Abstract:

Yishu fault zone is the Shandong section of the Tanlu fault zone, and it is the structural boundary between the Luxi fault block and the Ludong fault block. In history, the Tancheng M_S8¹/₂earthquake in 1668 and the Anqiu M_S7.0 earthquake in 70BC occurred with serious casualties and property losses. The Anqiu-Juxian Fault is a Holocene active fault in the Yishu fault zone. Predecessors have conducted a lot of research on the fault by means of field geological survey, magnetotelluric measurement, small earthquake activity analysis, GPS velocity field, etc. and found that the north section of the Yishu fault zone is currently in the locking stage and has the potential for strong earthquakes with M_S≥7.
The mobile gravimetry is an effective means for medium and long-term prediction of strong earthquakes. The gravimetry along the Yishu fault zone was conducted using the LCR-G and CG-5 relative gravimeters every half year from 2010 to 2020. The mobile gravimetry anomalies occurred in the north section of the Yishu fault zone in 2019, and the area was delineated as an earthquake risk area based on the mobile gravimetry anomaly. Then the anomaly was confirmed to be caused by groundwater decrement. Therefore, it is necessary to make clear whether the mobile gravimetry anomalies are caused by tectonic or non-tectonic background field variations.
In order to provide effective mobile gravimetry data for medium and long-term forecasting of earthquakes, we divide the Yishu fault zone area into plain area and mountainous hilly area according to the different geographical landforms to study the tectonic background and characteristics of stratum lithology, geomorphology and hydrogeology of the two types of areas. Based on the 22 periods of high-precision mobile gravity data from 2010 to 2020, the high temporal-spatial resolution GPS images and the distribution variation characteristics of groundwater are analyzed, the influence of groundwater and vertical crustal deformation on gravimetry is calculated quantitatively using the infinite plane layered model and the approximate proportional relationship between gravity and vertical crustal deformation, and the spatial distribution of gravity effects of groundwater and vertical deformation and the corrected gravity variation image are obtained. Then, combined with the leveling and GPS research results in recent years, the possibility of earthquake in the near future in the Yishu fault zone is judged. Finally, the time-sequence curve of gravity and gravity effects of groundwater and vertical deformation of Guangrao, Shouguang and Changyi stations are drawn to summarize the time-varying characteristics and synchronization of groundwater, vertical deformation and gravity.
Through the above correction processing, we find that:
(1)The multi-year gravity anomaly in the north section of the Yishu fault zone since August 2010 has a strong correlation with the decline of groundwater level and land subsidence in the region. The maximum impact of groundwater lowering on gravity is -102.21μGal, and the maximum impact of land subsidence is 190.56μGal. The superposition effect is 195.72μGal. The gravity field after correction is stable as a whole, and the variation in the positive and negative transition regions is not dramatic, indicating that there is no large differential movement of underground materials. Combined with the leveling and GPS research data, it is speculated that there is less possibility of strong earthquakes in the north section of the Yishu fault zone in the near future.
(2)The influence of groundwater and land subsidence on the mobile gravimetry mainly occurs in the north section of the Yishu fault zone, which is distributed in Quaternary strata, and there is little such interference in the bedrock-outcropped middle and south section of the Yishu fault zone. When gravity anomaly occurs in the plain area due to the variation of non-structural background field, it is likely to misidentify it as an earthquake precursor. Therefore, in the study of gravity anomaly, groundwater and vertical crustal deformation correction should be carried out first to eliminate the gravity anomaly caused by the variation of the non-tectonic background field.
(3)Deep confined water funnel area is often accompanied by land subsidence and obvious gravity variations. Phreatic decline generally does not cause land subsidence, but water loss will cause negative gravity change to a certain extent.
(4)In the daily correction of mobile gravimetry data, the mean annual correction constant can be calculated according to the magnitude variation law of groundwater and vertical crustal deformation in recent years, which has certain positive significance for the analysis, application and related research of mobile gravimetry data.

Key words: Yishu fault zone, gravity variations, water level variations, land subsidence, correction

摘要：

2010-2020年间利用LCR-G相对重力仪及CG-5相对重力仪对沂沭断裂带地区进行了每0.5a一期的相对重力观测, 通过对重力观测数据、 水文资料和形变资料的处理分析, 发现: 1)2010年8月以来沂沭断裂带北段出现的多年期重力异常与该地区地下水位下降及地面沉降有强相关性, 地下水位变化对重力的影响可达-102.21μGal, 地面沉降产生的最大影响为190.56μGal, 二者叠加影响可达195.72μGal; 2)流动重力观测在基岩出露的山地丘陵区一般不存在非构造背景场干扰, 而当第四系平原地区的重力场出现大幅变化时, 应重点明确是否为水位变化、 地面沉降等因素导致; 3)深层承压水漏斗区往往伴随地面沉降及显著的重力变化, 浅层潜水下降一般不引起地面沉降, 但水体流失会引起地面一定的重力负值变化; 4)日常对流动重力测量资料进行校正时, 可根据测点近年受地下水及垂直形变干扰的量级变化规律计算其年均校正常数, 这对流动重力资料的分析、 应用以及相关研究具有一定的积极意义。

关键词: 沂沭断裂带, 重力场变化, 水位变化, 地面沉降, 校正