighed against coda Q (Q (C)) estimates, using the coda decay technique, which is based on a weak backscattering of S-waves. A major observation in the study of body wave analysis is the low body wave Q (Q (0) < 200), moderately high value of the frequency-exponent, 'n' (> 0.5) and Q (S)/Q (P) ae 1, suggesting lateral stretches of dominant scattering mode of seismic wave propagation. This primarily could be attributed to possible thermal anomalies and spread of partially fluid-saturated rock-masses in the crust and upper mantle of the southern Indian region, which, however, needs further laboratory studies. Such physical conditions might partly be correlated to the active seismicity and intraplate tectonism, especially in SGT and EDC regions, as per the observed low-Q (P) and Q (S) values. Additionally, the enrichment of coda waves and significance of scattering mechanisms is evidenced in our observation of Q (C) > Q (S) estimates. Lapse time study shows Q (C) values increasing with lapse time. High Q (C) values at 40 s lapse times in WDC indicate that it may be a relatively stable region. In the absence of detailed body wave attenuation studies in this region, the frequency dependent Q relationships developed here are useful for the estimation of earthquake source parameters of the region. Also, these relations may be used for the simulation of earthquake strong ground motions which are required for the estimation of seismic hazard, geotechnical and retrofitting analysis of critical structures in the region.
机译：我们对印度南部P波、S波的频谱分析得出的地震衰减及其相关Q结构进行了系统研究。研究区域分为EDC（东达瓦克拉通）、西达瓦克拉通（WDC）和南部麻粒岩地形（SGT）的部分区域。这项研究是在1-20赫兹的频率范围内进行的，使用单站频谱比技术。我们利用大约32个宽带三分量地震台网中记录的45次地震，震级（M（L））在1.6到4.5之间，来估计平均地震体波衰减质量因子；Q（P）和Q（S）。观察到它们的估计平均值符合Q=Q（0）f（n）的幂律形式。印度南部地区（作为一个整体）的平均幂律关系为Q（P）=（95+/-a1.12）f（（1.32+/-0.01））；Q（S）=（128+/-A1.84）f（（1.49+/-0.01））。根据台站和记录的局部地震，对于EDC、WDC和SGT的部分地区，平均幂律估计值为：EDC地区的Q（P）=（97+/-A5）f（（1.40+/-0.03）），Q（S）=（116+/-A1.5）f（（1.48+/-0.01））；WDC区域的Q（P）=（130+/-a7）f（（1.20+/-0.03）），Q（S）=（103+/-a3）f（（1.49+/-0.02））；对于SGT区域，Q（P）=（68+/-A2）f（（1.4+/-0.02）），Q（S）=（152+/-A6）f（（1.48+/-0.02））。使用尾波衰减技术（基于S波的弱后向散射）将这些估计值与尾波Q（Q（C））估计值进行权衡。体波分析研究中的一个主要观察结果是低体波Q（Q（0）<200），频率指数'n'（>0.5）和Q（S）/Q（P）ae<1的中等高值，表明地震波传播的主要散射模式存在横向拉伸。这主要归因于南印度地区地壳和上地幔中可能存在的热异常和部分流体饱和岩体的扩展，但这需要进一步的实验室研究。根据观测到的低Q（P）和Q（S）值，这种物理条件可能部分与活动地震活动和板内构造作用有关，尤其是在SGT和EDC地区。此外，我们对Q（C）>Q（S）估计的观察证明了尾波的富集和散射机制的重要性。时间推移研究表明，Q（C）值随时间推移而增加。WDC中40秒消失时间的高Q（C）值表明它可能是一个相对稳定的区域。在该地区缺乏详细的体波衰减研究的情况下，本文建立的与频率相关的Q关系可用于该地区震源参数的估计。此外，这些关系还可用于模拟强震地面运动，这是该地区地震危险性评估、岩土工程和关键结构改造分析所需的。

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