Double-difference Earthquake Location Algorithm Research Articles

Crustal Structure and Seismogenic Background Beneath Zhumadian, Henan, China: Evidence from Magnetotelluric Data

Earthquakes threaten people’s lives and property, especially when cities are affected. Thus, detecting the crustal structure and seismogenic background beneath cities is important if we are to lessen losses. Because the city of Zhumadian in Henan Province, China, is a well-developed area with a large population, more attention should be paid to earthquake detection. We therefore collected data from 15 magnetotelluric (MT) stations along a 55-km survey line across Zhumadian. Then, subsequent to acquisition, processing, and data inversion, we developed a preferred electrical resistivity model. We accurately relocated original earthquakes during the period between 1981 and 2019 within the study area using the double-difference earthquake location algorithm. The relocated earthquakes were projected onto our preferred MT model. The new resistivity model presented here reveals that saturated rocks, including rocks with porosities of 4.31–19.69% and moderate salinities of 10 g/L pore fluid and/or graphite, form conductive features in the upper crust. We also show that aqueous fluids derived from metamorphic dehydration within the lower crust and mantle, not melts, constitute mid-crustal conductors and facilitate detachment structures. The data show that the relocated earthquakes are located adjacent to the boundaries between conductive and resistive areas as well as at the bottom of fault fracture zones. These outcomes imply that the combination of rock failure and fault slipping triggers earthquakes.

Spatial-temporal distribution of early aftershocks following the 2016 Ms 6.4 Menyuan, Qinghai, China Earthquake

The Lenglongling fault, located in the north Qilian fold zone along the northeastern margin of the Tibetan Plateau, plays an important role in accommodating the tectonic deformation between the Tibetan Plateau and the Alxa block. On 21 January 2016, the Ms 6.4 Menyuan earthquake occurred near the Lenglongling fault. In this study, we utilize the waveform matched filter technique and double-difference earthquake location algorithm to examine the spatial-temporal evolution of aftershocks following the Menyuan mainshock. Events from both China Earthquake Networks Center (CENC) routine catalog and relocated catalog are used as templates to scan through the continuous waveform data (21 January 2016 to 31 December 2016). We identify three and eleven times more events than listed in CENC routine catalog and relocated catalog, respectively. We find that the earliest aftershocks (e.g., in the first 10 min or so) mostly illuminated a steeply dipping plane, while aftershocks at later times outlined a near vertical dipping plane with mostly strike-slip focal mechanisms. The relocated aftershocks also suggest that the Menyuan mainshock occurred in a secondary fault rather than the major Lenglongling fault, which is consistent with geodetic and geological observations. Moreover, we find that aftershocks systematically expand with time along the fault strike and 16 repeating earthquake clusters occur in the rupture zone, which are likely driven by afterslip of the mainshock.

Seismologic characteristics of the 2017, <italic>M</italic><sub>s</sub>7.0 Jiuzhaigou earthquake, Sichuan, China

The M s7.0 Jiuzhaigou earthquake with the hypocenter depth of 20 km occurred at the NNW extension line of the Huya fault located next to the eastern boundary of the Bayan Har block. The focal mechanism solution of the mainshock based on the W-phase source inversion showed a sinistral slip and the moment magnitude was M w 6.5. Over 3000 aftershocks were recorded within six days, among which the largest was M 4.8 and three others were larger than M 4.0. The value of the aftershock frequency attenuation coefficient P was 1.03, and the b value of the magnitude-frequency relationship was 0.68. The aftershock activities were relatively inactive but the attenuation was normal. Relocation results of 603 aftershocks were obtained by using the double-difference earthquake location algorithm and the location errors of the EW, NS and the vertical directions were 0.81, 0.84 and 1.59 km respectively. The mainshock occurred in the center of the aftershock region and ruptured on both sides along the strike direction. The aftershocks were distributed linearly along a NNW direction, and the fault plane was nearly vertical with a length over 30 km and a width of about 15 km. Aftershocks were more concentrated in the southeast than in the northwest. The focal mechanism solutions of most aftershocks in the central and southeastern regions were consistent with the mainshock, while those in the northwest were thrust-types, which might be related to the Minjiang fault. Meanwhile, the regional stress was released more in the central and southeastern regions than in the northwest. The maximum intensity investigated in the damage zone was up to IX. The maximum intensity region was located in scenic areas, therefore few people were present when the earthquake occurred at night. The peak acceleration was highest in the NS records, followed by the EW and vertical records. The design spectra of intensity for Jiuzhaigou was VIII, which is higher than the acceleration response spectra obtained from the Jiuzhaibaihe station, the nearest station which had strong motion recordings. These aforementioned factors may have contributed to the minor casualties resulting from this earthquake. The aftershock regions of the Jiuzhaigou earthquake, the 1973 M s6.5 Songpan northeast earthquake, and the 1976 M s7.2 Songpan earthquake are connected along the Huya fault which may extend to the northwest in the deep earth. Considering the rupture displacement distribution of the mainshock, the focal mechanism of aftershocks, and the stress drop, strong aftershocks are unlikely to occur in the central and southeastern Jiuzhaigou aftershock regions; however, the development of seismicity in the northwest and the Minjiang River fault should be noted. Static Coulomb stress triggering of other 4 earthquakes larger than M 6.5 occurred near the Ganqingchuan border were calculated using the Coulomb 3.3 software. The results indicated that the Jiuzhaigou earthquake was triggered by the Wenchuan earthquake and related to the stress adjustment in the study area.

Development of double-pair double difference earthquake location algorithm for improving earthquake locations

Development of double-pair double difference earthquake location algorithm for improving earthquake locations

Complex deformation pattern of the Pamir–Hindu Kush region inferred from multi-scale double-difference earthquake relocations

The wide range of focal depths and the corresponding fault plane mechanisms in the Pamir–Hindu Kush region are believed to reflect the ongoing deformation along the Indian–Eurasian continental collision zone. Here we develop a multi-scale double-difference earthquake location algorithm to combine traveltime data recorded at local, regional and teleseismic distances. The 2906 relocated intermediate-depth earthquakes of MW≥4.5 we studied beneath the Hindu Kush show two groups of seismicity, with a separation of ca. 20–30km; these two groups exhibit thrust faults with different P-axes and different locations, reflecting the lateral geometry change of the seismic zone. Earthquakes beneath the Pamir present a clear south-dipping layer and unusual horizontal T-axes. There is a clear transition in direction of stress between the Hindu Kush and the Pamir regions. Here we focus on the complex deformation pattern beneath the continental collision zone arising from a combination of compression, tension, shearing and necking states.

Reservoir-induced seismicity in the Longtan reservoir, southwestern China

A pronounced increase in seismicity started in and around Longtan reservoir, southwestern China after October 1, 2006 when it began the impoundment, and by the end of May 14, 2010, about 3,233 earthquakes with −0.6 ≤ M L ≤ 4.2 had been located. This seismicity which occurred in five clusters mainly concentrated in the areas where few earthquakes had occurred before the first filling. There were four water filling periods in the Longtan reservoir, and the observed reservoir-induced seismicity (RIS) shows a strong correlation with the filling cycles. After the first filling, there appears to be an instant undrained response due to an elastic response to the reservoir load in the third and fourth cluster. Then, this seismicity is followed by a delayed, drained response due to pore pressure diffusion, with the seismicity migrating outwards in one or more directions in the second and third filling period. The seismic diffusivity (α s) we estimated is about 4.54 × 105 cm2/s. The activity levels in the five clusters are different due to differences in the structures and permeabilities of the faults. The delayed seismic response to the filling in the third cluster was due to the combined effects of the lack of local fault intersecting the reservoir and lower permeability of the rock. The b value we obtained for reservoir-induced events was significantly different and higher than that of pre-impoundment natural tectonic earthquakes in the Longtan reservoir. The results of relocated earthquakes based on double difference earthquake location algorithm showed that their focal depths were mainly shallower than about 10 km and the distribution of relocated RIS in four clusters had no relation with these intersecting faults in the Longtan reservoir except the fifth cluster. All these characteristics of RIS in the Longtan reservoir indicate that they may relate to the coupled poroelastic response that includes both pore pressure diffusion and an undrained response, but the pore pressure diffusion and the water permeation appear to play a more important role on inducing the earthquakes in Longtan reservoir.

High-resolution aftershock observations in the source region of the 2004 mid-Niigata Prefecture Earthquake

Abstract We deployed an extremely dense temporal seismic network in the source region of the 2004 mid-Niigata Prefecture Earthquake (thrust fault) on October 23, 2004, Japan. The seismic network consisted of 145 temporary seismic stations within a 30 km squared and had been kept within approximately a month after the mainshock. High accurate hypocenters of 708 events were determined by inverting the arrival times using double-difference earthquake location algorithm. The aftershocks along the mainshock (M w = 6.6) and the largest aftershock (M w = 6.3) rupture zones are distributed on two 60° westward-dipping planes, located approximately 5 km apart. Conversely, the Oct. 27 aftershock (M w = 5.8) occurred on an eastward dipping plane with a dip angle of 25° that was conjugate to the mainshock fault plane. Most of aftershocks at both northeastern and southwestern edges occurred at shallow depths with eastward-dipping planes. Epicenters of aftershocks in the southwestern region are aligned along N15°E, and rotate approximately 20° counterclockwise from the strike of the mainshock fault. This rotation of the aftershock alignments coincides with the rotation of anticline axes in the southwestern area of the source region. Furthermore, distributions of station corrections for a one-dimensional velocity model suggest that the seismic velocity at the western side of the Muikamachi-fault is lower than that at the eastern side. It is also inferred that the velocity structures in the hangingwall vary along the fault strike. The average velocity in the mainshock rupture area is higher than the periphery in the hangingwall, especially compared with the southwestern side of the hypocenter.

Study on S wave splitting in Dayao earthquake sequence with M=6.2 and M=6.1 in Yunnan in 2003

The polarization direction of fast wave and the delay time between fast and slow wave were measured for two earthquake sequences occurred continuously on 21 July (M=6.2) and 16 October (M=6.1) in Dayao, Yunnan in 2003 using cross-correlation coefficient method, after determining the high-resolution hypocentral locations of the earthquake sequences using the double-difference earthquake location algorithm. The results indicated that ① The phenomena of S wave splitting are obvious in the two earthquake sequences, and the average polarization directions of fast wave in most stations are almost consistent with regional maximum horizontal compressive stress direction except the station Santai. There are bimodal fast directions in the polarization directions at station Santai and the mean polarization direction is N80°E, indicating an inconsistent phenomenon referred to regional maximum horizontal compressive stress direction. ② There is no apparent relation between delay time and focal depth in the sequences, but the polarization direction show different character in different delay time range. ③ The comparison of S wave splitting results in the two earthquake sequences show that the polarization direction in M=6.2 earthquake sequence is more scattered and its average fast direction is 20° larger than that of M=6.1 sequence, and the delay times between two sequences show a little difference. ④ The spatial variation in S wave splitting polarization direction may be due to the stress disturbance imposed by the M=6.2 and the M=6.1 mainshocks on regional background stress field.

An improved double-difference earthquake location algorithm using s P phases: application to the foreshock and aftershock sequences of the 2004 earthquake offshore of the Kii peninsula, Japan (Mw=7.5)

AbstractWe have useds Pphases to improve the hypocentral locations of the earthquakes that occurred offshore southeast of the Kii peninsula in association with the Mw=7.5 mainshock on September 5, 2004. The earthquakes were more than 100 km from the onshore seismic network and, thus, their focal depths were poorly constrained. Thes Pphases were recorded about 7–11 s after the initialPphases, with both phases having almost the same apparent velocities. The computation of ray-paths revealed that the arrival times of the laters Pphases are sensitive to focal depths. We have recalculated the hypocenters and origin times for 36 events with more than sixs Pphase identifications on seismograms recorded at the High Sensitivity Seismograph Network by includings Pphases in a double-difference earthquake location algorithm, which eliminates the errors introduced by crustal heterogeneity. The relocation results were then compared with those from the Japan Meteorological Agency based on traditional absolute location techniques.

Fault Orientation in the Eastern Tennessee Seismic Zone: A Study Using the Double-Difference Earthquake Location Algorithm

We use the double-difference earthquake location algorithm to relocate approximately 1,000 earthquakes in the eastern Tennessee seismic zone (ETSZ). We examine the earthquake hypocenter relocations in an effort to resolve fault orientations and thereby gain insights into the tectonics of the seismic zone. The analysis involves visual comparison of three-dimensional perspective plots of the hypocenter relocations oriented accord ing to focal mechanism nodal planes derived from events within several clusters of earthquakes. Relocations in the central, most seismically active, portion of the ETSZ indicate a diffuse west-striking, north-dipping zone of hypocenters. The orientation of this zone of hypocen ters is consistent with structure imaged by a previous seismic reflection profile that shows a reflective mid-to-upper crust with apparent dips of approximately 35° to the north. Relocation of a smaller cluster of earthquakes near the Tennessee-North Carolina border suggests a steeply dipping northwest trending fault in that area.

The effect of distribution of stations upon location error: Statistical tests based on the double-difference earthquake location algorithm and the bootstrap method

AbstractIn this letter, we investigate the effect of station distribution (including the number and azimuthal gap of stations) upon location error based on the field data observed at Northern California Seismic Network (NCSN) using a double-difference earthquake location algorithm and a bootstrap method. The earthquakes relocated by all 117 stations are set as reference and the error of location is defined as the RMS of the difference to the reference. We find that the location error has a nonlinear relationship with the distribution of stations. The results may be used as guidelines for building seismic network.

Multi-planar structures in the aftershock distribution of the Mid Niigata prefecture Earthquake in 2004

AbstractA shallow destructive inland earthquake of M6.8 occurred on Oct. 23, 2004 (JST) in the middle part of Niigata prefecture, Japan. The aftershock activity was high as compared to those of the other shallow inland earthquakes in Japan. We relocated the sources of the main shock and aftershock activity by adopting the double-difference earthquake location algorithm. The precisely relocated hypocenter distribution shows that there are a doubleplanar structure with a distance 5 km in parallel dipping in WNW with about 50 degrees and a single-planar structure dipping in ESE with about 15 degrees. The upper and lower planes of the double plane began to be formed with the main shock and the largest aftershock of M6.5, respectively, while the ESE dipping plane began to be formed with the 3rd largest aftershock of M6.1. The three planes represent the fault planes of the large events mentioned above. The aftershock distribution is found to be a superposition of the fault planes of the main shock and the large aftershocks. The high aftershock activity is attributed to the formation of the fault planes.

Double-difference relocation of earthquakes in central-western China, 1992–1999

The double-difference earthquake location algorithm was applied to the relocation of 10,057 earthquakes that occurred in central-western China (21∘N to 36∘N, 98∘E to 111∘E) during the period from 1992 to 1999. In total, 79,706 readings for P waves and 72,169 readings for S waves were used in the relocation. The relocated seismicity (6,496 earthquakes) images fault structures at seismogenic depths that are in close correlation with the tectonic structure of major fault systems expressed at the surface. The new focal depths confirm that most earthquakes (91%) in this region occur at depths less than 20 km.

Relocation of the 1998 Zhangbei-Shangyi earthquake sequence using the double difference earthquake location algorithm

On January 10, 1998, at 11h50min Beijing Time (03h50min UTC), an earthquake ofML=6.2 occurred in the border region between the Zhangbei County and Shangyi County of Hebei Province. This earthquake is the most significant event to have occurred in northern China in the recent years. The earthquake-generating structure of this event was not clear due to no active fault capable of generating a moderate earthquake was found in the epicentral area, nor surface ruptures with any predominate orientation were observed, no distinct orientation of its aftershock distribution given by routine earthquake location was shown. To study the seismogenic structure of the Zhangbei-Shangyi earthquake, the main shock and its aftershocks withML≥3.0 of the Zhangbei-Shangyi earthquake sequence were relocated by the authors of this paper in 2002 using the master event relative relocation technique. The relocated epicenter of the main shock was located at 41.145°N, 114.462°E, which was located 4 km to the NE of the macro-epicenter of this event. The relocated focal depth of the main shock was 15 km. Hypocenters of the aftershocks distributed in a nearly vertical plane striking 180°200° and its vicinity. The relocated results of the Zhangbei-Shangyi earthquake sequence clearly indicated that the seismogenic structure of this event was a NNE-SSW-striking fault with right-lateral and reverse slip. In this paper, a relocation of the Zhangbei-Shangyi earthquake sequence has been done using the double difference earthquake location algorithm (DD algorithm), and consistent results with that obtained by the master event technique were obtained. The relocated hypocenters of the main shock are located at 41.131°N, 114.456°E, which was located 2.5 km to the NE of the macro-epicenter of the main shock. The relocated focal depth of the main shock was 12.8 km. Hypocenters of the aftershocks also distributed in a nearly vertical N10°E-striking plane and its vicinity. The relocated results using DD algorithm clearly indicated that the seismogenic structure of this event was a NNE-striking fault again.

Earthquake relocation and 3-dimensional crustal structure of P-wave velocity in cen-tral-western China

A simultaneous inversion of earthquake relocation and three-dimensional crustal structure of P-wave velocity in central-western China (21°N∼36°N, 98°E∼112°E) were performed in this paper. The crustal P-wave velocity model and earthquake relocation for this region are obtained using Pg and Sg phase readings of 9 988 earthquakes from 1992 to 1999 recorded at 193 seismic stations within central-western China by SPHYPIT90 and SPHREL3D90 programs. A lateral inhomogeneous structure of P-wave velocity in this region was obtained. Obvious contrast of P-wave velocities was revealed on both sides of active fault zones. Relocated epicenters of 6 459 events show clear lineation along active faults, which indicated a close correlation between seismicity and the active faults in this region. Focal depths of 82% relocated events ranged from 0 to 20 km, which is in good agreement with that from double-difference earthquake location algorithm.

Relocation of the 1998 Rueyli, Taiwan, earthquake sequence using three-dimensions velocity structure with stations corrections

To improve location identification, we employed layered mode joint hypocenters determination (JHD) and double-difference earthquake location algorithm (hypoDD), three-dimensions velocity structure location (3DLOC), and three-dimensions velocity structure with stations corrections location (3DCOR) for the Rueyli, Taiwan, earthquake sequence. We found that the 3DCOR can greatly improve the location identification precision of the Rueyli earthquake sequence. The Rueyli mainshock was relocated to 23.510˚N, 120.660˚E at the depth of 5.6 km. The focal depth is significantly deeper than the original CWBSN value of 2.8 km. Using this new location, a first-motion fault-plane solution shows that this earthquake is a thrust event with strike N53˚E, dip 42˚E, and rake 121˚. Based on the hypocentral distribution pattern ascertained after relocation and focal mechanism, the Rueyli earthquake was probably caused by faulting in the Tachienshan fault.

A Double-Difference Earthquake Location Algorithm: Method and Application to the Northern Hayward Fault, California

We have developed an efficient method to determine high-resolution hypocenter locations over large distances. The location method incorporates ordinary absolute travel-time measurements and/or cross-correlation P-and S-wave differential travel-time measurements. Residuals between observed and theoretical travel-time differences (or double-differences) are minimized for pairs of earthquakes at each station while linking together all observed event-station pairs. A least-squares solu- tion is found by iteratively adjusting the vector difference between hypocentral pairs. The double-difference algorithm minimizes errors due to unmodeled velocity struc- ture without the use of station corrections. Because catalog and cross-correlation data are combined into one system of equations, interevent distances within multiplets are determined to the accuracy of the cross-correlation data, while the relative lo- cations between multiplets and uncorrelated events are simultaneously determined to the accuracy of the absolute travel-time data. Statistical resampling methods are used to estimate data accuracy and location errors. Uncertainties in double-difference locations are improved by more than an order of magnitude compared to catalog locations. The algorithm is tested, and its performance is demonstrated on two clus- ters of earthquakes located on the northern Hayward fault, California. There it col- lapses the diffuse catalog locations into sharp images of seismicity and reveals hor- izontal lineations of hypocenters that define the narrow regions on the fault where stress is released by brittle failure.