The source characteristics of offshore seismic events, especially regular (or fast) and slow earthquakes, can provide key information on their source physics and frictional conditions at the plate boundary. Due to strong 3-D heterogeneities in offshore regions, such as those relating to sea water, accretionary prism and small-scale velocity heterogeneity, conventional methods using a 1-D earth model may mis-estimate source parameters such as the duration and radiation energy. Estimations could become severe inaccuracies for small offshore seismic events because high-frequency (>1 Hz) seismograms, which are strongly affected by 3-D heterogeneities, are only available for analysis because of their signal-to-noise ratio. To investigate the effects of offshore heterogeneities on source parameter estimation for small seismic events, we analysed both observed and simulated high-frequency seismograms southeast off the Kii Peninsula, Japan, in the Nankai subduction zone. Numerical simulations of seismic wave propagation using a 3-D velocity structure model clarified the effects of each heterogeneity. Comparisons between observations and model simulations demonstrated that the thick low-velocity accretionary prism has significant effects on high-frequency seismic wave propagation. Especially for shallow low-frequency tremors occurring at depths just below the accretionary prism toe, seismogram durations are significantly broader than an assumed source duration, even for stations with epicentral distances of approximately 10 km. Spindle-shape seismogram envelopes were observed even at such close stations. Our results suggest that incorporating 3-D heterogeneities is necessary for practical estimation of source parameters for small offshore events.

中文翻译：

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在海底地震仪上模拟高频地震图：小海上地震和浅低频地震的异质结构对震源参数估计的影响

海上地震事件的源头特征，尤其是定期（或快速）和慢速地震，可以提供有关其源物理和板块边界摩擦条件的关键信息。由于近海区域存在强烈的3-D异质性，例如与海水，增生棱镜和小尺度速度异质性有关的异质性，使用一维地球模型的常规方法可能会误估计源参数，例如持续时间和辐射能。对于小型海上地震事件，估计可能会变得严重不准确，因为受3-D异质性强烈影响的高频（> 1 Hz）地震图仅因其信噪比而可用于分析。要研究海上异质性对小地震事件源参数估计的影响，我们分析了日本南纪俯冲带东南部日本纪伊半岛东南部的观测和模拟高频地震图。使用3-D速度结构模型对地震波传播进行数值模拟，阐明了每种异质性的影响。观测值和模型模拟之间的比较表明，厚的低速增生棱镜对高频地震波的传播有重大影响。特别是对于发生在刚好在增生棱镜脚趾以下的深度的浅低频震颤，即使对于震中距离约为10 km的台站，地震图的持续时间也比假定的震源持续时间要宽得多。即使在这样的近站也观察到了纺锤形的地震波包络线。