Fling Effects from Near‐Source Strong‐Motion Records: Insights from the 2016 Mw 6.5 Norcia, Central Italy, Earthquake

Abstract

1. Introduction When an earthquake occurs, the elastic strain gradually accumulated for long time on either side of a fault is suddenly released. The elastic rebound generates the dynamic component of the motion and the static deformation of the ground. In the proximity of the source of large earthquakes, this tectonic displacement may be recorded by accelerometric instruments. The corresponding waveforms have characteristic shapes, producing a one-sided pulse in the velocity trace and an offset (fling-step) at the end of the displacement waveform (Figure 1a). The fling-step may be on the order of tens or hundreds of centimeters (Boore and Bommer 2005) and it is typically observed along the slip direction: it appears along strike-parallel direction for strike-slip faults (Kalkan et al., 2004), whereas it can be observed both in the strike-normal and vertical directions for dip-slip faults

Fling-Step Recovering from Near-Source Waveforms Database

“…This approach preserves the long-period near-source ground-motion featured by a one-side pulse in the velocity trace (fling-step) and an offset at the end of the displacement trace ( Figure 1). Example of earthquake waveforms processed by means of extended BASeline COrrection (eBASCO) (vertical component recorded at station T1214 during the Mw6.5 Norcia earthquake) from [9]. (a) analytical model of the fling-step in terms of acceleration, velocity, and displacement; (b) uncorrected waveforms; (c) waveforms corrected through a standard processing characterized by broad-band filtering and the application of a cosine taper both at the beginning and the end of the signal; (d) waveforms corrected by the eBASCO tri-linear detrend.…”

“…Indeed, near-source ground motion may be affected by two different effects, which can lead to large and long period pulses: rupture directivity and tectonic fling. The former produces short-duration and large-amplitude twoside pulses in the velocity trace along the fault-normal direction; the latter is the expression of the permanent tectonic offset of a rupturing fault and it is usually characterized by a one-side pulse in the velocity trace and by a non-zero final displacement along the slip direction (e.g., [7][8][9]). While directivity effects are usually fully recovered by conventional processing schemes, the accurate recovery of the flying is made difficult by the presence of noise and baseline offsets that, although small in acceleration, lead to artificial long period drifts in the displacement trace [9][10][11][12][13] (Figure 1).…”

“…Studies on how to recover permanent displacements from strong motion data and methods for baseline correction have been published since the 1970s, right after the development of the modern digital recordings [17]. Among the principal approaches based on piecewise correction of the signal baseline, noteworthy are Graizer [18], Iwan et al [19], Boore [10], Graves [11], Wu and Wu [17], Chen and Loh [20], Chao et al [21], Rupakhety et al [22], Wang et al [12] and D'Amico et al [9]. A review of some of such studies with emphasis on their limitations can be found in [22].…”

“…Despite the still ongoing research, properly estimating the uncertainties in the static deformation data is still a controversial issue; different baseline correction approaches may return variable PD since the seismic displacement is a very sensitive ground-motion characteristic. In such a context, GPS and InSAR data could be employed as a benchmark to obtain reliable displacement traces from accelerograms [9,12,13]. However, the availability of GPS receivers close to accelerometric stations is, generally, quite scarce.…”

“…In this work, we present the recent update of an automatic processing scheme, extended BASeline COrrection (eBASCO), for piecewise baseline correction of near-source records [9]. Its main advantage lies with the possibility to automatically process a considerable number of signals in a very short time.…”