请将下面这段话翻译成英文：
东汉时期，地震频繁发生。通过对地震的一系列仔细观测，张衡于公元132年发明了地动仪(seismograph)，它是世界上第一个用来识别和确定地震方向的仪器。 地动仪表面上铸(cast)有八条龙，每一条龙的嘴里都有一个铜球。八条龙身下的地面上有八只仰着头的铜蟾蜍（toad)，铜蟾蜍张着的嘴恰与龙口相对。当地震发生时，面朝地震方向的龙口会张开，口中的球将落入蟾蜍的嘴里，自动指明地震的方向。
参考答案:
During the Eastern Han period，earthquakes occurred frequently. Through a series of careful observations of earthquakes，Zhang Heng invented the seismograph ,in 132 AD, which was the world's first instrument to identify and ascertain the direction of earthquakes. The seismograph is cast with eight dragons on the surface,and each dragon has a copper ball in the mouth. On the ground below the dragons there are eight copper toads raising their heads and opening their mouths opposite to the dragons'mouths. When an earthquake occurred, the dragon facing the direction of the earthquake would open its mouth，and the ball in its mouth would fall into the toad's mouth, automatically indicating the direction of the earthquake.
1.东汉时期，地震频繁发生:“发生”可用occur表达；“频繁”可翻译为frequently。
2.通过对地震的一系列仔细观测:文中翻译为through a series of careful observations of earthquakes。
3.用来识别和确定地震方向:“识别”可用identify表达；“确定”可翻译为 ascertain。
4.面朝地震方向的龙口会张开:“面朝”可用face表达。
5.指明:文中翻译为indicate。
