The Herat cluster is named for the ancient city of Herat in northwestern
Afghanistan. It is based on a remarkable sequence of strong earthquakes
in October 2023. Three earthquakes of magnitude 5.9-6.3 occurred within
about an hour on October 7, followed by a 6.3 Mw event on October 11 and
another 6.3 Mw event on October 15. An active aftershock sequence lasted
until the end of 2023. The source region has experienced very little
seismicity in recent decades, other than this sequence. The nearby
seismograph station HRA was, unfortunately, non-operational at the time
of these earthquakes so there is no depth control (or origin time
control) from near-source arrival times. The next nearest seismograph
stations are ~150 km to the west and 600-700 km to the east. Therefore
direct calibration, based on azimuthal coverage by stations at local
distance is not possible. The InSAR signals from this sequence have been
extensively studied by several groups and provide a basis for indirect
calibration. This analysis is based on the work presented by Zhao et al.
models for the five largest events. The rupture models derived in that
study help constrain the epicenters and focal depths. Waveform studies
and analyses of teleseismic depth phases also are used to constrain
depth. Origin times are calibrated by performing a preliminary direct
calibration analysis which includes the 6.0 Mw earthquake near Mashad,
Iran on April 5, 2017 and its aftershocks, which were very well recorded
at short epicentral distances. The Herat sequence is ~180 km away from
those stations, however, so unknown variations in the intervening
crustal structure likely bias the epicenters. This is also true for the
origin times, but there is no better alternative. By comparing the
results of the direct and indirect calibration the level of epicentral
bias in the direct calibration solution can be estimated as several
kilometers (to the south).
InSAR modeling suggest shallow depths (<8-10 km) for the ruptures in the Herat sequence mainshocks. In most cases we have found that focal depths estimated from close-in stations lie near the deeper edge of the rupture zones. Waveform depths from multiple studies range from 4-17 km for the 5 largest events, concentrating around 9-10 km. In the case of the Herat mainshocks and many aftershocks, teleseismic depth phases suggest similar depths. Events for which there is no depth constraint were fixed at 9 km. Indirect calibration is based on locating the hypocentroid of the cluster using teleseismic P arrivals between 30° and 90°and shifting the cluster in space and time so that the hypocenter of the 6.3 Mw event on October 15 (the one for which there is the clearest InSAR signal) matches the specified calibration hypocenter. The calibration hypocenter is chosen such that the cluster fits the pattern of rupture models in the manner thought to be most likely, i.e., concentrated in the deeper parts of the individual rupture models. The calibration shift needed to achieve this result is ~15 km at 340°, and -0.4 s in origin time.