ABSTRACT. Seismic m on ito ring on three C ascad e volca noes in W ashington Sta te, U .S.A., has shown that small seismic events are generated by t he active glaciers found on each mount ain. D eta iled seismic experime nts have been conducted to invest iga te the sources for these icequakes. C onsidera ble eviden ce indicates t hat the even ts are the result of a sti ck-slip type of motio n ta king place at the b ed of the glaciers. T he few events we have been a ble to locate had d epths comparable with the glaciers' thickness. The similarity of wave fo rm from a n explosion a t the bottom of a glacier a nd natura l icequa kes suggests tha t the complexity of the seism ic wave form is due to the path a nd n ot the source. The events exhibit a n a nnual trend with more events being record ed in the summer tha n during the w inte r. A ten-fold in cr ease in the number o f events preced ed a la rge ice a vala nche tha t involved the entire glacier thickness, suggesting that seismi c monitoring m ay be useful in predicting ca tastrophic ice m ovem en ts. nom bre d'evenements p n:cederent un e gra nde avala nche de glace qui in teresse la tota lite d e l'epaisseur du gla cier , ce q ui permet d e penser que les rei eves sism ologiq ues peu vent e tre util es pour p revoir d es mou vem ents catas t rophiq ues de la glace. ZUSAMM ENFASSUNG. Seismischer NachweisJur diskrete Gletscherbewegung a ll der Gren zj/iiche zwischell Eis und Fels. Di e seism ische U berwachung dreier Vulka ne in der Cascad e R a nge, W as hin gton Sta te, U. S.A., hat erwiesen , dass kle ine seismische E r eig n isse von den akt iven G letschern, d ie a uf d iesen Bergen lieg en , a usgelost werde n . Ocr Untersuchung der Quell en solcher E rdbebe n gait en eingehe nde seismische Versu ch e; sie erbrachte n d en Beweis, dass d ie Ereignisse das Ergebnis ruckartiger Bewegu n gen sind , d ie am Bett d er G letsch e r statt find e n . Die wenige n E reig nisse, d ie loka lisiert werden konn ten, ka m en a us Tiefen, die der Gletscherd icke vergle ich bar waren. Die Ah nli ch keit der vVell enfo rmen bei einer E xplosion a m G rund e e ines G letschers und bei natiirl ichen Eisbebe n lasst verm uten, dass d ie komplexe Form d er seismischen W elle von d er Well enba hn herrOhrt , nicht a ber von d er Quelle. Die Er e ign isse zeigen ein e j a hreszeitliche T ende nz; sie treten im Sommer h a ufiger a uf a ls im Winter. E in Anstieg d er E reignisse a uf d as zehnfache ging einer gross en Eisla wine voraus, welche d ie ga n ze G letscherdicke erfass te; es ka nn da h er a ngen om men werden, d ass katastropha le Eisbewegungen sich durch seismische Uberwachung vorhersagen lassen.
INTROD UCTIONA distinct class of seism ic activi ty has long been associa ted with the seismic monitoring of three glaciated Quaterna ry Cascade R a nge volca noes in W ashington Sta te . First identified on helicorder records from a sta tion es ta blished on the southern flank of Mount R ainier in 1963, this cl ass of events was characterized by a low-frequ en cy coda (dom...