Variations of Seismicity in the Avachinsky Gulf (Kamchatka, Russia)

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  • Natural Hazards 19: 8796, 1999. 1999 Kluwer Academic Publishers. Printed in the Netherlands. 87

    Variations of Seismicity in the AvachinskyGulf (Kamchatka, Russia)

    VADIM SALTYKOV, VICTOR CHEBROV and JULIA KUGAENKOKamchatkan Seismological Department, Geophysical Service 9, Piip Av.,Petropavlovsk-Kamchatsky, Russia, 683006

    (Received 21 October 1997; in final form 21 April 1999)Abstract. Variations of seismic mode in the region of the Avachinsky Gulf (Kamchatka, Russia) areconsidered. Observed anomalies (seismic quiescence, the ring seismicity, reduction of the slope of theearthquake recurrence diagram) provide a basis to consider this region as a place of strong earthquakepreparation. The Kamchatka regional catalogues of earthquakes between 19621995 were used in theanalysis. A reduced seismicity rate is observed during 10 years in an area of 150 km 60 km in size.During the last five years, in the vicinity of the area considered, earthquakes with M > 5 occurredthree times more often than the average over thirty years. It is interpreted as ring seismicity. Theblock of 220 km 220 km in size, including the quiescence zone, is characterized by a continuousdecrease of the recurrence diagram slope, which has reached a minimum value for the last 33 yearsin this region.

    Key words: precursor, seismic gap, seismic quiescence, ring seismicity.

    1. Introduction

    The Kamchatka Peninsula, the northern part of the KurilKamchatka arc, is one ofthe most seismically active places on the Earth. Because of the small populationin this region, earthquakes with magnitude 8 (1904, 1923, 1959, etc.) have notcaused serious damage. However, PetropavlovskKamchatsky is the largest city inKamchatka and is close to the seismoactive zone. It could suffer in the event of anearby strong earthquake.

    A long-term forecast by Fedotov and Chernyshev (1990) suggests that oneof the most probable places for strong earthquakes (M > 7.75) in the KurilKamchatka arc is Avachinsky Gulf (Southern Kamchatka) (Figure 1). The long-term forecast is based on the fact that this zone is a seismic gap. A seismic gap isunderstood to be a part of a seismoactive area, which is not filled by the sources ofprevious strong earthquakes and is probably the location for the next earthquake(Mogi, 1968). The unraveling of such zones is the basis of Fedotovs method.However, it should be noted that, while the location is predicted, the time of theevent is still unknown.

    The data, according to the literature (e.g., Mogi, 1985), show that the regions ofpreparation of a strong earthquake exhibit variations of seismic mode over a long

  • 88 VARIATIONS OF SEISMICITY IN THE AVACHINSKY GULF (KAMCHATKA, RUSSIA)

    Figure 1. Sketch map of Southern Kamchatka, showing epicenters of earthquakes 19621995with magnitude 2.0 6 M < 3.0 (1), 3.0 6 M < 4.0 (2), 4.0 6 M < 5.0 (3), 5.0 6 M < 6.0(4), 6.0 6 M < 7.0 (5), 7.0 6 M (6) and with depth less than 70 km (7) and more than70 km (8). The dotted curve (6) marks the boundary of the region with reliable registration ofearthquakes with magnitude M = 2.0.

    period of time. This justifies the choice of the target region for the present work.The purpose of this work is the detection of changes in seismic mode, which arecharacteristic of the preparation of a strong earthquake in the Avachinsky Gulf. Themodes under examination are seismic quiescence, ring seismicity, reduction of therecurrence diagram slope.

    2. Data

    Regional Kamchatkan earthquake catalogues from 1962 to 1995 (the entire periodof operation of the Kamchatkan regional network) were used in the analysis ofseismicity (Figure 1). First, catalogues were cleared of aftershocks and swarms ofearthquakes by the method of Molchan and Dmitrieva (1991). The lower powerthreshold of these earthquakes was fixed according to the quality level for theregion under examination. The catalogue is complete for events of M > 2.6, butfor the Avachinsky Gulf the catalogue is complete for earthquakes of M = 2.0(Gordeev et al., 1998, Figure 1).

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    Figure 2. Left and middle: Central parts of circles (1, 2) with radius of 30 km (left) and 15 km(middle), in which seismic flow is calculated. 1 corresponds to circles without decrease inseismicity rate, 2 circles in which the decrease in seismicity rate is observed. The ellipse,approximating a zone of seismic quiescence, is drawn. Right: Set of zones (IV), in whichseismicity is investigated.

    3. Method of Analysis and Results

    Seismic quiescence. Seismic quiescence means the phenomena manifested as adecrease in seismic activity over time, compared to the background level for afixed spatial volume before strong earthquakes. Sometimes this is referred to as aseismic gap (Mogi, 1979), but it is necessary to understand, that this nomenclaturedescribes a completely different process than the one described above.

    The technique for detection of zones with anomalous seismicity rate (numberof earthquakes per unit time) consists of the following:(1) The region considered was covered by a set of circles with a radius of 30 km

    (Figure 2, left);(2) For each circle the cumulative seismic flow, i.e., the cumulative number of

    earthquakes with M > 2.6 as a function of time t , was evaluated;(3) Circles corresponding to a decrease in seismicity rate (i.e., a decrease in the

    slope of the cumulative seismic flow diagram) in recent years were identified(and are shown in Figure 2 (left) as solid dots). The shape of the diagram of thecumulative seismic flow in Figure 3 (zones I and II) is typical for these soliddots;

    (4) The region showing solid dots (Figure 2, centre) was in turn covered with aset of smaller circles (R = 15 km), and steps 2 and 3 were repeated. Thisprovides a more effective determination of the anomaly. The whole procedurewas followed using earthquakes with M > 2.0. This allowed us to outline anabnormal zone delineated by an ellipse in Figure 2 (centre), whose parame-ters are: 75 km and 25 km for half-major and half-minor axes respectively. Ifthis zone is actually a zone of seismic quiescence, then its size corresponds tothe source size of an earthquake of magnitude about 7.5.

  • 90 VARIATIONS OF SEISMICITY IN THE AVACHINSKY GULF (KAMCHATKA, RUSSIA)

    Figure 3. Cumulative number of earthquakes (left) and seismicity rate (right) as a function oftime in each zone. Magnitude threshold is M = 2.0 (zone IIV) or M = 2.6 (zone V).

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    To derive quantitative parameters for the processes occurring within and at theedge of the anomalous region, seismicity within a set of areas enclosed in con-centric ellipses with half-major/minor axes 50 km/20 km (ellipse I), 75 km/30 km(ellipse II), 100 km/40 km (ellipse III), 125 km/50 km (ellipse IV) and the rectangle51.553.5N and 158161E (Figure 2, right), is compared. Zone I is the area ofellipse I, zone II is the area between ellipses I and II, zone III between ellipses IIand III, zone IV between ellipses III and IV, and zone V between the rectangleand ellipse IV. The earthquakes considered were less than 70 km deep. A decreasein seismic rate is observed in zones I and II (Figure 3). Variations are clearer inzone I. In zone III, the seismicity rate does not exhibit an anomaly in recent years(visually), and in zone IV it increases; outside the ellipses (zone V), it is constant.

    For an estimation of the confidence on alleged changes in seismicity rate, theZ-test has been applied. The method (Habermann, 1983, 1991) consists of:(1) At any time t , averaged rates calculated over two intervals (from the beginning

    of monitoring t0 to instant t ; from instant t to the end of monitoring periodt1) are compared. t spans the whole interval (t0, t1). The parameter AS, whichcharacterizes the distinction of rates, is taken from standard deviate Z-test andis calculated by the formula:

    AS(t) = Z = (R1 R2) ( 21 /n1 + 22 /n2)1/2,where R1 is the mean rate in period 1 (t0, t) and R2 is the mean rate in period 2(t , t1); 1 and 2 are the corresponding standard deviations, and n1 and n2 arethe numbers of samples. The time t at which AS(t) is maximum separates atime period into two intervals with different rate, with a maximum of reliability.

    (2) If the function AS(t) accepts the maximum value Z0 at the time t = , it isnecessary to find out whether the anomaly, with duration d = t1 , is unique.For this purpose, the function LTA(t) = Z over a sliding window of width d(t d , t) and over the rest of the time period, is evaluated; the seismicity ratewithin time windows of duration d (R2) is compared to rate of the overallsample, minus the data within d (R1). If the Z-values obtained do not exceedZ0, then the identified anomaly, with duration d , can be considered as unique.

    In the present study, large positive Z-values (>6) were obtained in the first twozones (Figure 4). d for maximum AS decreased from 15 years to 10 years passingfrom zone I to zone II. This can be interpreted as follows: seismic quiescencestarted in zone I and then spread over zone II. No other similar anomalies wereobserved in these zones during the monitoring. As for the amount of change inrate in zone I, decrease was about 70% (Figure 3), that is close to decreases beforestrong earthquakes reported in the literature (Wyss and Habermann, 1988; Wiemerand Wyss, 1994). Apart from quiescence, some increase of seismicity in zones IIIand IV was observed during the last several years (2.53.0 yr); but this increase isnot unique, because LTA parameter has similar values at other times.

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    Figure 4. Diagrams of functions AS(t) (bold line) and LTA(t) (dotted line) for the five zones.The vertical dotted line shows a moment of change i