Turbidites as records of intense palaeoearthquakes in Lake Biwa, Japan

Some 20 turbidites were collected in the uppermost 3 m of surface sediments in Lake Biwa, southwest Japan. These turbidites correlate in age to historical earthquakes that caused severe loss of lives and property near Lake Biwa. The age of each turbidite was estimated precisely, based on sedimentation rate (by weight) and the age of tephra markers below the turbidites. The magnitudes and epicentres of major historical earthquakes during the last 1300 years are available from historical documents. Empirical equations relating intensity, distance to the epicentre and magnitude enable us to calculate the ground shaking at Lake Biwa from these historical earthquakes. Earthquakes having intensity stronger than grade IV_L on the Japan Meteorological Agency (JMA) earthquake scale, that is 44 gal acceleration at Lake Biwa, all coincide with the turbidite ages. This intensity is apparently the lower threshold for turbidity-current initiation in Lake Biwa.