集集大地震後蚯蚓大量爬出現象之解析

Analysis of the Earthworm Outbreaks after Chichi Earthquake

1999年9月21日臺灣發生921集集大地震，其後在地震連連中，多處地方有大量蚯蚓出現於地面。民眾疑其為地震前兆，引起恐慌。以屏東為主的調查發現，含臺北在內，各地在10、11月大量爬出的蚯蚓均為同一種：Metaphire posthuma（Vaillant），其發生地點都在學校或公共場所的開闊草地附近，容易引人注意。在屏東縣萬丹鄉興化國小操場找到大量M. posthuma藏身於草根下及排水溝中，這些蚯蚓有在夜間3：30以後爬出地面，於當天10：00左右再回到地下的日周性行為。蚯蚓數與地震的相關係數未達顯著水準。時間序列分析之交叉相關函數顯示蚯蚓的大量爬出與其後25天內的有感地震的相關性都不顯著；蚯蚓爬出量與地溫、相對濕度及日照時數的相關性都很顯著（p<0.01）。於萬丹及萬巒實際挖掘到與M. posthuma共棲的多種蚯蚓，又訪查出半年以前在瑪家國中操場即曾二度有M. posthuma大量爬出。這些事實証明屏東有不只一種蚯蚓、不只一日爬出地面。此次蚯蚓事件，應是同樣天候條件籠罩臺灣之下，蚯蚓表現的自然行為；M. posthuma在發生地為優勢種，其大量爬出地面之現象，雖於地震後人心驚惶時特別容易被人發現並予報導，但從其大量爬出的重複行為和量變動，與地震發生時間比對並不足以論斷為地震前兆。追蹤2000年9-11月臺灣多處蚯蚓大量爬出，均佐証相同結論。 Chichi Earthquake (magnitude of 7.3 ML) occurred on 21 September 1999, inflicting heavycasualties and severe property damages to Taiwan. In that autumn, there were the occurrencesof frequent aftershocks and of several outbreaks of earthworms, the emergence of a largenumber of earthworms from underground to the surface. The earthworm outbreaks werebelieved to be an omen of upcoming earthquake by many local people. To elucidate thephenomenon we investigated the outbreaks at six localities: Shinhua Preliminary School,Wandan, Chaochou Preliminary School, Wanlaun Preliminary School, Majia Middle School,Hengchuen Water Supply Treatment Plant, and Juvenile Police Branch of Taipei MunicipalPolice Department. It was found that the earthworm outbreaks at these localities were caused bya single species, Metaphire posthuma (Vaillant) . We selected the Hsinhua Preliminary Schoolplayground as a site to monitor the daily behavior of the earthworms in relation to aftershocksand meteorological changes. The results showed that the earthworms emerged in the earlymorning around 3:30 AM, and disappeared in the late morning around 10:00 AM. Thisconsistent rhythm of the earthworm emergence occurred daily in October and November, with aslight delay in the late morning disappearance for one or two hours in cloudy and rainy days.The earthworms disappeared were found to hide in the ditch around the playground and beneathgrass roots in the ground. There was no obvious association of the numbers of the earthwormsin the daily emergence to the occurrences and magnitudes of the aftershocks. In contrast, thenumbers were significantly correlated to soil temperature, relative humidity and sunshineduration. We concluded that the outbreaks of the earthworm M. posthuma during the postChichi earthquake period were merely a natural behavior of the species in response to themeteorological changes.