JinLai Hao

Rupture process of the M w 7.9 Nepal earthquake April 25, 2015

On April 25, 2015, a magnitude Mw7.9 earthquake occurred in the southern Himalaya, Nepal, at 14:11 local time (UTC 2015-04-25 06:11). Its epicenter was at 28.147°N, 84.708°E with a source depth of 15 km, as determined by the United States Geological Survey (USGS). The earthquake hazard and secondary disasters, including landslides and avalanches, resulted in serious damage to Nepal and surroundings (including Kathmandu and the northern Himalaya of China) and caused huge loss of life and considerable destruction of property. The April 25 earthquake occurred on the active tectonic arc of the Himalaya, which defines the subduction thrust interface between the Indian and the Eurasian plates. Across the Nepalese Himalaya, the northward motion of the Indian Plate relative to the Eurasian Plate is estimated to be about 40 mm/yr, which results from the northward under thrusting of the India Plate beneath the Eurasian Plate. The convergence between India and the Himalaya proceeds at a rate of about 18 mm/yr (Bilham et al., 1997; Bettinelli et al., 2006; Ader et al., 2012). The continental collision of the Indian and the Eurasian plates generates numerous and frequent earthquakes, and makes this area one of the most seismically hazardous regions in the world (Figure 1). The ongoing collision between the Indian and the Eurasian plates has built the Tibetan Plateau (the highest plateau region on Earth) and has induced the imbricate thrust belt as the plate boundary. From south to north, the major faults comprise the Main Frontal Thrust fault (MFT), the Main Boundary Thrust fault (MBT), and the Main Central Thrust fault (MCT). In deeper depth, the fold-thrust belt is connected with the Main Himalaya Thrust fault (MHT) with a low dip angle (Cattin and Avouac, 2000; Lavé and Avouac, 2000; Bettinelli et al., 2006). Research of historical earthquakes and GPS measurements has revealed the high-risk potential of generating great earthquakes in the Nepalese Himalaya (Bilham et al., 1998; Ambraseys and Douglas, 2004; Ader et al., 2012; Sapkota et al., 2013). Based on the convergence rate between the Indian and Eurasian Plates, and on historical seismicity, Ader et al. (2012) warned of the high possibility of the occurrence of a large earthquake in the central Nepal seismic gap; the occurrence of the April 25 earthquake confirmed this concern. Following the earthquake, preliminary results of the source mechanism and rupture process for this earthquake were prepared using the fast source inverse approach with real-time far-field seismograms (http://www.itpcas.ac.cn/ xwzx/zhxw/201504/t20150426_4344080.html). Here, a listric finite fault model is constructed to simulate the earthquake fault according to the tectonic setting. A new source process model is estimated by joint inversion of far-field seismograms and GPS coseismic displacements. The inverted results might help both in disaster mitigation and in research into seismotectonic and dynamic simulations of this region.

Slip history of the 2016 Mw 7.0 Kumamoto earthquake: Intraplate rupture in complex tectonic environment

Rupture history of the 2016 Mw 7.0 Kumamoto earthquake is constrained by using the waveforms of strong motion observations, teleseismic broadband body waves, and long-period surface waves. Its fault geometry is modeled with Hinagu (orienting 205° and dipping 73°) and Futagawa (orienting 235° and dipping 60°), two segments. The result reconciles the difference between moment tensor solutions and the surface fault trace. It reveals a complex rupture process that initiated on the Hinagu segment in dextral motion, propagated northeastward unilaterally, and after 15 s ceased near Aso volcano with normal fault motion. The average slip, rise time, and slip rate are 1.8 m, 2.0 s, and 1.2 m/s, respectively. The rupture broke through an ~30° fault intersection without notable delay, which can be a result of dynamic “unclamping.” The northeast boundary of the largest asperity might mark the bottom of the seismogenic zone, which becomes shallower gradually near Aso volcano.

Determination of regional earthquake source parameters in wavelet domain

Investigating source parameters of small and moderate earthquakes plays an important role in seismology research. For small and moderate earthquakes, the mechanisms are usually obtained by first motion of P-Wave, surface wave spectra method in frequency-domain or the waveform inversion in time-domain, based on the regional waveform records. We applied the wavelet domain inversion method to determine mechanism of regional earthquake. Using the wavelet coefficients of different scales can give more information to constrain the inversion. We determined the mechanisms of three earthquakes occurred in California, the United States. They are consistent with the previous results (Harvard Centroid Moment Tensor and United States Geological Service). This proves that the wavelet domain inversion method is an efficient method to determine the source parameters of small and moderate earthquakes, especially the strong aftershocks after a large, disastrous earthquake.

Restoration of clipped seismic waveforms using projection onto convex sets method

The seismic waveforms would be clipped when the amplitude exceeds the upper-limit dynamic range of seismometer. Clipped waveforms are typically assumed not useful and seldom used in waveform-based research. Here, we assume the clipped components of the waveform share the same frequency content with the un-clipped components. We leverage this similarity to convert clipped waveforms to true waveforms by iteratively reconstructing the frequency spectrum using the projection onto convex sets method. Using artificially clipped data we find that statistically the restoration error is ~1% and ~5% when clipped at 70% and 40% peak amplitude, respectively. We verify our method using real data recorded at co-located seismometers that have different gain controls, one set to record large amplitudes on scale and the other set to record low amplitudes on scale. Using our restoration method we recover 87 out of 93 clipped broadband records from the 2013 Mw6.6 Lushan earthquake. Estimating that we recover 20 clipped waveforms for each M5.0+ earthquake, so for the ~1,500 M5.0+ events that occur each year we could restore ~30,000 clipped waveforms each year, which would greatly enhance useable waveform data archives. These restored waveform data would also improve the azimuthal station coverage and spatial footprint.

Seismic characteristics of the 15 February 2013 bolide explosion in Chelyabinsk, Russia

The seismological characteristics of the 15 February 2013 Chelyabinsk bolide explosion are investigated based on seismograms recorded at 50 stations with epicentral distances ranging from 229 to 4324 km. By using 8–25 s vertical‐component Rayleigh waveforms, we obtain a surface‐wave magnitude of 4.17±0.31 for this event. According to the relationship among the Rayleigh‐wave magnitude, burst height and explosive yield, the explosion yield is estimated to be 686 kt. Using a single‐force source to fit the observed Rayleigh waveforms, we obtain a single force of 1.03×1012 N, which is equivalent to the impact from the shock wave generated by the bolide explosion.

Preliminary results for the rupture process of Jan. 10, 2018, Mw7.6 earthquake at east of Great Swan Island, Honduras

30◦ < ∆ < 90◦ At UTC 2018-01-10 02: 51: 31, an Mw7.6 earthquake occurred 44 km east of Great Swan Island, Honduras (location 17.469 °N, 83.520 °W, depth 10 km, according to the United States Geological Survey). We carried out studies of the focal mechanism and rupture process of the earthquake, using seismic data from the IRIS data center. For the focal mechanism solution, a point source model was used to invert 26 far-field P-waveforms and 26 SHwaveforms with high S/N ratio and relatively even azimuth coverage (epicentral distance ); then the result (Figure 1) was used to construct a finite fault model for rupture process inversion (Yao ZX and Ji C, 1997; Wang WM et al., 2008), resulting in a preliminary model of the slip distribution of this earthquake (Figures 2–4). The calculated seismic moment is 2.41×1020 N·m and the estimated earthquake magnitude Mw=7.5. The maximum slip is about 1900 cm.

Preliminary result for the rupture process of Nov.13, 2017, Mw7.3 earthquake at Iran‐Iraq border

30◦ < ∆ < 90◦ At UTC 2017-11-12 18:18:17, an Mw7.3 earthquake occurred at the border between Iran and Iraq (location 34.886°N, 45.941°E, depth 23 km according to USGS). We carried out focal mechanism and rupture process studies with the data from IRIS data center, using 26 far-field P-waveforms and 25 SH-waveforms with high S/N ratio and relatively even azimuth coverage (epicentral distance ) in a point source model to invert for the focal mechanism solution; the result (Figure1) was used to construct a finite fault model for rupture process inversion (Yao and Ji,1997; Wang et al., 2008), resulting in a preliminary slip distribution of this earthquake (Figures 2-4). The calculated seismic moment is 1.1×1020 N·m, Mw=7.3. The maximum slip is about 700 cm.

Globally optimized Chebyshev Fourier propagator: A wide‐angle dual‐domain one‐way method

We present a dual-domain one-way propagator using Chebyshev polynomials and global optimization. First, the square-root operator is approximated using Taylor expansion around the reference background velocity. Then, the first-kind Chebyshev polynomials are used to rearrange the partial derivative coefficients. Finally, the constant coefficients are optimized using simulating annealing by the globally-optimized scheme. We demonstrate the proposed method using theoretical error analyses and impulse responses. For various velocity contrasts, the accurate propagation angle of our method is about 60°, which allow us to handle wide-angle propagations and strong lateral velocity contrast simultaneously by purely using Fourier transform. Only four 2D Fourier transforms are required for each step of 3D wavefield extrapolation. Compared with globally optimized Fourier finite-difference method, this method has no splitting error for 3D cases and no numerical dispersion.