On 5 May 2014, a Mw 6.2 strike‐slip earthquake occurred in the Mae Lao region of Chiang Rai province in Thailand. This earthquake took place in a region of known faults and caused substantial damage and injuries, although the region had been previously identified as having a relatively low earthquake hazard. Detailed field reconnaissance and deployment of a dense, temporary, network of broadband seismometers allowed details of the damage and its relationship to seismicity to be analyzed. The aftershock sequence associated with this main shock occurs on two well‐defined trends, reflecting the two potential fault planes in earthquake mechanisms for the main shock and the majority of the aftershocks. The damage area was relatively large for an event of this magnitude, but building damage was largely limited to the primary rupture region, while liquefaction and other ground failure are spatially associated with the rupture area and along regional rivers. Stress modeling, combined with the time series and pattern of aftershock activity, leads us to propose that slip near the northern termination of the main shock rupture continued slightly onto a conjugate fault, helping to trigger the distinct pattern of two discrete, conjugate trends of aftershock activity that mirror the kinematics of the main shock fault mechanism.

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The Thai Meteorological Department (TMD) seismic network began development in 2008. There are a total of 71 seismic stations consisting of 26 borehole stations and 45 surface stations currently installed. The three-component data from the TMD seismic network have been widely used in previous seismological studies. In a recent analysis, we have found that sensor orientation as reported in the site metadata is sometimes significantly incorrect, especially for borehole stations. In this study, we analyze P-wave polarization data from regional and teleseismic earthquakes recorded in the network to estimate the true instrument orientation relative to geographic north and compare that to station metadata. Of the 45 surface stations, we found that at present, ~ 82% are well oriented (i.e., aligned within 0–15° of true north). However, 8 sites have sensors misoriented by more than 15°, and some stations had a temporal change in sensor orientation during an upgrade to the seismic system with replacement of the sensor. We also evaluated sensor orientations for 26 TMD borehole seismic stations, from 2018 to the 2022. For many of the borehole stations, the actual sensor orientation differs significantly from the TMD metadata, especially at short-period stations. Many of those stations have sensor misorientations approaching 180°, due to errors in the ambient noise analysis calibration techniques used during installation. We have also investigated how this sensor misorientation affects previous seismic studies, such as regional moment tensor inversion of earthquakes sources and receiver function stacking. We have found that the large deviations in sensor orientation can result in erroneous results and/or large measurement errors. A cause of the orientation error for borehole sites could be a combination of strong background surface ambient seismic noise coupled with an incorrect reference instrument response.

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