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2021 Vol. 34, No. 4

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Articles
Origin of ULVZs near the African LLSVP: Implications from their distribution and characteristics
An Fan, Xinlei Sun
2021, 34(4): 299-309. doi: 10.29382/eqs-2021-0042
Abstract:
Ultra-low velocity zones (ULVZs) provide important information on the composition and dynamics of the core-mantle boundary (CMB). However, their global distribution and characteristics are not well constrained, especially near African large low-shear velocity provinces (LLSVPs). Here, we used ScS precursor (SdS) and postcursor (ScscS) phases recorded by various seismic networks in Africa and South America to investigate the ULVZ characteristics underlying the South Atlantic Ocean. We found no evidence of ULVZs near the SE boundary of South America, but an ULVZ was found within the SW boundary of the African LLSVP, with thicknesses ranging from 11–18 km and reductions in S-wave velocities of 18%–34%. Our results, combined with the global distribution of ULVZs, suggest that thermal activity may be essential to ULVZ formation. Moreover, subducted slab and mantle flow may also play a key role, depending on the location of the ULVZs.
Data replenishment of five moderate earthquake sequences in Japan, with semi-automatic cluster selection
Yi Zheng, Bogdan Enescu, Jiancang Zhuang, Coleman Yu
2021, 34(4): 310-322. doi: 10.29382/eqs-2021-0030
Abstract:
Missing early aftershocks following relatively large or moderate earthquakes can cause significant bias in the analysis of seismic catalogs. In this paper, we systematically address the aftershock missing problem for five earthquake sequences associated with moderate-size events that occurred inland Japan, by using a stochastic replenishing method. The method is based on the notion that if a point process (e.g., earthquake sequence) with time-independent marks (e.g., magnitudes) is completely observed, it can be transformed into a homogeneous Poisson process by a bi-scale empirical transformation. We use the Japan Meteorological Agency (JMA) earthquake catalog to select the aftershock data and replenish the missing early events using the later complete part of each aftershock sequence. The time windows for each sequence span from 6 months before the mainshock to three months after. The semi-automatic spatial selection uses a clustering method for the epicentral selection of earthquakes. The results obtained for the original JMA catalog and replenished datasets are compared to get insight into the biases that the missing early aftershocks may cause on the Omori-Utsu law parameters’ estimation, characterizing the aftershock decay with time from the mainshock. We have also compared the Omori-Utsu law parameter estimates for two datasets following the same mainshock; the first dataset is the replenished sequence, while the second dataset has been obtained by waveform-based analysis to detect early aftershocks that are not recorded in the JMA catalog. Our results demonstrate that the Omori-Utsu law parameters estimated for the replenished datasets are robust with respect to the threshold magnitude used for the analyzed datasets. Even when using aftershock time windows as short as three days, the replenished datasets provide stable Omori-Utsu law parameter estimations. The p-values for all the analyzed sequences are about 1.1 and c-values are significantly smaller compared to those of original datasets. Our findings prove that the replenishment method is a fast, reliable approach to address the missing aftershock problem.
Joint analysis of b-value and apparent stress before the 2011 MW9.0 Tohoku-Oki, Japan earthquake
Yane Li, Xuezhong Chen, Lijuan Chen
2021, 34(4): 323-333. doi: 10.29382/eqs-2021-0036
Abstract:
Detecting tempo-spatial changes of crust stress associated with major earthquakes has implications for understanding earthquake seismogenic processes. We conducted a joint analysis of b-value and apparent stress in the source region before the March 11, 2011 MW9.0 Tohoku-Oki, Japan earthquake. Earthquakes that occurred between January 1, 2000 and March 8, 2011 were used to estimate b-values, while source parameters of events with magnitudes of Ms5.0–6.9 between January 1, 1997 and March 8, 2011 were used to calculate the apparent stresses. Our results show that the average b-value decreased steadily from 1.26 in 2003 to 0.99 before the Tohoku-Oki mainshock. This b-value decrease coincided with an increase in the apparent stress from 0.65 MPa to 1.64 MPa. Our results reveal a clear negative correlation between the decrease in b-value and increase in apparent stress, which lasted for approximately eight years prior to the 2011 mainshock. Additionally, spatial pattern results of the relative change in b-value show that the area associated with drastic b-value decreases (25% or greater) was concentrated near the 2011 mainshock epicenter. The joint analysis of b-value and apparent stress provides a promising method for detecting anomalies that could serve as potential indicators of large earthquakes.
An optimized finite difference method based on a polar coordinate system for regional-scale irregular topography
Jianxia Xu, Wei Zhang, Xiaofei Chen
2021, 34(4): 334-343. doi: 10.29382/eqs-2021-0022
Abstract:
The finite difference method (FDM) is an important numerical approach for simulating the propagation of seismic waves, and some FDMs can be used to study the impact of the Earth’s curvature and topography over large distances. To efficiently model the effects of the Earth’s irregular topography on the propagation of seismic waves, here we optimize a previously proposed grid mesh method and develop a novel two-dimensional boundary-conforming FDM based on a curvilinear polar coordinate system. This method efficiently simulates the propagation of seismic waves in an arc-shaped model with large variations in surface topography. Our method was benchmarked against other reported methods using several global-scale models. The consistency of the results confirms the validity of our proposed optimization strategy. Furthermore, our findings indicate that the proposed optimization strategy improves computational efficiency.
A multi-scale 3-D crust velocity model in the Hefei-Chao Lake area around the southern segment of Tanlu Fault Zone
Lingli Li, Huajian Yao, Song Luo, Junhui Li, Xiaoli Wang, Hongyu Ni, Ziwen Bao
2021, 34(4): 344-357. doi: 10.29382/eqs-2021-0020
Abstract:
Regional high-precision velocity models of the crust are an important foundation for examining seismic activity, seismogenic environments, and disaster distribution characteristics. The Hefei-Chao Lake area contains the main geological units of Hefei Basin, with thick sediments and the Chao Lake depression. Several major concealed faults of the southern NNE-trending Tanlu Fault Zone cross this area. To further explore the underground distribution characteristics of the faults and their tectonic evolutionary relationship with adjacent tectonic units, this study used ambient noise data recorded by a seismic array deployed in Hefei City and Chao Lake, constructing a 3-D velocity model at the depth of 1–8 km. Then a multi-scale high-resolution 3-D velocity model of this area was constructed by this new upper crustal velocity model with the previous middle and lower crustal model. The new model reveals that a high-velocity belt is highly consistent with the strike of the Tanlu Fault Zone, and a low-velocity sedimentary characteristic is consistent with the Hefei Basin and Chao Lake depression. The distribution morphology of high and low velocity bodies shows that the sedimentary pattern of Hefei-Chao Lake area is closely related to the tectonic evolution of the Tanlu Fault Zone since the Mesozoic. This study also identifies multiple low-velocity anomalies in the southeastern Hefei City. We speculate that strong ground motion during the 2009 Feidong earthquake (magnitude of 3.5) was related to amplification by the thick sediments in the Hefei Basin. We also discuss further applications of multi-scale high-resolution models of the shallow layer to strong ground motion simulations in cities and for earthquake disaster assessments.
Three-dimensional crustal P-wave velocity structure in the Yangbi and Eryuan earthquake regions, Yunnan, China
Jia Jia, Qingju Wu, Fuyun Wang
2021, 34(4): 358-366. doi: 10.29382/eqs-2021-0025
Abstract:
A magnitude 5.5 earthquakes occurred in Eryuan County, Dali Bai Autonomous Prefecture, Yunnan Province, China, on March 3. And a magnitude 5.0 earthquake occurred in the same place on April 17, 2013, i.e., 45 days later. Then, on May 21, 2021, multiple earthquakes, one with magnitude 6.4 and several at 5.0 or above, occurred in Yangbi County, Dali Bai Autonomous Prefecture, Yunnan Province, China. All of these occurred in the Weixi-Qiaohou-Weishan fault zone. In this study, 1,874 seismic events in Yangbi and Eryuan counties were identified by automatic micro-seismic identification technology and the first arrivals were picked up manually. Following this, a total of 11,968 direct P-wave absolute arrivals and 73,987 high-quality P-wave relative arrivals were collected for joint inversion via the double difference tomography method. This was done to obtain the regional three-dimensional fine crustal P-wave velocity structure. The results show that the travel time residuals before and after inversion decreased from the initial –0.1–0.1 s to –0.06–0.06 s. The upper crust in the study area, which exhibited a low-velocity anomaly, corresponded to the basin region; this indicated that the low-velocity anomaly in the shallow part of the study area was affected by the basin. Results also showed some correlation between the distribution of the earthquakes and velocity structure, as there was a low-velocity body Lv1 with a wide distribution at depths ranging from 15–20 km in the Yangbi and Eryuan earthquake regions. In addition, earthquakes occurred predominantly in the high-low velocity abnormal transition zone. The low-velocity body in the middle and lower crust may be prone to concentrating upper crustal stress, thus leading to the occurrence of earthquakes.
Receiver function structures beneath the Haiyuan fault on the northeastern margin of the Tibetan plateau
Yutao Shi, Yuan Gao, Laiyu Lu
2021, 34(4): 367-377. doi: 10.29382/eqs-2020-0055
Abstract:
We performed a receiver function analysis on teleseismic data recorded along two dense seismic profiles and from 4 broadband regional seismic stations across the northeastern Tibetan plateau. The crustal thickness and vP/vS ratio were measured by the H-κ domain search algorithm. The Moho discontinuity across the Haiyuan arc fault zone was also revealed by common conversion point (CCP) imaging. Our study results show that the crustal thickness and the vP/vS ratio were 42–56 km and 1.60–1.88, respectively. The crustal thickening on the northeastern margin indicates that the crust is shortening or that there was a superimposition of crusts during the collision of the Tibetan plateau with Eurasian block. Our results suggest that Haiyuan fault likely resulted from the interactions of high temperature and pressure conditions during the collision of the Indian and Asian continents. The Moho beneath the Haiyuan tectonic region exhibits an obvious offset and a vague discontinuity according to CCP imaging. This study suggests that the Haiyuan arc fault zone is a trans-crustal fault that cuts through the Moho in the northeastern Tibetan Plateau. Moreover, there are indications of strong deformation in the intensive crustal extrusion from the interior of the Tibetan Plateau to its northeastern margin.
Lower crustal attenuation in northeastern Tibetan Plateau from ML amplitude
Yanbing Liu, Shunping Pei
2021, 34(4): 378-386. doi: 10.29382/eqs-2021-0007
Abstract:
This study investigated the crustal attenuation structures of Sg and Lg waves of the northeastern Tibetan Plateau. We collected ML amplitude data recorded at 168 permanent stations between 1985 and 2016 and 11 temporary broadband stations between 2014 and 2016. Detailed Q0 variation maps of Sg and Lg waves were obtained by applying ML amplitude tomography. The average Q0 values of the Sg and Lg wave were 440 and 220, respectively. Relatively high attenuation anomalies of both waves appeared in the central and eastern regions of the Bayan Har Block and the east edge of the Qiangtang Block, which may be related to partial melting, high geotemperature, and strong tectonic processes. High attenuation anomalies were also found in the Qilian Orogenic Belt and Hetao Graben, which may be related to their active tectonic behavior and densely distributed faults. The relatively low attenuation anomalies of both waves were revealed in the Alax and Ordos blocks, Qaidam, Tarim, Qinghai Lake, and Gonghe basins, which can be explained by the tectonically stable properties and ancient composition of geological elements. These results indicate that the path between the highly attenuated lower crust of the Bayan Har Block and the Qilian Orogenic Belt is obstructed by three adjacent low attenuated areas (i.e., the Qilian, Qinghai Lake, and Gonghe basins); thus, it appears unlikely that a crustal flow channel from the interior of the Tibetan Plateau to the Qilian Orogenic Belt will form.