ALBERT

Description: INTRODUCTION A hundred years ago 2.4 cubic km of rock fell from a Pamir mountainside 〉700 m to the valley floor, releasing potential energy equivalent to an Mw 7.8 ± 0.1 earthquake. Its fall created the world’s highest dam, impounding a 17-km3 lake that remains to this day. Seismograms recorded in Europe and Asia registered an earthquake at the approximate time of the fall, and soon after the details of the landslide had been evaluated a controversy arose concerning whether these seismograms had recorded an earthquake that had triggered the landslide, or whether the seismograms had merely registered waves generated by the potential energy release of the landslide’s impact. Boris Galitzin (1915) reasoned that the radiated energy almost exactly equaled the potential energy released by the fall, and hence represented the unique case of the hypocenter and the epicenter being identical. Otto Klotz (1916) translated Galitzin’s article with a preface underlining its importance, and Harold Jeffreys, in a 1923 article, despite revealing flaws in Galitzin’s calculations, confirmed both the approximate coincidence in location of the two events and the equality of energy release computed by Galitzin. However, that same year Richard Oldham dismissed the implications of Jeffreys’s calculations, noting that the maximum epicentral damage was offset from the landslide and that the area of felt shaking was typical of a deep earthquake. Though Oldham’s arguments were eventually to win, it would take another decade before it was realized that it was the long duration of energy release in the landslide that accounted for its apparent absence in distant seismograms. The details of the causal earthquake, and the curious equality in landslide and earthquake energy, have never been fully resolved. We attempt to do so in this article. We quantify what is known of the earthquake and trace the history of...