Coseismic Dislocation Research Articles

Preliminary report of the September 5, 2022 MS 6.8 Luding earthquake, Sichuan, China

The 2022 MS 6.8 Luding earthquake is the strongest earthquake in Sichuan Province, Western China, since the 2017 MS 7.0 Jiuzhaigou earthquake. It occurred on the Moxi fault in the southeastern segment of the Xianshuihe fault, a tectonically active and mountainous region with severe secondary earthquake disasters. To better understand the seismogenic mechanism and provide scientific support for future hazard mitigation, we summarize the preliminary results of the Luding earthquake, including seismotectonic background, seismicity and mainshock source characteristics and aftershock properties, and direct and secondary damage associated with the mainshock. The peak ground displacements in the NS and EW directions observed by the nearest GNSS station SCCM are ∼35 mm and ∼55 mm, respectively, resulting in the maximum coseismic dislocation of 20 mm along the NWW direction, which is consistent with the sinistral slip on the Xianshuihe fault. Back-projection of teleseismic P waves suggest that the mainshock rupture propagated toward south-southeast. The seismic intensity of the mainshock estimated from the back-projection results indicates a Mercalli scale of VIII or above near the ruptured area, consistent with the results from instrumental measurements and field surveys. Numerous aftershocks were reported, with the largest being MS 4.5. Aftershock locations (up to September 18, 2022) exhibit 3 clusters spanning an area of 100 km long and 30 km wide. The magnitude and rate of aftershocks decreased as expected, and the depths became shallower with time. The mainshock and two aftershocks show left-lateral strike-slip focal mechanisms. For the aftershock sequence, the b-value from the Gutenberg-Richter frequency-magnitude relationship, h-value, and p-value for Omori’s law for aftershock decay are 0.81, 1.4, and 1.21, respectively, indicating that this is a typical mainshock-aftershock sequence. The low b-value implies high background stress in the hypocenter region. Analysis from remote sensing satellite images and UAV data shows that the distribution of earthquake-triggered landslides was consistent with the aftershock area. Numerous small-size landslides with limited volumes were revealed, which damaged or buried the roads and severely hindered the rescue process.

Research on deformation characteristics of the 2021 Qinghai Maduo MS7.4 earthquake through coseismic dislocation inversion

Over the past 70 years, the Qinghai Maduo Ms7.4 earthquake (May 22, 2021) has been the only earthquake with a magnitude of ≥7 that has occurred within the Bayan Har Block. To investigate the deformation process of this earthquake, we acquired the Interferometric Synthetic Aperture Radar (InSAR) coseismic deformation field based on Sentinel-1 SAR images, which showed that the maximum line of sight (LOS) displacement was ~1 m. By combining this information with the results of our field investigation, the fault slip distributions were obtained by inversion using the single- and double-fault models, wherein the latter model showed better inversion results than that of the former. According to the inversion results, the overall rupture pattern was predominantly left-lateral strike-slip, with a released seismic moment of ~Mw7.5. Specifically, the principal seismogenic fault had a strike of 283°, dip of 91°, and maximum slip of 4.87 m; the branch fault along the eastern segment of the rupture had a strike of 110°, dip of 94°, and maximum slip of 2 m. Conversely, the results derived from the Global Navigation Satellite System (GNSS) strain rate field before the earthquake indicated that the Maduo earthquake occurred in East Kunlun,which is characterized by the largest maximum shear strain rate. The strain accumulation rate (~2.5 × 10−8/yr) was significantly lower than that of the Xianshuihe fault zone on the southern boundary of the Bayan Har block and the East Kunlun fault zone on its northern boundary. Furthermore, pre-seismic deformation near the seismogenic fault was relatively gentle, showing partial late seismogenic characteristics.

Unique Pre-Earthquake Deformation Patterns in the Spatial Domains from GPS in Taiwan

Most earthquakes are considered to be caused by stress accumulating in and subsequently releasing from the crust. To extract non-linear and non-stationary earthquake-induced signals associated with stress accumulation, the Hilbert–Huang transform was utilized to filter long-term movements, short-term noise, and frequency-dependent (annual and semi-annual) variations from surface displacements measured by the global positioning system (GPS) in Taiwan. Earthquake-related surface displacements were expressed as horizontal directions (i.e., GPS azimuths) using the north–south and east–west components of residual GPS data to bypass influences resulted from the inhomogeneous nature of the crust. Analytical results showed that the relationships between earthquake occurrence and the aligned GPS azimuth passed the statistical test of the Molchan’s error diagram. Aligned GPS azimuths were in agreement with direction of earthquake-related P axes for 81% (26/32) studied events. Areas with the highest paralleling orientations of GPS azimuths appeared around epicenters several days to weeks before earthquake occurrence. Durations from aligned GPS azimuths to earthquake occurrence are roughly proportional to earthquake magnitude. Similar variations of the GPS azimuths were observed in GPS data containing or excluding co-seismic dislocation (i.e., one day before) in the temporal and spatial domain. These suggest that the aligned GPS azimuth could be a promising anomalous phenomenon for studying crustal deformation before earthquakes.

Numerical simulation study on the impact of the Wenchuan earthquake to the seismic environment of the Lushan earthquake

The finite element numerical simulation method was used to investigate the impact of the Wenchuan earthquake on the seismic environment of the southern Longmenshan Mountain. The three-dimensional viscoelastic geological model, including the Longmenshan front-range fault, central fault, back-range fault, the Bayan Kala block, and the South China block, was constrained by the results of the geological survey, coseismic dislocation, and the GPS observation of Wenchuan and Lushan earthquakes. We studied how the calculation results were influenced by gravity potential energy caused by high altitude topography, the low velocity layer in the middle crust and the rupture of the Beichuan-Yingxiu and Jiangyou-Guanxian faults, by modifying the parameters of a model. The Wenchuan earthquake was simulated by reducing the Young’s modulus of the possible generating fault. We calculated the Coulomb stress of the Dayi blind fault, Dachuang-Shuangshi fault, and the “Y”-shaped blind thrust fault between the two faults above. The simulation results show that the Coulomb stress has a similar distribution on the fault planes of the southern Longmenshan faults zone caused by the Wenchuan earthquake, considering the low-velocity layer or the double-rupture surface. The Coulomb stress on the top of the Dachuan-Shuangshi fault surface was about 0 MPa, and it gradually reduced towards the southwest direction. At the source of the Lushan earthquake, the Coulomb stress was about −0.02 MPa. For the west branch of “Y”-shaped fault, the Coulomb stress on the fault surface was significantly reduced and was about −0.02–−0.03 MPa at the source of the Lushan earthquake; after the Wenchuan earthquake, the Coulomb stress on the east branch of the “Y”-shaped fault plane was about −0.01–0.03 MPa; for the Dayi fault, the Coulomb stress was increasingly obvious on the fault surface from the southwest to the northwest after the Wenchuan earthquake. Therefore, the Wenchuan earthquake could reduce the risk of an earthquake on the Dachuan-Shuangshi fault and the west branch of “Y”-shaped fault earthquake, but it increases the risk on the east branch of “Y”-shaped fault and the Dayi fault. The maximum Coulomb stress near the source was located on the Dayi fault. Considering the terrain and the simultaneous failure of the Beichuan-Yingxiu and Jiangyou-Guanxian, the presence of the low velocity layer increases the Coulomb stress on the four faults, and obviously increases towards the east branch of the “Y”-shaped fault and the Dayi fault. If the terrain and the low velocity layer are considered, compared with only the ruptured Beichuan-Yingxiu fault, the Beichuan-Yingxiu and Jiangyou-Guanxian faults’ simultaneous rupture increased the risk of earthquakes on the Dachuan-Shuangshi fault, the east branch of “Y”-shaped fault, and the Dayi hidden fault, but reduced the risk of the west branch of “Y” type fault. In any of the above models, the Coulomb stress changes on the four fault planes of southern Longmenshan Mountains were influenced by terrain. In the case of certain crust, mantle medium properties, and spatial structure, the existence of the low velocity layer on the west of the Longmenshan fault zone, the topographic features of the Qinghai-Tibet Plateau to the Sichuan Basin, and the conditions for the simultaneous rupture of the central fault and the front-range fault during the Wenchuan earthquake, are the important factors that affect the Wenchuan earthquake to the Lushan earthquake. Among them, the terrain and low velocity layer have the most significant influence on the Coulomb failure stress for the area around the Lushan earthquake area caused by the Wenchuan earthquake. The effect of terrain features is more obvious in the north section of the selected section, yet the effect of the Wenchuan earthquake simultaneous rupture planes is smaller.

Coulomb stress evolution over the past 200 years and seismic hazard along the Xianshuihe fault zone of Sichuan, China

This study focuses on the M≥6.7 earthquakes that have occurred since 1816 on the Xianshuihe fault zone in southwest China. The interseismic Coulomb stress accumulation and the Coulomb stress changes caused by coseismic dislocation and postseismic viscoelastic relaxation of the previous shocks were computed for different periods on the relevant fault segments. Based on these results, we analyzed the relationship between time-adjacent strong shocks and the Coulomb stress evolution before every earthquake. The analysis suggests that strong earthquakes mostly occurred in the Coulomb stress enhancement region caused by coseismic dislocation and postseismic viscoelastic relaxation of the last earthquake. Considering the Coulomb stress evolution at the fault planes of the epicentral area before earthquakes, we found that the Coulomb stress accumulation caused by the interseismic tectonic loading was dominant for most strong earthquakes. For some other earthquakes the stress changes caused by coseismic dislocation and postseismic viscoelastic relaxation of surrounding earthquakes were very significant, which may be equivalent to the effect of interseismic tectonic loading lasting hundreds of years. Based on the time-dependent probabilistic risk model and the Dieterich (1994) model, we estimate the background seismic activity and the future earthquake probability for different fault segments, using long term seismic activity and strong earthquake recurrence cycles. It is shown that the Bamei, Selaha, and Kangding segments of the Xianshuihe fault zone have high earthquake probability, and are likely to have strong earthquakes. If energy is accumulated up to the year 2050, the magnitude of an event on these three segments could reach Mw 7.2, Mw 7.0, and Mw 7.1 respectively, while if the S7 and S8 cascades rupture, the event on these segments could reach a magnitude of up to Mw 7.2.

Source Model of the October 9, 1995 Jalisco-Colima Tsunami as constrained by field survey reports, and on the numerical simulation of the tsunami

A model of the seismic source of the October 9, 1995 Jalisco-Colima tsunami is obtained using the numerical modeling of the tsunami as constrained by field survey reports of tsunami run-up and the extent of flooding, by considering an earthquake (Mw∼8.0) with rupture area A = 9000 km2, L = 150km, W = 60km, with two different coseismic dislocation patches: 1 m the dislocation of the first 60km along the fault plane starting from its NW edge, offshore Chalacatepec and Chamela, and 3 m the dislocation of the next 90km; offshore Tenacatita, Navidad and Manzanillo. As expected, the seismic source obtained in this study is a simplified model in comparison of the seismic source obtained by inverting seismic and GPS data. However, the fact that this simplified model reproduces adequately the location of the rupture area and its coseismic dislocation is still important in order to provide some degree of certainty for the tsunami source of those historical tsunamis for which no seismic nor tide records are available, and only testimonies and vague relates of its effects are documented.

Static Stress Changes and Triggering Imposed by Wenchuan Earthquake on Lushan M7 Earthquake

Some faults rupture the surface followed by coseismic dislocation, which causes space stress changes and transmits or imposes static stress on or to neighboring faults.The static stress triggering leads to the adjacent fault ruptured in advance, which means a Coulomb stress increase and triggers a new earthquake event. In recent two decades, some studies on stress triggering became important field of seismotectonics. The occurrence of both Wenchuan M8.0 earthquake and Japan M9.0 eathquake have arisen interest of many researchers, and obtained much new result and understanding.

Seismotectonics of the 2008 and 2009 Qaidam Earthquakes and its Implication for Regional Tectonics

Abstract:Three magnitude >6 earthquakes struck Qaidam, Qinghai province, China, in November 10th 2008, August 28th and 31st 2009 respectively. The Zongwulongshan fault has often been designated as the active seismogenic structure, although it is at odd with the data. Our continuous GPS station (CGPS), the Xiao Qaidam station, located in the north of the Qaidam basin, is less than 30 km to the southwest of the 2008 earthquake. This CGPS station recorded the near field co‐seismic deformation. Here we analyzed the co‐seismic dislocation based on the GPS time series and the rupture processes from focal mechanism for the three earthquakes. The aftershocks were relocated to constrain the spatial characteristics of the 2008 and 2009 Qaidam earthquakes. Field geological and geomorphological investigation and interpretation of satellite images show that the Xitieshan fault and Zongwulongshan fault were activated as left lateral thrust during the late Quaternary. Evidence of folding can also be identified. Integrated analyses based on our data and the regional tectonic environment show that the Xitieshan fault is the fault responsible for the 2008 Qaidam earthquake, which is a low dip angle thrust with left lateral strike slip. The Zongwulongshan fault is the seismogenic fault of the 2009 earthquakes, which is a south dipping back thrust of the northern marginal thrust system of the Qaidam basin. Folding takes a significant part of the deformation in the northern marginal thrust system of the Qaidam basin, dominating the contemporary structure style of the northern margin of the Qaidam basin and Qilianshan tectonic system. In this region, this fault and fold system dominates the earthquake activities with frequent small magnitude earthquakes.

Runup Distribution for the 1908 Messina Tsunami in Italy: Observed Data versus Expected Curves

The source of the catastrophic 1908 Messina tsunami, southern Italy, is studied by best-fitting the available datasets of observed runup with a previously published empirical function (i.e., the expected runup distribution). The maximum runup is ∼12 m and was measured ∼30–40 km to the south of the area where the maximum coseismic dislocation and Mercalli–Cancani–Sieberg (MCS) intensities were recorded. The observed runup drops from ∼12 m to less than 1 m in a few tens of kilometers. The comparison between observed and expected runup distributions suggests that the main cause of the 1908 tsunami was a mass failure, thus supporting previously published evidence including tsunami arrival times, bathymetric maps, and chronicles reporting the interruption of submarine cables. This article adds a significant case history to the very limited database of thoroughly documented runup for landslide tsunamis.

Influence of 2008 Wenchuan earthquake on earthquake occurrence trend of active faults in Sichuan-Yunnan region

The Wenchuan earthquake coseismic deformation field is inferred from the coseismic dislocation data based on a 3-D geometric model of the active faults in Sichuan-Yunnan region. Then the potential dislocation displacement is inverted from the deformation field in the 3-D geometric model. While the faults’ slip velocities are inverted from GPS and leveling data, which can be used as the long-term slip vector. After the potential dislocation displacements are projected to long-term slip direction, we have got the influence of Wenchuan earthquake on active faults in Sichuan-Yunnan region. The results show that the northwestern segment of Longmenshan fault, the southern segments of Xianshuihe fault, Anninghe fault, Zemuhe fault, northern and southern segments of Daliangshan fault, Mabian fault got earthquake risks advanced of 305, 19, 12, 9.1 and 18, 51 years respectively in the eastern part of Sichuan and Yunnan. The Lijiang-Xiaojinhe fault, Nujiang fault, Longling-Lancang fault, Nantinghe fault and Zhongdian fault also got earthquake risks advanced in the western part of Sichuan-Yunnan region. Whereas the northwestern segment of Xianshuihe fault and Xiaojiang fault got earthquake risks reduced after the Wenchuan earthquake.

Finite fault inversion of DInSAR coseismic displacement of the 2009 L'Aquila earthquake (central Italy)

We define the geometric and kinematic characteristics of the fault activated during the Mw = 6.3, 6 April 2009 L'Aquila earthquake, from the modeling of Envisat and COSMO‐SkyMed (the first ever X‐band interferogram inverted for a coseismic dislocation study) DInSAR interferograms. Our best‐fit solution for the main shock is represented by a normal fault ∼16 km long and ∼12 km wide, with a small right‐lateral component, dipping 47°SW with a maximum slip of ∼90 cm. Although the seismic dislocation probably ended at 1 km depth, the updip projection of the fault plane corresponds to the northern segment of the mapped Paganica–S. Demetrio fault, where alignment of surface breaks was observed in the field. The absence of this fault in existing seismic source catalogues suggests that an improved approach, involving detailed surface and subsurface geological and geophysical investigations, is needed for a better assessment of the seismic hazard at the local scale.

A Multi‐Segment Analytic Modeling of Hypocentral Geometric Characteristic Parameters of the Ms8.1 Earthquake at the Kunlun Mountains

AbstractPrevious studies about the Kunlun Mountains Ms8.1 earthquake have a lot of differences in its hypocentral characteristic parameters such as the rupture length, direction of rupture plane (discontinuity plane), and the size of the rupture plane. First, utilizing the D‐InSAR technology we obtain the coseismic displacement field of the Kunlun Mountains Ms8.1 earthquake. Combining the field investigation, along the main rupture zone the displacement in the satellite line of sight (LOS) is decomposed. Synthesizing the results of InSAR interferogram, field investigation and satellite image interpretation, the rupture zone of the earthquake is re‐divided into segments. Meanwhile, a set of geometric parameters of the rupture are obtained. Second, analyzing the strain of north‐south walls of the fault, we find an asymmetric displacement distribution between the two sides of the rupture, and both of the walls are acted by compressive and tensile stress, respectively, a pattern that cannot be explained with the linear elastic theory. Laboratory experiments and in situ measurements in boreholes have shown that many crustal rocks exhibit a nonlinear elastic behavior in compression and tension with a dependence of the Young's modulus on the minimum principal stress. So we suggest that the nonlinear displacement distribution should be considered when dislocation and hypocentral parameters are analyzed. Finally, for simulating the earthquake displacement field, we present a model of multi‐hypocenter, heterogenous dislocation, and multi‐rupture section superposition. The simulated LOS interferogram by the model exhibits a more complete distribution characteristics of the earthquake displacement field than that of previous work. It provides a better explanation to the hypocentral characteristic parameters of the geometry, size of dislocation for a large surface rupture field, and the non‐homogeneity of coseismic dislocation.

Decomposing InSAR LOS displacement into co-seismic dislocation with a linear interpolation model: A case study of the Kunlun Mountain M S=8.1 earthquake

It has always been a difficult problem to extract horizontal and vertical displacement components from the InSAR LOS (Line of Sight) displacement since the advent of monitoring ground surface deformation with InSAR technique. Having tried to fit the firsthand field investigation data with a least squares model and obtained a preliminary result, this paper, based on the previous field data and the InSAR data, presents a linear cubic interpolation model which well fits the feature of earthquake fracture zone. This model inherits the precision of investigation data; moreover make use of some advantages of the InSAR technique, such as quasi-real time observation, continuous recording and all-weather measurement. Accordingly, by means of the model this paper presents a method to decompose the InSAR slant range co-seismic displacement (i.e. LOS change) into horizontal and vertical displacement components. Approaching the real motion step by step, finally a serial of curves representing the co-seismic horizontal and vertical displacement component along the main earthquake fracture zone are approximately obtained.

A deep strong reflector in the Nankai accretionary wedge from multichannel seismic data: Implications for underplating and interseismic shear stress release

We present the results of structural and stratigraphic interpretations for the multichannel seismic profiles to reveal the structure of the Nankai accretionary wedge off Shikoku Island. On the basis of reflection characteristics we identify three major seismic units that correspond to a unit consisting of both Oligocene‐Miocene accretionary prism and the sedimentary cover (unit A), a unit of underthrust Miocene‐Pliocene hemipelagic sediments (unit B), and a unit of subducting pre‐Miocene oceanic crustal rocks (unit C). According to relative strength of the topmost reflector of unit B the Nankai accretionary wedge can be divided into three different structural zones along the cross‐strike direction: apparent décollement reflector zone, low‐amplitude reflector (LAR) zone, and deep strong reflector (DSR) zone. Despite the velocity uncertainty involved in the depth conversion and therefore thickness estimation error, the décollement step‐down and the possible duplex structures in the LAR and DSR zones support that the landward thickness variation of unit B is caused by the structural thickening such as the underplating process. Structural features and reverse polarity suggest that the DSR as an upper interface of the underplated layer (unit B) corresponds to a shear plane or a fault zone with high pore fluid pressures. We found that most of the DSR was observed in a region which has not suffered from the 1946 coseismic dislocation even though it was locked during the interseismic period. The DSR as a shear plane or a fault zone constrains a regional variety of the coseismic dislocation around the updip limit of the Nankai seismogenic zone. Considering all of the DSR characteristics, we propose a hypothesis for the DSR's development and role. The DSR may play an important role as a possible releaser of the shear stress energy stored at the plate boundary around the updip limit of the Nankai seismogenic zone. A good correlation between the theoretical vertical displacement and seafloor topographic feature support the possible slip along the DSR as a shear plane.

Homogeneous vs heterogeneous subduction zone models: Coseismic and postseismic deformation

A finite‐element model (FEM) incorporating geologic properties characteristic of a subduction zone is compared with FEMs approximating homogeneous elastic half‐spaces (HEHS)s to investigate the effect of heterogeneity on coseismic and postseismic deformation predictions for the 1995 Colima‐Jalisco Mw =8.0 earthquake. The FEMs are used to compute a coefficient matrix relating displacements at observation points due to unit dislocations of contact‐node pairs on the fault surface. The Green's function responses are used to solve the inverse problem of estimating dislocation distributions from coseismic GPS displacements. Predictions from the FEM with heterogeneous material properties, loaded with either of the HEHS dislocation distributions, significantly overestimate coseismic displacements. Postseismic deformation predictions are also sensitive to the coseismic dislocation distribution, which drives poroelastic and viscoelastic relaxation. FEM‐generated Green's functions, which allow for spatial variations in material properties, are thus preferable to those that assume a simple HEHS because the latter leads to dislocation distributions unsuitable for predicting the postseismic response.

Interplate coupling along the Nankai Trough off southwest Japan derived from GPS measurements

We investigate interplate coupling between the Philippine Sea and the Eurasian plates at the Nankai Trough off southwest Japan during the interseismic period. Plate coupling is modeled by imaginary backward normal slip superposed on the steady‐state subduction. We divide the plate interface into several segments, and estimate coupling on each segment from GPS horizontal velocity data. Estimated coupling is in general agreement with the rate of steady‐state subduction predicted from a plate motion model. This implies that current coupling at the Nankai Trough is so strong that much of the relative plate motion is taken up by shortening of the leading edge of the overriding plate. It seems that stronger interseismic coupling is realized on the segment where larger coseismic dislocation occurred, and vice versa. Time fluctuation of plate coupling from 1995 to 1996 is found on a small segment.