ALBERT

Description: On the morning of 14 April 2010, the M s  7.1 Yushu earthquake struck the eastern Tibet Plateau and an M s  6.3 aftershock followed west-southwest of the mainshock epicenter one and a half hours later. The Yushu earthquake occurred on the northwestern continuation of the Ganzi–Xianshuihe fault zone and reactivated two segments of the fault. Ruptures associated with Yushu earthquake span a zone 70 km long, extending from south of Jielong in the west to south of Gyêgu (Yushu) in the east. Detailed mapping of the surface rupture zone shows that it consists of two strike-slip segments, the western and eastern segments, separated by the Longbao Lake step-over (pull apart) basin. Our analysis suggests that the 18-km-long western segment of the surface rupture south of the Longbao Lake basin may have been associated with the M s  6.3 aftershock. The eastern rupture segment, which is inferred to be associated with the mainshock, is 32 km long. The main surface rupture can also be divided into two sections, with a 2-km-long gap between them. The surface rupture exhibits left-lateral slip and is localized in a zone about 50 m wide along the present-day trace of the Yushu fault. A single break, without splays or branching traces, characterizes most of the surface rupture zone. The observed magnitudes of surface displacement are typically 0.5–1 m with a maximum slip of 2 m. Both field observations and seismic inversion results suggest, rupture of M s  7.1 Yushu earthquake nucleated near the Longbao Lake step-over basin in the west, propagated unilaterally eastward, and terminated within the east segment margin of the Yushu fault.

Description: Using the curved grid finite-difference method, we develop dynamic spontaneous rupture models of earthquakes on the Jiaocheng fault (JF) near Taiyuan, the capital and largest city of Shanxi Province in north China. We then model the wave propagation and strong ground motion generated by these scenario earthquakes. A map of the seismic-hazard distribution for a potential M  7.5 earthquake is created based on dynamic rupture and true 3D modeling. The tectonic initial stress fields derived from the inversion of focal mechanisms of historical earthquakes, a nonplanar fault, and a rough surface are considered in the dynamic rupture simulation. Based on the geological structure of the Taiyuan basin, normal faulting with a dipping angle of 60° is implemented for the scenario earthquake simulations. The largest uncertainty of a potential earthquake in the JF zone is the hypocenter. Four cases are used to nucleate the earthquake at different locations. Using these dynamic rupture sources for the JF, we further simulate and analyze both the seismic wave generated by the scenario earthquake and the strong ground motion. It is found that the low-velocity media of the Taiyuan basin redistribute the ground motion well. The effects of the regional stress fields on the dynamic rupture and hazard distribution are investigated and discussed further. Moreover, a scenario earthquake, which can cause great damage to the city of Taiyuan, is modeled and analyzed.

Description: 〈span〉〈div〉Abstract〈/div〉The 2008 Mw 5.2 Mt. Carmel earthquake is the largest earthquake in the last 50 yrs in southeastern Illinois, near the north termination of the north‐northeast‐trending Wabash Valley fault system (WVFS). The earthquake shows almost pure strike‐slip focal mechanism, but it is still uncertain which nodal plane (NP) is the ruptured fault plane. To resolve the fault plane, we determine rupture directivity of the earthquake via the relative centroid method. We begin with inverting the point‐source solution (strike 297°/dip 84°/rake 1° for NP1, strike 206°/dip 89°/rake 173° for NP2, and centroid depth 16 km) and then determine the relative location between the centroid and hypocenter via regional waveform fitting. Two 〈strong〉M〈/strong〉 4+ aftershocks are used as reference events, and the waveform time shifts of reference events with respect to the 1D velocity model are used to calibrate the path effects. The results show that the Illinois mainshock ruptured to east‐southeast along the 297° NP for about 2–3 km, consistent with relocated aftershock distribution, and we infer that the sinistral causative fault connects the north‐northwest‐trending La Salle anticlinal belt and the north‐northeast‐trending WVFS.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The 2008 Mw 5.2 Mt. Carmel earthquake is the largest earthquake in the last 50 yrs in southeastern Illinois, near the north termination of the north‐northeast‐trending Wabash Valley fault system (WVFS). The earthquake shows almost pure strike‐slip focal mechanism, but it is still uncertain which nodal plane (NP) is the ruptured fault plane. To resolve the fault plane, we determine rupture directivity of the earthquake via the relative centroid method. We begin with inverting the point‐source solution (strike 297°/dip 84°/rake 1° for NP1, strike 206°/dip 89°/rake 173° for NP2, and centroid depth 16 km) and then determine the relative location between the centroid and hypocenter via regional waveform fitting. Two 〈strong〉M〈/strong〉 4+ aftershocks are used as reference events, and the waveform time shifts of reference events with respect to the 1D velocity model are used to calibrate the path effects. The results show that the Illinois mainshock ruptured to east‐southeast along the 297° NP for about 2–3 km, consistent with relocated aftershock distribution, and we infer that the sinistral causative fault connects the north‐northwest‐trending La Salle anticlinal belt and the north‐northeast‐trending WVFS.〈/span〉

Description: Topography-dependent eikonal equation (TDEE) formulated in a curvilinear coordinate system has been recently established and is effective for calculating first-arrival travel times in an Earth model with an irregular surface. In previous work, the Lax–Friedrichs sweeping scheme used to approximate the TDEE viscosity solutions was only first-order accurate. We present a high-order fast-sweeping scheme to solve the TDEE with the aim of achieving high-order accuracy in the travel-time calculation. The scheme takes advantage of high-order weighted essentially nonoscillatory (WENO) derivative approximations, monotone numerical Hamiltonians, and Gauss Seidel iterations with alternating-direction sweepings. It incorporates high-order approximations of the derivatives into the numerical representation of the Hamiltonian such that the resulting numerical scheme is formally high-order accurate and inherits fast convergence from the alternating sweeping strategy. Extensive numerical examples are presented to verify its efficiency, convergence, and high-order accuracy.

Description: Spontaneous dynamic-rupture simulation in 3D is a difficult task in seismology, especially for the rupture dynamics of a fault with complex geometry and a free surface. In this study, we model an irregular fault that reaches the free surface and investigate the rupture dynamics of the intersection between the earthquake-induced fault and the Earth’s surface. We use the recently proposed curved grid finite-difference method (CG-FDM) to simulate a spontaneous dynamic rupture. However, this involves solving the inversion of an ill-conditioned matrix that is required in finite-difference method modeling at the point of intersection, which must be addressed to achieve stable simulation conditions. We achieve stable conditions at the intersection between the fault plane and the free surface by considering the continuity of the fault’s normal displacement, ensuring that the point of intersection meets the conditions of both the fault and the free surface. To verify our method, we simulate the spontaneous dynamic rupture of a rough fault in half-space and compare the results with those from another method. The good agreement between two methods validates our mathematical strategy for modeling the intersection between an irregular fault plane and a free surface using CG-FDM.

Description: 〈span〉〈div〉ABSTRACT〈/div〉Earthquakes can be detected by matching spatial patterns or phase properties from 1D seismic waves. Current earthquake detection methods such as waveform correlation and template matching (TM) have difficulty detecting anomalous earthquakes that are not similar to other earthquakes. In recent years, machine‐learning techniques for earthquake detection have been emerging as a new active research direction. In this article, we develop a novel earthquake detection method based on dictionary learning. Our detection method first generates rich features via signal processing and statistical methods, and further employs feature selection techniques to choose features that carry the most significant information. Based on these selected features, we build a dictionary for classifying earthquake events from nonearthquake events. To evaluate the performance of our dictionary‐based detection methods, we test our method on a labquake dataset, which contains 3,357,566 time‐series data points with a 400 MHz sampling rate. A total of 1000 earthquake events are manually labeled, and the length of these earthquake events varies from 74 to 7151 data points. Through comparison with other detection methods, we show that our feature selection and dictionary‐learning incorporated earthquake detection method achieves an 80.1% prediction accuracy and outperforms the baseline methods in earthquake detection, including TM and support vector machine (SVM).〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉Earthquakes can be detected by matching spatial patterns or phase properties from 1D seismic waves. Current earthquake detection methods such as waveform correlation and template matching (TM) have difficulty detecting anomalous earthquakes that are not similar to other earthquakes. In recent years, machine‐learning techniques for earthquake detection have been emerging as a new active research direction. In this article, we develop a novel earthquake detection method based on dictionary learning. Our detection method first generates rich features via signal processing and statistical methods, and further employs feature selection techniques to choose features that carry the most significant information. Based on these selected features, we build a dictionary for classifying earthquake events from nonearthquake events. To evaluate the performance of our dictionary‐based detection methods, we test our method on a labquake dataset, which contains 3,357,566 time‐series data points with a 400 MHz sampling rate. A total of 1000 earthquake events are manually labeled, and the length of these earthquake events varies from 74 to 7151 data points. Through comparison with other detection methods, we show that our feature selection and dictionary‐learning incorporated earthquake detection method achieves an 80.1% prediction accuracy and outperforms the baseline methods in earthquake detection, including TM and support vector machine (SVM).〈/span〉

Description: Irregular surfaces cause a number of problems for seismic processing and interpretation. The problems lie in the proper treatment of topography in the first-arrival travel-time calculation and ray-path tracing, both of which are subject to preconditions in ray-based seismogram synthesis, seismic tomography, and seismic migration calculations. Two treatment schemes for irregular surfaces have been used previously: (1) model expansion with the irregular surface treated as an inner discontinuity, and (2) flattening of the irregular surface using a transformation between curvilinear and Cartesian coordinates, while maintaining it as a free surface. In the first approach, first-arrival travel times can be calculated using an eikonal equation solver, and rays are traced backward from the receiver to the source along the direction of the gradient of the travel-time field. In the second scheme, a topography-dependent eikonal equation is used to calculate irregular-surface first-arrival travel times. We present a ray-path tracing scheme for irregular surfaces, which applies a travel-time field calculated using a topography-dependent eikonal equation. The scheme is realized using travel-time gradients in curvilinear coordinates. The validity of the scheme for tracing ray paths in the presence of irregular topography is illustrated by five models containing differing degrees of topographical complexity. Comparison of ray paths and first-arrival travel times between the two irregular-surface treatment schemes suggests the topography-flattened scheme avoids the difficulties of both discretizing the irregular surface and the velocity selection of the infill medium in the model expansion scheme. For the treatment of the inner discontinuity by model expansion, there is a possibility that ray paths will be traced outside the real physical model. Using the topography-dependent eikonal equation solver with our ray-path tracing scheme should provide an efficient means of dealing with irregular surfaces, which could be applied in the fields of seismic tomography, seismic migration, and tomographic static correction.

Description: 〈span〉〈div〉Abstract〈/div〉The Haiyuan fault is a major seismogenic fault on the northeastern edge of the Tibetan–Qinghai plateau. The 16 December 1920 Ms 8.5 Haiyuan, China, earthquake is the largest and most recent event along the eastern Haiyuan fault (the Haiyuan fault in the article). Because only a few near‐field seismic recordings are available, the rupture process remains unclear. To understand the source process and intensity distribution of the 1920 Haiyuan earthquake, we simulated the dynamic rupture and strong ground motion of said earthquake using the 3D curved‐grid finite‐difference method. Considering the differences in epicenter locations among various catalogs, we constructed two models with different source points. For each model, three versions with different fault geometries were investigated: one continuous fault model and two discontinuous fault models with different stepover widths (1.8 and 2.5 km, respectively). A dynamic rupture source model with a final slip distribution similar to that observed on the ground surface was found. The maximum displacement on the ground surface was ∼6.5  m. Based on the dynamic rupture model, we also simulated the strong ground motion and estimated the theoretical intensity distribution. The maximum value of the horizontal peak ground velocity occurs near Haiyuan County, where the intensity reaches XI. Without considering the site conditions, the intensity values in most regions, based on the dynamic scenarios, are smaller than the values from field investigation. In this work, we present physically based insights into the 1920 Haiyuan earthquake, which is important for understanding rupture processes and preventing seismic hazards on the northeastern boundary of the Tibetan plateau.〈/span〉