The incentive to speed up real-time location has motivated previous researchers to go beyond standard location procedures and use not only P-wave arrival at some network stations but also its nonarrival at others. In addition to being sensitive to velocity model and picking uncertainties, this approach is also highly dependent on time delays due to unknowns network latencies, processing, and packet size. Thus, seeking ways to add independent real-time constraints on earthquake location are important for earthquake early warning applications. In this study, we assess the robustness of three independent real-time back-azimuth (BAZ) determination schemes, using offline records of southern California earthquakes. We find that BAZ values computed by the three methods provide equivalent levels of accuracy. By sending the three BAZ estimates to a screening module that checks for coherency and signal-to-noise ratio criteria, we show that accurate BAZ estimates are obtainable in real time, with a standard deviation of 13°. Through examination of two earthquake scenarios that use offline data, we show that the inclusion of BAZ estimates into real-time location schemes improves the performance of real-time hypocenter determination, by cutting the time it takes to obtain well-constrained hypocenters.
Regional source-based earthquake early warning systems perform three consecutive tasks: (1) detection and epicenter location, (2) magnitude determination, and (3) ground-motion prediction. The correctness of the magnitude determination is contingent on that of the epicenter location, and the credibility of the ground-motion prediction depends on those of the epicenter location and the magnitude determination. Thus, robust epicenter location scheme is key for regional earthquake early warning systems. Available source-based systems yield acceptably accurate locations when the earthquakes occur inside the real-time seismic network, but they return erroneous results otherwise. In this study, a real-time algorithm that is intended as a supplement to an existing regional earthquake early warning systems is introduced with the sole objective of ameliorating its off-network location capacity. The new algorithm combines measurements from three or more network stations that are analyzed jointly using an array methodology to give the P-wave slowness vector and S-phase arrival time. Prior to the S-phase picking, the nonarrival of the S phase is used for determining a minimum epicentral distance. This estimate is updated repeatedly with elapsed time until the S phase is picked. Thus, the system timeliness is not compromised by waiting for the S-phase arrival. After the S wave is picked, an epicentral location can be determined using a single array by intersecting the back-azimuth beam with the S-minus-P annulus. When several arrays are assembled, the back azimuth and P and S picks from all arrays are combined to constrain the epicenter. The performance of the array processing for back azimuth and S-wave picking is assessed using a large number of accelerograms, recorded by nine strong motion sensors of the KiK-net seismic network in Japan. The nine stations are treated as three distinct seismic arrays, comprising three stations each. Good agreement is found between array-based and catalog-reported parameters. Finally, the advantage of the new array methodology with respect to alternative schemes for back azimuth and distance is demonstrated.
Most aftershocks occur in areas experiencing large co-seismic stress
changes, yet some occur long after the mainshock in remote
lightly-stressed regions. The triggering mechanism of these remote
delayed aftershocks is not well understood. Here, we study aftershocks
occurring in the Dead Sea (DS) area following the 2023 Mw7.8 and Mw7.6
Kahramanmaraş earthquakes. Most aftershocks cluster along previously
quiescent structures off the main the DS fault strand. Visual inspection
disclosed three aftershocks instantaneously triggered by the Mw7.6 in
the Northern DS basin, and match-filtering revealed a delayed
aftershock. Waveform similarity and temporal clustering suggest the
northern DS aftershocks re-rupture a stick-slip patch loaded by
surrounding creep. Velocity-gradient seismograms show the Mw7.6 exerted
larger transient stresses than the Mw7.8, which may explain triggering
by the Mw7.6, but not by the Mw7.8. This account of
instantaneously-triggered repeaters underscores the role of interactions
between aseismic and seismic slip in remote triggering.
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