摘要
由于印度-欧亚板块碰撞,位于板块边界带的喜马拉雅地区大震频繁,但对其活动性的认识仍十分有限.2015年4月25日尼泊尔中东部地区时隔80年再次发生8级地震,为研究板缘地震提供了一次难得机遇.本文用西藏和尼泊尔的GPS连续观测数据和全球分布的远震地震波记录联合反演此次特大地震的破裂过程,结果显示此次地震发生在印度板块与青藏高原接触边界面——喜马拉雅主滑脱断层上.北倾11°、近东西（295°）走向的断层面破裂约100km长（博卡拉到加德满都）,130km宽（从加德满都深入我国西藏吉隆县）,破裂以逆冲滑动为主,平均幅度达到2.4m,释放的地震矩高达9.4×1020 N·m.反演结果还显示,震源体主要破裂分布深度范围为5~25km,应无地表破裂,属于一次盲地震.基于GPS资料推测的地壳现今运动速率及1833年地震的震源位置,我们推测地震在此次地震破裂区域复发的周期可能为150~200a,而极震区以南的深部滑脱断层仍保持闭锁,未来仍有导致灾害性大震的可能性.
On 25 April 2015,a devastating（MS8.1）earthquake struck the central Nepal,causing severe damages in Kathmandu.The earthquake is believed to occur on a basal detachment faultalong which the Indian plate plunged under Tibet,providing a rare opportunity to understand seismicity of the continental plate boundary.Strong ground motions and permanent surface displacements induced by this event were observed unprecedentedly by continuous GPS networks in Nepal and Tibet,and these geodetic observations close to the rupture zone are important as such when a finite fault model of rupture is constructed to characterize rupture processes and source properties.In this work,we focus on retrieving the slip distribution and temporal history of this earthquake through a joint inversion of teleseismic waveforms and near-field GPS data.We derived 12 static coseismic offsets of GPS sites in Nepal and Tibet and retrieved 5seismograms of strong motions recorded by high-rate（1Hz）GPS sites in Tibet.In addition,we chose a total of 43P-wave waveforms from global seismic networks to enhance the spatiotemporal resolution of source model.The fault geometry is prescribed on a subsurface plane that is buried at 5~30km depths with a dip of 11°to the north and a strike of 295°,consistent with the USGS CMT solution and structural geology.This rectangular model plane in dimensions of 210km×160km was further divided into 21×8matrix of sub-faults.The finite source modeling assumes that the rupture processes can be approximated by abrupt rise of slip on these subfaults in the wake of rupture front that passages successively through them from the hypocenter.The rupture velocity across adjacent subfauts is assumed to be a constant at 2.5km·s-1.For each subfault,the slip growth is represented by a source time function that is parameterized by 5overlapping triangles with a 2sec half-time duration,each shifted by 2sec.Seismic moments of all triangles,each corresponding to a subevent,are unknown parameters to be solved with the non-negative least squares algorithm.The slip magnitude,rake and rise time for each subfault are derived from the estimates of the associated subevents,all together to minimize postfit residuals of the waveforms and static offsets while maintaining smoothness of seismic moment over the model plane for which a Laplace operator is used to achieve spatial regularization.Green′s functions were generated assuming a one-dimensional structure model. The frequency-wavenumber integration algorithm was used for GPS dynamic waveforms and static offsets,and a reflectivity method developed by Kikuchi for teleseismic data.The joint inversion shows that the detachment fault fails unilaterally from the hypocenter with slip extending eastward over an area of 100 km in along-strike length by 130 km in downdip width.The best-fitting model indicates that the seismic moments were released largely by thrusting motions within duration of 80 sec.In the first 40 sec,slip propagated essentially all the way to the Kathmandu.The slip model shows one major asperity between the hypocenter and Kathmandu,on which a peak slip of 4.3 mis found at 11 km depth,35 km away from the hypocenter.During 40 to 75sec,the rupture extends downward to the bottom of the model plane and slip attains the local maximum at 18 km depth and 60 km away from the hypocenter.Slip of0.5~1.0mis found at 25~30km depths beneath the Gyrong town.Slip continues also upward but stops approximately at 5 km shallow depth.The slip model does not indicate that the earthquake has broken the surface,suggesting that a significant fraction of the basal detachment fault remain locked at shallower depths.The unlocked part of the detachment fault has yielded an averaged slip of 2.4m with a total seismic moment of 9.4×1020 N·m,which gives MW=7.9.If the asperity of this event corresponds to the rupture zone of the 1833 MW7.7~7.8,its recurrence in the same rupture area would be every 150~200year.
出处
《地球物理学报》
SCIE
EI
CAS
CSCD
北大核心
2015年第11期4287-4297,共11页
Chinese Journal of Geophysics
基金
中国地震局地震研究所所长基金(IS201506204,IS201326127)
国家自然科学基金项目(41274027,41404016,41104024,41504011,41574017,41541029)联合资助
关键词
尼泊尔MS8.1地震
破裂模型
时空分布
GPS与远震体波联合反演
MS8.1Nepal earthquake
Slip model
Spatial-temporal rupture process
Joint inversion of GPS and teleseismic waveforms
