Normal Faulting in the 1923 Berdún Earthquake and Postorogenic Extension in the Pyrenees

The 10 July 1923 earthquake near Berdún (Spain) is the largest instrumentally recorded event in the Pyrenees. We recover old analog seismograms and use 20 hand‐digitized waveforms for regional moment tensor inversion. We estimate moment magnitude Mw 5.4, centroid depth of 8 km, and a pure normal faulting source with strike parallel to the mountain chain (N292°E), dip of 66° and rake of −88°. The new mechanism fits into the general predominance of normal faulting in the Pyrenees and extension inferred from Global Positioning System data. The unique location of the 1923 earthquake, near the south Pyrenean thrust front, shows that the extensional regime is not confined to the axial zone where high topography and the crustal root are located. Together with seismicity near the northern mountain front, this indicates that gravitational potential energy in the western Pyrenees is not extracted locally but induces a wide distribution of postorogenic deformation. Plain Language Summary When convergence across a mountain chain comes to a stop, topography and deep root are no longer maintained, starting off postorogenic extension. Here we analyze the 10 July 1923 earthquake near Berdún in the Spanish Pyrenees. The earthquake was recorded at early analog seismographs over Europe, and we collected and hand digitized these old recordings for waveform analysis. We estimate moment magnitude Mw 5.4, making it the largest Pyrenean earthquake recorded on seismographs. The earthquake responds to normal faulting, indicating extension perpendicular to the mountain chain, in agreement with earthquakes in other parts of the Pyrenees, as well as deformation inferred from Global Positioning System data. It shows that postorogenic deformation reaches the southern mountain front, far from the largest topography and crustal root, which is at odds with conventional models for postorogenic extension. Key Points The largest instrumentally recorded earthquake in the Pyrenees (Mw 5.4) corresponds to normal faulting at the south Pyrenean thrust front Earthquake distribution changes along strike of the chain, following the axial zone in the east and the mountain front in the west Western Pyrenean seismicity does not correlate with local gravitational potential energy, possibly due to stronger crust in this sector