Joint Inversion of Rupture across a Fault Stepover during the 8 August 2017 Mw 6.5 Jiuzhaigou, China, Earthquake

2021 ◽  
Author(s):  
Yong Zhang ◽  
Wanpeng Feng ◽  
Xingxing Li ◽  
Yajing Liu ◽  
Jieyuan Ning ◽  
...  
Keyword(s):  
Joint Inversion ◽  
Strong Motion ◽  
Focal Mechanisms ◽  
High Rate ◽  
Fault System ◽  
Interferometric Synthetic Aperture Radar ◽  
Coseismic Slip ◽  
Jiuzhaigou Earthquake ◽  
Rupture Model ◽  
The Right

Abstract The 8 August 2017 Mw 6.5 Jiuzhaigou earthquake occurred in a tectonically fractured region in southwest China. We investigate the multifault coseismic rupture process by jointly analyzing teleseismic, strong-motion, high-rate Global Positioning System, and Interferometric Synthetic Aperture Radar (InSAR) datasets. We clearly identify two right-stepping fault segments and a compressional stepover based on variations in focal mechanisms constrained by coseismic InSAR deformation data. The average geometric parameters of the northwest and southeast segments are strike = 130°/dip = 57° and strike = 151°/dip = 70°, respectively. The rupture model estimated from a joint inversion of the seismic and geodetic datasets indicates that the rupture initiated on the southeastern segment and jumped to the northwestern segment, resulting in distinctive slip patches on the two segments. A 4-km-long coseismic slip gap was identified around the stepover, consistent with the aftershock locations and mechanisms. The right-stepping segmentation and coseismic rupture across the compressional stepover exhibited by the 2017 Jiuzhaigou earthquake are reminiscent of the multifault rupture pattern during the 1976 Songpan earthquake sequence farther south along the Huya fault system in three successive Ms∼7 events. Although the common features of fault geometry and stepover may control the similarity in event locations and focal mechanisms of the 2017 and 1976 sequences, the significantly wider (~15 km) stepover in the 1976 sequence likely prohibited coseismic rupture jumping and hence reduced seismic hazard.

Slip Complementarity and Triggering between the Foreshock, Mainshock, and Afterslip of the 2019 Ridgecrest Rupture Sequence

2020 ◽  
Vol 110 (4) ◽  
pp. 1701-1715 ◽  
Author(s):  
Qiang Qiu ◽  
Sylvain Barbot ◽  
Teng Wang ◽  
Shengji Wei
Keyword(s):  
Joint Inversion ◽  
Stress Transfer ◽  
Strong Motion ◽  
Global Navigation Satellite Systems ◽  
Fault System ◽  
Coseismic Slip ◽  
Satellite Systems ◽  
The North ◽  
Global Navigation Satellite ◽  
Lower Crustal

ABSTRACT We investigate the deformation processes during the 2019 Ridgecrest earthquake sequence by combining Global Navigation Satellite Systems, strong-motion, and Interferometric Synthetic Aperture Radar datasets in a joint inversion. The spatial complementarity of slip between the Mw 6.4 foreshock, Mw 7.1 mainshock, and afterslip suggests the importance of static stress transfer as a triggering mechanism during the rupture sequence. The coseismic slip of the foreshock concentrates mainly on the east-northeast–west-southwest fault above the hypocenter at depths of 2–8 km. The slip distribution of the mainshock straddles the region above the hypocenter with two isolated patches located to the north-northwest and south-southeast, respectively. The geodetically determined moment magnitudes of the foreshock and mainshock are equivalent to moment magnitudes Mw 6.4 and 7.0, assuming a rigidity of 30 GPa. We find a significant shallow slip deficit (>60%) in the Ridgecrest ruptures, likely resulting from the immature fault system in which the sequence occurred. Rapid afterslip concentrates at depths of 2–6 km, surrounding the rupture areas of the foreshock and mainshock. The ruptures also accelerated viscoelastic flow at lower-crustal depths. The Garlock fault was loaded at several locations, begging the question of possible delayed triggering.

Comparison of local kinematic rupture joint inversion approaches for tsunami early warning: Examples of the 2017-2020 Mw > 6.3 East Aegean earthquakes

2021 ◽  
Author(s):  
Malte Metz ◽  
Marius Isken ◽  
Rongjiang Wang ◽  
Torsten Dahm ◽  
Haluk Özener ◽  
...  
Keyword(s):  
Early Warning ◽  
Joint Inversion ◽  
Moment Tensor ◽  
Strong Motion ◽  
Aegean Sea ◽  
High Rate ◽  
Source Inversion ◽  
Dynamic Rupture ◽  
Centroid Moment Tensor ◽  
Tsunami Early Warning

<p>The fast inversion of reliable centroid moment tensor and kinematic rupture parameters of earthquakes occurring near coastal margins is a key for the assessment of the tsunamigenic potential and early tsunami warning (TEW). In recent years, more and more multi-channel seismic and geodetic online station networks have been built-up to improve the TEW, for instance the GNSS and strong motion networks in Italy, Greece, and Turkey, additionally to the broadband seismological monitoring. Inclusion of such data for the fast kinematic source inversion can improve the resolution and robustness of its’ solutions. However, methods have to be further developed and tested to fully exploit the potential of such rich joint dataset.</p><p>In this frame, we compare and test two in-house developed, kinematic / dynamic rupture inversion methods which are based on completely different approaches. The IDS (Iterative Deconvolution and Stacking, Zhang et al., 2014) combines an iterative seismic network inversion with back projection techniques to retrieve subfault source time functions. The pseudo dynamic rupture model (Dahm et al., in review) links the rupture front propagation estimate based on the Eikonal equation with the dislocation derived from a boundary element method to model dislocation snapshots. We used the latter in both a fast rupture estimate and a fully probabilistic source inversion.</p><p>We use some Mw > 6.3 earthquakes that occurred in the coastal range of the Aegean Sea as an example for comparison: the Mw 6.3 Lesbos earthquake (12 June 2017), the Mw 6.6 Bodrum earthquake (20 July 2017), and the recent Mw 7.0 earthquake which occurred at Samos on 30 October 2020. The latter earthquake and the resulting tsunami caused fatalities and severe damage at the shorelines of Samos and around the city of Izmir, Turkey.<br>The fast estimates are based on only little data and/or prior information obtained from the regional seismicity catalogue and available active fault information. The large number of seismic (broadband, strong motion) and geodetic (high-rate GNSS) stations in local and regional distance from the earthquake with good azimuthal coverage jointly inverted with InSAR data allows for robust inversion results. These, and other solutions, are used as a reference for the comparison of our fast source estimates.<br>Preliminary results of the slip distribution and the source time function are in good agreement with modelling results from other authors.</p><p>We present our insights into the kinematics of the chosen earthquakes investigated by means of joint inversions. Finally, the accuracy of our first fast source estimates, which could be of potential use in tsunami early warning, will be discussed.</p>

Confirmation and Characterization of the Rupture Model of the 2017 Ms 7.0 Jiuzhaigou, China, Earthquake

2021 ◽  
Author(s):  
Xu Zhang ◽  
Li-Sheng Xu ◽  
Lei Yi ◽  
Wanpeng Feng
Keyword(s):  
Strong Motion ◽  
Early Time ◽  
Pulse Mode ◽  
P Wave ◽  
Fault System ◽  
Seismic Gap ◽  
Depth Range ◽  
Seismogenic Fault ◽  
Slip Pulse ◽  
Rupture Model

Abstract On 8 August 2017, an Ms 7.0 earthquake struck the Jiuzhaigou town, Sichuan Province, China, rupturing an unmapped fault, which is adjacent to the Maqu seismic gap in the Min Shan uplift zone in the easternmost part of the Bayan Har block. Having summarized the previous studies on the source of this earthquake, we confirmed the rupture model by jointly inverting the teleseismic P-wave and SH-wave data, Interferometric Synthetic Aperture Radar line-of-sight displacement data, and the near-field seismic and strong-motion data, a most complete dataset until now. The confirmation showed that a scalar seismic moment of 6.6×1018  N·m was released (corresponding to a moment magnitude of Mw 6.5), and 95% of the release occurred in the first 10 s. The slip area was composed of two asperities, with a horizontal extension of ∼20  km and a depth range of ∼2–15  km. A bilateral extending occurred at shallow depths, but the rupturing upward from deep depth dominated in the early time. The rupture process was found generally featuring the slip-pulse mode, which was related to the weak prestress condition. The aftershocks almost took place in gaps of the mainshock slip because of the coulomb stress change. Combining the aftershock relocations, aftershock focal mechanism solutions, and our confirmed rupture model, we suggest that the seismogenic fault was a northward extension of the mapped Huya fault. The occurrence of this earthquake made the Maqu seismic gap at a higher level of seismic risk, in addition to the moderate to high strain accumulation on the easternmost tip of the Kunlun fault system and the weak lower crust below.

scholarly journals Interplay of seismic and a-seismic deformation during the 2020 sequence of Atacama, Chile.

2021 ◽  
Author(s):  
Emilied Klein ◽  
Bertrand Potin ◽  
Francisco Pasten-Araya ◽  
Roxane Tissandier ◽  
Kellen Azua ◽  
...  
Keyword(s):  
Broad Band ◽  
Strong Motion ◽  
Velocity Model ◽  
High Rate ◽  
Aseismic Slip ◽  
Coseismic Slip ◽  
Narrow Strip ◽  
Rupture Area ◽  
3D Velocity Model ◽  
Largest Aftershock

An earthquake sequence occurred in the Atacama region of Chile throughout September 2020. The sequence initiated by a mainshock of magnitude Mw6.9, followed 17 hours later by a Mw6.4 aftershock. The sequence lasted several weeks, during which more than a thousand events larger than Ml 1 occurred, including several larger earthquakes of magnitudes between 5.5 and 6.4. Using a dense network that includes broad-band, strong motion and GPS sites, we study in details the seismic sources of the mainshock and its largest aftershock, the afterslip they generate and their aftershock, shedding light of the spatial temporal evolution of seismic and aseismic slip during the sequence. Dynamic inversions show that the two largest earthquakes are located on the subduction interface and have a stress drop and rupture times which are characteristics of subduction earthquakes. The mainshock and the aftershocks, localised in a 3D velocity model, occur in a narrow region of interseismic coupling (ranging 40%-80%), i.e. between two large highly coupled areas, North and South of the sequence, both ruptured by the great Mw~8.5 1922 megathrust earthquake. High rate GPS data (1 Hz) allow to determine instantaneous coseismic displacements and to infer coseismic slip models, not contaminated by early afterslip. We find that the total slip over 24 hours inferred from precise daily solutions is larger than the sum of the two instantaneous coseismic slip models. Differencing the two models indicates that rapid aseismic slip developed up-dip the mainshock rupture area and down-dip of the largest aftershock. During the 17 hours separating the two earthquakes, micro-seismicity migrated from the mainshock rupture area up-dip towards the epicenter of the Mw6.4 aftershocks and continued to propagate upwards at ~0.7 km/day. The bulk of the afterslip is located up-dip the mainshock and down-dip the largest aftershock, and is accompanied with the migration of seismicity, from the mainshock rupture to the aftershock area, suggesting that this aseismic slip triggered the Mw6.4 aftershock. Unusually large post-seismic slip, equivalent to Mw6.8 developed during three weeks to the North, in low coupling areas located both up-dip and downdip the narrow strip of higher coupling, and possibly connecting to the area of the deep Slow Sleep Event detected in the Copiapo area in 2014. The sequence highlights how seismic and aseismic slip interacted and witness short scale lateral variations of friction properties at the megathrust.

Complex behaviour highlighted by earthquake aftershock and swarm sequences in Ubaye Region (French Western Alps). 

2021 ◽  
Author(s):  
Marion Baques ◽  
Louis De Barros ◽  
Maxime Godano ◽  
Hervé Jomard ◽  
Clara Duverger ◽  
...  
Keyword(s):  
Fault Plane ◽  
Joint Inversion ◽  
Fluid Pressure ◽  
Western Alps ◽  
Focal Mechanisms ◽  
High Rate ◽  
Double Difference ◽  
Complex Behaviour ◽  
Aftershock Sequences ◽  
Damaged Zone

<p>The Ubaye Region, where the city of Barcelonnette is settled, is the most seismically active region in the French Western Alps since at least two centuries. Seismicity in this area exhibits a dual behaviour, with mainshock-aftershock sequences alternating with abnormally high rate of seismicity associated with seismic swarms. Understanding processes triggering such a peculiar seismic behaviour is of primary importance in order to assess the seismic hazard in this region. The latest swarm activity started on February 26, 2012, with an earthquake of moment magnitude 4.2. It was followed two years later (on April 7, 2014) by a shock of magnitude Mw 4.8. From the first earthquake to the end of 2016, the seismic level has not returned to the background level and shares the same characteristics as a seismic swarm.</p><p>With the aim to discuss the seismogenic processes involved in the area, we focused on the two months following the 2014 mainshock (Mw=4.8). During this period, a dense temporary network (7 stations) was operating at a maximal distance of 10km from the epicentre area. We analysed this period starting with a double-difference relocation of ~ 6,000 earthquakes previously detected by template-matching. These hypocentres did not align on the fault plane of the 2014 mainshock, but on conjugated structures belonging to the 2-km wide damaged zone of the main fault plane and on remote structures with various orientations further away. We then computed 99 focal mechanisms from a joint inversion of P polarity and S/P ratio to clarify the geometry of the active structures. Many nodal planes are inconsistent with the structures deduced from the alignments of the earthquake locations. The stress-state orientation obtained from those focal mechanisms (σ<sub>1</sub> trending N27°± 5°, plunging 50°± 9°, a σ<sub>2</sub> trending N215°± 5°, plunging 40°± 9°, and a sub-horizontal σ<sub>3</sub> trending N122°± 3°) is consistent with those previously calculated in the area (Fojtíková and Vavryčuk, 2018). Nevertheless, some structures are unfavourably oriented to slip within this stress-field, suggesting that additional processes are required to explain them. As the presence of fluids was highlighted for the 2003-2004 and the 2012-2015 crisis, we calculated the fluid pressure needed to trigger slip on the planes from the focal mechanisms using Cauchy's equation. We found that a median fluid-overpressure of ~20 MPa (range between 0 to 50 MPa) is needed to cause slip.  Although the origin of fluids and how they are pressurized at depth remains open. The fluid processes seem to be the most favourable additional processes and were also proposed to explain the 2003-2004 crisis.</p>

Locations and focal mechanisms of recent microearthquakes and tectonics of Livermore Valley, California

1982 ◽  
Vol 72 (3) ◽  
pp. 821-840
Author(s):  
Fred E. Followill ◽  
Joseph M. Mills
Keyword(s):  
Linear Trend ◽  
Source Region ◽  
Velocity Model ◽  
Strike Slip ◽  
Focal Mechanisms ◽  
Fault System ◽  
Lateral Velocity ◽  
Thrust Faulting ◽  
Northern Regions ◽  
The Right

abstract Over 100 well-recorded microearthquakes (local magnitude less than 3), which occurred between January and September 1980, have been relocated, and focal mechanisms have been estimated for six regions of recent seismic activity for the Livermore Valley. Each of these microearthquakes had a minimum of 10 stations distributed in at least three quadrants around the epicenter. Data from these microearthquakes were combined with data from three USGS refraction experiments and used to develop a velocity model for Livermore Valley. Using this model, together with source region and station corrections to compensate for lateral velocity variations in the upper crust, we recalculated the locations and focal mechanisms. Comparing the results from these regions, we found differences in focal depths, patterns of epicenter locations, and focal mechanisms. Focal depths in the northern regions were usually between 5 and 11 km. These were slightly greater than focal depths (2 to 8 km) in the southern regions. The pattern of strike-slip focal mechanisms with P axes trending NNE and the linear trend of epicenters along the right-lateral strike-slip Greenville fault system in the northern regions is in contrast with the pattern of a mixture of focal mechanisms in southern regions (which includes about one-third with thrust-type mechanisms where the T axis is nearer vertical than horizontal). In the southern regions, there is some indication of short offset (en echelon) segments and an absence of the extended linear trend found in the northern regions. We speculate that this more diffuse pattern of locations and focal mechanisms in the southern regions of the valley results from general north-south compressional tectonics with both strikeslip and thrust faulting occurring in a localized zone of deformation between the Livermore Valley block and the Diablo Range block to the south.

Using Seismic and Geodetic Observations in a Simultaneous Kinematic Model of the 2019 Ridgecrest, California Earthquakes

2020 ◽  
Author(s):  
Dara Goldberg ◽  
Diego Melgar ◽  
Valerie Sahakian ◽  
Amanda Thomas ◽  
Xiaohua Xu ◽  
...  
Keyword(s):  
Fault Zone ◽  
Kinematic Model ◽  
Plate Boundary ◽  
Slip Rate ◽  
Strong Motion ◽  
Seismic Source ◽  
Global Navigation Satellite Systems ◽  
High Rate ◽  
Fault System ◽  
Source Characterization

<p>The July 4, 2019 Mw6.4 and subsequent July 6, 2019 Mw7.1 Ridgecrest Sequence earthquakes in California, USA, ruptured orthogonal fault planes in the Little Lake Fault Zone, a low slip rate (1 mm/year) dextral fault zone in the region linking the Eastern California Shear Zone (ECSZ) and Walker Lane. This region is tectonically interesting because it accommodates approximately one fourth of plate boundary motion and has been proposed to be an incipient transform fault system that could eventually become the main tectonic boundary, replacing the San Andreas Fault. Additionally, large ruptures of such low slip rate faults are important to understand from the context of seismic hazard. We investigate the interaction within this fault system and demonstrate a novel kinematic slip method that inverts for both earthquakes simultaneously, allowing us to use Interferometric Synthetic Aperture Radar (InSAR) data that spans both earthquakes, along with seismic (strong-motion accelerometer) and geodetic (high-rate Global Navigation Satellite Systems (GNSS) and GNSS static offset) datasets that recorded each earthquake separately. We also present results of Coulomb stress change modeling to evaluate how the Mw6.4 earthquake may have affected the subsequent Mw7.1 event. Our findings suggest a complex rupture process and interactions between several fault structures, including dynamic and static triggering between splays involved in the July 4th Mw6.4 and July 6th Mw7.1 events. The integration of seismic and geodetic datasets provides constraints on rupture continuation through stepovers, as well as important context for regional models of seismic source characterization and hazard.</p>

Automatic Inversion of Rupture Processes of the Foreshock and Mainshock and Correlation of the Seismicity during the 2019 Ridgecrest Earthquake Sequence

2020 ◽  
Vol 91 (3) ◽  
pp. 1556-1566 ◽  
Author(s):  
Yijun Zhang ◽  
Xujun Zheng ◽  
Qiang Chen ◽  
Xianwen Liu ◽  
Xiaomei Huang ◽  
...  
Keyword(s):  
High Reliability ◽  
Strong Motion ◽  
Temporal Correlation ◽  
Earthquake Sequence ◽  
Coseismic Slip ◽  
Aftershock Activity ◽  
Strong Motion Data ◽  
Motion Data ◽  
Coulomb Failure Stress Change ◽  
Rupture Model

Abstract The 2019 Ridgecrest, California, earthquake sequence included an Mw 6.4 foreshock on 4 July, followed by an Mw 7.1 mainshock about 32 hr later. We determined the rupture patterns of the foreshock and mainshock by applying the automatic iterative deconvolution and stacking method to strong-motion records. The foreshock was characterized by a unilateral rupture toward the southwest, and the shallow portion had a relatively large slip with the maximum value of ∼1.4  m. The mainshock presents an asymmetrical bilateral rupture with an average rupture velocity of 2.0  km/s. More than 80% of the seismic moment was released on the northwest segment of the fault, producing a maximum slip of ∼5.2  m. With the two inferred slip models, we calculated the Coulomb failure stress change (ΔCFS) to analyze the spatial–temporal correlation of the seismicity activity in this sequence. The result shows that the epicenter of the Mw 7.1 mainshock was brought 0.4 bars closer to failure by the Mw 6.4 foreshock, and the stress-increased zone has a good spatial consistence with the coseismic slip distribution of the mainshock and the aftershock distribution of the foreshock. Besides, the positive ΔCFS induced by the mainshock also enhanced its aftershock activity, especially at depths of 4–10 km where the major rupture occurred, inferring that the mainshock-induced ΔCFS may be responsible for the occurrence of aftershocks. In addition, we test the effects of different cutoff frequencies and crust velocity structures on the inversion results. The result reveals that the main source rupture characteristics are almost independent of these factors, implying a high reliability of automation inversion of strong-motion data. Overall, this work indicates that automatic inversion of strong-motion data can provide reliable and rapid rupture model, which is essential for earthquake emergency responses and tsunami early warnings.

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