2060-33 Advanced School on Non-linear Dynamics and Earthquake Prediction 28 September - 10 October, 2009 Dating large infrequent earthquakes by damaged cave deposits Amotz Agnon Institute of Earth Sciences Hebrew University Jerusalem Israel Dating large infrequent earthquakes by damaged cave deposits Elisa J. Kagan* Institute of Earth Sciences, Hebrew University of Jerusalem, Jerusalem 91904, Israel, and Geological Survey of Israel, 30 Malkhe Israel Street, Jerusalem 95501, Israel Amotz Agnon Institute of Earth Sciences, Hebrew University of Jerusalem, Jerusalem 91904, Israel Miryam Bar-Matthews ⎤ ⎥ Geological Survey of Israel, 30 Malkhe Israel Street, Jerusalem 95501, Israel Avner Ayalon ⎦ ABSTRACT al., 1998). The sequence of deposition in the The long-term recurrence patterns of past earthquakes are of considerable consequence caves is at times disturbed by unconformities, for hazard assessments, and have implications for earthquake physics. We introduce a which are expressed by collapsed stalagmites, rigorously dated record of earthquakes from an extensive number of well-preserved pre- stalactites, speleothem pillars, and cave ceil- seismic and postseismic precipitates from caves located off the Dead Sea transform. We ings (Fig. 1B), overlain by regrowth. There dated events directly at the paleoseismic contact by means of a novel correlation method are standing stalagmites with severed tops with the oxygen isotope record of the speleothems recovered in one of the caves. Within (Fig. 1A), none in close proximity to fallen the 185 k.y. covered, we dated 38 seismite samples. These stem from 13–18 earthquakes ceilings. Cores drilled into calcite flowstone with a mean recurrence interval of ϳ10–14 k.y. We show that the deformational events deposits on floors of the caves (Fig. 1C) re- dated in the study caves complement independent near-fault paleoseismic records by tem- vealed distinct laminae encasing collapsed ob- poral correlation with the earthquakes recorded therein. This opens up a significant new jects (e.g., soda-straw stalactites, ceiling piec- avenue of earthquake research that will provide precise dating and observational con- es, detritus). Predominantly horizontal fissures straints on large infrequent earthquakes. in speleothems and walls of caves are wide- spread and range from completely closed to Keywords: earthquakes, U-Th dating, speleothems, wiggle matching, paleoseismicity, Dead Sea open a few centimeters. transform. Mapping of the caves revealed preferential orientation of collapsed long-axis speleo- INTRODUCTION mogenic element in the area (Fig. 2) is the thems; we measured orientations of 65 long Records derived from instruments, histori- Dead Sea transform, yet secondary intraplate axes of fallen speleothems calcified to a sub- cal documents, and soft sediments are inher- structures generate microseismicity (Salamon horizontal floor with frequent east-west and ently limited in temporal and spatial span. Re- et al., 2003). The location of the source of off- north-south directions (inset, Fig. 2). Dating gional paleoseismic histories are often fault paleoseismic evidence is never precisely revealed simultaneity of collapses in diverse incomplete because of lack of surface faulting, known, yet the most likely source in our study areas of a cave or in more than one cave (doc- scarcity of Quaternary strata, and difficulties area is the nearby segment of Dead Sea trans- umented here). These observations are inter- in recognizing and locating reliable and date- form. Although the Jerusalem area, close to preted as reflecting seismic origin for the dam- able earthquake markers (seismites). Cave de- the study sites, is not directly on any of the age. In addition, nonseismic sources of posits (speleothems) can undergo various seismic features, it has been affected by many damage are negated: whereas in higher lati- types of damage during earthquakes and offer earthquakes. Because this area has been con- tude caves, underground ice movement and significant advantages for recovering long his- tinuously populated and has been a major re- permafrost may have caused speleothem dam- tories of earthquakes (Forti, 1998; Gilli, 1999; ligious and political center throughout histor- age (Forti, 1998; Gilli, 1999; Delaby, 2001), Gilli et al., 1999; Delaby, 2001) (Fig. 1). A ical times, the record of these earthquakes has in this part of the Levant cave temperatures modern-day example is provided by a 1996 M been studied in detail (Ambraseys et al., 1994; were significantly above freezing during the 5.2 earthquake in France that caused the col- Amiran et al., 1994; Guidoboni et al., 1994). investigated period (Frumkin et al., 1999; Bar- lapse of thin stalactites in a cave 10 km from Paleoseismic studies have provided geological Matthews et al., 2000; Ayalon et al., 2002). the epicenter (Gilli et al., 1999). The cave en- evidence of earthquakes in the Dead Sea trans- Human and animal activity is ruled out as vironment is ideal for paleoseismological in- form region (e.g., Marco et al., 1996; Ken-Tor source of damage because the caves were vestigation because earthquake damage is of- et al., 2001; Amit et al., 2002; Migowski et closed to the surface until artificial opening in ten fossilized by postearthquake calcification al., 2004; Begin et al., 2005). the twentieth century. Sediment fill is scarce and preserved from erosion. The seismic re- and there is no evidence of water flow, sedi- sponse at different depths below ground varies SEISMITES IN THE CAVE ment creep, or subsidence. These lines of ev- greatly and may produce amplification or at- SPELEOTHEM RECORD idence are consistent with the criteria set by tenuation of as much as a factor of six (Kanai The Soreq and Har-Tuv carbonate caves are Forti (1998) and Gilli (1999) for establishing et al., 1966; Bard and Tucker, 1985). There- small (Ͻ5000 m2), shallow (12–50 m below a seismic source of damage in caves. Thus, fore direct magnitude assessment is difficult, the surface), developed in well-bedded to our working hypothesis is that the damage and although this caveat can ultimately be allevi- massive upper Cenomanian dolomite, and of unconformities in the speleothems represent ated by careful comparison with independent phreatic origin (Frumkin et al., 1999). During seismically induced features (here termed paleoseismic records. the past several hundred thousand years abun- seismites). The Soreq and Har-Tuv Caves, Israel, are dant speleothems have been growing in the located 40 km due west of the Dead Sea trans- study caves, providing a climate record of late CHRONOLOGY OF THE SEISMITES form, one of the major strike-slip fault sys- Pleistocene–Holocene time (Bar-Matthews et For this study, more than 90 speleothem tems in the world, active since the Neogene al., 1997, 2000, 2003; Ayalon et al., 2002). seismite samples were collected in the Soreq (Garfunkel et al., 1981). The dominant seis- The calendar chronology of the speleothems and Har-Tuv Caves by hammer and by core in the caves was established by U-series meth- driller (5–10 cm diameter) while striving for *E-mail: [email protected]. ods (Bar-Matthews et al., 1997; Kaufman et spatial randomness. We avoided unnecessary ᭧ 2005 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; April 2005; v. 33; no. 4; p. 261–264; doi: 10.1130/G21193.1; 3 figures; Data Repository item 2005046. 261 Figure 1. Speleothem seismites in study caves. Open stars and solid ellipses mark pre-earth- quake and postearth- quake deposits, respec- tively. A: Stalagmite with severed top and post- seismic regrowth. B: Collapsed ceiling with preseismic stalactites (below) and postseismic stalagmites (above). C: Core in flowstone expos- ing fallen ceiling pieces (1), thin stalactites (2), and detrital layers (3). D: Section of severed sta- lagmite with postearthquake unconformable regrowth; dashed line is paleoseismic contact. E: Schematic cross section of severed stalagmite with regrowth. U-Th-dated laminae indi- cated by ␣. Small circles are schematic representations of stable isotope drilling points. .cm 10 ؍ Age of event is defined by datum intersecting contact (largest circle). Scale bars damage to the natural splendor of the caves. contact (degree of age offset depends on rate Drilling positions were chosen to incorporate, of sedimentation). To overcome this obstacle, where possible, both the preseismic and and to improve the precision of paleoseismic postseismic material. The unconformity be- ages, a novel approach was devised and ap- tween preseismic and postseismic speleothem plied for the first time: after 234U/230Th dating, laminae is termed the paleoseismic contact. a high-resolution ␦18O profile was prepared Figures 1D and 1E depict broken speleothems for each seismite, for which the size of an al- with regrowth exposing the angular unconfor- iquot was 0.2–0.5 mg and therefore can come mity at the paleoseismic contact. For dating very close to the contact. The profile for each we chose 38 seismites with well-defined pa- seismite sample was compared (wiggle leoseismic contacts and adequate material. matched) to the Soreq Cave comprehensive Figure 2. Tectonic setting of Soreq and Har- Tuv Caves, located 40 km due west of Dead The laminae adjacent to the paleoseismic con- profile (SCCP) (GSA Data Repository Fig. Sea transform. Triangle—study cave sites; tact were separated into subsamples with a DR11), a continuous, detailed, and rigorously four-pointed star—city of Jerusalem; open 0.4