Statistical characteristics of enhanced seismicity before strong earthquakes based on earthquake cases in Chinese mainland

Before strong earthquakes, enhanced seismicity manifested by increased magnitude, frequency, or accelerated strain release generally appears within a specific temporal and spatial range of the source area. This significant enhanced seismicity is often observed before moderate-strong earthquakes occurred in Chinese mainland or abroad. The seismogenic processes of large earthquakes are multiscale and diverse, involving localization of deformation, fault heterogeneities, and variable local loading rate effects. Enhanced seismicity prior to moderate-strong earthquakes is closely related to such processes and exhibits different characteristics. An in-depth study of enhanced seismicity will help us to understand the seismogenic process of strong-large shocks, which may bring positive effect on predicting strong shocks based on them.

Many studies have been performed to analyze the characteristics of enhanced seismicity. A more thoughtful and systematic study is needed due to rapidly increased strong earthquake data in Chinese mainland and the urgent requirement for statistical predictive indicators. In this study, we intend to summarize the statistical characteristics of the prominent enhanced seismicities before moderate-strong earthquakes and attempt to seek the proper mechanism. Based on Earthquake Cases in China （1966−2017）, the spatio-temporal characteristics of the seismicity before strong earthquakes with magnitude M_S≥6.0 in the eastern Chinese mainland and M_S≥7.0 in the western Chinese mainland are summarized statistically. In the meantime, the regional features of enhanced seismicity before the strong earthquakes within the SichuanYunnan rhombic block, Bayan Har block, and North China block are also studied. The main contents and conclusions are as follows:

Among the 33 earthquake cases studied in this paper, 21 showed enhanced seismicity before the main shock, accounting for 64%, including five earthquakes of M_S6.0−6.9, 14 earthquakes of M_S7.0−7.9, and two earthquakes of M_S8.0 or above. The percentage of enhanced seismicity is 42% for sub-grade class from M_S6.0 to M_S6.9, 74% for for sub-grade class from M_S7.0 to M_S7.9 and 100% for sub-grade class from M_S8.0 or above. The possibility of the occurrence of enhanced seismic activity will increase with the magnitude of the main shock. The enhancement of seismicity appeared in 13 out of 18 cases in western Chinese mainland, accounting for 72%; 8 out of 15 cases in eastern Chinese mainland, accounts for 53%.

Secondly, in most cases for western Chinese mainland, the spatial extent of enhanced seismicities was observed within the intermediate or tectonic scale, and the probability of enhanced seismicity with a significant spatial scale increases with the magnitude of the main shock. Furthermore, the likelihood of enhanced seismicity with a large magnitude in western Chinese mainland is higher than in eastern Chinese mainland. The larger the average magnitude of enhanced premonitory seismicity, the more likely strong earthquakes with M_S>7.0 occur. The duration of seismic activity enhancement in the western Chinese mainland is directly proportional to magnitude of the main shock, while in the eastern Chinese mainland, the relatively more significant events tend to be associated with a mid-short-term enhanced seismicity.

Thirdly, the strong earthquakes in Sichuan-Yunnan rhombic block were preceded by the medium-long term intersected seismic strips, the various spatial-scale seismic gaps, and the enhancement of small-moderate earthquakes at medium-short-term scales. These features significantly indicate the location of further quakes, which deserves more attention. Different from the eastern border of the Bayan Har block, at the other three boundaries of the Bayan Har block, strong earthquakes are often attacked with seismic gaps encircled by premonitory medium-strong earthquakes. The seismic gap generally arises in medium-long-term time scales, and the mid-short-term scale enhanced seismicity is notable before strong earthquakes in the northern margin of North China block. In particular, the magnitude of the Haicheng earthquake is comparable to that of the Tangshan earthquake. Still, the Haicheng earthquake was not preceded by a significant and long seismicity enhancement, which suggests that the secondary blocks or adjacent tectonic influences may also control the enhancement of seismicity before earthquake.

Fourthly, enhanced seismicity prior to large earthquakes drives damage to the surrounding rocks. These enhanced seismicities are not limited to the faults that generate large earthquakes. Still, they drive distributed rupture and local rock mass deformation, ultimately resulting in major slip zones and large earthquakes. Laboratory studies of rocks and similar samples have shown that a relatively long period of distributed deformation precedes the onset of large ruptures. The enhanced seismicity manifested in foreshocks is the most significant signal for the subsequent occurrence of a larger seismic event at a similar time and space. However, the enhanced seismicity does not appear as a foreshock in every case related to the seismogenic mechanism. The cascade-up framework and pre-slip model are generally used to account for the occurrence of a foreshock, whereas the progressive localization framework is suitable for explaining the enhancement of seismicity without significant foreshocks.