Short-Term Foreshock and Aftershock Patterns of the 2021 Ms 6.4 Yangbi Earthquake Sequence

An Ms 6.4 earthquake struck Yangbi County in western Yunnan province, China, on 21 May 2021, causing damage in the nearby region. Intensive foreshock activity started three days before the mainshock, and numerous aftershocks followed along a northwest–southeast-trending right-lateral main rupture fault. Double-difference relocation of the foreshock and aftershock sequence shortly before and after the Ms 6.4 mainshock is conducted using the phase picks from the local seismic network. The focal mechanisms of relatively large foreshocks and aftershocks are also derived. The results not only delineate the ruptured fault geometry during the mainshock but also indicate the mechanism of static stress transfer according to the spatiotemporal evolution of foreshocks. The low background b-values around the mainshock are also consistent with the occurrence of the Yangbi earthquake sequence.

Two end-member earthquake preparations illuminated by foreshock activity on a meter-scale laboratory fault

The preparation process of natural earthquakes is still difficult to quantify and remains a subject of debate even with modern observational techniques. Here, we show that foreshock activity can shed light on understanding the earthquake preparation process based on results of meter-scale rock friction experiments. Experiments were conducted under two different fault surface conditions before each run: less heterogeneous fault without pre-existing gouge and more heterogeneous fault with pre-existing gouge. The results show that fewer foreshocks occurred along the less heterogeneous fault and were driven by preslip; in contrast, more foreshocks with a lower b value occurred along the more heterogeneous fault and showed features of cascade-up. We suggest that the fault surface condition and the stress redistribution caused by the ongoing fault slip mode control the earthquake preparation process, including the behavior of foreshock activity. Our findings imply that foreshock activity can be a key indicator for probing the fault conditions at present and in the future, and therefore useful for assessing earthquake hazard.

Recognizing the waveform of a foreshock

The 2011 Mw9.1 Tohoku, Japan, earthquake is the paradigmatic example of an earthquake anticipated by a significant foreshock activity, with a Mw7.3 earthquake occurred two days before, within about 10 km 1. Recent results 2 show that statistically relevant changes can be found in the magnitude distribution after the Mw7.3 foreshock but the discrimination between normal and foreshock activity still remains a scientific challenge 3. Here we show that the envelope of the ground velocity recorded after the Mw7.3 foreshock presents an atypical sawtooth profile very different from the one observed after other earthquakes 4. We interpret this profile as the consequence of the locked state of the mainshock fault which reduces the possibility of the foreshock to trigger its own aftershocks. We find a similar sawtooth profile after other Mw6+ foreshocks followed within 10 days by a larger earthquake, as in the case of the 2014 Mw8.1 Iquique, Chile, sequence. This observation allows us to define a level of concern, simply extracted from the first 45 minutes of the recording waveform, associated to the occurrence of a larger earthquake. A test of the method for 47 Mw6+ worldwide earthquakes gives precise warning in time and space after all the 10 earthquakes followed by a larger one with only 2 false alerts.

The 2019Durrës, Albania earthquake sequence – preliminary results from a post-seismic campaign

<p>On 26<sup>th</sup> of November 2019 an M<sub>w</sub> 6.4 earthquake ruptured near the port town of Durrës, only 25 km from Tirana, the capital of Albania. It caused major damage and killed 51 people, making it the deadliest earthquake in 2019 worldwide.</p><p>The earthquake occurred on the eastern Adriatic margin, where the Adriatic micro-plate collides with Eurasia causing widespread distributed deformation and crustal shortening that built the peri-Adriatic orogenic belts. Convergence is accommodated in the external Dinarides/Albanides by thrust faulting, mostlyalong E-dipping low-angle detachments with subordinate W-dipping back-thrusts in the most external thrust belt segment. The deformation front, particularly along the southeastern Adriatic coast, is seismically highly active, manifested not only by this most recent event, but also, e.g., by one of the largest instrumentally recorded earthquakes in Europe, the 1979 M7.1 Montenegro event slightly further north and a number of disastrous historic earthquakes.</p><p>The 2019 Durrës mainshock was apparently relatively deep (~25 km) and of thrust type. It was preceded by significant foreshock activity starting in September 2019 with two M<sub>w</sub> 5.6 and 5.1 earthquakes a few kilometres south of the mainshock that also had a thrust mechanism, however with nodal planes differing from the mainshock, indicating that these occurred on a different fault.</p><p>Approximately two weeks after the mainshock, we installed a 30-station short-period seismic network to densely cover the epicentral area. We will present a preliminary analysis of the mainshock and its aftershock sequencehopefully elucidating the fault network responsible for the earthquake sequence.</p>

Evaluation of Phenomena Preceding Earthquakes and Earthquake Predictability

Unusual phenomena sometimes precede a large earthquake and are considered by some as a telltale sign of that earthquake. Judging whether the phenomenon was indeed related to the earthquake is difficult for individual cases. However, the accumulation of data over time allows for statistical evaluation to determine whether there is a correlation between the occurrence of a certain type of phenomena prior to an earthquake. The focus of this study is to review such statistical evaluation. The aspects considered in this study include seismicity, crustal deformation, slow slip, crustal fluids, crustal properties, electromagnetic phenomena, and animal behaviors. The lead times range from minutes to a few decades. The magnitude of the earthquake-preceding tendency can be universally measured by the probability gain G, which is the enhancement ratio of earthquake probability suggested by the occurrence of the phenomenon. A preceding tendency is considered to exist if G is > 1 with reasonable statistical significance. Short-term foreshock activity, that is, temporarily heightened seismicity, produces by far the highest G > 100, sometimes exceeding 10000. While this strongly contributes to empirical forecasting, a considerable part of the predictive power of foreshocks is likely to derive from the mere aftershock triggering mechanism. This enhances the probability of small and large earthquakes by the same factor. It is fundamentally different from traditional expectations that foreshock activity signifies the underlying nucleation process of the forthcoming (large) earthquake. Earthquake-preceding tendency has also been proven significant for a number of other phenomena not ascribable to the aftershock-triggering effect. Some phenomena may be indicators of physical conditions favorable for large earthquakes, while some (e.g., slow slip) may represent triggering effects other than aftershock triggering. Phenomena not ascribable to aftershock triggering have a modest G of < 20 so far. However, these phenomena, including higher-order features of foreshocks, can be combined with the high G from aftershock-triggering effect, sometimes yielding a fairly scaring level of forecast. For example, say ∼10% chance of an M7 earthquake in a week in a few hundred km radius.