Modeling the ML4.7 mainshock of the February–July 2001 earthquake sequence in Aegion, Greece

An earthquake sequence comprising almost 2000 events occurred in February–July 2001 on the southern coast of the Corinth Gulf.Several location methods were applied to 171 events recorded by the regional network PATNET. The unavailability of S-wave readings precluded from reliable depth determination. For the mainshock of April 8, M_L= 4.7, the depth varied from 0 to 20 km. The amplitude spectra of complete waveforms at three local stations (KER,SER, DES; epicentral distances 17, 26 and 56 km) were inverted between 0.1 and 0.2 Hz for double-couple focal mechanism and also for the depth. The optimum solution (strike 220°, dip 40°, rake ‒160°, and depth of 8 km) was validated by forward waveform modeling.Additionally, the mainshock depth was further supported by the P- and S-wave arrival times from the local short-period network CRLNET (Corinth Rift Laboratory).The scalar seismic moment was 2.5e15 Nm,and the moment rate function was successfully simulated by a triangle of the 0.5 second duration. This is equivalent to a 1–1.5 km fault length, and a static stress drop 2–6 MPa. This value is important for future strong ground motion simulation of damaging earthquakes in Aegion region, whose subevents may be modeled according to the studied event. The T axis of the mainshock (azimuth 176° and plunge 67°), is consistent with the regional direction of extension N10°. However, none of the nodal planes can be associated to an active structure seen at the surface. The relationship of this earthquake sequence with deeper faults (e.g. possible detachment at about 10 km) is also unclear.     

Double-difference Relocation of the Aftershocks of the Tecom��n, Colima, Mexico Earthquake of 22 January 2003

On 22 January 2003, the M _w?=?7.6 Tecomán earthquake struck offshore of the state of Colima, Mexico, near the diffuse triple junction between the Cocos, Rivera, and North American plates. Three-hundred and fifty aftershocks of the Tecomán earthquake with magnitudes between 2.6 and 5.8, each recorded by at least 7 stations, are relocated using the double difference method. Initial locations are determined using P and S readings from the Red Sismológica Telemétrica del Estado de Colima (RESCO) and a 1-D velocity model. Because only eight RESCO stations were operating immediately following the Tecomán earthquake, uncertainties in the initial locations and depths are fairly large, with average uncertainties of 8.0?km in depth and 1.4?km in the north?Csouth and east?Cwest directions. Events occurring between 24 January and 31 January were located using not only RESCO phase readings but also additional P and S readings from 11 temporary stations. Average uncertainties decrease to 0.8?km in depth, 0.3?km in the east?Cwest direction, and 0.7?km in the north?Csouth direction for events occurring while the temporary stations were deployed. While some preliminary studies of the early aftershocks suggested that they were dominated by shallow events above the plate interface, our results place the majority of aftershocks along the plate interface, for a slab dipping between approximately 20° and 30°. This is consistent with the slab positions inferred from geodetic studies. We do see some upper plate aftershocks that may correspond to forearc fault zones, and faults inland in the upper plate, particularly among events occurring more than 3?months after the mainshock.     

Relocation of the 10 March 2011 Yingjiang,China, earthquake sequence and its tectonic implications

An earthquake with M_S5.8 occurred on 10 March 2011 in Yingjiang county, western Yunnan, China. This earthquake caused 25 deaths and over 250 injuries. In order to better understand the seismotectonics in the region, we collected the arrival time data from the Yunnan seismic observational bulletins during 1 January to 25 March 2011, and precisely hand-picked the arrival times from high-quality seismograms that were recorded by the temporary seismic stations deployed by our Institute of Crustal Dynamics, China Earthquake Administration. Using these arrival times, we relocated all the earthquakes including the Yingjiang mainshock and its aftershocks using the double-difference relocation algorithm. Our results show that the relocated earthquakes dominantly occurred along the ENE direction and formed an upside-down bow-shaped structure in depth. It is also observed that after the Yingjiang mainshock, some aftershocks extended toward the SSE over about 10 km. These results may indicate that the Yingjiang mainshock ruptured a conjugate fault system consisting of the ENE trending Da Yingjiang fault and a SSE trending blind fault. Such structural features could contribute to severely seismic hazards during the moderate-size Yingjiang earthquake.     

Comparison of a complex rupture model with the precursor asperities of the 1975 HawaiiM s-7.2 earthquake

A simplified multiple source model was constructed for the 1975 HawaiiM _s=7.2 earthquake by matching synthetic signals with three component accelerograms at two stations located approximately 45 km from the epicenter. Six major subevents were identified and located approximately. The signals of these are larger by factors of 1.4 to 3.2 than that of theM _L=5.9 foreshock which occurred 70 minutes before the main rupture and also triggered the SAM-1 recorders at the two stations. Dividing the rupture length (40 km) by the duration of strong ground shaking ( 50 sec) an, average rupture velocity of 0.8 km/sec (about 25% of S-velocity) is obtained. Thus it is likely that the rupture stopped between subevents. The approximate epicenters of the 6 major subevents, and of the foreshock, support the hypothesis that they were located in high stress asperities which rupture during the main shock, except for the last events which is interpreted as a stopping phase generated at a barrier. These asperities have been previously defined on the basis of differences in the precursor pattern before the mainshock. Thus, it appears that both the details of the precursors and of the main rupture depended critically on the heterogeneous tress distribution in the source volume. This suggests that main rupture initiation points and locations of high rupture accelerations may be identified before the mainshock occurs, based on precursor anomaly patterns. A satisfactory match of synthetic signals with the observations could be obtained only if the aximuth of the fault plane of subevents was rotated from N60°E to N90°E and back to N30°E. These orientations are approximately parallel to the nearest Kilauea rift segments. Hence the slip directions and greatest principal stresses were oriented perpendicular to the rifts everywhere. From this analysis and other work, it is concluded that this fault surface consisted of three types of segments with different strength: hard asperities (radius 5 km), soft but brittle segments between the asperities (radius 5 km), and a viscous half (10×40 km) which slipped during the mainshock, but where microearthquakes and aftershocks are not common.     

How deep was the Dudley (UK) earthquake of 22 September 2002?

On 22 September 2002, the largest UK earthquake (m_b4.3) of the last 10 years occurred near the town of Dudley in the West Midlands. Here we determine the earthquake focal mechanism and depth using data from stations at regional and teleseismic distances. Short-period teleseismic seismograms are interpreted in terms of P and surface reflections pP and sP. This analysis suggests that the source depth is deeper than the 9.7 km initially determined by the British Geological Survey (BGS). The relative amplitude method is applied to four teleseismic seismograms to support our interpretation of the surface reflections, and constrain the focal mechanism. Our preferred focal mechanism, a near vertical strike-slip with _s = 94°, = 88° and = –179°, is in reasonable agreement with a moment tensor determined by the Swiss Seismological Service. Synthetic regional surface wave seismograms match the observed seismograms for a model focal depth of 19.5 (±3.0) km and scalar moment, M₀, of 3.2 × 10¹⁵ N m. Our results emphasize that due to the well-known trade-off between depth and M₀ from inversions of long period (0.02–0.1 Hz) surface waves, it is preferable to combine long- and short-period data to constrain reliably the depth and hence estimate M₀. Our focal mechanism and depth are further validated by generating short-period synthetic seismograms that match the observations.     

2010年玉树地震序列ML≥4.0事件震源机制解特征分析

采用CAP方法反演2010年玉树7.1级地震序列前震、主震及余震19个M_L≥4.0事件的震源机制解,19个结果以走滑类型为主,前震、主震的震源机制解十分接近,反映出前震、主震之间密切的联系;震源深度集中在7~12 km,震源最浅（4.5 km）与最深（34 km）的两个余震事件具有明显的逆冲性质,表现出明显的边界特征;19个事件的震中分布在甘孜-玉树断裂北支玉树-隆宝断裂上,目前已经证明该断裂即为玉树地震的发震构造。自SE-NW沿玉树-隆宝断裂走向拉一剖面,观察震源深度沿剖面的变化情况,可看出玉树-隆宝断裂西北段震源深度要大于东南段,该段主要是余震活动的中后期,因此在地震活动的中后期,余震向地壳深部扩展,断裂累积的应变能得到更进一步的释放;P轴方位角优势分布集中在220°~230°,T轴方位优势分布集中在310°~320°,两个优势分布互相垂直性与单个事件的沙滩球应力轴一样,说明玉树地震的震源机制解类型较为简单;玉树周边地区应力场分布比较均匀,并不像汶川周边地区那么复杂,本次玉树地震为巴颜喀拉地块与羌塘块体边界处甘孜-玉树断裂应变能量的正常释放。     

Source properties of the 1997–98 Central Italy earthquake sequence from inversion of long-period and broad-band seismograms

We study source properties of the main earthquakes of the 1997–98 Umbria-Marche (central Italy) sequence by analysis of regional-distanceand teleseismic long period and broadband seismograms recorded by MedNet and IRIS/GSN stations. We use a modified Harvardcentroid-moment tensor (CMT) algorithm to allow inversion of long period waveforms, primarily Rayleigh and Love waves, for small earthquakes (4.2 M_W 5.5) at local to regional distances (<15^°). For the seven largest earthquakes (M_W>5.2) moment tensors derived from local and regional data agree well with those determined using teleseismic waveforms and standard methods of analysis. We also determine moment tensors for a foreshock and 12 other aftershocks, that were too small for global analysis. Focal depth and rupture propagation are analyzed for three largest shocks by inversion of teleseismic broadband body waves. The earthquakes are generally located at shallow depth (5 km or shallower) and are characterized by normal faulting mechanisms, with a NE-SW tension axis. The presumed principal fault plane dips at a shallow angle towards the SW. Only one of the events analyzed has an entirely different faulting geometry, indicating instead right-lateral strike-slip motion on a plane approximately E-W, or left-lateral faulting on a N-S plane. The other significant exception to the regular pattern of mechanisms is represented by the March 26, 1998, event, located at 51 km depth. Its connection with the shallow earthquake sequence is unclear and intriguing. The time evolution of the seismic sequence is unusual,with the mainshock accounting for only approximately 50% of the total moment release. The broadband teleseismic waveforms of the main, September 26, 09:40, earthquake are very complicated for the size of the event and suggest a complex rupture. In our favored source model, rupture initiated at 5 km depth, propagated updip and was followed, 3 seconds later, by a shallower subevent with a slightly rotated mechanism.     

M m: A variable-period mantle magnitude for intermediate and deep earthquakes

We extend to the case of intermediate and deep earthquakes the mantle magnitude developed for shallow shocks byokal andTalandier (1989). Specifically, from the measurement of the spectral amplitude of Rayleigh waves at a single station, we obtain a mantle magnitude,M _m, theoretically related to the seismic moment of the event through $$M_m = \log _{10} M_0 - 20.$$ The computation ofM _minvolves two corrections. The distance correction is the same as for shallow shocks. For the purpose of computing the frequency-dependent source correction, we define three depth windows: Intermediate (A) (75 to 200 km); Intermediate (B) (200–400 km) and Deep (over 400 km). In each window, the source correctionC _S is modeled by a cubic spline of log₁₀ T. Analysis of a dataset of 200 measurements (mostly from GEOSCOPE stations) shows that the seismic moment of the earthquakes is recovered with a standard deviation of 0.23 units of magnitude, and a mean bias of only 0.14 unit. These figures are basically similar to those for shallow events. Our method successfully recognizes truly large deep events, such as the 1970 Colombia shock, and errors due to the potential misclassification of events into the wrong depth window are minimal.     

Seismic relocation,focal mechanism and crustal seismic anisotropy associated with the 2010 Yushu MS7.1 earthquake and its aftershocks

The 2010 Yushu M_S7.1 earthquake occurred in Ganzi-Yushu fault, which is the south boundary of Bayan Har block. In this study, by using double difference algorithm, the locations of mainshock (33.13°N, 96.59°E, focal depth 10.22 km) and more than 600 aftershocks were obtained. The focal mechanisms of the mainshock and some aftershocks with M_S>3.5 were estimated by jointly using broadband velocity waveforms from Global Seismic Network (GSN) and Qinghai Seismic Network as well. The focal mechanisms and relocation show that the strike of the fault plane is about 125° (WNW-ESE), and the mainshock is left-laterally strikeslip. The parameters of shear-wave splitting were obtained at seismic stations of YUS and L6304 by systematic analysis method of shear-wave splitting (SAM) method. Based on the parameters of shear-wave splitting and focal mechanism, the characteristics of stress field in seismic source zone were analyzed. The directions of polarization at stations YUS and L6304 are different. It is concluded that after the mainshock and the M_S6.3 aftershock on April 14, the stress-field was changed.     

Seismic quiescence precursory to a past and a future Kurile island earthquake

A systematic search was made for seismicity rate changes in the segment of the Kurile island arc from 45°N to 53°N by studying the cumulative seismicity of shallow (h100 km) earthquakes within 11 overlapping volumes of radius 100 km for the time period 1960 through beginning of 1978. We found that in most parts of this island arc and most of the time the seismicity rate as obtained from the NOAA catalogue and not excluding any events is fairly constant except for increased seismicity in the mid 1960s in the southern portion due to the great 1963 mainshock there, and for seismicity quiescence during part of the time period studied within two well defined sections of the arc. The first of these is a volume of 100 km radius around a 1973 (M _s =7.3) mainshock within which the seismicity rate was demonstrated at the 99% confidence level to have been lower by 50% during 2100 days (5.75 years) before this mainshock. The second volume of seismic quiescence coincides with the 400 km long north Kuriles gap. In this gap the seismicity rate is shown (at the 99% confidence level) to be lower by 50% from 1967 to present (1978), in comparison with the rate within the gap befor 1967, as well as with the rate surrounding the gap. We propose that the anomalously low seismicity rate within the Kuriles gap is a precursor to a great earthquake, the occurrence time of which was estimated by the following preliminary relation between precursory quiescence time and source dimensionT=190L ^0.545. We predict that an earthquake with source length of 200–400 km (M>8) will occur along the north Kurile island arc between latitude 45.5°N and 49.2°N at a time between now and 1994.     

On the Problem of Deep Earthquakes in Crimea–Black Sea Region

It is a common opinion that only crustal earthquakes can occur in the Crimea–Black Sea region. Since the existence of deep earthquakes in the Crimea–Black Sea region is extremely important for the construction of a geodynamic model for this region, an attempt is made to verify the validity of this widespread view. To do this, the coordinates of all earthquakes recorded by the stations of the Crimean seismological network are reinterpreted with an algorithm developed by one of the authors. The data published in the seismological catalogs and bulletins of the Crimea–Black Sea region for 1970–2012 are used for the analysis. To refine the coordinates of hypocenters of earthquakes in the Crimea–Black Sea region, in addition to the data from stations of the Crimean seismological network, information from seismic stations located around the Black Sea coast are used. In total, the data from 61 seismic stations were used to determine the hypocenter coordinates. The used earthquake catalogs for 1970–2012 contain information on ~2140 events with magnitudes from–1.5 to 5.5. The bulletins provide information on the arrival times of P- and S-waves at seismic stations for 1988 events recorded by three or more stations. The principal innovation of this study is the use of the original author’s hypocenter determination algorithm, which minimizes the functional of distances between the points (X, Y, H) and (x, y, h) corresponding to the theoretical and observed seismic wave travel times from the earthquake source to the recording stations. The determination of the coordinates of earthquake hypocenters is much more stable in this case than the usual minimization of the residual functional for the arrival time of an earthquake wave at a station (the difference between the theoretical and observed values). Since determination of the hypocenter coordinates can be influenced by the chosen velocity column beneath each station, special attention is focused on collecting information on velocity profiles. To evaluate the influence of the upper mantle on the results of calculating the velocity model, two different low-velocity and high-velocity models are used; the results are compared with each other. Both velocity models are set to a depth of 640 km, which is fundamentally important in determining hypocenters for deep earthquakes. Studies of the Crimea–Black Sea region have revealed more than 70 earthquakes with a source depth of more than 60 km. The adequacy of the obtained depth values is confirmed by the results of comparing the initial experimental data from the bulletins with the theoretical travel-time curves for earthquake sources with depths of 50 and 200 km. The sources of deep earthquakes found in the Crimea–Black Sea region significantly change our understanding of the structure and geotectonics of this region.     

A note on the L’Aquila earthquake of 6 April 2009: Permanent ground displacements obtained from strong-motion accelerograms

On 6 April 2009, 01:32 GMT, an M_w 6.3 earthquake hit the Abruzzi region of central Italy causing widespread damage in the City of L’Aquila and its nearby villages. The mainshock of this earthquake was recorded by 57 digital strong-motion instruments, four of which are located on the hanging wall of the Paganica Fault near L’Aquila. These stations are no more than 6 km from the epicentre. We use accelerometric data from these four stations to estimate permanent ground displacements caused by the mainshock. Our numerical results reveal south-east and downwards directed permanent co-seismic displacements which are in fair agreement with the outcomes of GPS and InSAR measurements reported in preliminary Istituto Nazionale di Geofisica e Vulcanologia (INGV) reports.     

Spectral analysis and earthquake scaling of the Petatlan earthquake (M s =7.6) aftershocks,recorded at stations along the pacific coast and between the coast and Mexico City

Spectral parameters have been estimated for 214 Petatlan aftershocks recorded at stations between Petatlan and Mexico City and between Petatlan and Acapulco. The spectral parameters were used to obtain empirical relations for the estimation of seismic moment from coda length and fromM _L . Stress drops, using Brune's model, were calculated for these aftershocks. Six events with large stress drop are located within a previously suggested asperity, and seven more suggest a boundary zone at the intersection of the Petatlan and Zihuatanejo aftershock rupture volumes. Stress drops increase with increasing seismic moment up to 10²⁰ dyne-cm but appear to be constant at greater moment values. The peak horizontal velocity times distance of aftershocks recorded near the coast and between the coast and Mexico City (30 to 270 km away), scales linearly with seismic moment, and predicts well the peak horizontal values of large (M _s 7.0) coastal thrust events recorded on rock sites at Mexico City. Peak horizontal velocity is a straightforward measurement, thus this relation allows us to evaluate expected ground motion between the Pacific coast and Mexico City from the seismic moment of subduction related earthquakes along the coast.     

The M<Subscript>W</Subscript> 4.5 Vallorcine (French Alps) earthquake of 8 September 2005 and its complex aftershock sequence

On 8 September 2005 a moderate M_W 4.5 earthquake occurred in the north-western Alps midway between Chamonix (France) and Martigny (Switzerland). The focal mechanism corresponds to a right-lateral strike-slip on a N60°E fault plane. The foreshock–mainshock–aftershock sequence is investigated on the basis of data recorded by a temporary network of 28 stations deployed for 1 month just after the mainshock, and data from permanent, regional seismic networks. Absolute and relative locations of more than 400 events are obtained with a mean uncertainty of approximately 0.2 km. Small foreshocks, the mainshock, and early and late aftershocks are located relative to the main aftershock set. The seismic sequence exhibits a surprisingly complex structure, with at least five clusters on distinct fault planes. The main elongated cluster agrees with the location of the mainshock, its hypocenter being 4.3 km below sea level. We discuss the relationship between the right-lateral fault beneath the Loriaz peak (the source of the Vallorcine event), the nearby normal Remuaz fault, and the regional seismotectonic stress field.     

Long-term earthquake prediction in the Aegean area based on a time and magnitude predictable model

The Aegean and surrounding area (34°N–43°N, 18°E–30°E) is separated into 76 shallow and intermediate depth seismogenic sources. For 74 of these sources intervent times for strong mainshocks have been determined by the use of instrumental and historical data. These times have been used to determine the following empirical relations: $$\begin{gathered} \log T_t = 0.24M_{\min } + 0.25M_p - 0.36\log \dot M_0 + 7.36 \hfill \\ M_f = 1.04M_{\min } - 0.31M_p + 0.28\log \dot M_0 - 4.85 \hfill \\ \end{gathered} $$ whereT ₁ is the interevent time, measured in years,M _min the surface wave magnitude of the smallest mainshock considered,M _p the magnitude of the preceding mainshock,M _f the magnitude of the following mainshock, \(\dot M_0 \) the moment rate in each source per year. A multiple correlation coefficient equal to 0.74 and a standard deviation equal to 0.18 for the first of these relations were calculated. The corresponding quantities for the second of these relations are 0.91 and 0.22. On the basis of the first of these relations and taking into consideration the time of occurence and the magnitude of the last mainshock, the probabilities for the occurrence of mainshocks in each seismogenic source of this region during the decade 1993–2002 are determined. The second of these relations has been used to estimate the magnitude of the expected mainshock.     

High-precision relocation of the aftershock sequence of the January 8, 2022, MS6.9 Menyuan earthquake

The 2022 Menyuan M_S6.9 earthquake, which occurred on January 8, is the most destructive earthquake to occur near the Lenglongling (LLL) fault since the 2016 Menyuan M_S6.4 earthquake. We relocated the mainshock and aftershocks with phase arrival time observations for three days after the mainshock from the Qinghai Seismic Network using the double-difference method. The total length and width of the aftershock sequence are approximately 32 km and 5 km, respectively, and the aftershocks are mainly concentrated at a depth of 7–12 km. The relocated sequence can be divided into 18 km west and 13 km east segments with a boundary approximately 5 km east of the mainshock, where aftershocks are sparse. The east and west fault structures revealed by aftershock locations differ significantly. The west fault strikes EW and inclines to the south at a 71º–90º angle, whereas the east fault strikes 133º and has a smaller dip angle. Elastic strain accumulates at conjunctions of faults with different slip rates where it is prone to large earthquakes. Based on surface traces of faults, the distribution of relocated earthquake sequence and surface ruptures, the mainshock was determined to have occurred at the conjunction of the Tuolaishan (TLS) fault and LLL fault, and the west and east segments of the aftershock sequence were on the TLS fault and LLL fault, respectively. Aftershocks migrate in the early and late stages of the earthquake sequence. In the first 1.5 h after the mainshock, aftershocks expand westward from the mainshock. In the late stage, seismicity on the northeast side of the east fault is higher than that in other regions. The migration rate of the west segment of the aftershock sequence is approximately 4.5 km/decade and the afterslip may exist in the source region.     

Upper crustal structure of the western Corinth Gulf, greece, inferred from arrival times of the January 2010 earthquake sequence

The western part of the Corinth Gulf attracts attention due to its seismically active fault system and considerable seismic hazard. A moderate size earthquake occurred close to the town of Efpalio on January 18, 2010, followed by a sequence of smaller earthquakes. In the present paper we use this sequence to derive a local structural model for the region in the vicinity of Efpalio. The model is based on the minimization of traveltime residuals. In particular, we used arrival times from 51 selected events recorded on January 19 and 20 by at least 5 stations at epicentral distances less than about 25 km. A variant of the method of conjugate gradients has been used for this purpose. In comparison with several previous models, the new model is characterized by higher velocities to a depth of about 8 km. The velocity ratio in the model is v_P / v_S = 1.83. The hypocentres of the selected earthquakes lay at depths between about 5 and 9 km, but their distribution is rather irregular.     

Historical seismicity of the southeastern Caribbean and tectonic implications

We have relocated the twenty-eight largest magnitude (4.3M _s 7.3) historical (1922–1963) earthquakes of the southeastern Caribbean. We also present new focal mechanisms for seven of these events. The relocations are based on reported ISSP andS arrival times that we analyzed using generalized linear inversion techniques. The new focal mechanisms were constrained by first motionP polarities as reported by the ISS and as picked by us where records were available, and by the polarities and ratios ofSH andsSH, andSV andsSV arrivals that we determined from seismograms. The results of the relocations are commensurate with the distribution of seismicity observed in the recent era: hypocenters are shallow and intermediate in depth (0–200 km), and the events occur almost exclusively in areas known to be currently seismic. The frequent seismic activity in the vicinity of the Paria Peninsula, Venezuela, is clearly a persistent feature of the regional earthquake pattern; intermediate depth earthquakes indicative of subduction beneath the Caribbean plate occur here and along the Lesser Antilles arc. The Grenadines seismic gap is confirmed as an area of low seismic moment release throughout the historical era. Trinidad and the eastern Gulf of Paria were also largely quiescent.The new focal mechanisms, despite being a sparse data set, give significant insight into both subduction processes along the Lesser Antilles arc and into the shallow deformation of the Caribbean-South America plate boundary zone. The largest earthquake to have occurred in this region, the 19 March 1953 event (M _m =7.01), is a Lesser Antilles slab deformation event, and another earthquake in this region of the Lesser Antilles is probably a rarely-observed interplate thrust event. Shallow deformation in the plate boundary zone is complex and, near the Paria Penninsula, involves mixed southeastward thrusting and dextral strike-slip on east-striking faults, and secondarily, normal faulting. Bending of the subducting Atlantic-South American plate also seems to generate seisms. The rather high ratio of intraplate deformation to interplate deformation observed along the Lesser Antilles subduction zone in the more recent era seems to have been operative in the historical era as well.     

Source parameter and rupture process of the MW6.3 early strong aftershock immediately following the 2016 MW7.8 Kaikoura earthquake (New Zealand)

The 2016 M_W7.8 Kaikoura (New Zealand) earthquake was the most complex event ever instrumentally recorded and geologically investigated, as it ruptured on more than 12 fault segments of various geometries. To study the mainshock rupture characteristics, geodetic methods like InSAR and GPS play an essential role in providing satisfactory spatial resolution. However, early strong aftershocks may cause extra ground deformation which bias the mainshock rupture inversion result. In this paper, we will focus on studying the M_W 6.3 aftershock, which is the only M6+ thrust slip aftershock that occurred only 30 minutes after the Kaikoura mainshock. We will relocate the hypocenter of this event using the hypo2000 method, make the finite fault model (FFM) inversion for the detailed rupture processes and calculate the synthetic surface displacement to compare with the observed GPS data and figure out its influence on the mainshock study. Although we are not able to resolve the real ruptured fault of this event because of limited observation data, we infer that it is a west-ward dipping event of oblique slip mechanism, consistent with the subfault geometries of the Kaikoura mainshock. According to the inverted FFM, this event can generate 10–20 cm ground surface displacement and affect the ground displacement observation at nearby GPS stations.