Earthquakes with MS5.0 and MS6.0 were extremely active in the Xinjiang region from June, 2011 to March, 2013, exhibiting the characteristics of continuous occurrence in time and multi-band distribution in space, and as many as 20 earthquakes with MS≥5.0 occurred within the borders of Xinjiang (including 4 MS6.0 earthquakes). This group of MS≥5.0 earthquakes occurred mainly along the middle-eastern segment of the Tianshan Mountains where seismic activity was weak at the earlier stage, while in the Kalpin block, where seismic activity was more intensive, MS6.0 seismic quiescence had lasted for 8.7 years since the Wushi MS6.3 earthquake taking place in 2005, exceeding the maximum quiescence interval (6 years) for MS6.0 earthquakes in this area. From April 2013 to November 2013, only two MS5.0 earthquakes occurred in the Xinjiang region, basically falling back to the background level (Wang Qiong et al., 2014). Overall, the Kalpin earthquake occurred in a transition phase from the state of continuous occurrence of MS5.0 earthquakes to the relatively weak activity in Xinjiang. This paper introduces the basic parameters, focal mechanism solutions, and earthquake sequence characteristics of the Kalpin MS5.3 earthquake sequence, and preliminarily reviews and summarizes all kinds of seismological anomalies and the fixed-point precursor observation anomalies before the earthquake.1 BASIC SEISMIC PARAMETERS
The Kalpin MS5.3 earthquake occurred in a region with strong seismic monitoring capability in Xinjiang. Table 1 compares location results obtained from the Earthquake Administration of Xinjiang Uygur Autonomous Region, China Earthquake Networks Center, Harvard University and USGS, and basic parameters for the earthquake are basically the same. Therefore, the locating result from the earthquake catalogue of the Earthquake Administration of Xinjiang Uygur Autonomous Region is selected in this article, and the results of the earthquake magnitude and depth from the China Earthquake Networks Center are adopted. That is, the Kalpin MS5.3 earthquake occurred at 04:34:26 p.m., on December 1, 2013, with microscopic epicenter at 40°19′ north latitude and 78°54′ east longitude, and focal depth of 9km.
3 Earthquake Administration of Xinjiang Uygur Autonomous Region, Monthly Report Catalogue of Xinjiang, 2013-12.
4China Earthquake Administration, the National Earthquake Catalogue MS5.0, 2013.2 GEOLOGICAL TECTONIC SETTING AND FOCAL MECHANISM SOLUTION
According to on-site investigation, it is preliminarily determined that this earthquake occurred at the Kalpin fault, the southernmost tip of the Kalpin nappe where the seismotectonic environment is complex 2. The Kalpin fault developed along the piedmont of Kalpin Mountain, NEE-striking, with a length of 460km and dip angle of 40°-60°, which is a left-lateral strike-slip reverse fault (Zhu Lingren, 2002). Earthquakes with MS≥6.0 occur frequently along this fault, such as the strong earthquake swarm with MS6.0 in June, 1961, the MS6.2 earthquakes in 1972 and 1977 and the MS6.5 earthquake on February 25, 1991.
2 Earthquake Administration of Xinjiang Uygur Autonomous Region, 2013. Report of Assessment Results of Disaster Investigation of the Kalpin MS5.3 Earthquake in Xinjiang on December 1, 2013.
Focal mechanism solutions for the earthquake obtained by different institutions and individuals are listed in Table 2 (Fig. 1). All results show reverse fault type with left-lateral strike-slip component, which is consistent with the nature of seismogenic faults. Li Jin from the Earthquake Administration of Xinjiang Uygur Autonomous Region calculated the strikes of nodal planes of the main shock using the CAP method, and obtained the strike of nodal planeⅠ, 227°, and strike of nodal planeⅡ, 68°. The strike of nodal planeⅠis coincident with that of the seismogenic fault, and in combination with earthquake damage investigation results in the earthquake-stricken area 2 and spreading direction of aftershocks, analysis suggests that nodal planeⅠ, striking 227°, is the fracture plane caused by this earthquake.
According to the measurement of the Xinjiang Regional Digital Seismic Network, up until February 28, 2014, altogether 36 earthquakes of the Kalpin MS5.3 earthquake sequence occurred with ML≥1.0, including 1 earthquake with ML5.0-5.9, 1 earthquake with ML4.0-4.9, 2 earthquakes with ML3.0-3.9, 4 earthquakes with ML2.0-2.9 and 28 earthquakes with ML1.0-1.9. Among them, the largest aftershock had a magnitude of ML4.0, which occurred 7 days after the main shock (Fig. 2), with a magnitude difference of 1.8 with the secondary largest earthquake. The energy of the largest earthquake in the Kalpin ML5.3 earthquake sequence accounts for 99.89% of the total sequence energy, and according to the evaluation index of earthquake type, it is indicated that this earthquake sequence belongs to the mainshock-aftershock type (Wang Haitao, 1992).
It can be seen from the M-t and N-t maps (Fig. 2) that the daily frequency of aftershocks attenuated rapidly within 20 days after the main shock, which had basically decreased to the normal activity level, showing fast frequency attenuation. Among them, aftershocks with ML≥3.0 occurred 3 times, with the largest aftershock occurring 7 days after the main shock. Fig. 3 is the distribution map of epicenters of the Kalpin MS5.3 mainshock-aftershock sequence comprehensively positioned by the Xinjiang Regional Digital Seismic Network. The results show that the main shock and aftershocks spread in NE direction on the whole, basically consistent with the seismogenic structure, the Kalpin fault. The aftershocks are mainly distributed within 10km in the south of the main shock.
The minimum magnitude of completeness for aftershock sequence is determined to be ML1.5 based on 1gN-M chart. The Kalpin MS5.3 earthquake sequence includes only 20 ML≥1.5 earthquakes. Due to lack of sample numbers, the obtained sequence seismological parameters are unreliable, thus calculation and analysis are not carried out in this article.4 ANOMALY CHARACTERISTICS OF SEISMIC ACTIVITIES BEFORE THE KALPIN MS5.3 EARTHQUAKE 4.1 MS2.0-4.0 Earthquake Nesting Quiescence
Since March, 2013, there has been a MS4.0 seismic quiescence for 142 days in the Wuqia-Xinyuan region, which was shattered by the Nilka MS4.3 earthquake on August 3, 2013. Scans show that the possibility of the occurrence of MS≥6.0 earthquakes is 5/6 later in the quiescence zone. In addition, there was a seismic nesting quiescence of MS2.0-MS4.0 earthquakes in the MS4.0 seismic quiescence zone, for which, there was a MS3.0 seismic quiescence for 81 days in the Kashi-Xinyuan region from May 15 to August 3, 2013. The scanning also shows that the possibility of occurrence of moderate-strong earthquakes is 3/5 later in the quiescence zone, and there was a MS2.0 seismic quiescence for 76 days in the Bachu-Baicheng region from May 19, 2013 to August 3, 2013. The scan result shows that the possibility of the occurrence of moderate-strong earthquakes is 2/3 later in the quiescence zone. The Akqi MS3.7 earthquake broke the MS2.0 to MS4.0 seismic quiescence in the region, followed by 6 successive MS3.0 earthquakes in August-September (Fig. 4). Analysis suggests that the MS2.0 to MS3.0 seismically nesting quiescence is an anomaly of this earthquake.
After the Artux MS5.2 earthquake on March 11, 2013, there were 8.7 months of MS4.0 seismic quiescence in the Kalpin block (Table 3). The earthquake case study shows that the corresponding ratio of moderate-strong earthquakes along the western part of the south Tianshan Mountains is 68.4% (13/19) when the MS4.0 seismic quiescence in the Kalpin block is more than 4 months. This earthquake directly broke the MS4.0 seismic quiescence in the Kalpin block.
An 890km-long, NNE-trending seismic belt with magnitude over 4.0 has been formed in the middle part of the Kalpin-west Kunlun Mountains region (Fig. 5) since August, 2011. This seismic belt took a quite long time to develop, with the possibility of MS≥6.0 earthquakes nearby. Although the Kalpin MS5.3 earthquake occurred near the belt, given the long formation time, analysis suggests that this earthquake is not a target earthquake for the MS 4.0 seismic belt.
A 700km-long, NNE-trending seismic belt with magnitude of over 3.0 was formed in the Zhaosu-Yecheng region from June-November 2013 (Fig. 6), and intersects the MS4.0-MS5.0 seismic belts developed in the middle part of the Kalpin-west Kunlun Mountains region at the vicinity of the Puchang fault. The MS3.0 seismic belt took a shorter time to develop, with the possibility of occurrence of MS≥5.0 earthquakes. The Kalpin MS5.3 earthquake took place near the MS3.0 seismic belt, and analysis suggests that the Kalpin MS5.3 earthquake is the target earthquake of the MS3.0 seismic belt.
Previous studies (Wang Xiaorong, 2001) show that within the range of 2°×2° along the south Tianshan seismic belt, the occurrence of more than 4 earthquakes with MS3.5 within 3 months, which must include more than 3 MS4.0 earthquakes, shall constitute an anomalous increase of seismicity. The earthquake case study shows that of 16 moderate earthquakes along the south Tianshan Mountains, seismic enhancement appeared before the occurrence of 14 earthquakes.
From December, 2012 to March, 2013, moderate earthquakes were extremely active near the Puchang fault (Fig. 7), and altogether 13 MS≥3.0 earthquakes took place successively, including 8 earthquakes with MS3.0, 4 earthquakes with MS 4.0 and the Artux MS5.2 earthquake on March 11, 2013, forming a moderate seismic enhancement region. The Kalpin MS5.3 earthquake occurred in the vicinity of abnormal enhancement region on December 1, 2013.
According to various tectonic environments in different regions in Xinjiang, the anomaly characteristics of seismological parameters before moderate-strong earthquakes are analyzed by dividing the Xinjiang region into different zones by seismic belts. MS≥2.0 earthquakes occurring in the Aksu-Bachu region since 1980, excluding aftershocks, are selected and scanned, with a time window of 12 months and step length of 2 months, and the target earthquakes have magnitude of 5.0. It is agreed that the anomaly lasted for more than half a year, and the anomaly lasting for more than half a year after a corresponding earthquake is regarded as another anomaly. Scanning results show that before the Kalpin MS5.3 earthquake, the A(b) value, AC value, seismicity gap, seismicity gap of η value and 5 parameters for S value all showed some anomalies (Table 4), which disappeared after the earthquake. The anomaly is considered to correspond to this earthquake.
Fixed-point precursor observation anomalies were reported before the earthquake except for the hydrogen observation at Aksu fault, which was summarized after the earthquake. However, since this station was only set up in November, 2013, with short-term history data and relatively significant controversy, this observation item is not included in this study.
There are 9 fixed-point precursor observation stations within the range of 200km; Wushi, Wensu, Bachu, Aksu, Awati, Akqi, Kalpin, Halajun and Jiashi stations, of which only 3 stations lie within the range of 100km, with poor monitoring capability. Among the observation items of the 9 fixed-point precursor observation stations, the soil-layer stress instrument of the Bachu station broke down, and the gravity, geoelectricity, geomagnetism and water temperature at Wushi station, the electromagnetic disturbance at Awati station, water level, water temperature and water and electric conductance at Jiashi station, the Aksu fault creep deformation and Wensu electromagnetic wave all show no obvious anomalous changes, while other observation items displayed pre-earthquake anomalies to various degrees.5.1 Borehole Tilt Measurement in Kalpin
The borehole tiltmeter in Kalpin is 25km from the Kalpin MS5.3 earthquake on December 1. Continuous borehole tilt observation in Kalpin started in October, 2011. Since the observation, the E-W component of borehole tiltmeter has been inclining to the W direction, and the W-inclining rate slowed down from August to September 6, 2013, and accelerated later after September 7, deviating from normal trend change (Fig. 8).
The horizontal pendulum tiltmeter in Akqi is 84km from the Kalpin MS5.3 earthquake. Continuous observation of the horizontal pendulum in Akqi started from January, 1990, and was digitally reconstructed in 2007, with prediction efficiency rated B. Since 2007, the N-S component of the horizontal pendulum in Akqi has remained N-inclining, which was greatly influenced by the zero setting during the period. Since October, 2013, the N-S component of the horizontal pendulum in Akqi inclined backward to the S direction (Fig. 9).
The extensometer in Wushi is 102km from the Kalpin MS5.3 earthquake and the observation started officially in October, 2006. Because scale calibration for the E-W component on June 13, 2007 is flawed, the data of the component was incorrect from June 13 to November 30. Two components showed rapid extension from September 25-27, 2010, which was caused by water seepage in the cave from September 12 to 26, 2010.
The construction of piers for comparative measurement on August 27, 2013 also had some influence on the two components, so prediction efficiency is rated B. The E-W component of the extensometer in Wushi turns from compression to extension, with an anomaly amplitude of 4.2×10-7(Fig. 10).
The Wushi volumetric strain meter, 102km away from the Kalpin MS5.3 earthquake, was set up in 2007, and observation started officially in 2008. The main environmental interference comes from water pumping in a nearby water plant, and the influence is manifested in the form of distortion of the earth tidal curve, which has no obvious effect on trend change. The prediction efficiency is rated A. In early 2012, compressional strain rate of volumetric strain in Wushi was significantly slower compared to the same period in previous years. In order to compare the variation of velocity value at the same period each year, the difference of the velocity value with the average value of volumetric strain in Wushi is calculated. Since October, 2013, the high anomaly value of the velocity of volumetric strain in Wushi is anomalous (Fig. 11).
The new No.46 well of Wushi, 102km from the Kalpin MS5.3 earthquake, was built in 2007, with prediction efficiency rated B. During November 2-24, 2013, the water level of the new No.46 well rose rapidly by 63cm (Fig. 12). The Kalpin MS5.3 earthquake occurred 6 days after the water level resumed.
The Wushi component-type strain meter, 102km from the Kalpin MS5.3 earthquake, was set up in March, 2009. The main environmental interference came from water pumping in a nearby water plant, and the influence was manifested in the form as distortion of earth tidal curve, which had no obvious effect on trend change. The prediction efficiency is rated B. Since November 2, 2013, three components of the Wushi component-type strain have turned from extension to compression (Fig. 13).
The Halajun borehole tiltmeter, 186km from the Kalpin MS5.3 earthquake, was set up in December 2008, with prediction efficiency rated C. From June 26, 2013 to September 26, the borehole tilt vector in Halajun was knotted, and the anomaly lasted for 94 days (Fig. 14). The Kalpin MS5.3 earthquake occurred two months after the unknotting of the borehole tilt in Halajun. Since 2011, there have been a total of 6 knottings in Halajun borehole tilt, five of which were followed by moderate-strong earthquakes within three months.
In this article, by analyzing the characteristics of the Kalpin MS5.3 earthquake sequence on December 1, 2013 and the fixed-point precursor anomalies, the following conclusions are drawn.
(1) This earthquake occurred in the Kalpin block, where moderate-strong earthquake activity level was declining, starting after the Wushi MS6.3 earthquake in 2005. This earthquake occurred in the second half of 2011 when MS5 earthquakes took place continuously in Xinjiang, and seismic activity remained relatively weak in the relatively active Kalpin block.
(2) The seismogenic structure for the Kalpin MS5.3 earthquake on December 1, 2013 is the Kalpin fault. The mechanical nature of the earthquake source belongs to reverse faulting with left-lateral strike-slip component, and the strike of the fault plane is in line with the direction of long axis of the meizoseismal area, aftershock distribution and the Kalpin fault structure.
(3) Energy release of the main shock and aftershock attenuation show that this earthquake sequence is main shock-aftershock type. Aftershocks were mainly concentrated within 20 days after this earthquake, which shows rapid attenuation. The largest aftershock occurred 7 days after the main shock.
(4) Seismicity anomalies appearing before the earthquake mainly include the MS3.0 seismic belt, seismic quiescence of MS2.0, MS3.0 and MS4.0 earthquakes along the Wuqia-Xinyuan region, Kashi-Xinyuan region and Bachu-Baicheng region, abnormal enhancement areas of moderate earthquakes along the Puchang fault, as well as AC value, seismicity gap, A(b) value and S value anomalies in Aksu-Bachu region.
(5) Fixed-point precursory observation anomalies include mainly crustal tilting and crustal strain characterized by mutations. Except for the Halajun borehole tiltmeter, other fixed-point precursor observation anomalies exhibited features of migration towards the periphery from the epicenter, from near field to far field, and the number of abnormal observation items increased over time.
All the above anomalies were identified before the earthquake.
This paper has been published in Chinese in the journal of Inland Earthquake, Volume 29, Number 4, 2015.
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