RAPID: Seismo-acoustic radiation from a local earthquake aftershock sequence: how, when, and why seismic waves cross the ground-atmosphere interface

2020年3月31日晚（当地时间），爱达荷州中部发生了罕见的大地震（6.5级，深度约14公里）。这是爱达荷州有记录以来的第二大地震，由于这种震级的地震并不常见，因此记录有价值的余震序列的前景促使博伊西州立大学的研究人员第二天在震中附近地区建立了一个传感器网络。这次部署包括地震仪（用于测量地下地震波）和次声传感器（用于测量空气中的压力波）。这个新的台网大大提高了对已经被遥远的区域地震台网记录的283次2.5级以上余震的探测能力。此外，这个新的网络将大大提高余震定位的准确性，这将使我们能够了解和绘制该地区的断层分布，并更好地预测未来的地震灾害，包括未来几周和几个月可能发生的潜在大地震。除了追踪这些余震的地震辐射外，对次声（即大气中的低频压力波）监测的初步分析还揭示了大量地震“声音”的产生。这些空气波已经在地球物理网络的多个地点被识别出来，早期的分析表明它们起源于周围的山脉，因为山脉在地震波通过时震动。过去对山区产生的空气波的观测只进行了远距离研究；因此，科学家对地震声的产生过程所知甚少。因此，通过扩大和维护当地的地球物理传感器网络来监测爱达荷州斯坦利的余震序列，将提供一个独特而短暂的机会，不仅可以研究该地区的地震，还可以研究地震能量向空气的传输（反之亦然）。对地震产生的空气波的进一步了解可能会带来监测地震灾害的新方法，包括雪崩和落石等山区的二次灾害，从而使面临此类灾害的社区和必须对其作出反应的机构受益。爱达荷州中部最近发生的大地震（2020年3月31日，6.5级，深度~14公里）的余震为了解地形地震-声能转换提供了短暂而独特的机会。由于爱达荷州很少发生这种级别的地震，来自博伊西州立大学（BSU）的一组研究人员第二天开始在震中周围地区部署地震仪和次声传感器，从而建立了一个具有良好空间覆盖范围的传感器网络。BSU网络包括几个空间分散的站点，包括地震和声学阵列（可以识别相关信号并确定波的传播矢量的紧密间隔的传感器集群），使其能够识别可移动、低振幅、连续或突发的波源的反方位角。此外，由于一些次声传感器和地震仪放置在一起，因此可以确定地震和声学仪器对声波和地震波的相互反应。临时BSU网络的早期结果包括永久区域台站无法检测到的余震以及频繁的地震产生的次声。地震产生的次声可以在多个地点观测到，与岩石坠落或雪崩无关，似乎是在地震波通过时起源于附近的山区地形。这种次声以前只在大气传播效应强的区域距离观测到，这限制了源推断的分辨率；BSU网络不受此限制。在余震序列中维持和扩展BSU震声台网可以前所未有地详细阐明地面和大气之间的波能转换。这种对地震产生的次声的改进理解将包括以下主题：对次声产生的控制（例如，对地震震级和深度的依赖，地形特性和地震波类型），次声特性（例如，辐射模式和持续时间），地震产生的次声返回到地面的耦合，仪器灵敏度（例如，地震仪对次声和次声传感器对地面运动的响应），以及地震次声源定位的解析方法。地震相关地形次声可以作为一种新的监测方法应用于地震多发山区。由于需要将与地震相关的山区次声与雪崩或岩石坠落等次生地震灾害区分开来，因此提高对地震次声的理解将有助于受此类灾害影响的社区和机构。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Central Idaho experienced a rare large earthquake (M6.5, depth ~14 km) on the evening of March 31, 2020 (local time). This is the second-largest ever recorded earthquake in Idaho, and because earthquakes of this magnitude are uncommon, the prospect of recording a valuable aftershock sequence motivated Boise State University researchers to set up a sensor network the following day in an area near to the epicenter. This deployment included both seismometers (to measure seismic waves underground) and infrasound sensors (to measure pressure waves in the air). This new network greatly enhances the detection capacity to the more than 283 M2.5+ aftershocks that have already been recorded by the distant regional seismic network. In addition, this new network will significantly improve the accuracy of aftershock locations, which will allow us to understand and map the fault distribution in the area and better anticipate future earthquake hazards, including potential large earthquakes that could come in the following weeks and months. In addition to tracking the seismic radiation from these aftershocks, preliminary analyses from the monitoring of infrasound, or low frequency pressure waves in the atmosphere, has revealed the production of an incredible amount of earthquake ‘sounds’. These air waves have been identified at multiple sites across the geophysical network, and early analyses indicate that they originate in the surrounding mountains as the mountains shake during the passage of the seismic waves. Past observations of mountain-generated air waves have only been studied at long distances; therefore, scientists know very little about the process of earthquake sound generation. Thus, monitoring the Stanley, Idaho aftershock sequence by expanding and maintaining the local geophysical sensor network will provide a unique and fleeting chance to study not just this region’s earthquakes, but also the transmission of seismic energy to the air (and vice versa). An improved understanding of earthquake-generated air waves may lead to new methods of monitoring seismic hazards, including secondary hazards in mountainous regions like avalanches and rockfalls, benefiting the communities exposed to such hazards and the agencies that must respond to them.Aftershocks of Central Idaho’s recent large earthquake (March 31 2020, M6.5, depth ~14 km) provide an ephemeral and unique opportunity to understand topographic seismic-acoustic energy conversion. Motivated by the rarity of events of this magnitude in Idaho, a team of researchers from Boise State University (BSU) began deploying seismometers and infrasound sensors in the region surrounding the epicenter the following day, resulting in a sensor network with excellent spatial coverage. The BSU network includes several spatially dispersed sites including both seismic and acoustic arrays (closely-spaced clusters of sensors that can identify correlated signals and determine the wave’s propagation vector), enabling it to identify backazimuths to wave sources that can be mobile, low in amplitude, continuous, or with emergent onsets. Additionally, because some infrasound sensors and seismometers are co-located, it will be possible to determine the mutual responses of seismic and acoustic instruments to acoustic and seismic waves. Early results from the temporary BSU network include aftershocks that are not detected on permanent regional stations as well as frequent earthquake-generated infrasound. Earthquake-generated infrasound is observed at multiple sites, is not associated with rock falls or avalanches, and appears to originate in nearby mountainous topography during the passage of seismic waves. Such infrasound has previously only been observed at regional distances where atmospheric propagation effects are strong, limiting the resolution of source inferences; the BSU network does not suffer from this limitation. Maintaining and expanding the BSU seismo-acoustic network during the aftershock sequence can elucidate the conversion of wave energy between the ground and atmosphere in unprecedented detail. This improved understanding of earthquake-generated infrasound will include topics like controls on infrasound generation (e.g., dependence on earthquake magnitude and depth, topographic properties, and seismic wave type), infrasound properties (e.g., radiation patterns and duration), coupling of earthquake-generated infrasound back into the ground, instrumentation sensitivities (e.g., the response of seismometers to infrasound and of infrasound sensors to ground motion), and analytical methods for locating earthquake infrasound sources. Earthquake-associated topographic infrasound could serve as a new monitoring method in earthquake-prone mountainous areas. Due to the need to distinguish earthquake-related mountain infrasound from secondary seismic hazards like avalanches or rock falls, an improved understanding of earthquake infrasound will help communities and agencies affected by such hazards.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.