Waveguide effects in very high rate GPS record of the 6 April 2009, M_w 6.1 L'Aquila, central Italy earthquake

Abstract

A 10 Hz sampling frequency GPS station was installed near L'Aquila a few days before the 6 April 2009 M w 6.1 earthquake. It recorded displacement waveforms during the main shock and the largest M w 5.4 aftershock of 7 April. The horizontal components of the main shock contain a high-amplitude (43 cm peak-to-peak) nearly harmonic (1 Hz) wave train not evident in other nearby instrumental records. The persistency of this feature during aftershocks recorded by a temporarily colocated seismological station highlights a local site effect. Traditional models based on near-surface velocity structure and topography variations fail to reproduce the size and frequency band of the observed amplified motion. The amplified wave train can be explained by a low-velocity fault zone layer below the station. This model fits the delay of the large-amplitude nearly harmonic wave train after the S wave phase and is consistent with the variation in the fault excitation efficiency between the two shocks in relation to their different source depth and location. Synthetic calculation of trapped waves in a model consisting of two quarter spaces separated by a 650 m wide low-velocity zone with 50% velocity reduction and Q value of 20 fit well the observed anomalous record. The parameters of the model fault zone layer are consistent with geological evidence of a broad damage zone adjacent to the station and a similar site response found in this crustal zone with ambient noise. Results of shallow seismic surveys and sonic logs from deep wells provide independent constraints on the host rock velocities.

Toward reliable automated estimates of earthquake source properties from body wave spectra

“…Unrecognized directivity can lead to scatter and biases in strain/stress drops (and other properties) inferred from corner frequencies at stations affected by the directivity [e.g., Calderoni et al ., ]. Directivity of moderate and large events can significantly increase the amplitude of ground motion at stations in the forward direction, especially when coupled with structural effects [e.g., Olsen et al ., ; Avallone et al ., ], so it is important to clarify the possible existence of persistent earthquake directivity on given structures. Another fault‐specific process that can change the amount and frequency content of seismic radiation is coseismic rock damage in source volumes [ Ben‐Zion and Ampuero , ].…”

Low‐velocity zones along the San Jacinto Fault, Southern California, from body waves recorded in dense linear arrays

“…The damage zones are characterized by lower elastic moduli and seismic velocities compared to the host rocks. They reflect the behavior of past earthquakes [e.g., Dor et al, 2006;Ben-Zion and Ampuero, 2009;Xu et al, 2012] and can exert significant influence on properties of future ruptures [e.g., Harris and Day, 1997;Ben-Zion and Huang, 2002;Huang et al, 2014], amplification of ground motion near faults [e.g., Wu et al, 2009;Avallone et al, 2014;Kurzon et al, 2014], and long-term deformation processes [e.g., Finzi et al, 2009;. A detailed high-resolution imaging of fault zone (FZ) structure can therefore provide important constraints on the behavior of past and future earthquakes on the fault.…”

Coseismic displacement waveforms for the 2016 August 24 Mw 6.0 Amatrice earthquake (central Italy) carried out from High-Rate GPS data

“…For the moderate earthquakes occurred in Italy in the last 8 years (2009 Mw 6.3 L'Aquila; 2012 Mw 5.9 and Mw 5.8 Emilia main shocks, 2016 Mw 6.0 Amatrice) we observe a relatively good spatial coverage of the HRGPS stations around the epicenter, although the contribute of non-geophysical GNSS networks in this picture is still significant. Further efforts in developing denser HRGPS networks, with efficient coupling of the antenna installations with the solid Earth, and operating at sampling frequencies ≥ 10 Hz would provide additional constraints to characterize the source process and discriminate peculiar site effects (Avallone et al, 2014).…”