GRAVITY CHANGES BEFORE THE PINGYUAN MS5.5 EARTHQUAKE OF 2023

doi:10.3969/j.issn.0253-4967.2024.05.010

SEISMOLOGY AND GEOLOGY ›› 2024, Vol. 46 ›› Issue (5): 1172-1191.DOI: 10.3969/j.issn.0253-4967.2024.05.010

GRAVITY CHANGES BEFORE THE PINGYUAN M_S5.5 EARTHQUAKE OF 2023

LI Shu-peng¹⁾(), HU Min-zhang²^,³⁾^,^*(), ZHU Yi-qing⁴⁾, HAO Hong-tao²^,³⁾, YIN Hai-tao¹⁾, JIA Yuan¹⁾, CUI Hua-wei¹⁾, LU Han-peng¹⁾, ZHANG Gang¹⁾, WANG Feng-ji¹⁾, LIU Hong-liang⁵⁾

¹⁾ Shandong Earthquake Agency, Jinan 250014, China
²⁾ Key Laboratory of Earthquake Geodesy, Institute of Seismology, CEA, Wuhan 430071, China
³⁾ Gravitation and Earth Tide, National Observation and Research Station, Wuhan 430071, China
⁴⁾ The Second Monitoring Center, China Earthquake Administration, Xi'an 710054, China
⁵⁾ Mobile Survey Team of Hebei Earthquake Agency, Baoding 071000, China

Received:2023-09-20 Revised:2023-11-24 Online:2024-10-20 Published:2024-11-22

2023年山东平原M_S5.5地震前重力变化

李树鹏¹⁾(), 胡敏章²^,³⁾^,^*(), 祝意青⁴⁾, 郝洪涛²^,³⁾, 殷海涛¹⁾, 贾媛¹⁾, 崔华伟¹⁾, 陆汉鹏¹⁾, 张刚¹⁾, 王锋吉¹⁾, 刘洪良⁵⁾

¹⁾ 山东省地震局, 济南 250014
²⁾ 中国地震局地震研究所, 地震大地测量重点实验室, 武汉 430071
³⁾ 武汉引力与固体潮国家野外科学观测研究站, 武汉 430071
⁴⁾ 中国地震局第二监测中心, 西安 710054
⁵⁾ 河北省地震局流动测量队, 保定 071000

通讯作者: *胡敏章, 男, 1985年生, 研究员, 主要从事重力场均衡理论、重力场及其变化监测与地震分析预报研究, E-mail: huminzhang@126.com。
作者简介:
李树鹏, 男, 1991年生, 工程师, 主要从事流动重力观测方面的工作, E-mail: 570349661@qq.com。
基金资助:
山东省海洋地震观测研究团队(TD202401); 山东郯城巨震区低速率挤压逆冲构造野外科学观测研究站; 山东省地震局重点业务专项(YW2306); 中国地震局震情跟踪定向工作任务(2024010208)

Abstract

Abstract:

On August 6, 2023, an earthquake with M_S5.5 occurred in Pingyuan County, Dezhou City, Shandong Province, which is the largest earthquake in the Shandong region in the past 40 years. Before the earthquake, Shandong Earthquake Agency conducted biannual mobile gravity measurements near the epicenter, observed the spatiotemporal gravity field changes for the four years leading up to the earthquake, and made a certain degree of medium-term prediction, predicting that the epicenter location(36.00°N, 116.10°E)would be about 130km from the actual epicenter. This suggests that it is potentially feasible to carry out medium-term prediction of moderate earthquakes based on the temporal and spatial variations of the gravity field in the tectonically weak North China. Therefore, the study of the gravity changes before the 2023 Pingyuan M_S5.5 earthquake can help to deepen the understanding of the relationship between the time-space variations of the gravity field and the moderate earthquakes, enrich the database of “magnitude and gravity anomalies” in North China, and improve the science and accuracy of identifying and determining the medium- and long-term anomalies of earthquakes.

The mobile gravity data utilized in this paper were processed and calculated using the classical adjustment method in LGADJ software. This process involved corrections for earth tide, instrument height, monomial coefficient, air pressure, and zero drift, resulting in absolute gravity values for each measurement point. Eight absolute gravity points, including Jiaxiang, Tai'an, and Zibo, served as the starting reference points. The average accuracy of the observed data point values during each period ranged from 8.5 to 16.0μGal, indicating relatively high precision. Subsequently, the calculation results of the two data sets were subtracted to obtain the relative gravity change. This change was then interpolated on a continuous grid using the Surface module of GMT mapping software and subjected to 50-km low-pass filtering. Finally, the dynamic evolution image of the gravity field was generated.

Based on these results, this study analyzes the characteristics of regional gravity field changes since September 2019. These findings are integrated with information on deformation fields, seismic source mechanisms, and dynamic environments to explore the relationship between gravity changes before the earthquake and the seismic mechanism. The results indicate the following:

(1)Since May 2022, precursory anomalies have been detected in the gravity field changes around the epicenter. Between May 2022 and April 2023, there was a significant increase in positive gravity changes exceeding +50μGal and a spatial extent exceeding 160km in the south of the epicenter, with positive-negative differences exceeding 70μGal on both sides of the epicenter. However, the gravity changes near the epicentre remained stable and in a “locked” state. The magnitude, range, and duration of gravity changes before the earthquakes align with previously summarized indicators.

(2)Between September 2021 and September 2022, distinct four-quadrant distribution characteristics emerged in the regional gravity field changes. And the spatial distribution of regional gravity field changes corresponds to horizontal deformation fields, seismic source mechanisms, and coseismic displacement fields. Precisely, the compression zones of the seismic source mechanism and the inflow and subsidence areas of the coseismic displacement field correspond to regions of surface compression and gravity decrease before the earthquake. Similarly, the expansion zones of the seismic source mechanism and the outflow and uplift areas of the coseismic displacement field correspond to of surface expansion and gravity increase before the earthquake.

(3)The leading cause of the gravity changes anomaly before the Pingyuan M_S5.5 earthquake was the migration of deep-seated fluid materials, with the gravity effects generated by upper crustal deformation being a secondary factor. It is believed that the subduction of the Pacific Plate caused high-speed eastward migration of the relatively weak lower crust flow, dragging the upper crust eastward. The more rigid upper crust accumulated stress and strain during this process, developing numerous micro-fractures, while tectonic heterogeneity led to an east-west compression and north-south extension pattern. The fluid migration from compressed to expanded areas caused positive and negative differential changes in the gravitational field around the epicenter, culminating in the earthquake.

Key words: Pingyuan M_S5.5 earthquake, gravity changes, medium-term precursor, GPS

摘要：

2023年8月6日山东平原发生 M_S5.5 地震, 此次地震是山东地区40a以来发生的最大地震。震前山东省地震局在震中附近开展了每年2期的流动重力观测, 观测到了震前4a的重力场时空变化。文中利用流动重力观测资料, 系统分析了2019年9月以来的区域重力场变化特征, 并结合形变场、震源机制解、动力环境等探讨了震前重力变化与发震机理。结果表明: 1)2022-05—2023-04震中南部重力正值变化>50μGal, 区域直径>160km, 震中两侧正负差异变化>70μGal; 震中重力变化平稳, 处于“闭锁”状态。2)区域重力场变化与水平形变场、震源机制解、同震位移场的空间分布具有对应性——震源机制解的压缩区和同震位移场的流入区、沉降区对应了震前的面压缩区与重力下降区; 震源机制解的膨胀区和同震位移场的流出区、隆升区, 对应了震前的面膨胀区与重力上升区。3)此次地震前重力变化异常的主要原因是深部流体物质迁移, 次要原因是上地壳形变产生的重力效应。

关键词: 平原M_S5.5地震, 重力变化, 中期前兆, GPS

LI Shu-peng, HU Min-zhang, ZHU Yi-qing, HAO Hong-tao, YIN Hai-tao, JIA Yuan, CUI Hua-wei, LU Han-peng, ZHANG Gang, WANG Feng-ji, LIU Hong-liang. GRAVITY CHANGES BEFORE THE PINGYUAN M_S5.5 EARTHQUAKE OF 2023[J]. SEISMOLOGY AND GEOLOGY, 2024, 46(5): 1172-1191.

李树鹏, 胡敏章, 祝意青, 郝洪涛, 殷海涛, 贾媛, 崔华伟, 陆汉鹏, 张刚, 王锋吉, 刘洪良. 2023年山东平原M_S5.5地震前重力变化[J]. 地震地质, 2024, 46(5): 1172-1191.

Figures/Tables 11

Fig. 1 The gravity surveying route and tectonic outline of Shandong and its surrounding areas.

Table 1 Information of the gravity survey in the Shandong region

使用仪器	观测年月	点值平均精度/μGal	使用仪器	观测年月	点值平均精度/μGal
C451、C452	2019-09	16.0	C451、C452	2022-05	13.1
C216、C511	2020-04	8.5	C451、C452	2022-09	14.4
C216、C511	2020-09	8.7	C451、C452	2023-04	8.7
C451、C452	2021-04	9.1	C451、C452	2023-07	10.2
C451、C452	2021-09	12.7

Fig. 2 Gravity change patterns between two adjacent observation campaigns and variation accuracy.

Fig. 3 One-year sliding changes of the gravity field and variation accuracy.

Fig. 4 Accumulated Change Images and Change Aceuracy of the Gravity Field.

Fig. 5 Single-period variations and time series variation charts of gravity points in the southern of the epicenter.

Fig. 6 Single-period variations and time series variation charts of gravity points within 60km.

Fig. 7 Single-period variations and time series variation charts of gravity points s in the northeast of the epicenter.

Table 2 Quantitative indication of gravity variation and magnitude

指标名称	来源	M_S4.0	M_S5.0	M_S6.0	M_S7.0	M_S8.0
重力变化量级/μGal	胡敏章等(2019a),模型A	25	49	73	96	120
	胡敏章等(2019a),模型B	28	50	72	93	115
	祝意青等(2018)		≥50	≥80	≥100	≥120
重力变化平面范围/km	贾民育等(2000)		160	300	440	580
	祝意青等(2018)		100	200	400	600
	胡敏章等(2019a)		140	220	350	660
异常时间/a	祝意青等(2018)		0.5~1	1~3	3~5	≥5

Fig. 8 Comparison diagram of vertical deformation rate and gravity changes in the Shandong area.

Fig. 9 Comparison diagram of surface expansion rate and gravity changes in the Shandong area, and focal mechanism solution and co-seismic displacement field of the Pingyuan MS5.5 earthquake.

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