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

当地时间2020年3月31日晚，爱达荷州中部发生罕见的大地震(6.5级，震源深度~14公里)。这是爱达荷州有记录以来第二大地震，由于如此规模的地震并不常见，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的一个地区建立了传感器网络。这一部署包括地震仪(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)。这一新台网极大地提高了对远区地震台网已记录到的283个2.5级余震的探测能力。此外，这个新的网络将显著提高余震位置的准确性，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危险，包括未来几周和几个月可能发生的潜在大地震。除了跟踪这些余震的地震辐射外，对次声或大气中低频压力波的监测的初步分析显示，地震产生了令人难以置信的“声音”。这些气波已经在地球物理网络的多个地点被发现，早期的分析表明，它们起源于周围的山脉，因为山脉在地震波传播过程中发生摇晃。过去对山脉产生的空气波的观测只在很远的距离进行研究；因此，科学家们对地震声音产生的过程知之甚少。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更好的理解可能会导致监测地震灾害的新方法，包括山区的次生灾害，如雪崩和崩塌，使暴露在此类灾害中的社区和必须对其做出反应的机构受益。爱达荷州中部最近发生的大地震(2020年3月31日，6.5级，深度约14公里)的余震为了解地形地震-声能转换提供了一个短暂而独特的机会。受爱达荷州罕见地震的鼓舞，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，从而形成了具有良好空间覆盖的传感器网络。BSU网络包括几个空间上分散的站点，包括地震和声学阵列(可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)，使其能够识别波源的后方位角，这些波源可以是移动的、低幅度的、连续的或具有紧急情况的。此外，由于一些次声传感器和地震仪位于同一位置，因此可以确定地震和声学仪器对声波和地震波的相互响应。BSU临时网络的初步结果包括永久区域台站没有检测到的余震以及频繁的地震次声。地震产生的次声在多个地点被观测到，与岩石坠落或雪崩无关，似乎起源于地震波通过期间附近的山区地形。这种次声以前只在大气传播效应很强的区域距离处观测到，限制了源推断的分辨率；BSU网络不受这一限制。在余震序列期间维持和扩大北大地震声学台网，可以前所未有地详细地阐明地面和大气之间的波能转换。这种对地震次声的进一步了解将包括以下专题：对次声产生的控制(例如，对地震震级和深度、地形特性和地震波类型的依赖)、次声特性(例如，辐射模式和持续时间)、地震次声返回地面的耦合、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反应)，以及确定地震次声来源的分析方法。与地震相关的地形次声可以作为地震多发山区的一种新的监测方法。由于需要区分与地震相关的山次声和次生地震危险，如雪崩或崩塌，对地震次声的更好理解将有助于受此类危险影响的社区和机构。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.