2018 Vol.2(2)

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Chinese ionospheric investigations in 2016–2017
LiBo Liu, WeiXing Wan
2018, (2): 89-111. doi: 10.26464/epp2018011
[Abstract](400) [FullText HTML] (168) [PDF 9468.6KB](45)
After the release of the previous report to the Committee on Space Research (COSPAR) on progress achieved by Chinese scientists in ionospheric researches (Liu LB and Wan WX, 2016), in the recent two years (2016–2017) many interesting new investigations into various ionospheric related issues have been completed. In this report, about 100 publications are summarized. The topics highlighted are as follows: Ionospheric space weather, ionospheric dynamics, ionospheric climatology and modelling, ionospheric irregularity and scintillation, Global Navigation Satellite System (GNSS) related ionospheric issues and other techniques, and radio wave propagation in the ionosphere. An outstanding feature is that more and more observations from the Meridional Project supported the ionospheric investigations.
A statistical study of the likelihood of a super geomagnetic storm occurring in a mild solar cycle
Bin Zhuang, YuMing Wang, ChengLong Shen, Rui Liu
2018, 2(2): 112-119. doi: 10.26464/epp2018012
[Abstract](265) [FullText HTML] (97) [PDF 533.13KB](17)
The activities of geomagnetic storms are generally controlled by solar activities. The current solar cycle (SC) 24 is found to be mild; compared to SCs 19–23, the storm occurrence and size derived by averaging the occurrence number and Dst around the solar maximum are reduced by about 50–82% and 36–61%, respectively. We estimate separately, for SC 19 to 24, the repeat intervals between geomagnetic storms of specific Dst, based on fits of power-law and log-normal distributions to the storm data for each SC. Repeat intervals between super geomagnetic storms with Dst≤–250 nT are found to be 0.36–2.95 year(s) for SCs 19–23, but about 20 years based on the data for SC 24. We also estimate the repeat intervals between coronal mass ejections (CMEs) of specific speed (VCME) since CMEs are known to be the main drivers of intense storms and the related statistics may provide information about the potential occurrence of super geomagnetic storms from the location of the Sun. Our analysis finds that a CME with VCME≥1860 km/s may occur once per 3 and 5 months in SC 23 and 24, respectively. Based on a VCME-Dst relationship, such a fast CME may cause a storm with Dst=–250 nT if arriving at the Earth. By comparing the observed geomagnetic storms to storms expected to be caused by CMEs, we derive the probability of CME caused storms, which is dependent on VCME. For a CME faster than 1860 km/s, the probability of a CME caused storm with Dst≤–250 nT is about 1/5 for SC 23 or 1/25 for SC 24. All of the above results suggest that the likelihood of the occurrence of super geomagnetic storms is significantly reduced in a mild SC.
Co-seismic slip distribution of the 2011 Tohoku (MW 9.0) earthquake inverted from GPS and space-borne gravimetric data
Xin Zhou, Gabriele Cambiotti, WenKe Sun, Roberto Sabadini
2018, 2(2): 120-138. doi: 10.26464/epp2018013
[Abstract](264) [FullText HTML] (87) [PDF 7285.66KB](34)
Data obtained by GRACE (Gravity Recovery and Climate Experiment) have been used to invert for the seismic source parameters of megathrust earthquakes under the assumption of either uniform slip over an entire fault or a point-like seismic source. Herein, we further extend the inversion of GRACE long-wavelength gravity changes to heterogeneous slip distributions during the 2011 Tohoku earthquake using three fault models: (I) a constant-strike and constant-dip fault, (II) a variable dip fault, and (III) a realistically varying strike fault. By removing the post-seismic signal from the time series, and taking the effect of ocean water redistribution into account, we invert for slip models I, II, and III using co-seismic gravity changes measured by GRACE, de-striped by DDK3 decorrelation filter. The total seismic moments of our slip models, with respective values of 4.9×1022 Nm, 5.1×1022 Nm, and 5.0×1022 Nm, are smaller than those obtained by other studies relying on GRACE data. The resulting centroids are also located at greater depths (20 km, 19.8 km, and 17.4 km, respectively). By combining onshore GPS, GPS-Acoustic, and GRACE data, we obtain a jointly inverted slip model with a seismic moment of 4.8×1022 Nm, which is larger than the seismic moment obtained using only the GPS displacements. We show that the slip inverted from low degree space-borne gravimetric data, which contains information at the ocean region, is affected by the strike of the arcuate trench. The space-borne gravimetric data help us constrain the source parameters of a megathrust earthquake within the frame of heterogeneous slip models.
The 13 November 2016 Kaikoura, New Zealand earthquake: rupture process and seismotectonic implications
Yi-Ching Lo, Li Zhao, XiWei Xu, Ji Chen, Shu-Huei Hung
2018, 2(2): 139-149. doi: 10.26464/epp2018014
[Abstract](298) [FullText HTML] (166) [PDF 4939.23KB](36)
The 13 November 2016 Kaikoura earthquake occurred in the northeastern coastal region of the South Island, New Zealand. The Mw 7.8 mainshock generated a complex pattern of surface ruptures, and was followed within about 12 hours by three moderate shocks of Mw ≥ 6.0. Here we use teleseismic waveforms to invert for the source rupture of the Kaikoura earthquake. The resulting slip-distribution model exhibits insignificant slip near the hypocenter and three pockets of major slip zones with distinct senses of motion. The mainshock started from a rupture near the hypocenter, grew into thrust on shallow crustal faults ~50 km northeast of the hypocenter, and then developed into two slip zones: a deeper one with oblique thrust and a shallower one with almost purely right-lateral strike-slip. Locations of the thrust and strike-slip motions in the slip-distribution model agree well with reported coastal uplifts and horizontal offsets. The overall slip pattern is dominated by horizontal motion, especially at shallow depth, due to the partitioning of thrust and strike-slip motions above the subduction zone megathrust. Aftershock distribution suggests that most aftershocks tend to occur near the edges of the major slip zones of the mainshock. This observation on aftershock locations may provide useful information for seismic hazard assessments after large earthquakes.
A test on methods for MC estimation based on earthquake catalog
YiJian Zhou, ShiYong Zhou, JianCang Zhuang
2018, 2(2): 150-162. doi: 10.26464/epp2018015
[Abstract](228) [FullText HTML] (99) [PDF 892.13KB](27)
This study tested five methods widely used in estimating the complete magnitudes (MC) of earthquake catalogs. Using catalogs of observed earthquake properties, we test the performance of these five algorithms under several challenging conditions, such as small volume of events and spatial-temporal heterogeneity, in order to see whether the algorithms are stable and in agreement with known data. We find that the maximum curvature method (MAXC) has perfect stability, but will significantly underestimate MC unless heterogeneity is absent. MC estimated by the b-value stability method (MBS) requires many events to reach a stable result. Results from the goodness of fit method (GFT) were unstable when heterogeneity lowered the fitness level. The entire magnitude range method (EMR) is relatively stable in most conditions, and can reflect the change in MC when heterogeneity exists, but when the incomplete part of the earthquake catalog is dismissed, this method fails. The median-based analysis of the segment slope method (MBASS) can tolerate small sample size, but is incapable of reflecting the missing degree of small events in aftershock sequences. In conditions where MC changes rapidly, such as in aftershock sequences, observing the time sequence directly can give a precise estimation of the complete sub-catalog, but only when the number of events available for study is large enough can the MAXC, GFT, and MBS methods give a similarly reliable estimation.
Which velocity model is more suitable for the 2017 MS7.0 Jiuzhaigou earthquake?
LiSheng Xu, Xu Zhang, ChunLai Li
2018, 2(2): 163-169. doi: 10.26464/epp2018016
[Abstract](163) [FullText HTML] (69) [PDF 1601.57KB](20)
On Aug. 8, 2017, an MS7.0 earthquake struck Jiuzhaigou, a county of Sichuan province, China. A number of investigations and studies have been conducted, some of which involved local velocity models. However, the suitability of these models has not been properly addressed. Here we collect 11 already-existing models, including those used in studies of the 2017 MS7.0 Jiuzhaigou earthquake, choose 10 local stations surrounding the earthquake, and employ the same technique (TRIT) to relocate the hypocenter. And furthermore, we choose a more suitable model from the 11 already-existed models by analyzing the relocation process and the relocated results for reasonability. Finally, our conclusion is that the model Fang2018 is more suitable and the hypocenter parameters, 103.801°E, 33.192°N and 15.8 km for longitude, latitude and depth, respectively, and 2017-08-08 13:19:46.66 for its origin time, based on this model should be recommended for the 2017 MS7.0 Jiuzhaigou earthquake.
A preliminary analysis of the Shangri-La Bolide on 2017 Oct 4
Quan-Zhi Ye
2018, (2): 170-172. doi: 10.26464/epp2018017
[Abstract](218) [FullText HTML] (187) [PDF 1154.67KB](16)