INTRODUCTION

There are two tectonic faults in Dien Bien Province, northwest Vietnam: the Lai Chau–Dien Bien and Ma River–Shon La faults. Generally, northwest Vietnam is a region with complex tectonics and intense seismic activity. Recently, earthquakes have occurred quite frequently in Dien Bien; an unusually large number of events have been recorded since the beginning of 2017. One of the latest earthquakes (M = 4.7) occurred in Muong Ang District, Dien Bien Province, on January 8, 2018, at 23:21:21.8 GMT. This earthquake stirred great public interest in the possible devastating effects to the population of the province.

Shortly before the earthquake, the dense network was deployed in the epicentral zone to provide recording of earthquakes with magnitudes less than 1.0 without omission (Burmin et al., 2010, 2018). Immediately after the Muong Ang earthquake, the Geophysical Institute, Vietnam Academy of Sciences and Technologies, sent a group of researchers to collect macroseismic data, i.e., information about how seismic oscillations were perceived by people and to what extent civilian facilities were damaged.

The Vietnam seismic network recorded the foreshocks and aftershocks of this earthquake from January 2018 through May of 2018. Comparative analysis of the macroseismic and instrumental data makes it possible to study in the Muong Ang earthquake in sufficient detail and characterize the seismic process in the earthquake zone. The study area lies between 20.70° and 22.80° N and between 102.10° and 105.30° E (Fig. 1).

Fig. 1.
figure 1

Map of study area: (1) study area; (2) faults of different levels: (a) level I, (b) level II, (c) level III; and (3) state borders.

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Instrumental data were collected in this area through May 2018, so that a catalog of foreshocks and aftershocks of the Muong Ang earthquake could be produced for further research. Importantly, the foreshocks, mainshock, and aftershocks were recorded not only by the Vietnam seismic network, but also by foreign seismic stations (see Seismological Bulletin of the International Seismological Centre, 2001. URL: http://www.isc.ac.uk/iscbulletin/search/bulletin/) and a temporary seismic network (23 stations) set up under the project “Investigation into the Issue about Deployment of a Seismic Monitoring Network and Assessment of Features of Earthquakes Triggered by Reservoirs of the Song Da Hydroelectric Power Stations” (Le Van Zung et al., 2018) (Table 1).

Table 1. Vietnam seismic stations that recorded the January 8, 2018, Muong Ang earthquake
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This paper analyzes the spatiotemporal distribution of earthquakes in Dien Bien Province and neighboring areas using data on foreshocks and aftershocks. Their spatiotemporal interrelationship is also examined, as well as their correlation with the tectonic and geomorphic features of Dien Bien Province. Similar analyses can be found in publications on earthquakes in Vietnam and Southeast Asia (Kondorskaya and Ngo Thi Ly, 1994; Ngo Thi Lu, 1990, 1998, 1999, 2003; Ngo Thi Lu and Nguyen Quang, 1997; Ngo Thi Lu et al., 2000, 2010).

MACROSEISMIC DATA

Table 2 presents macroseismic data on the January 8, 2018, Muong Ang earthquake. These data describe a wide range of local seismic impacts. Only those events felt by highly sensitive people are presented (shocks of intensity I ≥ III on the MSK-64 scale (Medvedev et al., 1965)). Table 3 describes the perceptions of people and phenomena observed in the study area during the Muong Ang earthquake. Based on the macroseismic data, an isoseismic map was compiled for the January 18, 2018, earthquake (Fig. 2).

Table 2.   Macroseismic data on January 8, 2018, Muong Ang earthquake
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Table 3.   Scale of earthquake intensities (Medvedev et al., 1965)
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Fig. 2.
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Isoseismic map of January 8, 2018 Muong Ang earthquake: (1–3) Isoseismals of (1) I = VI, (2) I = V, and (3) I = IV; (4) level I fault; (5) level II fault; (6) level III fault; (7) earthquake epicenter with magnitude M < 3.0; (8–10) epicenters of earthquakes with (8) M = 3.0–3.9, (9) M = 4.0–4.9, and (10) M > 5.0; (11) boundary of the aftershock zone; (12) the main shock; (13–16) reported seismic intensities of (13) I = III, (14) I = IV, (15) I = V, and (16) I = VI. Faults of different levels: I.1 Lai Chau Dien Bien; II.1 Tuan Zao; II.2 Shon La; II.3 Shong Ma; II.4 Fumaitun; III.1 Chieng Hyong; III.2 Than Nua-Sop Kop; III.3 Muong Chung-Nam Tu.

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INSTRUMENTAL DATA

As noted above, the January 18, 2018, Muong Ang earthquake with magnitude M = 4.708 occurred in Muong Ang District of Dien Bien Province in northwest Vietnam at 23:21:21.8 GMT. The coordinates of the earthquake were 21.3740° N, 103.290° E; the source depth was ~10 km.

The isoseismic map of the January 8, 2018, Muong Ang earthquake is based on data collected in the epicentral zone (see Fig. 2). The elongated axes of the isoseismal lines are generally oriented north–south, deviating northeast-southwest parallel to the trend of the Tuan Zao fault.

From January to April 2018, the Vietnam seismic network recorded four foreshocks with magnitudes of M = 1.7–4.1, the main shock, and 23 aftershocks with magnitudes of M = 1.5–4.0. The main parameters of the Muong Ang earthquake, its foreshocks, and aftershocks are presented in Table 4.

Table 4.   Main parameters of January 8, 2018, Muong Ang earthquake
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CHARACTERISTICS OF THE MUONG ANG EARTHQUAKE

The spatiotemporal evolution of foreshocks and aftershocks of the Muong Ang earthquake (Fig. 3) shows that their epicenters are concentrated along the same direction as the trend of the Tuan Zao fault where the latter intersects the Shong Ma fault. The period of foreshock and aftershock activity can be divided into three time intervals.

Fig. 3.
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Spatial distribution of foreshocks and aftershocks of January 8, 2018, Muong Ang earthquake. For legend see Fig. 2.

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Interval 1 includes foreshocks, the main shock, and the first aftershocks concentrated in an ellipsoidal region with an area of about 9.52 km2 (the major axis is 5.48 km, and the minor axis, 2.20 km). The major axis is oriented northeast–southwest (Fig. 3). Note that the aftershock epicenters of Interval 1 lie in the area bounded by the intensity VI isoseism with the major axis oriented in the same direction as the general trend of the isoseisms in the epicentral region.

Interval 2 includes aftershocks concentrated in a narrower area, also shaped like an ellipse about 5.17 km2 in are (the axes are 3.8 and 1.7 km). The major axis of the ellipse is oriented in the same direction as the trend of the Tuan Zao fault where this fault intersects the Shong Ma fault. Note that the aftershock epicenters of Interval 2 are located north of the mainshock epicenter and the zone of foreshocks and aftershocks of Interval 1 (see Fig. 3). This area is bounded by the intensity V isoseism, whose major axis is oriented in the same direction as the general trend of isoseisms in this area.

Interval 3 comprises the latest series of aftershocks that occurred in the epicentral zone of the Muong Ang earthquake from mid-2018 to mid-2019. This series includes six aftershocks with epicenters concentrated in a narrow ellipsoidal zone about 8.40 km2 in area (the axes are 5.75 and 1.80 km). The major axis is oriented north–south, in the same direction as the trend of the Tuan Zao fault where this fault intersects the Shong Ma fault. The aftershock epicenters of Interval 3 are located east of the main shock epicenter and the zone of foreshocks and aftershocks of Intervals 1 and 2 (see Fig. 3). This zone is bounded by the intensity VI isoseism with the major axis oriented in the same direction as the general isoseismic trend.

The above considerations show that there are three stages of the aftershock generation process. The first series of aftershocks occurred at the end of the left wall of the Tuan Zao fault in Muong Bam, Sha Zung. Afterward, aftershock activity gradually migrated north and northeast, then, at the third stage, to the south. This type of the evolution of seismic activity perfectly fits the seismic and tectonic data and confirms the modern tectonics of the Tuan Zao and other faults in this area. This also explains why strong earthquakes followed by a series of aftershocks most often occur in Dien Bien Province and neighboring areas.

Note that, according to the tectonic data, the Tuan Zao fault can be traced only to Muong Bam and Sha Dung. However, the aftershock distribution, the isoseismic pattern, and the specifics of the focal process of the Muong Ang earthquake indicate that the Tuan Zao fault extends further south to Chieng Sho, not to Muong Bam and Sha Dung (see Figs. 2 and 3), as shown on the tectonic map (Nguyen Ngok Thi et al., 2005).

CONCLUSIONS

Analysis of macroseismic and instrumental data on the January 8, 2018, Muong Ang earthquake in northwest Vietnam not only revealed some specific characteristics of this earthquake, but also showed good agreement between the macroseismic and instrumental data and the tectonic structure of the region.

Based on a detail study of the Muong Ang earthquake, the following can be stated.

The evolutionary process of the Muong Ang earthquake took place in three stages. The first stage comprises foreshocks, the main shock, and aftershocks on the left wall of the Tuan Zao fault in Muong Bam and Sha Zung Ban. Then aftershock activity gradually migrated north and northeast. The third stage comprises aftershocks that migrated south, as opposed to the events of the previous stages. This confirms that the Tuan Zao and other faults in this region are active. It can therefore be suggested that the Tuan Zao fault extends rather far south to the Chieng Sho District, instead of terminating in the Sha Zung and Muong Ban Districts, as shown on modern tectonic maps.