大陆型山谷冰川冰震的规律和影响因素——以老虎沟12号冰川为例

Rule and affectingfactors of seismic events in valley glacier with continental features:A case studyon Laohugou Glacier No.12

2015年10月,利用便携式宽频带地震仪在祁连山老虎沟12号冰川上进行了9d的冰震观测,在台站的震动信号中识别出大量与冰川动态特性相关的冰震。根据信号的频率等特征,可将其分为短周期冰震和长周期冰震。短周期冰震的持续时间大多在0.1s以下,频谱集中在20～100Hz之间,其数量可达每天6 600个,占所检测到冰震的绝大部分。长周期冰震持续时间可达几十秒至数小时,其地震波形频率集中在0.1～10Hz,观测期间检测到296次长周期冰震。对比冰震发震时间和当地气温发现:气温下降,短周期冰震事件数量快速增加;气温升高,短周期冰震数量逐渐减少。短周期冰震的发震时间分布呈现出以1d为周期变化的特征。利用麦克斯韦模型模拟的冰川在气温影响下的应力变化可以解释观测到的冰震日变性:当气温下降,冰川温度下降,冰体积收缩并产生应变,相应的拉张应力使得冰裂隙破裂而产生短周期冰震信号。

Icequakes on Laohugou Glacier No.12were observed by operating a portable digital seismometer for 9days in Oct.2015.Lots of seismic signals relating to glacier dynamic characteristics were found.On the basis of signal frequency and duration time,two types of seismic event can be divided.Short-period icequakes have time duration less than 0.1s,the frequency domains are among 20-100Hz and the numbers are up to 6600per day.The majority of seismic events are short-period icequakes.Long-period icequakes have time duration from several minutes to several hours,the frequency domains are among 0.1-10Hz and the numbers are up to 296during the observation.Comparing the time distribution of short-period icequakes with local air temperature,it is found that once temperature drops,the numbers of short-period icequake increase rapidly. And once temperature rises,the numbers of short-period icequake decrease.The time distributions of short-period icequakes have a diurnal cycle.It is inferred that once air temperature decreases,temperature of glacier ice decreases and as a result,the fracture at the surface of glacier under tensile thermal stress was happened. Using Maxwell Model to calculate glacier stress change,the diurnal cycle of short-period icequakes can be explained.