Spatio-temporal variations of shallow seismic velocity changes in Salton Sea Geothermal Field,California in response to large regional earthquakes and long-term geothermal activities

We measure spatio-temporal variations of seismic velocity changes in Salton Sea Geothermal Field,California based on cross correlations of daily seismic traces recorded by a borehole seismic network from December 2007 to January 2014.We find clear co-seismic velocity reductions during the 2010 M 7.2 El Mayor–Cucapah,Mexico earthquake at~100 km further south,followed by long-term recoveries.The co-seismic reductions are larger with longer post-seismic recoveries in higher frequency bands,indicating that material damage and healing process mostly occurred in the shallow depth.In addition,the co-seismic velocity reductions are larger for ray paths outside the active fluid injection/extraction regions.The ray paths inside injection/extraction regions are associated with smaller co-seismic reductions,but subtle long-term velocity increases.We also build 3D transient water flow models based on monthly injection/extraction rates,and find correlations between several water flow parameters and co-seismic velocity reductions.We interpret the relative lack of co-seismic velocity changes within the geothermal region as unclogging of fracture network due to persistent fluid flows of geothermal production.The long-term velocity increase is likely associated with the ground water depletion and subsidence due to net production.

A 15-year-Long catalog of seismicity in the Eastern Tennessee Seismic Zone(ETSZ)using matched filter detection

We present a detailed catalog of 13671 earthquakes in the Eastern Tennessee Seismic Zone(ETSZ)that spans January 1,2005 to July 31,2020.We apply a matched filter detection technique on over 15 years of continuous data,resulting in arguably the most complete catalog of seismicity in the ETSZ yet.The magnitudes of newly detected events are determined by computing the amplitude ratio between the detections and templates using a principal component fit.We also compute the b-value for the new catalog and comparatively relocate a subset of newly detected events using XCORLOC and hypoDD,which shows a more defined structure at depth.We find the greatest concentration along and to the east of the New York-Alabama Lineament,as defined by the magnetic anomaly,supporting the argument that this feature likely is related to the generation of seismicity in the ETSZ.We examine seismicity in the vicinity of the Watts Bar Reservoir,which is located about 5 km from the epicenter of the M_(W) 4.4 December 12,2018 Decatur,Tennessee earthquake,and find possible evidence for reservoir modulated seismicity in this region.We also examine seismicity in the entire ETSZ to search for a correlation between shallow earthquakes and seasonal hydrologic changes.Our results show limited evidence for hydrologicallydriven shallow seismicity due to seasonal groundwater levels in the ETSZ,which contradicts previous studies hypothesizing that most intraplate earthquakes are associated with the dynamics of hydrologic cycles.

利用三分量背景噪声互相关技术测量2008年汶川震中区的波速变化

2008年汶川地震导致龙门山断裂带中段的北川-映秀断裂和灌县-江油断裂分别产生了约240 km和72 km 的地表破裂，地震引起地表显著破裂的同时，也一定会使地下介质属性和状态发生改变，研究这些变化对于了解汶川地震机理、断层带演化和地震危险性分析都有重要意义。近年来迅速发展的利用背景噪声测量波速变化的方法，为研究地震引起地下介质随时间变化提供了新的思路。国内外几个研究小组已经利用该方法对汶川地震引起的地壳介质波速变化情况进行了研究，这些研究应用了四川区域台网或川西台阵的数据。这两个台网台站平均间距较大，四川区域台网的平均台间距大于50 km，川西台站则约为20～30 km；而且这些台站距离汶川地震震中和地表断裂带相对较远，难以获得震中区地震波速度随时间变化的过程。而距离汶川地震震中较近的紫坪铺水库台网台间距较小（小于10 km），且在汶川地震前后均有良好的连续观测资料，为研究震中区介质波速的时空变化提供了宝贵的数据。

Under the surface:Pressure-induced planetary-scale waves,volcanic lightning,and gaseous clouds caused by the submarine eruption of Hunga Tonga-Hunga Ha'apai volcano

We present a narrative of the eruptive events culminating in the cataclysmic January 15, 2022 eruption of Hunga Tonga-Hunga Ha’apai Volcano by synthesizing diverse preliminary seismic, volcanological, sound wave, and lightning data available within the first few weeks after the eruption occurred. The first hour of eruptive activity produced fast-propagating tsunami waves, long-period seismic waves, loud audible sound waves, infrasonic waves, exceptionally intense volcanic lightning and an unsteady volcanic plume that transiently reached-at 58km-the Earth’s mesosphere. Energetic seismic signals were recorded worldwide and the globally stacked seismogram showed episodic seismic events within the most intense periods of phreatoplinian activity, and they correlated well with the infrasound pressure waveform recorded in Fiji. Gravity wave signals were strong enough to be observed over the entire planet in just the first few hours, with some circling the Earth multiple times subsequently. These large-amplitude, long-wavelength atmospheric disturbances come from the Earth’s atmosphere being forced by the magmatic mixture of tephra, melt and gasses emitted by the unsteady but quasicontinuous eruption from 0402±1–1800 UTC on January 15, 2022. Atmospheric forcing lasted much longer than rupturing from large earthquakes recorded on modern instruments, producing a type of shock wave that originated from the interaction between compressed air and ambient(wavy) sea surface. This scenario differs from conventional ideas of earthquake slip, landslides, or caldera collapse-generated tsunami waves because of the enormous(~1000x) volumetric change due to the supercritical nature of volatiles associated with the hot,volatile-rich phreatoplinian plume. The time series of plume altitude can be translated to volumetric discharge and mass flow rate. For an eruption duration of ~12 h, the eruptive volume and mass are estimated at 1.9 km^(3) and~2 900 Tg, respectively, corresponding to a VEI of 5–6 for this event. The high frequency and intensity of lightning was enhanced by the production of fine ash due to magma-seawater interaction with concomitant high charge per unit mass and the high pre-eruptive concentration of dissolved volatiles. Analysis of lightning flash frequencies provides a rapid metric for plume activity and eruption magnitude. Many aspects of this eruption await further investigation by multidisciplinary teams. It represents a unique opportunity for fundamental research regarding the complex, non-linear behavior of high energetic volcanic eruptions and attendant phenomena, with critical implications for hazard mitigation, volcano forecasting, and first-response efforts in future disasters.

Seismic detections of the 15 February 2013 Chelyabinsk meteor from the dense Chin Array

ChinArray is a dense portable broadband seismic network to cover the entire continental China, and the Phase I is deployed along the north-south seismic belt in southwest China. In this study, we analyze seismic data recorded on the ChinArray following the February 15, 2013 Chelyabinsk （Russia） meteor. This was the largest known object entering the Earth＇s atmosphere since the 1908 Tunguska meteor. The seismic energy radiated from this event was recorded by seismic stations worldwide including the dense ChinAn＇ay that are more than 4000 km away. The weak signal from the meteor event was con- taminated by a magnitude 5.8 Tonga earthquake occur- red ~ 20 min earlier. To test the feasibility of detecting the weak seismic signals from the meteor event, we compute vespagram and perform F-K analysis to the surface-wave data. We identify a seismic phase with back azimuth （BAZ） of 329.7° and slowness of 34.73 s/deg, corre- sponding to the surface wave from the Russian meteor event （BAZ ） 325.97°）. The surface magnitude （Ms） of the meteor event is 3.94 ±0.18. We also perform similar analysis on the data from the broadband array F-net in Japan, and find the BAZ of the surface waves to be 316.61%. With the different BAZs of ChinArray and F-net, we locate the Russian meteor event at 58.80°N, 58.72°E. The relatively large mislocation （-438 km as compared with 55.15°N. 61.41°E by others） may be a result of theweak signals at teleseismic distances.

北京房山岩体附近随采矿活动增强的背景地震活动和动态触发事件研究

北京西部房山岩体多次发现动态触发事件,但之前的研究都受限于台站分布稀疏的影响。本文系统地分析了2011年3月11日日本东北近海MW9.1地震和2012年4月14日印度洋MW8.6地震之前和之后各40天内固定台站和流动台站记录的连续波形。首先,本文采用短时平均比长时平均的检测方法构建模板数据库;其次,本文运用波形模板匹配滤波技术对连续波形进行互相关来检测地震事件。最后,本文分别检测得到日本东北近海地震主震后和印度洋地震主震后1 956个和950个地震事件。多数检测到的地震属于浅源(<5km)地震并群集出现在北京西部房山岩体北岭向斜构造,这个地区邻近一个正在运行的煤矿。从两次主震的强振幅面波中分别检测出7个和10个地震事件,但对应的强前震和余震波形中则没有检测到类似的多事件爆发模式。多种统计检验表明两次主震后短时爆发事件是动态触发事件。本文认为采矿相关的活动可能扰动了地下应力环境,从而使该地区比其他地区更容易被动态应力触发。

利用断裂带首波获得2010年Mw6.9玉树地震断裂带的速度差异

断裂带首波（FZHW）沿着存在岩性差异的断裂面传播，为断裂带特征研究提供了高精度的信息。本文利用2010年4月13日（世界时）Mw6．9玉树地震的余震资料对甘孜-玉树断裂带（GYF）进行了定量分析。与直达P波波形相比，断裂带首波具有振幅低、周期长和初至极性相反等特征。结隆盆地是一个拉张性盆地且被隆宝湖填充，在盆地边缘两侧断裂带附近的4个台站观测到了断裂带首波。直达P波和断裂带首波之间的走时差随着沿断裂面的距离呈线性增加，相应的速度差异为5％～8％，但我们没有观测到走时差随震源深度的变化，结果显示拉张盆地形成了一个地表浅层几千米的低速区。主震震中往东南方向的速度差异很小（1％～3％），与野外现场调查和地震成像的结果较一致。

Detecting remotely triggered temporal changes around the Parkfield section of the San Andreas fault

Detecting temporal changes in fault zone properties at seismogenic depth have been a long-sought goal in the seismological community for many decades. Recent studies based on waveform analysis of repeating earthquakes have found clear temporal changes in the shallow crust and around active fault zones associated with the occurrences of large nearby and teleseismic earthquakes. However, repeating earthquakes only occur in certain locations and their occurrence times cannot be controlled, which may result in inadequate sampling of the interested regions or time periods. Recent developments in passive imaging via auto- and cross-correlation of ambient seismic wavefields （e.g., seismic noise, earthquake coda waves） provide an ideal source for continuous monitoring of temporal changes around active fault zones. Here we conduct a systematic search of temporal changes along the Parkfield section of the San Andreas fault by cross-correlating relatively high-frequency （0.4-1.3 Hz） ambient noise signals recorded by 10 borehole stations in the High Resolution Seismic Network. After using stretch/compressed method to measure the delay time and the decorrelation-index between the daily noise cross-correlation functions （NCCFs）, we find clear temporal changes in the median seismic velocity and decorrelation-index associated with the 2004 M6.0 Parkfield earthquake. We also apply the same procedure to the seismic data around five regional/teleseismic events that have triggered non-volcanic tremor in the same region, but failed to find any clear temporal changes in the daily NCCFs. The fact that our current technique can detect temporal changes from the nearby but not regional and teleseismic events, suggests that temporal changes associated with distance sources are very subtle or localized so that they could not be detected within the resolution of the current technique （-0.2%）.

NEW RESULTS FOR A CLASS OF UNIVALENT FUNCTIONS

Let A denote the family of all analytic functions f(z) in the unit disk D={z ∈C:|z|<1}, normalized by the conditions f(0) = 0 and f'(0) = 1. Let U denote the set of all functions f ∈ A satisfying the condition |(z/f(z))^2f'(z)-1|<1 for z∈D.Let Ω be the class of all f∈A for which |zf'(z)-f(z)|<1/2, z∈D.In this paper, the relations between the two classes are discussed. Furthermore, some new results on the class Ω are obtained.

Possible triggering relationship of six Mw > 6 earthquakes in 2018–2019 at Philippine archipelago

Philippine archipelago (PA) has strong background seismicity,but there is no systematic study of earthquake triggering in this region. There are six earthquakes (M_(w) > 6) occurred between 2018/12/29 and 2019/09/29 in PA,which provides an excellent opportunity to investigate the triggering relationship among these events. We calculate the static Coulomb stress changes of the first five events,and find that the local seismicity after the 2018/12/29 M_(w) 7.0 earthquake is mostly associated with positive Coulomb stress changes,including the 2019/05/31 M_(w) 6.1 event,suggesting a possible triggering relationship. However,we cannot rule out the dynamic triggering mechanism,due to increased microseismicity in both positive and negative stress change regions,and an incomplete local catalog,especially right after the first M_(w) 7.0 mainshock. The dynamic stresses from these M_(w) > 6 events are large enough (from 5 kPa to 3532 kPa) to trigger subsequent events,but a lack of seismicity and waveform evidence does not support delayed dynamic triggering among these events,even the shortest time interval is less than 24 hours. In the past 45 years,the released seismic energy shows certain peaks every 5–10 years. However,earthquakes with M_(w) > 6.0 were relatively infrequent between 2004 and 2018 at PA. Hence,it is possible that several regions are relatively late in their earthquake cycles,which would enhance their susceptibility of being triggered by earthquakes at nearby and regional distances.

A multi-channel approach for automatic microseismic event association using RANSAC-based Arrival Time Event Clustering(RATEC)

In the presence of background noise,arrival times picked from a surface microseismic data set usually include a number of false picks that can lead to uncertainty in location estimation.To eliminate false picks and improve the accuracy of location estimates,we develop an association algorithm termed RANSAC-based Arrival Time Event Clustering(RATEC)that clusters picked arrival times into event groups based on random sampling and fitting moveout curves that approximate hyperbolas.Arrival times far from the fitted hyperbolas are classified as false picks and removed from the data set prior to location estimation.Simulations of synthetic data for a 1-D linear array show that RATEC is robust under different noise conditions and generally applicable to various types of subsurface structures.By generalizing the underlying moveout model,RATEC is extended to the case of a 2-D surface monitoring array.The effectiveness of event location for the 2-D case is demonstrated using a data set collected by the 5200-element dense Long Beach array.The obtained results suggest that RATEC is effective in removing false picks and hence can be used for phase association before location estimates.

Rapid communications of preliminary results for the recent magnitude 6.6 Menyuan,Qinghai,China earthquake helps scientists better study intraplate earthquakes

Within one month of the magnitude 6.6 Qinghai,China Earthquake on 01/07/2022,several articles were published online in peer-reviewed journals and websites focusing on different aspects of this significant event.

Earthquake Research Advances(ERA)opens a new era in earthquake research

In May 2021,two strong earthquakes occurred in west and southwest China,causing ground cracks,building damages,along with human injuries.These events remind us again,that earthquakes and earthquakeinduced disasters always intertwine with human history.Reducing the losses caused by earthquakes and subsequent hazards(e.g.,landslides,tsunamis)is one of the primary goals that geoscientists keep striving for.